Cover Story — Injection Molding: Large as Life
For the uninitiated, a structural plastics part is considered to be a part or assembly of parts, generally longer than 12 in., that enclose, support or bear a load. Parts and components that fall into...
For the uninitiated, a structural plastics part is considered to be a part or assembly of parts, generally longer than 12 in., that enclose, support or bear a load. Parts and components that fall into this classification are manufactured for a wide variety of markets, ranging from automotive to construction, and many more. These parts are made from a variety of “low pressure” processes, including structural foam, gas-assist (full-shot, short shot, structural web), co-injection and RIM.
Design engineers must consider the effects of wall thickness, resin/fillers, ribs and external reinforcement in order to make large, structural parts with appropriate stiffness. It’s for achieving the right balance of design, cost and performance factors, as well as the use of innovative materials or processes, that recognition is given to the top parts submitted in 13 categories of the SPI Structural Plastics Conference’s annual design competition.
Molder Horizon Plastics, based in Coburg, ON, received the top prize in the Automotive category for its 2002 Ford Expedition spoiler U22. The part was made by the structural web process, which injects gas under low pressure behind the plastic gas to produce a lightweight part with a high-quality surface finish. Horizon also won the top prize in the Industrial & Military category as molder of the Fuelmaker Natural Gas Refueling System. Horizon molded the system’s half-bucket and top cowling, compressor housing as well as numerous internal parts from copolymer polypropylene, using the structural foam molding process. According to Horizon Plastics president Brian Read, the use of plastic internals provides good vibration stability and eliminates rusting.
“The housing is designed to snap fit, so the tolerances are tight,” says Read. “This is a good example of the abilities of structural foamed parts to replace metal, reduce costs and improve performance.”
Additionally, Horizon won the Recreation & Leisure category as molder of the Easy-Entry Ladder system for above-ground pools. The system incorporates an A-frame with large steps, all of which snap-fit together for easy installation. The system’s 15 main parts are molded from HDPE in two large molds using the structural foam process. Tooling for the Easy-Entry system was built by F.G.L. Precision Works in Concord, ON, and MSI Mold Builders (Cedar Rapids, IA). F.G.L. also built the tooling for both the spoiler and the natural gas refueling system.
F.G.L. won the Building & Construction category for making the tooling used to mold Hancor’s High Capacity Envirochamber. The chamber is part of a waste management housing system and has 1/8 in. wall thickness with louvered slots in the side wall to accommodate ventilation. According to F.G.L. vice-president Tom Meisels, the tool was made of aluminum with P20 steel inserts and required 8000 lb. side cores to create the slots.
F.G.L. also took the top honors in the Materials Handling category for supplying the tooling used to make the Spectralite Case. The case replaces plywood cases currently used to ship stage lighting for concerts and plays. F.G.L. built the 21-cavity mold from aluminum for Horizon Plastics, the molder of the case. The case is structural foam molded from 20% glass-filled polypropylene
“F.G.L. was particularly innovative in building a mold capable of molding all the case’s various dividers and sections in one mold,” says Brian Read.
DESIGNS THAT SHINE
One good example of a way in which cost, performance and design considerations are balanced in the development of large, structural parts is Bemis Manufacturing’s John Deere Lightening Series Garden Tractors Project, which won the Lawn & Garden category of this year’s design competition. As the molder, Bemis was involved at the ground level of helping to implement John Deere’s corporate strategy of building all its lawn and garden tractors from a single platform, much like the plan used by automotive OEMs to design and build vehicles. The project resulted in 35 molded parts (hoods, grilles, dashes and other parts) which in turn go into 20 distinct models in four series of tractors under two separate brand names. The project was formidable, given most parts have to fit either aesthetically or functionally with a number of mating parts, despite color or material differences and associated shrink/expansion differentials.
One of the keys was the development of a new material to make the hood, the single largest part and one of the most visible.
“We pretty much started with a clean sheet of paper,” says Steve Larson, Bemis’ vice-president market and business development. “We gave our materials supplier a delta E, modulus and impact, along with a price point and asked them to come up with a material.”
GE Plastics developed a first-of-its-kind ASA/SAN/PC alloy that met cost and modulus requirements, however did not meet impact demands. The alloy also would not fill the part properly. Both problems were solved by the use of a co-injected ABS, which boosted flow and added core reinforcing to improve impact strength. The part is co-injected on 2200-ton Milacron press.
The top award winner in the design competition was Steelcase Inc.’s Cachet Chair, which won the prize in the Furniture category, as well as the People’s Choice and overall Conference Awards. The Cachet Chair is designed as non-primary seating (for example, conference room or cafeteria seating), and comes in swivel and four-legged varieties. Except for a few metal fasteners, the chairs are entirely plastic. A 35% long glass-fibre filled nylon was used to mold the frame components. The parts are molded hollow using the “overspill” gas-assist molding process in which the mold is completely filled, then allowed to cool partially before the molten core is evacuated into the overspill by injected gas. The process was carefully tuned to achieve the required wall thickness and permit the glass matrix to be maintained in the center of the part. The process delivers a part that is strong and light, with a pleasing appearance.
The seat and back are gas-assist molded on a two-shot rotary platen injection molding machine, using an unfilled polypropylene for the comfort surface and a glass-fibre reinforced for the polypropylene perimeter frame. The comfort surface is shot first, the mold is opened, rotated 180 degrees, and closed for the second shot of the perimeter frame. Injected gas is used to evacuate the molten cores of both parts.