In the production process of PVC profile, the product quality is directly related to the extrusion and plasticization quality of PVC material. The so-called PVC material refers to the product (powder solid) of PVC resin mixed with various auxiliaries. The extrusion quality of PVC materials generally includes two aspects, internal quality and external quality. Internal quality refers to the plasticity of PVC materials and the physical properties of PVC profiles, while external quality refers to the external surface, section structure and size of profiles. This paper focuses on the discussion of the mechanism and influencing factors of PVC extrusion from hardware (extruder) and software (formula and process), which is of positive significance to improve the quality of PVC extrusion and the product quality of PVC profile.
Extruder is the guarantee of PVC extrusion
Extruder is the hardware part of PVC extruding and plasticizing, which is the guarantee of extruding and plasticizing. The industrial extruder was invented in 1795. The synthetic polymer industry developed in the middle of the 19th century promoted the application of extrusion processing. In 1925, various types of PVC were extruded. In 1935, the first single screw extruder specially designed for thermoplastic plastics manufactured by Germany marked the beginning of modern extrusion technology. From the first single screw extruder to today's single screw and double screw extruders, there are many models Rod extruder, extruder has experienced 70 years of development. It can be said that the development history of extruder is the continuous improvement of PVC extrusion quality.
As we all know, there are many kinds of extruders in industrial production, which can be divided into molding extruder, mixing and pelletizing extruder and feeding extruder according to the purpose of extruder; single screw extruder, twin screw extruder and multi screw extruder according to the number of screws; exhaust extruder and non exhaust extruder according to whether they can exhaust; screw position in space It can be divided into horizontal extruder and vertical extruder. 　　
The development process of twin-screw extruder also reflects the demand for the continuous improvement of the plasticizing quality of PVC materials. The development process of twin-screw extruder can be divided into three stages: the first stage is 1950-1960, as a new equipment, the function of twin-screw extruder is not perfect; the second stage is 1960-1970, a high performance and high cost twin-screw extruder; the third stage is after 1970, pay attention to the economic rationality of processing equipment. According to the relative position of two screws, twin-screw extruder can be divided into meshing type and non meshing type. According to the structure and shape of the two screws, they can be divided into parallel twin-screw extruder and conical twin-screw extruder. According to the different relative rotation directions of the two screws engaged with each other, they can be divided into two categories: the same rotation and the opposite (reverse) rotation. In the production of PVC profile, most of the manufacturers use the meshing conical counter rotating twin screw extruder. 　　
The screw is the main part of the extruder which affects the extrusion plasticization of PVC material, and its improvement process is also the process to meet the requirements of PVC material plasticization. The main parameters of screw are: screw diameter; length diameter ratio; compression ratio; screw section (feeding section, compression section, metering section); screw groove depth; screw pitch; screw angle; screw edge width; screw head number; screw section; screw head type; screw cooling mode. When the extruder manufacturers Manufacture the screw, the above parameters are generally considered. This is because different parameters above will lead to different extrusion quality of PVC materials. In other words, the above parameters should be changed to meet the quality requirements of PVC extrusion. 　　
The barrel is another main part that affects the extrusion and plasticization of PVC materials in the forming process of extrusion mechanism. There are three heating modes: heat carrier (oil, steam) heating, resistance heating and induction heating. Considering the extrusion and plasticization effect of stable PVC material, the two kinds are better after use. There are two cooling methods: oil cooling and blower blast cooling. Through practical observation, the author believes that although the oil cooling is uniform, the oil pipe is easy to form carbon. After the oil pipe is carbonized, it will not only clean up the trouble but also affect the cooling effect. Therefore, it is better to control the plasticizing temperature of PVC materials by using blower air. At present, most extruder manufacturers use resistance heating, blower blast cooling and rectangular feeding port. 　　
For the production of PVC profile, the plasticization and temperature uniformity of PVC materials extruded by single screw extruder are not as good as that of twin screw extruder due to the processing of PVC powder. This is because the transportation of materials in Single Screw Extruder mainly depends on the friction between the materials and the inner wall of the barrel and between the materials and the surface of the screw. The twin screw extruder is "forward transportation", which has the function of forcing the materials forward The role of. The material in the twin-screw extruder is fed into the barrel by the feeding hopper and reaches the die through the screw. In this process, the material is driven by the thread and moves through the circumference of each part to carry out shearing, rolling and kneading, as well as moving towards the die direction. The flow of materials in the twin-screw is not due to friction and traction, but to mechanical forced transportation. This kind of conveying is equal in quantity, which reduces the countercurrent, shortens the time for the material to stay in the barrel and reduces the temperature fluctuation. It has been proved that in the twin-screw extruder, the residence time of the material is only half of that of the single screw extruder with the same diameter and rotation, while the heat transfer efficiency is 4 times higher, thus improving the plasticizing quality of PVC material. 　　
It should be noted that the assembly accuracy of screw and barrel has a great influence on the extrusion quality of PVC materials. The assembly accuracy is mainly shown in the four clearances of twin-screw extruder, which are the clearance between two screws, the clearance between screw ribs, the clearance between screw and barrel, the distance between screw tip and barrel head section (clearance). The fit clearance between the screw and the barrel is generally 0.1-0.6mm. According to the different screw diameters, the clearance increases with the increase of the diameter, but the clearance cannot be too large or the clearance between each point of the outer circle of the screw and the barrel cannot be too large. Otherwise, the extruding quality of PVC material will decrease. Extrusion mechanism of PVC material
According to the three-stage Plasticization Process of PVC material in the barrel of twin-screw extruder, the screw is divided into three areas: solid conveying area, melting area and melt conveying (extrusion) area. The mechanism of extrusion plasticization of PVC material is the mechanism of three-stage plasticization of screw in the barrel.
In the barrel, the flow, preheating and compaction areas of solid polymer (PVC) and its additives are defined as solid conveying areas. First of all, the plasticization mechanism of the solid conveying area. First of all, the flow of solid polymer particles in the hopper to the barrel is realized by gravity. With the rotation of the screw, the particles move towards the head direction, while the particles in the hopper flow continuously. The polymer and small molecule particles in PVC material in the solid conveying area (C1 area of barrel) are heated gradually. At the same time, the screw shear and friction between particles also increase the heat of particles, so that particles can fully contact, diffuse and penetrate in the dense state.
In this area, due to the change of screw pitch and screw edge width, PVC particles are compacted compactly to form a solid bed or solid plug sliding on the screw groove. The movement of the solid plug depends on the friction between the barrel surface and the solid plug, while the friction between the screw and the solid plug prevents the movement of the solid plug. Therefore, in the barrel, the particles of PVC materials do not advance in the same direction, but roll and slip from time to time, rotate with the screw, "bridge", pile up after the "bridge", break through the "bridge", and the process repeats with the extrusion of PVC materials and the flow of PVC materials in the hopper. Make the particles fully contact, diffuse and permeate in the dense state.
In this area, the better extrusion plasticizing quality of PVC material is that PVC changes from glassy state to high elastic state in terms of state; in terms of aggregate structure, 50% - 60% of PVC resin particles break into primary particles, and the surface of various auxiliary particles fully contacts and diffuses with primary particles.
It is worth noting that for stable operation, the height of solid material in the hopper must be above a certain critical value at any time. Above this critical value, the change of the feed height will not affect the performance of the extruder, but if the feed height is lower than the critical value, it becomes a strong unstable factor. The change of solid level will cause the change of bottom pressure, change the operation of extruder, and lead to the deterioration of PVC extrusion quality.
2. Plasticization mechanism in melting zone
In the barrel, the coexistence region of solid polymer and melt is defined as melting region or phase transformation region. This area is C2 and C3 heating area. Melting zone is the main part of extruder, especially the setting of temperature (C2 zone, C3 zone, screw core) and screw rotation
When the pressure is large, these pressures, together with the softening effect of the surrounding thermal medium, turn the compacted particles into a dense "solid bed". At this time, the solid bed is composed of a part of PVC high elastic state, a part of PVC glass state and a small amount of PVC viscous flow state. The solid bed has the shape of a spiral groove and slides in the groove. Because of this relative motion, the velocity distribution is generated in the molten film between the solid bed and the barrel surface. Therefore, the melt in the melt film starts to push forward to the thread and flow. When it meets the screw edge, the screw edge "scrapes" the melt from the barrel and gathers in the melt pool at the back of the screw groove in front of the push thread. When the solid bed moves along the spiral groove, more and more molten materials are brought into the molten pool. Therefore, the size of the molten pool increases, while the size of the solid bed decreases. The solid bed is destroyed gradually and becomes a viscous flow state.
In this area, the extrusion quality of PVC material is better. In terms of state, PVC changes from high elastic state to viscous flow state; in terms of aggregate structure, it changes from 60-70% of PVC primary particles to primary particles, and various auxiliary molecules contact with PVC primary particles to form a combination of physics and chemistry.
In the melting zone, the factors to improve the extrusion quality of PVC materials are as follows:
(1) increase screw speed
(2) increase the set temperature of the barrel in the melting zone
(3) proper clearance between screw and barrel
For a specific PVC profile production formula, there should be an optimal set of melting zone barrel temperature.
3. Plasticizing mechanism of melt conveying zone
In the barrel, the solid polymer is completely converted into melt, and the part of the melt forced to the head is defined as the melt conveying area (barrel C4 heating area). In this region, the melt macromolecules react and homogenize with various auxiliaries under the shear action. With the constant quantitative extrusion of PVC viscous fluid, the melt pressure is formed to ensure the compactness of the final product of PVC material. In this area, the better extrusion plasticization quality of PVC material still keeps the viscous flow state of PVC macromolecule from the state. From the aggregation structure, it is a crystal composed of PVC primary particles and a small number of primary particles, which can improve the strength and toughness of the final material. When the material containing this kind of crystallite is extruded and cooled, under the action of external force, the primary particles can block the movement of the primary particles and achieve the improvement of strength; and because of the large surface area of the primary particles, part of the impact energy can be absorbed and the toughness can be improved when impacted.
The best formula and technology are the basis and condition of PVC extrusion
There are more than ten kinds of raw materials for the production of PVC profiles,