Mold forms were built for each fan rib using epoxy putty.
The SL master and epoxy putty forms for the fuel cell impeller were encapsulated in RenCast epoxy as the first step in building the injection mold.
The two mold halves are pictured here with the even-numbered mold inserts cast.
The odd-numbered mold forms were removed prior to casting.
PADT casts its epoxy tools using the RenCast 2000 system because it can withstand thermoplastic injection pressures and temperatures for prototypes and initial parts.
After the epoxy was cast, the mold sections were cured in a pressure chamber overnight.
Final Ultem blower parts are shown here with the epoxy inserts and mold halves in the center and background.
The warm epoxy mold was then removed from the injection press to demold the completed part.
A series of short shots was used to warm the mold until it was ready to produce the Ultem fan blades.
Pressure to decrease the length of product development cycles continues in todayâs 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.
âWeâve optimized the ability to quickly build epoxy injection molds that can be used with engineering thermoplastics as well as metals,â explains Mark Schanze, molding technologies manager at PADT. âWith 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.â
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.
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, âOnce 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.â
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).
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.
âWith 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,â Schanze explains.
âWorking with the RenCast aluminum-filled epoxy, weâre 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,â Schanze adds.
Metal prototypes in epoxy tools
Processing the metalâa low-melt binder mixed with 60% to 70% metal powderâis similar to molding with plastics. Schanze explains, âLike plastics injection molding, each MIM project must be carefully evaluated before beginning to analyze shape and design areas that might pose molding or demolding challenges. In addition, variables in feedstock formulations including the type of binder being used, metal particle/grain size and shape distribution, and the mix between water- and gas-atomized particles must be considered.â
Once molded, âgreenâ metal parts are sent out for debinding and sintering. Most of the binders used by PADT are debound using water; thermal and catalytic debinding are also sometimes used. Parts are then placed in a sintering furnace and heated at progressively increasing temperatures until the metal begins to melt at about 2550F (1400C). At this point, the temperature is reduced to about 77F (25C) over a 30- to 40-minute period to ensure that the parts hold their shape as molded.