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Additive Manufacturing 24 MoldMaking Technology —— MARCH 2018 This two-cavity mold was printed for a short-run of threaded eyelets. The mold was fitted with twin cooling tubes to improve cycle time. The tube fittings were epoxied to the mold to ensure a water-tight fit. Water was run at 65°F. Cycle times were less than 90 seconds. The threaded bolts were added to ensure high-precision alignment with the knock-out pin assembly. The mold face was sanded to 0.005 inch to provide a tight, no-leak closure with the top mold half. After 125 cycles of molding HDPE parts, there was no visual wear in the cavity and gate. The threaded parts met spec and worked well. A completed two-cavity plastic composite mold was 3D-printed on a pro- prietary high-precision desktop 3D printer using a carbon nanotube-reinforced, high-performance composite filament. The upper half shows the surface finish prior to post processing and the lower half is partially sanded. Material Matters Zollo now understood that the industry was ill-equipped to make significant advancements in 3D printing due to a lack of technical understanding of plastics and how to apply material science. He also identified a gap between his cus- tomers' engineering knowledge of part and tooling design and his company's expertise in designing for 3D printing using FDM extrusion technology and optimizing printing strategies for high-precision parts. There are thousands of experienced engineers trained in designing parts and molds using traditional, subtractive manufacturing technology, and there is a growing number of engineers with some level of experience or training in 3D printing parts. However, at this time, there are very few engineers who have both skill sets and fewer yet who have relevant experience in designing, printing and using plastic 3D-printed molds, according to Zollo. Zollo believes that this lack of know-how in designing injection molds for 3D printing is the next major obstacle to a broader adoption manufacturers 3D-printing molds. Designing injection molds is all about managing plastics flow and trans- ferring heat efficiently. Printed plastic composite molds differ from metal molds in their surface energy, mechanical strength and heat conductance, Zollo says, and these differences must be accounted for when designing the mold. "Here's a simple example: Increase the draft of internal pins and structures to ensure smooth movement between mold halves. If 3 percent draft was specified in the CAD drawing for metal molds, revise this to 4 to 5 percent for printed plastic molds," Zollo says. Holes should be printed slightly smaller in diameter, so they can be postprocessed to smooth the interior. A few other examples of principles that need learning and application include how to adapt the infill and outer layer print strategy to optimize mechanical strength; how to ori- ent the print job for optimal parting lines; how to properly dimension holes for alignment and knock-out pins; and how to properly design printed-in cooling channels.