Relationship Between Plastic Shrinkage Rate and Mold Design

Shrinkage rate is one of the most fundamental and critical parameters in plastic mold design. It refers to the percentage change in dimensions when a plastic part cools from a molten state to room temperature. Inaccurate shrinkage compensation directly leads to dimensional deviation, warpage, poor assembly, and even rejection of molded parts. Mold design must be based on accurate shrinkage data, combined with material characteristics, product structure, and processing parameters to achieve precise dimension control. This article systematically explains the definition, classification, influencing factors of shrinkage rate, and its guiding role in mold design, providing reliable standards for mold design and debugging.

Basic Parameters and Classification of Shrinkage Rate

The shrinkage rate is calculated as:Shrinkage rate S = (Mold cavity dimension − Actual part dimension) / Mold cavity dimension × 100%

Mold dimension compensation formulas:Cavity dimension = Designed part dimension × (1 + S)Core dimension = Designed inner dimension × (1 − S)

Amorphous Plastics

Amorphous plastics have low and stable shrinkage, generally ranging from 0.4% to 0.8%. Typical values include PC at 0.5%–0.7%, ABS at 0.4%–0.6%, PMMA at 0.5%–0.8%, and PPO at 0.5%–0.7%. These materials are less affected by cooling conditions and are easier to control in precision molding.

Crystalline Plastics

Crystalline plastics have high shrinkage due to molecular ordering during cooling, ranging from 1.0% to 3.5%. Typical values include PP at 1.5%–2.5%, HDPE at 1.8%–2.5%, PA6 at 1.5%–2.5%, PA66 at 1.2%–2.0%, and POM at 1.8%–2.2%. Shrinkage of crystalline plastics is more sensitive to mold temperature, cooling rate, and holding pressure.

Reinforced and Modified Plastics

Glass fiber or mineral filling reduces shrinkage by 50%–70%. Glass‑filled materials show shrinkage rates of 0.3%–0.8%, while mineral‑filled materials range from 0.6%–1.2%. Flame‑retardant modified grades usually have slightly higher shrinkage than standard grades.

Anisotropic Shrinkage

Glass‑filled plastics show obvious anisotropic shrinkage. Shrinkage in the flow direction is smaller than that in the vertical direction, with a difference of 0.3%–0.5%. This characteristic often causes warpage and distortion in long, flat, or large‑area parts.

Impact of Shrinkage on Mold Dimension Design

Cavity and Core Dimension Compensation

High‑precision parts require shrinkage values provided by material suppliers, with an additional 0.05%–0.1% reserved for mold modification. For general parts, the median value of standard shrinkage ranges can be used. For complex structures, different shrinkage values must be applied according to wall thickness and flow direction.

Effect of Wall Thickness on Shrinkage

Shrinkage increases with wall thickness. Parts with wall thickness of 1–2 mm use lower shrinkage values; those of 2–4 mm use medium values; and those above 4 mm use higher values. Thick sections require higher shrinkage compensation and improved cooling systems to avoid sink marks and dimensional instability.

Treatment of Anisotropic Shrinkage

For long, flat parts or glass‑filled materials, mold design must adopt different shrinkage values in flow and vertical directions. Symmetric gating systems help reduce uneven shrinkage and warpage. Reasonable gate position and quantity can effectively improve flow uniformity and reduce anisotropic effects.

Impact of Mold Structure on Shrinkage Control

Gating System Design

Gate size and position directly affect packing effect and shrinkage uniformity. Large gates improve packing and reduce shrinkage. Gates located at thick sections ensure effective compensation. Multi‑point gates help balance flow and reduce differential shrinkage.

Cooling System Design

Mold temperature affects crystallization rate and shrinkage. For crystalline plastics, every 10°C increase in mold temperature raises shrinkage by 0.1%–0.3%. Cooling channels must be evenly distributed to maintain consistent cooling across the part. Reasonable cooling design stabilizes shrinkage and improves dimensional repeatability.

Ventilation and Ejection Systems

Poor ventilation causes uneven pressure and abnormal shrinkage, leading to short shots, burns, and internal stress. Ejection components must be arranged to avoid stress concentration and deformation after shrinkage. Proper ejection design ensures stable dimensions after release.

Common Defects and Solutions

Undersized parts are usually caused by insufficient shrinkage compensation and require increased cavity scaling. Warpage is mainly caused by anisotropic shrinkage and can be improved by optimizing gating and cooling systems. Sink marks result from uneven shrinkage in thick areas and require enhanced packing and cooling. Unstable shrinkage is often caused by uneven cooling or insufficient holding pressure and can be solved by adjusting process parameters.

Practical Guidance for Mold Design

Mold design must be based on accurate shrinkage data. Designers should select appropriate shrinkage values according to material type, product structure, wall thickness, and precision requirements. During trial molding, actual shrinkage should be measured and used to modify the mold. Stable shrinkage control relies on coordinated design of mold structure, gating, cooling, and ejection systems. Only through systematic design and debugging can high‑precision and high‑stability plastic parts be produced consistently.