Extrusion & Post
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Thickness deviation of PP film (machine direction/transverse direction)
Problem Phenomenon
CPP/IPP film shows periodic thickness fluctuations in the machine direction, or one side thick and one side thin in the transverse direction; after bag making, strength and heat sealing are uneven.
Cause Analysis
Machine direction (MD) deviation mostly comes from extrusion output pulsation (unstable feeding, screw wear, screen pack clogging) or haul-off speed fluctuation; transverse direction (TD) deviation mostly comes from uneven die lip gap, uneven die head temperature across the width, or asymmetric cooling.
Solution
①MD direction: stabilize feeding and extrusion output, check screw wear and screen pack pressure difference, stabilize haul-off; ②TD direction: readjust the die lip bolts, calibrate the temperature of each die head zone, and even out cooling; ③configure online thickness measurement with closed loop; ④establish thickness SPC monitoring.
Key Takeaway
PP film thickness must be 'treated by direction': for MD check the stability of extrusion and haul-off, for TD check the symmetry of the die lip and temperature field; adjusting randomly without distinguishing directions often presses down one lump only for another to pop up.
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