In a general temperature control system, a control temperature point needs to be set according to the use requirements, that is, the set temperature. The temperature control system can automatically control the temperature within this temperature range.

In the technology of the plastic extrusion industry, there are basically two ideas about the process temperature setting and control of the conical twin-screw extruder: one is the low-temperature process, the temperature setting is roughly around 165¡æ~175¡æ; the other is the normal temperature process. The temperature setting is roughly around 175¡æ~185¡æ; in terms of the temperature setting trend, there are saddle-type processes with high, medium and low in the front and high in the back, and there are also step-type processes that gradually increase from front to back.

Optimization of plastic extrusion process temperature

Setting of process temperature

Specific settings of various process temperature indicators for extrusion are as follows:

Feeding section: 185¡æ, depending on the shear performance of the extruder and the size of the extrusion volume, ensure that the displayed temperature is £¾180¡æ;

Compression section: 180¡æ;

Melting section: 180¡æ:

Measuring section: 170¡æ~180¡æ, depending on the shear performance of the extruder and the size of the extrusion volume, ensure that the displayed temperature is ¡Ü185¡æ. If necessary, the screw temperature, feeding speed and other methods can be used to adjust them respectively;

Head temperature: 185¡æ:

Die temperature: 190¡æ~200¡æ, depending on the cross-section molding and wall thickness of the profile, make corresponding adjustments.

Optimization mechanism of process temperature

According to the specific functions of each heating section, plastic extrusion can be roughly divided into three areas: heating, constant temperature and insulation. Heating and constant temperature are mainly in the extruder, which is divided into two relatively independent and interrelated parts with the exhaust hole as the boundary. The insulation zone process is composed of the confluence core, die and other parts. There are two heat sources for plastic extrusion, one is the external heat provided by the electric heater, and the other is the internal heat generated by the shearing, calendering and friction of the twin screw. The two heat sources play different roles in different stages of extrusion. The temperature control device controls only the external heat. The temperature of the die head and die without internal heat is generally easy to control; with internal heat, the shearing effect is strong, but the compression section that has not exceeded the material plasticization demand and the melting section that mainly serves the exhaust are relatively stable and easier to control.

The shear is relatively weak and mainly relies on external heating, but the feeding section (extruder with low external heating power configuration) where external heating is difficult to meet the material plasticization demand and the metering section where the shear heat has exceeded the material plasticization demand are often not controlled by the temperature control device. Therefore, the feeding section, especially the metering section, is the focus and difficulty of temperature control. The main body of extrusion control is the material temperature, and the set temperature is only a means. The displayed temperature has different corresponding relationships with the material temperature under different working conditions, and is the basis and benchmark for setting the temperature.

1. Feeding section temperature

The displayed temperature of the feeding section is only the temperature transmitted to the barrel by the electric heater, not the material temperature. The material temperature is often much lower than the displayed temperature. When the material just enters the extruder through the feeding screw, the temperature is only about 30¡ã, and the shear heat generated by the screw is also far from the plasticizing temperature. At the same time, the material will pass through the exhaust hole through the compression section. The material needs to complete the transformation process from the glass state to the viscous flow state in the heating area. It is required to be basically "orange peel-like", without powdery substances, and tightly wrapped on the surface of the screw groove, so that it will not be vacuumed out of the exhaust hole or blocked. Therefore, the function of the feeding section is to focus on external heating, and the set temperature should be as high as possible so that the electric heating ring can provide sufficient external heat to the material. At this time, the electric heater is turned on and off frequently, and even works without stopping. Since the material enters the feeding section, there is still some time before it is extruded from the die. In order to prevent the material from "bridging" at the feeding port or "sticking to the wall" in the machine, the set temperature should not be too high, and the display temperature should be above 180¡æ. Although the setting temperature of the feeding section is lower, such as the temperature is set to about 170¡æ, profiles with internal quality standards can also be produced. However, due to the relatively small amount of external heat supplied, excessive reliance on shear heat to increase the melt temperature will increase the wear of the barrel and affect the life of the extruder, which is not worth the loss.

Practice has proved that under the premise of unchanged feeding, extrusion speed and metering section setting temperature, increasing the setting temperature of the feeding section can effectively reduce the temperature difference between the metering section display temperature and the setting temperature, which fully shows that the feeding section temperature plays a role in adjusting shear heat to a certain extent.

2. Compression section temperature

When the material enters the compression section with a large shearing effect, it heats up faster under the action of the screw shear force. Setting a higher temperature helps to reduce the viscosity of the material and accelerate fluidity. Like the feeding section, it can reduce the harm of shear heat.

3. Melting section temperature

The material in the melting section is basically melted. Due to the change in the screw groove volume (generally the compression ratio is less than 1), the melt pressure drops suddenly, which can fully play the role of constant temperature and exhaust. The set temperature is consistent with the compression section, which helps to prevent the melt from cooling down.

4. Metering section temperature

The metering section display temperature is not the material temperature. It is only the temperature of the material transferred to the barrel under the action of shear heat, and the material temperature is often higher than the displayed temperature. The purpose of setting the temperature is not to provide external heat, but mainly to stop external heating in time and use cooling to transfer excess heat to prevent material degradation. Therefore, the set temperature should not be too high, and the displayed temperature should be ¡Ü185¡æ. When the extrusion volume is too small and the displayed temperature is too low, the set temperature of the barrel and screw or the feeding speed can be increased in time according to the situation.

5. Confluence core temperature

The melt enters the die head and is completely in a molten state. It begins to change from a spiral motion with variable speed and pressure to a uniform linear motion, and establishes melt pressure through the die to make the temperature, viscosity and flow rate more uniform, making the final preparation for molding. Due to the change in the direction of movement, a certain amount of energy must be sacrificed to establish melt pressure, and at the same time, the internal heat generated by the shear action in this area no longer exists. Therefore, the temperature setting should be higher to slow down the heat loss of the material. There are great differences of opinion on the setting of the confluence core temperature in the industry. Some people advocate setting the confluence core temperature between 165¡æ and 175¡æ, believing that increasing the set temperature of the die head will lead to a decrease in the main engine power and the melt pressure of the parison, thereby affecting the physical and chemical properties of the extruded product. In fact, the amount of heat provided or output is not entirely determined by the set temperature, but is mainly related to the difference between the actual temperature of the heating object and the set temperature. When the set temperature is much higher than the material temperature, such as the material temperature in the feeding section, increasing the set temperature can provide a large amount of external heat to the material; when the set temperature is lower than the material temperature, it not only does not heat the material, but instead has a cooling effect.

As mentioned earlier, the actual temperature of the melt passing through the metering section is higher than the displayed temperature. If the displayed temperature is around 185¡ãC, then the material temperature is also roughly above 185¡ãC. The purpose of setting the temperature of the confluence core is not to heat, but to protect the heat of the melt from being lost due to the low temperature of the die head. At the same time, when the melt is extruded in the machine, the melt near the barrel will flow at a lower speed than the center of the melt due to friction with the inner wall of the barrel, resulting in the so-called "marginal" effect. Setting a higher temperature can effectively adjust the flow velocity of the melt cross section.

When the set temperature is lower than the actual temperature of the melt at the confluence core, the melt will not only not get external heat, but will be in a state of complete heat dissipation, and the flow rate of the surface melt will slow down, causing unbalanced flow with the core melt, which will affect the molding quality of the die extrusion product. Even in the part where the flow cross-section resistance of the die diverter cone is large, yellow lines appear due to material retention. Of course, raising the set temperature of the confluence core refers to the melt temperature of the metering section. If the set temperature of the confluence core is too high, the surface melt flows too fast, which will also make the cross-sectional flow rate unbalanced.

6. Die temperature

The die setting temperature is mainly for molding and adjusting the flow rate and wall thickness. As the melt enters the die, it has been transformed from a cylinder into a thin-walled melt in the shape of a profile under the guidance of the diverter cone. By relying on external heating, the temperature of the parison melt can also be evenly raised to the optimal plasticization area. Since the die temperature is directly related to the external molding quality of the profile, it is worth pointing out that when the profile is poorly plasticized, it is inappropriate to rely too much on increasing the die temperature to adjust. Due to excessively high temperature, the melt will be extruded from the die and expand unevenly.

7. Screw temperature

There are generally two settings for screw temperature. One is the screw self-temperature adjustment, which uses the heat pipe principle to implement balanced heat exchange inside the screw without external energy, but the heat exchange efficiency is low. At present, most of the conical twin-screw extruders in my country below 55 types are of this configuration; one is the external heating and cooling device, which adjusts the temperature of the screw heating zone and the constant temperature zone by adding external energy.

The setting of the screw temperature is mainly determined by the setting and displayed temperature difference between the heating zone and the constant temperature zone. Its main function is to assist the heating of the feeding section or cool the metering section to balance the temperature difference between the two. Judging from the problems existing in the industry's extruders, the latter is mainly used.

8. Process temperature automatic control mechanism

The reason why the extrusion temperature setting is required to be "saddle-shaped" is mainly to ensure that the material and melt temperature are "step-shaped", from low to high, always in a stable rising and balanced plasticization state, so as not to cause the material to fail to plasticize to the exhaust hole due to the setting temperature being too low in the heating area, and the material to leak out from the exhaust hole; in the constant temperature area, the setting temperature is too high, causing the material to degrade.

Some people in the industry set the setting temperature in a "step-shaped" way, which is obviously a misunderstanding. When the displayed temperature is in a controlled state, the external heat and the internal heat can be adjusted and balanced with each other. Under certain conditions of the set temperature, when the shearing effect is large and the internal heat is high, the external heating coil will automatically reduce the working time and heating amount, and provide air cooling (or oil cooling) from the outside and oil cooling from the inside to prevent material decomposition; when the shearing effect is small and the internal heat is low, the external heating coil will also automatically increase the working time, thereby automatically maintaining the balance between the heat supplied and the required heat. Increasing the set temperature will increase the external heat supply, while the viscosity of the material will decrease and the fluidity will increase, resulting in a decrease in shear heat; lowering the set temperature will reduce the external heat supply, while the viscosity of the material will increase and the fluidity will decrease, resulting in an increase in shear heat.

The energy provided by the extruder is always consistent with the set temperature. It does not change due to the strength of the extruder shear performance or the size of the extrusion volume. Under higher processing temperatures and lower shearing effects, the same degree of plasticization can be obtained as under lower processing temperatures and higher shearing effects. Therefore, regardless of the strength of the extruder shearing performance and the size of the extrusion volume, the setting of the extrusion process temperature should be basically the same and there should be no difference.

Overload extrusion, temperature uncontrolled state and countermeasures

The above new ideas are based on a premise, which is established under normal extrusion conditions and is based on the display temperature being in a controlled state. If the extrusion efficiency is not properly improved, the heat supplied by the feeding section will be difficult to meet the heat demand for material plasticization, the displayed temperature will be uncontrolled and often lower than the set temperature, the material will not be well plasticized at the exhaust hole, and some powder will be sucked away from the exhaust hole by the vacuum; the heat supplied by the metering section exceeds the heat demand required for the constant temperature of the melt, the displayed temperature will be uncontrolled and often higher than the set temperature, resulting in partial overheating and decomposition of the extruded product. This phenomenon varies with the degree of improvement in extrusion efficiency. The higher the extrusion efficiency, the greater the temperature difference between the set temperature and the displayed temperature, and the more serious the adverse consequences.

This phenomenon is particularly prominent in extruders with low screw shear heat or external heating power configuration in the feeding section. When the displayed temperature is not controlled by the set temperature, the so-called process optimization is difficult to achieve practical results. The above phenomenon is a manifestation of the imbalance between the heat supplied by the extruder and the heat required for material plasticization. The temperature difference between the set temperature and the displayed temperature of the feeding section is a sign of the degree of lack of external heating or shear heat, and the temperature difference between the set temperature and the displayed temperature of the metering section is a sign of the degree of excess shear heat. At present, two measures have been taken for the heat matching of the feeding section of the extruders produced in my country: one is to increase the power of the heating coil, such as the power configuration of the feeding section of the 65/132 conical twin-screw extruder has reached 9kW; the second is to reform the screw thread structure, and set a single-head thread after the double-head thread in the feeding section or compression section to effectively improve the compression ratio of the screw groove. The phenomenon of insufficient heat supply in the feeding section of the extruder has been significantly improved compared with the past. However, the excess shear heat in the metering section still restricts the improvement of extrusion efficiency.

In addition to being restricted by the screw structure, the shear heat is also directly affected by the ratio of the feeding speed to the extrusion speed. When the set temperature of the metering section is lowered, the heating coil has stopped heating, the cooling device does not stop working, and the temperature control is invalid, according to the needs, corresponding measures can be taken according to the following procedures to effectively reduce the displayed temperature of the metering section: First, lower the screw set temperature and use oil cooling to transfer the excess shear heat of the metering section. However, lowering the screw set temperature will also reduce the material temperature in the feeding section. When the heating coil of the extruder feeding section is equipped with a low power, the screw setting temperature should be lowered, and the need for temperature control in the feeding section should be taken into account, and the two should not be neglected; second, the feeding speed should be appropriately reduced to reduce shear heat. Under the condition of a certain extrusion speed, increasing or reducing the feeding speed is an effective means to adjust the shear heat. However, reducing the feeding speed will also reduce the material temperature in the feeding section, and the shear heat needs of the feeding section and the metering section materials are contradictory. Like the screw temperature setting, when the heating coil of the extruder feeding section is equipped with a low power, the feeding speed should be reduced, and the need for temperature control in the feeding section should also be taken into account. At the same time, excessively reducing the feeding speed will cause the melt in the metering section to fail to completely wrap the screw groove, and will also increase the wear of the screw and the barrel, resulting in the so-called "sweeping" symptom; third, appropriately reduce the ratio of extrusion speed to feeding speed. Feeding speed and extrusion speed are both concepts related to extrusion volume, and each has different functions. The feeding speed should be coordinated with the external heat supply to adjust the shear heat and the degree of material plasticization; the extrusion speed should be coordinated with the traction speed to adjust the extrusion volume and wall thickness. When the feeding speed is used to adjust the metering section display temperature, and the feeding section display temperature cannot be taken into account, it is necessary to reduce the ratio of extrusion speed to feeding speed. On the one hand, it reduces the shear heat of the melt in the metering section, and on the other hand, it prolongs the residence time of the material in the feeding section to facilitate plasticization.

It should be pointed out that lowering the set temperature of the metering section is mainly to control the shear heat and prevent material degradation, not the lower the display temperature, the better. When the heating coil has stopped heating and the cooling device has not stopped working, it is meaningless to set the temperature lower. Although the metering section display temperature is higher than the set temperature, it is also within the normal range within the 185¡æ range and no adjustment is necessary. When the extruder produces small-sized profiles, the extrusion volume is low, resulting in too little shear heat, and the metering section display temperature is lower than 180¡æ, it is also necessary to increase the barrel and screw set temperature or feeding speed in time according to the situation to keep the material temperature always running in the ideal temperature area.

Under the condition that the compression ratio of each section of the extruder screw is allowed, the shear heat can only be exerted by increasing the feeding speed.