Solving Sink Marks in Injection Molding

 Solving Sink Marks in Injection Molding requires addressing multiple aspects, including material selection, process optimization, mold design, and product design. Below are some specific solutions:

 

1. Optimize Product Design

Uniform Wall Thickness: Ensure the product has uniform wall thickness to avoid localized thick areas. If thick walls are unavoidable, use coring or ribs to reduce material accumulation.
Transition Design: Use gradual transitions between thick and thin sections to avoid sudden changes in thickness.
Rib Design: The thickness of ribs should be 50%-60% of the main wall thickness to minimize sink marks.

 

 2. Adjust Injection Molding Process

Increase Holding Pressure:
Increasing holding pressure can compensate for material shrinkage during cooling, reducing sink marks.
Note: Excessive holding pressure may cause flash or increase internal stress.
Extend Holding Time:
Ensure the holding time is sufficient to allow adequate material packing during cooling.
Optimize Cooling Time:
Extend cooling time to allow uniform cooling of the product, reducing differential shrinkage.
Lower Melt Temperature:
Reduce the melt temperature appropriately to minimize material shrinkage.
Adjust Injection Speed**:
Increasing injection speed can improve mold filling, but avoid excessive speed to prevent other defects (e.g., flow marks or burning).

 

 3. Optimize Mold Design

Gate Design:
Position gates near thick sections to ensure adequate material filling and packing.
Increase gate size or number to improve material flow and packing.
Cooling System:
Optimize cooling channel design to ensure thick sections are adequately cooled.
Improve cooling efficiency in thick areas to avoid significant temperature differences between inner and outer regions.
Venting Design:
Ensure proper mold venting to avoid air traps that can cause insufficient filling or sink marks.

 

 4. Material Selection

Choose Low-Shrinkage Materials:
Use materials with low shrinkage rates (e.g., amorphous plastics like ABS, PC).
For crystalline plastics (e.g., PP, PA), additives like glass fibers can be used to reduce shrinkage.
Dry the Material:
Ensure the material is thoroughly dried to avoid shrinkage caused by moisture evaporation.

 

 5. Post-Processing

Mold Surface Treatment:
Add textures or sandblasting to the mold surface to mask minor sink marks.
Product Post-Processing:
For products with existing sink marks, heating or surface treatments (e.g., painting) can improve appearance.

 

 6. Simulation Analysis

Use mold flow analysis software (e.g., Moldflow) to simulate the injection molding process, predict the location and severity of sink marks, and optimize process parameters and mold design.

 

Summary

Solving sink marks requires a comprehensive approach, including product design, mold design, injection molding process, and material selection. By optimizing wall thickness, adjusting holding pressure and cooling time, and improving gate design, sink marks can be effectively reduced or eliminated. If the issue persists, further optimization of process parameters can be achieved through mold flow analysis.