Extrusion & Post
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Dimensions of Block Copolymer Polypropylene Injection Molded Products Too Large Exceeding the Upper Limit
Problem Phenomenon
The actual dimensions of the product exceed the upper tolerance limit of the drawing, and the gap during assembly is too large or tight fitting is impossible.
Oversized dimensions are usually related to insufficient shrinkage, and are prone to occur after changing the material batch number or adjusting the process.
Cause Analysis
（1）The holding pressure is too high or the holding pressure time is too long, and the product is over-compensated
（2）The mold temperature is low, and the fast cooling rate and low crystallinity lead to reduced shrinkage rate
（3）The barrel temperature is low, the melt density is large, and the shrinkage rate is small
（4）The cooling time is too long; the product fully shrinks in the mold and its size becomes smaller, and the shrinkage allowance after demolding is small
（5）The shrinkage rate value taken in mold dimension design is too large
Solution
（1）Appropriately reduce the holding pressure or shorten the holding pressure time
（2）Appropriately increase the mold temperature to a higher value within the 40-90°C range to increase shrinkage
（3）Appropriately increase the barrel temperature to a higher value within the 200-270°C range
（4）Verify the mold dimensions according to the actual shrinkage rate of 1.2-2.0, and repair the mold when necessary
（5）Establish a process parameter recording system, and test the first-piece dimensions at each shift change
Key Takeaway
Oversized dimensions are mainly caused by insufficient shrinkage, and the actual shrinkage rate is controlled within the tolerance range by adjusting the holding pressure and temperature parameters.
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