Congruent relationship between the different types of extruders and parameters of temperature and melt pressure.
The shear performance of the extruder is determined by the screw structure of the extruder. However, the quality of the extrusion and the efficiency of the extrusion are also dependent on the extrusion process and the shear performance of the extruder. Otherwise, the low-shear extruder is extruded at an excessively high extrusion speed, and it is difficult to produce an extruded high-quality profile product, and the high-shear extruder operates at an excessively low extrusion speed, and it is difficult to effectively exert the extrusion efficiency. Different shear performance extruders have a certain range of process control, which is limited. The extrusion process route advocated by the industry is “saddle type”, that is, the set temperature of the heating zone is higher, the set temperature of the constant temperature zone is lower, and the set temperature of the heat preservation zone is higher. However, different shear performance extruders operate at different extrusion speeds, and the "saddle" "saddle" and "seat" are completely different.
1, the heating path and temperature measurement principle of the extruder.
In the extrusion of plastic profiles, the extrusion efficiency of the extruder with different shear performance should be maximized, and the heat required for the screw heating zone (feeding section, compression section) and constant temperature zone (melting section, metering section) should be established. The balance of the heat supplied is the key. According to the shear performance characteristics of the extruder, different shear performance extruders, extrusion of different specifications of plastic profiles, should adopt different extrusion processes to meet the needs of product quality performance.
Plastic profile extrusion, the material is converted from glass to molten state. There are two kinds of heat sources. One is the external heating provided by the electric heater, and the other is the rolling, friction and shearing of the material during the rotation of the screw.
the heat. In the start-up production, the melting of the material is mainly based on heating. In the normal production stage, the melting of the material is mainly caused by the internal heat generated by the screw rolling, friction and shearing of the material. According to the relevant data, the proportion of heat supplied by the extruder in the extrusion of the profile is about 65%.
The external heating temperature control system mainly implements temperature setting and display through electrical meter components. When the display temperature exceeds the set temperature index parameter, the heating coil is immediately powered off, the heating is stopped, and forced cooling is performed by the screw oil cooling device and the screw air cooling device; when the display temperature does not reach the set temperature index parameter, The heating coil continues to work uninterrupted.
Since the internal heat is mainly restricted by the screw characteristics, feeding and extrusion speed of the extruder, it is not affected by the external heating temperature control system. When the extrusion speed of the low shear extruder is too high, even if the working frequency of the heating coil outside the feeding section and the compression section is increased, the intermittent time is very short, and the display temperature may not reach the set temperature; even the melting section and the metering The heating outside the segment stops working and starts the screw and barrel cooling device operation, indicating that the temperature may still be much higher than the set temperature.
At the same time, since the temperature measuring point (thermocouple) reflecting the display temperature is installed on the wall of the extruder barrel, there is a certain distance from the material in the barrel, and the meter shows that the temperature and the actual temperature of the material have a certain gradient under different working conditions. Different correspondence. Under normal circumstances, there is external heating in the feeding section and the compression section, and there is shear heat, which is two-way heating, indicating that the temperature is basically equal to the material temperature; when the melting section and the metering section material display temperature does not reach the set temperature, it is also two-way. heating. When the display temperature exceeds the set temperature, heat begins to transfer from the inside to the outside, which can be called reverse heat transfer, indicating that the temperature is lower than the material temperature. It can be seen that when the extrusion speed of the low shear extruder is high, the actual temperature of the molten section of the screw and the metering section is not only higher than the set temperature, but also higher than the display temperature.
Therefore, when the display temperature is operated in the set temperature region, the set temperature parameter is basically equivalent to the material temperature, which is the control target and basis for the plasticizing and melting of the material. When the display temperature deviates from the set temperature region, the display temperature can be assumed to be the material temperature, that is, instead of the set temperature, it becomes the control target and basis for the plasticizing and melting of the material. Setting the temperature is only a means of increasing or decreasing the external heating.
2, low shear extruder
For the low-shear extruder, since the compression of the feed section and the compression section is relatively small, the internal heat provided can not meet the plasticizing requirements of the glassy material, so the temperature setting of the feed section and the compression section should be higher, because the formula
different, roughly 190 ~ 200 ° C, although the display temperature is still low in the case of increasing the extrusion speed, but the purpose of increasing the set temperature is for the feeding section, the compression section electric heating coil, has been working uninterrupted, As long as the display temperature is in the range of 180-185 °C, the material is tightly wrapped in the screw, in a micro-melting state, and there is no venting phenomenon, which can be regarded as normal; the set temperature of the melting section and the metering section should be lower, due to different formulas. , about 165 ~ 175 ° C, although the display temperature is still high in the case of increasing the extrusion speed, but the purpose of lowering the set temperature is to stop the heating in the melting section, the metering section electric heating coil, and start the screw oil Cold and screw air cooling to cool the material, as long as the temperature is in the range of 180 ~ 185 ° C, there are no symptoms such as pores and pitting in the extruded parison section, which can be regarded as normal extrusion speed.
On the contrary, even if the temperature of the feeding section and the compression section is set higher, the heating coil will work continuously, the venting material is loose, and it is in the shape of bean curd. The screw is not wrapped, and the material is exposed from the venting hole of the screw barrel; The metering section sets the temperature lower and the electric heating coil has stopped working. The screw oil cooling and the barrel air cooling have been cooling the material, and the extruded parison has symptoms such as pores and pitting, which can be regarded as the extrusion speed.
At the limit, the extrusion speed or feed and extrusion speed should be reduced in time.
venting is a sign of poor plasticization of low shear extruders. But not the venting holes are caused by low shear extruders.
The main reasons for the venting of the venting hole are as follows:
1) The feed rate is too fast, and the increased shear heat is insufficient to balance the heat required for the increased feed amount, resulting in poor plastic plasticization;
2) The extrusion speed is too fast, the increased shear heat is insufficient to balance the loss of heat in the feed section and the compression section, and the plastic plasticization is poor;
3) When the formula adopts CPE impact modifier, the addition amount of processing aid is less, the friction property of the material is poor, and the plasticization is poor when it reaches the vent hole;
4) In the formula, the lubricant is excessive, the material moves out in the extruder too fast, and the plasticization is poor when it reaches the vent hole;
5) The axial gap between the screw and the barrel of the extruder is too large, the leakage is serious or the temperature of the screw feeding section and the compression section is too high, which causes the material to be “over-plasticized”. The material that has been converted into the melt has undergone the first compression section. After the secondary pressure peak, the stress is released to the vent hole, the volume expands, adheres to the end face of the screw edge, and the screw barrel of the exhaust section is scraped off the wall of the vent hole with the rotation of the screw, accumulating to a certain extent from the vent hole overflow.
The first two types of venting holes are related to the poor shear performance of the extruder. The third and fourth types of venting holes are mainly related to the formulation. The fifth type of venting holes are mainly related to the extruder wear. It is related to high shear performance. When judging the cause of venting of the venting hole, it should be considered comprehensively and should not be blindly determined. If the vent hole of the test formula occurs, the formula should be adjusted; if it belongs to the extruder wear, the gap between the screw and the barrel of the extruder should be adjusted; if the material is found to be “over plasticized” in the vent, the feed should be adjusted. Speed ratio; the first three types of vents generally show an increase in torque, and the latter two types of vents generally show a torque reduction.
3, high shear extruder
For the high shear extruder, because the feed section and the compression section are relatively high in compression, the internal heat provided can meet the melting requirements of the glassy material, so in general, the feed section and the compression section set the temperature ratio to the low shear. The cutting extruder set the temperature to be lower. It depends on the specific shape of the material in the venting hole of the barrel. Not only should we pay attention to whether the vent hole is material or not, but also pay attention to whether the vent material is “over-plasticized”; the same reason, because the melting section and the metering section have relatively low compression and less shear heat, the melting section and the measurement are generally under normal conditions. The set temperature is higher than the set temperature of the low shear extruder.
Also according to the type of material, and pay attention to whether the vent material is "over plasticized"; the same reason, because the melting section, the metering section compression is relatively low, the shear heat is less, so in general, the melting section, the metering section set temperature ratio The low shear extruder set the temperature higher. It is also determined by the shape of the parison at the exit of the die. Not only should there be any porosity in the cross section of the parison, but also whether the parison is “under plasticized”. The high shear extruder sets the temperature profile relatively flatter than the low shear extruder.
The most common problem in high-shear extruder extrusion is not the venting of the venting hole, but the shearing heat of the feed section and the compression section is too high, causing the material to have a "sticky wall" phenomenon in the vent hole. After starting for a while, the parison has a yellow line, which is difficult to produce normally. Therefore, the set temperature of the two sections should be reduced, the external heat supply should be reduced or the formula should be adjusted, and the lubricant should be increased or the processing aid should be reduced. If the effect is not significant or the formulation lubricant and processing aid change causes the quality of the profile to change, and at the same time use different shear performance extruder production, the same mixing, storage, and conveying system is impossible for high shear extrusion. The other ingredients are ordered separately, only the extrusion speed is reduced, and the operation is performed in the lower extrusion efficiency range. Therefore, it is also possible to remind the company to purchase a machine with the same or similar screw structure and performance when adding or updating the extruder.
In the extrusion process, the process of converting the material from the glass state to the molten state, in addition to the balance between the heat required for the plasticization of the material and the heat supplied, so that the material is ideally plasticized, the melt pressure is also very important. Control indicators. Due to the influence of the die resistance and the compression ratio of each section of the screw during the extrusion process, the material itself is not present at normal pressure. For different die, the compression ratio of each segment of the screw is basically constant and immutable. The compression ratio of each section of the screw is only to distribute and adjust the pressure of each section of the screw material, and it is impossible to increase or decrease the total pressure of the melt during the extrusion process. The total pressure adjustment is mainly carried out by a process such as extrusion speed, feed rate and extrusion speed ratio. Adjusting the extrusion speed, feed and extrusion speed ratio is not only an important measure to adjust the extrusion temperature, but also the main measure to adjust the melt pressure and extrusion efficiency.
Under the premise of the same extrusion speed, the feeding speed is increased or decreased, the volume of the screw material in the feeding section changes, and the volume of the material in the exhaust section remains unchanged, so the feeding section and the compression section change with the compression ratio, and the melting pressure follows Increase or decrease; increase or decrease the extrusion speed under the premise of the feed rate, and the screw material volume in the feed section also changes. The material volume of the exhaust section remains unchanged, so the feed section and the compression section change with the compression ratio, and the melt pressure thereof increases or decreases; the feed rate increases or decreases synchronously with the extrusion speed, because the screw material volume of the feed section is not Change, only the change in melt pressure caused by the increase or decrease in speed.
It can be seen that in the first two cases, the melt pressure changes synchronously with the change of the melt volume, and the change is large; in the latter case, the melt pressure changes little due to the constant melt volume. Therefore, when the temperature is adjusted, the low-shear extruder or the high-shear extruder should pay attention to the change of the material's melt pressure. The extruded parison should be guaranteed not to be "less plasticized" or "over-plasticized". Next, try to increase the melt pressure to ensure that the profile has a good finish, compactness, dimensional change rate and impact performance.
Regardless of whether the high-shear extruder or the low-shear extruder is operated for a period of time, the axial gap wear of the screw and the screw increases, and leakage occurs. When it is still within the allowable range, there is a problem of process and formulation adjustment. Generally, without affecting the physicochemical properties and appearance quality of the product, the method can be processed by appropriately reducing the extrusion speed, setting the temperature, or appropriately reducing the lubricant, increasing the filler, and reducing the melt fluidity. Of course, it is undoubtedly the most effective solution to stop the adjustment of the axial gap between the screw and the barrel and carry out the polishing process to maintain the original rheological state of the material.
Regarding the control of the head and the transition section and the die temperature, the difference in shear performance between the low-shear extruder and the high-shear extruder is mainly in the screw structure, and has little to do with the machine head, transition section and die. . At the same time, the set temperature of the machine head, transition section and die is only for the melt material to change the flow direction, adjust the material flow rate of the material section and improve the appearance of the product. Therefore, there is no big difference in temperature control between the two different shear performance extruders. It is mainly adjusted according to the shape and appearance color of the export parison.
The use of high shear or low shear extruders should be compatible with the extrusion profile specifications. The high shear extruder produces more heat and a larger amount of extrusion. If the extrusion profile is too small, the speed of the die and the die speed can be adjusted, and the pulling speed cannot be too high, so that the extrusion efficiency cannot be obtained. Effectively, it can not effectively use the thermal energy converted by the extruder's own mechanical energy to achieve the purpose of energy saving; low shear extruder produces less heat, and the extrusion amount is small. If the extrusion profile is too large, the extrusion speed When it is improved, the quality of the product is difficult to guarantee.
Therefore, when the extruder is equipped with a mold, the high-shear extruder should be equipped with a large-profile profile mold, and the low-shear extruder should be equipped with a small-profile profile mold. To ensure that the heat supplied is balanced with the required heat under similar process conditions.
Regardless of the high-shear or low-shear extruder extrusion production profile, the quality of the tested products meets the requirements of the standard indicators, the process parameters are determined, must be strictly enforced, generally do not change easily.
When testing the new formula, try to use the original process parameters as the reference system. According to the rheology of the material, especially the appearance quality such as the melt pressure and the dimensional change and smoothness of the product, adjust the process temperature or the processing aid and lubricant in the formula. the amount.
1). The shearing performance of the extruder is determined by factors such as the taper of the screw, the number of threads, the pitch, the width of the screw, the depth of the groove, and the helix angle. The screw taper, pitch, screw width and groove depth are mainly changed by the volume of the screw material to increase or decrease the thread shear area and shear heat; the spiral angle change is increased by increasing or decreasing the material stroke. Or reduce the heat of shearing.
The number of thread heads is increased. For example, the feeding section and the compression section are changed from single head to double head. Mainly, the screw material flow in the heating zone is changed from the series flow to the parallel flow, the flow speed is increased, and then the single-head thread is blocked through the exhaust section. Increase its compression ratio and shear heat.
Practice has proved that increasing the number of threads in the first two sections of the screw leads to a decrease in material stroke, but the increased shear heat is much greater than the heat lost by the material stroke, which is the most effective measure to improve the shear performance of the extruder;
2). The shear heat of the extruder is configured according to the characteristics, shape and heat required for plasticizing. Since the material of the feeding section and the compression section is substantially in a glass state, and the exhaust section is required to be “micro-melted” and tightly wrapped around the screw, it is not peeled off by the screw barrel, and the heat required is large; therefore, the heating zone is relatively compressed. Because the material of the melting section and the metering section is basically in a viscous flow state, but the locality is not very uniform, further constant temperature is needed, and the required heat is less, so the compression in the constant temperature zone is relatively small.
3). The extrusion process should control the heat demand according to different specifications of the profile extrusion to ensure the balance between the heat supplied by the extruder and the volume of the material of different specifications.
For low-shear extruders, the set temperature of the feed section and the compression section should ensure uninterrupted work of external heating, and the shear heat of the extruder is still insufficient to supplement. The set temperature of the melting section and the metering section should be such that the external heating stops at a proper time, and the display temperature is still in the control range of the set temperature. Otherwise the extrusion speed should be reduced.
For the high-shear extruder, the process control is basically the same except that the two sections are basically the same. The biggest difference is that the first two sections of the cutting heat may be insufficient or excessive, and should be adjusted according to the direction and amplitude of the display temperature deviating from the set temperature. If the display temperature is lower than the set temperature, increase the set temperature and keep the outer heating ring uninterrupted.