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How to Increase/Decrease Injection Molding Shrinkage?

Injection molding processing can affect shrinkage, but not as much as fillers.
Increase shrinkage: shorter cooling time in injection mold. Higher mold temperature. Greater product thickness. Higher resin melt temperature during injection. More plasticizer in resin. Less injection speed. Lesser packing / holding pressure and volume.

Decrease shrinkage: longer cooling time in the injection mold. Lower mold temperature. Thinner product walls. Lower resin melt temperature during injection. Greater injection speed. Greater packing / holding pressure and volume. Fillers such at talc, glass beads, or glass fibers (more anisotropic shrinkage with fibers).

Injection rates, packing, and mold temperatures can significantly affect molded-in stress or allow voids and sink marks. Molded-in stress can affect warp, solvent sensitivity, dimensional stability, and impact resistance; so, these tertiary effects need to be considered.

To change the shrinkage of the material,
  • an easy way is to leave it to your vendor to change formulation;
  • another way is to change the mold design to get the dimension you want;
  • the last way is to optimize the processing parameter such as molding temperature, melt temperature, and injection speed/pressure/time, cooling time.
Polypropylene is a semi crystallization polymer, higher molding temperature might result in higher shrinkage, but it might lead large crystals thus worse properties. Higher melt temperature might result in higher shrinkage, but it will consume higher energy and may result in flashing.

Injection speed/pressure/time is the most complex part in the injection molding because these parameters are inter played each other greatly. The key is to design the processing parameter according to the product structure to control the melt filling in the injection mold. Normally, less weight will result in higher shrinkage and higher weight will get low shrinkage. Also, the cooling design is also very important.

As the literature said, injection mold shrinkage of polypropylene is a very complex problem influenced by the inherent structure of the material and also by the heat transfer dynamics that occur during the injection molding process. Consideration of these factors, in conjunction with flow and cooling analysis, are important means of minimizing shrinkage and ensuring it is as uniform as possible throughout the part.

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