Guerrilla guide to CNC machining, mold making, and resin casting

As discussed in section 1.3 of this guide, resin casting is a pretty amazing, simple, and user-friendly process that comes equally handy in CNC prototyping, 3D printing, and in manual DIY work; all its sophisticated uses aside, you will end up using it to replace broken parts in appliances or toys, make unique gifts, or even encapsulate backyard flowers or bugs (don't deny it).

Alas, the online market for moldmaking and casting supplies is dominated by several companies that cater chiefly to artistic users, and sell expensive products with poor mechanical properties and little utility in high-precision engineering work. In that spirit, even if you are familiar with artistic resin casting using epoxies, polyester resins, or polyurethanes from Alumilite, Smooth-On, and similar sources, you will be probably surprised by how much better your results can get.

4.1. Raw materials

4.1.1. Machinable stock

Well, before we dive into the world of casting resins, we should briefly revisit the choice of materials you can use to make master patterns. This part of this chapter is specific to projects that rely on CNC milling, so if you are interested in replicating manually crafted or 3D printed parts, you may want to skip ahead a page or two (this site provides a good overview of how to build patterns by hand - and if you have any other questions, feel free to drop me a mail or stop by /r/resincasting).

Still here? All right! Of course, milling machines are not particularly fussy, and will cut almost anything that is softer than tungsten carbide, but rigid enough to stay in one place; still, some materials are more predictable than others, and produce better results. Prime choices include rigid engineering plastics such as polyurethane, epoxy, polyester, ABS, polyamide (Nylon), or acetal (Delrin); many varieties of hard woords; aluminum, brass, and other soft metals and alloys; and more exotic choices such as printed circuit boards, hard waxes, plaster, etc.

Conversely, common materials that machine with greater difficulty or offer sub-par surface finish include stringy, low-melt thermoplastics (PET, polycarbonate, some grades of polyethylene); rubbers and other stretchy or squishy polymers (including some grades of PVC and most polystyrene foams); plywood and particle boards (including MDF); and exceptionally hard or highly abrasive stuff, such as steel, stone, or glass. Of course, many of these materials can be still cast or formed using CNC-machined molds and dies.

Poor choice of working materials is one of the most common mistakes made by hobbyist machinists; quite a few people stick to workpieces that offer poor accuracy, get damaged easily, gum up the tool, or simply cost way too much. For moldmaking purposes, your best bet is one of the little-known materials: an extremely accurate, low cost piece of plastic known as a medium-density modelling board, originally devised for the automotive industry. It's essentially a mix of medium strength polyurethane, and a combination of soft fillers such as calcium carbonate and aluminum hydroxide. There are many types of machinable boards, but the one we are interested in has a density of about 0.70-0.78 g/cm³, and vaguely resembles wood:

Prototyping boards of this particular variety include Huntsman RenShape 460 (or sligthly less dense BM 5460), Axson ProLab 65, Sika SikaBlock M700, BCC MB2001, Necuron Necumer 651, and several more. The material is typically sold in bulk, in sheets of about 50 x 150 cm, 25 mm thick. This may sound like a lot, but I recommend buying a full board, rather than grossly overpriced cut-to-size bits. The material lasts me for about a year, and costs about $12 per liter (roughly $250 for the whole thing). It's much less than what you'd pay for a similar slab of HDPE or acrylic - and it machines easier, too. (Planks of dense hard wood, when glued together and planed, may be a cheaper alternative for uncomplicated parts.)

Buying prototyping boards is actually pretty easy. If you are in the US, you can simply go to Freeman Supply, and order RenShape 460 online (search for item #075229). In other places, simply look at the manufacturers' websites and find local distributors, then send out several e-mails or make some calls (online ordering isn't common in the industrial world). Be aware that prices may vary significantly, so shop around.

Alongside with the board, you may want to order a matching board repair putty; it's a fast-curing, polyester compound that can be used to fix minor damage to your molds, or even completely fill a previously created cavity to reuse a particular workpiece for a new project. If you are ordering online with Freeman, go with their Quik-Fil; otherwise, ask the distributor for a matching product - they will be able to advise.

What else? Oh, about the only minor drawback of the medium-density boards is that they have a very fine but perceptible grain, as shown in this magnified image:

This grain normally has no appreciable effect on dimensional accuracy, but imparts a satin finish that will transfer to any transparent, water-clear parts. Of course, you can create high-gloss molds by coating the pattern with paste wax or a similar sealer (carnauba wax is particularly good); or you can always simply polish the final part - but both these options affect dimensional accuracy, and can be annoying when working on complex molds. The alternative is to use a more expensive material known as a tooling board - made out of solid, dense polyurethane, with no perceptible grain. Boards such as RenShape 5169 or BM 5272 cost up to 50% more, need to be machined 20-40% slower, and cause some wear to the tool - but they scratch this particular itch.

Note: RenShape 460 is relatively easy to cut with a hand saw; in fact, it's comparable to soft woods. That said, the extra labor may be annoying in the long haul, so it makes sense to have a decent jigsaw nearby. You can get one for around $35; blades designed for hard woods will cut the material very quickly and last for years.

4.1.2. Silicone rubbers

In order to replicate the parts laid out inside your pattern cavity, you need a flexible and durable substance to take an impression of the desired shape, and use it as a mold for the final product of your work.

There are several types of castable rubbers that could be useful for this purpose, but silicones are hard to beat. There are quite a few formulations that combine ease of use, excellent mechanical properties, perfect dimensional accuracy, no odor, no toxicity, and temperature resistance up to 300° C. On top of that, silicones come with an inherently non-stick surface, which helps greatly in casting work.

Almost all the silicone formulations you can find on the market come as a viscous goo consisting of long, linear, partly polymerized chains of siloxanes; that nominally non-reactive soup is then combined with a suitable cross-linker and a catalyst. The reaction between these components quickly turns the goo into a very bouncy solid; this can be initiated in several ways:

Room temperature vulcanizing, one component rubber (RTV-1): these substances undergo hydrolysis when exposed to atmospheric humidity. This reaction creates unstable molecules that promptly bind to each other, and release acetic acid or a simple alcohol as a byproduct (hence the trademark smell of the silicone sealants from a hardware store). This reaction, known as condensation polymerization , has the unfortunate effect of subtly affecting dimensional accuracy of the part - simply due to the release of volatile molecules. That said, the main reason why this process isn't popular for moldmaking is more prosaic: it's just that thicker layers take forever to set, owing to their limited exposure to air.

High-temperature vulcanizing silicones (HTV): here, condensation polymerization occurs in presence of free radicals, which are liberated from a heated organic peroxide. The suitable curing temperature hovers around 100-150° C. This method is sweet and simple, but also has a fatal side effect: the coefficient of thermal expansion for silicones is pretty high, around 0.025%/°C - and so, after cooling down, the dimensions will be off by several percent. This makes HTV rubbers completely useless for precision work. (It is possible to use radiation or UV light instead of heat, but that's not very practical in most cases.)

Room temperature, two-component rubber (RTV-2): Condensation cure: these rubbers polymerize at room temperature in presence of an organotin compound (typically dibutyltin dilaurate), which is mixed into the system shortly before pouring it all into a mold. The catalyst itself is fairly harmful, has a characteristic smell, and will leach out of the rubber in small amounts. Tin-cured silicones are popular in hobby work, but I don't recommend them; organotin aside, they exhibit measurable shrinkage (around 0.5%), have inferior mechanical properties, and are prone to cure reversal after 1-2 years or so. Addition cure: these silicones are catalyzed with a platinum complex, and polymeryze with no byproducts whatsoever. The materials are safe and odor-free, and the finished product has practically no shrinkage, exhibits superb mechanical characteristics, and can be stored indefinitely. Platinum cure rubbers are commonly used in medical and food handling applications, and are the prime choice for flexible molds.



In other words, you almost certainly want to stick to RTV-2 platinum-catalyzed silicones, unless you are working on life-sized castings (at that point, the cost of silicone can become prohibitive).

Before discussing specific products, let's have a quick look at the notable characteristics that will come up in product datasheets for these rubbers, and review their significance to our work:

Stiffness: the rubber we are going to use needs to be flexible enough to allow easy demolding of finished parts, but must be sufficiently rigid to maintain dimensional accuracy when laid on a flat surface, filled with liquid resin, and clamped or weighed down until the plastic is fully cured. There is no single, consistently advertised parameter that would give you a good idea of how flexible a particular rubber is, but this can be inferred in two relatively simple ways: Indentation hardness: this parameter is measured using an ad hoc scale known as "Shore A", named so after its inventor. The test involves pressing a flat-tipped 0.8 mm needle into a sample with a modest force (800 gf). Deflection of 2.5 mm or more corresponds to 0 on the scale, while no deflection whatsoever is denoted with the value of 100. Indentation hardness is a relatively poor proxy for rigidity, but is the primary key by which silicones are sorted in any catalog. In general, silicones under 10 Shore A are jello-like, and are used chiefly for special effects, soft bicycle seats, and - why not - sex toys. Compositions around 20-40A are fairly stretchy and squishy, similar to a rubber band. Silicones around 60-80A are still flexible, but begin to resemble a pencil eraser or a tire - you can flex them, but you won't stretch them easily. What to buy: rubbers starting from 20 Shore A are marketed as suitable for moldmaking, but I strongly recommend using around 60A for small, dimensionally accurate molds; rubbers under 40 Shore A are not advisable, except for large-scale work or for certain manual moldmaking techniques. Tensile strength vs elongation at break: if you need a more accurate picture of flexibility, looking at these values is a better approach - especially when comparing silicones to other types of rubber-like elastomers that we'll talk about soon. In essence, tensile strength tells you the stress (force divided by the area of cross-section) that causes the material to either snap, or to develop a localized defect known as necking. Elongation at break, on the other hand, describes the extent to which a standardized specimen can be stretched before breaking apart. If you divide tensile strength expressed as MPa (1 MPa = 145 psi) by elongation at break expressed as a ratio ( 1 + elongation / 100% ), you will get a somewhat arbitrary but useful value that will be low for squishy rubbers, and high for rigid ones. (There is also a proper way to measure stiffness - Young's modulus - but it's not very commonly seen in silicone datasheets.) What to buy: unless you are working with very large or very deep molds, try to select a rubber where this calculated "stiffness coefficient" is at least 2 MPa. Ratios up to 3 MPa are OK for most uses; values around 4 MPa make sense only for relatively shallow molds where dimensional accuracy is paramount.

Tear strength: removing cured parts from the mold inevitably involves some non-uniform pulling in the vicinity of sharp corners and other tight spots. In these cases, tensile strength does not accurately describe the resilience of the molding material; tear strength testing, which involves pulling a sample apart near the edge, offers a much better insight. The test used in the States is usually ASTM D624 die B, which involves a sample nicked with a razor; in Europe, a different method, similar to ASTM D624 die C, is more popular. There is no general mapping between these tests, but for moldmaking silicones specifically, the results given by the European approach (BS903 part A3) tend to be 30% lower or so. What to buy: tear strength of at least around 15-20 kN/m, as per ASTM method, is highly desirable (the value corresponds to force divided by the thickness of the sample; 1 kN/m = 5.7 ppi). Formulations up to 30 kN/m are available on the market, and are certainly not a waste of money. It's best to stay away from products where the advertised tear strength less than 12 kN/m or so.

Mixed viscosity: rheology of a liquid is a complex topic and remains very difficult to fully parametrize, but within a single family of products, formulations with lower dynamic viscosity are generally easier to mix, pour, and degas - and will be more inclined to conform to complex shapes without the aid of vacuum or pressure. Silicone rubbers suitable for moldmaking start around 10,000 mPa*s; this viscosity resembles a runny syrup, and is very easy to work with. Compositions that go over 35,000 mPa*s or so are more prone to air entrapment when mixing the components, and when pouring them into a mold; the use of vacuum degassing is advisable for any complex work. Finally, once you cross 100,000 mPa*s, almost any silicone becomes fairly painful to work with. What to buy: all other things being roughly equal, go with the system with the lowest viscosity. Products below 50,000 mPa*s are advisable, but not strictly a must.

Pot life and demold time: RTV-2 formulations begin to polymeryze the moment you mix the two components together; the point at which it the mix becomes so thick that it is no longer free-flowing or self-leveling is known as pot life or gel time . In the same vein, the point at which it is sufficiently cross-linked to be safe to remove from the master is referred to as demold time . Both of these values must be chosen so that the product can be worked with at a reasonable pace. What to buy: pot life of around 30 minutes in the bare minimum for low viscosity compositions; for medium and high viscosity, 50-90 minutes is preferable, because every step of the casting process will take a bit longer. Demold time should be as short as possible; I'd stay clear of any formulations that require more than 24 hours to set. Note: the speed of this and many other chemical reactions roughly doubles with every 10° C. If you are in a rush and don't care about some minor thermal expansion, using moderate heat is a great way to have your molds ready in 2-3 hours or so.

Shrinkage: addition cure silicones used for moldmaking purposes should have no measurable shrinkage. Some manufacturers simply say "none", while others say "less than 0.1%" or so. Seeing a higher value in the datasheet is a warning sign; shrinkages between 0.2% and 0.6% are commonly seen in condensation cure silicones, and should be avoided in precision work.

Color: moldmaking silicones are usually opaque beige in their virgin state, and can be pigmented easily. Systems in such a neutral color come handy if you wish to use the rubber not only for molds, but also to make functional parts (tires, etc). Sadly, many of the moldmaking compositions are artificially dyed blue, red, or green, to help novice users mix the components properly. That aside, there are several silicone formulations that are nearly transparent. The product is never water clear, and will have a slight milky haze; it will also lose transparency if exposed to high temperatures. Because of this, it's not particularly suitable for decorative purposes, but can be indispensable for non-CNC replication work, where you often need to see the submerged master part, and carefully cut the mold to recover it. The major downside of transparent silicones is that they have somewhat different flow characteristics, and can be difficult to degas. What to buy: I would not consider color to be the most important factor - if necessary, polyurethane elastomers offer a good alternative for making functional parts. That said, if you have a choice, it certainly doesn't hurt to go with something that can be pigmented easily.

All right, that's it! Other parameters are either uninteresting, or are not advertised consistently. To help you with the selection process, my top recommendations for mechanical projects would be:

Best overall properties: Quantum Silicones QM 262. This is a pale blue, opaque rubber with low viscosity (35,000 mPa*s), hardness around 65A, 12 hour demold time, and high strength. Hobby Silicone is a reputable distributor, so give them a try. In Europe and some other markets, Bluestar Silicones RTV 3460 is a decent, but considerably more viscous, alternative.

Transparent / pigmentable: Silicones Inc P-592 (formerly known as XP-592). Truly exceptional tear strength, good price, medium viscosity (50,000 mPa*s), hardness in the vicinity of 60 Shore A, more challenging to degas. Cures in about 12 hours. You can order this product through Innovative Polymers, along with some of the other items we'll cover in the next section.

High rigidity: Quantum Silicones QM 270, also sold as ACC Silicones QM270 in Europe. Neutral beige or turquoise rubber around 75 Shore A, excellent mechanical properties and manageable viscosity (50,000 mPa*s). Remains easy to degas. Somewhat less suitable for deep molds, undercuts, and in other applications requiring a stretchy material - but its great for small, high-precision parts.

Super-low viscosity: Quantum Silicones QM 237. Around 40 Shore A, very easy to mix and pour (10,000 mPa*s), properties are still acceptable for typical molds. If you live in Europe, you may want to check out ACC Silicones MM242, which is pretty similar and comes in a transparent variety.

You should pick just one of these; if you're undecided, go with QM 262 or (X)P-592. If you need other options... well, Silicones Inc and Quantum are the most interesting US-based companies I know of. Other choices include Polytek, GT Products, BJB, and Smooth-On - but in my opinion, their selection is much less impressive, and the options I have tried pale in comparison with the ones mentioned on the recommended list.

US market aside, globally, Bluestar Silicones (Rhodia) is fairly ubiquitous; you may also want to check out ShinEtsu, Wacker, Zhermack, Huntsman, Axson, or Dow Corning - depending on where you are located, they all offer some interesting choices. But then, one reader living in Norway reported that placing an international order with Hobby Silicone for QM 262 actually turned out to be much cheaper than locally available alternatives (whoa).

As far as pricing goes, platinum cure silicones cost around $35-$45/liter when bought in one liter cans, or about $30-$35/liter in 4-5 liter pails; for example, (X)P-592 costs around $120 for 4 l, while QM 270 and QM 262 in 5 l quantities fetch $140 and $170, respectively. Unreacted liquids should survive at least 2-3 years without significant deterioration, as long as you keep them in tightly closed containers, away from sunlight, moisture, and excess heat - so getting a full pail is not a bad idea.

Note: resin manufacturers in the States use a somewhat confusing scheme for describing the size of their two-component kits: "1 gallon kit" usually means that you are getting about one gallon of whichever component is needed in greater quantity; and a matching amount of the other one. If the mix ratio is 10:1 (as is the case with most platinum cure silicones), the gain is minimal - but for resins mixed 1:1, you are actually buying two gallons or so. To further confuse you, the same does not apply to sizes specified in pounds - "15 lbs" means that you are getting just enough to cast a 15 lbs blob of plastic or rubber. Be sure to account for these differences when shopping around: a lower price is not always a better deal. To further confuse you, the same does not apply to sizes specified in pounds - "15 lbs" means that you are getting just enough to cast a 15 lbs blob of plastic or rubber. Be sure to account for these differences when shopping around: a lower price is not always a better deal.

4.1.3. Rigid polyurethanes

All right, let's talk about the materials you can employ to actually make final parts!

Polyurethanes are an incredibly interesting and versatile class of two-component, addition cure polymers. They use two principal reagents, mixed in comparable quantities: a non-volatile isocyanate and a complex alcohol (polyol). Some formulations trade some or all of the polyol for a polyetheramine, resulting in a material that is more properly called a polyurea. In any case, the two components are usually combined with variable amounts of chain modifiers, usually chemically similar to the primary polyol; and possibly surfactants, plasticizers, fillers, and so on. The whole thing is then catalyzed with a wide variety of organometallic compounds (bismuth, zinc, tin, zirconium, aluminum, or similar); with tertiary amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO / TEDA); with diazoles such as 1,2-dimethylimidazole (DMI); or with something pretty close to that.

Modern polyurethane chemistry lets you manufacture everything from soft foams, to high-performance rubbers, to faithful, often superior imitations of many other rigid engineering plastics - all that without having to go bankrupt on injection molding equipment, and while using only fairly safe and predictable chemicals. They greatly outperform more familiar resins, such as epoxies or polyesters, and in hobbyist workshops, are much less dangerous to work with.

The only downside of high-performance polyurethane systems is that they generally require a basic vacuum rig - a small pump and a suitable container to remove any dissolved gases from the mix. Products that do not require degassing are readily available, but usually don't perform as well as their peers. (We'll talk about the required harware later on, but it's pretty compact and doesn't cost a lot.)

Anyway - if you are aiming to make functional prototypes, it is probably prudent to start by stocking up on a polyurethane resin that lets you produce hard, rigid, and shock-resistant parts. Once more, let's have a look at some of the key things worth highlighting in a datasheet:

Stated purpose: there is a lot of variety in the world of polyurethanes, and not all the properties of a material can accurately captured with several cold, dry numbers alone. You should always pay attention to what the manufacturer is trying to tell you: if they are talking about simulating engineering plastics such as ABS, polyamide, or polyolefins - great. If they are instead paying undue attention to tooling fixtures, conceptual prototypes, scale models, etc, be wary. This may mean that the resin has disproportionately low strength in thin sections, is very brittle, or suffers from other malady. If in doubt, simply talk to them and clarify. For the same reason, tread carefully if the resin is designated strictly for meter-mix machines, vacuum assisted casting equipment, or something of that sort. It may mean that the material is very difficult to degas, has an annoyingly short pot life, or requires a heated mold. That label is not always a problem, but 75% of the time, it's there for a reason. As usual, when in doubt, ask. It's often also possible to get a sample and try it out yourself.

Hardness: indentation hardness of rigid plastics is typically measured using a scale called "Shore D". This approach is very similar to the one used for rubbers, except that it relies on a sharper tip (0.1 mm) and a greater force (4,500 g). Indentation hardness doesn't necessarily map to rigidity, or any other useful property - but since it's always listed prominently, use it as a very rough selection criteria: plastics under 75 Shore D are unlikely to be strong enough for making thin-walled parts.

Flexural properties: almost all the prototyping work you may end up doing will probably benefit from a plastic that withstands high bending forces without snapping in half; and that maintains as much rigidity as possible under such a load. These qualities are measured using two values: flexural strength and flexural modulus . The first one describes the force in relation to part dimensions at which a standardized specimen breaks; the other one tells you the ratio between the stress the material is under, and its degree of deformation. The higher it is, the more rigid the part will seem. In comparison to these two, compressive and tensile properties usually matter a lot less in small-scale prototyping work. What to buy: aim high. Look for flexural strength around 90-120 MPa, and for flexural modulus somewhere between 2.5 and 3.1 GPa. This is roughly comparable or better than tough materials such as ABS (Lego bricks), polyamide (plastic gears), polycarbonate (CDs and DVDs), or acrylic glass. For a comparison of common plastics, this website is a great starting point.

Impact resistance: flexural strength of a polymer is important, but doesn't paint a complete picture. For example, polypropylene is a relatively flimsy plastic, with flexural strength less than 40% that of acrylic glass; yet, Tupperware made out of PP easily survives being dropped to the floor, while acrylic glass of the same thickness and shape would likely shatter right away. The difference is that, especially in thin sections, polypropylene has a much better ability to flex and dissipate the energy of a localized shock; this is usually measured using a test known as notched Izod impact strength, where a pendulum swings and breaks an upward-facing, notched sample in its path; the test estimates the energy lost by the pendulum, in proportion of the thickness of the specimen (minus the depth of the notch). Lo and behold - the strength of acrylic glass is barely 15 J/m (1 J/m = 0.0187 ft-lbs/in), while various grades of polypropylene vary between 100 and 300 J/m. Impact strength matters more often than you might expect. For example, tiny gears subjected to sudden torque variations when braking will be more prone to failing catastrophically if the material has poor resistance to such shocks. Most rigid polyurethanes have Izod impact strength around 30-50 J/m, resembling Nylon, polystyrene, and so forth: they will perform well, but if you step on a medium size, very thin-walled part, it will probably break. There are quite a few formulations closer to 100-300 J/m, resembling plastics such as ABS; if you ever stepped on a Lego brick, you probably know that it prevails, at the expense of your foot. What to buy: your primary resins used for functional prototypes should have impact strength of at least 30 J/m; if the value is not provided by the manufacturer, it's probably lower than that. You typically don't need much higher figures, but if you have a choice, getting something closer to 100 J/m doesn't hurt.

Shrinkage: this parameter is somewhat important in polyurethanes, because in some circumstances, it may significantly affect the dimensions of your parts. Alas, the standardized test is essentially meaningless. Polyurethane polymerization reaction has virtually no inherent shrinkage, but can be fairly exothermic; thermal expansion of the liquid, and the response of the flexible mold that surrounds it, is what may mess up the final result. In general, coefficient of thermal expansion for rigid polyurethanes is about five times lower than that of silicone (perhaps 0.005%/°C); when using sensibly selected resins, the exotherm in thin parts will not exceed 5-10° C above room temperature, and the resulting shrinkage will be practically nil, no matter what the datasheet says. On the flip side, when dealing with parts that have thick cross-sections, the temperature may peak 50° C over ambient - and at that point, dimensional accuracy in the most easily deflected portion of the mold will suffer. For example, when casting a solid block of resin with dimensions of 10 x 10 x 30 mm, and using a sealed rubber mold and a resin with 10 minute pot life, the most vulnerable point in the mid-section of the mold may be off by as much as 0.3 mm (3%). The rest of the part will be probably spot on. Luckily, it is within your power to change this. We'll talk about this in more detail later on, but in essence, you can: Slow down the reaction. The boring option is to switch to a system with a pot life of 30-50 minutes, and overnight cure - as these will usually have negligible exotherm even for bulkier castings. The more exciting option is to get rid of some of the catalyst present in the product, which usually involves adding a common and reasonably safe chemical to the system. It's simpler than it sounds. Partly prepolymerize the resin. Pre-mixing the full amount of isocyanate with about 10-20% of the intended amount of polyol, and setting this mixture aside for about an hour or two, will minimize the exotherm later on, likely reducing shrinkage by 70-90%. This has several other benefits, too, and the only side effect is increased viscosity. Add an inert filler to act as a moderator. Well-chosen additives allow you to reduce shrinkage by at least 30% without significantly compromising flexural properties of the system; going up to 80-90% is possible if you are willing to make some performance trade-offs. Allow some extra resin to flow back into the mold as the system cools down. Adding sprues or vents allows shrinkage to be offset without deforming the mold, and can result in dimensional accuracy improved by 80% or more. Cast in several layers. This eliminates shrinkage almost completely, but works well only with relatively simple shapes. The approaches can be combined, often with synergistic results. It is also possible to let shrinkage happen, but direct it toward non-essential locations; this can be accomplished by intentionally weakening a section of the mold, for example by using thin walls. The bottom line is that you don't have to worry a lot - but you need to be aware of this behavior, and need to know how to reduce it when necessary. What to buy: all in all, don't read too much into the datasheet, but be wary of resins with very high stated shrinkage - unless that shrinkage is measured by the manufacturer for a sample that is cured at an elevated temperature. For samples that spend the first few hours or days at room temperature, advertised shrinkage should be less than 0.3% or so. Beyond that, comparisons are somewhat pointless.

Mixed viscosity: flow characteristics of polyurethanes are very different from silicones, possibly due to higher surface tension; on top of that, polyurethanes usually have much shorter pot life. Because of that, resins under 1,000 mPa*s are preferred; products over 2,000 mPa*s or so will be more challenging to degas and pour - and venturing past 4,000 mPa*s should be done at own risk. The parameter is not critical, but it's good to avoid extremes.

Pot life and demold time: many polyurethanes will have fairly short pot life, between 30 seconds and 4 minutes. That's usually not enough for manual casting, unless you are doing something very simple. Around 7-8 minutes is highly advisable for low viscosity resins, and closer to 10-15 minutes makes sense for when dealing with resins over 1,500 mPa*s or so. As for demold time, it is usually preferable to keep it short; between 2 and 6 hours is ideal, as it lets you crank out multiple batches in a single day. That said, overnight cure (12-24 hours) is not the end of the world. Any slow-curing resin can be accelerated in a simple way, and we'll cover that a few sections down.

Presence of fillers: some resins are pre-mixed with fillers to improve their flexural properties, abrasion resistance, or fire rating. In general, don't go there unless you are sure that you can't approximate this effect by manually adding the appropriate filler, and that you want to be permanently stuck with all the drawbacks (e.g., higher viscosity, lower impact strength, and the tendency for fillers to settle in storage).

Color: avoid dyed or non-pigmentable resins. Transparent, translucent, neutral white, and slightly off-white resins (light beige, lightly straw-colored, etc) can be usually easily pigmented to your liking. Colors such as amber, caramel, or tan are more of a gamble; ask for a photo or a sample of the material to determine how bad it really is. Some of these colors are faint and easily overcome, and some are there to stay.

Compatibility with platinum-cure silicones: there is a small number of transparent resins or other specialty formulations may be inhibited by the platinum catalyst present in silicone molds. Always scan the datasheet for any mentions of that risk. If it's not mentioned, you are good to go.

All right, ready for some recommendations? Here we go:

Best all-around resin: Innovative Polymers IE-3075. Really, nothing else comes close. Translucent, high-strength plastic with a reasonable pot life (8 minutes) and quick demold time (2-3 hours for smaller parts). Around 83 Shore D, flex strength 120 MPa, modulus 2.9 GPa, decent impact resistance. Very low cost - around $17 per liter. The system can be slowed down in a simple way if you need a longer cure time or lower exotherm. A good fallback option in the US and several other markets is BJB Enterprises TC-854 A/B, which is about 15% less rigid and cures a tad faster, but is otherwise indistinguishable from IE-3075 and costs about the same. The other US choice is Huntsman RenCast 6591, but it has noticably shorter pot life, is harder to pigment, and costs much more. For those of you in Europe, RenCast 5146 (also sold as RenPIM 5219) is a very similar, low-cost resin. This one has a pot life of around 30 minutes, and requires overnight cure (unless you add some extra catalyst). There are some distributors for BJB products in the EU, too, so you may also have luck with TC-854 A/B.

Water clear: Innovative Polymers OC-7086. Like other mercury-free compositions, it is extremely sensitive to moisture in thin layers. When making small castings in single-part molds, you may end up getting relatively low yields unless you preheat the mold, perform pressure casting, or add a small amount of a separately purchased catalyst (see section 4.4.4). The price of the resin is relatively steep, around $45 per liter.

High-impact plastic: Innovative Polymers TP-4052. This product is fairly similar to IE-3075 in most aspects. Flexural parameters are somewhat worse, but impact strength is almost three times higher. Price is in the vicinity of $30 per liter. Note that as opposed to IE-3075, I have not found a way to slow this system down when low shrinkage in mass casting is desired.

At this point, unless you have a specific itch to scratch, it's perfectly OK to order just IE-3075 directly from IPI. Their "1 gallon kit" (actually around 6.5 l) sells for about $110. "Quart kits" are also available if you need to try it out first.

If you want to shop around, I don't think it makes sense to look beyond Innovative Polymers - not if you're in the States; they have a remarkable selection of unique, user-friendly products designed specifically for manual casting, and great customer service. They take direct orders, and also have several local distributors. If you're skeptical, you can have a look at products from Smooth-On, Alumilite, BJB Enterprises, or Freeman - but you will not find anything that even comes close to that selection. If you are in Europe, checking out Huntsman and Axson is not a bad plan.

As with silicones, polyurethane resins are pretty stable and have a fairly long shelf life - but they are fairly sensitive to sunlight, humidity, heat, oxygen, and moisture. It is a good practice to buy several 100-250 ml polypropylene or HDPE bottles (example) for intermediate storage of the amounts of resin you plan to use within a month or so - and keep the original containers sealed and blanketed with inert gas. The inert gas can be just a burst of "canned air" (difluoroethane or tetrafluoroethane), but if you want to save money in the long haul and have some room in your workshop, it makes more sense to invest around $150 in a small nitrogen tank, a regulator, and a hose. This setup will last for months, and it's only about $10 to refill at a nearby Praxair location or so.

In any case, if you store the resins properly, you can expect most of them to maintain their properties for at least 2-3 years; manufacturers usually give much more conservative guarantees, but take them with a grain of salt. Remember to agitate the containers if the components separate. Oh, some formulations may crystallize if kept below 10° C or so, but this process can be reversed easily. Prolonged storage in crystallized form is not advisable, as it may lead to the formation of insoluble dimers.

Caution: although most of the polyurethane formulations you will encounter are reasonably safe, there are some unfortunate exceptions. We'll talk about this a bit more in chapter 7 - but for now, definitely watch out for: Volatile isocyanates: all isocyanates are dangerous if inhaled, but most of the prototyping formulations use prepolymers or monomers with negligible evaporation rates - so it would take serious effort to put yourself at risk (intentionally aerosolizing the resin would be one bad idea). In particular, MDI, HMDI / DMDI, and IPDI isocyanates are pretty safe. That said, some industrial resins use more volatile and more reactive compounds, such as toluene diisocyanate (TDI) or hexamethylene diisocyanate (HDI) - and these are best avoided.

Toxic catalysts: minutiae amounts of harmful organomercury salts, such as phenylmercuric neodecanoate, were found to be excellent, highly selective catalysts for polyurethanes; less concerning but still somewhat nasty tin(IV) compounds (e.g., dibutyltin dilaurate) offered some hard-to-replicate benefits, too. Needless to say, you probably want to avoid unnecessary risks. Such catalysts are largely phased out in favor of compounds of zinc, bismuth, tin(II), titanium, aluminum, and so on - but you can occasionally find them in products available in some markets (for example, Smooth-On, BJB, Alumilite, and Freeman still use them frequently).

Possibly harmful amines: several amine curatives once-popular in certain high-performance elastomers - most notably 4,4'-methylenebis(2-chloroaniline) (MBOCA / MOCA) - were implicated as probably carcinogenic in humans. Be watchful for products that haven't switched to safer, modern alternatives. You won't find any of the problematic components in resins from Innovative Polymers - but other manufacturers sometimes show less restraint. Request and study material safety datasheets (MSDS) when in doubt.

4.1.4. Flexible polyurethanes

Flexible polyurethane elastomers are an interesting alternative to silicones. You don't necessarily need to buy any, but they may come handy if you wish to make functional rubber parts, and your silicone is not pigmentable, or has insufficient strength.

Key advantages of these rubbers include 30-50% lower price, and much better performance toward the upper end of Shore A (above 40 or so): several times higher tear strength, much less pronounced tear propagation, excellent abrasion resistance, lower coefficient of thermal expansion, lower viscosity, and the ability to pigment the system as seen fit. On the flip side, polyurethanes exhibit some exotherm-caused shrinkage in larger castings, and adhere to many other plastics, making them less desirable for creating negative molds; they also tend to have slightly worse rebound characteristics, and limited temperature resistance (they get soft around 70-90° C, and deteriorate somewhere between 150 and 200° C - so casting low-melt metals, for example, is out of question).

The parameters to look for in these compositions are similar to these for silicones, with the exception of pot life and viscosity - here, the advice provided for polyurethanes is more pertinent. When reviewing the datasheets, pay close attention to the ratio of tensile strength to elongation at break, because there can be striking differences in the rigidity of various products, especially those rated 70 Shore A or above; some of them are only somewhat flexible, and will not be suitable for making parts such as tires, rollers, or transmission belts.

Note: This goes both ways: not all Shore A polyurethanes are particularly rubbery, and not all Shore D polyurethanes are necessarily very rigid. For example, some 60 Shore D polymers are highly elastic and can be stretched up to three times their original length, even though their surface feels hard as nails. They resemble the rubbers used in certain garden hoses, shopping cart wheels, etc.

As for recommendations: you should go with Innovative Polymers HP-21xx series; I've tried quite a few other products, and nothing else comes close. They are relatively inexpensive ($25 per liter) and feature superb "true rubber" mechanical characteristics, long pot life, and good cure profiles. For example, HP-2170A is a super-stretchy 70 Shore A rubber with tear strength of 42 kN/m, far surpassing most silicones. There are also softer variants, down to 50 Shore A (HP-2150A); and more rigid but still surprisingly flexible ones, up to around 60 Shore D (HP-2160D). In fact, it's possible to blend them to achieve intermediate properties as needed for a particular project.

All the products in the HP-21xx line take several days to polymerize at room temperature; if that's too slow and heating them up is not an option, you can add some separately purchased catalyst to get overnight cure with no real trade-offs.

4.1.5. Epoxies, polyesters, and so on

If you have dabbled in resin casting before, chances are, you used epoxy or styrene-based polyester resins, rather than polyurethanes. These options are popular with hobbyists because of their broad availability, low price, and less onerous processing requirements (i.e., less sensitivity to mixing ratios, moisture contamination, etc). That said, I think it's a bad idea to use these materials in precision casting work, for a couple of reasons:

High shrinkage. Both polyesters and epoxies cure by condensation, and usually exhibit very high exotherm compared to polyurethanes with the same demold time. Shrinkage as high as 10% is common in unfilled polyesters; epoxies are better, but not dramatically so.

Mold damage. Epoxies and polyesters use corrossive, oxidizing, or permeating components, and this reduces the service life of silicone molds quite a bit. It may be difficult to get 5-10 pulls out of a complex mold.

Brittleness. Most epoxies and polyesters are designed to be cheap and to bond well, and not to have outstanding flexural parameters or impact strength. When superior characteristics are required, fiberglass, carbon fiber, or aramid cloth is typically used for reinforcement, but that's not very feasible for tiny, complex parts.

Low UV resistance and clarity. Transparent epoxies and polyesters usually have lower clarity than water-clear polyurethanes, and are more prone to developing a yellow hue when exposed to sunlight. In comparison, many polyurethanes exhibit high UV stability.

Workshop safety concerns. Components of a polyester resin are extremely volatile, flammable, and are an inhalation risk. Epoxies are more user-friendly, but use corrossive chemicals, and contain non-trivial amounts of bisphenol A. Bisphenol A hasn't been conclusively implicated in being harmful to humans, but is suspected of being an endocrine disruptor based on some subset of animal studies. Limiting your non-essential exposure may be a wise step.

Of course, I don't want to demonize these polymers. Polyesters are sometimes useful for bonding and repair applications; and epoxies are extremely useful as high-performance glues and laminating or potting resins. Both epoxies and polyesters can be superior in lay-up composite applications, too, in part owing to their improved bonding capabilities. You can have a look at the products sold by Freeman Supply, or check out the low-cost, water-clear epoxies available from Polymer Composites Inc; just don't expect them to be a sensible match for PU for the processes discussed in this guide.

4.1.6. Pigments and dyes

Adding colors to your castings is fairly easy. One option is to find an artist store, and shop for dry, non-toxic organic pigments. Such pigments will work equally well with polyurethanes, silicones, and just about anything else - but may be relatively painful to disperse or to blend with any accuracy. The other choice is to purchase coloring pastes where pigments are already dispersed in a non-reactive (plasticizer) or reactive (polyol) medium - but these won't work in silicones.

For dry pigments, Kremer is probably the best source online. For reactive dispersions, you can ping Innovative Polymers (send them your picks from this RAL chart); Eager Plastics has a pretty good selection of non-reactive pigments and dyes, too.

Beyond that... selecting your palette is a matter of personal preferences, but here are some quick tips:

Be sure to get some titanium-based white (Kremer 46200, IPI Bright White) and deep black (Kremer 47400, IPI Black 201). These two are used not only as standalone pigments, but are also useful as opacifiers for controlling transparency of the material, and the saturation produced by other dyes.

When browsing artist stores, try to avoid most mineral-, plant-, and animal-based pigments; stick to modern synthetic dyes where possible. In particular, be wary of traditional products containing lead, mercury, arsenic, cadmium, chromium, and nickel.

It's easy to go from vivid hues to muted ones, but not the other way round. Focus on punchy pigments with simple names; "Bohemian moonlight blue" is not what you need.

You can't just buy three primary colors, and hope to make every other hue with that. It won't work, especially if you are aiming for a vibrant pallette. Aim to get at least 6-8 different, useful colors to begin with.

Whatever you do, keep your pigments moisture-free. Store them in closed containers and in a dry place.

If you need inspiration, here are some nice picks:

Daylight fluorescent colors: Kremer has a line of day-glo pigments that result in extremely vivid colors that form translucent dispersions; Innovative Polymers carries a good selection, too.

Basic opaque pigments: in addition to black and white, you may want to start with vibrant yellow (Kremer 55100, IPI Yellow 802), cyan (Kremer 55500, IPI Blue 303), pure red (Kremer 55300, IPI Bright Red), magenta (Kremer 55470, IPI Magenta), green (Kremer 55700, IPI Signal green), orange (Kremer 55200, IPI Orange 602).

Transparent dyes: useful only if you are planning to make transparent or translucent parts. ORASOL dyes produce brilliant transparent colors, but pre-made dispersions from Eager Plastics are more convenient to use and have very high yields.

Fancy pearlescent pigments: check out Paint With Pearl. Their prices are hard to beat.

A small bag or bottle of a pigment will cost somewhere between $5 and $20, and should last for years.

Tip: You can, of course, opt to paint your parts instead of adding pigments to the resin; acrylic and polyurethane lacquers can be used alike. For adding text or other ornamental elements to machined parts, you may also want to consider a low-cost vinyl cutter, such as Silhouette SD or Roland Stika SV-8. These devices are fairly affordable, and the results look amazingly good - especially if a layer of clear coat is applied on top. If another cutting machine is too much, you can also simply equip your CNC mill with a specialized If another cutting machine is too much, you can also simply equip your CNC mill with a specialized drag knife to get comparable results.

Adding colors aside, many other properties of cured plastics can be altered in profound ways by introducing certain easily available, low-cost additives. It's almost impossible to provide an exhaustive overview of all the available choices, but several use cases are definitely worth calling out:

If you want to reduce shrinkage, there are many fillers that increase the distance between reactive molecules of the resin, slowing down the reaction and buffering some of the produced heat - but the ones that do so without significantly affecting viscosity or mechanical properties of the finished part are of particular interest. I found that 3M Scotchlite iM30K, a type of high-strength, hollow glass microspheres (less than 0.02 mm in diameter) works great when added at a ratio of about 4% by weight; when used in conjunction with the pre-mixing trick discussed earlier on, shrinkage is eliminated almost completely. When flexural properties are of utmost importance, tiny milled glass fibers, around 0.8 mm long, significantly reduce shrinkage at a radio between 10 and 30% by weight. Both of these fillers sell for less than $20 per liter.

When you need to reduce weight, Scotchlite S15 can be added to any system at ratios as high as 3:1 by volume, resulting in a free-flowing resin with effective density less than 0.4 g/cm³. Compressive strength will be pretty reasonable, but flexural properties will suffer. This may come handy especially for hulls of model airplanes or boats.

When you want an extremely hard plastic, the aforementioned variety of short glass fibers can substantially improve flexural strength, more than double flexural modulus, and significantly increase hardness and abrasion resistance - especially when used in conjunction with a silane coupling agent (see later on). Using longer or thicker fibers is a possibility, but may affect surface finish or complicate the casting process. For short fibers, ratios around 30% by weight are a good starting point. The downside of this trick is a significant reduction in impact strength; starting with a high-impact resin, such as TP-4052, may give you a bit more wiggle room.

For improved impact strength, lower hardness, or lower viscosity, plasticizers can be introduced to the system to effectively add "padding" in the resulting polymer structure. Because they don't participate in the polymerization reaction, they reduce tensile strength and may increase creep (the tendency for the material to permanently deform under relatively minor but sustained load), but that trade-off is acceptable in many uses. For polyurethanes, one of the safer and more versatile plasticizers is dipropylene glycol dibenzoate (DPGDB), available from Eager Plastics under the name of EP9009. For silicones, any low-viscosity silicone oil will do; you can get it from the same source that you are purchasing the resin from. Plasticizer levels between 5 and 10% are common, although in silicone rubbers, up to 50% may be useful. You shouldn't have to pay more than $20-$40 per liter or so.

To simulate select properties of metals: powdered metals, such as copper, aluminum, or iron, can be added to resins in significant amounts to increase their weight, thermal conductivity, and remarkably, give them a highly metallic appearance and feel when polished. You can find suitable 300-600 mesh metal powders on eBay or from sources such as ArtMolds. Prices vary, but aren't prohibitive. Flexural properties will suffer.

Of course, multiple types of fillers can be combined; in particular, it may be useful to add some glass fibers to lightweight materials filled with hollow glass spheres, to maintain acceptable flexural strength. If you are wondering what to buy up front, it's not a completely bad plan to get some Scotchlite iM30K, plus 0.8 mm glass fibers, and a bit of DPGDB (also useful for preparing pigment dispersions and such). Other fillers are not nearly as essential.

4.2. Casting workshop

Now that we have the selection of resins, pigments, and fillers sorted out, it's time to briefly chat about the workshop equipment you will need to get the ball rolling. The list isn't particularly long, but even when it comes to something as inconsequential as mixing cups, making the wrong choice will unnecessarily complicate your life.

4.2.1. Vacuum pump and chamber

Insufficient mixing may cause a range of problems with finished parts - but vigorous stirring will almost always introduce some air into the resin. This problem aside, bubbles of air may get trapped inside mold crevices as you pour the mixture in - even if your mixing skills are beyond reproach. Last but not least, some resins may simply liberate some amount of dissolved gases once the polymerization reaction begins; IE-3075 is an example of that. Vacuum degassing solves all these problems, and is not as scary as it may sound.

Even if you are on a tight budget, you should get a low-cost vacuum pump capable of getting pulling around -1000 mbar of vacuum (that's about 10 mbar absolute, or -29.5 inches of mercury). I ordered mine from VIOT: click here for an entry-level model that sells for around $100, and should work fine; I have a $140 model with a higher flow rate, and it served me well. You will also need some sort of a vacuum chamber to hold the mixing container and the mold itself; it's possible to rig something together on your own, for example using a sturdy cooking pot and a cover made out of thick polycarbonate - but low-cost vacuum dessicator chambers work fine in that capacity, and start around $50 or so. If you are planning to work on relatively small projects, or opted for a pump with a relatiely low flow rate, Bel-Art #420100000 is a good choice; otherwise, model #420430000 will accommodate larger pieces, too.

(For a bit more money, you can also find some good-looking purpose-built degassing chambers from several sellers on eBay.)

About the only other piece of this puzzle is a hose to connect the pump with the chamber. I use 1/4" Kuriyama K7160 Polyspring, which I ordered from these guys; it's about $4 per meter. Other hoses can be used, too, but they need to be vacuum-rated, which in practice means that they need to be either fairly rigid, or reinforced with a metal spring to avoid collapsing as soon as you turn the pump on. Beyond that... well, getting some vacuum grease for o-rings and other parts of the vacuum chamber is not a bad plan (link); it also comes handy for several other purposes every now and then - for example, for preventing caps on resin bottles from seizing in storage.

4.2.2. Auxiliary tools

It goes without saying that for any sort of precision work, you need a reasonably well-equipped workshop to begin with. I am assuming that you already have that - and that basic tools such as clamps, needle files, or several grades of fine sanding paper, are always within reach.

With that in mind, there are several less obvious, minor items that will be useful specifically when casting mechanical parts, and that we haven't mentioned before:

Precision scale: observing proper mix ratios is extremely important for addition cure silicones - errors of as little as 2% may affect important properties of the rubber. Polyurethanes are a bit more tolerant, but it is important to stay within 4% or so. To that effect, in order to accurately mix components when working with relatively small quantities of resins (10-50 ml), you should try to get a scale with a resolution of at least 0.05 g, and measurement range of at least 0-200 g. You can get something usable for under $40 (I ordered from wholesale-scales.com, and can't complain). If you are watching your budget, you can also get away with a regular, gram-resolution kitchen scale - but in this case, you shouldn't attempt to mix less than about 30 ml of polyurethane, or 100 ml of silicone. In the long haul, this may be wasteful, depending on the size of your parts.

Mixing containers: any plastic cup should do, but consider getting a box of one hundred 100 ml polypropylene beakers from Ted Pella or any other outlet with lab supplies; the price should be around 20 cents a piece ($20 per box). PP beakers are nearly indestructible, will not crack during mixing, will not melt when heated up, and will not fall apart when exposed to solvents. But perhaps most importantly, resins don't stick to them, so you can reuse the same cup over and over again. This actually leads to savings after a month or two of casting work!

Mixing sticks: you can mix the resin with just about anything, as long as it's dry, clean, and has the right shape to reach all the spots inside the mixing cup. Many people use tongue depressors, which cost something around 1-2 cents a piece (try Ted Pella), and can be wiped clean and reused indefinitely. If you choose that option, just keep them away from moisture, and be sure to clip the business end to get a flat tip for better reach. Tongue depressors aside, chopsticks also work well, as do proper laboratory spatulas and flattened mixing sticks (link). The choice is yours.

Freezer paper or thick aluminum foil: it's convenient to pour a bit extra resin into the mold, and let it overflow before sealing everything; this minimizes the risk of air entrapment. Any non-permeable, cheap material can be used to protect the surrounding working area against the overflowing resin; freezer paper and heavy-duty aluminum foil from a grocery store will do the trick.

Deburring tools: there are several inexpensive hand tools that prove very useful for quickly cleaning up the thin film of resin that may be present around the mold's parting line, and for fixing other minor casting artifacts. I recommend grabbing one miniature swivel deburrer, such as Noga RB1000 ($8); one fixed-blade scraper, such as Noga SC8000 ($20); and perhaps a couple of small, hard Nylon brushes, such as this kit (already mentioned before). You can do without them, and just use a toothbrush, a pin, and a needle file - but your life will be a bit more difficult.

Dental picks / wax carvers: a low-cost set of picks, hooks, and spatulas - something like this kit ($9) - will be extremely useful for removing silicone molds from masters, and for assorted other precision work. In fact, if you don't have that in your workshop already, you are missing out.

Sheet polypropylene or HDPE: flat, smooth, non-stick plastics are useful for covering single-part molds to ensure dimensionally accurate and aesthetic castings. Polypropylene is the best choice, and you can get it in cut-to-size pieces from many sources, including Professional Plastics; HDPE is a second option, but it is less temperature-resistant, and it's more common to see warped sheets. In any case, get at least 15-20 cut-to-size pieces, around 1-3 mm thick, with dimensions corresponding to the scale of parts you are expecting to routinely make (10 x 15 cm is probably a good starting point). You can get individual larger pieces later on. Note: make sure that the material you are buying is machined to a flat, smooth surface; HDPE in particular is sometimes available in patterned varieties - suitable for making cutting boards, but not so much for casting work. The material may come with some minor scratches, but these can be buffed out easily with fine sanding paper. If the pieces are badly gouged or warped, however, request a replacement!

Brushes: a set of small, soft-bristled brushes (example) will be useful for cleaning master patterns and applying mold releases and other treatments to molds.

That's about it! For working with small quantities of resin and pouring it into complex multi-part molds, you may also find it useful to get a box of single-use, two-element 10 ml syringes with no rubber seals (e.g., from eNasco), and some blunt-tip dispensing needles.

4.2.3. Mold releases and other useful chemicals

Mold release is a material that forms a protective barrier between the mold and the resin you will be pouring in, and makes it easier to demold final parts. The use of mold releases is optional when casting silicones in polyurethane patterns, or polyurethanes in silicone molds, because these materials don't adhere to each other in a particularly strong way - but still, a properly selected release agent makes demolding easier, and prolongs the life of any mold by reducing its exposure to reactive chemicals. On top of that, if you ever want to cast polyurethanes in polyurethane molds, or silicones in silicone molds, a robust adhesion barrier is simply a must.

I have tried many different mold releases over the years, and my top pick is, unquestionably, Stoner A324; this spray-on agent beats most of the silicone, PTFE, or zinc stearate releases that I have tried before. They sell it for about $5 per can, and this should last you for about 1-3 months of hobby work. Stoner ships for free if you order a full box of 12 cans ($60).

If you can't obtain this product in your market, silicone-based mold releases are your second best choice, although some varieties may gradually permeate and swell silicone molds. Releases based on mineral oils, PTFE, zinc stearate, polyvinyl alcohol, and so on, usually don't perform well in high-precision, vacuum-assisted casting work. The manufacturer of your silicone rubbers may be able to recommend some specific, locally available picks.

That aside, you may want to also grab a wax-based mold release: they dry to form a hard, polishable, permanent layer that not only serves as a barrier, but should improve the surface aspect of polyurethane patterns. Low-viscosity brush-on formulations, such as Synlube 531, should work well; more viscous liquids, such as AdTech MR-1, may need to be diluted with naphtha when working with intricate shapes, but it's not a hard thing to do. Hardware store paste waxes that contain carnauba wax (link) work fine for simple shapes where dimensional accuracy is not critical, but high-gloss finish is desired - and if you dilute them to a water-like consistency, you can use them for complex models, too. In all cases, once the wax is dry, you can simply buff it with a soft cloth or a brush.

Note: waxes should not be applied to silicone molds, because they will inevitably crack and peel off - and the solvents used are usually damaging to silicone rubbers, too. In general, with any new mold release, always test it by soaking a piece of silicone in it for about 5-10 minutes; any substantial or permanent change in dimensions would be a reason for concern.

Mold releases aside, you may find it useful to get some of the following, largely optional chemicals:

Basic solvents: it's good to have the following two options at hand: Acetone: when used sparingly, works as a great degreaser for plastics; if you don't make a habit of using it, you will not get robust bonds with cyanoacrylate or epoxy glues, especially on molded parts that may be still covered with release agents or so. It is also useful as a solvent for certain resin additives, and helps degas stubborn or high-viscosity resins (1-2 drops make a huge difference). It is advisable to get reagent-grade acetone from eBay (link); hardware-store variety is usually also OK, but may cause problems if it contains too much water. VM&P naphtha: useful for diluting mold releases and for cleaning up silicone spills. Ethyl acetate (an acetone-like solvent used in nail polish) works for the last two purposes, too - and may be a more environmentally friendly alternative.

Siloxane surfactants (very optional): concentrated siloxane surfactants make it easier to use polyurethane resins as surface coatings; they do that improving wetting characteristics and eliminating pinhole defects. You will likely need them if you wish to use resins such as HP-21xx or IE-3075 as high-performance surface coats. Look for a product called Satur~8 on eBay; when added to resin at around 1.5%, it should make quite a difference.

Silane couplers (ditto): these may come handy if you wish to add glass-based fillers to polyurethane resins, or improve bonding to glass surfaces. They modify the surface of these materials to make them form strong, covalent bonds with polyurethanes, epoxies, and similar plastics. The coupler is applied as a 2% solution in alcohol or acetone, and then left to dry in the presence of atmospheric moisture (water molecules are needed for the actual activation process to occur). The most common and useful compounds are called 3-glycidoxypropyltrimethoxysilane (GLYMO) and (3-aminopropyl)-triethoxysilane (APTES). Concentrated variants can be ordered from Chemsavers.com (just drop them a mail), or get a smaller quantity from SPI Supplies (link for GLYMO, link for APTES). Diluted APTES is also available from crafts stores. Note that you don't always have to go through all this trouble; for example, milled glass fibers available from Fibreglast are already pre-treated in this manner, and there is no point in soaking them in a coupling agent again. On the other hand, Scotchlite glass microspheres come in a virgin state and will benefit from this step.

4.2.4. Optional: temperature-controlled oven

Temperature-controlled ovens are not essential in casting work, but they let you perform several time-saving tasks:

Drying molds, pigments, fillers, and other supplements if necessary. In most cases, this can be also accomplished with any cheap, $40 toaster oven.

Accelerating the cure of resins when dimensional accuracy is not critical. This is sometimes preferred to chemical acceleration, because there is no need to purchase any reagents, and the behavior can be fine-tuned more easily. Thermal acceleration requires accurate temperature control between around 30-70° C, and will usually let you demold parts in under 1 hour.

Post-curing finished parts, so that they achieve their ultimate properties sooner than normal (it may take up to a week at room temperature for all the reactions to cease). This requires accurate temperature control around 80-120° C.

Low-cost toaster ovens are the obvious solution, but somewhat unfortunately, they have extremely poor temperature accuracy and stability, especially on the lower end of the scale. If you are an accomplished DIYer, you may be able to grab one and simply equip it with a more accurate, digital temperature control circuit. That said, if you are willing to part with a few hundred bucks and have some floor space, you can also try getting a real laboratory oven, such as this one ($400); hot air sterilizers and dryers may offer a smaller and slightly more affordable alternatives in some markets, too (example, around $300).

If you don't have that much money, or simply don't have enough room, don't worry; you can do just fine without this piece of equipment, provided you are willing to wait a bit longer for your castings every now and then.

4.2.5. Optional: pressure pot

There are some complex, multi-part molds where it may be hard to consistently avoid air entrapment, even with the aid of vacuum; on top of that, there are some resins that tend to be difficult to fully degas, or that will develop bubbles of carbon dioxide when not mixed perfectly well, or when exposed to residual humidity. That latter set of problems is particularly evident in mercury-free water clear poulyrethanes, such as the vanilla version of Innovative Polymers OC-7086.

To improve your odds when dealing with such tricky cases, it helps to have a pressure pot; the idea is to increase ambient pressure surrounding the mold to about 3-4 bar, thus crushing and dissolving back any existing bubbles, and discouraging the formation of new ones. Sure, it's a brute-force solution - but can you argue with its results?

Pressure casting equipment is more bulky, more expensive, and somewhat more dangerous than vacuum pumps (due to much higher pressure differentials) - and for most part, isn't necessary; get it only if you have plenty of room, and you are either forced to work with water clear resins that don't tolerate non-pressurized casting, or you are willing to spend at least $200 to improve your yields a tiny bit (say, from 85% to 97%).

If you want to take this route, pressure pots start around $80 (link); nicer ones fetch as much as $300 (link). To operate it, you will also need a compressor - and these start from $100 from fairly noisy units from the hardware store, to $180 for relatively quiet-running ones; standard pressure hoses and fittings add another $20 or so.

4.3. Your first casting projects

All right, all right - enough with all the theory and shopping tips. It's time to dig out that wax pattern that you have made before, and turn it into a finished work of art! This section is all-text, but if you need visual aids, this photolog is probably good to look at.

Caution: similarly to many workshop and household chemicals, casting resins can be harmful if misused. In particular, they may react violently when mixed with incompatible substances; cause severe irritation or lasting damage if splashed into your eyes; and will emit dangerous vapors if overheated, burned, or intentionally aerosolized. Please refer to product safety datasheets (MSDSes), and to section 7 of this guide, for an overview of known risks, material handling recommendations, and disposal procedures. Do not proceed with any hands-on experiments until you have done so.

4.3.1. Making a silicone mold

Resin casting is fun, but you need to remember that once the components are mixed, the reaction will proceed no matter what; it's important to plan accordingly: read this section fully and memorize all the steps beforehand, and have all the necessary supplies and information within reach.

First, you need to estimate the amount of resin you will need: for CNC-machined or 3D-printed shapes, simply ask your CAD application to calculate the volume of the master, and subtract this value from the volume of a dummy box of the same height, width, and depth. To have a comfortable margin, add about 15% to the result, or 10 ml, whichever is greater. Then, multiply that volume by the density of the resin (check the datasheet; it's usually between 1.1 and 1.4 g/cm³). The resulting figure is the weight of the material you need to prepare.

With these calculations out of the way, prepare the following stuff:

A roll of paper towels. Tear away about 5-10 pieces ahead of time, and place them within reach; this comes handy for wiping mixing tools, cleaning up minor spills, etc.

An easily accessible trash bag for dirty towels and other things you want to get rid of in a hurry.

Mixing container that fits within your degassing chamber, and holds at least 3 times the volume of resin you will be working with.

A clean, flat, and clutter-free work surface, covered with aluminum foil or freezer paper.

A kitchen timer, a stopwatch, or something else that would let you keep track of time.

Although accidents are unlikely, you should still try to minimize potential damage: if there are any LCDs or other expensive gizmos in your workshop, consider moving them a bit farther away, or covering them with plastic sheeting. Don't wear your best clothes, and if you have a carpeted floor or expensive hardwood, cover the area that is most likely to suffer in case of a spill.

When you are ready to go, place the mixing cup on a scale, then tare it. Agitate both components of the resin in their original containers, and then use a clean tongue depressor or a spoon to pour about 20 g of silicone into the mixing cup. Add a suitable amount of catalyst (the ratio is usually 10:1, but check the datasheet), start the timer, and begin mixing thoroughly for honest 3 minutes; be sure to repeatedly scrape the sides and the bottom to avoid leaving any unmixed resin in these spots.

Next, place the cup under vacuum; the mix will initially rise as the bubbles expand, and then collapse back; you should keep it under vacuum for another 1-2 minutes past that point. If the resin gets dangerously close to overflowing during the initial rise, simply release the vacuum (i.e., yank out the hose), and try again; several cycles of that should do the trick - and next time, use a larger cup.

The next step is to pour some of the resin into the mold created in chapter 3; use about half of the required volume or so. Place the mold on a strip of aluminum foil, and put that under vacuum; this will help the resin conform to even the most intricate shapes with no effort on your end. After about 2-3 minutes, you can slowly release the vacuum, pour the remaining amount of resin (use a bit more than necessary to get a convex surface at the top of the mold), and set the whole thing aside for several minutes to allow any bubbles to rise to the surface (or collapse back into the resin). If there are any stubborn bubbles on the surface at that point, you can apply a gentle burst of compressed air to get rid of them.

Finally, cover the entire contraption with a flat, clean sheet of polypropylene; lay it down gradually, starting at one side, to avoid air entrapment. When done, weigh it down with something reasonably heavy - around 500 g should do - and brace the whole thing, so that the cover doesn't slide off. Be sure to check the timer at this point - has the entire process taken more than about 12-15 minutes? If yes, why?

In any case, give it about 12 hours or so, until the resin remaining in the mixing cup is firm and tack-free. If you are impatient, placing the mold in a temperature-controlled oven at around 50° C will cut the curing time down to 1-2 hours or so - but don't go too high, given that this particular master is made out of wax. When the rubber is ready to demold, pull off the cover, and then use a dental hook or a similar tool to pry the rubber off near the corner of the mold. Remove it fully and inspect the result. if it looks flawless - as it should - you may want to briefly post-cure it at around 100° C for 30-60 minutes, and in the meantime, pat yourself on the back!

Here are several questions that may be on your mind:

What about the application of a mold release? Waxes already have a non-stick surface, so if you are following the instructions from chapter 3, you should be fine. When using prototyping boards or 3D-printed materials, you should apply a thin layer of mold release beforehand, and allow it to dry. Use a soft brush to spread the substance evenly, and don't overdo it: you want to wet the workpiece, but do not let the liquid pool in hard-to-reach spots.

The resin didn't cure! What now? Ow, bummer. The most likely reason is insufficient mixing (especially if there are sticky patches or streaks in an otherwise well-cured material); bad mixing proportions; contact with incompatible materials (platinum-cure silicones are particularly sensitive to sulfur compounds); or low temperature (the resin will take forever to cure below 18° C or so). If you have ruled these causes out, feel free to drop me a mail, and we'll try to get to the bottom of it.

What if I'm having trouble demolding? This takes some practice - try to find the right tools, and locate a spot where you can grab the rubber with a hook without causing any damage. For particularly challenging shapes, introducing pull tabs and draft angles in the mold, as discussed earlier, is the way to go.

What if the mold is too big to put under vacuum? For simple shapes, you can just gently pour the degassed rubber in, starting at the lowest point of the mold. If there are any tight spots, it's useful to fill a syringe with a small amount of catalyzed resin, put a needle on, and use that to force the resin into these problem areas beforehand. When working with truly oversize pieces, you may find it useful to rely on a process called glove molding : brush on several thin layers of silicone onto the part, and then coat this flexible skin with a rigid, thixotropic polyurethane, epoxy, or polyester resin to create a backing shell. This is a great way to conserve silicone, although the mold takes more time and effort to prepare. (For optimal results, the silicone resin used for this purpose may need to be thickened to a non-flowable consistency with an additive such as Bluestar 22646 or with fumed silica. Fumed silica can be also used as a thickener for the rigid resin used as the outer coat.)

Vacuum degassing only made the bubbles worse, what now?! The resin should rise and collapse pretty quickly; some bubbles will continue to appear, but they should collapse back shortly after the system is returned to room pressure. If that doesn't happen, check the ballast valve on your pump (it should be closed) and examine the entire setup for any leaks. Make sure that the pump is in good working order (many pumps require the right amount and type of mineral oil for lubrication). Verify that the pump is well-suited for the size of your vacuum chamber, too. If you're stumped, getting a cheap vacuum gauge from Amazon may help.

How long will the mold last? Depends. When following the advice provided in this guide, around 50 pulls would be the norm. Lower-strength silicones, more aggressive resins (epoxies, polyesters, very fast or very slow polyurethanes), and complex geometries (deep molds with intricate detail) will lower the lifespan of silicone molds.

Okay, okay - time for some polyurethane fun!

4.3.2. Casting plastic parts

Polyurethane casting is not dramatically different from working with silicones, but you have to be swift, and pay more attention to detail. Quite simply, with a resin such as IE-3075 or TP-4052, you will probably only have about 6 minutes to go through all the steps - so there is no time to look around for paper towels or a mixing stick.

In general, before starting, you should go through all the preparation steps outlined in the previous section; and when done, you should also do the following:

Make sure that the resin hasn't separated or crystallized. In almost all formulations, the isocyanate component should be clear, and both components should be completely homogenous. Agitate the containers if necessary; crystallization may require heating the containers slightly (usually not more than 40° C); always check with the manufacturer first, and exercise caution. Note: in filled systems, the filler settles out of the solution and hard-packs at the bottom of the container; this is particularly true for heavy fillers, such as glass. You need to use a small paint mixer attached to a power drill to properly homogenize the contents of larger containers; it's not a big deal, but watch out for spills. Manual mixing is viable only for small quantities, up to perhaps 250 ml or so.

If you haven't done so already, get two smaller polypropylene bottles, and fill them up with a more manageable amount of isocyanate and polyol, so that you do not have to constantly agitate, open, and tip over gallon-size jugs. When done, blanket the original containers with "canned air" or nitrogen, and close then tightly. Be sure to label the new bottles clearly, too.

Make sure that the mold and any hygroscopic fillers you want to add are perfectly dry - especially before first use, or after extended storage. Placing solid materials at 110-150° C for about 10-20 minutes, or applying vacuum for 5-10 minutes, is a good option if you want to play it very safe.

Prepare a perfectly flat, spill-proof surface on which the silicone molds will be laid out while the resin cures. You can use a sheet of polypropylene, or a neat piece of aluminum foil with no creases. Whichever option you choose, applying some mold release to that surface is a good idea.

Grab a small, flat sheet of polypropylene or HDPE to serve as a top cover for the mold. Mark one side with a permanent marker, apply a thin coat of mold release to the other, and place the sheet within reach.

Apply a very light coat of mold release to the silicone mold, and spread it evenly with a brush. Repeat the process every 5-10 pulls, but don't overdo it; for casting polyurethanes in silicone molds, too little is better than too much. Allow the release to dry for 15 minutes or so.

With all the preparations taken care of, you are ready to roll. Place the mixing cup on the scale, tare it, and pour the required amount of isocyanate; in our case, 10 g will do. Next, gently pour the appropriate amount of polyol (for IE-3075, this will be 8.9 g), start the timer, and begin mixing very thoroughly, frequently scraping the walls. Most polyurethane resins, IE-3075 included, require at least 90 seconds of mixing to progress from a dispersion to a proper solution when mixed in a small quantity - and if you stop sooner, the cured material may not cure properly. Do it right.

When done, place the container under vacuum, and turn on the pump. The resin should take no more than about a minute to rise and collapse back; if it's taking much longer, your vacuum chamber may be too big, or the pump may be inadequate or malfunctioning (e.g., due to an open gas ballast valve, or due to contamination). If, on the other hand, everything went as expected - and the bubbles have collapsed in a timely manner - you should now pour some of the resin into the mold cavity (to fill it roughly halfway, taking care to cover any detail where air entrapment is likely), and place the mold under vacuum for another minute or so. Don't worry if it never stops bubbling at this stage; that's OK.

After a brief round of degassing, release the vacuum gently, place the mold on a previously prepared flat surface, and add the remaining resin, until it overflows and forms a convex surface (this is important - otherwise, air entrapment is a lot harder to avoid). Grab the polypropylene cover and carefully lay it on top of the mold, using the technique outlined for silicones. You should brace the cover against something, so that it doesn't slide off, and weigh it down with around 200 g (larger molds can be clamped with several kg of force, but this particular one is relatively easy to squish).

Well, that's it! Before you go, check the timer; if the process has taken more than six minutes, you should figure out how to improve it. In any case, leave the mold alone for at least 3 hours (or more, depending on the resin used), and when you come back, confirm that the material left over in the mixing cup is tack-free and hard as nails. Next, gently flex the polypropylene cover to detach it from the part, and extract your casting from the mold.

Hopefully, the result is perfectly fine; that said, the likelihood of mishaps is higher for polyurethanes than it is for silicones - so if something isn't right, don't despair. Here's a quick summary of the most common issues I have seen:

The resin is still sticky or gooey: assuming you waited long enough, the culprit may be low ambient temperature (try to stay close to 20-25° C), incorrect mixing proportions, or mixing the wrong components (the containers all look almost the same). A less likely cause may be the introduction of a problematic filler or dye, or the use of a resin that is incompatible with platinum cure silicones (this is rare; the datasheet should have said so). Treating the surface of the mold with a separately purchased polyurethane catalyst is a possible workaround for that last problem - see later on.

The material turned into a foam: this may be caused by using a resin that crystallized during storage and hasn't been heated up to dissolve the crystals; by adding a filler or other additive that contained a significant amount of water; or by similar type of gross moisture contamination (wet hands, condensation, etc). In some formulations, failing to properly degas the catalyzed resin, mixing for way too short, or not allowing the mold release to dry properly before pour, may also cause a similar symptom. (If you suspect your vacuum setup, check out the tips in the previous section.)

Top surface is soft and scratches easily: this is almost always due to insufficient mixing. Dispersed droplets of polyol have coalesced and, being lighter than the isocyanate component, floated up. More thorough mixing or higher ambient temperature will usually fix the issue. Premixing - as discussed later on - should also help.

Discoloration or clusters of tiny bubbles running around edges or in other well-defined streaks: usually also a matter of insufficient mixing - see above.

Several individual, round bubbles that floated to the top or are stuck in tight corners at the bottom of the mold: air entrapment during the casting process. If there are stubborn bubbles still present in the liquid as you are pouring it into the mold, and your vacuum system is in good working order otherwise - mix in 1-2 drops of acetone per 10 ml of resin before degassing, and see if that helps. Oh - when in-mold degassing is not possible due to the size of the part, applying the resin with a syringe or a brush may help with any tricky spots.

Giant tear-shaped bubble near the top surface: air sucked into the mold due to applying too much force when placing the cover or insufficient clamping. It may also be that you haven't poured enough resin to begin with, or that the cover or your work surface is warped.

Several oddball bubbles in thin sections, especially if suspended in the middle of the part: often caused by using a highly moisture-sensitive and slowly curing resin, such as OC-7086; the bubbles form as the resin is already partly-solid, contributing to their sometimes elongated or meandering shapes. If all other options fail, pre-heating the molds, adding some extra catalyst (see later), or using pressurized casting will help.

If you are seeing any issues, it's important to narrow the problem down right away, while the number of variables is still fairly low. If you are out of ideas, don't hesitate to ping me at lcamtuf@coredump.cx; I may be able to help. A good place to discuss your experiences or showcase your work may be /r/resincasting, too!

Anyway - if everything turned out to be just fine, you may want to briefly post-cure the part and any leftover material. Place it at around 100° C for one hour, and then play with it to get a sense of its physical properties: try drilling a hole in or scratching the surface of one of the leftover bits, and see how hard it is to break it.

4.4. Miscellaneous polyurethane casting tips

This section is just a quick a collection of random notes that should come handy in real-life projects, but that I wanted to keep out of your first casting job. Enjoy!

4.4.1. Meta: not all resins are alike

Before we dive into various advanced topics, you should know that there are significant differences in the handling characteristics of various polyurethane resins, even if the advertised cure times and ultimate physical properties of the material are roughly the same. In particular, be aware of the following:

The choice of catalysts will affect the resin's sensitivity to moisture, often dramatically so. Some resins with highly selective catalysts will cure pretty well in open air, and can be used as surface coats (e.g., HP-21xx); some have moderate sensitivity that can be greatly lowered using the premixing technique outlined later in this chapter (IE-3075, TP-4052); and others, such as OC-7086, really need to be modified with an aftermarket catalyst, or pressure-cast, to maintain sanity when making precision parts.

The use of blocked or temperature-sensitive catalysts helps achieve long pot life followed by a snap cure when casting larger models; unfortunately, the same trick may prolong demold time when working with tiny parts that never reach the activation threshold.

The exact composition of the system will greatly affect miscibility of the formulation at lower temperatures, before the reaction really kicks in. Some resins will require very little stirring when working with quantities around 10-20 ml, but a few may need as much as 5 minutes of work, unless you are willing to increase the temperature slightly (to ~30° C) or employ a similar trick.

Depending on the composition, the resin may have a brittle stage, at which point it is very vulnerable to cracking even under minimal stress; in other systems, incomplete polymerization is simply marked by susceptibility to plastic deformation.

Because of all these striking differences, don't take everything you see in this guide as universally applicable to every formulation on the market; and in the same vein, don't expect your own experiences with product A to be fully applicable to product B. If in doubt, request a sample of any new product you are considering, or simply ask.

4.4.2. Adding pigments and fillers to resins

Let's start with something simple. Non-reactive (i.e., plasticizer-based) liquid dyes and coloring pastes can be mixed into the working amount of isocyanate, before adding any polyol; when taking this route, just try to stay under 2% by volume (around 8-10 drops per 10 ml); if you find yourself routinely having to add more, consider switching to dry pigments or a higher-yield dye or a reactive carrier - because past this point, solvents used in the dye will be affecting the properties of the part.

Reactive coloring pastes designed specifically for polyurethanes use a polyol as a base; that's the case for pigments from Innovative Polymers. Their main benefit that they can be added at much higher levels without completely messing up the properties of your parts. In principle, you should subtract the weight of the added dye from the required weight of polyol - but in practice, this varies from one formulation to another. In a quick experiment with IE-3075, I found that using the nominal amount of polyol results in improved strength. Results in other resins may vary.

When working with dry pigments, there is a bit more legwork involved. If you simply dump the pigment unceremoniously into the liquid, it will probably clump together - and stay that way. To avoid this, you need to place the desired amount of material in an empty mixing cup, tare it, and start adding isocyanate drop by drop, mixing constantly, until you end up with a homogenous, runny paste (siloxane surfactants can make the process easier, too). Once the paste looks good, you can gradually add the remaining isocyanate while constantly mixing - and you should be all set.

Tip: if you have a high-yield, hard-to-disperse powdered pigment that you keep coming back to, it may make sense to make a custom coloring paste for future use. Simply disperse it thoroughly in an inert plasticizer (e.g., dipropylene glycol dibenzoate, discussed earlier; silicone oil works for platinum cure rubbers) or in a suitable polyol, and pour that into a dropper bottle.

Of course, as noted earlier, you should ensure that the material is moisture-free; in tricky cases, premixing the resin or adding zeolite should help. Glass-based fillers, such as Scotchlite or milled fibers, may benefit from being pre-treated with a silane coupler, too.

4.4.3. Premixing the system

Premixing is one of the simplest and lesser known tricks that can help solve many of the problems that crop up in some polyurethane casting jobs. For example, it can dramatically reduce shrinkage without affecting cure time; lower the risk of cure inhibition; halve the time needed to fully mix the resin in small batches; and greatly reduce the sensitivity to moisture, to the point of making many systems suitable for surface coating applications. These benefits stem from the reduced reactivity and improved compatibility of a partly polymerized liquid. It's not a silver bullet, but for many formulations, it's pretty close to being one.

The only price to pay for premixing is an increase in viscosity, which typically isn't a big deal if you have started with a low-viscosity system such as IE-3075; and the added expense of about 5 minutes of work and about one hour of waiting per every batch that you intend to cast.

If you want to try it out, the recipe is very simple: measure the desired amount of isocyanate, add all the fillers and dyes you want to have, and then introduce between 10% and 20% of the necessary amount of polyol; this mix needs to be stirred thoroughly, degassed - and then stored in a covered cup, blanketed with an inert gas, for about 1-2 hours. At that point, the remaining polyol can be mixed in, and the resin can be cast.

Keep in mind that premixed resins will have short shelf life: the viscosity will keep increasing, and in presence of a catalyst, the isocyanate will more aggressively react with ambient humidity and deteriorate. You should premix only the amount you intend to use right away.

Oh - in case you are curious, this graph shows the impact of premixing on the exotherm for 10 g of IE-3075, and how it compares to the use of fillers. The measurements were taken using a thermocouple submerged in a small, insulated polypropylene cup, approx. 30 mm in diameter:

Here's another tidbit you won't find in any other hobbyist reference on resin casting: it is possible, and in fact fairly easy, to chemically slow down many polyurethane systems to significantly reduce shrinkage; and to accelerate slow-curing ones to get your parts sooner or have fewer artifacts in thin sections of your molds. Why bother, you may ask? Well, it not only saves you money, compared to buying several resins for different applications - but perhaps more interestingly, it enables you to come up with custom-tailored cure profiles that are of no commercial interest to the manufacturer.

In essence, there is a wide variety of catalysts used in castable polyurethane resins. Every catalyst behaves differently: some are highly active at room temperature, some kick in only later on, when the resin has warmed up due to exotherm. Some are better at driving the early stages of polymerization, but stop shortly thereafter; some have a sustained effect until the very end. Some are highly selective toward the desirable isocyanate-polyol reaction, and some don't mind catalyzing the isocyanate-water reaction - which leads to the formation of bubbles of CO 2 . Some are very stable, and some deteriorate when exposed to open air and other substances, which may cause inhibition or poor surface cure. But there is no single product that gives you the very best on all fronts.

For this reason, manufacturers combine various catalysts to reach a compromise that makes sense for their intended customers - but these parameters aren't necessarily ideal for your needs. For example, OC-7086 is a resin designed for larger castings; when dealing with tiny parts, it will cure too slowly, and with far too much sensitivity to ambient moisture.

Thankfully, you can fix this on your own.

4.4.4.1. Speeding things up

Ideally, if you wish to use OC-7086 or HP-21xx - or accelerate any other finicky resin - you should get bismuth neodecanoate from Santa Cruz Biotechnology or Krackeler. The cost is around $35 for 250 g, and that amount will last you forever. The catalyst isn't dangerous, but both of these places have a blanket policy of shipping only to commercial addresses. If you can't have it shipped to work, ping the folks who run Chemsavers.com - they should be able to get it for you and ship it to your home for a very modest premium.

Bismuth neodecanoate is a syrupy liquid which needs to be diluted with a plasticizer (e.g., DPGDB), a suitable polyol, acetone, or something else of that sort. Depending on the resins you are working with, you may have to experiment with dosage, but typically, levels between 50 and 500 ppm will be enough. For example, to "fix" OC-7086, you can prepare a 4% solution in plasticizer, and add it at about 1-2 drops per 10 ml of isocyanate as you are getting ready to mix it with a polyol. To speed up HP-21xx, you will need a solution closer to 50%, added in similar quantities.

Now, if this particular bismuth compound is hard to find where you live, don't despair! A decent alternative is tin(II) 2-ethylhexanoate, also known as stannous octanoate. This substance is commonly sold as an accelerator for condensation-cure silicones; for example, a nearly pure form is available under brand names such as Smooth-On Accel-T, Quantum Silicones QSil STO, or Bluestar VICURE #2. Just be careful not to buy anything based on dibutyltin dilaurate, dimethyltin dineodecanoate, or a similar tin(IV) compound: they will work great, but also happen to be a lot more toxic.

As with bismuth, the appropriate dosage varies depending on the resin; for OC-7086, a 1% solution, added at 1-2 drops per 10 ml of isocyanate, is a good starting point. Note that the compound is a bit more harmful than bismuth - handle it with care.

Bismuth and tin aside, there are several other, more exotic options to choose from. They may offer very specific benefits, such as improved curing characteristics in particular systems, or no subsequent inhibition of platinum silicones (which are somewhat sensitive both to bismuth and tin). If you need additional guidance, click here to expand a section with some rough notes.

Alternative catalysts: there is a large number of other catalysts used in polyurethane formulations. The list below includes some of the more common choices, along with an explanation why they aren't featured more prominently in this chapter: Other complexes of bismuth. According to scientific literature and tons of published patents, various other alcohol-soluble soaps of bismuth, including 2-ethylhexanoate and naphthenate, perform very well. Nevertheless, they are less common and tend to be more expensive.

Complexes of zinc. Various zinc-containing compounds - including 2-ethylhexanoate, neodecanoate, and acetate - are commonly used as polyurethane catalysts. On the flip side, they are less potent than bismuth, tend to be less selective, and have a more non-linear temperature response - leading to higher exotherm in bulky castings and slower cure in thin sections.

Triethylenediamine. A very potent catalyst, actually offering superior surface cure in products such as HP-21xx - so if you are doing a lot of work with amine-based formulations, go for it. On the flip side, it messes up resins such as OC-7086.

Other non-metal catalysts. A fair number of other amines, imidazoles, and similar substances exhibit strong catalytic activity in polyurethanes, although most of them tend to favor the reaction with water over the reaction with polyols. There are several exceptions, but they tend to be difficult to find. Something like 1,2-dimethylimidazole or n-(3-aminopropyl)imidazole may work.

Zirconium acetylacetonate. Reported as an excellent catalyst for some formulations, and a weak one for others. More exotic zirconium complexes are said to work more consistently