How to Use Composite SL Molds to Produce Injection Molded Plastic Parts
Can composite stereolithography molds be used to offer fast delivery of prototype or low-volume production plastic parts?
Throughout the past fifteen years many RP technologies and materials have been evaluated for producing injection molded plastic parts. In general, the results have not been successful. Mold life has been short, tolerances were not maintained or mold post processing added significant time to the process. Therefore, a trial was conducted to determine if composite stereolithography (SL) molds could be used to offer fast delivery of prototype or low-volume production plastic parts.
Although composite SL materials were designed for tooling applications—such as injection molding—processing parameters were not established for many plastics. Critical parameters—such as tool preparation, tolerances, cycle times, operating temperatures and injection pressures—needed to be established. The other major questions were: how many parts can be obtained from the mold before it could no longer be used and how does the part geometry affect tooling capabilities? To answer these questions, a handle design with some complex features was chosen for testing.
Processing of the composite SL inserts proved to be quite straightforward. After producing the mold inserts on a SL machine, they can be easily squared off by conventional grinding or milling using carbide tools. Lubrication for these processes is accomplished using a mixture of dishwashing liquid and water. The mold inserts are then subjected to a simple silicone oil pretreatment to facilitate mold release during use. Standard mold release agents, such as zinc stearates, are recommended in addition to the pretreatment.
Molding trials began by using high temperatures and low pressures until confidence was gained with the composite SL inserts. As the trials progressed, the temperatures were reduced and pressures increased. For reference, ABS was processed at 275°F and 450 psi injection pressure, and Xenoy was processed at 475°F and 400 psi injection pressure. These conditions are similar to those used in molding with conventional steel and aluminum molds. Between shots, the open mold faces were cooled using a compressed air gun. Precise surface temperature monitoring with an infrared temperature sensor is recommended. Cycle times for most materials were about one minute. This is longer than those typically encountered when using aluminum and steel molds, but it is acceptable for prototyping and short-run production. About thirty Santoprene parts and sixty ABS parts were produced using one set of inserts, and one hundred Xenoy parts were produced on a second set of inserts. The inserts showed little evidence of wear, and it is suspected that many more parts could have been produced with each mold set.
The molding trials proved that composite SL molds could be successfully used to produce significant quantities of injection-molded parts. Using SL to make the mold allows the creation of features—such as square inside corners—that cannot be machined into metal tools. This allows the end user to have parts produced to the exact design rather than having to compromise on details or spend more money and time to have the features added by secondary operations—such as electrical discharge machining (EDM). Using composite SL molds, significant quantities of detailed, close-tolerance parts can be produced in just a few of days.