Everyone is talking about how 3D printing is revolutionizing the way companies prototype and iterate designs faster and with more freedom than ever before. One thing that doesn’t get the attention it deserves is 3D printing for injection molding; not the most glamorous of applications but one which is making meaningful changes in the manufacturing industry. This is an area which is beginning to see even more innovation as molding goes from the machine shop, to the industrial 3D printer, to the desktop 3D printer.

This post will breakdown what happened before, what’s happening now, and what‘s to come.

Traditional Injection Molding

A mammoth 2700kn Plastic Injection Molding Machine (FL270)

Injection molding was first patented back in 1872 by two business savvy brothers (John & Isaiah Hyatt) who built a basic machine that worked like a hypodermic needle: pushing plastic through a hole into a heated cylinder. It was created to make billiard balls for a manufacturer. James Hendry, in the the 40’s, went a step further building the first screw injection molding machine. Instead of a plunger pushing the plastic through a hole, an auger is placed in the cylinder and mixes the injection material before pushing forward and injecting the material into the mold.

This is the process most commonly that is used today and has led to higher accuracy thanks to the pre-mixing and increased speed due improvements in automation. Injection molding is now used in all manner of industries and to create a large range of high volume products. Why is it used for the purposes of mass production? Well you can create a mould in which you can inject low cost plastic into repeatedly creating the same shape making it cheapest way to create 1,000’s of the same product. The more products you produce the cheaper the products become as you begin offsetting the great expense of the mold.

How it works: Material granules are fed via a hopper into a heated barrel , melted using heater bands and the frictional action of a reciprocating screw barrel (auger). The plastic is then injected through a nozzle into a mold cavity, where it cools and hardens to the configuration of the cavity. The mold tool is mounted on a moveable platen — when the part has solidified, the platen opens and the part is ejected out using ejector pins. AV Plastics

4 Stages of Screw Injection Molding

You cannot create any geometry with injection molding and there are many design limitations to consider when creating a mold. These include part and mold material, component geometry and mold design requirements (draft angles, ribs and gussets, wall thickness and split lines). The geometries especially for injection molding are a challenge as you have to make sure you can get the part out of the mold. Ejector pins can be found when looking at most mass manufactured goods, this is one of the considerations you need to make when creating your mold design. Injection molding is best used when you need to create the same part 100’s or 1,000’s or even 10,000’s of times as you can take advantage of the economies of scale.

Injection Pins

Traditionally, molds are made by a mold making company that machines p-20 steel bars which are then placed into a block, known as a base. This base is then milled to the required mold geometry. Both halves of the mold are then drilled for the bushings and guide pins, which hold both parts together during the injection process. Grinding is then performed to produce a smooth and accurate surface, after which the CNC’ing begins to create the final outline of the mold.

This is a long process, with the CNC machining taking up to 20 hours alone, and involves a variety of manufacturing techniques, often resulting in the need to outsource the process to a company specifically equipped to deal with the stringent requirements of mold production. Going from block, to outline, to detail, to fine detail while changing manufacturing methods creates long lead times for engineers to deal with when waiting to get their parts.

Water CNC cutting machine adding details to the mold (How it’s made)

Injection molding is and will continue to be a manufacturing method regularly employed by industry, thanks to its ability to mass produce parts at a high speed for a low cost. Evidently, it’s not as simple and straightforward as producing a metal mold and popping it into the machine, as strict design considerations are needed (material, design, process) and a wide variety of manufacturing steps are required. So how does 3D printing fit into this complex world and how can on-demand manufacturing play a role in mass production?

PolyJet

The first 3D printing technology to step into the world of mold-making was material jetting, via Stratasys Connex PolyJet machines back in 2011. Material Jetting (Stratasys PolyJet) technologies are similar to inkjet printing, but instead of jetting drops of ink onto paper, these 3D printers jet layers of liquid photopolymer onto a build tray and cure them instantly using UV light. It uses a support material which easily breaks away to create complex geometries that would otherwise be difficult with traditional tooling.

PolyJet Schematic

The biggest hurdle for 3D printing to be applied in the molding industry was the materials. Traditionally, 3D printing materials lacked the properties to withstand the forces and heat continuously exerted throughout the injection molding process. From these material limitations, Stratasys launched a material to in the form of Digital ABS (Simulated ABS), a photo-reactive resin capable of withstanding high impact designed specifically for short injection molding runs.