In the world of fused deposition modeling (FDM), or fused filament fabrication (FFF) as the technology for non-Stratasys-branded systems is called, one family of materials may be considered king of the crop: polyaryletherketone (PAEK). A class of semi-crystalline plastics, PAEK withstand high temperatures while maintaining mechanical strength.
For FDM and FFF 3D printing, PAEK is primarily used in the form of polyether ether ketone (PEEK) filament and its much more affordable alternative polyetherimide (PEI), better known under the brand name ULTEM.
Developed by General Electric’s Plastics Division, specifically by Joseph Wirth in the 1980s, PEI is a durable thermoplastic with important physical properties that include high heat, solvent and flame resistance, as well as high dielectric strength, thermal conductivity and overall strength.
After SABIC acquired GE’s Plastics Division in 2007, ULTEM became the property of the largest public company in Saudi Arabia. The material is a more affordable alternative to PEEK, but has a lower impact strength and usable temperature. Perhaps more importantly, 3D printing-compatible ULTEM 9085 has received a number of aerospace certifications that have made it the go-to material when 3D printing performance plastic parts for civil aircraft.
At first, the only company capable of 3D printing with the material was Stratasys, which not only holds patents related to extrusion additive manufacturing (AM), but also the mechanisms required to safely heat a 3D printer sufficiently and stably to melt temperature-resistant materials like ULTEM.
However, in 2009, key patents related to the extrusion process expired, giving members of the open -source 3D printing movement known as RepRap the courage to commercialize their inventions. Since then, numerous FFF companies sprang up and many have since shuttered their doors. During that time, very few explored the possibility of printing with ULTEM and, now, only a handful of companies have produced printers capable of handling PEI or PEEK.
Why is that? ENGINEERING.com wanted to know more, so it turned to a number of experts.
PAEK, PEEK and PEI
As described above, PAEK is strong and resistant to a number of environmental hazards. It has a continuous operating temperature of 250 °C (482 °F) and can even handle loads for a short period of time in temperatures of up to 350 °C (662 °F). When burned, PAEK puts out a low amount of heat and its fumes are the least toxic and corrosive. PAEK also has good chemical resistance.
The material does not break during an unnotched Izod impact test, has a tensile strength of 85 MPa (12,300 psi), a Young’s modulus of 4,100 MPa (590,000 psi) and yield strengths of 104 MPa (15,100 psi) at 23 °C (73 °F) and 37 MPa (5,400 psi) at 160 °C (320 °F).
An aircraft duct 3D printed from ULTEM 9085 using FDM. (Image courtesy of Stratasys Direct Manufacturing.)
Whereas ULTEM is the only brand of PEI available on the market, PEEK is produced by a number of companies, including several large manufacturers. This may limit the variation between numerous PEI filaments, as the base resin will always be from the same supplier.
As for PEEK, the material has been available as a powder for selective laser sintering for some time, but is relatively new to extrusion 3D printing processes, as are the printers that can handle it. PEEK has a higher impact strength and usable temperature than ULTEM. As a whole, however, PEEK is many times more expensive than ULTEM. Therefore, when that added strength and temperature resistance is not absolutely critical, ULTEM may be the more cost-effective option. Therefore, the applications of the materials often overlap.
The Applications of PAEK
To better understand its use in 3D printing, we spoke with Phillip Keane, a Singapore-based engineer with expertise in 3D printing for aerospace. He received his Master of Science studying ULTEM CubeSat structures, worked with Stratasys to 3D print a drone made from ULTEM and is currently pursuing a Masters of Engineering with a focus on ULTEM.
Keane pointed out that one reason for ULTEM 9085’s use in 3D printing for aerospace is the high number of certifications it has received, which include Flammability, Smoke and Toxicity; Federal Aviation Administration; Joint Aviation Requirements and Airbus certifications. “It's easier to pick a material and process that has the required certifications than it is to certify a new material,” Keane said. “Also, ULTEM has a high specific strength (strength-to-weight ratio) which is comparable to certain types of aluminum.”Keane noted that while ULTEM has not yet been used in any critical part on an aircraft, for noncritical items it makes a good replacement for aluminium and for heavier/weaker plastics that may have been used in the past.
Keane also noted that, given the long lifespan of an aircraft, it is sometimes necessary to replace legacy parts for an aircraft component OEM that no longer exists. This makes 3D printing these replacement parts a desirable option. “When it comes to retrofitting or replacing legacy parts on these older aircraft, you sometimes find that not only are the companies extinct, but so are the original machines and tools,” Keane said. “No problem. Just find the old technical drawing, convert it to CAD, and use a Fortus [from Stratasys] to print it out in ULTEM. It's cheaper, stronger and lighter than the original part in many cases.”
One company that seems happy with the decision to 3D print parts from ULTEM over manufacturing them via traditional means is United Launch Alliance. The company’s Atlas V rocket launched last year with ULTEM parts used to replace previously used metal components in the rocket’s ducting system. Printed by Stratasys on a Fortus 900mc system, these parts included brackets, nozzles and panel closeouts.
Greg Arend, manager of Additive Manufacturing at United Launch Alliance, explained that 3D printing with ULTEM ultimately made it possible to reduce the costs in part production significantly. “At United Launch Alliance, we have completed design on more than 60 additively manufactured ULTEM parts that will fly on our Atlas, Delta and Vulcan Centaur launch vehicles,” Arend said. “In addition to achieving typical cost savings of 50 percent to 75 percent over traditional parts, we’re seeing additional benefits such as part consolidation, lead time reductions, and usually a small mass reduction is achieved as well.”
According to Keane, aluminum and titanium alloys may still be preferable for critical applications in which part failure could result in loss of life, injury, system failure or the loss of expensive hardware. “That's not to say that ULTEM can't live up to the more critical tasks,” he said. “It just hasn't been tested in such a manner yet. Until then, it's best suited for aircraft interiors, and fittings suchas electrical boxes and for duct work.”
3D Printing a Drone from ULTEM
One of Keane’s most recent accomplishments was the creation of a quadcopter 3D printed with embedded electronics. After previously launching a CubeSat company that relied on 3D printing with ULTEM, Stratasys approached Keane to determine whether or not electronics could successfully be incorporated into a heat-resistant drone within the high temperature environment of a machine printing with ULTEM.
Keane holding his drone, 3D-printed in ULTEM with embedded electronics. (Image courtesy of Stratasys.)
The goal was to integrate off-the-shelf electronics into a drone made from FAA-certified plastic and see if they could survive the high-temperature environment. To pull it off, the quadcopter was designed with self-supporting angles of 45° so that there would be no internal support structures that might get in the way of installing the printed circuit board, electronic speed control, receiver, flight controller, wiring harnesses and batteries within.
The printing process took about 14 hours with the machine pausing only three times to install the electronics. Everything but the motors were incorporated during the print, which had to be installed after the print was complete; otherwise, the printer’s extruder would collide with the vertically protruding motor shaft. Although the parts were purchased “offtheshelf,” it was necessary to ensure that they could withstand high heats by testing the survivability of components in high temperature and by refitting the boards with high temperature components where needed.
Although the drone was just a prototype, the project demonstrated the possibility of automated fabrication of complex, functional objects with 3D printing.
Making the Filament
Historically, ULTEM filament has been one of the few that Stratasys itself has sold under its own brand, but, with the FFF explosion that began in 2009, other manufacturers have sought to sell their own ULTEM materials. One of the few is 3DXTech, producers of engineering-grade filaments.
According to 3DXTech Founder and President Matt Howlett, the production process isn’t much harder than it is for making other filaments. “Generally speaking, making the filament isn't that much different from standard filaments,” Howlett said. “Except that the extrusion temperatures are much hotter—north of 350°C—and the speeds are generally much slower.”
He continued, “We specified certain materials of construction in the extruders to be able to handle 400°C and above with very aggressive fillers (carbon fiber and glass fiber, for example). This makes the extruders more expensive to buy and maintain, but that's the cost of doing business in niche materials.”
This may partially explain why there are so few manufacturers of ULTEM resin. Howlett added that price may also play a role. “The other factor is that the ULTEM base resin is very expensive and has an 18+-week leadtime from the supplier, Sabic,” Howlett said. Another important reason may be that there are few FFF 3D printer manufacturers that make machines capable of handling the material, limiting demand to Stratasys customers.
Howlett has been in the plastics business, including the high-performance materials market, for about 25 years, working at chemical companies like Solvay, DSM and Bayer in technical sales and marketing, as well as global management. In addition to pure ULTEM 9085 and 1010—the former’s stronger, more heat and chemical resistant sibling—3DXTech also manufactures unique composites, such as a carbon fiber-ULTEM composite.
To do so, the company uses a twin-screw extruder to compound the plastic ULTEM pellets with raw carbon fiber. Once compounded, the filament is processed through a single-screw extruder and, voilà, you’ve got the strength and resistance of ULTEM with the stiffness dimensional stability of carbon fiber.
Working with ULTEM
In addition to his own experience with ULTEM, Keane described some of the challenges that ULTEM poses during the printing process. “Because of the high extrusion temperature of ULTEM, the print chamber must be kept at an evenly distributed high temperature,” he said. “That’s because any cool areas will cause the ULTEM to shrink unevenly. This can result in warping and even a lack of bonding to the previous layers.”
With a melting temperature between 345 and 400°C, designing a nozzle capable of withstanding the heat is not so difficult. There are already all-metal hotends on the market that can reach temperatures of 350°C and beyond. The issue is really the print chamber itself, according to Keane.
“The engineering challenge comes from maintaining this high temperature in the chamber without damaging the machinery within the printer,” Keane explained. “It's not an impossible task. Stratasys has mastered it by use of a thermal curtain system which protects the top part of the printer (the gantry) from heat damage. The gantry is effectively external to the print chamber, with only the heads being exposed to the chamber heat.”
A primary issue with 3D printing at such high temperatures is the potential damage that can be done to the components within the machine, as well as the effect that thermal expansion might have on the tolerances of the printing process. This process must be tackled by either placing those components outside of the actual print chamber or other methods.
FFF 3D Printing with PAEK
So far, the number of FFF 3D printer manufacturers that have embarked on the development of PEI and PEEK-capable 3D printers can be counted on two hands. Here’s an informal list of all the companies that we’re aware of: Roboze, Apium (formerly Indmatec), AON3D, Tractus3D, Rokit and Verashape.
We spoke with representatives from several of these companies to learn how they’d managed to develop printers capable of handling high temperature materials like PEEK and ULTEM. As it turns out, their solutions are trade secrets.
Ben Schilperoort, CEO of Tractus3D, mentioned that one obstacle to overcome while developing the T650P RTP 3D printer involved stable printing temperatures. “Managing the temperatures in the printhead and in the object layers to get them solid are the biggest challenges. That it is why it is hard to print objects with substantial volume and still be strong with high resolution,” Schilperoort said.
For Canadian-based AON3D, the challenge wasn't so much in developing the AON-M 3D printer itself, but in making sure it remained affordable. In order to fill the niche between entry-level desktop printers and existing industrial machines, AON3D had to build a printer that worked with high-performance materials.
"As with all engineering challenges, there are some trade-offs that need to be made," said Kevin Han, CEO of AON3D. "What we've done is cut away as much as possible that is unnecessary to delivering performance, and what you're left with is a very lean product which meets all the requirements but remains affordable”
Apium actually started as a manufacturer of PEEK filament, before it expanded to other materials like PEI and a printer capable of using these materials as well. “As we started with PEEK in the first place, our idea was to combine a technology that is capable of printing high complex geometries with a very short setup time and zero material loss during processing high-performance polymers that can be used in the most challenging industries and application,” said Philip Renner, applications researcher at Apium.“After a very successful introduction of our PEEK filament, we decided to expand our product range for materials due to many requests from manufacturers.”
Apium was one of the first companies to release a commercial FFF 3D printer capable of printing with PEEK and ULTEM, back when it went by the name of Indmatec. According to Renner, this led the company to design a number of parts from scratch. “The biggest challenge in the beginning of developing our Apium P155 3Dprinter was the design of the printhead and our device to control the solidification process of the polymers,” Renner said. “All of those parts had to be designed by ourselves because there was nothing available in the market that could satisfy our demands. The second thing was finding the right mechanical operating system. We had to find a way, where temperature sensitive parts are not loaded with the heat, coming from our designed parts. And, finally, writing our own software to be able to operate the printer to the desired performance.”
A LulzBot TAZ 4 modified to 3D print with PEI. (Image courtesy of NASA.)
For those looking to join the quickly growing segment of industrial-grade FFF 3D printers, NASA researchers detailed their work
in modifying a LulzBot TAZ 4 3D printer for printing with PEI. Necessary modifications included:
- Replacing the hotend with an all-metal hotend
- Replacing the thermistor with one capable of detecting temperatures up to 500°C
- Developing 3D printing cooling mechanisms for the printer’s stepper motors
- Replacing the DC-powered bed with an AC-powered bed that could heat up to 230°C
- Repositioning electronics and lengthening the cables
- Changing the firmware to reach higher temperatures with the bed and hotend
The most important piece of the project was the method of heating the print chamber. Keane mentioned that, with Stratasys machines, the entire build chamber is usually heated beneath a specialty curtain system. The NASA team saw the use of a convection oven around the printer as impractical and instead used directed infrared heating on the printed part itself. Twelve 35 W halogen light bulbs were situated around the build chamber as shown in the image below.
IR heating lamp unit. (Image courtesy of NASA.)
As a result, the temperature of the printing environment does not increase substantially, but the part being printed stays at a near uniform temperature. It was still necessary to create an enclosure, made with an aluminum frame, cardboard foam walls and a rubber seal around its door.
Layout of IR heating lamps with respect to the printbed. (Image courtesy of NASA.)
The Future of PAEK
With the major manufacturers of PEEK resin seeing the AM space as a potentially lucrative one, more suppliers of PEEK material will, in turn, cause prices to drop. This will encourage more users of PEEK materials to enter the field, as well. What may drive the cost down even further is the use of industrial plastic pellets in 3D printing, rather than filaments.
A new start-up called DPP Technologies has developed a pellet-driven 3D printer called the XL DPE. High-temperature materials are still in the works, but it is something that the company is experimenting with.
“Using our DPE (Direct Pellet Extrusion) method of delivery has produced results better than expected,” explained Bill Roberson, developmental manager at DPP Technologies. “We found using our cast aluminum heated build platform and a little ‘special’ deck prep is a good start. This combination is working well during test prints. It lets the extrusion adhere to the plate and keeps the warping to a minimum. Currently, we are working with a master batch plant to refine a pellet that has the base properties of ULTEM and additional additives to allow it to perform as it was intended and to curb the warp and curl effects associated with ‘high temp’ polymers.”
If DPP can pull it off, it’s possible that the pricing between pellets and filaments will be significantly different. “[A] couple of online retailers [sell] ULTEM 9085 at basically $75.00 per pound. You can order pellets in bulk, with no spools, reels or packaging, for $20.00 per pound,” Roberson said.
Randeep Singh, head of Business Development at AON3D, pointed out that the cost-effectiveness of additive manufacturing, as compared to subtractive manufacturing, won’t be lost on users. Whereas, with CNC machining, it’s necessary to cut away a thick block of expensive PEEK material, 3D printing more or less uses only the material required to print the part.
“PEEK is pushing the price of silver right now. It’s really expensive. I think that AM is going to not only open up more capabilities for AM users, but it will make it so much cheaper to use that material, so you’ll probably see it used in more places,” Singh said.
For Roboze, manufacturers of the One+400 3D printer, users may turn to the machine to replace CNC machining and metal parts. “PEEK and PEI represent a new chance to produce end parts replacing metals,” said Ilaria Guicciardini, marketing director for Roboze. “What actually surprises people about these materials is that, even if they're ‘plastics,’ they still maintain the intrinsic advantages of their nature—extreme workability and lightness. Thanks to unique thermal and chemical properties, they can be used for end parts, today made of metal alloys, with accessible manufacturing costs compared to ceramic and especially metal materials.
“Our goal is to offer in this context the chance to support the traditional methods by partially replacing today's CNC production,” she added.
Outside of FFF, medical-grade PEEK and polyetherketoneketone (PEKK) have been used for the creation of medical implants. For instance, Oxford Performance Materials has developed its own brands of medical-grade and aerospace-grade PEKK. The company’s OsteoFab material has been FDA-approved to create patient-specific cranial devices, facial devices and spinal implants.
SLS 3D printing, however, is typically much more expensive than FFF or FDM and, when applied to performance materials in the PAEK family, this price only increases. Therefore, PAEK in filament form may be potentially attractive for the creation of custom implants at a price that may be much less expensive than the laser-sintered variety.
Regardless of the method of implementation, it’s evident that a growing number of manufacturers have begun to fill the niche left between low-cost desktop 3D printers and professional systems capable of handling PAEK materials. Therefore, the impact those materials have on the world of AM may only just be beginning.