Missing link found in PIM press control

Milko Guergov of M&C Advanced Processes Inc. (Ann Arbor, MI) says his gas counter-pressure (GCP) closed loop process control system can prevent powder/binder separation, and all of the problems it can cause. It can prevent jetting and knitline formation. It can improve part density. Most importantly, it can prevent part failures.

GCP has proven effective in the high-volume molding of extremely complex medical MIM parts to 4 Sigma limits in several dimensions. These parts have tolerances of ±.0005 inch and accuracies to within ±.1 percent. Just ask Thermat Precision Technology Inc. (Corry, PA). M&C has also proven the technology’s effectiveness in CIM, to which Carpenter EPG Certech (Wood-Ridge, NJ) can attest.

Guergov applied advanced mathematical analytical principles to the molding process and discovered a critically important processing variable that had been overlooked. He calls this missing link internal melt pressure (IMP). Guergov discovered that in addition to all of the commonly recognized shrinkage forces encountered by the melt in a mold cavity during fill and solidification, one had escaped detection—air resistance (Figure 1).

The IMP of a shot must be counterbalanced to compensate for air resistance, as well as all of the other commonly known factors influencing stress, sink, filler and fiber orientation, warp, and shrinkage. The IMP of a shot also must be counterbalanced to prevent powder/binder separation.

His solution is to prepressurize the molding system from the nozzle to the mold cavity (Figure 2). Shop air prepressurization at only 200 psi works well, but an inert gas like nitrogen also can be used. Mold cavities are sealed in rubber. In real time, pressure transducers in the injection nozzle and at the last place to fill in the cavities accurately measure all gas and all melt pressure displacement transitions during fill, pack, and hold (Figure 3).

These measurement signals are sent back to the machine controller that automatically adjusts the process so that the IMP of the feedstock is maintained at a static level—the minimum static pressure within the feedstock to counterbalance shrinkage forces and air resistance. GCP closes the loop between the feedstock, the mold, and the molding machine, and it does so during the shot.

Guergov began his investigations into the applicability of GCP more than four years ago when he was introduced to MIM by Engel, a molding machinery suppler. Engel is one of a growing number of suppliers offering GCP as a fully integrated control option. Others include Milacron, Van Dorn Demag, and HPM. M&C licenses GCP methods and technology ($10,000 to $20,000 per system). It also designs, engineers, assembles, and starts up GCP mold and machine retrofit systems.