Injection Molding
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Low hardness of general-purpose polyethylene injection molded products
Problem Phenomenon
The surface hardness test value of the product is lower than the standard requirement, and the surface is easily scratched or indented.
Low hardness affects the wear resistance and surface scratch resistance of the product, and affects the use performance in products requiring surface hardness.
Cause Analysis
（1）The mold temperature is low, and the crystallinity of general-purpose polyethylene is low, resulting in decreased hardness
（2）The barrel temperature is high, and the material is partially degraded and the molecular weight is reduced
（3）The holding pressure is insufficient, and the product density is low
（4）The cooling time is insufficient, and the crystallization process is not completed
（5）The material grade is improperly selected, and the hardness index does not meet the requirements
Solution
（1）Appropriately raise the mold temperature to the higher value within the range of 30-80°C to increase the crystallinity
（2）Control the barrel temperature at a reasonable value within the range of 160-260°C to avoid overheating
（3）Raise the holding pressure to 80-120MPa to ensure that the product is dense
（4）Extend the cooling time to ensure sufficient crystallization
（5）Select a suitable grade, and add a nucleating agent to increase the crystallinity if necessary
Key Takeaway
Low hardness is mainly caused by insufficient crystallinity or material degradation, and needs to be improved by increasing the mold temperature and holding pressure.
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