Screw Speed vs. Recovery Time

Many times I have visited molder to help them with processing issues. In reviewing their setup parameters, I find many times that the screw speed is not setup to operate at its most effective operation.

There have been times, for example, when a customer was processing Xenoy— Sabic’s blend of semi-crystalline polyester (typically polybutylene terephthalate, or PET, and polycarbonate)—and the molder was complaining that the screw was degrading the resin. The screw was turning at a fairly moderate speed and was recovering in 5 seconds, then sitting in the back position before injecting for an additional 8 seconds. Then during the injection stroke, degassing and smoke was evident at the nozzle.

After witnessing several cycles of operation in the above manner, I suggested that the screw rotation be reduced to the point where the screw got back just in time so that a count of “3” could be obtained.  Then the screw was stroked forward to fill the mold. The screw speed was practically reduced by half, and within about three shots the degraded material was flushed out of the barrel, and the smoking stopped.

This simple change in the processing parameter reduced the shear on the resin and allowed the process to be improved. Not only was the degrading eliminated, but most of all the overall melt temperature was reduced which in turn reduced the cooling time and, therefore, reduced the overall cycle time.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com