Expert summary: setting and optimization of PVC extrusion process temperature

October 10, 2019

Expert summary: setting and optimization of PVC extrusion process temperature
 

This article is divided into the following eight points:
introduction
Process temperature optimized benchmark
Process temperature setting
Optimization mechanism of process temperature
Overload extrusion, temperature uncontrolled state and countermeasures
Fault status and countermeasures of equipment and electrical appliances
Influencing factors and countermeasures of raw materials, formula and kneading
to sum up

 

First, the introduction
In the technical literature related to PVC extrusion in the plastic extrusion industry, there are basically two ideas for the process temperature setting and control of the conical twin-screw extruder:
One is a low temperature process, and the temperature is set at about 165 ° C - 175 ° C;
One is a normal temperature process, and the temperature is set at about 175 ° C - 185 ° C;
In the trend of temperature setting, there is a “saddle type” process with high, medium, low and high heights (I agree with the “saddle type” process mode, the company's production also adopts this process mode), and it also gradually rises from front to back. High "ladder" process mode. In the company's different product series, there is also an ultra-high temperature process with the screw barrel temperature above 200 °C (this is the case for our company's threading pipe production), and the ultra-low temperature process with a screw barrel temperature of about 150 °C (some screws of our company) , the screw is near the end of the equipment). It cannot be said that these processes can produce products with high quality standards, but some completely different processes can produce products of the same quality, but it is an indisputable fact.

 

Therefore, I feel that it is necessary to carry out a comprehensive, systematic analysis and research on the merits of these process temperatures, so that from the table and the inside, to the false and true, from a variety of different process temperature parameters, a set of scientific and reasonable can really guide the production. Process temperature setting method.
In fact, after years of development in China's extruder manufacturing industry, both the screw structure pressure configuration and the external heating coil power configuration provide conditions for the good and balanced plasticization of PVC-U plastics.

 

Practice has proved: It can completely solve the problem that most people think that "the extrusion process should be compatible with the extruder", regardless of the specifications, shear performance of the conical twin-screw extruder, the amount of extrusion, Under the temperature controllable state, it can basically achieve the extrusion under the same process temperature condition by optimizing the process temperature, laying a good foundation for the company's three extrusion process (uniform equipment, uniform mold, uniform formulation). Thereby achieving improved product quality, reducing extruder wear, extending its working life, further reducing formulation costs, facilitating management, timely detection and effective handling of multiple levels of targets such as failures. The basic idea of ​​this optimized process temperature and the production process practice are summarized below. Due to my limited level, mistakes and embarrassment are inevitable. 


Second, the benchmark of process temperature optimization
 

To optimize the extrusion process temperature, you should first understand and master the benchmark for setting the process temperature. Mass production practices have proven that the following three conditions can be used as benchmarks:
2.1 Thermal stability of PVC resin: PVC resin is a heat sensitive polymer. The pure PVC resin begins to degrade at 100 °C, and the degradation accelerates at 150 °C. In turn, PVC begins to transform from a vitrified state to a viscous state at 160 °C. Therefore, pure PVC resin cannot be directly processed at all, and it is necessary to improve the thermal stability of the resin by adding a heat stabilizer. The stabilizer test for general PVC resin was carried out at 180 ° C, 30 min and 200 ° C for 20 min. Therefore, the plasticizing temperature and time of the PVC resin should not exceed this range.

 

2.2 Plasticity: The degree of plasticization, also known as the degree of gelation. In PVC plastics, the degree of plasticization is a sign of the degree of crystallization of the product and the degree of fusion of PVC primary particles. A large number of research and test data show that the unmodified PVC-U plasticity is 60%-65%, that is, the primary particles in the product have not been completely plasticized, and most of the fusion, the impact performance is the strongest. When the degree of plasticization is 60%, the breaking strength is the highest, and the degree of plasticization is 65%, and the elongation at break is the largest. When the temperature of the melt is below 150 ° C, the degree of plasticization is zero; when the melt temperature is below 190 ° C, the primary particles in the product are clearly visible, the degree of plasticization is below 45%; when the melt temperature is around 200 ° C, Most of the primary particle boundaries disappeared in the product, only a few primary particles were visible, and the degree of plasticization was 70%. When the melt temperature was above 200 °C, the primary particles of the product were completely plasticized, and the degree of plasticization was over 80%. Related links:|Technology|PVC product production process--plasticity
 

2.3 Processing temperature of CPE blending system: All PVC products of our company are added with CPE blending toughening modification, while the temperature band of CPE impact modifier is relatively narrow. A large number of tests prove that PVC modified by CPE The microstructures formed at 190 ° C and 200 ° C have very different microscopic morphology. At 190 °C, the modifier particles form a network structure of PVC primary particles, which can obtain good impact and toughening effect. At 200 °C, the PVC primary particles are completely melted, and the network structure disappears into spheres, which are dispersed in PVC. In the resin matrix, the impact resistance is greatly reduced. It can be seen from the above discussion that the processing conditions of PVC modified by CPE blending are more demanding. At the same time, PVC plastics are "undetermined" polymers, and PVC degradation is not only temperature dependent but also time dependent. The higher the temperature, the shorter the degradation time, and the lower the temperature, the longer the degradation time. The temperature of the barrel melt should be controlled between 180 ° C and 185 ° C (note here the melt temperature, not the barrel display temperature, the two are very different) to prevent the high temperature melt The residence time in the machine is too long and decomposition occurs. The remaining melt temperature difference is completed by the die, and the melt temperature of the die section should be controlled from 190 ° C to 200 ° C or even higher, so that the melt can reach the optimum degree of plasticization and immediately extrude from the die. In order to achieve the formation from the best plasticization state, and not to be decomposed due to the high temperature time.


 

Third, the process temperature setting
The specific temperature settings of each section of the extruder barrel and the combined core and mold are as follows:

 

3.1 Feeding section: 185 ° C - 195 ° C, depending on the shearing performance of the extruder and the amount of extrusion, to ensure that the display temperature is at least > 185 ° C; the larger the amount of extrusion, the higher the temperature required, so that the powder It can be quickly heated and vitrified to form small pieces. Our company's extrusion production of threading pipe and drain pipe are high-speed extrusion, especially the threading pipe. In the materials I have consulted, we have not seen such rapid production, so our company's threading pipe and drainage pipe extrusion The temperature of the feeding section of the equipment is extremely high, generally above 195 °C, and the individual machines even reach 210 °C-220 °C. The actual internal material temperature is only between 100 °C and 130 °C, which is close to the end of the feeding section. The glassy state requires a temperature of about 150 °C.
 

3.2 Compression section: generally 180 ° C; can also be properly increased according to the actual extrusion speed, our company's threading pipe production in this section is more than 180 ° C, reaching 190 ° C -195 ° C drainage pipe production is roughly 180 ° C.
 

3.3 Melting section: generally at 180 ° C; can also be properly increased according to the actual extrusion speed, our company's threading pipe

production in this section is more than 180 ° C, reaching 190 ° C -195 ° C drainage pipe production is roughly 180 ° C.
 

3.4 Metering section: The temperature of the metering section is very important during the over-extrusion process, and its importance in some sense even exceeds the feed section. The temperature should generally be set at 170 ° C -180 ° C, depending on the shear performance of the extruder and the amount of extrusion, to ensure that the display temperature ≤ 185 ° C. Because the internal shear heat of the metering section is very large, it is easy to cause the melt to heat up, and too high melt temperature will accelerate the decomposition of PVC to form yellowing, discoloration lines, foaming, etc., which affect the quality of the product. Therefore, if necessary, the screw temperature and the feed rate can be separately adjusted.
 

3.5 Extrusion die section temperature: The extrusion die temperature setting is relatively simple, mainly to prevent the melt from cooling in the mold body, generally set at about 185 ° C, during the production process of most products, the temperature is set at This area is no problem, individual products (corrugated tubes) are higher than this, reaching 190 °C.
 

3.6 die section temperature: 190 ° C -210 ° C, depending on the surface brightness and extrusion pressure of the product extrusion. Generally speaking, increasing the temperature of the die can appropriately increase the brightness of the surface of the product, and can also reduce the internal pressure of the extruder to a certain extent, the internal pressure of the extruder is lowered, and the frictional shear force is naturally lowered. In other words, by appropriately increasing the die temperature, the frictional heat of the inside of the extruder can be reduced by a small amount (when the internal friction shear heat is excessive), and vice versa. I have been engaged in PVC extrusion production for 20 years, and there are already some experienced extrusion operation masters. Most of them have already known the temperature regulation through the die to meet the production process required by the product.


Fourth, the optimization mechanism of process temperature
 

According to the specific functions of each heating section, the PVC-U extrusion production is carried out by a conical twin-screw extruder. The whole process can be roughly divided into three areas: heating, constant temperature and heat preservation. The heating and constant temperature are mainly in the extruder, and are divided into two relatively independent and interrelated parts by the vent hole. The heat preservation zone process is composed of a merging core, an extrusion phantom and an extrusion die.
Here we should first understand that there are two kinds of heat sources in the PVC-U extrusion process, one is the external heat provided by the electric heater, and the other is the shearing, calendering and friction of the PVC-U material by the twin screw, and The internal heat generated by the friction between the PVC and the U itself. 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 extruder head and the die part without internal heat is generally easy to control (some parameters are designed to be super-normal extrusion molds, and internal heat is also generated); there is internal heat, and the shearing effect is strong, but it has not exceeded the material. The compression section of the demand and the melting section, which is mainly the exhaust service, are relatively stable and easy to control. The shearing is relatively weak, mainly relying on external heating, but the external heating is difficult to meet the materialization requirements of the material feeding section (extruding machine with lower external heating power configuration is particularly prominent); the shearing heat has exceeded the measurement of plasticizing demand of materials. Segments are often not controlled by temperature control devices.
Therefore, in the temperature control of the entire extrusion process, the feeding section and the metering section are the key points and difficulties of temperature control. The extrusion control body is the temperature of the material, not the temperature of the barrel and mold. Setting the temperature is only a means, and the display temperature is different under different working conditions, and the material temperature has different correspondence (the material temperature of the feeding section is lower than the display temperature, the temperature of the measuring section is higher than the display temperature), and the thermoelectricity is added. The relationship between the installation position and the display temperature can only partially reflect the material temperature, but only the basis and benchmark for setting the temperature. The following is a detailed description of the temperature setting mechanism and key points of each segment.

 

4.1 Feeding section temperature: The feeding section is the electric heater that transfers heat to the screw barrel. The displayed temperature is the temperature of the section of the barrel, not the material temperature. The material temperature is often much lower than the display temperature. When the material just enters the extruder through the feeding screw, the temperature is only about 30 °C -40 °C, and the temperature rise of the shearing tropical material caused by the screw is also far from the plasticizing (vitrification) temperature. At the same time, the material passes through the venting section and will pass through the venting hole, and the material needs to be converted from the glassy state to the viscous flow state in the heating zone. The requirement is basically “orange peel”, no powdery substance exists, and tightly coated. On the surface of the groove, the vacuum is not taken out from the vent hole or the vent hole is blocked. Therefore, the function of the feeding section is to heat the outside, and the set temperature should be as high as possible so that the electric heating ring provides sufficient material for the material. heat. At this time, the electric heater is turned on and off more frequently, and does not even stop working. Since the material enters the feeding section, the distance is still extruded from the die for a certain period of time. In addition, in order to prevent the material from “building the bridge” at the feeding port or “sticking the wall” in the machine, the set temperature should not be too high, so it should be displayed. The temperature is preferably 185 ° C or higher. Although the feed section is set to a lower temperature, for example, the temperature is set to about 170 ° C or even lower, it can produce products with inherent quality standards. However, because the external heat supplied is relatively small, too much dependence on the shear heat to raise the melt temperature, the wear of the screw barrel will increase, which will affect the service life of the screw barrel of the extruder, which is not worth the loss. Through our long-term observation in the maintenance of extrusion equipment, it is found that after only one or two years (some even less than one year), the screw barrel will be seriously worn, and most of the wear will be concentrated on the double-headed screw with a large compression ratio. After the smashing, the first single-head snail or the second single-head snail part and the wide working area such as the metering section, the maximum wear amount is 2mm~3mm. At this time, the yellow line will appear in the extrusion production (due to material reflow) In the high temperature state, the residence time is too long. If the gap is adjusted, the screw and the screw will be partially sharply rubbed. The black line of the product and the equipment will make an abnormal sound, and it will not work properly. Only the screw and the screw have to be replaced. . Although the occurrence of this phenomenon is closely related to the improper use of steel and heat treatment methods used by manufacturers, the important reason is that the extrusion temperature is too low, resulting in strong shearing at these parts. Increased wear and tear. The higher set temperature of the feed section not only facilitates material melting, but also makes full use of external heat to reduce the wear of the extruder by shearing. A large number of practices have proved that under the premise of constant temperature of feed, extrusion speed and metering section, appropriately increasing the set temperature of the feed section can effectively reduce the temperature difference between the display temperature of the metering section and the set temperature, fully illustrating The feed section temperature plays a role in adjusting the shear heat to some extent.
 

4.2 Compressed section temperature: The material enters the compression section with large shearing effect, and the temperature rises faster under the action of the shearing force of the screw. The set temperature is higher, which helps to reduce the viscosity of the material and accelerate the fluidity. As with the feed section, the heat of shearing can be reduced.
 

4.3 Melting section temperature: The material of the melting section is basically melted. Due to the change of the volume of the screw groove (the general compression ratio is less than 1), the melt pressure is suddenly reduced, and the function of sufficient constant temperature and exhaust can be exerted. The set temperature and the compression section are consistent or slightly higher, which helps to prevent the melt from cooling down, and the melt temperature also tends to decrease as the melt pressure decreases.
 

4.4 Metering section temperature: The temperature displayed in the metering section is not the material temperature. It is only the temperature at which the material is transferred to the barrel under shear heat. The material temperature is often higher than the display temperature. The purpose of setting the temperature is not to provide external heat, but mainly to stop external heating in time, and to use the barrel cooling device and appropriate adjustment of the screw oil temperature to transfer excess heat to prevent material decomposition. Some of the cooling devices with severely worn barrels will be in a long-term working state soon after the equipment is turned on to barely keep the temperature from rising. Therefore, the set temperature should not be too high, so that the temperature is ≤185 °C. When the extrusion amount is too small and the display temperature is too low, the screw barrel, the screw set temperature or the feed rate may be increased as appropriate to increase the shear.

 

4.5 Combined core and extrusion die temperature: The melt enters the merged core, which is completely in a melt state, and begins to change from a spiral motion of variable speed and variable pressure to a uniform linear motion, and establishes melt pressure through the die to make the temperature, The viscosity and flow rate are more uniform, making final preparation for product molding. Due to the change of the direction of motion, the establishment of melt pressure is at the expense of a certain amount of energy, and the internal heat generated by shearing in this region no longer exists. Therefore, the temperature setting should be higher to slow the heat loss of the material. From the large number of industry literatures I have consulted, the opinions on the temperature setting of the combined cores are quite different in the industry. Some people advocate that the temperature of the combined core should be set between 165 °C and 175 °C. Will cause the host power and parison melt pressure to decrease, thus affecting the physical and chemical properties of the extruded product. It is a misunderstanding by me to combine actual production analysis and test, because the supply or output of heat is not completely determined by the set temperature, mainly related to the difference between the actual temperature and the set temperature of the heating object. When the set temperature is much larger than the material temperature, such as the material temperature of the feed 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, not only does the material not heat, but instead The role of cooling. As already mentioned, the actual temperature of the melt passing through the metering section is higher than the display temperature. If the display temperature is around 185 °C, the material temperature is also above 190 °C. The purpose of setting the temperature of the merging core and the mold body is not to heat, but to protect the heat of the melt from being lost due to the temperature of the merging core and the mold body being too low. At the same time, when the melt is extruded in the machine, the melt near the barrel is rubbed against the inner wall of the barrel, and the flow speed is lower than the center speed of the melt, causing a so-called "marginal" effect. Therefore, the set temperature is higher, but the flow velocity of the melt section can be effectively adjusted. When the set temperature is lower than the actual melt temperature of the core part, the melt will not get external heat, but will be in a completely heat-dissipating state, the surface melt flow speed will be slowed, and the core melt will be uneven. Flow will affect the molding quality of the die extrusion products. Even in the part where the resistance of the flow cross section of the mold is large, the yellow line appears due to the retention of the material. Of course, increasing the combined core set temperature is for the metering section melt temperature. If the convection core is set too high and the surface melt flows too fast, the cross-section flow velocity will be unbalanced. Others think (most of my company's operators think so too): the temperature of the combined co