The temperature control system consists of heating and cooling. Nowadays, extrusion machines usually use electric heating, which is divided into resistance heating and induction heating. The heating plates are installed in the body, neck, and head of the machine. The heating device is made of plastic inside an external heating cylinder to raise the temperature required for process operation.
　
The cooling device is set to ensure that the plastic is within the temperature range required by the process. Specifically, it is to eliminate excess heat generated by shear friction during screw rotation, in order to avoid the difficulty of plastic decomposition, charring, or shaping due to high temperatures. Machine barrel cooling is divided into two types: water cooling and air cooling. Generally, air cooling is more suitable for small and medium-sized extrusion machines, while for large machines, water cooling or a combination of the two forms of cooling are often used; The screw cooling mainly adopts central water cooling, with the aim of increasing the solid conveying rate of materials, stabilizing the glue output, and improving product quality; But the cooling at the hopper is to strengthen the conveying effect of solid materials, prevent plastic particles from sticking and blocking the material outlet due to temperature rise, and to ensure the normal operation of the transmission part.
　
1、 Temperature control system
1. Temperature control mechanism
Install thermocouple temperature measuring elements at several appropriate positions close to the plastic layer. During the heating or extrusion process, the thermoelectric potential signal measured by the temperature measuring element is sent to the temperature controller for amplification and comparison with the temperature setting value. If it does not reach the set value, continue heating. If it exceeds the set value, start the cooling fan to cool the body and return to the set temperature. Repeat this process and adjust it automatically or manually.
2. Temperature control part of the extrusion machine
It mainly includes six temperature control parts: front nose, middle nose, neck, front body, middle body, and rear body.
3. The role of temperature control parts
As long as the temperature is appropriate and the mold is properly matched, the plastic surface of the front and middle heads will be flat and smooth.
⑵ The neck has a smaller volume than the barrel, and there is also a filter plate, which has high resistance. Higher temperatures are required to facilitate the entry of plastic melt into the machine head
The front and middle fuselage are homogenization and plasticization sections, which achieve uniform plasticization under the influence of temperature.
(4) The temperature control of the rear body includes preheating and discharging excess gas from the feeding port, with the lowest temperature in this area.
　
2、 Heating control instrument
There are two forms of control: manual control and automatic control.
1. Operation of heating control instruments
There are two types of temperature display for temperature instruments - pointer scale meters and digital display meters.
The temperature determination method of a pointer scale: Rotate the positioning needle to the desired temperature scale, and read the actual temperature according to the indicating needle;
The temperature setting method for digital display meters: Set the instrument to the upper and lower heating limits, turn the digital control knob or press the digital scale control key to maintain the temperature in a certain area. Can directly read the actual temperature on the temperature instrument; It can also be determined from the position of the ammeter pointer and the temperature deviation meter. When the pointer points to "zero", it means that the temperature has risen to the predetermined temperature control value.
2. Several issues to pay attention to when heating
(1) Regularly check whether the instruments, switches, circuits, plugs, wiring posts, and thermocouples in each heating zone are in good condition, and promptly find a dedicated person to repair any problems.
(2) When heating, it is necessary to see whether it is manual or automatic heating, and do not leave the post during heating.
When adjusting the mold and cleaning the machine head, it is necessary to prevent damage to the plug, wiring post, and thermocouple, and only prevent electric shock.
(4) Use a small screw driver to adjust the positioning needle
When abnormalities are found in manual and automatic control, a dedicated person should be promptly appointed for repair.
During heating, the instrument pointer always indicates the maximum or minimum value, and a dedicated person should be promptly appointed for repair.
　
3、 The impact of temperature setting on product quality
(1) If the temperature is too high and exceeds the optimal plasticizing temperature, the plastic is prone to scorching and aging, resulting in air holes and poor shaping. Plastic is prone to "slipping" inside the barrel, affecting the stability of extrusion volume and causing thickness fluctuations.
(2) If the temperature is too low and below the optimal plasticizing temperature, the plastic plasticization is not good, and there are rigid blocks and small particles on the surface of the plastic sleeve, poor sealing, and phenomena such as detachment, cracks, and broken adhesive.
(3) When setting the temperature, it is also necessary to consider the impact of environmental temperature, with a difference of 5-10 ℃ between winter and summer
　
4、 Preheating device
Preheating the cable core is necessary for both insulation extrusion and sheath extrusion. For the insulation layer, especially the thin layer insulation, the presence of air holes cannot be allowed. The wire core can be thoroughly cleaned of water and oil stains on the surface by high-temperature preheating before extrusion. For the extrusion of the sheath, its main function is to dry the cable core and prevent the possibility of air holes in the sheath due to the effect of moisture (or moisture surrounding the cushion). Preheating can also prevent the residual internal pressure of the plastic during extrusion due to sudden cooling. During the extrusion process, preheating can eliminate the significant temperature difference formed when the cold line enters the high-temperature machine head and comes into contact with the plastic at the mold mouth, avoiding the fluctuation of plastic temperature and resulting in the fluctuation of extrusion pressure, thereby stabilizing the extrusion quantity and ensuring the extrusion quality. Electric heating wire core preheating devices are used in the extrusion molding unit, which requires sufficient capacity and ensures rapid heating to achieve high efficiency in wire core preheating and cable core drying. The preheating temperature is limited by the discharge speed and is generally similar to the temperature of the machine head.
　
5、 Cooling after extrusion molding:
The formed plastic extrusion layer should be immediately cooled and shaped after leaving the machine head, otherwise it will deform under the action of gravity. The cooling method usually adopts water cooling and is divided into rapid cooling and slow cooling according to different water temperatures. Quenching refers to direct cooling with cold water. Quenching is beneficial for shaping the plastic extrusion layer, but for crystalline polymers, due to sudden heat cooling, residual internal stress can easily occur within the extrusion layer organization, leading to cracking during use. Generally, PVC plastic layers are subjected to quenching. Slow cooling is to reduce the internal stress of the product by placing water at different temperatures in sections in the cooling water tank, gradually cooling and shaping the product. The extrusion of PE, XLPE, and PP is carried out using slow cooling, which involves three stages of cooling: hot water, warm water, and cold water.
　
4、 Precautions for using cooling water for screws
(1) The cooling water flow rate of the screw should not be too large. Touch it with your hand to feel the water warm;
(2) When using screw cooling water, close attention should be paid to the changes in the outer diameter in order to control the cooling water;
(3) When shutting down, water should be cut off to prevent equipment accidents;
(4) The situation of screw cooling water should be clarified during the shift handover.
　
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