Setting and optimization of temperature in PVC extrusion process

December 7, 2017


This article is divided into eight points as follows:


Datum of process temperature optimization

Setting of process temperature

Optimization mechanism of process temperature

Overloaded extrusion, temperature uncontrolled state and Countermeasures

Fault status and Countermeasures of equipment and electrical equipment

Factors and Countermeasures of raw material, formula, kneading and so on




In the technical literature related to the extrusion industry of plastics and PVC extrusion, there are two basic ideas about the temperature setting and control of the conical twin screw extruder.

One is low temperature process, the temperature setting is about 165 -175 C.

Another is the normal temperature process, the temperature setting is about 175 C -185 C.

On the trend of temperature setting, there is a saddle shaped technology with high and low back height. (I agree with the saddle type process mode. The company's production is also applied in this process mode), and there is a "ladder like" technology mode which is rising from the front to the rear. In the company's different product lines, there are superhigh temperature technology of screw tube temperature above 200 degrees and the ultra-low temperature process of the screw barrel temperature of 150 degrees or so. It can't be said that these technologies can produce products of the highest quality, but some of the completely different processes can produce products of the same quality, but it is an undisputable fact.

Therefore, I think it is necessary for the process of the temperature the pros and cons of comprehensive and systematic analysis and research, in order to eliminate the false and retain the true, from the outside to the inside, from difference process parameters, refine the method of setting a set of scientific and reasonable to guide the production process temperature.

In fact, after years of development, the extruder manufacturing industry in China has provided the conditions for good and balanced plasticization of PVC-U plastics, both in the configuration of screw structure or in the power configuration of external hot ring.

Practice proves that can break past most people think of "extrusion process and extruder should adapt to directional thinking problems, no matter what size and shear properties of conical twin-screw extruder, extrusion amount how many, in the temperature controlled conditions, can optimize the process temperature, basically the same temperature conditions under the extrusion, the extrusion process for the three United Company (United equipment, mold, unified unified formula) to lay a good foundation. So as to improve the quality of products, slow down the wear of extruders, extend their working life, further reduce the cost of formulation, facilitate management, timely detect and effectively handle failures, and so on. The basic idea of the optimum process temperature and the practice of production process are summarized as follows. Due to my limited level, mistakes and defects are unavoidable, I would like to invite all the technicians, managers and operators to criticize and correct them so as to update them in time. Thank you.


二、Datum of process temperature optimization

In order to optimize the extrusion process temperature, first of all, we should understand and master the datum of setting process temperature. A large number of production practices have proved that the following three conditions can be used as a benchmark:

2.1The thermal stability of PVC resin, PVC resin is a heat sensitive polymer resin, PVC only began to degrade under the condition of 100℃, 150 ℃ under the condition of accelerated degradation. In turn, PVC is converted from a glass state to a viscoelastic state by a high elastic state at 160 ℃. Therefore, the simple PVC resin can not be processed directly, and heat stabilizers must be added to improve the thermal stability of the resin. In general, the stabilizer test of PVC resin was carried out under the conditions of 180℃, 30min, 200℃ and 20min.  Therefore, the plasticizing temperature and time of PVC resin should not exceed this range.

2.2 plasticization degree: plasticization degree, also called gelation degree, in PVC plastic, plasticization degree is the mark of the degree of product crystallization and the degree of fusion of PVC primary particles.  A lot of research and test data show that without modification of PVC-U plasticizing degree at 60%-65%, the products of primary particles is not yet fully plasticized, only most fusion, the strongest performance impact, the plasticizing rate at 60%, the highest tensile strength, fracture elongation of plasticizing degree in 65% maximum. When the temperature of the melt at 150℃, the plasticizing rate is zero; melt temperature at 190 ℃, products of primary particles is clearly visible, plasticizing degree below 45%; melt temperature at 200 ℃ Celsius, the boundaries of primary particles in most products disappear, only a few primary particles can be seen, plasticizing degree is 70%; melt temperature to 200 ℃ above, the primary particles are completely plastic products, the plasticizing rate can reach more than 80%.

2.3 and the processing temperature of CPE blends: all of our products are PVC CPE mixing toughening modification, and impact modifier of CPE temperature zone is relatively narrow, a large number of experiments proved that products modified by CPE PVC in the formation of 190 ℃and 200 ℃ under the conditions of the morphology of phase difference great. 190 ℃ of modifier particles formed a reticular structure coated PVC primary particles, can obtain good anti impact toughening effect; 200℃PVC primary particles melted completely disappeared into mesh sphere, dispersed in the PVC matrix, resulting in a decline in anti shock performance. From the above discussion, we can see that the processing conditions of PVC modified by CPE blend are more stringent. At the same time, PVC plastics are "indeterminate" polymers, and the degradation of PVC is not only related to temperature but also time. The higher the temperature, the shorter the degradation time, the lower the temperature and the longer the degradation time. The melt temperature of the screw tube should be controlled between 180℃ and 185 ℃. Here we should pay attention to the melt temperature rather than the temperature of the screw cylinder, and the two are very different), so as to prevent the decomposition of the high temperature melt in the machine for a long time. The melt temperature remaining to complete by the die, the die section of the melt temperature should be controlled at 190℃ to 200 ℃ or even higher, in order to arrive at an instant best melt plasticizing degree, immediately extruded from the die, in order to achieve both molding from the best plasticizing degree of state, and not because of too high the temperature and time of decomposition.


三、The temperature of each section of the extruder barrel, the confluence core and the die is set as follows:

3.1 feeding section: 185℃ -195 ℃, depending on the cutting performance and extrusion volume of the extruder to ensure the display temperature is at least >185 ℃. The higher the extrusion temperature is, the higher the temperature is, so that the powder can be heated and vitrified quickly and form a small block. Our company extrusion line pipe and drainage pipe production are high speed extrusion wear, especially in threading pipe, I have consulted the data has not been so fast production, so our company's threading pipe and drainage pipe extrusion equipment for ultra high temperature material, generally less than 195 ℃, individual machine even up to 210 ℃ to 220 ℃, the temperature inside the actual is only between 100 ℃-130 ℃, to feed the end of the section to be close to the glass transition temperature is about 150 ℃ to the state.

3.2 The compression section is generally at 180 ℃, and it can also be increased according to the actual extrusion speed. The production of the threading pipe in our company is over 180 ℃ at this stage, reaching 190℃-195 ℃, and the drainage pipe production is almost 180 ℃.

3.3 The melting section is generally at 180℃, and it can also be increased according to the actual extrusion speed. Our production line is over 180 ℃, reaching 190℃ -195℃, and the drainage pipe production is almost 180 ℃.

3.4 measurement section: the temperature of the measurement section is very important during the whole process of extrusion, and its importance is in a sense even more than the feeding section. The temperature should be set at 170℃ -180℃, depending on the shear performance of extruder extrusion amount and size, to ensure the display temperature are less than 185℃. Because of the large shear heat inside the metering section, it is easy to cause the melt to heat up, and too high melt temperature will accelerate the decomposition of PVC to form yellow, discolouration line, foaming and so on, which will affect the quality of products. Therefore, the screw temperature and feed speed can be adjusted respectively when necessary.

3.5 extrusion mould body temperature: extrusion mould body temperature setting is relatively simple, mainly to prevent melt cooling in the mold body, generally set at 185 ℃ Celsius, most of the products in the production process, the temperature set in this interval no problem, individual products (bellows) would be higher than this, up to 190 ℃.

3.6 The temperature of die section: 190 ℃-210 ℃, depending on the luminance of the surface and the pressure of extrusion. In general, the higher the die temperature, can increase the brightness of the surface of the product, but also can decrease the internal pressure of the extruder, extruder, internal pressure reduces, friction force naturally reduces, in other words, appropriate to increase the die temperature, can produce less internal friction reducing extruder shear heat (when the internal friction shear heat in large time), and vice versa. I have been engaged in PVC extrusion for 20 years. I have experienced some experienced extruder operators. Most of them know that the production process can meet the needs of products by adjusting the temperature of the die.


四、The optimization mechanism of process temperature

According to the specific functions of each heating section, PVC-U extrusion is produced by conical twin screw extruder. The whole process can be divided into three parts: heating, constant temperature, heat preservation and so on. Heating and constant temperature are mainly divided into two independent and interrelated parts in the extruder, with exhaust holes as the boundary. The insulation zone is composed of confluence core, extrusion die and extrusion die.

Here, we should first know that there are two kinds of heat sources in PVC-U extrusion process, one is the external heat provided by electric heaters, one is the internal heat generated by the twin screw's shearing, calendering and friction action on PVC-U material, and the friction between PVC-U parts. The two kinds of heat sources play different roles in different stages of extrusion. The temperature control device controls only external heat. No, die die part of the temperature heat there are generally easy to control (extrusion mould, design parameters of ultra conventional will produce heat); heat, stronger shear, but not beyond the compression section and the main demand for plasticizing exhaust clothing melting section service, relatively stable it is easy to control. The shear is relatively weak, mainly depends on external heating, but the external heating is difficult to meet the plasticizing demand of materials. (the extruder with low external heating power is particularly prominent); the metering section that shear heat has surpassed the plasticizing demand is often not controlled by the temperature control device.

Therefore, in the temperature control of the whole extrusion process, the feeding section and the measurement section are the key and difficult points of the temperature control. The main body of the extrusion control is the temperature of the material, not the temperature of the cylinder and the mold. The set temperature is only a means of temperature in different conditions and display, and material temperature and different (feeding section material temperature below the display temperature, measuring section material temperature is higher than the temperature display), and the relationship between the thermocouple installation position, display temperature can only partly reflect the temperature of the material, only the set temperature. The basis and reference. Below, the temperature setting mechanism and emphasis of each section.

4.1 feeding section temperature: the feeding section is the electric heater transfer heat to the cylinder, the temperature of the display is the temperature of this section, not the material temperature. The temperature of the material is often far below the display temperature. When the material feeding screw by entering the extruder, the temperature is only 30 ℃ -40 ℃Celsius, away from the plastic material temperature rise shear and the screw to produce tropical (glass) temperature has a big gap, at the same time the material through the compression section, through the vent hole, the material needs to complete transformation process from glass to the viscous flow in the heating area, the basic requirements of a "orange peel", no powdery substance, and tightly coated on the surface of the screw groove, not to be pumped or clogged vent from the vacuum exhaust hole, so the feeding section is focusing on the functions of heating, setting temperature should be as high as possible, so that power the heating ring material to provide enough heat to outside. At this time the electric heater is open and close more frequently, even without pause. As the material enters the feeding section, the distance from the die extrusion is still in a period of time, and the setting temperature should not be too high for preventing material from bridging in the feeding port or sticking to the wall in the machine. It is advisable to show the temperature above 185 ℃. Although the setting temperature of the feeding section is low, for example, the temperature is set to about 170 ℃ or even lower, it can also produce the products with internal quality standard. However, because the external heat of supply is relatively small, too much reliance on shear heat to raise the melt temperature and increase the wear of the screw can affect the service life of the screw cylinder of the extruder. Through our long-term in the extrusion equipment maintenance was observed only after one or two years (some even less than a year), the spiral tube made will have serious wear, work wear area are mostly concentrated in the wider than after the compression of stud Ling big the first single head screw Ling or second single head Luo Ling site and metering section of the maximum wear volume of 2mm ~ 3mm, when extrusion will appear yellow line (because of material return, under the condition of high temperature and long residence time by adjusting the gap), and because of the screw and screw barrel local point friction products appear black and equipment issue abnormal the sound does not work, only to replace the screw barrel and screw. Although the occurrence of this phenomenon is closely related to the improper steel and heat treatment methods adopted by manufacturers, its important reason is also due to the low setting of extrusion temperature, which results in the relatively strong shearing effect of these parts and the aggravation of wear.  The higher setting temperature in the feeding section is not only beneficial to the melting of the material, but also the external heat can be fully utilized to reduce the wear of the extruder. A lot of practice has proved that the speed and the metering section set temperature invariable in extrusion feeding, under appropriate increase to set temperature material, can effectively reduce the temperature difference between the measuring section display temperature and the set temperature, fully explain the feeding temperature plays a role in adjusting the shear heat to a certain extent.

4.2 the temperature of the compression section: the material enters the larger shear section, and the heating is faster under the shear force of the screw. Setting a higher temperature can help reduce the viscosity of the material and speed up the fluidity, which can reduce the damage of the shear heat as the feed section.

4.3 melting section temperature: melting material is basically melting. Because of the change of spiral groove volume (the compression ratio is less than 1), the melt pressure suddenly decreases, which can give full play to the function of constant temperature and exhaust. Setting the temperature and the compression section consistent or slightly higher will help prevent the melt from cooling, and the melt temperature will also decrease because of the decrease of the melt pressure.

4.4 temperature of the measurement section: the temperature of the measurement section is not the material temperature. The temperature is usually higher than the display temperature when the material is transferred to the cylinder under the shear heat. The purpose of setting temperature is not to provide external heat, but mainly to stop heating in time, and to transfer excess heat with proper adjustment of screw cooler and screw oil temperature, so as to prevent material decomposition. The cooling device, which has a serious wear and tear, will be in a long working condition before the equipment is opened to keep the temperature not rising. Therefore the set temperature should not be too high, in order to show the temperature are less than 185. When the extrusion quantity is too small and the display temperature is too low, the setting temperature of the screw and the screw or the feeding speed can be increased in time to increase the shear.

4.5 The confluence of core and die temperature: the melt into the confluence of core, has been completely in melt state, and started by the change tendency of spiral motion into uniform linear motion, and through the die set of melt pressure, temperature, viscosity and flow velocity more uniform, making final preparations for molding. As a result of changing the direction of motion, the formation of melt pressure must be sacrificed at the expense of certain energy, and the internal heat produced by the shear action in the region has no longer exist. Therefore, the temperature setting should be higher to slow down the heat loss of the material. I refer to the literature from a large number of industries, in the industry have different opinions about merging core temperature setting is large, some people advocate the confluence of core temperature set at between 165℃ to 175℃melting section temperature: melting material is basically melting. Because of the change of spiral groove volume (the compression ratio is less than 1), the melt pressure suddenly decreases, which can give full play to the function of constant temperature and exhaust. Setting the temperature and the compression section consistent or slightly higher will help prevent the melt from cooling, and the melt temperature will also decrease because of the decrease of the melt pressure.

4.6 die temperature: die set temperature is mainly for forming and adjusting the flow rate and the surface brightness of the service, because of the melt into the mould in shunt cone oriented, has been transformed into a thin-walled cylinder by melt products need to shape, rely on external heating, can also be parison to the best plastic melt temperature uniformity the degree of regional. Therefore, the die temperature is directly related to the external molding quality of the product. It is worth noting that when the extruding products are slightly plasticized, it can also be solved by increasing the die temperature appropriately. But when the extruded products are badly plasticized, it is also inappropriate to overrely on increasing the die temperature. Due to the high surface temperature, melt extrusion from dies, uneven expansion, but also because of reduced melt pressure change inside the device of friction and shearing degree, but increased material plasticating, at this time or through the spiral tube and the temperature of each section of comprehensive regulation to solve.

4.7 screw temperature: the control of screw temperature usually has two devices. One is screw self tempering, and the heat pipe convection principle is applied to realize the equilibrium exchange of heat inside the screw without additional energy, but the heat transfer efficiency is low. The conical twin-screw extruder below 55mm is basically configured in this way. One is the external heating and cooling device, which regulates the temperature of the heating area and constant temperature zone of the screw by external energy. The setting of the temperature of the screw is determined mainly by the temperature difference between the setting and display of the heating zone and the constant temperature zone. Its main function is to add temperature to the feeding section or to cool down the measurement section to balance the temperature difference between the two (our twin screws are used in this device). From the current situation of the actual extruder and the existing problems, it is mainly to play the role of the latter.

4.8 temperature control mechanism summary: extrusion temperature setting is required for the "saddle", mainly to ensure materials and melt temperature showed "ladder type", from low to high, always in a steady rise, the equilibrium state and not because the plastic, the material in the heating area setting temperature is too low, the material to the exhaust hole failed to take material plasticization, from the exhaust hole; in the constant temperature region by setting the temperature is too high, causing material degradation. In the industry literature, some people claim to set the setting of temperature "staircase", which is obviously a misunderstanding. When the temperature is in a controlled state, external heat and internal heat can be adjusted and balanced. Set the temperature under certain conditions, when the shear is large, high heat, and heat circle will automatically reduce the working time and the amount of heating, to provide air from the external auxiliary (or oil cooling), internal cooling, oil cooling, to prevent material decomposition; when the shear effect is small, low heat the outer ring, the heat will automatically increase the working time, thereby automatically maintain the heating quantity and the required heat balance. Improve the set temperature, and heat in the increase at the same time, reduce material viscosity, increase liquidity, resulting in reduced shear heat; conversely, if reduce the set temperature, the decrease of heat supply at the same time, due to the increase of viscosity, reduced mobility, leading to increased shear heat. The energy provided by the extruder is always consistent with the setting temperature.  It does not change because of the strength of the extruder's shearing performance and the size of the extruder. Under higher processing temperature and lower shear effect, the same plasticizing degree can be obtained under lower processing temperature and higher shear action. Therefore, no matter the strength of the extruder, the size of the extrusion and the temperature of the extrusion process should be basically the same, and it should not be too different. This is also a reference to a lot of industry literature and material plasticization. At the same time, I should consider how to make use of external heating to slow down the shear heat and ensure the quality of extruded products, so as to reduce the wear and prolong the service life of the screw.


五、overloading extrusion, temperature uncontrolled state and Countermeasures

The new idea is based on the condition that the temperature is controlled under the condition of normal extrusion. If it is not appropriate to raise the efficiency of extrusion, also happens to the heating material is difficult to meet the demand of heat plasticizing, display temperature is not controlled, often below the set temperature, the material to vent to good plasticizing, there are still some powder by vacuum pumped from the exhaust hole; at this time most of the operating personnel after the temperature will increase to compensate the compression section and dissolve harm is not the main financial section, hazards in the metering section, the total heat metering section was beyond the required heat melt temperature, is because of the increase of extrusion speed brought about a substantial increase in the metering section of shear frictional heat caused by the temperature controlled display, tend to be higher than the set temperature, cause local overheating and decomposition of extruded products.

This phenomenon changes with the increase of extrusion efficiency. The higher the extrusion efficiency is, the greater the temperature difference between the setting temperature and the display temperature is, the more serious the adverse consequences will be. This phenomenon is particularly prominent in the extruder with lower shear heat or low external heating power. When the display temperature is not controlled by the set temperature, the process optimization is difficult to achieve. The above phenomenon is the characterization of the heat imbalance between the heat supply of the extruder and the material plasticization. The temperature difference between the temperature and the display temperature in the feeding section is a sign of the degree of the external heating or shearing heat. The setting temperature of the metering section and the temperature difference between the display temperature and temperature are the signs of the excess degree of shear heat.

At present, two measures are taken to match the heat of the feeding section in the extruder produced in our country.

One is to improve the power of the heating ring, for example, the power distribution of the feed section of the 65/132 conical twin screw extruder has reached 9kW.

Second is to reform the screw thread structure. A single head thread is set after the double head thread in the feeding section or the compression section, which can effectively improve the compression ratio of the slot.

The lack of heat supply in the feeding section of the extruder has been significantly improved than in the past. However, the excess of shear heat in the measurement section still restricts the increase of the extrusion efficiency. We also made a special research on this problem. Now all extruders are special made with twin screws. Their parameters are adjusted, and the gap between Lo Ling and lo Ling is increased appropriately, so as to adapt to our ultra high speed extrusion. In addition to the structure of the screw, the shear heat is influenced directly by the feed rate and the ratio of the extrusion speed. When the setting temperature of the metering section is lowered, the heating ring has stopped heating, and the cooling device has not stopped working. When the temperature control is invalid, the corresponding measures can be taken according to the requirements and the following procedures should be taken to effectively reduce the display temperature of the metering section.

5.1 one is to reduce the temperature of the screw setting, reduce the setting temperature of the screw, and the method of oil cooling to transfer the excess shear heat in the measurement section. But reducing the setting temperature of the screw also reduces the material temperature of the feed section. When feed configuration of low power heating ring extruder, screw should reduce the set temperature, the need to take into account the feeding control temperature, do not care for this and lose that.

5.2 two is appropriate to reduce the feeding speed and reducing feed speed, can reduce the shear heat (we call to reduce torque). Under the certain conditions of the screw speed of the extruder, improving or reducing the feeding speed is an effective means to adjust the shear heat.  However, the reduction of feed rate will also reduce the material temperature of the feed section, and the demand for the shear heat of the feed section and the material in the measurement section is contradictory.  The same as screw temperature setting, when the power supply section of the extruder is equipped with low heating power, the feed speed is reduced, and the temperature control needs of feeding section is also taken into account. At the same time reduce the excessive feeding speed, resulting in complete package cannot melt metering section of screw groove, will also increase the screw and screw barrel wear Ling, "Saul zetoms so-called" (the middle part and screw the symptoms of excessive wear).

5.3 three is the concept of reducing the rate of extrusion and feeding speed appropriately, the feed rate and the extrusion speed are the same as the ex