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Temperature control scheme of twin-screw extruder

by:TENGDA     2023-01-28
​ Twin-screw extrusion equipment is mainly used for thermoplastics. It is one of the three major types of plastic machines (injection molding/extrusion/blow molding). It occupies an extremely important position in plastic processing and can be used to form pipes, rods, wires, Plates, films, coatings of cables and wires, and other profiles of various cross-sectional shapes.
In the actual production process of plastics, the stability of temperature control directly determines the quality of the product, and is closely related to the energy consumption of the whole machine. The temperature control of twin-screw extruder/extrusion production line often involves a dozen to dozens of control loops (multiple hosts + auxiliary machines), and the related control mainly includes barrel temperature (heating/cooling control, multiple temperature zones) , Die head temperature (heating control, multiple temperature zones), melt temperature, melt pressure and main motor speed and feeding motor speed. Among them, the accurate control of the temperature of each part of the cylinder body and the machine head is of great significance to improve the output and quality of plastic products: if the extrusion temperature is low, it will cause excessive load and poor discharge; if the extrusion temperature is too high, It may lead to changes in material properties and scrap the finished product. Therefore, for twin-screw extruders, the selection of temperature-controlled products is particularly important.
At present, there are generally three temperature control schemes for twin-screw extruders:

1. Discrete instrument:

According to the requirements of processing technology, each zone is controlled separately by means of temperature control meter + solid state relay (electric heating) + solenoid valve (air cooling or water cooling). This kind of control scheme works well under static conditions, but the control under dynamic conditions (normal operation), even if it is air-cooled, can only be said to be average, let alone water-cooled equipment, and the discrete structure is not conducive to heating current monitoring The implementation of the function (because each circuit needs a separate current transformer), in addition, the use of multiple temperature control meters makes the circuit structure of the whole twin-screw extruder complicated, and the installation, wiring and debugging are more troublesome. On the one hand, the labor cost is increased, and on the other hand, the probability of failure is also increased.

2. Pure PLC control:

Using the built-in soft PID module of PLC for temperature control, its integration level is higher than that of discrete instruments, and the failure rate is also low, but it requires higher technical level of personnel, and the on-site debugging is much more troublesome than that of a single meter. However, its actual control effect is not even as good as that of a single meter (because of its very simple PID function), and this solution does not support the heating current monitoring function.

3. Special loop control module:

In order to make up for the shortcomings of the first two schemes, more and more dedicated PID control modules have been used in recent years. Use a dedicated closed-loop multi-loop control module to centralize temperature control, each module has 2-4-way control, which simplifies the installation, commissioning and wiring of the twin-screw extruder, but the essence of these modules is to cancel the display of multiple single meters, Integrating into one module, there is no substantial improvement in the algorithm.

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