Plastic shrinkage rate refers to the percentage reduction in size of a plastic product relative to the mold cavity size after it has cooled to room temperature after being removed from the mold. It is a crucial parameter in plastic molding, affecting dimensional accuracy, appearance quality, and many other aspects of the plastic product.
Influencing Factors
**Properties of the Plastic Material Itself:** Different plastics have different molecular structures and crystallinity, leading to variations in shrinkage rates. Generally, crystalline plastics have a higher shrinkage rate than amorphous plastics. For example, polyethylene (PE) is a crystalline plastic with a relatively high shrinkage rate, while polystyrene (PS) is an amorphous plastic with a lower shrinkage rate.
**Molding Process Conditions:**
**Temperature:** Higher mold and melt temperatures slow down the cooling rate of the plastic melt, giving the molecular chains more time to align. For crystalline plastics, this increases crystallinity, resulting in a higher shrinkage rate. For example, when injection molding polyoxymethylene (POM), increasing the mold temperature from 80°C to 100°C significantly increases the shrinkage rate.
**Pressure:** Injection pressure and holding pressure also affect the shrinkage rate. Sufficient holding pressure allows molten plastic to continuously replenish the mold cavity during cooling, reducing volume shrinkage. Insufficient holding pressure results in insufficient molten plastic replenishment during cooling, leading to increased shrinkage.
Product Shape and Size
Uneven wall thickness in plastic products causes different cooling rates in different areas. Thicker walls cool slower and have relatively larger shrinkage rates, while thinner walls cool faster and have smaller shrinkage rates. For example, in an injection-molded part with significant wall thickness variations, noticeable shrinkage depressions may appear in the thicker sections.
Larger product dimensions result in greater total shrinkage. This is because larger dimensions mean a larger total volume of molten plastic, leading to a greater absolute value of volume change during cooling.
General Plastics
Polyethylene (PE):
High-density polyethylene (HDPE): Shrinkage rate 2%–5.0%.
Low-density polyethylene (LDPE): Shrinkage rate 1.5%–5.0%.
Polypropylene (PP): Shrinkage rate 1%–2.5%, with reinforced PP+30% GF having a shrinkage rate of 0.4%–0.8%. Polyvinyl chloride (PVC): Rigid PVC has a shrinkage rate of 0.6%–1.0%, while flexible PVC has a shrinkage rate of 1.5%–2.5%.
Polystyrene (PS): Shrinkage rate is 0.4%–0.7%, while high-impact polystyrene (HIPS) has a shrinkage rate of 0.2%–0.6%.
Acrylonitrile-butadiene-styrene copolymer (ABS): Shrinkage rate is 0.3%–0.8%, while reinforced ABS with 30% GF has a shrinkage rate of 0.2%–0.6%.
Engineering Plastics
Polyamide (PA):
Polyamide 6 (PA6): Shrinkage rate is 0.6%–1.4%, while reinforced PA6 with 30% GF has a shrinkage rate of 0.3%–0.7%.
Polyamide 66 (PA66): Shrinkage rate 0.8%–1.5%, with reinforced PA66 + 30% GF shrinkage rate at 0.2%–0.8%.
Polycarbonate (PC): Shrinkage rate 0.5%–0.7%, with reinforced PC + 30% GF shrinkage rate at 0.2%.
Polyoxymethylene (POM): Copolymer POM shrinkage rate 1.5%–3.5%, homopolymer POM shrinkage rate 1.5%–3%.
Polybutylene terephthalate (PBT): Shrinkage rate 0.44%, with reinforced PBT + 30% GF shrinkage rate at 0.2%.
Polyethylene terephthalate (PET): Shrinkage rate 1.8%, with reinforced PET + 30% GF shrinkage rate at 0.2%–0.9%.
Specialty Plastics
Polyphenylene oxide (PPO): Shrinkage rate 0.7%. Polyphenylene sulfide (PPS): Shrinkage of reinforced PPS + 40% GF is less than 0.12%.
Polysulfone (PSF): Shrinkage rate is 0.6%.
Polyethersulfone (PES): Shrinkage rate is 0.6%.
Liquid crystal polymer (LCP): Shrinkage rate is 0.006%.
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