Part Shrinkage In Injection Molding

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

The part shrink material data sheet provides the shrinkage of the material that will be used for the part. However, this rate is only a starting point. The thickness of the specimen at a given rate was 0.125 inches. If parts to be made through the injection molding process are thicker or thinner than .125 inches, you must change the shrinkage used during the mold design phase. Thicker parts will shrink more than thinner parts. When these situations occur, injection molding experience comes in handy. You can compare previously molded parts and see shrinkage.

After the injection mold is completed, sampling is performed. After the part is visually filled, a dimensional check is performed to determine how the part shrinks. Processing adjustments can be made to change the shrinkage. Making adjustments is a tricky process. By making adjustments, care should be taken to ensure that the parts are still full.

You can change the excessive shrinkage of the part by ensuring that the part is filled and the gate is sealed during the injection process. Both the filling or holding phase of the cycle and the holding pressure can be increased. The cooling phase of the cycle can be increased to reduce part shrinkage. Mold temperature also plays a large role in mold shrinkage. Colder molds will cause less shrinkage, but may also cause unfilled parts due to material freezing. Faster injection speeds also reduce part shrinkage.