Material Analysis
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COC Excessive Back Pressure Degradation
Problem Phenomenon
After COC increases back pressure to improve mixing, melt yellowing, increased black specks and viscosity decrease appear, product brittleness rises, and color is unstable between batches.
Cause Analysis
The COC back pressure upper limit is about 150 bar specific pressure; beyond the limit, shear heat increases sharply and causes thermal degradation; high back pressure prolongs plasticizing time and aggravates the heat history of the melt; degradation molecular chain scission leads to embrittlement.
Solution
①Control back pressure within 150 bar specific pressure; ②when mixing is insufficient, prefer switching to color masterbatch or increasing melt temperature; ③monitor the actual melt temperature to prevent shear overheating; ④reduce speed and purge the machine after degradation.
Key Takeaway
COC back pressure 150 bar is the red line drawn by the supplier; mixing demand cannot be solved by exceeding back pressure; the damage of shear heat to high-viscosity materials is often underestimated.
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