Material Drying
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Insufficient Impact Strength of High-Density Polyethylene Injection-Molded Products
Problem Phenomenon
The product breaks in drop tests or impact tests, and the impact strength is lower than the standard requirement.
The fracture shows brittle fracture characteristics without obvious plastic deformation, and it is easy to be damaged by impact in actual use.
Cause Analysis
(1) The melt temperature is too low, the molecular chain orientation is severe, and the residual internal stress is large
(2) The mold temperature is too low, and rapid cooling leads to low or uneven crystallinity
(3) The holding pressure is too high, and over-packing generates residual stress
(4) The moisture content of the material is high, and hydrolysis at high temperature leads to a reduction in molecular weight
(5) The regrind ratio is too high or the regrind is used too many times; each thermal history of the material reduces the molecular weight
Solution
(1) Increase the melt temperature to the higher value within the range of 180-260°C to reduce molecular orientation
(2) Increase the mold temperature to the higher value within the range of 30-80°C to improve crystallization quality
(3) Appropriately reduce the holding pressure to avoid residual stress from over-packing
(4) Fully dry the raw material, and control the moisture content of high-density polyethylene below 0.02%
(5) Control the regrind ratio to not exceed 20%, and the regrind is not used more than 2 times
Key Takeaway
Insufficient impact strength is usually caused by excessive internal stress or material degradation, and needs to be addressed from temperature parameters, drying, and regrind control.
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