While the two material categories most frequently used in 3D printing are metals and plastics, there is a wide variety of sludge, powder and other gunk that just doesn’t quite fit into these classifications. Applications include everything from the manufacturing mundane, like 3D printing sand cores for sand casting, to those bordering on science fiction, like 3D-printed food, organs and buildings.

Sheikh Mohammed bin Rashid Al Maktoum inaugurates the world’s first 3D-printed office building, which was 3D printed by WinSun. (Image courtesy of the Government of Dubai Media Office.)

Whether it’s an office building in Dubai or a ceramic mold in a foundry you’re interested in, ENGINEERING.com’s latest guide on 3D printing materials has you covered.

From 3D Printing Paper to People

In previous editions of this guide, ENGINEERING.com has covered some of the most exciting or widely used materials. Our metals article detailed each metal powder that is being increasingly used in industries like aerospace and medicine. The polymer powders guide focused specifically on plastic powders used in powder bed processes, like selective laser sintering and Multi Jet Fusion (MJF). We also wrote up guides on thermoplastic filaments for fused deposition modeling, photopolymers for photopolymerization processes and composite materials for a variety of 3D printing processes.

That leaves us with a grab bag of unique materials that don’t quite fit into any of the aforementioned categories. As a result, the processes vary as well. In some cases, there will be material extrusion and, in others, we’ll be discussing photopolymerization. As we break down each material and its applications, we’ll try our best to break down the processes used as well. We’ll also focus on some of the more widely used and industrial materials (ceramics, clay and sand) before getting into materials that, at this point in human progress, might have less immediate applications for your standard engineer (paper, food and tissue).

Traditional Clay and Ceramics

Ceramics are one of the oldest materials utilized by humanity and play an important role in additive manufacturing (AM). They can be broken down into two broad categories of more traditional materials used for construction and aesthetic applications, as well as high-performance ceramics, for use in electronics and medical applications.

A model made with a 3D Systems ColorJet Printing 3D printer. (Image courtesy of 3D Systems.)

One of the more commonly used ceramics in 3D printing is gypsum, which is found in the binder jetting process invented by MIT and licensed to 3D Systems. The process uses piezoelectric inkjet printheads to deposit a liquid binding agent and colored ink onto a layer of gypsum powder, resulting in full-color, sandstone-like printed objects. These full-color objects are typically used for aesthetic purposes, such as architectural or medical modeling, as well as art and consumer production applications.

Ceramic objects made via ceramic 3D printing technology from Figulo, which was acquired by 3D Systems. (Image courtesy of 3D Systems.)

Binder jetting can also be used to 3D print other forms of ceramics. Although the product no longer seems to be heading to market, the CeraJet 3D printer, from 3D Systems, was capable of depositing a binding agent onto a specialty ceramic powder. The resulting green print could then be fired in a kiln, creating a fully dense ceramic object.

The seemingly most straightforward method for 3D printing with ceramics may be via paste extrusion. Most often, this process relies on modifying a fused deposition modeling (FDM) style system or building a gantry system to deposit concrete or clay, rather than plastic. A printhead must be used that combines an air pump with a specialty deposition head that ensures the material flows in a controlled manner.

The World’s Advanced Saving Project (WASP) has created a clay extruder for its desktop 3D printer, which has seen been blown up for use with a large-scale “house” printer. This large-scale BigDelta 3D printer is being used to print a mud hut in Italy as a proof of concept. WinSun Global, in China, has developed a method for 3D printing large-scale concrete walls and structural components that are then assembled on-site.

High-Performance Ceramics

When 3D printing high-performance ceramics, the most commonly used 3D printing processes are forms of vat photopolymerization, in which either an ultraviolet (UV) laser or a projector is used to cure photosensitive resin layer by layer. To print with ceramics, the photopolymer is loaded with ceramic particles, such as alumina, zirconia or hydroxyapatite. Once printed, these green parts are usually sintered in a furnace to achieve fully dense ceramic parts.

High-performance ceramics have highly specialized applications due to unique properties that include good thermal and electrical properties; high temperature, corrosion and wear resistance; low density and thermal expansion; and good biocompatibility.

Several companies manufacture digital light processing (DLP) machines for use with high-performance ceramic materials, including Lithoz, Admatec and 3DCeram. Prodways sells a rebranded version of a 3DCeram machine as well.

Michiel de Bruijcker, managing director of Admatec, elaborated on some of the applications that these materials can be used for. “These types of ceramics are used to make high tech components that are wear resistant, high temperature resistant, have zero coefficient of thermal expansion, and/or act as electro isolators. Think about sensors housings, micro reactors, cooling housings, etc. In the dental market, the material can be used for all dental restorations, crowns, copings and bridges. There is huge potential there.”

A bracket and turbo pump wheel 3D printed with the ADMAFLEX 130 ceramic 3D printer from Admatec. (Image courtesy of Admatec.)

“Ceramic 3D printing may also be used to produce investment castings, including the direct printing of investment casting shells, which will potentially eliminate foam printing in this market, and printing cores for rotor and vane blades,” de Bruijcker added. “This is a bit longer out, but hydroxyapatite materials will come into play for 3D printing bone replacements. Admatec joined an EU-sponsored program in this field called cerAMfacturing. Ceramics are used for aesthetic reasons, as well. The material can be used to make jewelry or, say, for an Apple ceramic watch.”

In terms of advantages, de Bruijcker pointed to a few: “By nature, ceramic is a very fine powder. This produces smooth surfaces and a high level of detail. Standard pressing, CIM powders can be used, which are relatively cheap,” he said.

As for limitations, de Bruijcker explained, “Compared to metal and plastic, ceramics is a small market. Also, in order to obtain high densities, post-processing such as debinding and sintering are required. Additionally, lots of chemical/process knowledge is required to understand the shaping, debinding and sintering of ceramics. Finally, ceramic 3D printing is associated with relatively long throughput times.”

The technology is expanding beyond DLP technologies, with HP demonstrating that a version of its MJF process can 3D print ceramic parts, as well. XJet’s nanoparticle jetting technology, first demonstrated to use metals, can also be used to 3D print with ceramics, as well.

Although they have not been used with high-performance ceramics thus far, ceramic-loaded photopolymers have also been used with desktop stereolithography (SLA) and DLP machines, as well. Tethon 3D and Formlabs have developed ceramic resins for use in vat photopolymerization, but the materials seem difficult to work with so far.

Sand

Binder jetting can be a flexible technology, which can not only be used to 3D print full-color models, but for industrial applications as well. ExOne and Voxeljet also license binder jetting technology from MIT to 3D print with sand, among other materials. By depositing a liquid binder onto particles of sand, it’s possible to create sand cores and molds for the sand casting process.

In sand casting, a sand core or mold can be placed into a molding box, which is filled with molten metal. Once cooled, the box is broken apart and the final metal object remains. Traditionally, a sand mold is made at a foundry by hand-packing sand around a piece of tooling. The tooling can take up to 12 weeks to produce, while the foundry process might take another 14 or 15 weeks.

A graphic of the traditional sand casting process. (Image courtesy of Wikimedia.)

By 3D printing the sand core or mold, it’s possible to have the final metal part in just a week or two. In some cases, it’s not necessary to 3D print the entire sand mold or core, but just some inserts that may be used to modify an existing piece of tooling.

A large 3D-printed sand core for sand casting. (Image courtesy of HPI.)

Both Voxeljet and D-Shape have blown up binder jetting to create extremely large, 3D-printed sand objects for architectural purposes. So far, the 3D-printed components made by these processes are more artistic than structural in nature, as there is still much work to be done in proving the integrity of 3D-printed sand objects.

Paper

Founded by brothers Conor MacCormack and Fintan MacCormack, Mcor has developed a process called Selective Deposition Lamination, in which paper is the feedstock. In the company’s previous generation of machines, a standard inkjet printer would print a shape onto a piece of standard office paper that would then be glued onto the sheet below it. Then, a tungsten carbide blade would trace that shape, cutting it out from the surrounding paper. The process would be repeated until the object was complete and stripped out of the cut paper.





A paper print made with the Mcor IRIS HD 3D printer. (Image courte A paper print made with the Mcor IRIS HD 3D printer. (Image courtesy of Mcor.)

Last year, however, the company swapped out the standard office paper for a roll of paper and the inkjet printer for integrated printheads to create a more compact, less expensive system. By using paper, the process is said to be more eco-friendly than other processes. The use of CMYK color makes it possible to 3D print vivid models for marketing purposes, visual modeling and visual prototyping.

Deirdre MacCormack, chief marketing officer at Mcor, was able to speak to some of the benefits and applications of paper 3D printing. "Due to the inherent qualities of paper our parts are lowest cost (10-20 percent that of competing technologies), best color in the industry (we have the highest measured color in the industry – 2 million colors) and the most eco-friendly solution," MacCormack explained. "Therefore, there are a myriad of applications suited to our technology including, medical, design, packaging, GIS [Geographic Information System] mapping, architecture, casting, CFRP [carbon fiber reinforcement polymer] and more."

Side by side images of two 3D printed models made of a municipality in Turkey. On the left, the current state of the land and, on the right, the same area with buildings and other features representing city growth. (Images courtesy of Mcor and SBK3D Ltd.)

MacCormack outlined each of these applications in detail, pointing out that the color accuracy of the paper printing technology has been key for use in plastic surgery, for instance. Before and after models of a patient's face both show the patient what the surgical outcome will be and enable "the surgeon to set their expectations and come to an agreement with them about the changes that will take place," MacCormack said. For GIS and topographical mapping, it's possible to model map out real estate hot zones and growth trends or manage and plot visual data for collaboration between crews and regulators.

Perhaps most interesting for engineering purposes is the use of paper 3D printing for CFRP. Honda has been using paper 3D printing for the construction of extremely strong and lightweight carbon fiber-reinforced plastic molds. Production of CFRP parts can be expensive, but high performance industries like racing and aerospace rely on them when a high strength-to-weight ratio and rigidity are required. Previously working with CNC machines to make them, Honda turned to paper 3D printing to create CFRP molds to avoid the expense and labor of CNC machining.

On the left, a CAD model for a 3D-printed paper mold, which is subsequently coated with carbon fiber reinforcement. On the right, the final part. (Image courtesy of Mcor.)

"This is where our 3D printing technology comes in," MacCormack explained. "The mold is printed on our technology in the 3D shape required. The model is then covered with a release agent and the carbon fibre formed over this. The model is then placed in an autoclave which cures composites with heat (135 degrees Celsius) and pressure (0.5Mpa) for two hours. Obviously using paper based 3D printed models as the moulds for this process reduces the cost of producing CFRPs considerably. In fact, it is ten times cheaper than molds produced using CNC methods."

MacCormack pointed out that, not only are the paper molds ten times cheaper than those made with CNC machines, but that Honda has said that there is no difference between using paper 3D printing and plastic 3D printing for the production of parts for wind tunnel testing. "They also found that toughness is higher in our parts when compared with stereolithography and about one-tenth of the cost to produce," she added.

Other benefits to the technology that MacCormack described include the fact that paper parts don't warp like plastic parts or shatter like ceramics. Whereas paper doesn't have a glass transition temperature and auto ignites at 250°C, ABS melts at 100°C. They can be relatively strong as well, with paper parts exhibiting and Ultimate Tensile Strength of 32.5MPa, compared to 22MPa seen with parts made via fused filament fabrication. Paper is also highly porous as a material, containing as much as 70 percent air, making it possible to to infiltrate the material to transform it, causing the material to take on 70 percent of the bulk characteristics of any infiltration material. This enables such new physical properties as increased strength and flexibility.

This is in addition to the eco-friendly nature of the material. The paper used in Mcor systems is made to conform to the ISO 9706 standard and carries the EU Ecolabel, representing products and services with a reduced environmental impact throughout the entire lifecycle of the product. Chlorine free, the paper has been certified by the Forest Stewardship Council as a product that uses paper from well-managed forests and/or recycled material.

Bioprinting

Bioprinting is one of those technologies that pushes the bounds of science into the realm of science fiction. The first commercial bioprinter was the 3D-Bioplotter from EnvisionTEC. Since the machine’s release in 2000, the technology has proliferated with a number of FDM-style systems being developed, including some for just $5,000.

Most frequently, bioprinters use a pneumatic extrusion process to deposit hydrogel loaded with stem cells to create a structure that is then incubated, causing the cells to proliferate and form the desired tissue. It’s also possible to 3D print particle-laden inks to create structures from functional materials like hydroxyapatite.

The lab of Ramille Shah at Northwestern University uses both types of materials—hydrogels and particle-laden inks—to achieve remarkable breakthroughs. In the case of hydrogels, Shah’s lab was able to 3D print functioning ovaries transplanted into mice. With her particle-laden inks, Shah 3D printed flexible bone as well as neuron-type cells made with 3D-printed graphite.

With Shah’s platform, it is possible to create flexible graphene structures (A), to cut smaller graphene objects from larger ones (B), and create highly detailed objects or fuse multiple parts together (C). (Image courtesy of the Shah TEAM Lab and Northwestern University.)

Organovo may be the most widely known commercial organization working on bioprinting, having begun selling bioprinted liver and kidney tissues. The company’s next endeavor is to develop 3D-printed liver patches that can be used to extend the life of a damaged liver while a patient awaits a transplant. The holy grail, however, is to 3D print functioning organs.

In some cases, researchers are combining bioprinting with more traditional forms of mechanical engineering. For instance, Shuvo Roy, with the Kidney Project at the University of California San Francisco, has developed an artificial kidney device that uses both silicon nanomembranes and actual kidney proximal tubule cells to replicate the functions of a kidney.

The artificial kidney will be roughly the size of a coffee cup and powered by the pumping of the body’s own blood. (Image courtesy of UCSF.)

Both Roy’s work point to an emerging field in which tissue engineering and mechanical engineering might be combined to create new hybrid technologies. When asked about this prospect, Roy responded, “Mechanical engineering, as we think of it, is an established field going back hundreds, if not thousands, of years. On the other hand, tissue engineering is nascent, but it promises to ultimately deliver functional tissues and organs that conventional engineering technologies will likely never be able to. The key word is ‘ultimately,’ and we are still ways from routine replacement of full tissues and organs via tissue engineering methods.”

He continued, “Consequently, there is an opportunity to get to patients faster following a hybrid pathway, where you take the advances of tissue engineering to date, meaning functional constructs short of full tissue, and combine them with conventional engineering components to arrive at medical devices that provide a biological function somewhat similar to a healthy tissue, but in a shorter time frame.”

There are still some challenges when it comes to 3D printing with biological materials, such as proteins. “The ability to keep the properties of biological materials stable in a conventional/external device can be a challenge since the biochemical milieu is missing,” Roy said. “However, if we can integrate them with conventional mechanical components successfully, the biocompatibility of the overall device is improved and could result in a medical device better suited for implantation.”

Food

Almost as sci-fi as 3D printing organs is the concept of 3D printing food, though perhaps not quite as lifesaving. Although that may depend on who you ask. If it’s cows and the planet, 3D-printed food may be capable of saving both.

That’s because some researchers are working to 3D print steak through the same processes used to print liver tissue. The goal is to replace cattle farming as a means of producing beef, thus reducing the unnecessary death of cows and the amount of methane contributing to global warming caused by the cattle industry.

Though the company is not focused on 3D printing meat at the moment, Modern Meadow may have been among the first to popularize the idea. Currently, Modern Meadow is focused on producing leather through a process it refers to as biofabrication.

Natalia Krasnodebska, Head of Communications for Modern Meadow, explained, “Our research is focused on biofabricating new materials that can enable new design and performance possibilities while minimizing harm to animals and the environment. Our initial focus is on producing biofabricated leather, which we see as an ideal first product to showcase the innovation possible with biofabrication.”

According to Krasnodebska, biofabrication relies on innovations at the subcellular level, editing DNA with tools for synthetic biology, and macroscopic level, by organizing higher order structures with material science. The result is the ability to “build a material from the ground up with truly tunable properties,” she said.

“We design, grow and assemble collagen, the main protein in skin (and leather),” Krasnodebska said. “Using the latest tools of biotechnology, we engineer cells to produce collagen, and create biofabricated leather materials. We then tan our materials like leather using a proprietary process. We practice ecologically sound tanning and use all available tanning chemistry.”

The hope for companies like Modern Meadow is the reduction of the impact of meat and leather on the environment. “Livestock production is one of the greatest stressors on the planet,” Krasnodebska explained. “It uses 40 percent of the world's land, 30 percent of the world’s water and produces methane gas, a contributor to climate change. Alternatives to meat and leather would greatly lessen the impact on our environment, but Modern Meadow is only a small part of this movement. We don't really see ourselves as replacing leather, but providing an alternative material that has additional performance and aesthetic capabilities.”

Outside of meats, there are more fanciful foods being printed. The open-source Fab@Home 3D printer, developed at Cornell University, was the first printer to use food as a feedstock in 2005. With a simple syringe, the FDM-style printer could 3D print with chocolate, cheese and dough.

Others have since taken the Fab@Home concept to create new businesses, many of which don’t ultimately succeed. These have produced printers dedicated to printing chocolate, such as the Choc Creator, or savory foods, such as the Foodini. Savory foods often have to be subsequently cooked, which may be one reason why food 3D printing hasn’t taken off quite yet. Although NASA is considering taking this concept to space with a pizza 3D printer that could give astronauts something other than freeze-dried edibles to eat while floating around the ISS.

A full-color 3D-printed sugar cake topper made with the ChefJet. (Image courtesy of Volim Photo.)

Binder jetting has also been used to 3D print food, specifically sugar. The ChefJet was another never-released product from 3D Systems that deposited a food-safe binder onto powdered sugar to create beautiful and ornate objects, like cake toppers.

Although the ChefJet may never make it to market, a Dutch company called 3dChef still produces custom sugar objects for customers. Sing is expanding from his first location in the Netherlands to begin 3D printing full-color food objects for people in Australia.

Julian Sing, founder of 3dChef, was able to elaborate on some of the applications of food printing. “Food printing in general I believe is multifaceted, depending on your use,” Sing said. “Yes, restaurants are always looking to add an engaging experience to a guest’s dining. 3D food printing becomes another tool in the kitchen to add to that process. On a global level 3D printing has a good fit in world food aid. The materials last longer with little spoilage and you use only what you need. In a medical application, there is the PERFORMANCE Project, in which meals are printed for people with eating issues. This has been tested and rolled out across Germany. Right now, 3D food printing is like the first 3D printers: slow and in use, but there is no reason they cannot be as fast or faster than an HP Fusion 3D printer.”

A start-up in the UK has developed an entirely novel food printing technique that relies on modernist cooking. Through the process of spherification, a sodium-rich gel is encapsulated in a thin membrane by introducing it to cold calcium chloride.

The Future of 3D Printing Materials

The materials available for 3D printing are only growing. Not only are plastics and metals expanding rapidly, but as people like Shah at Northwestern University have demonstrated, organic and functional materials like graphite are being increasingly developed as well.

As this occurs, the functionality of 3D printing only increases to the point where we will not be limited to 3D printing individual components, but complete assemblies. Mentioned in our edition on metals, conductive metal inks are enabling the 3D printing of electronic circuits directly into plastic parts.

In the future, we won’t just have metals and plastics to thank for these fully functional devices, but the hodgepodge of miscellaneous materials, as well.