Injection Molding
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Low weld strength of random copolymer polypropylene injection molded products
Problem Phenomenon
The strength of the product at the weld line is significantly lower than other areas, and it breaks at the weld line under stress.
Low weld strength is usually 50-80% of the base material strength, affecting safety in use in load-bearing and sealing products.
Cause Analysis
(1) Melt merging temperature is low, and the two melt streams do not fuse fully
(2) Insufficient injection pressure transmission; holding pressure shrinkage compensation at the weld line is inadequate
(3) Mold temperature is low; the melt cools rapidly at the merging point
(4) Poor cavity venting; gas at the merging point hinders fusion
(5) The weld line is located in a load-bearing part of the product
Solution
(1) Increase melt temperature to the higher value within the 190-260°C range
(2) Increase mold temperature to the higher value within the 40-80°C range
(3) Increase injection pressure and holding pressure to ensure the weld line area is fully compacted
(4) Improve mold venting, setting vent grooves or overflow wells at the weld line
(5) Adjust gate position to move the weld line to a non-load-bearing part
Key Takeaway
Low weld strength is caused by insufficient melt fusion and needs to be enhanced by increasing temperature, pressure and improving venting.
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