Introduction and production process of PVC foam boardIntroduction and production process of PVC foam board
1. Definition of foam material
Foamed plastics, also called foamed plastics, are based on plastics and contain a large number of bubbles. It can be said to be a composite material filled with gas.
2. Classification of foam sheet
According to the different foaming ratio, it can be divided into high foaming and low foaming. According to the degree of softness and rigidity of the foam body, it can be divided into hard, semi-rigid, and flexible foams. According to the cell structure, it can be divided into closed-cell foam and open-cell foam. Common PVC foam sheets are hard closed-cell low foam sheets.
3. Application of PVC foam sheet
PVC foam sheet has the advantages of chemical resistance, weather resistance, flame retardancy, etc. It is widely used in many aspects, including display panels, signs, billboards, partitions, construction boards, furniture boards, etc.
4. Key factors for evaluating the quality of foamed sheet
For foamed materials, the size and uniformity of the cells are the key factors affecting the quality of the sheet. For low-expansion foam sheet, the cells are small and uniform, the foam sheet has good toughness, high strength and good surface quality. From the perspective of reducing the density of the foamed sheet, only if the cells are small and uniform, there is the possibility of further reducing the density. If the cells are large and scattered, it is difficult to further reduce the density.
5. The 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 foam sheets with chemical foaming agents, the expansion of the bubble core has a decisive influence on the quality of the foam sheet. 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 the processing characteristics of polymer materials, there are many ways to improve the strength of the 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. Melt strength decreases as the melt temperature increases, whereas melt strength increases as the melt temperature decreases, 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, the addition of a high-viscosity foaming regulator has a more obvious effect on improving the melt strength.
6. The role of processing aids in PVC foam sheet
ACR processing aids can promote the melting of PVC, improve the surface finish, improve the elasticity of the melt, and enhance the elongation and strength of the melt. It is good for covering air bubbles and preventing the collapse of cells. The molecular weight and amount of the foaming regulator have a great influence on the density of the foamed sheet: as the molecular weight increases, the strength of the PVC melt increases, the density of the foamed sheet can be made lower, and the amount of the regulator is increased Has the same effect. However, this effect does not have a linear relationship. If you continue to increase the molecular weight or increase the amount, the effect on reducing the density is not very obvious, and the density will tend to be constant.
There is an important relationship between the foaming regulator and the foaming agent. The density of the foaming sheet and the foaming regulator have an equilibrium point. Outside this equilibrium point, the density of the foaming sheet is not affected by the content of the foaming agent. , To maintain a constant, that is, increasing the amount of foaming agent can not reduce the density, the reason for this phenomenon is that with a certain amount of foaming regulator dosage, the melt strength of PVC is limited, excessive gas leads to cell collapse or merge.
During the extrusion of PVC foam sheet, the problems encountered can be basically classified into 4 categories.
One is the stability problem;
The second is the melt strength problem;
The third is the lubrication problem;
The fourth is dispersion.
These four types of problems, especially the first three types of problems, will be mutually restrictive and cross-influential. From the surface, it is sometimes difficult to distinguish them immediately. It is necessary to observe and analyze the situation and find the root cause of the problem before it can be solved.
(1) Insufficient stability will affect the entire board surface, the board surface will be yellow, and the foamed sheet will be brittle.
(2) Insufficient melt strength will result in large foam cells in the foam 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 bounce 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.
(3) 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 plate. The external slip is too little. The temperature in the 5 zone of the extruder is not easy to control and it is easy to heat up. As a result, the temperature of the confluence core is high, large bubbles, stringing, yellowing, etc. occur in the middle of the plate, and the surface of the plate is not smooth; if there is too much slip, the precipitation will become serious. It will also show that 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. If there is too much inner slip, the phenomenon of high temperature of the confluent core is easy to occur.
(4) If the dispersion is not good, the surface of the plate will not be 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 late external lubrication, need to increase the amount of external lubrication added; increased temperature 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 As for the screw with weak ability, uneven dispersion sometimes occurs