Applications
In the extrusion process , the extruder flow rate can be adjusted using gravimetric feeders or by a gear pump positioned before the die, which maintains a constant flow regardless of the quantity fed by the feeders.
The melt pump is particularly used in extruders for profiles or pipes, where flow stability is essential for product quality.
Requirement:
Thermoregulation
Stable rheological behaviour and the consequent dimensional consistency of the extruded material are obtained through correct temperature control. It is essential to guarantee thermal stability in these areas, so that the system can respond quickly to variations in thermal or process load, avoiding potential defects (such as surface instability, thickness variations or alterations in the optical quality of the product).
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Gefran’s temperature control technology is based on solid state relays, PID controllers and thermocouples:
- Each resistor is managed in a closed loop via PID control, integrated into the PLC or entrusted to dedicated controllers such as the 1850, which allows the melt temperature to be maintained within ±0.5 °C of the setpoint, ensuring constant viscosity and the absence of surface or dimensional defects
- The GRP-H solid state relay with integrated heat sink ensures precise current control up to 120 A. IO-Link communication also reduces the number of I/O cards required, thanks to the integrated management of commands, alarms and current consumption
The solution is scalable and compatible with PIDs integrated in PLCs or remote modules. Recipe storage and historical trend functions support rapid production changes and continuous quality improvement.
Requirement:
Flow rate adjustment
The speed of the pump and the corresponding force with which the polymer is pushed towards the die are regulated by the drives. The control of the latter is usually entrusted to the PLC of the extruder itself. The pump is regulated by closed-loop control, based on upstream and downstream pressure readings.
Solution:
Gefran uses two high-precision MELT sensors (upstream and downstream of the pump) to regulate the pump speed and maintain stable pressure and constant flow of the molten polymer. The Impact series MELT pressure sensors are ideal for critical environments and integration into safety logic.
Main features:
- Designed with a reinforced membrane, piezoresistive sensing element and no transmission fluid
- Thermal stability less than 1% FS between 20 and 350 °C, also perfect for regenerated or abrasive polymers
- PLc certification and electrical output improve safety and reliability
- The absence of internal fluid makes the sensor suitable for medical and food applications
Operating principle
The melt pump is a volumetric device consisting of two counter-rotating gear wheels inside a closed body. The molten polymer enters between the upstream teeth, is captured in the cavities and transported to the outlet area. During movement, the melt is progressively compressed and pushed towards the die, with constant pressure regulated by the control system. Clearances and tolerances are optimised to minimise internal losses and ensure high volumetric efficiency (>98%).
The speed of the gears and the outlet pressure are controlled by a closed loop with PID regulation. The main components – such as wheels, casing and bearings – are designed to operate with the melt as a lubricant and withstand high temperatures and pressure differentials of up to over 300 bar.
The integration of the pump allows the rotation speed of the extrusion screw to be decoupled from the amount of material extruded from the die, stabilising the pressure and improving the quality of the extruded product.
Requirements and technology
The melt pump must ensure a constant and precise flow of molten polymer to the die. Accurate temperature control prevents dimensional variations or surface defects, while monitoring the polymer pressure upstream and downstream of the pump closes the pressure control loop by providing feedback to the PID algorithms for the drives.
Gefran offers a complete range of control platforms, pressure sensors, melt temperature sensors and modular solutions thatcan be easily integrated into any plant to ensure efficiency, safety and consistent quality in the extrusion process.
