Yes! It CAN Be Injection Molded
While not for the faint of heart, a large part with both structural and highly cosmetic requirements can be injection molded. And Mack’s Southern Operations recently proved it!
The part is the base of an external refrigeration unit for semi-trailer trucks that sits between the truck cab and front of the trailer. It’s big – 76 in. long x 22.4 in. wide x 0.180 in. thick — and weighs 15.3 lbs.
Previously produced by twin-sheet thermoforming of ABS/PC with high gloss on one of the two sheets, the part suffered a high failure rate in the field. “Impact failures and cracking from abuse during maintenance were the primary culprits,” says Ken Kincaid, technical engineering manager of Mack’s Southern Operations, Inman, S.C.
The customer, a major producer of temperature and climate controls for the transportation industry, came to Mack with a challenge. Eliminate the cracking. And produce a Class A part right out of the mold that would be comparable to a painted panel.
Given the size of the part, which also had ribs and bosses, this was a tall order for conventional injection molding. Sink marks and cycle time would be big challenges.
Long-time veterans of the gas-assist process, Kincaid and his crew combined internal and external gas-assist in the same cycle to mold the large structural part and produce a high-gloss, Class A surface.
Here’s the processing sequence. Twenty-five seconds after injecting a full shot of Geloy ASA resin into the tool, Mack injects internal gas to core out the part’s 2.5-in-thick rim and bosses. This displaces material, creating a gas channel of 1.6 in. diameter around the rim, and accelerates cooling. The rim then seals off the periphery of the part, preventing gas seepage during the external gas phase. External gas is applied on the core side, forming a pillow of nitrogen that packs out the part and results in a 90 gloss rating on the cavity or show side, right out of the mold.
Dual process yields dividends
Using two forms of gas-assist technology to produce this large truck panel resulted in large savings in tool costs, press size and material, according to Kincaid.
“With conventional injection molding, we would have needed to design a multi-drop hot-runner system and sequential valve gating to manage the flow lengths because of the size of the part,” he explains. “With gas-assist, we only need one drop, because the gas helps to push the resin and pack out the tool. We also avoided tooling movements – a lifter and slide system – cutting at least $30,000 from the tool cost.”
With gas-assist, Mack is also running the part on a 3000-ton press, rather than the 5000 tons necessary for conventional injection molding. And because of the material displacement inherent with gas-assist molding, Mack not only uses less resin, but also a less expensive resin (Geloy ASA) to maintain both structural and cosmetic requirements.
Contact: Ken Kincaid, Mack Technical Engineering Manager