No. 36 - Cavity-Pressure Sensing

The collection of real-time data from inside the mold first occurred through cavity-pressure sensing and marks the origins of closed-loop injection machine control. Until this point, the injection process was monitored via machine signals such as timer-based injection or screw position after injection.

Open-loop control methods could not quickly compensate for changes in the process, making product quality susceptible to variations in materials, moisture content, machine hydraulic-oil temperature, or barrel temperature. These undocumented process changes affected mold pressure, fill time, material temperature, and ultimately the characteristics of the part.

The earliest thinking on cavity-pressure control was presented as a technical paper at the SPE ANTEC in 1957 by Dow Chemical’s Carl Beyer. He put strain gauges on the ejector pins, resulting in a crude monitoring and control device. That paper inspired a university student, Rodney J. Groleau, to look at the variables of the molding process from the machine to the mold. His 1967 thesis paper was a statistical study that examined the correlation between part and mold dimensions and cavity pressure. He monitored cavity pressure with postage-stamp-size strain gauges or slide transducers. Their output data payload was delivered to a strip-chart recorder. That data was manually entered onto computer punch cards and then run through a computer. The statistical analysis took four months.

The breakthrough occurred with the enhancement of computing power, which brought with it the ability to condition and display the pressure signal and to provide an immediately usable output. In 1967, Groleau co-founded Control Process Inc. with his thesis advisor, Donald Paulson, and developed the first commercial strain-gauge cavity-pressure sensor, which made possible the industry’s first process-control system based on monitoring plastic in the mold. To prove out the technology, Groleau worked with injection machine manufacturers such as New Britain, which made the technology an option on its presses. The analog signal from the cavity sensor was used to adjust injection so that the melt experienced the same pressure on each cycle. The pressure profile in the cavity for the first time became the control variable for optimizing part quality. Other benefits of the cavity-pressure system included the ability to correct for melt-flow imbalances in the runner system. It also protected the mold from excessive pressure. The later ability to store injection-pressure profiles led to faster mold setups.

One of the first applications of the new process-control technology was molding optical lenses for Polaroid cameras in 1968-69. Cycle times of the thick parts averaged 20 minutes, yet there was no possibility to determine if the process had been carried out correctly to generate a usable part. Placing pressure sensors near the gate and at the end of cavity fill provided a view into the mold and helped to reduce scrap parts significantly.

In 1985, Groleau founded RJG Inc. to promote the concept of peak cavity pressure as an accurate indicator of part weight and size. Controlling peak cavity pressure thereby is the means to effective quality control. Groleau began training molders in the concept of “decoupled molding,” whereby mold filling, packing, and holding are separated into distinct process steps, each requiring its own level of control.