What does a melt pump do in BOPP film production? It pressurizes and meters polypropylene melt, reducing the impact of upstream extrusion fluctuations on the melt supply to the die. This helps control variations in cast sheet thickness in the machine direction and provides more stable conditions for subsequent biaxial stretching.
BOPP film production involves extrusion and plasticization, sheet casting and cooling, machine-direction stretching, and transverse-direction stretching. Changes in raw material characteristics, screw speed, processing temperature, and filtration resistance can affect the pressure and flow rate at the extruder outlet. An unstable melt supply to the die can cause thickness variations, making subsequent stretching and process adjustment more difficult.
A melt pump is typically installed between the extruder and the die. As the gears rotate, melt fills the spaces between the gear teeth and the pump housing and travels along both sides of the pump chamber to the outlet. The theoretical flow rate depends on pump displacement and rotational speed. Actual flow is also affected by inlet filling, melt viscosity, and the operating pressure differential, so sufficient feed and a suitable temperature must be maintained.
During operation, pump speed controls the flow rate, while outlet pressure varies with resistance from the die and downstream flow channels. Adjusting extruder speed using pump inlet pressure feedback helps maintain the melt supply to the pump. By taking on part of the pressure-building load, the melt pump helps coordinate plasticization and pressurized melt delivery. Film thickness control also requires coordinated adjustment of the die, cooling conditions, and stretching parameters.
Pump selection should account for the throughput range, melt temperature and viscosity, inlet and outlet pressures, connection dimensions, and other operating conditions. The drive, temperature control system, and pressure protection must also be appropriately matched. The pump should be thoroughly preheated before use, and pump inlet pressure and filter differential pressure should be monitored during operation. Tianjin Ruicheng Pump Industry Co., Ltd. provides melt pump selection and equipment integration services for BOPP film production lines based on actual operating requirements.
