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Comparison of shrinkage rate of common plastics

2025-01-10 13:13:54 Shrinkage rate of plastic
The shrinkage rate of plastics refers to the percentage by which the size of a plastic product shrinks relative to the size of the mold cavity after it is taken out from the mold and cooled to room temperature. It is a very important parameter in plastic molding processing and will affect many aspects such as the dimensional accuracy and appearance quality of plastic products.

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

Uneven wall thickness of plastic products will lead to different cooling rates in different parts. The thick-walled part cools slowly and has a relatively large shrinkage rate; the thin-walled part cools quickly and has a small shrinkage rate. For example, in an injection-molded part with a large difference in wall thickness, obvious shrinkage dents may occur in the thick-walled area.

The larger the size of the product, the greater the total shrinkage amount. Because a larger size means a larger total volume of the plastic melt, and the absolute value of the volume change during the cooling process is also larger.

NameShrinkage RateNameShrinkage RateMaterial TypeMaterial Type
High-density polyethylene (HDPE)2% - 5.0%Polypropylene (PP)1% - 2.5%General PlasticsGeneral Plastics
Low-density polyethylene (LDPE)1.5% - 5.0%PP with 30% glass fiber reinforcement (PP+30% GF)0.4% - 0.8%General PlasticsGeneral Plastics
Rigid Polyvinyl chloride (PVC)0.6% - 1.0%Flexible Polyvinyl chloride (PVC)1.5% - 2.5%General PlasticsGeneral Plastics
Polystyrene (PS)0.4% - 0.7%High impact polystyrene (HIPS)0.2% - 0.6%General PlasticsGeneral Plastics
Acrylonitrile-butadiene-styrene copolymer (ABS)0.3% - 0.8%ABS with 30% glass fiber reinforcement (ABS+30% GF)0.2% - 0.6%General PlasticsGeneral Plastics
Polyamide 6 (PA6)0.6% - 1.4%PA6 with 30% glass fiber reinforcement (PA6+30% GF)0.3% - 0.7%Engineering PlasticsEngineering Plastics
Polyamide 66 (PA66)0.8% - 1.5%PA66 with 30% glass fiber reinforcement (PA66+30% GF)0.2% - 0.8%Engineering PlasticsEngineering Plastics
Polycarbonate (PC)0.5% - 0.7%PC with 30% glass fiber reinforcement (PC+30% GF)0.2%Engineering PlasticsEngineering Plastics
Copolymer Polyoxymethylene (POM)1.5% - 3.5%Homopolymer Polyoxymethylene (POM)1.5% - 3%Engineering PlasticsEngineering Plastics
Polybutylene terephthalate (PBT)0.44%PBT with 30% glass fiber reinforcement (PBT+30% GF)0.2%Engineering PlasticsEngineering Plastics
Polyethylene terephthalate (PET)1.8%PET with 30% glass fiber reinforcement (PET+30% GF)0.2% - 0.9%Engineering PlasticsEngineering Plastics
Polyphenylene oxide (PPO)0.7%Polyphenylene sulfide with 40% glass fiber reinforcement (PPS+40% GF)Less than 0.12%Special PlasticsSpecial Plastics
Polysulfone (PSF)0.6%Polyethersulfone (PES)0.6%Special PlasticsSpecial Plastics
Liquid crystal polymer (LCP)0.006%--Special PlasticsSpecial Plastics

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%.

injection mould

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