materials | 3D printing
Right: Teknor Apex has developed
tailored PLA compounds
specifically for 3D printers
performance beyond standard levels had generated resins with either higher HDT or greater impact strength – but not both in the same grade,” the company says.
FDM advances There is still a big difference between desktop 3D printing machines and the industrial fused deposition modelling (FDM) equipment built by Stratasys, although the company clearly feels the need to protect its position in the market. Last year, for example, it bought MakerBot, a leader in 3D printing machines. It is also investing heavily in materials development of its own. Stratasys is currently in advanced development with
composite materials for use on its FDM machines. “We showed what is possible at NPE and got positive feedback,” says Jim Orrock, vice president for FDM materials development at the company. Orrock notes that the potential ability to use
reinforced materials is an important advantage for FDM over another established AM technology, selective laser sintering (SLS), which uses very fine powders with particle diameters of up to 70 micron; FDM equipment produces parts with a resolution of 127 micron. He says it is possible to use long fibre reinforced compounds that enable a modulus in finished FDM parts as high as 20 GPa, which is higher than is normally obtained with injection moulding. One of the reasons for this is that the process produces extreme levels of anisotropy, so the high modulus can only be achieved in the in-plane (x-y) direction. Extruding composites through very fine nozzles
obviously leads to fibre breakdown, but even so, material that starts off containing fibres 6 mm long ends up in the FDM part with fibres between 150 and 200 micron long. Much of the materials development for FDM is done in-house at Stratasys, and cooperation with outside compounders is very limited, although not entirely unwelcome, Orrock emphasizes. That situation looks
The properties of Teknor Apex PLA compounds for 3D printing compared to standard PLA
unlikely to change: Stratasys has developed materials expertise that it wants to keep to itself, and prefers to buy standard materials that it can modify in-house. Last year, Stratasys acquired Interfacial Solutions, a provider of thermoplastics R&D and production services that had been a production partner to Stratasys for several years. Interfacial Solutions has now been renamed the Stratasys Advanced Materials Center. Stratasys is using its in-house expertise to, among other things, modify materials to improve through-plane mechanical properties of FDM parts. “With an unfilled amorphous resin, we can obtain properties in the z axis between 50 and 80% of x-z properties,” Orrock says. “With a semi-crystalline polymer, we can reach 90%, through manipulation of crystallisation kinetics to get better heat retention [and hence interlayer bonding].” To demonstrate his point, Orrock says that an (amorphous) ABS bead exits the nozzle of an FDM machine at over 200°C and reaches local ambient temperature (80°C with Stratasys machines, which produce parts in enclosed environments) in just 20 ms. By contrast, a (semi-crystalline) polyamide 12 can be formulated to take eight seconds. Stratasys is close to adding more semi-crystalline materials to its product slate. Orroch says acetals and polyesters are likely to be commercialised later this year. Not far behind is polyamide 6. “The problem with semi-crystalline polymers is that
Source: Teknor Apex Company 44 INJECTION WORLD | June 2015
standard materials shrink too much for the FDM system,” Orrock says. Shrinkage is compensated for in injection moulding by holding pressure, but that option is not available with FDM (or in fact any additive manufacturing technology).
www.injectionworld.com
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