injection moulding sink mark

 In plastic injection molding, a sink mark is a surface defect that appears as a depression, dimple, or dent on the surface of a molded part. It typically occurs in thicker sections of the part or near features like ribs, bosses, or other areas where there is a significant variation in wall thickness. Sink marks are caused by uneven cooling and shrinkage of the plastic material during the molding process.

 

Causes of Sink Marks

1. Material Shrinkage:
As the molten plastic cools and solidifies, it shrinks. If the outer surface cools and hardens faster than the inner material, the inner material continues to shrink, pulling the surface inward and creating a sink mark.

2. Insufficient Packing Pressure or Time:
During the holding/packing phase, additional material is pushed into the mold to compensate for shrinkage. If the packing pressure or time is too low, the material shrinks excessively, leading to sink marks.

3. Thick Sections:
hicker areas of the part cool more slowly, increasing the likelihood of shrinkage and sink marks.

4. High Melt Temperature:
Higher melt temperatures can increase the material's shrinkage as it cools, contributing to sink marks.

5. Improper Gate Location or Size:
If the gate (the entry point of molten plastic into the mold) is too small or poorly located, it may restrict material flow, preventing adequate packing and causing sink marks.

6. Material Properties:
Some materials, especially semi-crystalline plastics (e.g., polypropylene, nylon), have higher shrinkage rates and are more prone to sink marks.

 

How to Identify Sink Marks

Sink marks are visible as shallow depressions on the surface of the part.

They are most common in areas with thicker walls or near structural features like ribs, bosses, or mounting points.

 

How to Prevent Sink Marks

1. Optimize Part Design:
Maintain uniform wall thickness throughout the part.
Use ribs or coring to reduce material volume in thick sections.

2. Adjust Process Parameters:
Increase holding/packing pressure and time to ensure adequate material packing.
Optimize cooling time to allow uniform cooling of the part.
Reduce melt temperature to minimize shrinkage.

3. Improve Mold Design:
Position gates near thick sections to ensure proper material flow and packing.
Optimize cooling channels to ensure even cooling, especially in thick areas.

4. Select Appropriate Materials:
Use materials with lower shrinkage rates, such as amorphous plastics (e.g., ABS, PC).
For crystalline plastics, consider using filled or reinforced materials to reduce shrinkage.

 

Corrective Actions for Existing Sink Marks
Increase holding pressure and time.
Reduce melt temperature.
Extend cooling time.
Modify part design to reduce wall thickness or add coring.
Adjust gate size or location.

 

By addressing these factors, sink marks can be minimized or eliminated, resulting in higher-quality injection-molded parts.

plastic injection molding sink mark

 

The reasons for sink mark in plastic products are as follows:

Reasons for injection molding sink mark

Sink marks in injection molding are common defects that occur when the surface of a molded part shrinks and creates a depression or dimple. This typically happens because the inner part of the material cools and shrinks faster than the outer surface, pulling the surface inward. Sink marks are most noticeable on thicker sections of the part or near ribs, bosses, or other features that create variations in wall thickness.

Causes of Sink Marks:

1. Insufficient Cooling Time:

• If the cooling time is too short, the outer surface solidifies while the inner material is still cooling and shrinking, causing sink marks.

1. Inadequate Holding Pressure or Time:

• Holding pressure helps pack additional material into the mold to compensate for shrinkage. If the pressure or time is insufficient, the material shrinks excessively, leading to sink marks.

1. Excessive Wall Thickness:

• Thicker sections cool more slowly, increasing the likelihood of shrinkage and sink marks.

1. High Material Shrinkage Rate:

• Some materials, such as semi-crystalline plastics (e.g., polypropylene or nylon), have higher shrinkage rates, making them more prone to sink marks.

1. Improper Gate Location or Size:

• If the gate is too small or poorly located, it can restrict the flow of material, preventing adequate packing and leading to sink marks.

1. High Melt Temperature:

• Excessive melt temperature can increase material shrinkage as it cools, contributing to sink marks.

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How to Prevent Sink Marks:

1. Optimize Wall Thickness:

• Design parts with uniform wall thickness to ensure even cooling. If thick sections are unavoidable, use coring or ribbing to reduce material volume.

1. Increase Holding Pressure and Time:

• Apply sufficient holding pressure and extend the holding time to pack more material into the mold and compensate for shrinkage.

1. Adjust Cooling Time:

• Ensure adequate cooling time to allow the entire part to solidify uniformly.

1. Use Low-Shrinkage Materials:

• Select materials with lower shrinkage rates, such as amorphous plastics (e.g., ABS, PC) or filled resins.

1. Optimize Gate Design:

• Use larger gates or position them near thicker sections to ensure proper material flow and packing.

1. Control Melt Temperature:

• Avoid excessively high melt temperatures to minimize shrinkage.

1. Add Mold Features:

• Incorporate mold features like gas vents or cooling channels to improve cooling efficiency and reduce shrinkage.

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Corrective Actions for Existing Sink Marks:

• Increase holding pressure and time.
• Reduce melt temperature.
• Extend cooling time.
• Modify part design to reduce wall thickness or add coring.
• Adjust gate size or location.

By addressing these factors, you can minimize or eliminate sink marks and improve the quality of injection-molded parts.

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.