Material Drying
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Bubbles/voids appearing on the surface of polypropylene injection molded products
Problem Phenomenon
Round or elliptical bubbles of different sizes appear inside or on the surface of the product, with bulges or dents visible on the surface.
Bubbles not only affect the appearance but also significantly reduce the density and mechanical strength of the product, and thick-walled products are particularly serious.
Cause Analysis
（1）The raw material is not sufficiently 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 drawn into the melt
（3）The holding pressure is insufficient or the holding time is too short, and the voids produced by shrinkage in the thick-walled parts are not compensated
（4）The barrel temperature is too high (exceeding the 200-280°C upper limit), and the material thermally degrades to produce gas
（5）Air is drawn in when the screw retracts to suck material, and the back pressure is insufficient, resulting in insufficient venting
Solution
（1）Fully dry the raw material; polypropylene is dried at 60-80°C for 2-4h, and the moisture content is controlled below 0.02%
（2）Use multi-stage injection, slow first, then fast, then slow, reducing the injection speed at the initial and final stages of mold filling to reduce gas entrainment
（3）Raise the holding pressure to 80-120MPa and extend the holding time to 15-25s to ensure sufficient shrinkage compensation of the thick walls
（4）Appropriately lower the barrel temperature to a reasonable value within the range of 200-280°C to avoid thermal degradation
（5）Raise the back pressure to 5-10MPa to ensure sufficient venting during screw pre-plasticization
Key Takeaway
Bubbles and voids are mainly caused by moisture vaporization, gas entrainment, or insufficient shrinkage compensation, and need to be systematically solved from raw '2079851475616104451, injection process, and holding pressure parameters.
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