Material Drying
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PET Preform Bubbles (Insufficient Drying)
Problembeschreibung
Round bubbles and water-blister-like cavities appear inside or on the surface of transparent preforms; after blowing, the bubble areas are stretched thin and even form pinholes.
Ursachenanalyse
PET pellets absorb moisture and the moisture content exceeds the standard; during melting, water vaporizes and induces hydrolysis; drying dew point not meeting standard or insufficient drying residence time is the main cause, and an excessively high regrind ratio also brings in moisture and volatiles.
Lösung
①Dehumidifying drying 150~180℃×4~6h, moisture content reduced below 50ppm; ②calibrate dryer dew point and air volume; ③reduce the regrind ratio and convey sealed; ④re-dry damped pellets before use.
Wichtigste Erkenntnis
Preform bubbles are almost all related to moisture management, and qualified drying is the primary prerequisite; bubbled preforms will inevitably be stretched thin during blowing, so they should be rejected online and the drying records traced, and after confirming the measures are effective, solidify them into the work instruction.
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