Extrusion & Post
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Dimensions of general-purpose polyethylene 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 is too large or tight fitting is impossible during assembly.
Oversized dimensions are usually related to insufficient shrinkage, and are easy to occur after changing the material batch or adjusting the process.
Cause Analysis
（1）The holding pressure is too high or the holding time is too long, and the product is over-fed
（2）The mold temperature is low, the cooling rate is fast, and the crystallinity is low, resulting in reduced shrinkage
（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 shrinks fully in the mold and the size is small, and the shrinkage margin after demolding is small
（5）The shrinkage rate value used in the mold size design is too large
Solution
（1）Appropriately reduce the holding pressure or shorten the holding time
（2）Appropriately raise the mold temperature to the higher value within the range of 30-80°C to increase shrinkage
（3）Appropriately raise the barrel temperature to the higher value within the range of 160-260°C
（4）Verify the mold dimensions according to the actual shrinkage rate of 1.5-3.0, and repair the mold if necessary
（5）Establish a process parameter recording system, and check the first article 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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