Injection Molding
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Severe internal stress birefringence in PET injection molded parts
Problem Phenomenon
Transparent PET products show dense colored fringes under a polariscope, indicating a high internal stress level.
Cause Analysis
Fast filling speed causes high molecular chain orientation that is not fully relaxed; mold temperature is low; cooling rate is too fast.
Solution
Reduce injection speed; increase mold temperature to 140-160°C; extend cooling time; anneal to eliminate stress.
Key Takeaway
PET birefringence is the optical manifestation of internal stress, especially important for optical-grade PET.
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