Are you ready for another update on faster molds from the world of rapid prototyping? Until recently, the latest developments were rapid tools made from a resin and powder metal mixture using DTM's selective laser sintering process. Now, there are cavity and core inserts created in 3D System's SLA machine, then secured in a mold frame where molten plastic is directly injected onto them.
Skeptics take note: the cavity and core are built in SLA epoxy, which can withstand temperatures over its HDT of 250F when the mold is supercooled. Materials such as polyurethane, PP, PS, and ABS can be molded successfully in the SLA epoxy cavity. As for wear, vapor deposition techniques used to plate the SLA cavity with nickel are proving successful.
One of the first hints that this feat could be achieved came at 3D Systems' SLA Users Group meeting held last December. There, and months later at SME's "Rapid Prototyping and Manufacturing '96," Jeff Heath from Xerox, reported on the company's SLA direct tooling efforts. Xerox is using the SLA cavities for small part protoypes with 20-second cycle times, saving time and money, verifying hard tool designs, and performing accurate fit and function tests.
IMM spoke with Mark Horner, technical sales engineer for Astro Model Development Corp.(Eastlake, OH). Horner believes his company along with rapid molding partners Frantz Tool and Frantz Industries(Mentor, OH), is one of two or three companies including Xerox who have publicly acknowledged success using SLA direct tools. Astro Model operates a service bureau that specializes in rapid prototypes and engineering models along with machining capabilities to make production molds.
For SLA direct tooling, currently limited to under 50 parts, Astro first creates the part in a 3D solid modeling system (Pro/E) or imports the CAD file from the customer. The design takes into account shrinkage of the SLA process as well as typical molding shrink. Part size must fit within the SLA envelope (currently 20 by 20 by 24 inches for an SLA 500). Next, the software is used to "bury" the part in a block, subtracting part geometry to create a cavity and core. After converting this to an STL file, Astro sends data to an SLA machine to build the mold halves.
Horner admits to some softening with unplated cavities. Heat transfer can be problematic because epoxy does not have high thermal conductivity. Frantz Tool uses supercooled air to chill the mold before each shot. For additional cooling, the SLA process allows flexibility in designing water lines, so that cooling channels can conform to part geometry. Plating the cavity and core surfaces with nickel (.0001 inch thickness) is a slight help in heat transfer and keeps the epoxy from softening. But the real goal of plating, Horner says, is to improve wear during repeated cycles.
Part of the excitement over this technology is the potential reduction in lead times. For one customer, Astro was able to model a part in 6 hours and run the SLA cavity overnight. "The entire process, from concept to prototype part, can take less than five days," says Horner.
Horner affirms that each of the currently available rapid tooling processes has merit for specific applications. Using rapid prototypes as pattern masters for RTV silicone tooling, for example, works well for parts in lot sizes ranging from 20 to 50. After the RTV molds cure, a two-part urethane is cast to create prototypes. Astro uses a 50-psi injection system that mixes the two component urethane in a vacuum, then injects the mix under the same vacuum to avoid trapped air.
The Keltool process, created by 3M more than 20 years ago and currently owned by Keltool Inc., replicates SLA masters in A-6 tool steel for production mold cavities that can run millions of parts. Using this method, Astro/Frantz cut the cost of one mold by $14,000. To build cavities from SLA parts, Keltool applies reverse generation: silicone rubber is poured around the SLA master to create a temporary mold, and that mold becomes the master for the next rubber temporary mold.
This second mold is filled with a mixture of metal powders and resin binder to make a green part. After firing to sinter the metal particles and remove the plastic, the part is infiltrated via furnace with a copper alloy to produce a 100 percent dense cavity. The rubber mold can produce up to 200 identical cavities. Size is limited to 5 by 5 by 12 inches, and a shrink rate of .006 inch/inch is required to account for sintering. Speed and tolerances (±.001 inch) are the main benefits of this process, Horner says.
