According to the plasticizing process of PVC material in the twin screw extruder, the screw is divided into three areas: solid conveying zone, melting zone, and melt conveying (extrusion) zone.

1. Plasticizing mechanism in solid conveying area

In the machine barrel, the flow, preheating and compaction area of solid polymer (PVC) and its additives is defined as the solid conveying area. First of all, the flow of solid polymer particles in the hopper to the barrel is achieved by gravity. With the rotation of the screw, the particles in the hopper flow continuously while the particles are propelled towards the machine head. In the solid conveying area (barrel C1 area), the particles such as polymers and small molecules in the PVC material are gradually heated, and at the same time, the shearing action of the screw and the friction between the particles also increase the heat of the particles, so that the particles are fully in the dense state. Contact, Diffusion, Penetration.

In this area, due to the change of screw pitch, flight width, etc., PVC material particles are densely compacted to form a solid bed or solid plug that slides 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 solid plug from moving. Therefore, in the barrel, the PVC material particles do not advance in the same direction, but roll and slip from time to time, rotate with the screw, "bridge", accumulate behind the "bridge", break through the "bridge", and squeeze out with the PVC material. The flow of out and PVC material in the hopper is repeated.

In this area, the extrusion and plasticization quality of PVC materials is better. From the perspective of state, PVC is transformed from a glass state to a high elastic state; from the perspective of aggregate structure, 50% to 60% of PVC resin particles are broken and changed into primary Particles, the surfaces of various additive particles fully contact and diffuse with the primary particles.

It is worth pointing out that for stable operation, the height of solid material in the hopper must be above a certain critical value at all times. Above this critical value, changes in the level height will not affect the performance of the extruder, however, if the level height is below the critical value, it becomes a strong destabilizing factor. The change of the height of the solid material surface will cause the change of the bottom pressure, which will change the operation status of the PVC extruder, and lead to the deterioration of the extrusion and plasticization quality of PVC materials.

2. Plasticizing mechanism in melting zone

In the barrel, the coexistence area of solid polymer and melt is defined as melting zone or phase transition zone. This zone is C2, C3 heating zone. The melting zone is the main part of the extruder, especially the temperature setting (barrel C2 area, C3 area, screw core) and screw speed setting, the gap between the screw and the screw, the gap between the screw and the barrel, etc. The extrusion quality of PVC materials has an important influence. When the PVC material reaches the melting zone, due to the change of the pitch of the screw and the width of the flight, the PVC material particles have already produced considerable pressure while being compacted, and these pressures are combined with the softening effect of the surrounding heat medium. Turn compacted particles into a dense "solid bed". At this time, the solid bed is a mixed state composed of a part of PVC high elastic state, a part of PVC glass state, and a small amount of PVC viscous fluid state. The solid bed has the shape of a helical channel and slides within the channel. Due to this relative motion, a velocity distribution is created in the melt film between the solid bed and the barrel surface. As a result, the melt in the melt film begins to flow toward the screw, and when it encounters the screw flight, the screw flight "scrapes" the melt from the barrel and gathers it at the rear of the screw groove in front of the advancing screw thread. in the molten pool. As the bed of solids moves along the channel, more and more molten material is 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 gradually destroyed and becomes a viscous fluid state and is transported forward.

In this area, the extrusion and plasticization quality of PVC materials is better. From the perspective of state, PVC transforms from a high elastic state to a viscous fluid state; from the perspective of aggregated structure, it changes from 60 to 70% of PVC primary particles to primary particle crushing. Particles, various additive molecules are in contact with PVC primary particles to form a physical and chemical combination.

In the melting zone, the factors that improve the extrusion and plasticization quality of PVC materials are:

(1) Increase the screw speed;

(2) Increase the set temperature of the barrel in the melting zone;

(3) Appropriate clearance between screw and barrel.

For a specific PVC profile production formula, there should be a set of optimal barrel temperatures in the melting zone.

3. Plasticizing mechanism in melt conveying area

In the barrel, the solid polymer is completely converted into a melt, and the part where the melt is forced to be transported to the head is defined as the melt delivery zone (barrel C4 heating zone). In this area, the molten macromolecules react and homogenize further with various additives under the action of shearing, and the viscous fluid of the PVC material is continuously extruded quantitatively to form a melt pressure to ensure the compactness of the final formed product of the PVC material Spend. In this area, the extrusion and plasticization quality of PVC materials is better, and the state still maintains the viscous flow state of PVC macromolecules. From the perspective of aggregate structure, it is a crystal composed of PVC primary particles and a small amount of primary particles. This part of the primary Particles can increase the strength and toughness of the final material. When the material containing this crystal is extruded and cooled, under the action of external force, the primary particles can hinder the movement of the primary particles to achieve an increase in strength; and due to the large surface area of the primary particles, they can absorb part of the impact energy when impacted. , improve toughness.