A Molding Alternative

When it comes to producing innovative structural plastic components, injection molding usually emerges as the process of choice, but it isn't the only way to go. Consider, for example, the entries in the recent New Product Design Competition held by the SPI's Structural Plastic Division. In addition to top-notch injection-molded parts, the competition also showcased the capabilities of 12 products made using low-pressure structural foam molding. That this process can produce large parts, some weighing more than 300 lbs, will come as no surprise to those who know plastics. Neither will the fact that it uses low cost tooling compared to injection molding, which can improve its value proposition on short production runs. What may be less well known is that it can increasingly make big parts that have tough-to-integrate features and good cosmetics. Here's a look at three winners:

Everkote Armored Wood

Which is the better material for outdoor structures, wood or plastic? Thanks to this new armored wood product from Buckhorn Inc., no one need answer that question again. This construction material, which won the competition's lawn and garden category, consists of a strong, creep-resistant engineered wood core encapsulated by a weather-proof, colorful, UV-resistant polypropylene skin. Like ordinary lumber, the armored version can be nailed, screwed and cut. The company's engineers turned to structural foam molding as a way to encapsulate the wood core for three reasons. For one, the molding process bonds the plastic to the wood, which prevents any shifting of the wood core inside its plastic sleeve. For another, structural foam's low injection pressures keep the wood core from shifting on its positioning pins in the tool during the molding process. Finally, the aluminum tooling used in this structural foam application helps keep costs low. Visit www.everkote.com for more information about armored wood's applications and manufacturing process.

Citadel container

By combining low-pressure structural foam molding with hot plate welding, Buckhorn Inc. has created a new container for storing bulk liquids or pastes—including foods. Measuring 48 x 45 x 46.5 inches, this Citadel container is nothing if not strong. It can hold up to 3,000 lbs, or a bit more than 307 gallons of liquid, and be stacked up to five units high for up to two years at temperatures up to 150F. All the more remarkable is that Buckhorn's engineers managed to accommodate these loads in a collapsible structure made from commodity plastics like polypropylene or HDPE—rather than the rigid wood, metal, or combination structures traditionally used. The Citadel, which won in the industrial and military category, gets its strength from a clever arrangement of internal supports. Rather than using a single-walled part with ribs, Buckhorn employs structural foam molding to make a matched pair of ribbed single-walled parts. It then hot-plate welds these single walls into a double wall structure with the ribs on the inside. This arrangement not only improves the strength but also creates a smooth interior wall—important for food handling applications. For more information, visit http://rbi.ims.ca/4391-551.

Half In-Ground Junction Box

This big junction box from Horizon Plastics Co. at first glance seems like a natural fit for structural foam molding as most engineers think of it. And in some ways it is. The polyethylene part—which measures about 50-inch square, 42 inches deep and weighs about 55 lbs—has the right combination of commodity materials, large dimensions, and heft to make structural foam an obvious choice. But this part, which won the competition's award for building and construction products, has a twist that shows the design freedom possible with structural plastics. The part has integrated feet, a feature that is not typically found on this type of product. These feet eliminate the need to put this box on a pallet for shipping. In addition, they also made it tough to fill without a careful optimization of gate location.For more information, visit Horizon Plastics at http://rbi.ims.ca/4391-552.