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Position sensors pace injection molding
By Rick DeMeis -- Design News, June 22, 1998
Cologno Monzese, Italy--Negri & Bossi, a leading European injection-molding machine maker, was looking for a way to measure the force holding the dies in its machines together--while at the same time lowering the number of costly mechanical and electrical components. Previously, accessory clamps and strain-gauge load cells directly measured these forces. Too much clamping force can damage a mold; too little results in plastic "flash" bleeding into the mold, resulting in a crudely finished part.
Maurilio Meschia, the company's technical manager, was familiar with the high-accuracy positioning possible with Temposonics magnetostrictive sensors from MTS Systems Corp. (Cary, NC). These could indirectly measure force by directly measuring the strain on the load frame. And MTS Temposonics III sensors are microprocessor based with special embedded software and distributed-microprocessor Control Area Network (CANbus) interface support. All these factors came together to convince Meschia to try the devices. "By installing the multiple magnet sensors, we were able to use fewer sensors and eliminate the use of special strain gauges while still achieving equal or better data," he says. "We tested various sensor products and found only the Temposonics III could give us the performance and flexibility we needed."
The Temposonics III electronics communicate via CANbus to quickly send accurate and repeatable measurements. With 2 microns standard resolution, the devices can measure small changes in the molding machines' platen and tie bar positions. Both open and DeviceNet(TM) version of CANbus can support the sensors.
On the molding machines, the magnetostrictive sensors monitor the positions of four axes: the plastic injector; the injector carriage; the finished-product ejector; and the mold clamp halves. The injector screw and injector carriage are measured by a single sensor, cutting the number of sensors needed from four to three. The clamp-monitoring sensor uses two magnets, one of which determines the most extended position during each mold cycle. This value is compared to the commanded position, and the difference used as feedback to determine stretch in the load frame columns. The controller recalculates the strain, and thus force, to adjust the mold closure pressure on successive cycles. The results were so encouraging that Meschia says, "We are switching all of our series to the new Temposonics III system."
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