Extrusion & Post
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Flash/Burrs on High-Density Polyethylene Injection-Molded Products
Problem Phenomenon
Thin sheet-like overflow appears at the parting line, ejector pin holes, slider joints, etc. of the product, namely flash or burrs.
Flash affects appearance and dimensional accuracy, increases post-processing workload, and in severe cases causes the product to stick in the mold and makes demolding difficult.
Cause Analysis
(1) The clamping force is insufficient, and the mold is forced open by the melt pressure during injection, creating a gap
(2) The injection pressure is too high or the holding pressure is too large, exceeding the mold clamping force
(3) The mold parting line is worn or does not fit well, and there is a gap
(4) The barrel temperature is too high, the melt viscosity is low and the fluidity is too good, making it easy to overflow
(5) The mold vent groove is too deep, and the melt overflows into the vent groove to form flash
Solution
(1) Check and increase the clamping force to ensure that the clamping force is greater than 1.2-1.5 times the melt pressure in the cavity
(2) Appropriately reduce the injection pressure and holding pressure, using the lowest pressure on the premise of ensuring quality
(3) Reduce the barrel temperature to the lower value within the range of 180-260°C to increase the melt viscosity
(4) Check the mold parting line, re-lap the mold closing surface, and ensure the gap is less than 0.02mm
(5) Check the vent groove depth; the vent groove depth for polyolefin materials does not exceed 0.03mm
Key Takeaway
Flash is a manifestation of a mismatch between the mold clamping force and the injection pressure, and needs to be controlled from three aspects: clamping force, injection pressure, and mold precision.
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