Extrusion & Post
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Dimensions of Block Copolymer Polypropylene Injection Molded Products Too Small Exceeding 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 particularly common in thick-walled products or highly crystalline materials.
Cause Analysis
（1）The holding pressure is insufficient or the holding pressure time is too short, and insufficient shrinkage compensation leads to a large shrinkage amount
（2）The mold temperature is high, and the high crystallinity of block copolymer polypropylene leads to 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 prematurely, and the holding pressure cannot be transmitted to the cavity
Solution
（1）Increase the holding pressure to 80-120MPa and extend the holding pressure time to 15-25s
（2）Appropriately reduce the mold temperature to a lower value within the 40-90°C range
（3）Appropriately reduce the barrel temperature to a lower value within the 200-270°C range
（4）Extend the cooling time to ensure the product is fully cooled and set
（5）Increase the gate size to 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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