When the customer wants prototypes in a high-temperature material, the answer to lower cost and faster lead times may be aluminum-filled epoxy tools.

Pressure to decrease the length of product development cycles continues in today’s marketplace with an added twist: Many OEMs now expect engineering prototypes made from production materials so that the resulting parts can be tested more accurately. When the final parts are to be produced from high-temperature engineering thermoplastics like polyetherimide (Ultem, GE Plastics) or PTFE (Teflon, DuPont), or from metal, the challenge of building a cost-effective prototype tool in a short time becomes daunting.

Engineers and tooling specialists at Phoenix Analysis & Design Technologies (PADT) believe they have found an answer in aluminum-filled epoxy tools. The material they use, called RenCast and made by Huntsman Advanced Materials (formerly Vantico), has a heat deflection temperature (HDT) of 446F (230C), allowing it to handle high-temperature resins.

“We’ve optimized the ability to quickly build epoxy injection molds that can be used with engineering thermoplastics as well as metals,” explains Mark Schanze, molding technologies manager at PADT. “With the epoxy molds, we can provide customers with unique, fast, and economical solutions to their new product development needs, particularly on complex projects for which steel tooling would be cost-prohibitive.”

PADT put its rapid prototyping capabilities to a test recently when it molded production-quality Ultem fans for installation in fuel cells being tested and installed on operating automobiles. The RenCast 2000 (resin)/Ren2000 (hardener) epoxy selected for the tools is designed to withstand the temperatures and pressures required to injection mold engineering thermoplastic prototypes and short-run parts. In addition to the HDT mentioned, the material has an ultimate compressive strength of 38,000 psi and a tensile strength of 11,000 psi.

Fan Design

For the fuel cell project, PADT was asked to mold 50 to 70 functional, multi-finned blowers. Engineers developed the design according to customer specifications, producing a fan-like blade that provides for high airflow at low pressure. When virtual simulation tests indicated that the design satisfied the performance criteria for the part, a stereolithography (SL) master was built. The model was then used to confirm the finite-element analysis results, verifying that the 3-D solid blower had the same airflow characteristics as the virtual part. The SL master performed as expected and technicians began building an epoxy injection mold. Schanze says, “Once we had the SL master, it took us about two weeks to build the epoxy tooling and mold 25 parts for initial testing. Developing the tools was the most difficult task because of the multiple fins on the part.”

Tool Production

Moldmakers started by fabricating mold forms that would be used in casting the 10 blower fins. For the forms, epoxy putty was packed under each blade on the SL model. Each section cured for 5 minutes before being demolded and then sequentially numbered. When all mold forms were complete, edges were machined smooth. Then, an aluminum box was assembled around the fully released SL master and epoxy putty mold forms.

Next, the moldmakers poured mixed RenCast 2000 epoxy around the pattern so that it was completely encapsulated. The mold half was placed in a pressure chamber and allowed to cure overnight until the epoxy reached a rigid state.

After the cured mold was removed from the aluminum box, registration shapes were machined into the parting surface. The mold half was then thoroughly released and set back into the box. Epoxy was mixed and poured over the SL master, the forms, and the first mold half, and then allowed to cure overnight in the pressure chamber. Time to set up the master, make the two mold halves, and build the inserts totaled about 40 hours.

To cast epoxy fins for the injection mold, all mold surfaces were thoroughly released. Then, moldmakers installed the five even-numbered epoxy putty mold forms in the top half of the mold and cast the odd-numbered sections. The cast sections cured overnight in the pressure chamber and then the even-numbered epoxy putty mold forms were removed. Next, the remaining fins were cast between the odd-numbered epoxy sections and cured overnight in the pressure chamber.

To complete the mold, all tool pieces were post-cured in an oven for 3 hours at 140F (60C), followed by 6 hours at 302F (150C).

Part Processing

To prepare for the injection of the Ultem parts, the two mold halves were installed in steel jackets. Mold inserts were then put in place and the completed tools were placed in the injection press. To warm the mold and optimize its durability, technicians started by making a series of short shots, slowly bringing the epoxy tool up to the desired 180F to 200F (82C to 93C) surface temperature.

At this point, part production began using the fastest possible cycle time to ensure a relatively constant, warm mold temperature. (See Table 1, opposite, for processing parameters.) At the outset of the project, engineers had planned to incorporate heating cartridges in the tools to control mold temperature. However, this would have added time and expense to the tool production. Ultimately, the decision was made to simply cast the tool from the aluminum-filled epoxy.

“With the filled epoxy, we get some thermal conductivity in the tool. This allows us to inject resin until it skins and then pressurize and pack the plastic until the part is complete. Using this technique, we reduce shrinkage and ensure high-quality parts,” Schanze explains.

“Working with the RenCast aluminum-filled epoxy, we’re able to quickly cast injection molds and make from 100 to 1000 complex thermoplastic parts for prototyping as well as low-volume and initial part production,” Schanze adds.