How to Prevent Shrinkage Errors in Injection Molding
Shrinkages are a common quality issue in plastic injection molding. To minimize these errors, optimizations can be made in material selection, mold design, process parameter adjustments, and post-processing.
1. Select Materials Properly
Use Low-Shrinkage Materials
Different materials have varying shrinkage rates. For instance, glass-fiber-reinforced plastics, crystalline plastics (e.g., PA, PP), and amorphous plastics (e.g., PC, PMMA) exhibit distinct shrinkage behaviors. Choosing materials with low and stable shrinkage can effectively reduce errors.
Control Material Moisture Content
Excessive moisture can cause bubbling, warping, and uneven shrinkage. Moisture-absorbing materials (e.g., PA, PBT) should be thoroughly dried before processing.
2. Optimize Mold Design
Design Proper Mold Dimensions
Mold cavity dimensions should account for material-specific shrinkage and include appropriate compensation space.
Improve the Cooling System
Uneven mold temperature can lead to inconsistent shrinkage in different parts of the product. Designing well-distributed cooling channels ensures uniform mold temperature.
Ensure Uniform Wall Thickness
Uneven wall thickness causes cooling rate differences, leading to uneven shrinkage. Optimize product design to maintain consistent wall thickness.
Design an Appropriate Gating System
The gate and runner design should ensure that molten material fills the mold cavity uniformly, reducing pressure loss and shrinkage errors.
3. Adjust Process Parameters
Increase Injection Pressure and Holding Pressure
Higher injection and holding pressures help compensate for volume shrinkage during material cooling. However, excessive pressure may damage the mold or warp the product.
Extend Holding Time
Insufficient holding time can lead to incomplete mold filling or sink marks during cooling. Extending the holding time reduces shrinkage errors.
Optimize Mold and Barrel Temperatures
Raising the mold and barrel temperatures moderately can decrease material viscosity, improve flowability, and reduce internal stresses during molding.
Adjust Cooling Time
Extending cooling time ensures the product is fully set before ejection, preventing shrinkage caused by temperature differences.
4. Control Production Conditions
Maintain Stable Production Conditions
Fluctuations in temperature, pressure, or other parameters during production can result in inconsistent product dimensions. Consistency in machine settings and environmental parameters reduces errors.
5. Minimize Internal Stress in Products
Internal stresses can cause dimensional changes in the product during use. Adjusting molding processes and optimizing mold structure can reduce these stresses.
Post-Processing and Optimization
Post-Molding Annealing
Annealing the product after molding helps eliminate internal stresses and stabilize dimensions by slow heating.
