Injection Molding
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PC Birefringence (Rainbow Pattern)
Problem Phenomenon
Transparent PC parts show rainbow-shaped interference fringes under polarized light, optical parts (lenses, light guides) are unqualified.
Cause Analysis
Polymer orientation frozen by filling and holding pressure forms internal stress, causing birefringence; high injection speed, high holding pressure, and low-temperature molds all aggravate orientation stress, which appears optically as rainbow patterns.
Solution
①Reduce injection speed and holding pressure; ②Increase mold temperature and melt temperature; ③Enlarge the gate to reduce shear; ④Optimize the flow path design for optical parts; ⑤Anneal to eliminate stress when necessary.
Key Takeaway
PC rainbow pattern = 'optical development of internal stress': reducing pressure, raising temperature, and annealing can alleviate it; optical parts must minimize orientation stress from flow design, this is more of a design problem than a process problem.
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