Views: 0 Author: Site Editor Publish Time: 2026-05-20 Origin: Site
If you work in injection molding, product design, or plastic manufacturing, you must have encountered these common problems: inconsistent product dimensions, surface sink marks, uneven warping, or unstable mass production sizes. In most cases, the root cause is plastic shrinkage.
Plastic shrinkage is an unavoidable physical change during the injection molding process. When molten plastic fills the mold cavity and cools down from a high-temperature fluid state to a solid state, its molecular structure tightens and the volume shrinks. However, shrinkage is not fixed—it fluctuates greatly according to materials, mold designs, and production processes.
Today, we will break down four core factors affecting plastic shrinkage rate, helping you quickly troubleshoot defective products and optimize molding quality.
The material itself determines the basic shrinkage range of plastic parts. All plastic materials are divided into two categories with completely different shrinkage performances:
Crystalline plastics (PP, PE, PA, POM, PET) have a high shrinkage rate, usually between 1.5%–3.0%. During cooling, the molecular chains are regularly arranged and crystallized, which greatly reduces the internal volume and causes obvious shrinkage. These materials are more likely to produce sink marks and warping.
Amorphous plastics (ABS, PC, PS, PVC) have a low and stable shrinkage rate, generally 0.4%–0.8%. The molecular structure is disordered after cooling, with small volume changes, bringing better dimensional stability.
In addition, modified materials will greatly change shrinkage: addingglass fiber, talc powder, or mineral fillers can significantly reduce the shrinkage rate and improve the uniformity of product shrinkage. Unqualified drying and raw material impurities will also lead to abnormal local shrinkage.
A good mold design can restrain abnormal shrinkage, while unreasonable design will directly lead to defective products, even with perfect materials and processes.
Gate location and size are the most critical points. The area near the gate maintains high pressure for a long time with low shrinkage; the far end of the cavity suffers from insufficient pressure supplementation, resulting in larger shrinkage. Too small a gate will freeze prematurely, cutting off the pressure supply and causing overall excessive shrinkage.
Besides, mold temperature and cooling system layout matter a lot. Uneven cooling water distribution leads to inconsistent cooling speed of different product areas. The slow cooling area has higher crystallinity and larger shrinkage, eventually causing warping and torsion of finished parts.
For mass production, process parameters are the most direct way to control shrinkage deviation. Even the same mold and material can get completely different shrinkage results with different parameters.
Holding pressure and holding time are the core parameters to adjust shrinkage. Sufficient holding pressure can continuously supplement molten material to the cavity to compensate for the volume reduction caused by cooling shrinkage. Insufficient holding will directly lead to large shrinkage, sink holes, and size deviation.
Melt temperature and mold temperature also play a key role. Higher temperature makes plastic molecules more active and increases the crystallization degree after cooling, which raises the shrinkage rate of crystalline plastics. Properly lowering the temperature can effectively reduce excessive shrinkage.
Moreover, injection speed, back pressure, and cooling time will affect the melting uniformity and molding density of plastics, further causing shrinkage fluctuation.
Product structure is the prerequisite for shrinkage performance. Wall thickness is the biggest structural factor: the thicker the wall, the longer the cooling time, the higher the crystallization degree, and the more obvious the shrinkage. Thick walls are the high-incidence area of sink marks.
Uneven wall thickness will cause differential shrinkage between thick and thin areas, resulting in internal stress and product warping. In addition, structural features such as ribs, boss columns, and grooves will block the flow of molten plastic, leading to inconsistent local shrinkage.
For glass fiber reinforced plastics, fiber orientation will cause anisotropic shrinkage: the shrinkage along the flow direction is small, while the shrinkage perpendicular to the flow direction is large, which is the main reason for part torsion.
Plastic shrinkage is a comprehensive result of materials, molds, processes, and structures. To solve shrinkage problems in actual production, we can follow this priority:
1. Optimize injection process parameters (fastest and most effective adjustment) 2. Check mold gate and cooling system to improve pressure and cooling uniformity 3. Appropriately adjust product wall thickness and structural transition 4. Replace modified materials with lower shrinkage if necessary
Understanding shrinkage factors helps us control product dimensional accuracy, reduce defective rates, and improve production stability—which is essential for high-quality plastic part manufacturing.
