Avanced Epoxy Injection Molds
Rapid epoxy tooling yields production-quality polycarbonate parts with outstanding optical clarity.
To injection mold 400 optically clear polycarbonate (PC) prototypes of a new security device under a three-week deadline, Ralph S. Alberts Company (Montoursville, PA) specified an advanced epoxy casting system to build its tooling. The Cast-ITTM 2000 aluminum-filled epoxy, supplied by Vantico (East Lansing, MI) - formerly Ciba Specialty Chemicals - had the physical characteristics required for the project because the cured material can be polished to a high gloss. The product also exhibits properties that are significantly better than conventional epoxies, including a higher glass transition temperature, greater compressive strength and flexural modulus, and improved thermal conductivity.
The Alberts Company has been involved in casting epoxy injection molds for prototypes, initial parts and short-run production since 1963. Today, the company is renowned for its ability to quickly injection mold hundreds of production-quality parts from difficult-to-mold engineering thermoplastics in soft tools. Its rapid manufacturing capabilities have made the Alberts Company a preferred supplier for leading appliance, hardware and consumer electronics manufacturers throughout the country.
According to Mike Downs, injection molding supervisor at the Alberts Company, "Our customers continually come to us with projects that can't be handled with traditional methods and materials. As a result, we're always experimenting and looking for new approaches to molding parts. Cast-IT 2000 has proved to be different than any other epoxy we've used and provides us with unprecedented capabilities in the rapid injection molding of challenging part designs like the clear security device." (See Figure 1).
Project Parameters
To test the new design for a two-part electronic security device, the customer, XLVision
(Sebastian, FL), needed optically clear parts injection molded from the
same polycarbonate that would be used for production. The slightly
contoured "cover" measured four inches (10 cm) long x two inches (five
cm) wide x 0.015 inch (0.04 cm) thick and featured two "snap" fittings.
The part's rectangular housing was two inches (5 cm) long x three
inches (7.6 cm) wide x 1.5 inches (3.8 cm) deep with a 0.090-inch
(0.23-cm) thick wall.
Downs explains, "Running clear PC parts is difficult under any circumstances. Given the tight project deadlines, we needed to use rapid epoxy tooling that could be finished to a surface comparable to that of highly polished aluminum."
Moldmaking
Once the tooling material was selected, moldmaking began. All surfaces
on the stereolithography pattern were smoothed to a fine finish.
Aluminum inserts that were needed to form the snap-fit functions were
fitted and then all surfaces were sealed with several coats of Ren
Shape® Sealer 1. Next, several coats of RP 805 paste wax were applied.
After all of the components were attached to the parting surface, the
mold box was put into place to prepare for casting.
To blend the highly aluminum-filled Cast-IT 2000 epoxy system, technicians redispersed filler in the resin system using a mechanical mixer and then incorporated and thoroughly mixed in hardener, scraping pail sides frequently during blending. The mixture was vacuum de-aired and then poured continuously into the aluminum box and over pattern surfaces until the model was completely covered. After the Cast-IT 2000 system became almost tack-free, the tool back-support was poured over the insert using Vantico's Ren® RP 4037 R/H epoxy casting material. The completed mold was postcured in an oven to attain full mechanical properties.
To polish the tool to a high gloss, moldmakers began by hand-sanding the mold with 220-, 400- and 600-grit sand-paper. They continued sanding using an oil lubricant and successively finer sandpaper up to 2,000 grit, washing surfaces after each step. To finish the mold, it was coated with automotive polish, cleaned with a solvent and sealed with Sealer 1.
Molding Parts
To run polycarbonate parts, the mold was mounted in a 50-ton (45-tonne)
press. It was heated using short shots to bring the tool up to
temperature. Then, several test parts were injected at different
temperatures and pressures, with the best results achieved using a melt
temperature of 580°F (304°C) and injection pressure of 7,000 psi/48 MPa
(see Figure 2).
Cycle time was two minutes and 15 seconds, with 45 seconds for part
injection and curing and 90 seconds for mold cooling. (Compressed air
was used to maintain tool surfaces at temperatures of about 190°F
(88°C). Following these molding parameters, the Alberts Company
produced 400 production quality polycarbonate parts with outstanding
optical clarity.
