Extrusion & Post
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Thickness deviation of PP film (machine direction/transverse direction)
Fenómeno del problema
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.
Análisis de la causa
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.
Solución
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.
Conclusión clave
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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