Expert summary: setting and optimization of PVC extrusion process temperature
This article is divided into the following eight points:
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 core is set higher, which will lead to the "mixing" of the core. In fact, there is a “paste” in the core, which is mainly due to the low finish of the inner wall of the confluent core, the unevenness of the joint or the transitional shoulder, so that the material is retained or the temperature is increased after the start-up, the connection bolt is not tightened, and the joint is broken. It is not caused by setting the temperature too high. In order to prevent the merging core paste, it is unreasonable to intentionally reduce the set temperature of the merging core. The too low condensing core temperature will cause the material close to the merging core wall to slow down due to cooling hardening, and the corners may not even flow. It causes the decomposition of the paste due to the long heating time, which has the opposite effect.
4.6 Die temperature: The die set temperature is mainly for shaping and adjusting the flow rate and surface brightness. As the melt enters the die, under the direction of the split cone, it has been transformed from a cylinder into a thin-walled melt in the shape required by the product. The body, by external heating, can also uniformly raise the parison melt temperature to the optimum plasticization area. Therefore, the die temperature is directly related to the external molding quality of the product. It is worth noting that when the extruded product is slightly plasticized, it can also be solved by appropriately increasing the temperature of the die. However, when the extruded product is severely plasticized, it is not appropriate to rely too much on increasing the temperature of the die to solve it. Because the surface temperature is too high, the melt is extruded from the die, and uneven expansion occurs. At the same time, the friction and shearing degree inside the equipment are changed due to the decrease of the melt pressure, which in turn worsens the plasticization of the material. It is still necessary to solve the temperature through the comprehensive adjustment of the various sections of the barrel.
4.7 Screw temperature: There are generally two kinds of devices for screw temperature control. One is self-temperature adjustment of the screw. The principle of convection of the heat pipe is used to implement the balanced exchange of heat inside the screw. No additional energy is required, but the heat exchange efficiency is low. At present, the conical twin-screw extruders below 55 in China are generally of such a configuration; one is an external heating and cooling device that adjusts the temperature of the heating zone and the constant temperature zone of the screw by applying energy. The setting of the screw temperature is mainly determined by the temperature difference between the setting of the heating zone and the constant temperature zone and the display. Its main function is to assist the heating of the feed section or to cool the metering section, balancing the temperature difference between the two (this type of device is used in our twin-screw). Judging from the current actual extruder situation and existing problems, it is mainly to play the role of the latter.
4.8 Process Temperature Control Mechanism Summary: The reason for the extrusion temperature setting is “saddle type”, which is mainly to ensure that the material and melt temperature are “stepped”, from low to high, always in a steady rise, balanced plasticization state. Therefore, because the material is set to a temperature too low in the heating zone, the material can not be plasticized when it reaches the vent hole, and the material is discharged from the vent hole; in the constant temperature region, the material is degraded due to the set temperature being too high. Some people in the industry literature advocate that setting the temperature to a "stepped" setting is obviously a misunderstanding. When the display 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 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, assist in providing air cooling (or oil cooling) from the outside, and provide oil cooling inside. Cooling is carried out to prevent the material from decomposing; when the shearing effect is small and the internal heat is low, the external hot ring will automatically increase the working time, thereby automatically maintaining the balance between the supplied heat and the required heat. Increasing the set temperature, while increasing the external heat supply, as the viscosity of the material decreases, the fluidity increases, resulting in a decrease in shear heat; conversely, if the set temperature is lowered, the external heat supply is reduced while the material is The viscosity increases and the fluidity decreases, resulting in an increase in shear heat. The energy supplied by the extruder is always in harmony with the set temperature. It is not due to the shear strength of the extruder and the amount of extrusion. At higher processing temperatures and lower shear, the same degree of plasticization can be obtained with lower processing temperatures and higher shear. Therefore, no matter the strength of the extruder shearing force, the amount of extrusion, the setting of the extrusion process temperature should be basically the same, and should not be too different. This is also my recent reference to a large number of industry literature and full consideration of material plasticization, taking into account how to use external heating, slowing down the shear heat, on the basis of ensuring the plasticized quality of extruded products, reducing the wear of the barrel and extending its work New ideas for life. Related links: Detailed explanation of PVC profile ingredients, hot and cold mixing technology
Fiive Overloading, uncontrolled temperature and countermeasures
The above new ideas are premised on the basis of normal extrusion conditions to show that the temperature is under control. If the extrusion efficiency is not properly improved, it will also occur that the heat supplied by the feed section is difficult to meet the heat demand for plasticizing the material. The display temperature is not controlled, and often lower than the set temperature, the material to the vent hole fails. Good plasticization, there are still some powders, which are evacuated from the venting holes by vacuum; at this time, most operators will increase the temperature in the rear section to make up for the damage of the compression section and the melting section. The main hazard is measurement. In the section, the total heat of the metering section originally exceeds the heat demand of the melt temperature, because the increase of the extrusion speed brings about a large increase in the shear friction heat of the metering section, so that the display temperature is not controlled, and tends to be higher than Setting the temperature causes the extruded product to locally overheat and decompose.
This phenomenon varies with the increase in extrusion efficiency, and the higher the extrusion efficiency, the greater the temperature difference between the set temperature and the display temperature, and the more serious the adverse consequences. This phenomenon is particularly prominent in extruders where the feed section has a low shear heat or an external heating power configuration. When the display temperature is not controlled by the set temperature, the so-called process optimization is difficult to achieve. The above phenomenon is a sign of the heat imbalance between the heat supplied by the extruder and the plasticization of the material. The temperature difference between the set temperature and the display temperature of the feed section is a sign of the degree of external heating or shear heat deficiency. The temperature difference between the set temperature and the display temperature of the metering section is a sign of the degree of excess heat of shearing.
At present, the extruders produced in China have taken two measures in the heat matching of the feeding section:
First, to increase the heating coil power, such as the 65/132 conical twin-screw extruder feed section power configuration has reached 9kW;
The second is to reform the screw thread structure, and set a single-head thread after the double-head thread of the feeding section or the compression section to effectively improve the compression ratio of the screw groove.
The lack of heat supply in the feed section of the extruder has changed significantly from the past. However, excessive shear heat in the metering section still restricts the improvement of extrusion efficiency. We have also carried out special research on this issue. Now all of our extruders are specially made with twin screws. The parameters are adjusted to increase the gap between the measuring section and the screw. To adapt to our ultra-high speed extrusion. In addition to the constraints of the screw structure, the shear heat is directly affected by the ratio of feed rate to 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 when the temperature control is invalid, the corresponding measures can be taken according to the following procedures to effectively reduce the display temperature of the metering section:
5.1 First, reduce the screw set temperature, reduce the screw set temperature, you can use oil cooling method to transfer excess shear heat in the metering section. However, lowering the screw set temperature will also reduce the material temperature of the feed section. When the power of the heating section of the extruder feeding section is low, the screw setting temperature is lowered, and the temperature of the feeding section should be taken into account, so please do not care about it.
5.2 The second is to reduce the feed rate appropriately, reduce the feed rate appropriately, and reduce the heat of shear (we call the reduction of torque). Increasing or decreasing the feed rate under certain conditions of the extruder screw speed is an effective means of adjusting the shear heat. However, reducing the feed rate will also reduce the material temperature of the feed section, and the need for the shear heat of the feed section and the metering section is contradictory. As with the screw temperature setting, when the power of the heating section of the extruder feed section is low, the feed rate is reduced, and the temperature control of the feed section is also required. At the same time, the feeding speed is excessively lowered, so that the melt of the metering section cannot completely wrap the groove, and the wear of the screw and the screw barrel is increased, and the so-called "broom" (and the excessive wear of the middle portion of the screw barrel) occurs.
5.3 Thirdly, the ratio of extrusion speed to feed rate is appropriately reduced. The feed rate and extrusion speed are the same concepts as the extrusion amount, and each has different functions. The feed rate should be coordinated with the external heat supply to adjust the shear heat size and the plasticization degree of the material; the extrusion speed should be coordinated with the traction speed to adjust the extrusion amount and wall thickness. When the feed rate is adjusted to adjust the temperature of the metering section, and the temperature of the feed section cannot be taken into consideration, it is necessary to reduce the ratio of the extrusion speed to the feed rate. On the one hand, the shear heat of the metering section melt is reduced, and on the other hand, the heat is extended. The residence time of the material in the feed section is facilitated by plasticization. It should be pointed out that lowering the set temperature of the metering section mainly controls the shearing heat to prevent material degradation, and the lower the temperature, the better. When the heating coil has stopped heating, the cooling device does not stop working, and in this case, the temperature is set lower, which is meaningless. When the metering section shows that the temperature is higher than the set temperature, it is also in the normal range in the 185 °C range and it is not necessary to adjust. When the extruder produces small-sized products, the extrusion amount is low, resulting in too little shear heat. When the metering section shows that the temperature is lower than 180 °C, it is necessary to increase the setting temperature or feed of the screw barrel and screw according to the situation. Speed to keep the material temperature always running in the ideal temperature range. Under the conditions of the compression ratio of each section of the extruder screw, the feed rate can be increased to exert the effect on the shear heat. On the contrary, there will be two different results: when the feeding amount is larger than the groove volume of the feeding section, there will be a phenomenon of “feeding” of the feeding hole, so that the raw material directly overflows from the feeding port and spills on the equipment table and the ground, both of which are polluted. The environment wastes raw materials; when the volume of the screw groove in the feeding section is larger than the volume of the melting section, the phenomenon of “blowing” of the vacuum hole occurs, thereby clogging the vacuum exhaust pipe, resulting in failure to exhaust, affecting product quality and failing to produce normally. Therefore, increasing the feed rate is also limited.
Six. Fault status and countermeasures of equipment and electrical appliances
In addition to correctly setting the temperature throughout the extrusion process, the key is to effectively control the display temperature (melt temperature). In addition to the overload operation of the extruder, when the equipment, electrical appliances, etc. fail, the display temperature will also be in an uncontrolled state, directly bringing about changes in the melt temperature.
6.1 Extrusion machine barrel and screw are seriously worn, the extruder barrel and screw are seriously worn, resulting in increased radial clearance, which causes the material to flow from the high pressure area to the low pressure area during the extrusion process. "positive flow" or "countercurrent" phenomenon. According to the analysis of the extruder with the screw structure of 2-2-l-3-3, it is known that when the material is moved in parallel from the double-headed groove of the feeding section to the first single-headed groove, the series motion is started, and the pressure is started. Then, the single-headed groove is moved in series to the double-headed groove, and the parallel movement begins. The pressure drops suddenly. When the single-headed groove is once again started to move in series, the pressure rises. When the first and second single-head bolts and the corresponding parts of the barrel are worn under shearing, some materials may leak from the single-headed groove to the front double-headed groove, that is, reverse flow may occur, or The back of the three head slots leaks, that is, the forward flow occurs; when the melt flows from the larger three slots of the melting section to the smaller volume of the three heads of the metering section, due to the screw of the metering section and the screw of the corresponding part, Wear under shearing, part of the melt may leak from the metering section screw groove to the melting section screw groove, that is, reverse flow occurs. Irregular flow of material or melt, especially reverse flow, results in prolonged residence time in the machine, "over plasticization", local degradation, and "yellow line" along the axial direction of the product. Therefore, at this time, some experienced operators have to reduce the set temperature, increase the viscosity of the material, and reduce the countercurrent method, and barely maintain the continuous extrusion production. In fact, this phenomenon is also common in the industry category, the so-called "ultra-low temperature process", the original reason is derived from this. Because the melt temperature is too low, the plasticization is not balanced, and the quality of the extruded product is difficult to be effectively guaranteed.
6.2 The screw processing and assembly of the extruder are improper, resulting in the minimum clearance of the two screws in the axial direction is small. The axial design of the two-screw axial direction of the extruder is generally more than 2mm. However, due to the machining deviation, the actual serial movement of many screws is only about 1mm, that is, the axial minimum clearance of each functional section of the screw can only guarantee 0.5mm. . If it is not carefully controlled and adjusted during the assembly process, the minimum axial clearance of a functional segment may be around 0.2 mm, or even smaller or direct collisions may cause a fight. The extruder works for a period of time. If the thrust bearing wears, the axial movement of the screw will also occur, and the axial clearance will change. This is because the degree of wear of the two-displacement thrust bearing cannot be exactly the same. Local overheating occurs during the extrusion process. Some so-called masters generally use methods to increase the set temperature, reduce the viscosity of the material, and enhance the fluidity of the material, and barely maintain production. The so-called "ultra-high temperature process", the original reason is also derived from this. Because the temperature is too high, it not only affects the quality and color of the inside and outside of the product, but also causes the precipitation of hydrogen chloride due to the partial decomposition of the material, and the reaction of the ultramarine (referring to the products with ultramarine blue, our profile adds ultramarine), resulting in lead contamination of the product. Discoloration. At the same time, hydrogen chloride has super strong water absorption, and combines with water to form hydrochloric acid, which has strong corrosive effect on equipment and mold. Every time we remove the black paste block from the mold that has been dismantled by the paste, after the surface is placed for a while, something like water drops will appear on the surface. In fact, it is the small hydrochloric acid formed by hydrogen chloride absorbing the moisture in the air.
6.3 Electrical instrument failure, causing the display temperature to be out of control, there are roughly the following situations:
a) Thermocouples: Thermocouple failures can be roughly divided into two types. The first one is not installed in place, or there is a slight short circuit in the mounting hole and the thermocouple line. The temperature of the barrel cannot be transmitted as it is. It often shows that the temperature is lower than the set temperature and causes uninterrupted heating, so that the actual temperature of the material is too high or even paste. material. The second type is the thermocouple open circuit (open circuit). At this time, the display temperature will reach the full scale or directly display the break, causing the heating controller to stop outputting the heating command. The heater will stop heating because the contactor will be disconnected. Slow materials will not be able to continue production due to no external heat heating (the occurrence of broken couples is particularly evident in the screw feed section and the mold and the combined core).
b) The electric heater coil part or the connection with the wire is burnt due to poor contact, the actual power of the heater becomes small or directly to zero power, and the display temperature is low. This phenomenon often occurs because the heating coil is opened and closed more frequently or even for a long time. The mold section is often disassembled and assembled, and the wiring is poor.
c) AC contactors are frequently turned on, and each time they open and close, arcs are generated. The temperature of the arc is very high.
You can think about the welding of the welding machine, which is a typical example of using arc. Arc sometimes causes the surface of the AC contactor to melt and the clutch is glued, resulting in uninterrupted operation of the electric heater, indicating a high temperature; repeated adhesions for a long time, mechanical force disengagement, re-adhesion will gradually burn the contacts The open circuit causes the heater to be inoperable, and the display temperature is lower than the set temperature.
d) Heating the main line insurance open circuit or circuit breaker trip, mostly due to the transient high current generated by the rear line or heater short circuit. At this time, although the heating indicator is on, the display shows heating, but the meter shows that the number or the pointer does not rise and fall, indicating that the temperature is getting lower and lower.
6.4 Improper installation of the heating coil: The heating coil is improperly installed, and the screw or die is not in tight contact and there is a gap, so that the heat of the heating coil is lost and cannot be transmitted to the screw barrel or the die. The heating coil does not stop working, indicating that the temperature is still biased. Low, affecting material plasticization. In this case, there is also the danger of burning the heater, and the installation of the heater should be paid enough attention.
6.5 Poor vacuum exhaust: The vacuum is too low or the venting hole is blocked by the raw material. The exhaust is poor, causing the material to entrain air or volatiles, which not only affects the plasticization of the material, but also causes the product to foam.
6.6 Screw temperature: The so-called screw temperature shows not the true temperature of the screw, but the temperature of the high-temperature heat-conducting oil that the screw oil pump feeds into the screw to flow. Some equipment is simply labeled as “screw oil temperature”. The abnormality is also divided into two situations. The first one is the oil circuit blockage, and there is no oil in the screw. However, according to the actual situation that the production speed is generally fast, most of the screw oil heating function is closed. At this time, the screw oil temperature display is often very low, about 50 °C (because the high temperature oil does not flow from the screw at all, the temperature of each section of the screw and the temperature of the material in each section is the same), causing the screw This function is lost, and the directional temperature adjustment function cannot be implemented, which causes the feed section to display a low temperature, and the metering section shows that the temperature is running out of control. The second type is the blockage of the waterway. Because the waterway is cooling the oil passage, the function is to take away the excess heat from the oil passage.
Therefore, the blockage of the waterway is not easy to find due to the blockage of the oil passage, and the damage is not so big. However, the heat generated by the oil circuit can not be dissipated, resulting in a gradual increase in the temperature of the high temperature fuel tank, which is uncontrollable, and finally loses the function of heat dissipation and temperature adjustment, and only retains the function of balancing the feed section and the metering section temperature. Excessive oil temperature will also cause leakage of high-temperature heat-conducting oil caused by damage of oil circuit, high-temperature solenoid valve and oil pump seal (mostly fluorine-containing rubber), which wastes high temperature oil and pollutes the working environment. It can be seen that to optimize the setting and control of the extrusion process temperature, the working quality of the extruder and the temperature control system must first be ensured.
6.7 Replacement of the screw: Before replacing the new screw, the quality of the machining must be strictly checked and assembled carefully to ensure that the minimum radial and axial clearance of the screw assembly is in an optimal state; the extruder is operated for a period of time, and the screw is adjusted periodically. Radial clearance with the screw barrel, gradually shift the wear part, widen its wear space, to improve the working life of the extruder barrel and screw; and regularly check whether the extruder thrust bearing is in series and timely repair. When the screw barrel and the screw are worn to the maximum amount, the repair and repair shall be carried out in time; when the temperature control system is out of control, the cause treatment shall be promptly found to prevent the disease from working; when installing the heating ring, it must be kept flat, and the screw barrel and the combined flow core, The die is tightly jointed, and there is no heat dissipation gap. During the extrusion production process, it is necessary to closely observe whether the vent hole is blocked. Is the vacuum too low? Found an abnormality and dealt with it in time. When the screw shows that the temperature is running high and the running is low, the cooling water line and the oil circuit should be cleared at any time to ensure that the display temperature runs along the ideal track.
Seven, Influencing factors and countermeasures of raw materials, formula, kneading, etc.Changes in raw materials, formulations, blending and other factors will have an impact on the plasticity of PVC-U materials.
7.1 Raw materials: The molecular weight of PVC resin is too high or too low, which directly affects the degree of melt plasticization. The low molecular weight resin has a higher degree of plasticization than the high molecular weight resin. Therefore, if the raw material is mixed or has its own quality problem, the molecular weight distribution area is too wide, which is undoubtedly fatal to the extrusion production of PVC-U. According to the actual production situation, our company takes into account the production difficulty and the comprehensive performance of the product. The extruded products are all loose SG5 (molecular weight 1000-1100) PVC resin to ensure high intrinsic performance. Most of the injection molding products use loose SG8 (molecular weight 650-750) PVC resin and a small amount of loose SG7 (molecular weight 750-850) PVC resin, which fully considers the intrinsic performance and processing technology.
7.2 Formulation: The processing aid and lubricant in the formula are not properly matched, or the amount added is not suitable, and the plasticization of the material is advanced or pushed back. The amount of filler in the formula also directly affects the physical and chemical properties of the product. index. The formula is undoubtedly the most important part of the production of PVC products. The formulas used by our company are generally used on a large number of tests and after trial production. The amount of processing aids and lubricants added in the formula are tested and verified to ensure the relative balance of friction and lubrication properties. And there are on-site process personnel to fine-tune at any time according to the actual situation, as well as the inspection of the quality of the formulation process, etc., to ensure the validity and suitability of the formulation in the production process.
7.3 Kneading process: the kneading (mixing) discharge temperature is set improperly, the hot mixing (stirring) temperature is too high or too low, the kneading time is too long or too short, and the cold mixing (stirring) discharge temperature is too high will affect the mixing. The degree of gelation is closely related to the degree of plasticization of the extruded product. Therefore, as the first step in the extrusion process of plastic products, the operator should be strictly prevented from reducing (changing) the mixing temperature and shortening the mixing time. At the beginning of the mixing, because the hot mixing pot is cold, the mixing time per pot is longer, and as the mixing pot increases, the time required for each pot to reach the discharge temperature will become shorter and shorter. When the discharge time is too short, the material components will be unevenly dispersed. The temperature of the heat discharge material (the method we use) should be increased appropriately, or the continuous mixing material should be changed into batch mixing to extend the mixing time. Guaranteed normal discharge (this has not been tested for effectiveness, and then we can consider step-by-step testing in the case of ample kneading equipment, and effectively promote it).
7.4 Kneading equipment: As the kneading machine works for a long time, the blades are worn, and the kneading equipment of our company adopts the self-friction heat generation type. Therefore, the wear of the pulp leaves will prolong the mixing time of each material. When the time required to reach the discharge temperature is too long, local overheating decomposition of the material may occur, and the blade should be replaced in time; the thermocouple should be carefully inspected before each mixing to avoid material wrapping or short circuit, open circuit, etc.
Occurs, does not reflect the true temperature, causes the kneading for a long time to cause the paste; when mixing, should pay close attention to whether the hot mixing (kneading) pot or the cold mixing pot discharge valve leaks, to prevent uneven dispersion of local material components, temperature cooling is not Equal phenomenon occurs. Especially for kneading (hot mixing) pots, if there is leakage of the discharge door, the material that has not been kneaded at a temperature will be slowly leaked into the cold agitating pot, and some unsynthesized composite stabilizer will also leak. This caused a similar situation that was not pinched at all, and the extrusion process could not proceed normally. After each mixing, check whether there is any residual material in the mixer that is stuck to the wall of the pot and the venting bag, and dispose of it in time. In order to eliminate all kinds of factors affecting the quality of the mixture.
Eight, summaryIn this paper, the idea of process temperature optimization of cone twin-screw extruder mainly covers the following contents:
8.1 Setting and control of extrusion process temperature: based on PVC-U melt plasticization degree of 60%-65%, the screw melt temperature should be controlled between 180 °C and 185, and the die temperature should be controlled. Between 190-210 ° C, so that the melt reaches the optimum degree of plasticization, it is immediately extruded from the die. Thus, it is possible to extrude from the optimum degree of plasticization, and prevent the material from overheating for too long, causing decomposition and lowering certain properties.
8.2 Appropriately increase the set temperature of the feeding section: to provide sufficient external heat to the material to ensure the material is well plasticized, effectively reduce the temperature difference between the set temperature and the display temperature of the metering section, and slow the shearing effect on the extruder barrel and The wear of the screw; lowering the set temperature of the metering section, not as low as possible, but based on the display temperature not exceeding 185 ° C -190 ° C, to ensure that the material is stable, balanced plasticization, to prevent local overheating decomposition. When the extrusion amount is large, the external heating has been stopped, and the cooling system does not stop working. When the display temperature is still higher than 190 °C, the screw setting temperature or feeding speed may be appropriately reduced according to the situation; when the extrusion amount is small, the temperature is displayed. When the temperature is lower than 180 °C, the screw barrel, screw setting temperature or feeding speed should be appropriately increased.
8.3 Under certain conditions of extrusion speed, the feed rate is an effective means to control the shear heat: while reducing the feed rate, adjusting the set temperature of the metering section and the temperature difference of the display temperature, the set temperature and display of the feed section are also caused. When the temperature difference is greater, it is necessary to reduce the extrusion speed in order to prolong the residence time of the material in the feed section, absorb the external heating, and promote plasticization.
8.4 The core is set to ensure the temperature of the melt section is balanced and consistent: if the set temperature is too low, the flow rate of the melt surface will be too slow, which will affect the molding quality of the extruded product from the die; the set temperature is too high. Will cause the melt surface to flow too fast, which also affects the molding quality of the extruded product from the die.
8.5 Replace the screw barrel and screw: When replacing, be sure to strictly check and adjust the axial clearance of each section of the screw to prevent the material or melt from being overheated due to too small a certain gap. The extruder works for a certain period of time and adjust the screw and screw in time. The cylinder has a radial gap to extend its working life.
8.6 Optimized extrusion process temperature: extruders with different specifications and shear performance can be realized. Under different extrusion conditions, extrusion can be carried out at the same process temperature as long as the display temperature is in a controllable state. Thereby achieving improved product quality, reducing extruder wear, extending its working life, further reducing costs, facilitating management, timely detection and effective handling of multi-layer targets of failure.
8.7 The optimization process has the preconditions: when the extruder is overloaded, the equipment and electrical appliances and instruments are in a fault state, indicating that the temperature is not controlled; the raw materials, formula related additives, mixing materials and other factors affect the plasticizing degree of the materials. At the same time, appropriate countermeasures should be taken to lay a good foundation for optimizing the process.