Injection Molding
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COC Optical Distortion Birefringence
Problem Phenomenon
COC optical lenses and window parts show excessive birefringence, colored fringes are visible in polarized light inspection, and imaging quality is unqualified.
Cause Analysis
Filling flow orientation and internal stress cause amorphous COC to produce stress birefringence; high injection speed, high holding pressure and low mold temperature all increase orientation stress freezing; birefringence is most severe near the gate.
Solution
①Adopt a low-stress process: staged speed reduction, low holding pressure, high mold temperature; ②optimize gate position and form to reduce shear orientation; ③annealing treatment to release stress; ④full inspection of the optical zone with a polariscope.
Key Takeaway
The birefringence of COC optical parts shares the same origin as internal stress; a low-stress process is a prerequisite for optical performance; polarized light inspection should be incorporated into the standard inspection process for optical parts.
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