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. Other molding methods excel at structural parts too. Consider, for example, the entries in the recent New Product Design Competition held by the Society of the Plastics Industry's Structural Plastic Division. In addition to a collection of top-notch injection molded parts, the competition also showcased the capabilities of 12 different products made using low-pressure structural foam molding. That this process can produce large parts, some weighing over 150 lb, 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 relatively 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 (see sidebar). Here's a look at a couple of winning applications that make good use of this alternative molding 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 lb or a bit over 307 gallons of liquid and be stacked up to five units high for up to two years at temperatures up to 150 F. 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 for this kind of liquid handling containers. 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.
Which is the better material for outdoor structures, wood or plastic? Thanks to Buckhorn's new armored wood product 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 weatherproof, 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--as do the robotics used to load the wood-core insert into the machine and to take out the finished posts. Visit www.everkote.com for more information about armored wood's applications and manufacturing process.
For more on low-pressure structural foam and other winners from the competition, check out the May 16 issue of Design News. Or visit the SPI's Structural Plastics Division at www.plasticparts.org.
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