Navigating the world of 3D printing materials is no easy task, which is why companies like Senvol have created databases dedicated to every material associated with industrial 3D printing and 3D Matter is doing the same with desktop materials.
To contribute to the growing knowledge base around this topic, we are in the process of creating several in-depth guides examining the most widely used and most specialized 3D printer materials. The topic is a broad one, which is why we have chosen to break it down according to specific 3D printing processes, such as powder bed metal processes and fused filament fabrication.
This article in particular will take a look at that most intriguing class of materials known as photopolymers, essential to vat photopolymerization 3D printing processes—such as stereolithography (SLA), digital light processing (DLP) and the newer generation of continuous DLP technologies, such as continuous liquid interface production (CLIP) from Carbon—and material jetting processes, specifically PolyJet printing from Stratasys and MultiJet Printing (MJP) from 3D Systems.
What Are Photopolymers?
A photopolymer is a type of polymer that changes its physical properties when exposed to light. In the case of 3D printing, these are typically liquid plastic resins that harden when introduced to a light source, such as a laser, a lamp, a projector or light-emitting diodes (LEDs). While most of these light sources project ultraviolet (UV) light, that is not always the case, with some very exciting new material chemistries allowing for curing with visible light.
Unlike the thermoplastics used in material extrusion technologies, like fused deposition modeling, photopolymers are thermosets, meaning that, once the chemical reaction takes place to harden the material, it cannot be remelted. While they may include a number of ingredients, such as plasticizers and colorants, the key elements necessary for the photopolymerization process are photoinitiators, oligomers and/or monomers.
When hit with a light source, photoinitiators will transform light energy into chemical energy, causing the oligomer (also referred to as “binders”) and monomer mixture to form three-dimensional polymer networks. To alter the physical properties of the material, such as the stiffness or viscosity, the chemistry might include a variety of oligomers and monomers, such as epoxies, urethanes and polyesters.
Additionally, fillers, pigments and other auxiliary chemistries may be thrown into the mix to change the color of the material or further augment the functionality of the printed part.
Out of any 3D printing material category, photopolymers represent the largest market segment in the additive manufacturing (AM) materials market. This is in part due to the fact that the first commercial AM systems were SLA printers from 3D Systems, as well as the practical uses that they serve today, which can be as diverse as printing castable models of jewelry or dental crowns to creating biomedical ceramic bone implants.
To better understand the versatility of this material class, ENGINEERING.com reached out to experts in the field, who were able to provide insight on some of the most practical and most unique applications of photopolymers for 3D printing.
The Birth of 3D-Printable Photopolymers
3D Systems is, in many ways, responsible for introducing the world to photopolymer 3D printing, as it was the first company to commercialize the technology in 1986 with the first SLA machine. Scott Turner, senior researcher at 3D Systems, may be the perfect person to begin an investigation into 3D printing photopolymers, as he’s been with the company since two years after its founding when, as he put it, “there was only one usable photopolymer with poor physical properties and major distortion of the printed part.”
Turner believes that the broader adoption of photopolymers began in the early 1990s when the first “hybrid” photopolymer formulations were introduced.
“The combination of this new class of material along with advances in imaging methods placed SLA parts on par with the accuracies that were being produced using injection molding of thermoplastics,” Turner said. “Although these photopolymers often had similar tensile strength and flex modulus as thermoplastics, they could not match thermoplastics in the areas of impact resistance and long-life physical properties.”
QuickCast prints can be investment cast in metal with high resolution. (Image courtesy of 3D Systems.)
The researcher explained that, in the 1990s, the company went on to develop photopolymers engineered for applications that did not require the same longevity as injection-molded thermoplastics, including materials for the company’s QuickCast Patterns: “This application is the practice of building hollow photopolymer parts using stereolithography that were capable of surviving the ceramic investment process but fragile enough to burn out of the ceramic without damage or residue. Using today’s photopolymers, QuickCast is the standard production method used for low-volume metal investment casting used by the aerospace industry.”
Over time, the properties of photopolymers have been further improved to more closely match those of injection-molded thermoplastics so that parts can survive weeks and even months. One such specialty material from 3D Systems is called CeraMAX, a ceramic-reinforced composite meant to have good temperature, chemical, moisture and abrasion resistance.
About the material, Turner said, “It has been my experience that most engineers don’t consider use of photopolymers because there is a perception they can’t get the physical properties they need for their use case. In the case of CeraMAX, once you educate someone about the speed and cost with which they can get a jig or fixture made in this high-performance material, they will typically choose it over having to produce a metal object that a) costs more to fabricate, b) has a longer lead time and c) is required to be stored between use cycles. Modern manufacturing companies are looking for ways to transition their tooling assets from physical to digital so they can be deployed when and where needed.”
3D-Printable Ceramics
Both Scott Turner and Alban D’Halluin, co-head of R&D at Prodways, were quick to point out that, when thinking about 3D printing materials and processes, the application is of prime importance.
As D’Halluin explained, “At Prodways, we define and create materials as a companion to a 3D printer and with a proposed process for a specific application. Creating photocurable materials is easily doable, but creating materials which are ultrafast, have the right mechanical properties, do not yellow after a while, are not absorbing water and are compatible with the desired post-process … this is an art.”
Prodways is a French manufacturer of 3D printers that use a unique photopolymerization process called MOVINGlight, which sees a set of UV LED lights photocure large swaths of resin at a time. With a dedicated materials production department, the company creates photopolymers for its MOVINGlight machines in-house, though the printers are also open to third-party materials. Among the photopolymers Prodways creates are biomedical ceramic composites.
D’Halluin elaborated on the formulation of the company’s ceramic materials: “To be precise, we are not talking about ‘ceramic photopolymers’ but rather a ‘photopolymer filled with ceramic powder.’ These materials are then composed of the photocurable organic part (binder) and the inorganic filler. We 3D print the material, then the binder is burnt out (debinding) and the filler is heated to densify (sintering).
In general, we start with the filler and the goal is to design a binder that (a) photocures fast and deep enough, circumventing the light masking and light absorption of some fillers, (b) keeps the filler well distributed (no sedimentation or agglomeration effects) and (c) burns well for a nice debinding phase without polluting the filler or damaging the desired geometry,” D’Halluin continued.
Currently, Prodways offers two industrial ceramic materials, zirconia and alumina, and two biomedical ceramics, hydroxyapatite and tricalcium phosphate, but also engineers custom binders for specific applications and has a number of custom ceramic-filled materials for some of its customers. D’Halluin explained that the biomedical ceramics are most often used for either resorbable or permanent bone implants.
Resins for 3D Printing Microscopic Parts
The German firm Nanoscribe is known for taking the photopolymerization process down to the nanoscale, allowing users to fabricate the smallest 3D-printed parts ever made. In fact, these prints are so small that one artist even lost his entirely, adding a new dimension of impermanence to his 3D-printed artwork.
Nanoscribe’s Photonic Professional GT system uses a process known as two photon polymerization, in which a high-powered laser directs two photons of near-infrared (NIR) light in ultrashort pulses at photocurable resin. Combined with piezo-driven actuators and focusing optics, this allows for the 3D printing of objects with details finer than 200 nm (7.9 µin).
Though the prints are tiny and the technology advanced, Michael Thiel, cofounder and chief science officer of Nanoscribe, said that the photopolymers used for the process aren’t always all that different from resins used with more standard photopolymerization processes like SLA and DLP.
“For example, SU-8, one of the standard resins for microsystems, is exposed very effectively by Nanoscribe’s Photonic Professional GT system,” Thiel explained. “The reason is that we use NIR laser light to expose UV-curable photopolymers with two- or multi-photon absorption, for example low-energy NIR photons allow for excitation of the photoinitiators that are made to be excited by 405 or 365 nm, which are the wavelengths typically used in SLA or DLP printers as well.”
The company wants its customers to be able to use standard photopolymers, such as the aforementioned negative-tone SU-8 or positive-tone AZ resins, but Nanoscribe has also developed its own specialty resins. A material dubbed IP-L780, the first in Nanoscribe’s IP series, allows for twice the resolution of SU-8 and is also biocompatible, making it ideal for 3D printing microscaffolds for understanding cell growth and proliferation.
Thiel also explained that there are many nonpolymerizable materials that cannot be directly 3D printed in a one-step procedure. For that reason, the company employs methods to deposit materials onto 3D-printed structures, such as atomic layer deposition and chemical vapor deposition, or for casting them into other materials, such as silicon-single-inversion and silicon-double-inversion for casting prints in silicon.
Galvanization or electroless plating is also used to apply thin layers of metal, such as gold, nickel or copper, onto polymer structures, while casting structures in polydimethylsiloxane (silicone) can allow for producing copies of the print in other materials.
PolyJet 3D printing
Photopolymers for 3D printing are not limited to vat polymerization technologies, like SLA and DLP, but are the key ingredient to material jetting processes, including PolyJet from Stratasys and MJP from 3D Systems. By jetting photosensitive inks and then curing them under a UV lamp with each layer, these processes are capable of producing some very amazing prints.
PolyJet inks are low-viscosity materials that are described as more reactive and faster curing than photopolymers used for other processes, like SLA and DLP. Additionally, printed parts require no or very little post-curing. Made up of a large number of ingredients, the materials can be manipulated to create parts with a wide variety of physical properties and in a huge palette of colors. The use of a hydrogel-type support material allows for built-in support structures that can be washed away with high-pressure water.
Zehavit Reisin, vice president of the materials business at Stratasys, explained just how PolyJet enables material flexibility that’s impossible with other technologies: “Other than the technical differentiators (viscosity, curing duration, etc.) between PolyJet and SLA/DLP, there are two unique capabilities of PolyJet technology that affect the application world that they enable: The first is the creation of Digital Materials, available only with PolyJet. This is the ability to produce new materials with new properties by sophisticated mixtures and compositions, “on-the-fly”, during printing. The second is the creation of Elastomeric materials at different Shore scale A values, wide range of tear resistance values and elongation at break.”
The ability to manipulate materials on the fly opens up exciting possibilities for 3D printing multi-material parts. As Reisin noted, “This is done through the digital material’s generation capability, which enables compositions of rubber and rigid materials for new rigid properties all the way to polypropylene simulation. It also enables a wide range of Shore scale A rubber-like materials, high temperature and high toughness compositions for ABS properties, transparency and opaque combinations. Recently, we added full color using the latest J750, which enables thousands of shades and colors on the same part. On top of that, J750 is able to produce textures and gradients using a VRML file input—hence it provides superior appearance capabilities.”
As evidenced with the latest J750 PolyJet machine, it’s obvious that Stratasys is working on improving the technology all of the time. The technology has numerous practical uses, for both aesthetic and functional applications, according to Reisin.
She pointed out that PolyJet has applications for true-to-life models throughout the prototyping process, including concept verification, presentation, and fucntional testing. Potentially more powerful is the ability to create parts for tooling, such as the fabrication of molds for the injection of plastic materials for batch production of 50 to 100 parts, all without any deterioration to the mold. Resin added, “PolyJet also uses dental materials, which serve a variety of dental applications, including stone models, surgical guides and veneers. The PolyJet offering also includes biocompatible materials that are used when prolonged skin contact is required (hearing aids, orthopedics surgical guides, etc.) or during contact with mucosal membranes of up to 24 hours (dental surgical guides).”
Ultrafast 3D Printing Resins
Carbon astonished the world with its lightning-fast CLIP technology, which can produce engineering-grade parts in a matter of minutes. The use of an oxygen-permeable window enables Carbon’s M1 3D printer to 3D print in a layerless fashion quickly, but perhaps equally important are the photopolymers with which the M1 can print.
Not only is CLIP technology fast, but the parts it produces have physical characteristics that rival those made with injection molding. Jason Rolland, vice president of materials at Carbon, explained that this is in part due to the physical makeup of components made with CLIP.
“Traditional additive approaches to photopolymerization have been unable to create objects with properties similar to injection molding, because the parts created are typically weak and brittle,” Rolland said. “Additionally, they are not isotropic—parts have different mechanical properties in different directions. Carbon’s parts are isotropic because of the unique continuous nature of our process, which doesn’t generate internal layers.”
He went on to add that an additional element to the CLIP process is a post-print heating step that activates further properties within Carbon’s photopolymers. “We address the mechanical shortcomings of traditional photopolymers by integrating a second reactive chemistry in many of our materials. We use a two-stage curing process—first growing parts with our CLIP technology and then heating them—to activate a secondary thermal chemistry.”
Rolland pointed out that this approach allows Carbon to use a much wider range of chemistries to create materials for end use. “They are tough and resilient, isotropic and machinable. Through the second-stage thermal curing process, our materials adapt and strengthen, resulting in high-resolution parts with engineering-grade mechanical properties and agnostic directionality,” Rolland said.
This has enabled Carbon to release seven proprietary resins so far, all of which maintain unique characteristics for specific applications, such as high heat resistance, thermal stability, durability and elasticity. Rolland mentioned one material in particular as an achievement for additive manufacturing.
One exciting material from Carbon is EPU 60, the first in its elastomeric polyurethane (EPU) family. EPU 60 is engineered to create highly elastic, resilient and tear-resistant parts that have been difficult to achieve with previous AM techniques. Rolland described these parts as “similar to what you might find in an athletic shoe or gasket- or seal-compatible parts similar to what you might find in automotive, aerospace and industrial applications that use traditional polyurethane elastomers.” With Carbon’s EPU family, however, this material is now available with AM.
Carbon released its first seven materials with the M1 3D printer earlier this year, but the list of resins will continue to grow. The company has partnered with Kodak to develop new materials and applications and recently appointed Ellen Kullman, the former chair and CEO of chemical giant DuPont, to its board of directors.
Materials for Low-Cost, High-Resolution 3D Printing
Most of the photopolymers described so far have been designed for industrial 3D printers, typically defined as systems with a price of over $5,000. However, Wohlers Report 2016 pointed out that Formlabs may defy this categorical distinction with its $3,499 SLA 3D printer, the Form 2: “One could argue that the capabilities of the company’s products match or exceed the capabilities of other photopolymer systems classified as industrial systems.”
In addition to selling a low-cost SLA machine capable of high-resolution prints, Formlabs produces its own resins, more recently expanding its line of functional materials to include castable, tough, flexible and even biocompatible dental photopolymers.
Dávid Lakatos, head of product for Formlabs, was able to elaborate on how the company’s Flexible and Tough resins, in particular, may be useful for engineering purposes: “Flexible Resin offers a soft-touch texture for tactile applications. Engineers can create parts that are bendable, compressible and impact resistant, so this is excellent for prototyping, product design and engineering. Tough Resin is a durable ABS-like material that’s developed to withstand high stress or strain, so it’s ideal for engineering challenges like snap-fit joints and other rugged prototypes.”
These allow a desktop 3D printer manufacturer to increase the functionality of low-cost technology through specialty resins. While, in the past, those looking to prototype parts may have had to rely on expensive industrial equipment, now they can purchase something like the $3,499 Form 2 3D printer and achieve similar results. In fact, the capabilities of low-cost machines and the accompanying materials are improving to such an extent that small end-part manufacturing can even be produced on these systems.
According to market analysis firm SmarTech Markets Publishing, the low-cost 3D printing market will generate more than $4 billion by 2021, including $1.7 billion from materials alone.
DIY 3D Printing Resins and the Future of Photopolymers
Numerous companies sell third-party resins for 3D printing, both for industrial systems and desktop machines. For instance, Tethon3D recently began selling a ceramic-infused material for 3D printing porcelain objects. New photopolymers are being developed all of the time. Thanks to Autodesk, some of these materials may even be open source.
In an effort to spur the evolution of desktop 3D printing, Autodesk released its Ember DLP 3D printer with an open-source model, going so far as to provide the formula for its photopolymer resin as well. For those with the chemical know-how, the company’s standard clear resin can be concocted at home or in a lab and modified to produce new materials. For instance, Amy Karle, one-time artist in residence at Autodesk, developed her own nontoxic cell growth media to 3D print on the Ember.
The 3D printing materials market is set to reach $8.3 billion by 2025, if the predictions of market research company IDTechEx are accurate. Currently, Wohlers Associates has photopolymers representing 45.5 percent of that industry. It may be difficult to estimate what portion of that $8.3 billion market will be occupied by photopolymers, but the materials are clearly quite powerful.
For instance, there are new developments taking place around materials cured by visible light. While PhotoCentric 3D has developed a series of "daylight resins", Solido, out of Italy, has developed the $99 OLO 3D printer, which uses a smartphone as the light source to harden resin sensitive to visible light.
This firm is not the only one working on this technology, as Jeng Ywam-Jeng, a researcher at the National Taiwan University of Science and Technology, has developed his own version of the technology and is aiming to commercialize it soon. The same visible-light resin could then be used for larger devices, such as tablets and TVs.
If it were possible to infuse similar reinforcing materials and other additives to the resin, we might even imagine a day in which large-scale objects could be 3D printed from functional materials at home. That day may be far off, but whether it be through inkjetting, a high-powered laser or an array of LED lights, the potential for 3D printing photopolymers is seemingly endless.