Moisture and shear are the two primary causes of splay on an injection molded part. Let’s first discuss moisture, as most of the time splay occurs because the resin has not been properly dried. Proper resin is a function of time/temperature. Most drying specifications for various resins are at temperatures where the resin will not agglomerate, and then held for a given amount to time during which the moisture in the resin will be reduced to a processable level. Splay caused by moisture can typically be identified by the fact that it will move around from part to part. It will never be in the same place.
Splay due to shear can come from two sources. One can be from the shearing action caused by the screw, i.e. shear due to high screw speed, or from a mixing section that has too tight of a clearance on a mixing section. It can be determined if the splay is caused by screw shear by slowing the rpm of the screw. By reducing the screw speed, the shear rate in the screw will be reduced and therefore should eliminate the splay seen in the part.
The other type of shear splay is due to shear at the gate of the mold. This splay will typically show on the part directly in front of the gate of the part. This is cause by the fact that the gate is improperly sized; during the injection of the part, the shearing action on the resin occurs due to the high shear rate at the gate area. It can be determine if this is the cause of the shear by reducing the injection speed of the resin into the mold. If the splay disappears, then the mold needs to be reworked to have the gates modified by either fanning the gate or possible adding additional gates into the part.
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
