PVC wood plastic production process and common problems in productionPVC wood plastic production process and common problems in production
Factors affecting the performance of wood-plastic composites (WPC) in raw materials
1. Melt flow rate: To some extent, the melt flow rate reflects the larger relative molecular weight. The smaller the melt flow rate, the greater the relative molecular weight. The larger the relative molecular weight, the larger the billet has better melt strength, which can change the weight of the parison to sag. The tensile strength, impact strength, heat deformation temperature and other properties of the product are improved, which is advantageous, but the larger the relative molecular weight, the higher the viscosity. The worse the fluidity, the more difficult it is to process. At the same time, the parison has a high "retractability", and the parison will shrink more before closing the mold. Under the same conditions, the unstable flow of the parison is intensified, and even the melt is broken. Therefore, considering the equipment processing capacity and process feasibility, almost all manufacturers of 200L plastic drums choose HMWHDPE resin, the melt flow rate can meet the product quality requirements, generally about 2.0 (g / 10min, 21.6kg).
2. Molecular weight distribution: From the perspective of molding processing, a wide molecular weight distribution is better than a narrow distribution, with better fluidity and easy processing control, and a wide molecular weight distribution can reduce die pressure, reduce parison melt cracking tendency, and improve processing Performance, can increase the extrusion speed under the same conditions. However, the wide molecular weight distribution also indicates that there are relatively low and relatively high molecular weight parts. When the proportion of the relatively low molecular weight parts is too high, the mechanical properties and thermal stability of the product are reduced, and the classification effect in the flow process , So that the low molecular weight grades of the polymer are more concentrated on the surface of the extruded parison, even clear from the surface, the surface of the parison appears to be scattered with some fine white particles, the inner wall of the blow molded product is rough, and the white particles fall off It is often easy to block the gas valve, causing the gas circuit system to malfunction. When the proportion of the relatively high molecular weight part is too high, plasticization is difficult, incompletely plasticized particles appear on the surface of the parison, and the appearance quality decreases. At present, the bimodal distribution resin has a tendency to replace the unimodal distribution. Under the same conditions, it has excellent processing performance and melt strength, and the resistance to environmental stress cracking is also significantly improved.
Molding process and equipment
Good processing technology and equipment should ensure that the materials and blowing agent are mixed evenly, and maintain a sufficiently high and stable head pressure, so that the die pressure is large enough and the pressure drop is fast enough to obtain good-shaped cells.
1. Mixing and feeding
Production process of raw materials:
Wood fiber is a material with strong water absorption. The water content is generally about 15% or even higher, and the inorganic filler also reaches about 5%. The water content is the natural enemy that affects the stability of plastics, especially wood plastics production, and product quality. It affects the expansion ratio, the water absorption rate of the product, the deformation of the product, the weather resistance and other physical properties of the product. Therefore, it is best to control the moisture content within 1%. Fibers and fillers with reasonable moisture control-add to the high-mixer-mix to 100 degrees-add the appropriate amount of coupling agent-mix to 110-115 degrees- --Open the large cover of the mixer for 2-3 minutes when the motor is turned off and do not move ------ Add PVC and stabilizer ----- Mixed to 80 degrees ----- Add lubricant and other additives-- --100 degrees plus processing aids, foaming agent --- 120-125 powder discharge ------ After the powder is discharged, the motor stops and then open the big cover for 2-3 minutes before mixing the next batch (It is best to mix five hand-made materials to clean up the mixing machine once) ----- The powder can be granulated in the mixing bucket so that it can be granulated to 40 degrees or added with twin screw production (the mixing bucket must have circulating cooling water). It is necessary to pay attention to observe the temperature and time in the mixing, such as the general mixing of the mixing material in the boiling pot, about 20-25 minutes, the hot pot is about 12-15 minutes (temperature setting is about 125 degrees), if the time difference is 3 minutes or longer It may be that there is a problem with the temperature sensing line or the temperature control is inaccurate, or the horn knife of the mixer is badly worn. This will affect the stability of the expansion ratio and color.
When granulating, it is often necessary to pay attention to whether the temperature of each temperature control is accurate, whether the fan is operating normally, whether the size of the current, the particle size, the surface, and the length of the material are sticky, whether the material is stable, and the material must not be broken.
The mixing process affects the dispersion of each component by affecting the contact and reaction between different components, which in turn affects the properties of the material. When mixing, the proper order, temperature and time should be selected. Due to the fluffy wood flour powder, the phenomenon of "bridging" and "holding pole" easily occurs during the feeding process. Unstable feeding will make the extrusion fluctuate and reduce the quality of the extrusion. Therefore, the feeding method and feeding amount must be strictly controlled. Generally, forced feeding devices or hungry feeding are used to ensure the stability of extrusion. PVC / wood powder composite extrusion foam molding is generally divided into two steps and one step: two-step method is to granulate first and then to form; one-step method is to save the granulation process and use surface-modified wood Powder and PVC powder are directly fed and extruded after being mixed at high speed. Studies have shown that the masterbatch method (two-step method) is beneficial to improve the mechanical properties of PVC / WPC.
2. Molding temperature
Mainly pay attention to the following aspects
a. The key to setting the temperature of each section of the extruder is the compression section, the metering section and the die. The accuracy of each temperature control, such as inaccurate temperature control, affects the expansion ratio and color. Foaming is mainly the temperature setting of the compression section and the metering section, and molding is mainly the mold temperature. b. Normal operation of fans in each section.
c, the size and stability of the host current. If the current fluctuation of the host is large, it indicates that the slip agent, processing aid, fiber and PVC components are not well dispersed.
d. The speed of the host must be the same, because the speed will affect the foaming ratio and color.
The setting of the extrusion molding temperature should take into account the physical effects and chemical reactions of the materials in the extruder barrel. The temperature of the feeding section should not only ensure that the material can be quickly melted, to prevent the escape of the decomposition gas, but also to prevent the blowing agent from decomposing in advance; the temperature setting of the compression section and the metering section needs to consider the decomposition temperature and decomposition rate of the chemical blowing agent, wood powder Factors such as scorching and PVC decomposition; the temperature of the machine head should keep the melt flowable while having sufficient melt viscosity to maintain the melt in the machine head under high pressure so that it does not foam in the machine head . Yao Zhuping believes that under the conditions of full plasticization, low temperature extrusion should be used. Screws and forming dies should also have low-temperature extrusion characteristics to ensure that the cells have a good shape and a small diameter. The temperature of the feeding section should be controlled below 165 ℃, the compression section and homogenization section should be between 160 ~ 180 ℃, and the head and die should be set below 160 ℃.
3. Screw speed
The influence of screw speed on extrusion foaming is mainly reflected in the following aspects: First, it affects the extrusion pressure, the higher the speed, the greater the pressure in the extruder, which is more conducive to nucleation and the number of nucleated cells The more, the higher the foaming rate. However, when the pressure is too high, the growth of nucleated cells is inhibited, which affects the full growth of the cells; the second is that the higher the screw speed, the stronger the shearing effect. When the shearing effect is too strong, it is easy to merge or rupture the cells, which affects the hair growth. The quality of the foam and the formation of low-density foam; the third is that the screw speed is too high or too low, so that the residence time is too short or too long, and it is easy to cause early foaming or insufficient decomposition of the foaming agent, which is not conducive to the formation of uniform and fine. Cell structure. Therefore, when other influencing factors remain unchanged, there is an optimal value for the screw speed, generally between 12 ~ 18r / min.
4. Extrusion pressure
Insufficient extrusion pressure will cause the surface of the product to be rough and low in strength, and higher extrusion pressure can not only control the gas-containing melt in the head from foaming in advance, but also make the pressure difference between the inside and outside of the die of the head large, so that the pressure The high rate of decrease is conducive to the nucleation of bubbles, the number of nucleated bubbles increases, and the foaming rate also increases, which is conducive to obtaining a uniform and fine cell structure. However, too high extrusion pressure is detrimental to cell growth. To obtain proper head pressure, it can be achieved by adjusting the screw speed, head temperature and die shape.
5. Molding equipment
The single screw extruder mainly relies on friction to transport materials, and the mixing effect is poor. The wood powder stays in the barrel for a long time and is easy to scorch. Therefore, it is greatly restricted in the extrusion of PVC / wood plastic composite materials. In order to improve the mixing effect of PVC / wood-plastic, the single screw used for PVC / wood-plastic processing should be equipped with a mixing zone, or pelletized first, and then extruded with pellets, but this process consumes additives and reduces PVC performance.
Key points of PVC foaming process control
Plastic foam molding is divided into three processes: bubble core formation, bubble core expansion and bubble solidification. For PVC foamed sheets added with chemical foaming agents, the expansion of the bubble core has a decisive influence on the quality of the foamed sheets. PVC is a straight-chain molecule. The molecular chain is short and the melt strength is low. During the expansion of the bubble core into the bubble, the melt is not enough to cover the bubble. The gas is easy to overflow and merge into a large bubble, which reduces the product quality of the foamed sheet. .
The key factor to improve the quality of PVC foam sheet is to improve the melt strength of PVC. From the analysis of processing characteristics of polymer materials, there are many ways to improve the strength of PVC melt, but the most effective way is to add additives to improve the melt strength and reduce the processing temperature. PVC is an amorphous material. The melt strength decreases with the increase of the melt temperature, and the melt strength increases with the decrease of the melt temperature, but the cooling effect is limited and only plays an auxiliary role. ACR processing agents have the effect of improving the strength of the melt, of which the foaming regulator is the most effective. The melt strength increases as the content of the foaming regulator increases. Generally speaking, as long as the screw has sufficient dispersing and mixing ability, adding a high-viscosity foaming regulator has a more obvious effect on improving the melt strength. Methyl methacrylate resin molecular formula C5H8O2; CH2C (CH3) COOCH
Common problems in the extrusion process
In the extrusion process of PVC foamed sheet, the problems encountered can be basically classified into four categories, one is the stability problem; the second is the melt strength problem; the third is the lubrication problem; the fourth is the dispersion problem. These four types of problems, especially the first three types of problems, will restrict and cross each other.
The impact is sometimes difficult to distinguish immediately from the surface. It is necessary to observe and analyze the situation and find the root of the problem to solve it. Insufficient stability will affect the entire board surface, yellowing the board surface, and making the foamed sheet brittle.
Insufficient melt strength will result in large cells in the foamed sheet and very long longitudinal section. The most direct way to judge whether the melt strength is insufficient is to press the sheet wrapped on the middle roll with your fingers after the three rolls. When the melt strength is good, you can feel the elasticity. If it is difficult to pop up after pressing, it indicates that the melt strength is poor. Because the screw structure and cooling method are quite different, it is difficult to judge whether the temperature is reasonable. Generally speaking, within the allowable load of the extruder, the temperature in zone 3-5 is better.
In order to obtain foamed uniform products in foamed pipes, it is also necessary to ensure that PVC materials have good melt strength. The foaming additives developed by the company are developed to solve this problem of foamed products, which can effectively improve the quality of foamed pipes.
Lubricants are divided into external lubricants and internal lubricants. The external slip is good for demoulding, which is good for the surface finish of the sheet. The external slip is too little. The temperature in the 5 zone of the extruder is not easy to control and easy to heat up, which will cause the confluence core High temperature, large bubbles in the middle of the plate, stringing, yellowing, etc., and the surface of the plate is not smooth; too much slippage, precipitation will become serious, the structure shown in the mold and the precipitation of the slide surface of the plate will also show Some individual phenomena move back and forth from time to time on the board. Inner slip is good for plasticization and melt fluidity. Insufficient inner slip is difficult to control the thickness of the board surface, which is shown as the middle thickness of the plate is thin on both sides. The more inner slip, the higher the temperature of the confluent core.
Poor dispersion will cause the surface of the sheet to be not smooth.
Process temperature control problem: The four problems mentioned above are fundamental problems, they are the foundation and the deep problems. Compared with the above four problems, process temperature control is much more intuitive and is a surface problem, but poor temperature control will induce the emergence of fundamental problems. Increasing the processing temperature, the material stability time will be reduced, and stability problems will occur; the original lubrication balance will be broken, generally manifested as insufficient external lubrication, especially in the later period of external lubrication, the need to increase the amount of external lubrication; temperature increase will also lead to The melt strength is reduced, the foaming sheet foam cells are increased, the number of cells is reduced, the sheet is brittle and easy to break; increasing the temperature reduces the melt strength and also reduces the melt viscosity, the viscosity is reduced, the shear dispersion ability is reduced, the dispersion In the case of a screw with a weak capacity, uneven dispersion sometimes occurs.
Hidden Dangers of Wood-plastic Composite Materials and Their Solutions
1. The water content of the wood-plastic composite material is too high. Because the wood-plastic material has a porosity of 16% to 21%, it is easy to fold and pollute. The solution is to strictly control the temperature and processing speed during production and processing to reduce the water content.
2. Because the amount of antioxidant is not enough, the wood-plastic board is easily oxidized, the surface is too brittle, and it is easy to fall off. The solution is to add an appropriate amount of antioxidants
3. Wrinkles appear on the plastic-wood material after production and processing, which mostly occurs during the rapid cooling of the material. The solution is to allow sufficient time for cooling, shrinking, and placing the finished WPC material, and try not to perform high-speed production in order to increase productivity.
4. The product fades. This phenomenon is caused by insufficient pigment or polishing transition. The solution is to reduce sanding or add inorganic pigments.
5. The surface of wood-plastic products is too smooth. The solution is to print the texture on the surface of the material, change the plastic content to increase the traction, and use special high-quality special paint to solve such problems.
Effect of machine head pressure on the processing performance of wood-plastic composite materials
The temperature of the extruder is high and the pressure of the head is small, which makes the extruded profile not dense, which leads to defects in product performance, destroys the excellent properties of rice husk powder as a filler, and seriously affects the appearance of the image. When the head pressure is low, streaks appear on the surface of the product, and there is a phenomenon of segmentation. Extrusion does not form, and there is often a phenomenon of material accumulation caused by the material not forming. Continuity.
As long as the pressure is within the allowable range, the higher the pressure, the denser the extruded product, the better the extrusion quality. For vented extruders, the head pressure is related to the full length of the second metering section. The filling degree of this segment depends on the amount of feed. When the filling length exceeds the exhaust port, the screw torque of the extruder rises and feeds from the exhaust port, which affects the stability of extrusion. The extruded products appear "corrugated". That is, the unstable pressure prevents the material from flowing uniformly through the flow path of the machine head. This fast and slow melt flow causes cracks in the extruded products, which seriously affects the mechanical and physical properties of the products. Effect of Rotational Speed on Processing Properties of Wood-plastic Composites
When the temperature of the machine head decreases, the pressure of the machine head increases, then the extrudate has better moldability and the surface of the product is smoother. However, when the head pressure is very high, the material extruded from the head is cooled well, the product is harder, and the material that has not been cooled is softer, and the hard product in front cannot be lifted, causing most of the material to overflow at the exhaust , Make the feed at the machine head uneven, the extrusion is unstable, the product surface has segmented stripes, which affects the appearance quality.
Increasing the rotation speed of the screw can increase the extrusion output, reduce the production cost and improve the production efficiency, which is the need of industrial production. However, as the rotation speed increases, the material gradually transitions to slip flow in the die. If the slip flow is not smooth and blocked, there will be product quality problems. Continue to increase the screw speed, the output has a sudden process. But at this time, due to the shortening of the heating process and the fusion effect in the die, the extruded sheet
There will be internal stress, and the surface of the product will be rough or even cracked after the die is exported. At higher speeds, the material is ejected before it cools, so that the cooling of the product is uneven, causing ripples on the surface of the product, which affects the appearance of the product and the quality of the extrusion molding. Intermittent production has seriously affected the continuity of production.
When the rotation speed is very low, the material advances in a laminar flow, the material can be fully cooled and shaped, the surface of the product after the extrudate exits the die is smooth, the appearance quality is good, the product is uniform, but the output is low.
Effect of temperature on processing performance of wood-plastic composites
The wood-plastic composite material is mainly affected by the barrel temperature and the head temperature during the extrusion process. The barrel temperature has a decisive influence on the mixing and plasticizing effect of the composite material. The head temperature has an important influence on the extrusion molding.
As the barrel temperature increases, the apparent viscosity of the melt of the composite system decreases. For the extrusion processing of materials, increasing the temperature will help increase the fluidity. In the first group of experiments, the temperature of the extruder was set higher, but the increase in temperature caused the wood fiber to burn, causing the viscosity to be too low to produce enough The pressure of the machine head and the high temperature of the extruder lower the viscosity of the material, which is not conducive to cooling and shaping. It is easy to cause weld marks on the surface of the product. The surface of the product is rough, the strength is poor, and the extrusion quality is affected. The course is uneven, and the wave shape appears due to the elastic recovery of the melt after the die is exported, which affects the appearance quality of the product. The phenomenon that the material is not formed often occurs, resulting in discontinuous production.
Therefore, on the basis of satisfying the plasticizing quality of the materials, the temperature of the extruder should be reduced as much as possible.
The temperature control of the transition section of the die from the die to the cooled and shaped die has a significant effect on the quality of the extruded product. If the temperature in this section is too low, the viscosity of the wood-plastic composite material increases, and the flow is difficult. The flow at the wall of the flow channel will be cooled and solidified prematurely, so that the material cannot fill the flow path of the machine head and it is difficult to extrude; As the temperature of the segment increases, the surface quality of the extruded product is greatly improved. The material is in a molten state when flowing into the flow path of the shaping section through the transition section. In order to allow the molecules to be fully shaped, the temperature of the head should be controlled in stages, that is, the temperature gradually decreases. It can be seen that for the extrusion molding of wood-plastic composite materials, the temperature of the head is very critical, which directly affects the quality of the extrusion molding.
The corresponding relationship between the extrusion process and the extruder
The shear performance of the extruder is determined by the screw structure of the extruder. However, the quality of extrusion and the efficiency of extrusion also depend on the extrusion process and the shear performance of the extruder. Otherwise, the low-shear extruder is extruded at too high an extrusion speed, making it difficult to produce high-quality profile products. The high-shear extruder operates at an excessively low extrusion speed, and it is difficult to effectively exert the extrusion efficiency. Different shear performance extruders have a certain process control range, which is limited. The extrusion process route advocated by the industry is "saddle-shaped", that is, the set temperature of the heating zone is higher, the set temperature of the constant temperature zone is lower, and the set temperature of the heat preservation zone is higher. However, the extruders with different shear performance operate at different extrusion speeds, and the height of the "saddle" and "seat" of the "saddle type" is completely different.
When extruding plastic profiles, it is necessary to maximize the extrusion efficiency of extruders with different shear properties, and establish the heat and heat requirements of the screw heating zone (feeding section, compression section) and constant temperature zone (melting section, metering section). The balance of the heat supplied is the key. According to the shear performance characteristics of the extruder, different extruders with different shear performances and different specifications of plastic profiles should be extruded separately to meet the needs of product quality and performance.
Plastic profile extrusion, the material is converted from the glass state to the molten state. There are two heat sources, one is the external heating provided by the electric heater, and the other is the calendering, friction, and shearing of the material during the rotation of the screw. Of heat. During start-up production, the melting of the material is mainly outside heating. In the normal production stage, the melting of the material is mainly based on the internal heat generated by the screw rolling, friction and shearing of the material. Relevant data show that: in the extrusion of profile, internal heat accounts for the proportion of heat supplied by the extruder,
Roughly above 65%.
The external heating temperature control system mainly implements temperature setting and display through electrical instrument components. When the display temperature exceeds the set temperature index parameter, the heating ring is immediately powered off to stop heating, and the screw oil cooling device and the screw cylinder air cooling device are forced to cool; when the display temperature does not reach the set temperature index parameter, The heating coil has been working continuously. Since the internal heat is mainly restricted by the characteristics of the extruder screw, feeding and extrusion speed, it is not affected by the external heating temperature control system. When the extrusion speed of the low-shear extruder is too high, even if the working frequency of the heating ring outside the feeding section and the compression section is increased and the intermittent time is very short, the displayed temperature may not reach the set temperature; even if the melting section and the metering Out-of-segment heating stops working and starts the screw and barrel cooling device to run, the display temperature may still be much higher than the set temperature.
At the same time, because the temperature measuring point (thermocouple) reflecting the displayed temperature is installed on the wall of the screw barrel of the extruder, and there is a certain distance from the material in the screw barrel, the instrument display temperature and the actual temperature of the material have a certain gradient under different working conditions. Different correspondence. Under normal circumstances, there is both external heating and shear heat in the feeding section and compression section, which is bidirectional heating, and the display temperature is basically equal to the temperature of the material; when the temperature of the melting section and the metering section shows that the temperature has not reached the set temperature, it is also bidirectional heating. When the displayed temperature exceeds the set temperature, the heat starts to transfer from the inside to the outside, which can be called reverse heat transfer, and the displayed temperature is lower than the material temperature. It can be seen that when the extrusion speed of the low-shear extruder is high, the actual temperature of the materials in the screw melting section and the metering section is not only higher than the set temperature, but also higher than the display temperature.
Therefore, when the display temperature is running in the set temperature range, the set temperature parameter is basically equal to the temperature of the material, which is the control target and basis of the plasticization and melting of the material. When the display temperature deviates from the set temperature range, the display temperature can be assumed to be the material temperature, that is, instead of the set temperature, it becomes the control target and basis for the plasticization and melting of the material. Setting the temperature is just a means of regulation to increase or decrease external heating.
For low-shear extruders, due to the relatively small compression of the feed section and compression section, the internal heat provided can not meet the plasticizing requirements of glassy materials, so the temperature setting of the feed section and compression section should be higher, due to the formula Different, roughly around 190 ～ 200 ℃. Although the display temperature is still low when the extrusion speed is increased, the purpose of increasing the set temperature is for the electric heating coil of the feeding section and compression section to work continuously. As long as the display temperature is in the range of 180 ~ 185, the material is tightly wrapped in the screw, is in a slightly molten state, and there is no exhaust hole feeding phenomenon, it can be regarded as normal; the setting temperature of the melting section and the measuring section should be lower, due to different formula It is about 165 ～ 175 ℃. Although the display temperature is still high when the extrusion speed is increased, the purpose of lowering the set temperature is to stop the heating of the electric heating coil in the melting section and the metering section and start the screw oil cooling. Cool the material with the air cooling of the screw cylinder, as long as the temperature is displayed in the range of 180 ~ 185 ℃, the extrusion parison section does not show symptoms such as porosity, pitting, etc., which can be regarded as normal extrusion speed. Conversely, even if the temperature of the feeding section and compression section is set higher, the heating ring works continuously, the material of the exhaust hole is loose, and it is in the form of bean curd residue. 1. The setting temperature of the metering section is even lower, the electric heating ring has stopped working, the screw oil cooling and the screw cylinder air cooling have been cooling the material, and the extrusion parison has appeared symptoms such as porosity and pitting, which can be regarded as the extrusion speed To the limit, the extrusion speed or feeding and extrusion speed should be reduced in time (3).
The vent material is a sign of poor plasticization of the low-shear extruder. But it is not that the exhaust vents are caused by low-shear extruders. There are mainly the following reasons for the exhaust hole feeding: ①The feeding speed is too fast, the added shear heat is not enough to balance the heat required by the increased feed volume, resulting in poor plasticization of the plastic; ②The extrusion speed is too fast, The increased shearing heat is not enough to balance the heat lost by the reduction of the residence time of the feed and compression sections, resulting in poor plasticization of the plastic; ③When the formula uses CPE impact modifier, the amount of processing aid added is too small , Poor friction performance of the material, poor plasticization by the exhaust hole; ④ excessive lubricant in the formula, the material moves too fast in the extruder, and the plasticization by the exhaust hole is poor; ⑤ the extruder screw and The axial gap of the screw barrel is too large, the leakage is serious, or the temperature of the feeding section and compression section of the screw is too high, resulting in the material "over plasticizing". The material that has been converted into melt undergoes the first pressure peak in the compression section. When the vent hole is released, the stress is released, the volume expands, and it adheres to the end surface of the screw rib. It is scraped by the screw barrel of the exhaust section on the wall of the exhaust hole tube as the screw rotates, and it accumulates to a certain extent to overflow from the exhaust hole. The first two types of exhaust vent material are both related to the poor shear performance of the extruder, the third and fourth types of exhaust material are mainly related to the formulation, and the fifth type of exhaust material are mainly related to the extruder wear. And high shear performance. When judging the cause of the exhaust vent material, it should be considered comprehensively, not blindly. If it belongs to the exhaust hole of the test formula, the formula should be adjusted; if it belongs to the extruder wear, it should be adjusted
The gap between the screw and the barrel of the extruder; if the material is found to be "overplasticized" in the exhaust hole, the feed extrusion speed ratio should be adjusted; the first three types of exhaust holes generally show increased torque, and the latter two types of exhaust holes The risk is generally reduced torque.
For high-shear extruders, due to the relatively high compression of the feed section and compression section, the internal heat provided can meet the needs of the melting of glassy materials, so in general, the set temperature of the feed section and compression section is lower than the shear Cut the extruder set temperature is lower. According to the specific shape of the material in the exhaust hole of the screw cylinder. Not only should we pay attention to whether the exhaust hole is feeding, but also whether the material of the exhaust hole is "over-plasticized"; the same reason, because the compression of the melting section and the metering section is relatively low, and the shear heat is small, the melting section and the metering are generally The set temperature of the section is higher than that of the low shear extruder. Also according to the molding material, and pay attention to whether the material of the exhaust hole is "overplasticized"; the same reason, because the compression of the melting section and the metering section is relatively low, and the shearing heat is small, the setting temperature ratio of the melting section and the metering section is generally The setting temperature of the low-shear extruder is higher. It also depends on the shape of the parison at the exit of the die. Not only should pay attention to whether there are porosity in the parison section, but also whether the parison is "underplasticized". The set temperature curve of the high-shear extruder is relatively gentler than that of the low-shear extruder.
The most common problem with high-shear extruders is not the exhaust hole feeding, but the shearing heat of the feeding section and compression section is too high, causing the material to appear "sticking" symptoms in the exhaust hole. After starting for a period of time, the parison has a yellow line, which is difficult to produce normally. Therefore, the two-stage set temperature should be reduced as much as possible, the external heat supply should be reduced or the formula should be adjusted, and the lubricant should be increased or the processing aid should be reduced. If the effect is not significant or changes in the formulation lubricant and processing aids lead to changes in the quality of the profile, and simultaneous production with different shear performance extruders, the same mixing, storage, and conveying system cannot be squeezed for high shear Separate ingredients for the machine only reduce the extrusion speed and operate in the lower extrusion efficiency range. Therefore, it can also remind enterprises to purchase models with the same or similar structure of the extruder screw when the extruder is added or updated.
During the extrusion process, the process of converting the material from the glass state to the molten state. In addition to the balance between the heat required for the plasticization of the material and the heat supplied, so that the material can complete the ideal plasticization, the melting pressure is also very important. Control indicators. Since the material is affected by the die resistance and the compression ratio of each section of the screw during the extrusion process, it does not exist at normal pressure. Different die, the compression ratio of each section of the screw is basically constant, not variable. The compression ratio of each section of the screw is only for the distribution and adjustment of the pressure of the screw material in each section, it is impossible to increase or decrease the total pressure of the melt in the extrusion process. The total pressure adjustment mainly depends on the process methods such as extrusion speed, feedstock and W extrusion speed ratio. Adjusting the extrusion speed, the ratio of feed and extrusion speed is not only an important measure to adjust the extrusion temperature, but also the main measure to adjust the melt pressure and extrusion efficiency.
Under the premise that the extrusion speed is unchanged, increase or decrease the feed rate, the material volume of the screw in the feed section changes, and the volume of the material in the exhaust section remains unchanged, so the feed section and compression section change with the compression ratio, and the melt pressure varies with Increase or decrease; increase or decrease the extrusion speed under the premise that the feed rate is unchanged, the screw material volume in the feed section also changes. The material volume of the exhaust section remains unchanged, so the feeding section and compression section change with the compression ratio, and their melt pressure increases or decreases accordingly; the feeding speed increases or decreases synchronously with the extrusion speed. The change is only due to the increase or decrease in speed, resulting in changes in the melt pressure. It can be seen from this that in the first two cases, the melt pressure changes synchronously with the melt volume change, and the change is large; in the latter case, the melt pressure changes little because the melt volume remains unchanged. Therefore, when adjusting the temperature of the low-shear extruder or the high-shear extruder, pay attention to the change of the melt pressure of the material. The extruded parison should ensure that neither "underplasticization" nor "overplasticization" is required. Next, increase the melt pressure as much as possible to ensure that the profile has a good finish, compactness, dimensional change rate and impact performance.
Regardless of whether the high-shear extruder or the low-shear extruder is operated for a period of time, after the axial wear of the screw and the barrel increases, a leakage phenomenon occurs. When it is still within the allowable range, there is a problem of process and formulation adjustment. Generally, without affecting the physical and chemical properties and appearance quality of the product, it can be processed by appropriately reducing the extrusion speed, setting the temperature, or appropriately reducing the lubricant, increasing the filler, and reducing the melt fluidity. Of course, shutting down to adjust the axial gap between the screw and the barrel and performing polishing treatment to maintain the original rheological state of the material is undoubtedly the most effective solution.
Regarding the temperature control of the head, transition section, and die, the difference in shear performance between the low-shear extruder and the high-shear extruder is mainly in the screw structure, and has little relationship with the head, transition section, and die. . At the same time, the set temperature of the machine head, transition section and die is only for changing the flow direction of the melt material, adjusting the material flow rate of the material cross section and improving the appearance of the product. Therefore, there is no big difference in temperature control between the two different shear performance extruders. The adjustment is mainly based on the shape and appearance color of the export parison. For specific adjustments, see the author's "UPVC Profile Extrusion Process and Dynamic Thermal Balance of Materials" It should be noted that:
When using high-shear or low-shear extruder, it should match the specifications of the extruded profile. The high-shear extruder generates more heat and has a larger amount of extrusion. If the size of the extruded profile is too small, the traction speed cannot be too high due to the restriction of the linear speed of the die and the shaping die, so the extrusion efficiency cannot be obtained Effectively use, can not effectively use the heat energy converted by the extruder's own mechanical energy, to achieve the purpose of energy saving; low shear extruder produces less heat and small extrusion volume, if the extrusion profile is too large, the extrusion speed When improving, the product quality is difficult to guarantee. Therefore, when the extruder is equipped with a die, the high-shear extruder should be equipped with a large-sized profile die, and the low-shear extruder should be equipped with a small-sized profile die. In order to ensure the balance between the heat supplied by the extrusion and the required heat under similar process conditions.
Regardless of whether the high-shear or low-shear extruder is used to produce the profile, the quality of the tested product meets the requirements of the standard indicators. The process parameters must be strictly implemented and generally not easily changed. When testing new formulations, try to use the original process parameters as the reference system, and adjust the process temperature appropriately according to the rheological conditions of the materials, especially the appearance quality such as melt pressure and product size change, smoothness, etc.
Or the amount of processing aids and lubricants in the formula.
① The shearing performance of the extruder is determined by factors such as the taper of the screw, the number of thread heads, the pitch, the width of the screw edge, the depth of the screw groove, and the helix angle. Among them, the screw taper, screw pitch, screw edge width and screw groove depth are mainly increased or decreased by the screw groove material volume; the screw angle change is increased by increasing or decreasing the material stroke Or reduce the shear heat. The number of threaded heads is increased. For example, the feed section and compression section are changed from single head to double head. The main purpose is to change the flow of the screw material in the heating zone from serial flow to parallel flow, and the flow speed is accelerated. Improve its compression ratio and shear heat. Practice has proved that increasing the number of screw heads in the first two sections of the screw leads to a reduction in the material stroke, but the increased shear heat is much greater than the heat lost by the material stroke reduction, which is the most effective measure to improve the shear performance of the extruder;
② The shear heat of the extruder is configured according to the characteristics, shape and heat required for plasticizing of the extruded material. The materials in the feeding section and compression section are basically in a glassy state, and the exhaust section is required to be in a "micro-melt state" and tightly wrap the screw without being peeled off by the screw barrel, which requires more heat; therefore, the heating zone is compressed more ; Because the materials in the melting section and the metering section are basically in a viscous flow state, but the locality is not very uniform, further constant temperature is required and less heat is required, so the compression in the constant temperature area is relatively small.
③ The extrusion process should be controlled according to the heat requirements of different profile extrusion to ensure the balance between the heat supplied by the extruder and the heat required by the material volume of different specifications. For low-shear extruders, the temperature setting in the feed section and compression section should ensure uninterrupted operation of the external heating, and supplement the shear heat of the extruder. The set temperature of the melting section and the metering section should ensure that the external heating stops working in time, and the display temperature is still within the control range of the set temperature. Otherwise, the extrusion speed should be reduced. For the high-shear extruder, except that the process control of the last two stages is basically the same, the biggest difference is that the heat of the first two stages may be insufficient or excessive, and should be adjusted according to the direction and amplitude of the display temperature deviating from the set temperature If the display temperature is lower than the set temperature, the set temperature should be increased to keep the outer heating ring in an uninterrupted heating state. If the display temperature is higher than the set temperature, the set temperature should be reduced to make the outer heating ring stop working properly and Start the screw and screw cooling device to implement forced cooling.
④ Melt pressure is the main index to ensure the performance of profile compactness, dimensional change rate, impact strength and so on. When adjusting the extrusion temperature and speed, the melt pressure should be kept as high as possible on the premise of ensuring the appearance and molding quality of the profile.
⑤ The strength of the extruder's shearing performance should be configured according to the specifications of the profile to give full play to the extrusion efficiency of the extruder. ⑥, different shear performance extruder has different process parameters. The process parameters that have been proved to ensure the quality of the profile should not be easily changed to avoid fluctuations in the profile quality.
⑦ The experimental formula should be adjusted based on the original process parameters, especially the torque, based on whether the vent hole of the extruder is extruded and the shape of the die extruded parison.
⑧The radial gap between the screw and the barrel of the extruder changes. Without affecting the quality of the profile, the extrusion temperature, speed and formula can be adjusted to reduce the melt fluidity and properly control the material leakage.
The current shortcomings of domestic ecological wood products
1. Product discoloration and deformation problems
A, the problem of discoloration (poor weather resistance).
There are many reasons for its discoloration: ultraviolet rays in sunlight, high temperature, oxygen, air and the pH of the environment in which the product is located. How to prevent the discoloration of the product can be started from the following aspects: a. Choose materials and additives that have as little discoloration as possible or can improve its weather resistance, such as color fastness, anti-migration, high temperature resistance, and heat resistance. Good toner. b. Add some antioxidants and ultraviolet absorbers to improve the weather resistance of the products. c. If the production and processing requirements are met, reduce the processing temperature as much as possible. d. Spray weathering paint with good weather resistance on the front of the product. e, co-extrude a thin layer of material with good weather resistance on the front of the product. B, deformation problem (product bending and shortening or lengthening)
The deformation is mainly due to the poor thermal deformation of the Vika product. To improve the thermal deformation of the ecological wood, the following aspects should be used: a. Add as little or as little material as possible that will affect the deformation of the ecological wood, such as low melting point lubricants, CPE, DOP, epoxy soybean oil, etc. b, reasonable lubrication system and dosage. c, reasonable production process such as production speed, cooling water temperature. d, how to release the internal stress of ecological wood products. e, the rational design of the mold. f. The placement and storage of ecological wood semi-finished products. g, the reasonable mix of types and amounts of fillers in order to improve the related mechanical properties of ecological wood such as impact strength, flexural strength, modulus, etc. C, flame retardant
As far as the composition of ecological wood is concerned, the key point of its flame retardant performance is to increase its oxygen index and reduce its smoke emission during combustion.
If you want to know more about it please do not hesitate to contact me. WhatsApp:+86-15966835076.