DSM SOMOS WATERSHED SL RESIN VERIFIES ACCURACY REQUIREMENTS FOR INVESTMENT CASTING
27 October 2005 - DSM Coating Resins

In the largest accuracy study ever conducted for QuickCast parts, Chicago-area service bureau Express Pattern used Somos WaterShed 11120 stereolithography resin to produce investment cast patterns resulting in a metal part yield of close to 95%, similar to yields achieved by traditional wax patterns made from tooling.

The company built more than 500 QuickCast patterns from WaterShed, ranging in size from 1 to 20 inches. Each pattern was measured multiple times in three to four dimensions, ultimately making 4,300 individual measurements on finished patterns. Based on these efforts, Express Pattern was able to verify that over 98% of all measurements were within the acceptable tolerance band required by manufacturers.

"This study is significant in that it demonstrates the capacity of high-accuracy, high-stability stereolithography resins like WaterShed 11120 to be successfully used in demanding manufacturing applications like investment casting with greater yields than ever before," says DSM Somos Marketing Manager Eva Montgomery. "With the proper SL resin, so much more than just part prototyping can be achieved."

"QuickCast" refers to a hollow build style used in stereolithography to create parts that are subsequently sent to foundries for investment casting. The hollow structure is needed in order to accommodate expansion of the material as it is heated during the casting process. This percentage of hollow space to solid space is referred to as the "void ratio." The higher the void ratio, the better the part's success rate will be at the foundry.

"Today's QuickCast build styles result in a theoretical 87% void ratio, though some resin always fails to drain out of the pattern, causing the void ratio to be less than 87%," says Montgomery. "A low-viscosity stereolithography resin such as Somos WaterShed 11120 is attractive because of its improved resin drainage and subsequent reliability in building thin-walled parts."

Tom Mueller, partner at Express Pattern, explains additional factors which led them to choose WaterShed for their study: "One of the key factors affecting the accuracy of QuickCast parts is their ability to resist growth when exposed to air moisture. In the past, we discovered that patterns would expand due to humidity before ever reaching the foundry, which substantially skewed the accuracy of the final product. WaterShed's high humidity-resistance provides a major advantage in this area."

Other WaterShed advantages include: high initial green strength to prevent distortion during the post-cure cleaning process, heat deformation characteristics conducive to the investment casting burn-out process, and the overall transparency of the resin. "During the finishing process, WaterShed's clarity aids in visually inspecting the models for any trapped resin remaining, says Mueller. "Trapped resin expands and cracks the shell if not removed properly and is one of the key causes of failure in the foundry process."

Based on their study, Express Pattern believes that the production use of stereolithography patterns will have a significant impact on the way investment casting is viewed and the applications for which it is considered. "Without the cost and lead-time required for tooling, investment casting can now be very attractive, cost-effective option for small quantities, even as small as one," says Meuller.

Those interested in learning more about Somos WaterShed 11120 are invited to contact Somos Marketing Manager Eva Montgomery. In addition, Express Pattern has written a manual for foundries on how to process QuickCast patterns. To obtain a copy, please contact Tom Mueller at Express Pattern (email: tmueller@expresspattern.com)

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