Influencing Factors

Properties of the Plastic Material Itself

Different plastic materials have different molecular structures and degrees of crystallinity, resulting in differences in shrinkage rates. Generally speaking, the shrinkage rate of crystalline plastics is larger than that of non-crystalline plastics. For example, polyethylene (PE) is a crystalline plastic, and its shrinkage rate is relatively large; while polystyrene (PS) is a non-crystalline plastic, and its shrinkage rate is relatively small.

Molding Process Conditions

• Temperature: Higher mold temperature and melt temperature will slow down the cooling rate of the plastic melt, allowing the molecular chains more time to arrange neatly. For crystalline plastics, this will increase the degree of crystallinity, resulting in an increase in the shrinkage rate. For instance, when injecting polyoxymethylene (POM), if the mold temperature is increased from 80°C to 100°C, the shrinkage rate will increase significantly.

• Pressure: Injection pressure and packing pressure also have an impact on the shrinkage rate. Sufficient packing pressure can enable the plastic melt to continuously replenish into the cavity during the cooling process, reducing the volume shrinkage. If the packing pressure is insufficient, the plastic product will not receive enough melt replenishment during the cooling process, and the shrinkage rate will increase.

Shape and Size of the Product

General Plastics

• Polyethylene (PE):

• High-density polyethylene (HDPE): The shrinkage rate is 2% - 5.0%.

• Low-density polyethylene (LDPE): The shrinkage rate is 1.5% - 5.0%.

• Polypropylene (PP): The shrinkage rate is 1% - 2.5%, and the shrinkage rate of PP with 30% glass fiber reinforcement (PP+30% GF) is 0.4% - 0.8%.

• Polyvinyl chloride (PVC): The shrinkage rate of rigid PVC is 0.6% - 1.0%, and the shrinkage rate of flexible PVC is 1.5% - 2.5%.

• Polystyrene (PS): The shrinkage rate is 0.4% - 0.7%, and the shrinkage rate of high impact polystyrene (HIPS) is 0.2% - 0.6%.

• Acrylonitrile-butadiene-styrene copolymer (ABS): The shrinkage rate is 0.3% - 0.8%, and the shrinkage rate of ABS with 30% glass fiber reinforcement (ABS+30% GF) is 0.2% - 0.6%.

Engineering Plastics

• Polyamide (PA):

• Polyamide 6 (PA6): The shrinkage rate is 0.6% - 1.4%, and the shrinkage rate of PA6 with 30% glass fiber reinforcement (PA6+30% GF) is 0.3% - 0.7%.

• Polyamide 66 (PA66): The shrinkage rate is 0.8% - 1.5%, and the shrinkage rate of PA66 with 30% glass fiber reinforcement (PA66+30% GF) is 0.2% - 0.8%.

• Polycarbonate (PC): The shrinkage rate is 0.5% - 0.7%, and the shrinkage rate of PC with 30% glass fiber reinforcement (PC+30% GF) is 0.2%.

• Polyoxymethylene (POM): The shrinkage rate of copolymer POM is 1.5% - 3.5%, and the shrinkage rate of homopolymer POM is 1.5% - 3%.

• Polybutylene terephthalate (PBT): The shrinkage rate is 0.44%, and the shrinkage rate of PBT with 30% glass fiber reinforcement (PBT+30% GF) is 0.2%.

• Polyethylene terephthalate (PET): The shrinkage rate is 1.8%, and the shrinkage rate of PET with 30% glass fiber reinforcement (PET+30% GF) is 0.2% - 0.9%.

Special Plastics

• Polyphenylene oxide (PPO): The shrinkage rate is 0.7%.

• Polyphenylene sulfide (PPS): The shrinkage rate of PPS with 40% glass fiber reinforcement (PPS+40% GF) is less than 0.12%.

• Polysulfone (PSF): The shrinkage rate is 0.6%.

• Polyethersulfone (PES): The shrinkage rate is 0.6%.

• Liquid crystal polymer (LCP): The shrinkage rate is 0.006%.