Extrusion & Post
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Low density polyethylene injection products with undersized dimensions below the lower limit
Problem Phenomenon
The actual dimensions of the product are smaller than the lower limit of the drawing tolerance, making assembly difficult or impossible.
Undersized dimensions are usually related to excessive shrinkage, and are especially common in thick-wall products or highly crystalline materials.
Cause Analysis
(1) Insufficient holding pressure or holding time too short; insufficient shrinkage compensation leads to large shrinkage
(2) Mold temperature too high; high crystallinity of low density polyethylene leads to large post-shrinkage
(3) Barrel temperature too high; low melt density and large volume shrinkage
(4) Insufficient cooling time; the product continues to shrink after demolding (post-shrinkage)
(5) Gate solidifies too early; holding pressure cannot be transmitted to the cavity
Solution
(1) Increase holding pressure to 80-120MPa, extend holding time to 15-25s
(2) Appropriately reduce mold temperature to the lower value within the 30-60°C range
(3) Appropriately reduce barrel temperature to the lower value within the 160-220°C range
(4) Extend cooling time to ensure the product is fully cooled and set
(5) Enlarge gate size, extend gate freezing time
Key Takeaway
Undersized dimensions are caused by excessive shrinkage; the shrinkage rate needs to be controlled by strengthening holding pressure, lowering temperature, and extending cooling time.
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