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New Polymer Applications in Additive Manufacturing

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

New materials, new ways to create fiber-reinforced plastics give manufacturers more options

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Using 3D Systems’ new Figure 4 Production Black 10 photopolymer, a single DLP engine produced these 1,200 plastic, finished, end-use components in 48 hours.

The 3D printing of polymers has been around for over 30 years. And as Patrick Dunne, vice president of advanced application development for 3D Systems Inc., Berkeley, Calif., put it, there are significant applications across many different industries, “ranging from large investment casting patterns for Inconel rocket components at Space X all the way to the mass custom manufacturing of Invisalign Clear Aligner tooling for straightening teeth. We have customers 3D printing 400,000 unique pieces of plastic every single day.”

Even so, new applications are coming on line, largely owing to innovations in materials, according to Dunne.

One of the newest materials addresses what Dunne referred to as the “mother lode” of additive manufacturing: The mass production of simple plastic parts. To do that, 3D Systems has just released Figure 4 Production Black 10 (PRO-BLK 10), which Dunne said is the first “direct” photopolymer that produces parts with production-grade properties.

3D Systems is using its non-contact membrane digital light processing (DLP) technology with Figure 4 Production Black 10. DLP is an inherently fast technique, but competing photopolymers have required long periods in an oven after printing in order to achieve “production part” properties. That’s not the case with this new photopolymer. It is simple. “Wash it, dry it, cure it with some light, and you have a functional plastic component,” said Dunne, who added that such functionality requires two essential material properties, the first of which is an elevated level of elongation to yield.

“The industry often cites figures for elongation to break,” explained Dunne. “But a functional end-use plastic part must also have a high elongation to yield. Because once [a part is stressed] beyond the yield point of a plastic, [it is] in a permanent state of deformation and distortion. And almost all the polymers, stereolithography resins, and inkjet resins … all the different plastics historically used in the market … have had a low elongation to yield. They are essentially in a permanent state of distortion when in use.”

Conversely, Figure 4 Production Black 10 has an elevated level of yield with 12 percent elongation break. “It feels like plastic,” said Dunne. “It behaves almost identically to a thermoplastic. When it flexes, it springs back with the rebound characteristic [desired] in production plastic parts. And as the stress and strain approaches the break point, it doesn’t break suddenly or unexpectedly. It has a yielding ductile mode of failure. Again, like you would expect from production plastics.”

The second critical material property necessary for a production part is environmental stability. As Dunne put it, “It’s all very well having a really tough piece of plastic. But if sitting in sunlight, or exposure to rain, or cycles of cold and heat cause it to change … If it becomes more brittle, discolors, or loses its transparency or translucency in the case of clear plastic … then it is not a plastic that can address production applications. It is just prototype material.” Dunne said a combination of factors contribute to Figure 4 Production Black 10 hitting both material requirements, but the primary driver was a breakthrough in chemistry.

More Needed for Mass Production

Of course, the requirements don’t end with these two material properties. The process must also produce parts with dimensional accuracy and an excellent surface finish. And, said Dunne, scalability also depends on being able to quickly remove support structures that were required during the build automatically. That’s achieved with 3D Systems’ Figure 4 Production printer, which delivers “blended micro pixels and pinpoint support structures.” After removing those structures, you’re left with “production grade sidewall quality” according to Dunne. The machine is also able to express unique “digital” textures on the surface of parts, such as friction grips on dials. And because the economics of 3D printing don’t necessitate high quantities, a manufacturer could offer custom textures for boutique applications like luxury automobiles. Or, as Dunne put it, “When ordering a Lamborghini one could choose between buffalo hide or ostrich skin. There are no time or cost constraints on the textures that can be applied to interior decorative cladding.”

On the other hand, Dunne pointed out that although there is almost no correlation between complexity and cost with additive manufacturing, that is not necessarily an advantage in achieving viability for mass production because “99.99 percent” of the plastic parts produced today are simple in design. “You can’t be exclusively tied to the need for complexity in the design in order to drive an economic justification,” said Dunne. He added that the technology is currently directed towards small plastic parts, meaning parts that fit in the palm of a hand, but said that isn’t much of a compromise since “80 percent of plastic parts fit on the palm of your hand.”
Figure 4 Production Black 10 produces matte, opaque, black parts that look like black ABS plastic. 3D Systems plans on releasing an opaque white version of the material, and has “ideas that will completely change the whole equation when it comes to available colors,” reported Dunne.

Composite Materials Expand FDM

Invented by S. Scott Crump, the co-founder of Stratasys Ltd., Eden Prairie, Minn. in 1988, Fused Deposition Modeling (FDM) is the most common type of 3D printing. Like DLP, many important recent advances in FDM stem from new materials. In this case, composites. As Dave Veisz, vice president of engineering for MakerBot Industries, Brooklyn, N.Y. (a Stratasys subsidiary) explained, one factor making FDM parts suitable for tools and jigs in a demanding manufacturing environment, or other long-term, real-world applications, is the addition of reinforcement to the base polymer. The reinforcement is “usually a certain percentage of carbon fiber or glass fiber,” explained Veisz. “It combines the attributes of the base material, whether that’s chemical resistance, impact, strength, or temperature resistance, and then adds increased strength and stiffness from flowing these stiff fibers into the material.”

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MakerBot’s Method machines start at $4,999 yet deliver finished part accuracy of ±200 µm across all three dimensions, regardless of geometry, thanks to a sealed material bay, heated chamber, and other features, according to the manufacturer.

MakerBot printers don’t add the fibers during the print process. Instead, MakerBot partners with leading filament companies that add the fiber fillers to the industrial screw extrusion line, producing the 1.75-mm diameter filaments that go into the MakerBot machines. These filaments would otherwise be purely plastic, but in this case include a pre-determined percentage of chopped carbon of fiberglass strands. MakerBot offers a specialized LABS Experimental Extruder for printing several of these filaments from partner suppliers. The hardened steel nozzle of the extruder reduces the filament to a diameter of 0.4 mm when making the object. In particular, Veisz highlighted a Kimya ABS to which they add carbon fiber, yielding both a much higher stiffness-to-weight ratio and protection from electrostatic discharge (ESD). The ability to dissipate an electrostatic charge is important when manufacturing jigs and fixtures for electronics, explained Veisz.

Joe Roy-Mayhew, director of materials for Markforged Inc., Watertown, Mass., echoed these thoughts, while adding that the longer the fibers in these composites, the better the mechanical properties. And Markforged has two machines, the X5 and X7, that can incorporate continuous fiber. They do so by using two different nozzles. One deposits a layer of melted plastic, followed by a second nozzle that lays in continuous carbon, Kevlar, or fiberglass, ironing it in as it goes.

“You get the real strength of these fibers,” explained Roy-Mayhew, “which is an order of magnitude better than the mechanical properties of a filled material with short, chopped fibers. That’s why we’re able to match or exceed the properties of aluminum in a plastic composite part.”

The contrast is huge. The Markforged X3, using chopped fibers, produces material that’s 20 percent stronger and 40 percent stiffer than ABS, while the X5 produces material that is 20 times stronger and 10 times stiffer than ABS. The X7, Markforged’s top-of-the-line industrial machine, produces material that is “stronger than 6061 aluminum and 40 percent lighter.” Using flexural strength figures, Roy-Mayhew cited 50 to 100 megapascals (MPa) for filled nylon (i.e. reinforced with chopped fibers), or a 50-100 percent boost versus unfilled nylon, while continuous fiber would deliver over 500 MPa.

To explain this phenomenon, he offered an analogy with Lego bricks you can try at home: Build a beam with a set of short bricks and it will break apart rather easily under a load. But add a brick—even a thin one—that extends across the entire beam and it holds up under heavy load. In the first case, each connection is like the plastic between the chopped fibers in the plastics used in the X3 and similar systems. Each is a possible point of failure. The second case has similar gaps, but also an area with a continuous “brick” that stiffens the entire structure.

However, all FDM machines print in layers and the vertical adhesion between layers is not as strong as that within any given layer. And the continuous fiber printers do not apply fibers vertically. Therefore, FDM material is not isotropic and the strength figures cited above apply to one plane, the horizontal X-Y plane. This also means that while a continuous fiber printer can achieve metal levels of strength in the X-Y plane, the material will only be as strong as that produced by other plastic printers in the Z plane. Veisz said the degree of Z-axis adhesion depends on both the material and the machine, but is always a fraction of the X-Y strength, 60 to 70 percent being a common figure. Having said that, there are ways to improve Z-axis strength to make them better suited to functional parts. Perhaps more importantly, argued Roy-Mayhew, most applications do not require fully isotropic properties.

Veisz explained that a heated chamber, like that of MakerBot’s Method and Method X machines, enables printing of engineering-grade material formulations that are similar to what is used for injection molding, so engineers can prototype in the same material that is later used for production. The heated chamber also creates much better bonding in the Z plane compared to a heated bed or open air desktop printer. “We heat the build plane to about 100oC (212ºF) for ABS and ASA, which is just below the glass transition temperature of the material. This lets the part meld and slowly cool, which has an annealing effect on the material.”

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MarkForged’s X7 machine can print parts that are stronger than 6061 aluminum, yet 40 percent lighter, by laying in continuous fiber reinforcement, according to the manufacturer.

Compare this to casting a metal part and then quenching it. This changes the grain structure, making the part brittle and susceptible to cracking. In contrast, annealing the metal gives it different mechanical properties, notably increased strength and toughness. “A heated chamber in FDM printing has this impact,” said Veisz. He added that while key features—such as a heated chamber, optimized Z strength, reduced warping, and dimensional accuracy—had “traditionally only been available on industrial FDM printers, MakerBot has made that technology more accessible in terms of size, price point, and ease of use.”

MakerBot’s Method machine starts at $4,999. Yet it’s also the first machine for which they claim a finished part accuracy of ±200 µm across all three dimensions, regardless of geometry. Such a claim is “virtually unheard of,” said Veisz, because the absence of things like a sealed material bay, a heated chamber, rigid frame and soluble support allow for too many sources of variability to be sure of finished part dimensional accuracy.

Roy-Mayhew observed that, for most applications, parts will be stressed from specific directions rather than every direction. “With 3D printing, you don’t have to build a part with the bottom down. We can print it sideways or diagonally to best account for the forces it’s going to face. We’re able to design a part such that very few actually delaminate, which could come from weakness between layers. [It can be designed] so that the full strength of the fibers bears the stress. That’s really important and powerful.” What’s more, Roy-Mayhew said, most applications don’t require reinforcement throughout the part. Users of its continuous fiber machines tend to build with a lot of fiber when they first get the printers, then learn that selective reinforcement is more cost-effective and still able to deliver the required properties. “We have customers lifting tons of weight with printed components they used to make with metal components,” said Roy-Mayhew.

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Haddington Dynamics cut costs by 58 percent and reduced the number of parts in its robotic arm from 800 to less than 70 by printing most of them on a MarkForged machine. Reinforced with continuous carbon fiber filament material, the arm is stiff and lightweight enough for the robot to have a precision of 50 µm.

One important example is the U.S. military, which is using these printers to create replacement parts for tactical units in the field, parts that would otherwise be made of metal. These parts are so good, he argued, that the need for 3D metal printing is largely driven by higher temperature applications, rather than the need for strength at “room temperature.” Lest that be viewed as a critique, he hastened to add that Markforged also makes a line of metal printers.

Other Advances in FDM

Besides the addition of fibers to broaden the applicability of FDM produced parts, Veisz credited big plastics companies like BASF, Mitsubishi Chemical, Polymaker, Kimya, and DSM with formulating special filaments that can be run in FDM printers to make end-use parts “to the same standards that people have traditionally used injection molding for.”

Some of these new materials meet the UL standard of V-0 for flammability (V-0 being relatively flame retardant). This has made FDM a viable method for additional end-use applications, such as aircraft interior parts.

The Airbus A350, for example, has over 1,000 parts that are printed on Stratasys machines. “Siemens makes a lot of train parts on FDM printers, and replacement parts as well,” said Veisz.

Other improvements focus on lowering costs and speeding up the process so that the economies of scale continue to shift, making FDM printing viable for higher and higher volumes. Veisz said one such improvement is water soluble support structures: “print an object, put it into an agitation tank, and wash away the support without any manual effort, leaving just the model geometry.”

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HP’s line of Jet Fusion machines print fusing and detailing agents across the entire work area in one pass for each layer, combining the printing with the fusing energy. HP says this makes the machines up to 10 times faster than FDM and SLS printer solutions.

This has also eliminated the need for much of the 3D printing knowledge previously required to get a good part. There is no need to worry about orienting the part in just the right way to eliminate the breakaway support, he added.

In contrast, Roy-Mayhew said Markforged prefers breakaway supports as an even faster way to get to a finished part. Plus, he said, “with our printers, printing anything at roughly a 45º angle possibly means not needing supports at all.” To him, this speaks to the “great distinction between polymer printing and metal printing.”

All forms of metal printing require extensive post-processing while MarkForged’s approach to polymer and composite printing produces a usable part “right when it comes off the printer,” said Roy-Mayhew.

HP Claims Speeds 10x Faster Than FDM

Just when you thought there couldn’t be another approach to 3D printing (or another set of acronyms) HP Inc., Palo Alto, Calif., charged into the market with MultiJet Fusion (MJF). The process is analogous to powder bed fusion in that a bed of powder is laid down for each layer. But HP Jet Fusion machines then print fusing and detailing agents across the entire work area in one quick pass for each layer, combining the printing with the fusing energy.

HP’s proprietary architecture is capable of printing 30 million drops per second along every inch of the bed. That, plus the combined action of the fusing and detailing agents, is said to produce “extreme accuracy” and smooth, well defined edges at speed. How fast? Based on internal testing and simulation, HP claims to be up to 10 times faster than FDM and selective laser sintering (SLS) printer solutions in the $100,000-300,000 price range, with a 50 percent lower cost per part.

New automotive applications include a “topology-optimized active coolant distributor for electric vehicles” developed by EDAG. Adient, a global automotive seating manufacturer for major automakers, “is creating a new seat headrest optimized for size and lightweighting without compromising material strength and flexibility.” Glaze Prosthetics is creating customizable HP 3D-printed prosthetics that are lighter, less expensive and more comfortable, improving the lives of its patients around the world. And in a challenge to Invisalign (mentioned at the beginning of this article), SmileDirectClub uses 49 HP Jet Fusion 3D printing systems to produce more than 50,000 unique mouth molds a day.

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