The Best 3D Printer Materials: Polymer Powder Edition

Powder bed additive manufacturing (AM) technologies like selective laser sintering (SLS) and selective laser melting (SLM) are already used to produce complex geometries for prototypes and end parts. With the market introduction of the Multi Jet Fusion (MJF) platform from HP and the coming commercialization of high speed sintering (HSS) from Xaar, the world may be primed to see powder bed 3D printing leveraged for mass manufacturing.

As powerful as they are, these technologies are only as useful as the materials they use. While a form of Nylon may be tough enough for a detailed prototype or noncritical part, something much stronger, like polyetherkeytonekeytone (PEKK), can be used in critical applications, such as constructing a spacecraft.

To learn about the wide range of polymers found in the world of powder bed AM, ENGINEERING.com spoke with a number of companies in the field. Here’s what we learned about the best and most unique polymers for powder bed 3D printing processes.

How Do Powder Bed Processes Work?

SLS was invented by Carl Deckard at the University of Texas at Austin in the 1980s, originally as part of his undergraduate studies and later became part of his PhD research. The process involves directing a high-powered CO₂ laser at a bed of powder and fusing it into a shape determined by a CAD file. With each completed layer, the bed is dropped and the next layer is printed. This process is repeated until the object is complete.

The chambers of SLS machines are usually heated to bring the powder to a temperature just below melting. Most often, SLS machines rely on a laser pulse, rather than continuous exposure, to melt the powder. SLM differs from SLS in that the particles are not just sintered, but fully melted together, resulting in overall stronger parts. More recently, new powder bed technologies have been developed to speed up the printing process. Both HSS and MJF rely on infrared lamps to sinter particles together.

Plastic powder bed processes have a unique benefit in that the powder itself can act as a support material for subsequent layers. This makes it possible to 3D print moving parts, overhangs, underhangs and cavities without the need for a separate support material to be washed away or forcibly removed upon printing. However, fully hollow objects are impossible to create with this process because a hole must be incorporated into the design in order to remove trapped powder.

Gert Claes, product manager of Magics at Materialise, pointed out that, combined, these qualities make SLS particularly advantageous for batch manufacturing. For this reason, SLS is heavily used by service bureaus like Materialise, Sculpteo and Shapeways to produce a variety of objects at once. 

"One of the advantages of SLS for batch production is that you can position several parts on top of each other without the need to create support structures," Claes said. "Manually nesting parts can take a lot of time, however, so that’s why Materialise offers software that automatically nests parts—even on multiple platforms—taking into account a predefined nesting height and density. Users can nest hundreds of parts in a few minutes."

As a service bureau, Materialise uses its Magics software to subnest a series of parts from the same order. This makes it possible to "calculate the representative build volume and thereby offer customers a correct quotation," according to Claes. Cages can also be made around fragile parts to protect them or make them more retrievable.  

He also noted that thermal distortion can be a risk in SLS parts, but said that software like Materialise Magics can be used to analyze the surface area for each layer of a print and distribution within a build to protect against the big differences in temperature between two consecutive layers and ensure high surface quality.

Although polymer powder bed fusion processes can be used to sinter a wide variety of materials, including ceramics, metal and glass, this article will focus mostly on thermoplastics. These differ from thermosets in that, once melted, thermoplastics can be remelted.

The World of Nylon

By far, the most widely used plastic in polymer powder bed processes is polyamide (PA), also known as Nylon. Tougher and more resistant than some other plastics, like ABS, some form of PA is offered by every manufacturer of SLS machines or powders. While the most basic varieties are PA11 and PA12, there are also numerous types of PA composites that combine PA powder with beads of glass, carbon fiber and alumide to provide certain added properties.

The material is also often dyed to allow for colored forms of Nylon, though, as of yet, most PA sintered parts cannot be printed with more than one color at a time. For this reason, companies like EOS have invested in technologies for dying SLS parts. There is an exception for inkjetting technologies, like MJF, which can deposit colored ink onto 3D-printed parts in order to create vibrant, full color objects.

Evonik has been a long-time manufacturer of SLS powders, dating back almost 20 years. Sylvia Monsheimer, business director of 3D Printing at Evonik, described the company’s process for manufacturing powders.

“Each supplier has its own method to prepare polymer powder,” Monsheimer said. “It can be grinding, precipitation or other suitable methods. We have been using precipitation to produce powders for powder bed fusion processes for decades and have proven to be able to supply material in a consistent high quality and [in] the volumes needed. We check the quality in each production step and, thus, we are able to supply materials in a narrow range with regards to medium particle size; only approved materials that pass all tests will leave our house.”

Of its products, Monsheimer elaborated, “PA12 turned out to be a reliable standard for laser sintering materials. However, Evonik is working to expand the portfolio, according to a change of the markets from prototyping to more specialized applications or series production, which require more different materials.”

In particular, Evonik has begun producing a flame-retardant PA12, as well as new rubber like materials and PA613, which combines high temperature resistance, high elongation and high stiffness, which can be used to open up new applications, particularly in the area of transportation.

PA 11 and 12 from Prodways

To learn more about some of the entry-level PA powders, ENGINEERING.com spoke with Luca Veneri, head of the SLS program at Prodways. Prodways began as a manufacturer of industrial digital light processing (DLP) technology, but expanded its systems portfolio by partnering with Chinese SLS maker Farsoon Technology and by acquiring Veneri’s start-up, Norge Systems. Now, the company offers a wide range of SLS and DLP printers, as well as engineering services and materials.

“Our basic materials are PA11 and P12, which are quite standard for our users,” Veneri said. “Then, there is rigid carbon fiber and rigid glass fiber.” Glass-filled PA is unique in that it has higher temperature and chemical resistance than standard PA. Prodways’ PA12-GFX 2550 is described as being akin to polypropylene 20 percent injection molded parts. PA12-CF 6500 is a carbon fiber-filled PA12 that has improved strength with a modulus range of 4700–6500Mpa and heat deflection.

Prodways also offers other materials, like a mineral-filled PA12, for circumstances when toughness and resistance to thermal deformation are needed. But, before embarking on these composites, Veneri said that customers may first be introduced to an entry-level material. “Usually the easiest to use is a special grade we have that is PA12-L 1600, which is very fine and has really wide parameter windows, so it’s really easy to process.”

Due to the flexible parameter window for PA12-L 1600, users should have less difficulty using the material. At the same time, the powder is described as having good mechanical properties, recyclability, color stability, as well as low water absorption. This makes it suitable for a range of applications, including production and prototype parts for the aerospace and automotive industries.

For those looking to experiment with materials or even SLS technology itself, Veneri explained that the company’s ProMaker P1000 is open in terms of materials and operating parameters. As the first industrial SLS system priced at under EUR€100,000 (USD$113,170), the P1000 is “perfect for investigating new materials,” Veneri said. “The process is open. You can change the way that you use the laser, the speeds, the gain of all the different lamps, the curve inside of the process. Basically, it’s our R&D machine. It’s not just an R&D machine; it’s production machine, but fully open.”

This is a standout feature in the world of SLS, where most machines are closed to the operator and allow for limited modification of the printing parameters. “The point of that trend is to try to keep the range of modification as low as possible. For commercial reasons and because playing with the tool could be dangerous,” Veneri explained. “Our vision, however, is that, if you pay to have an open license, you are responsible for your machine, so you are fully aware of your goal. If your goal is to explore new material, there is nothing that we need to keep hidden just for us. If it’s open, it’s open. Even with the risk that you are going to maybe use the machine with too much material. That’s the way it is. It’s a trade-off.”

PA 6 from BASF

Chemical giant BASF has also entered the world of 3D printing by developing a PA 6 powder for Prodways and Farsoon. According to Kara Noack, Market Development manager of AM at ‎BASF, the material was meant to bridge the performance gap between polyaryletherketone (PAEK) powder (more on this material in a separate section) and PA11 and PA12.

“We also know from our traditional engineering plastics business that PA6 has a very large share of the injection molding polyamides market, so there are many customer needs that best fit PA6 chemistry,” Noack said. “This led us to partner with Farsoon to develop a PA6 SLS powder while they developed the equipment and processes that would best work with our material.”

PA6 is a more robust Nylon that is used in the automotive and electronics industries to replace metal for the creation of lighter-weight, less expensive parts. It does require higher temperatures to print than some SLS machines, such as the aforementioned P1000. Operating at a temperature of 220°C, the ProMaker P2000 HT is able to print with PA6 and ensure that the proper mechanical properties are achieved.

“Our Ultrasint PA6 powder for SLS improves the temperature resistance over other PAs, offers higher strength and modulus, and also burst resistance and pressure retention,” Noack added. “We think this product will open the door for our customers to be able to succeed in many additional 3D printing applications that previously wouldn’t have been achievable. Prodways is a NA distributor for the BASF Ultrasint PA6 and for the Farsoon HT SLS machines.”

Windform from CRP

CRP Group is a firm that specializes in manufacturing AM for high-performance applications and engineering parts, as well as producing SLS materials. The company’s SLS powder brand is called Windform, which began as a carbon-filled PA powder but has since expanded to a larger portfolio of PA composites.

Windform SP, for instance, is a carbon fiber reinforced PA that is meant to have strong mechanical properties like its other carbon fiber materials, but with added resistance to shock, vibrations and deformation. Other properties include a high impact strength and elongation at break, in addition to resistance to high temperatures.

In a case in which CRP USA, the firm’s U.S. branch, helped develop parts for DeltaWing Racing Cars, the company had to print runner lengths that attached to the base of an engine’s plenum. The material for the task needed to have the proper characteristics to perform during a race. Stewart Davis, director of Operations for CRP USA, said of the project, “Windform SP’s toughness and heat deflection temperature allow the part to be built and then raced in the endurance series. The engine is run under boost, so it sees pressure variation in addition to the vibration, shock and temperatures changes associated with racing.”

More recently, the company released a thermoplastic elastomer material to create rubber-like parts. Designed to exhibit chemical, heat and tear resistance, Windform RL is meant for applications in the automotive industry, as well as footwear and athletic equipment, for parts such as gaskets, hoses and shock absorbing components.

PEEK from Victrex and EOS

PAEK has been available for SLS 3D printing for some time. Developed by British chemical company Victrex in collaboration with EOS, EOS PEEK (polyether ether ketone) HP3 is based on VICTREX PEEK polymer. The material can only be processed with high-temperature systems and is for use specifically in the EOSINT P 800.

A semi-crystalline thermoplastic, the PAEK family is known for its strength, stiffness, flame retardant capability, sterilizability, smoke and toxicity performance,as well as its high temperature and chemical resistance. Parts printed with EOS PEEK HP3 in particular are described by the manufacturer has having a tensile strength up to 95 MPa and a Young’s modulus up to 4400 MPa, while maintaining mechanical properties in motion in temperatures up to 180 °C, while static in temperatures as high as 240 °C and keeping electrical properties in temperatures up to 260 °C.

For these reasons, EOS PEEK HP3 is meant to replace metal parts in order to reduce costs while maintaining a high strength-to-weight ratio.

PEKK from OPM

Perhaps the toughest material for powder bed processes is PEKK. Even stronger than polyetherimide(PEI) for FDM, PEKK has the highest performance under high temperatures of any powder polymer while also exhibiting strong chemical resistance.

So far, there is only one company that can use a powder bed process to print PEKK: Oxford Performance Materials (OPM). In fact, the material can’t even be sintered, according to OPM CEO Scott DeFelice. It must be melted. “PEEK is not actually a laser sinterable material,” DeFelice said. “We don’t laser sinter. We laser melt. Most people in polymers are sintering.”

OPM has developed the material for use in aerospace, industrial and medical applications. This has led to the use of OPM’s brand of PEKK in applications as diverse as patient-specific medical devices and on spacecraft.

DeFelice pointed out that, unlike PEEK HP3, PEKK is highly recyclable. At the same time, it may actually have better mechanical performance than PEEK HP3. This is in part due to the Z-axis properties of parts printed with OPM’s technology as compared to laser sintered parts. In SLS, the bond between layers on the Z-axis is traditionally weak because the previous layer is cooled past the melting point by the time that the laser melts the subsequent layer. “No one in the aerospace or biomedical [industry] cares about your XY performance. They only care about Z because that’s the weakest link,” DeFelice said.

PEKK may also be compared to ULTEM or PEI, a more affordable alternative to PEEK and PEKK that is used in FDM 3D printing. These components also lack Z-axis strength, so that they may not be used for structural applications in the aerospace industry.

Materials for MJF

MJF works by depositing a binding ink and a detailing ink to a bed of polymer powder before a set of infrared lamps fuses the powder together. The use of these inks is meant to allow “voxel-level” control over the physical properties of each point on the print bed, meaning that the color, stiffness, conductivity, flexibility and other characteristics can be controlled pointbypoint. Many of these features were not released when the technology first hit the market, however.

As a new process, MJF still features a lot of unknown variables and very few materials. In fact, to start, MJF can print with only two varieties of Nylon on the market, VESTOSINT 3D Z2773, a PA-12 developed by Evoink, and HP 3D High Reusability PA 12, an 80 percent recyclable Nylon developed by HP.

Sylvia Monsheimer of Evonik explained the difference between how materials behave in SLS as compared to MJF. “The technology is different,” Monsheimer said. “Parts are being built definitely faster, but the technology is slower with respect to the energy input into a voxel, which is good for the material. It will be exciting to see approvals for new applications that can be addressed with this new technology. The open materials platform matches long year’s customer requests—they want to have direct contact with the material suppliers. And the market needs lots of players working on lots of new applications at the same time to make the expected growth true. I hope that we will see a huge number of them very soon.”

Through an open materials platform, however, the company aims to quickly expand its materials portfolio. By allowing powder manufacturers access to various stages of the MJF printing process, HP hopes that more materials can be developed more quickly. So far, HP has four manufacturers already listed in this endeavor: Evonik, BASF, Lehmann&Voss&Co and Arkema.

Kara Noack at BASF relayed that her company is developing a thermoplastic polyurethane material for flexible parts. “For the Multi Jet Fusion system, we can currently share that we are developing TPU sintering powder for this platform,” Noack said. “This is very exciting to be able to offer a durable elastomeric product that can be used by a technology to produce functional parts with production speed and affordability. The possibilities of voxel-level customization are also very interesting to be able to apply to BASF engineering plastics.”

Of the open materials platform, Noack said, “Powders for Multi Jet Fusion require different attributes than for SLS, and BASF has benefited from being a founding member of the HP open materials platform. This approach has given us accessibility to work closely with HP technical staff to be able to accelerate our material development specific to that equipment and process. It also enables us to work directly with end users to drive our development towards their specific application needs, so I think customers will really win with more and better material choices.”

The Future of Polymer Powders

As MJF evolves, it’s likely that we’ll see even more polymers added to the technology’s material handling capabilities. An open approach may even become one for others in the industry to follow. We are also awaiting the first HSS systems to hit the market, at which point we may learn what sort of materials that process can work with.

At the moment, it seems as though it would be difficult for either of these technologies to reach the temperatures needed to melt plastics in the PAEK family. OPM, however, is in the process of expanding its own materials portfolio. Through a partnership with composites company Hexcel, the company has released a carbon fiber reinforced PEKK and plans to add more powders in the near future.

Victrex also announced last year that it would be developing new forms of PAEK through the British government–run Innovate UK program. One key goal of the project is to increase the reusability of PAEK powders. The company hopes to showcase technology demonstrators related to these new AM materials by 2018.