The machine injecting high-density polyethylene into a two-cavity mold to form thin-walled parts consisted of a hopper holding polymer pellets, a screw driving pellets and melt through a heated barrel toward the injector nozzle, the injector, and a horizontal press in which the mold was installed.
Soon after purchasing the machine specifically to make these parts, the injection-molding company found the unit’s injection capacity was too low to make two parts at once. Switching to a larger screw and barrel would not have generated enough pressure to force polymer melt into the thinnest areas of the parts.
Instead, the machine’s manufacturer tweaked the operating software to allow the existing screw to build up melt for the next part in the injector while one part was in the press. The fix, which was set up so the machine injected melt for a specified period of time, effectively boosted injector capacity.
At the time of the accident, the machine was cycling but wasn’t producing parts. The worker opened the operator’s gate to see what was wrong and found a runner stuck in the mold that was blocking the flow of plastic.
Representatives from the machine’s manufacturer came out to review the incident. They found no problem with the machine or the software.
Warning signs did alert operators to areas with high temperatures, potential pressure buildups, and pinch points. The operator’s manual also warned that hot, pressurized melt could release when the operator unblocked a runner and recommended retracting the injector head before attempting to remove parts from the mold.
Investigators found the machine had not stopped adding melt to the injector or cycling through its production steps because no process alarms were set. Such alarms could have alerted operators to the presence of excessive pressure or melt volume in the injector.
After the incident, the company set up a cushion alarm. If the volume of melt in the injector exceeds a preset level, an alarm lets operators know a problem exists that may involve trapped pressure.
This month’s safety violation comes from the files of Lanny Berke, a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a safety violation to share? Send your images and explanations to firstname.lastname@example.org.
Edited by Jessica Shapiro