August 22, 2016
by Chris Curran
As resolutions improve, material choices expand, and methods to control their properties evolve, 3-D printing will find uses beyond rapid prototyping.
For 3-D printing to move beyond prototyping into fabricating finished products and components, the materials used in printing must undergo several developments. Resolutions must continue to improve. More types of materials must become available and especially include more metals. And materials and printing processes must be optimized so multiple materials can be combined in a single fabrication process; for example, metals combined with plastics to make circuits, batteries, and so on.
The good news: innovation is happening in all these areas.
The industry is also likely to find ways to customize materials, adding another dimension to the design process. Such customization creates new materials by using chemical recipes to achieve desired mechanical, thermal, electrical, and other properties.
As discussed in the article “The road ahead for 3D printers,” expectations are high for 3-D printing, also known as additive manufacturing. Meeting these expectations depends on the industry’s ability to pivot from rapid prototyping to printing finished products and components. Making this pivot depends not only on advances in hardware and software; it also requires innovation in materials. A robust choice of materials and the ability to control and predict their performance are essential to achieve broader use of 3-D printing. This article examines three trends in 3-D printing materials, which are sometimes called inks. (See Figure 1.)
- Improving resolution: Achieving greater resolutions to print finer details
- Moving beyond plastics: Expanding the range of materials used
- Mixing materials and controlling their properties: Working with multiple materials to create new combinations that have unique properties and to fabricate complete systems
Figure 1: Tends in materials that are advancing the 3-D printing industry
Toward higher-resolution printing
Resolution, which is the fineness of detail during printing, is a function of material and the printing process. In 3-D printing, resolution is measured by the micron (internationally called the micrometer and denoted μm), which is one-thousandth of a millimeter. Today’s 3-D printers offer resolutions ranging from 100 μm for typical desktop printers to 0.1 μm for the most advanced machines.
A sheet of office paper is about 100 μm thick. An object fabricated with a 100-μm resolution is composed of stacked layers of material, and each layer is 100 μm. Therefore, no feature on the object can have a height smaller than 100 μm, and the height of larger features must be multiples of 100 μm. An object of 100-μm resolution has a layered texture reminiscent of fine plywood. Whether the texture is acceptable without further finishing depends on the application.
When the resolution is smaller than 50 μm, the layered texture is no longer discernible and the results become indistinguishable from objects created by using the injection molding process.
For most 3-D printing processes, the size of each layer is gated by the size of the droplet extruded from the printhead.
A resolution of 16 μm has proven sufficient for making anatomical models realistic enough for use in the operating room to guide surgeons, says Dima Elissa, co-founder and CEO of ProofX. “But I can’t say that will be sufficient forever,” she adds.
Below the micron scale is nanoscale territory, measured in nanometers (nm) or one- thousandths of a micron. That territory is also being pioneered in research labs. “The smallest nozzles we’ve printed with are 500 nanometers in diameter. That’s pretty small. With post-processing, the finest features we’ve demonstrated are down to 200 nanometers,” says Eric Duoss, a materials scientist at Lawrence Livermore National Laboratory.
Those are germ-sized features. (See Figure 2.) Even smaller particles may be in the offing-and their adoption could revolutionize fabrication. OWL Nano, a desktop stereolithography (SLA) printer, can already print at resolutions of 0.1 μm, or 100 nm.
Figure 2: Logarithmic chart of the dimensions that 3-D printing materials are being asked to achieve or that feedstocks need to attain
Moving beyond plastics
Currently, 3-D printing is dominated by thermoplastics used in fused filament fabrication (FFF) printers and by photopolymer resins that harden when exposed to certain wavelengths of light and that are used in stereolithography apparatus (SLA) printers. Both materials are undergoing development beyond the previously mentioned resolution issues.
“There is a lot of chemical space to explore to make stronger end products. And if you name a physical metric, there is something better out in chemical space for that metric.” –Lance Pickens, MadeSolid
Thermoplastics are desirable because they soften enough for deposition when heated and then solidify at room temperature. The two most commonly used are acrylonitrile butadiene styrene (ABS) and polylactide acid (PLA). ABS is a structural plastic widely used in products ranging from toy building bricks to crash helmets. PLA is made from organic material, compostable, and biodegradable in a human body over time, which makes it usable for implants.
The problem with ABS is that it tends to shrink and warp when cooling; successful use requires a printer that has a heated fabrication chamber, increasing the cost and complexity of the process. Successful printing is easier with PLA, but the resulting models tend to be brittle. Consequently, the industry is seeking warp- free thermoplastics that have high tensile and impact strength.
The use of thermoplastics involves many tradeoffs, but those tradeoffs also present opportunities for a range of experiments. “There is a lot of chemical space to explore to make stronger end products. And if you name a physical metric, there is something better out in chemical space for that metric,” says Lance Pickens, CEO and founder of MadeSolid, a developer of advanced materials for 3-D printers. In this regard, Pickens’ company has produced a material called PET+ that has strength comparable to ABS but ease of use like PLA.
Work is also under way in the field of complex fluids and their possible application to FFF. Complex fluids are a mixture of particles and a suspension media, and they are often viscoelastic-that is, they behave like solids under standard conditions but undergo a change to more liquid-like behavior under shear, Duoss explains. A common example is polymer paint, which flows when pressed under a brush but otherwise clings to the wall, where it subsequently cures and hardens. Similarly, these complex fluids would solidify once extruded from a nozzle and maintain their shape at room temperature. Such advancements open up the FFF method to many more materials rather than the subset of thermoplastic materials that it is limited to today.
In time, the availability of lower-cost inert ceramic piezoelectric printheads, which can deposit any suitable liquid without using heat, is also expected to enable FFF printers to accommodate a wider variety of materials, according to Pickens. Today these heads cost about $1,500 each, so the industry concentrates on far cheaper thermal printheads, which are limited to using materials that have a specific range of viscosity and boiling points.
One near-term possibility in the FFF and SLA methods is the fabrication of full-color, photorealistic models. The company OVE has developed ultraviolet-curable color inks and has integrated their use in the FFF and SLA processes. Each 3-D printed layer is followed by a 2-D print layer for color, offering as much color choice as inkjet printing offers.
The slow part of the SLA process is the release and recoating cycle. Resins used for SLA printing likely will be optimized for fabrication speed, specifically to drain from the part faster, says Andrew Boggeri, lead engineer at FSL3-D, a 3-D printer vendor. “We can perhaps double-maybe triple-the speed of this [release-recoat] cycle and still maintain the quality,” he estimates.
Far greater use of metals in 3-D printing is expected in the near future. Currently, metal fabrication is achieved primarily on devices that use selective laser sintering (SLS) or similar technology. These are generally large machines that have expensive, powerful lasers, which offer significant advantages in industrial settings. But the use of emerging materials and printer heads that work at nanoscale may soon permit fabrication of metal objects, possibly using desktop FFF printers.
Nanoscale technology may be the key, because the temperature at which metal particles will fuse together falls off sharply as the particle sizes become smaller than 50 nm. This size puts them in the range of cigarette smoke particles and viruses. (See Figure 2.)
For example, copper normally fuses at about 1000°C (1832°F). But 20 nm copper particles will fuse at about 280°C (536°F), which a kitchen oven can reach. Consequently, this little-known physical property opens the possibility of desktop 3-D metal fabrication.
“We believe this will work with steel and other metal alloys” at temperatures specific to them, Pickens says. “All you need to do is apply energy. Nanotechnology is the future of materials in 3-D printing and will turn it from a cottage industry into a widespread manufacturing technology. It will lead to new products we can’t foresee.”
The widespread adoption of nanoscale technology assumes the availability of industrial quantities of reasonably priced, graded, and consistently sized metallic nanoparticles. There is no natural source of such particles. The search has begun for the necessary industrial- scale manufacturing processes, either by scaling up the processes used to make nanoparticles for films and coatings, or through entirely new proprietary processes.
Nanoparticles will not be a priority for 3-D printers that use metal in workshop settings, because high-temperature equipment is already common in such settings. For industrial applications, such installations may be the norm because fabricating the object is often only the first step in creating a metal part. Additional steps can involve the removal of support structures and surface finishing, both of which need heat treatment that requires a machine shop.
“Generally we will see a step-by-step adoption of metal, starting with aluminum and brass,” due to their low melting points, predicts David Alan Grier, a computer science professor at George Washington University. “High-tensile steel may be one of the last things we get to.”
Organics and other materials
Printing a body part from living tissue for implantation has already been demonstrated. Doctors can harvest tissue from one area of the patient and use it to print a part the patient needs elsewhere. Such methods have been used to print skin grafts for burn victims and to grow bone replacements.
An expansion of this technique integrates blood vessels into the design and fabrication of the object. At Harvard University, Professor Jennifer A. Lewis has overseen the development of bio-inks that create living tissues with blood vessels. Using multiple printheads, the system lays down an intercell matrix and living material. Another printhead creates 35 μm blood vessels by laying down a material that melts as it cools, rather than as it warms. After being printed as an interconnected network of filaments, the material is melted by cooling and then drained away, leaving hollow tubes to form blood vessels.
Such development is part of what Elissa at ProofX calls the trend toward personalized precision medicine. But aside from living material, anything that goes into the operating room must be certifiably able to withstand sterilization in an autoclave, or oven.
Machines that can print with fiber, even ordinary cotton, are being explored, as they would be useful in wound dressings. MarkForged, a 3-D printer vendor, has announced a machine that can lay down carbon fiber filaments to create high-impact parts.
Multi-material road to fabrication of systems
“We are creating materials via microarchitected design that have properties or property combinations that were previously unobtainable.” –Eric Duoss, Lawrence Livermore National Laboratory
Current 3-D printers build components that can be assembled into systems, such as an individual gear that can be installed in a gearbox or a cylinder that can be filled with chemicals to make a battery. The Holy Grail of 3-D printing is to fabricate an entire gearbox, gaskets and all, or a ready- to-use battery that has the necessary chemicals- in other words, to fabricate complete systems. Obviously that goal would require the printing device to work with multiple materials, especially metal and plastic.
Combining metal and plastic fabrication on one machine is a daunting challenge, if only because of differing melting points. However, new approaches are evolving. “For us, something that has worked out very well is to build structures with one process and add some functionality with another process, and then add more functionality again with a third process,” explains Duoss of Lawrence Livermore National Laboratory.
He calls that multi-step process deterministic deposition, in which material is added to a preexisting 3-D structure. The process requires more sophistication than building on a flat substrate. By using different material in each step, designers can mix materials and embed one material in another.
Desktop alloys and microarchitectures
With 3-D printers that lay down material at micron or nanometer scale, different materials can be combined to create an object that has customized physical properties. By combining materials, designers can add toughness, elasticity, conductivity, and other such characteristics exactly where needed, and the printer can add internal voids-hollowed out spaces inside an object to reduce weight.
Lawrence Livermore and other labs are working at this scale, and designers are tuning coefficients of thermal expansion and other properties by mixing materials and voids. “We are creating materials via microarchitected design that have properties or property combinations that were previously unobtainable,” Duoss says. Integral to the process are Ashby charts, which let designers select materials on the basis of their properties. Named for Professor Michael Ashby at the University of Cambridge, such charts compare a range of materials according to selected properties, such as strength versus density, strength versus cost, resistivity versus cost, and so forth. (See Figure 3.) It is likely that future fabrication systems will have the Ashby chart data built in, and establishing the properties of the material will be as integral to the design process as establishing its shape.
Or going even further, future fabrication systems might decide what material needs to go where in a fabricated object, and then add it. But there is a fundamental difference between fabricating with feedstock from a supplier who warrants its properties and relying on calculated properties from ingredients combined on the fly.
“You must be able to take [an object] out [of the 3-D printer] and be assured it is the right quality, or the technology will have limited appeal for commercial use,” Grier warns.
The rose oval for stretch-dominated lattices shows that some materials developed by using microarchitected design, populate a previously nonpopulated area of the Ashby material selection chart, signifying new properties that can be designed into materials.
Figure 3: This example of an Ashby chart shows the kind of comparative information future designers might use to customize an object’s physical properties.
Standards and economics
There are no industry standards to ensure that the same part made on different machines will have the same properties or dimensions. This significant challenge will hinder the industry in moving beyond prototyping to printing finished products. The Additive Manufacturing Consortium, operated by the nonprofit EWI (formerly Edison Welding Institute), is taking initial steps toward setting standards.
Grier says one thing seems certain: The materials that become most widely used will mirror the needs of the industries that adopt 3-D printing with the most enthusiasm. These industries presumably will be those that have costly problems that 3-D printing can solve. Such problems probably include the requirement to stock expensive, specialized, and individual replacement parts and then deliver them immediately to distant points where they are suddenly and unexpectedly needed.
“For airlines, truck fleets, HVAC vendors, and certain medical specialties, needed parts can suddenly be made locally,” Grier says. “Everything will flow from the economics of the situation. Those doing it because it’s cool will die out.”
The pace of change
Stereolithography, the first form of 3-D printing, was invented in 1984. Since then, the power of computers has increased by a factor of more than 30,000, thanks to the computer industry’s adherence to Moore’s law, by which the number of transistors in a computer chip doubles about every two years. The result is that millions of people now pursue digital, online lifestyles very different from 1984.
But computers only need to manipulate almost- mass-free electrons. For physical objects, there can be no Moore’s law or any exponential rise in functionality. Consequently, the 3-D printing field is still experimenting with materials and probably always will be. No perfect solutions exist for materials, only tradeoffs calculated with ever-increasing precision. Bronze, a copper alloy, was invented at least 6,500 years ago, yet development of new copper alloys continues to this day.