One of the most common problems in injection molding is determining the shrinkage rate of a part that is going to be injection molded.

Material data sheets provide a shrinkage rate for the material that is going to be used for a part.  However, this rate is only a starting point.  The test specimen for the given rate is .125″ thick. If the part that is going to be made through the injection molding process is thicker or thinner than .125″, then the shrinkage rate used in the mold design stage must be changed. A part that is thicker will have more shrink then a thinner part. Experience in injection molding comes in handy when these conditions arise.  You can compare previous parts that have been injection molded and see what the shrink rate is.

Once an injection mold is complete, then first sampling takes place.  Once the part is visually filled, a dimensional check will be made to determine how the part is shrinking. There are processing adjustments that can be made to alter the shrink rate. Making adjustments is a touchy process. By making adjustments, care should be taken to make sure the part is still filled and the cosmetics are still acceptable.

A part that is shrinking too much can be changed by making sure the part is being filled and the gate is sealed during the first stage injection. The pack or hold phase of the cycle can be increased and also the hold pressure. The cooling phase of the cycle can be increased to reduce the part shrinkage. The mold temperature plays a big part in mold shrinkage as well.  A colder mold will produce less shrinkage but also might cause unfilled parts due to the material freezing off. Increasing the injection speed also decreases the part shrinkage but might also increase the risk of gas burns. Increasing the melt temperature will also reduce the shrink rate even though you may think a higher melt temperature would increase the shrink rate.

These are just a few parameters that affect part shrinkage in injection molding. A combination of these mentioned can be used.  The last resort is to alter the mold dimensions. Again, having experience in injection molding and injection mold building is the most important virtue to have to solve the problems of shrinkage in parts. There are also little tricks that have been developed at Toth Mold/Die Inc. over the years to remedy injection molding part shrinkage problems without having to change cavity and core dimensions.

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