How to control the temperature of the injection molding machine barrel
At present, resistance heating is widely used in the barrel of injection molding machine. Heating devices include cast aluminum heaters, ceramic heaters, mica heaters, etc., which use the combined action of a heating source and mechanical shear to plasticize plastics in a steady state. screw and barrel The barrel temperature is controlled in sections to obtain a temperature distribution that meets the process requirements, as well as a sufficiently fast heating speed and temperature control accuracy, and meet energy-saving requirements. Generally, when the shear heat of the processed plastic is small, it is approximated that the melt temperature mainly depends on the heating temperature of the barrel. In recent years, a new electromagnetic dynamic plasticization method has emerged, which basically does not require external heating in the stable working process, and has the characteristics of high plasticization efficiency and low energy consumption.
Because the barrel temperature of the injection molding machine and its control effect directly affect the quality of injection molded products, such as surface residual stress, shrinkage rate, and product weight stability, the latter has now become an important basis for measuring the quality of precision injection molded products. This has attracted the attention of the industry. The reliability of the barrel temperature control device and its temperature control accuracy have become the key to precision injection molding. Due to the large inertia of the heating system, coupled with the influence of environmental factors such as power supply voltage fluctuations, it is often difficult to obtain the ideal quality factors, and corresponding measures need to be taken. Based on the relay-type switch control of the injection molding machine barrel heating system, the barrel adopts the front, middle and back three-stage heating method, and its performance indicators such as overshoot, transition process time and steady state deviation are not ideal.
Although the contactless thyristor is an ideal device for controlling high-power components, it is also used to control the heating of the barrel and adjust the output power of the thyristor by changing the size of the conduction angle. Practical applications have confirmed that this method still has three obvious defects:
(1) Instant turn-on will cause great pollution to the grid, which in turn will cause strong interference to the control system itself, especially in the case of high power, which is more serious;
(2) The control circuit is more complicated, especially for the trigger pulse circuit to meet the requirements of precise power adjustment;
(3) There is a non-linear relationship between the output voltage and the conduction angle of the control quantity, which brings a lot of inconvenience to the adjustment of the control system, and also deteriorates the performance of the control system.
Therefore, most of today use non-contact zero-crossing solid state relays as high-power control devices, and control the heating process based on various new control strategies, and control the temperature by adjusting the heating power.