11. May 2012 09:47
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 is due to the fact that the resin has not been dried properly.
Resin drying is a time/temperature element. Most drying specifications for various resins are at temperatures where the resin will not agglomerate, and held there for a given amount to time for which the moisture in the resin will be reduced to a processable level. Splay caused by moisture can typically be easily spotted because 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 source can be from the shearing action caused by the screw. This is due to high screw speed, or from a mixing section that has too tight of clearance on a mixing section. It can be determined if the splay is due to 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.