Molds & Tooling
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PC Product Yellowing
Problembeschreibung
Natural-color or transparent PC products turn yellow, light transmittance decreases, brown streaks appear in severe cases, and color difference between batches is obvious.
Ursachenanalyse
Too high melt temperature or too long barrel residence time leads to thermal degradation; shear heat generated by excessive screw speed/back pressure makes the actual melt temperature far exceed the setting; carbonized material accumulated in hot runner dead corners also causes local yellowing.
Lösung
①Lower the barrel temperature to the lower limit of the grade-recommended range; ②Shorten the melt residence time in the barrel to reduce thermal cumulative degradation; ③Reduce screw speed and back pressure to control shear heat; ④Clean hot runner dead corners; ⑤Use temperature measurement means to actually measure the melt temperature and calibrate the process.
Wichtigste Erkenntnis
PC yellowing = 'thermal history exceeding the standard': lower temperature, shorten time, and reduce shear all together; the actual melt temperature is often higher than the set value, shear heat must be actually measured; PC has strong thermal sensitivity, and temperature management must leave sufficient safety margin.
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