Injection Molding
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Uneven color/color difference appearing on polypropylene injection molded products
Problem Phenomenon
Areas of different color shades appear on the product surface, or there is a color difference from the standard color sample.
Color difference may appear between different parts of the product, different mold cavities, or different batches, affecting the appearance consistency of the product.
Cause Analysis
（1）The color masterbatch is unevenly dispersed, the mixing time is insufficient, or the mixing ratio is unstable
（2）The barrel temperature fluctuates greatly, and the coloring effect of the color masterbatch differs at different temperatures
（3）The screw speed and back pressure are set improperly, and the plasticization and mixing are insufficient
（4）The barrel or screw is worn, and the stagnation zone produces discoloration or uneven coloring
（5）The mold surface conditions are inconsistent, and the gloss differences in different areas cause visual color difference
Solution
（1）Ensure the color masterbatch is fully mixed with the raw material, with a mixing time of 5-10min, using weighing equipment for accurate metering
（2）Stabilize the barrel temperature, with the temperature difference of each section controlled within ±3°C (200-280°C range)
（3）Appropriately raise the back pressure to 5-10MPa and optimize the screw speed to ensure uniform plasticization and mixing
（4）Check the wear of the screw and barrel, and replace the screw head or non-return ring when necessary
（5）Regularly clean the mold cavities to ensure consistent surface conditions of each cavity
Key Takeaway
Color difference is caused by uneven colorant dispersion or fluctuations in process parameters, and consistency needs to be ensured from three aspects: mixing, plasticization, and temperature control.
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