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Additive Manufacturing 26 MoldMaking Technology —— MARCH 2018 These molded HDPE parts were produced in the 3D-printed composite mold. Blue spray paint makes it easier to see that the threads molded well and the part is functionally correct. There are faint print lines in the head, so this mold would require additional burnishing of the cavity to optimize aesthetics, if required by the customer. The composite material is formulated to be sanded, burnished, drilled, tapped and threaded in post processing, allowing these printed molds to be prepared in a similar manner to aluminum mold inserts. provide print bed leveling, this printer performs a 25-point measurement of the entire print bed surface before each print job, creating a 3D profile of any deviation in flatness. This profile is used to create offsets in the print commands, automatically compensating for any imperfections in the Z axis across both X- and Y-axis movements of the printer nozzle. The printer uses an ARM Cortex processor-based controller board that runs an embedded real-time operating system (RTOS). This real-time operating system supports multi-threaded printing operations to optimize printing per- formance, which minimizes any latencies that could impact precise movement of the printer's extruder assembly. The printer also utilizes proprietary printer driver controls of the stepper motors. Finer motor control, combined with real- time processing of G-code with automatic offsets for imper- fections in the print bed results in more precise printing. "We also have customized the STL file-slicing engine soft- ware to deliver more consistent plastic flow on the external surfaces of each printed part, resulting in a denser, smoother finish than what is typically seen with FDM printers," Zollo says. The printer's nozzles are optimized for printing low layer height extrusions of the company's proprietary compos- ite filament. The solid outer layers of the printed mold are designed with postprocessing in mind. The printed mold is sanded using conventional abrasives to 0.005-inch tolerance. The printed mold halves are quite stiff, but still exhibit a small amount of flex under pressure, Zollo says, which allows the mold halves to close tightly to prevent flashing. On a printed cubic-inch comparison, the UV-curable digital ABS used on third-party industrial printers for producing molds is fairly dense, has a relatively low rate of heat transfer and costs roughly $5.00 per printed cubic inch. Each layer of this material must be UV-cured to create the thermoset and print the molds solid, which requires very long print times on expensive, indus- trial printing systems with long cycle times. "Depending on the in-fill printing strategy, our high-performance composite fila- ment runs in the range of $4.00 to $7.00 per printed cubic inch on printers costing between $4,000 to $15,000," Zollo says. When he originally tested printing molds in his lab, he expected to be able to print small, simple molds capable of molding 10 to 20 units before wearing out. He was pleasantly surprised to learn that his printed molds could easily support 100+ cycles, depending on the design of the mold and the mate- rial being molded. While none of his clients have attempted to mold more than 125 cycles to date, inspection of the molds after this many cycles indicates no visible wear on the cavity, gates or sprues. Zollo sees no technical reason why molding parts in HDPE, ABS or PP would not hold up to 500+ cycles. He believes this compares quite favorably with molds produced on third- party printing systems. He is hearing from service providers that typical life expectancy of printed plastic molds ranges from 5 to 25 parts, depending on the plastic to be molded. Zollo does note that there are a few practical limitations. "So far, we have focused on designing and printing simple, two-part, one- and two-cavity injection molds. The largest outer dimension printed to date is approximately 6 by 6 by 4 inches. We are working on methods to print larger molds that exhibit the desired dimensional precision, and expect to expand the range of dimensions in the near future through improved printing process control," Zollo says. A second limitation has to do with aesthetics. Although molds produced using his company's composite materi- als are designed to be sanded and polished, it is difficult to completely remove visibility of the print lines in the surface of the molded part. He continues to research new methods to improve surface finish. For parts that have an aesthetic requirement, he recommends embossing the surface with a pattern, either in the print design or via post processing. Gaining Ground To widen the field of compatible printers for users, Zollo's company tested and validated precision printing of the high- performance composite filament on a number of new open- source printers. To date, he determined that European-based German RepRap, the Italian printer maker Roboze and the American manufacturer Airwolf produce printer models with