How to Design Injection Mold Based on Plastics Shrinkage Rate?

In the injection molding process, plastic undergoes dimensional shrinkage as it cools and solidifies from a molten state. This phenomenon is known as plastics shrinkage rate. If the plastics shrinkage rate is not considered during mold design, the final product dimensions may deviate from the intended specifications, leading to assembly issues or functional defects. Therefore, accurately calculating the shrinkage rate and optimizing mold design are crucial.

Factors Affecting Plastics Shrinkage Rate

The shrinkage rate of plastic is not a fixed value but is influenced by multiple factors, including:

Material Properties: Different plastics exhibit varying shrinkage rates (e.g., ABS: ~0.4%–0.7%, POM: ~1.5%–2.5%).
Filler Types: Glass-fiber-reinforced plastics (e.g., PA66+GF30) have lower shrinkage rates (0.2%–0.5%), while pure resins shrink more.
Molding Process: Injection pressure, holding time, and cooling rate all affect shrinkage.
Product Structure: Non-uniform wall thickness, ribs, and inserts can cause localized shrinkage variations.

Calculation and Compensation Methods for Shrinkage Rate

Basic Shrinkage Calculation

The mold cavity dimensions must be enlarged to compensate for shrinkage using the following formula:

Mold Dimension=Product Design Dimension×(1+Shrinkage Rate)Mold Dimension=Product Design Dimension×(1+Shrinkage Rate)

Example:

For a 100mm-long ABS part with a shrinkage rate of 0.5%, the mold cavity dimension should be:

100×(1+0.005)=100.5mm100×(1+0.005)=100.5mm

Anisotropic Shrinkage Adjustment

Some plastics (e.g., glass-fiber-reinforced materials) exhibit different shrinkage rates along the flow direction and perpendicular direction:

Flow Direction: Lower shrinkage (due to fiber orientation).
Transverse Direction: Higher shrinkage.

Solutions:

Use mold flow analysis software (e.g., Moldflow) to predict shrinkage trends.
Apply targeted compensation for critical dimensions during mold design.

Post-Shrinkage and Secondary Shrinkage

Some plastics (e.g., PA, PBT) continue to shrink slowly after molding (0.1%–0.3% within 24 hours), so high-precision parts require additional compensation.

Key Adjustment Strategies in Mold Design

Cavity and Core Dimension Optimization

For high-precision parts (e.g., gears, connectors), apply shrinkage compensation factors.
For complex features (e.g., threads, snap-fits), add empirical adjustments (±0.05mm).

Cooling System Design

Uniform Cooling: Prevents warpage (e.g., conformal cooling channels).
Rapid Cooling: Reduces shrinkage variation (e.g., POM requires strong cooling).

Gate and Runner Optimization

Gate Location: Affects flow direction and shrinkage uniformity (e.g., multi-gating reduces localized shrinkage).
Runner Balancing: Ensures consistent filling across cavities to minimize shrinkage differences.

Draft Angle Adjustment

Shrinkage can cause parts to stick to cores, so increase draft angles (typically 0.5°–2°), especially in high-shrinkage areas.

Future Trends: Higher Performance and Sustainability

As NEVs transition to high-voltage platforms (800V and above), the demand for high-temperature plastics will grow further. Future material development will focus on:

Composite Materials: Such as carbon fiber-reinforced PPS, enhancing strength while reducing weight.
Recyclable Plastics: Including bio-based high-temperature nylons, aligning with environmental sustainability goals.