Material Drying
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Bubbles/Voids Appearing on the Surface of High-Density Polyethylene Injection-Molded Products
Fenómeno do problema
Round or elliptical bubbles of various sizes appear inside or on the surface of the product, and bulges or depressions can be seen on the surface.
Bubbles not only affect appearance but also significantly reduce the density and mechanical strength of the product, which is especially serious in thick-walled products.
Análise das causas
(1) The raw material is not fully dried, and the moisture vaporizes at high temperature to produce water vapor bubbles
(2) The injection speed is too fast, and the air in the cavity has no time to be discharged and is involved into the melt
(3) The holding pressure is insufficient or the holding time is too short, and the voids produced by shrinkage in thick-wall sections are not compensated
(4) The barrel temperature is too high (exceeding the upper limit of 180-260°C), and the material thermally degrades to produce gas
(5) Air is brought in when the screw retracts and sucks material, and insufficient back pressure leads to inadequate venting
Solução
(1) Fully dry the raw material; dry high-density polyethylene at 60-80°C for 2-4h, and control the moisture content below 0.02%
(2) Adopt multi-stage injection, slow first, then fast, then slow again; reduce the injection speed at the initial and final stages of filling to reduce gas involvement
(3) Increase the holding pressure to 80-120MPa and extend the holding time to 15-25s to ensure full shrinkage compensation of thick walls
(4) Appropriately reduce the barrel temperature to a reasonable value within the range of 180-260°C to avoid thermal degradation
(5) Increase the back pressure to 5-10MPa to ensure full venting during screw pre-plasticization
Conclusão principal
Bubbles and voids are mainly caused by moisture vaporization, gas involvement, or insufficient shrinkage compensation, and need to be systematically solved from raw '2079851475616104451, injection process, and holding parameters.
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