Feature Articles - Automated Mould Industries

Mold Evacuation Solves Short Shots

Automated Mould Industries (AMI), a custom molder in Richmond, Ill., used a mold purge/evacuation system to achieve zero defects when molding a critical part, an oil slinger for an automotive fuel pump. The part had not been consistently filling out in a 16-cavity mold, and despite 100% quality inspection, a small percentage (0.26%) of defective parts was slipping through undetected. AMI ultimately concluded that the problem could be corrected only by fixing the molding process, not by better inspection.

The thin-walled, glass-filled polyethylene part is one of about 50 components of a fuel pump manufactured by BG Automotive Motors (BGAM), a joint venture of Robert Bosch GmbH of Germany and General Electric Co. It was injection molded in a 16-sec cycle and had to be produced for only a penny a part. After being molded, the washer-like part was incorporated into BGAM's automated assembly line. Production records showed that just one off-spec part would shut the assembly line down for half an hour, at a cost of $1200.

AMI was informed about its short-shot problem by BGAM, which declared that it would not tolerate even one shutdown of the assembly line per month. The defective parts had to be eliminated. AMI's visual inspection system had achieved 99.74% defect-free rate, but at an inspection rate of one part per second, nearly 80 defective parts in 30,000 were slipping past the inspection station.

In an effort to address the problem, AMI implemented a more rigorous visual inspection system, but this did not significantly improve quality within an 8-hr shift because of what AMI quality assurance manager Sam Makwana describes as the "fatigue factor." The molder considered an automatic high-speed sorting machine, but the $50,000-plus price tag of such an optical inspection system would adversely affect part cost.

Fix the process

AMI's management concluded that the molding process had to be made 100% reliable in order to eliminate human error and the need for inspections altogether. The company systematically checked all of the usual tooling and process control parameters for a solution, but to no avail. It even considered buying sophisticated machine controls that would automate and computerize the process. But the problem appeared to be inherent in the thin-walled part and the high-speed, multi-cavity molding process.

Interestingly, the solution to the problem was already on the company's molding room floor. Months earlier, AMI had purchased a Mold-Vac 4000 system from CAE Services Corp., Bloomingdale, Ill. This unit had eliminated a problem for another of AMI customers by using a blowback feature to clean and purge vented ejector and core pins between molding cycles. However, the principal use of the Mold-Vac system is to prevent short shots, voids, and burns by eliminating trapped air and gas within the mold cavities. The vacuum cycle actively assists in filling the cavity by reducing cavity pressure and improving the heat-transfer capability of the mold, especially for thin-walled parts like the BGAM oil slinger.

In order to accommodate the Mold-Vac system, AMI needed only to modify the mold venting, which required about 5 hr of tool-room time. The Mold-Vac device is self-contained, operates with shop compressed air rather than needing its own pump and motor, and has few moving parts. Equally important, the system's evacuation time is substantially less than the mold fill time, so fast cycles were maintained.

According to AMI, the evacuation system provided a cost-efficient way to achieve zero defects for the oil slinger.