PETG can soften and form continuous strings when trimming heat is not removed from the cut. Dull tools, high rubbing speed, low chip load, poor chip evacuation, unsupported walls, and trimming before the part has cooled can leave melted edges or material that reattaches behind the cutter.
The edge should be examined for a smeared glossy surface rather than brittle chips. Tool temperature, spindle speed, feed, flute condition, support, and part temperature should be reviewed together. Increasing speed alone often adds friction without improving cutting.
A sharp tool and cutting condition that produces clean chips should be selected for the sheet thickness and geometry. Airflow or extraction may improve chip removal but should not blow debris onto appearance surfaces. The part must be cool and supported enough to resist vibration and local deflection during trimming.