PVC wood plastic extrusion production technology, Foshan Nanhai Yuanjin Plastic Machinery Factory will tell you now:
Due to the increasing shortage of global forest resources, the technological development and development of wood plastic products at home and abroad Applications are developing rapidly. Wood-plastic composite materials have the advantages of hardness, strength, durability, wear resistance, and dimensional stability. Generally speaking, the hardness of wood-plastic composite materials is 2 to 8 times higher than that of untreated wood, and the wear resistance is 4 to 5 times higher. The application of various additives also gives it many special properties. It is also an environmentally friendly material that can be recycled and reused, and the raw materials are cheap and abundant. It has good benefits in reducing environmental pollution, protecting forest resources, and promoting economic development, and has attracted the attention of many researchers. In addition, wood-plastic composite materials also have secondary processability, and various products produced have beautiful appearance. It is one of the ideal substitutes for wood.
Wood-plastic composite materials have been studied abroad for a long time, but high-content powder filling has only developed significantly in recent years. For example, there is the famous "Einwood" in Japan; the Austrian Cincinnati Company and the PPT Mold Company have developed various wood-plastic board products; some companies in the United States are also actively developing and promoting such products. In China, Tangshan Plastics Research Institute, National University of Defense Technology, Guangdong University of Technology, etc. have conducted some research on low-content wood powder modified filling resin systems. Beijing University of Chemical Technology is also developing special equipment for wood-plastic products.
Wood-plastic composite extrusion technology is based on the formula of traditional plastic profiles, adding fillers such as wood chips, shavings, edge waste and crop fibers to obtain low-cost green materials, and designs Molds suitable for this formula use advanced and unique extrusion processing methods to make wood plastic products. Here the author discusses the extrusion technology of polyvinyl chloride (PVC)/wood flour composite materials.
1 Experimental part
1.1 Raw material selection
Wood powder is mainly made from leftover materials and sawdust from the wood products processing industry, which is mechanically crushed and ground. have to. PVC selects raw materials with a K value of 57 to 60 (average molecular weight 650 to 750). In addition, foaming agents, coupling agents, assistant foaming agents, plasticizers, nucleating agents, lubricants, colorants, UV stabilizers and other additives should be added.
1.2 Sample preparation
The sample preparation process is shown in Figure 1. [m] The temperature settings of each section of the extruder are shown in Table 1. [m] 2 Determination of formula and process parameters
2.1 Determination of formula
The formula design is based on the performance of the product, raw and auxiliary materials, molding process and equipment. This is a complex and tedious task. To be on the safe side, we usually just make some small changes based on experience based on the original mature formula, and then use experiments to determine the optimal solution that meets the requirements. The author is based on the formula of ordinary PVC door and window profiles, adding wood powder, foaming agent, assistant foaming agent, colorant, etc., and then determines the dosage of different raw and auxiliary materials based on orthogonal experiments.
The addition of wood flour generally worsens the flow properties of the material. As the wood powder content increases, the plasticization time increases and the fluidity becomes lower and lower. If the fluidity of the material is too poor, the wood powder will be subject to greater shearing force, which will increase the residence time in the extruder and make the wood powder easily burnt, which is not conducive to extrusion; conversely, if the fluidity is too large, Failure to form sufficient extrusion pressure will also cause strength defects and surface defects of the product. Therefore, during the extrusion process, the rheological characteristics of the system have a great impact on the processing process and various properties of the final product. Table 2 shows the processing properties of composite materials with different wood flour contents. [m] Due to the large particle size and low density of the wood powder used in the test, as the filling amount increases, the volume ratio of the wood powder filler in the system increases, which affects lubricants, plasticizers, processing aids, etc. Large adsorption capacity. Although large friction heat can be generated during the processing to speed up the plasticization speed, it is not enough to offset the impact of slowing down the plasticization speed and increasing the plasticization time due to the adsorption of plasticizers, processing aids, etc., thus making PVC plasticization delay. The greater the wood powder content, the more processing aids are absorbed, which will increase the plasticization time and worsen the processing performance. It was finally determined that the wood flour content was 30 parts.
The dosage of other raw materials is 100 parts of PVC, 3 parts of tribasic lead sulfate, 1.5 parts of dibasic lead sulfate, 0.5 parts of lead stearate, 0.4 parts of calcium stearate, and stearic acid 0.8 parts, polyethylene wax. ． 3 parts, 5 parts acrylic polymer, 6 parts chlorinated polyethylene, 30 parts CaCO, 0.9 parts AC foaming agent, 5 parts ACR-530, 0.31 parts iron yellow,
Iron 0.15 parts brown.
2.2 The influence of screw speed on extrusion molding
From the solid transport theoretical formula and the viscous fluid transport theoretical formula, it can be known that the production capacity is directly proportional to the rotation speed.
Increasing the rotational speed can effectively increase extrusion output, reduce costs and improve production efficiency, which is a need for industrial production. However, the increase in rotation speed is limited by power, plasticizing quality and extrusion temperature. It can be seen from the power calculation formula in the viscous fluid transport theory that as the rotational speed increases, the power consumption increases.
In the test, it was also found that when the screw speed gradually increases, the following phenomena occur: ① When the screw speed is very small, the material advances forward in laminar flow, and the surface of the product after the extrudate exits the mold is smooth, but the output is very low ; ② As the screw speed increases, the material gradually transitions to slip flow in the die. If the slip flow is not smooth or blocked, product quality problems will occur. Therefore, as the screw speed continues to increase, the heating history of the material is shortened and the fusion effect in the die becomes poor, resulting in internal stress, which causes the surface of the product to be rough or even cracked after exiting the die. Secondly, the increase in screw speed will cause the material to The residence time in the extruder is shortened, and the mixing quality of the material is reduced, which affects the strength of the final product; thirdly, PVC and wood powder are both heat-sensitive materials, and excessive screw speed can easily lead to degradation and gelatinization of the material; finally, For wood-plastic extrusion molds, a cooling plate is designed at the exit section of the die. If a higher screw speed is used, the material will be pushed out before it has time to cool. This will cause uneven cooling of the product and cause ripples on the surface of the product, affecting the product. The appearance and quality of extrusion molding may cause the product to not be formed in serious cases, causing production to be interrupted and making continuous production impossible. Therefore, only on the premise of meeting the extrusion temperature of the material, mixing quality and economic indicators of the production line design, can the rotation speed be maximized to improve productivity. The final feeding speed was determined to be 8.2 r/min; the main engine speed was 8.8 r/min.
2.3 The influence of extrusion temperature on extrusion molding
Due to the high water absorption rate of wood powder, the general moisture content is above 40a, which easily causes the material to become unstable when heated or left for a long time. Deformation, especially the evaporation of water and the decomposition of lignin and other components during the molding process, is prone to "burning" and turning brown when the temperature is above 180°C, resulting in poor appearance of the product and reduced bending strength and impact strength. Therefore, the temperature control of the extruder is very important.
The test found that the viscosity of the wood-plastic composite system is very sensitive to temperature. As the temperature increases, the melt apparent viscosity of the composite system decreases greatly. At the same shear rate, the melt viscosity at 160°C is an order of magnitude higher than the melt viscosity at 200°C. From the perspective of molecular motion, viscosity is related to factors such as internal friction, diffusion and orientation of molecules when the material flows. When the temperature increases, the mobility of the molecular segments increases, the volume expands, and the interaction between molecules weakens, causing the flow to The property
increases and the viscosity decreases.
Wood-plastic composite materials are often affected by barrel temperature and die temperature during the extrusion process.
The barrel temperature has a decisive influence on the mixing and plasticizing effect of composite materials, while the die temperature has an important influence on extrusion molding. Since the functions and viscous flow conditions of each section of the extruder are different, it is not appropriate to use full cooling or full heating one-sidedly. The temperature range of each section of the extruder must be appropriately selected. From the analysis of friction coefficient in solid conveying theory and combined with experimental experience, it can be found that the temperature of the feeding section will affect the friction coefficient between the material and the barrel, and the friction coefficient changes with the change of heating temperature. During production, the temperatures of the barrel and screw should be selected and controlled accordingly according to different materials to ensure greater friction between the material and the barrel to meet the needs of solid transportation.
For the plasticizing section, since it is the transition zone from the feeding section to the extrusion section, the impact on the production capacity is not very obvious. However, since this section plays a plasticizing role, its heating temperature must be maintained at the viscosity of the material. within the flow temperature range to ensure that the requirements for plasticizing temperature and starting temperature of the extrusion section are met. The temperature of the extrusion section is affected by the temperature of the first two sections. Generally, according to different materials, it can be selected within a certain range between its corresponding viscous flow temperature and decomposition temperature.
If the die temperature is too high or too low, it will cause melt rupture. If the temperature is too low, it will increase the friction between the melt and the flow channel, affect the slippage, and cause the melt to break; it will also increase the viscosity of the wood-plastic composite material, causing difficulty in flow, and causing the material on the wall of the flow channel to The flow is cooled and solidified prematurely, which cannot fill the machine head flow channel, making it difficult to extrude. It will also cause the material to be poorly plasticized and cannot fully wrap the wood powder, which will affect the strength of the product. If the temperature is raised, the surface quality of the extruded products will be greatly improved, and the material will be in a molten state when passing through the transition section and entering the flow channel of the shaping section. In order to ensure smooth extrusion, the temperature of the machine head should be controlled in sections, that is, the temperature should be gradually reduced.
2.4 The influence of extrusion pressure on extrusion molding
The extrusion pressure of the extruder is closely related to the temperature. High extrusion temperature and low machine head pressure make the extruded profile not dense, thus leading to product performance defects, destroying the excellent properties of wood powder as a filler, and seriously affecting the appearance. When the machine head pressure is low, stripes will appear on the surface of the product, and segmentation will occur. Extrusion will not be shaped, and material accumulation will occur. Continuous products with good appearance and quality will not be obtained, which will affect the continuity of production. Within the allowable pressure range, the higher the extrusion pressure, the denser the extruded product, and the better the extrusion quality. For vented extruders, the head pressure is related to the filling length of the second metering section. The degree of filling of this section depends on the amount of material supplied. When the filling length exceeds the exhaust port, the screw torque of the extruder increases and material comes out of the exhaust port, affecting the stability of extrusion, and the extruded product will appear "corrugated". That is, the unstable pressure prevents the material from flowing uniformly through the die flow channel. This fast and sometimes slow melt flow causes cracks in the extrusion molded products, which seriously affects the physical and mechanical properties of the products.
When the die temperature drops, the die pressure increases. At this time, the extrudate has better formability and the surface of the product is smoother. However, when the pressure of the machine head is very high, the material extruded from the machine head is well cooled, and the product is hard. The uncooled material at the back is soft and cannot push the hard product in front, causing most of the material to overflow at the exhaust port. , causing uneven material supply at the machine head, unstable extrusion, and segmented stripes on the surface of the product, affecting the appearance quality of the product.
Therefore, reasonably increasing the extrusion pressure of the extruder can ensure the smooth and smooth extrusion of the molten material, which can not only ensure the appearance quality of the product but also make the product dense and strong.
3 Mold Design
3.1 Die Design
The die is a component connected to the interface of the extruder. Its main function is to make the molten material rotate by It changes into linear motion, generates the necessary molding pressure, and molds plastic products with the required cross-sectional shape. In addition to the above functions, the wood-plastic composite mold must also provide sufficient pressure to the molten material before the straight section of the mold to ensure that the material does not foam in the extruder, converging core, machine head and other mold entrance parts.
Slowly release the machine head pressure at the exit part of the die, and form uniform microbubbles around the nucleating agent before exiting the die.
The flow channel structure of the special profile machine head is generally divided into four parts: transition section, splitting section, compression section and shaping section.
3.2 Design of shaping device
The shaping device is used to shape the material from a high elastic state to a solid state according to the product requirements. It should be considered that the temperature of the material after exiting the mold is still very high, and the foaming in the setting mold has not completely stopped, and the material continues to expand; the wood-plastic composite material has a poor cooling shrinkage and a large shrinkage in the later period.
The styling device is divided into dry and wet styling parts. The dry shaping part is commonly known as the shaping mold, and the wet shaping part refers to the water tank.
When the profile passes through the shaping mold, it relies on vacuum adsorption to make the profile closely contact with the shaping mold. The cooling water takes away heat through the water holes and completes heat exchange with the profile to cool the profile, but the profile itself does not come into contact with water. . The wet setting part uses a immersion vacuum vortex water tank to cool and set the product to the maximum extent. Figures 2 and 3 are the final assembly drawings of the final mold and water tank respectively. [m] 3:3 Software Analysis
Flow2000 is an industrial application software developed by Compu Plast International Company specifically for the plastic extrusion industry. The system has 12 modules (including three-dimensional flow, extruder, profile die, profile cooling, etc.). The profile die system is now used to analyze the die design plan. The special-shaped profile die system is based on the finite element method for the design and analysis of arbitrary-shaped profile die heads and is suitable for PVC door and window frame profiles. The cross-section at the die exit (Figure 4) is now used for analysis. The analysis process is: ① Input. Save the cross-section diagram in dxf format and input it; ②Finite element division. Divide the cross-section into small units in the environment of editing geometry, and automatically generate a grid; ③ Calculation. Enter parameters such as material, equipment extrusion volume, and extrusion speed to perform calculations. The calculation results are shown in Figure 5. [m] 3.4 Determination of mold plan
As can be seen from Figure 5, due to the boundary effect, the flow rate of the material near the outer and inner walls of the mold cavity is slower, and the flow rate in the center part is faster. The material flow rate is also faster at the intersections and corners.
From the analysis results of Flow2000, the material flow is basically uniform with the original design plan, which can meet the extrusion requirements, and this plan is determined to be the final plan. If there are still defects in the product during the debugging process, they can be repaired during the debugging process.
4 Conclusion
Extrusion processing is one of the important molding methods of modified plastics. During the extrusion processing process, the performance of the processed object is inevitably involved. The performance of the processed materials not only plays a decisive role in the performance of the product, but also has a great impact on the extrusion quality and output. Therefore, studying the relationship between the properties of modified plastics and the extrusion process has important practical significance. The extrusion process of wood-plastic composite extrusion technology is very complex and has many influencing factors. The combined effects of process, formula and mold cause uncertainty among the variables. The key issues in wood-plastic composite extrusion technology are: ① raw material selection (such as wood powder varieties) and how to improve the interface bonding force between the plastic matrix and wood powder; ② molding equipment and molding process selection; ③ molding mold design. Wood-plastic composite extrusion technology is one of the most important development directions in the future. Its extrusion products have great market potential, but before the final production of window profiles, it still requires the hard work of many people.