Extrusion & Post
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Dimensions of general-purpose polyethylene injection molded products too small, below the lower limit
Problem Phenomenon
The actual dimensions of the product are smaller than the lower tolerance limit of the drawing, making assembly difficult or impossible.
Undersized dimensions are usually related to excessive shrinkage, and are especially common in thick-walled products or highly crystalline materials.
Cause Analysis
（1）The holding pressure is insufficient or the holding time is too short, and the feeding is insufficient, resulting in a large shrinkage
（2）The mold temperature is high, and the crystallinity of general-purpose polyethylene is high, resulting in large post-shrinkage
（3）The barrel temperature is high, the melt density is small, and the volume shrinkage is large
（4）The cooling time is insufficient, and the product continues to shrink after demolding (post-shrinkage)
（5）The gate solidifies too early, and the holding pressure cannot be transmitted to the cavity
Solution
（1）Raise the holding pressure to 80-120MPa, and extend the holding time to 15-25s
（2）Appropriately lower the mold temperature to the lower value within the range of 30-80°C
（3）Appropriately lower the barrel temperature to the lower value within the range of 160-260°C
（4）Extend the cooling time to ensure that the product is fully cooled and shaped
（5）Increase the gate size and extend the gate freezing time
Key Takeaway
Undersized dimensions are caused by excessive shrinkage, and the shrinkage rate needs to be controlled by strengthening the holding pressure, lowering the temperature, and extending the cooling time.
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