What does a melt pump do in cast film extrusion? Its main functions are to pressurize and meter the polymer melt, reduce the effect of extrusion fluctuations on die feeding, and help control film thickness variations in the machine direction.
During cast film production, uneven screw feeding, changes in processing temperature, and increased filter screen resistance can cause pressure and flow fluctuations. When the amount of melt entering the die varies, film thickness can fluctuate even if the haul-off speed remains constant. Before repeatedly adjusting temperature and screw speed, operators should check material feeding and melt pressure to identify the source of the fluctuations.
A melt pump is generally installed between the extruder and the die, working alongside a screen changer and connecting flow channels. As the gears rotate, the melt fills the spaces between the teeth and travels along both sides of the pump housing to the outlet. With sufficient inlet feeding and properly plasticized material, adjusting the pump speed controls the flow rate and helps reduce the impact of upstream fluctuations on downstream feeding. By taking on part of the pressure-building duty, the pump also provides more flexibility for adjusting the extruder’s operating conditions.
However, uneven film thickness is not always caused by feeding problems. Cross-direction thickness variations also require checks of the die lip gap, die temperature, and flow channel condition. Chill roll temperature and haul-off speed also affect film formation. A melt pump therefore needs to work in coordination with the controls of the entire production line.
Pump selection should consider the resin type, processing temperature, actual throughput, and operating differential pressure to determine the displacement, speed, and drive configuration. Changes in filtration resistance should also be taken into account. Tianjin Ruicheng Pump Industry Co., Ltd. provides melt pump selection and system configuration recommendations based on the operating conditions of cast film lines, supporting continuous feeding and film thickness control.
