Prototype molds for large parts

By casting prototype molds in epoxy rather than machining metal molds, Troy saves up to 66 percent in lead time for those critical sample parts. Cavity and core halves are cast from a heat-resistant epoxy supplied by Ciba Specialty Chemicals, Performance Polymers, E. Lansing, MI. Mold halves are installed in either welded aluminum or steel chases designed with features including ejector pins, heated sprues, and/or multidrop manifolds for shooting large parts.

Using these techniques, Troy has produced as many as 500 parts from materials including SMA, polypropylene, ABS, and TPO. The only materials Troy avoids molding in epoxy tools are highly glass filled resins, where the glass fibers will wear away the tool surface after a few parts.

According to Art Osentoski, models and prototype manager at Troy, "We have developed the expertise to cast epoxy injection molds for very large, highly contoured thermoplastic parts to meet needs for hundreds of prototypes in a short time frame."

Among the projects Troy has completed recently with Ciba's Ren epoxy are molds to form a polypropylene third door for a pickup truck, rubber-like automotive side-window seals, and a TPO child safety seat.

Truck Door

To make the 60-by-38-inch injection mold for a truck door and produce 200 polypropylene parts in less than six weeks, Troy moldmakers had an SLA part produced from customer CAD surface files and then cast the mold from RenRP 4036R/RP 1511H epoxy filled with RP 40 aluminum grain.

Osentoski reports, "We built the SLA part in six sections and glued it together in about a week. Then we made the cast epoxy mold in five weeks, saving a considerable amount of time compared with the 16 to 18 weeks it would have taken to machine a metal mold."

To start the mold, Troy moldmakers built a wood parting line on the SLA part and then added a wooden frame into which the first mold half would be poured. All surfaces were coated with mold release and two layers of heat-resistant surface coat were applied to the model. Next, water cooling lines were installed to be cast in place by the epoxy along with 15 metal inserts that would facilitate demolding the undercut areas. The mold was poured over a two-day period, allowing the casting to cure at room temperature overnight between pours. Then, the completed mold cured on the pattern overnight followed by an unsupported oven postcure.

After separating the pattern from the cavity, Troy moldmakers inverted the pattern and parting line, built a new frame, and poured the second mold half following the same process used for the first section. Osentoski reports, "We cast cooling lines into both sides of the large mold to help us run molds at a steadily controlled temperature."

To complete the molds, each mold half was installed in a steel chase with welded corners. The jacket for the core side of the mold was designed with a full ejection plate and then holes for pins were drilled directly through the steel and epoxy to prepare the mold for use. On the cavity side of the mold, a six-drop manifold was installed in the chase to allow for injection of the large part from six different drops. "We use manifolds on large parts that require high injection pressure," Osentoski explains.

Thermocouples were added to monitor mold temperature and guide use of cooling lines. Polypropylene parts were shot at 380F and 3000 psi with a 5-minute cycle time. The epoxy used for the molds withstood the temperatures and pressures with a glass transition temperature of 350F, compressive strength of 25,800 psi, flexural strength of 9300 psi, and tensile strength of 6300 psi.

Window Seal Mold

To build an injection mold to make automotive window seals from an experimental rubber-like material, Troy produced a multicavity epoxy mold using the same process as for the truck door. For these parts, however, large loose pieces that supported the full dimension of the parts were built and bolted in place to permit easy demolding of the highly contoured seals.

"Using loose pieces on molds increases assembly and cycle times, but for prototype runs up to 300 parts, it's well worth the effort to make a fast, inexpensive mold with the pieces than to wait the many weeks to produce a metal mold with mechanical slides," Osentoski states.

To cast the mold, Troy used an SLA model and poured the epoxy over the metal inserts that were installed for increased strength and an aluminum block into which the sprue would be drilled. The window seal mold measured 60 inches long by 49 inches wide by 20 inches deep, including the metal chase. The completed mold ran at 250F using a cycle time of 4 minutes.

Child Seat

Another difficult-to-form epoxy mold recently built at Troy was for a TPO child seat. The injection mold measured 44 inches long by 20 inches wide by 32 inches high, with much of the mold depth created to accommodate the pressures involved in injecting the deep-draw part. Tools were cast on an SLA model and were designed with a single heated sprue that provided for injection of TPO at 420F from the center of the part. As with other epoxy molds, cooling lines were cast in place and thermocouples were used during the molding process to ensure a consistent mold temperature.

For the child seat project, the epoxy mold was mounted in an aluminum chase with welded corners. Aluminum was chosen rather than steel because it is faster to machine and the part did not require the high injection pressures that were needed to form the truck door.

"In total, we pulled 150 seats off the mold during two separate runs of 75 parts each," Osentoski reports. "The mold withstood the injection with little mold wear."