Common problems of PVC foam sheet extrusion

Common problems of PVC foam sheet extrusion In the extrusion process of PVC foam sheet, the problems encountered can be basically classified into 4 categories.

One is the stability issue;

The second is the melt strength problem;

The third is the lubrication problem;

The fourth is the problem of dispersion.

These four types of problems, especially the first three types of problems, will restrict each other and affect each other. From the surface phenomenon, it is sometimes difficult to distinguish them immediately. Only by observing and analyzing, can the root cause of the problem be found.

a. Insufficient stability will affect the entire board surface, the board surface will turn yellow, and the foamed sheet will be brittle

b. Insufficient melt strength will lead to large cells in the foam sheet and long longitudinal section. The most direct way to judge whether the melt strength is insufficient is to press the sheet wrapped on the middle roller with your fingers behind the three rollers. When the melt strength is good, you can feel the elasticity when pressed. If it is difficult to bounce after pressing, 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 zones 3-5 should be lower.

 c. In order to obtain a uniform foamed product in the foamed pipe, it is also necessary to ensure that the PVC material has a good melt strength. The foaming aid developed by the company is developed to solve this problem of foamed products, which can effectively improve the quality of foamed pipes.

d. Lubricants are divided into external lubricants and internal lubricants. The external sliding is conducive to demoulding and is good for the smoothness of the surface of the sheet. If the external sliding is too small, the temperature of the 5th zone of the extruder is not easy to control, and it is easy to heat up, which will lead to If the temperature of the confluence core is high, there will be problems such as large bubbles, string bubbles, and yellowing in the middle of the plate, and the surface of the plate will not be smooth; if there is a lot of external slippage, the precipitation will become serious. It will be manifested as some individual phenomena moving back and forth on the board irregularly. Internal slippage is beneficial to plasticization and the fluidity of the melt. If the internal slippage is insufficient, it is difficult to control the thickness of the board surface, which means that the plate is thick in the middle and thin on both sides. If the internal slippage is too much, the phenomenon of high confluence core temperature is easy to occur.

e. Poor dispersion will bring about the phenomenon that the surface of the sheet is not smooth f. Process temperature control problem: The four problems mentioned above are fundamental problems, basic problems, and deep-seated problems. Compared with the above four problems, the process temperature control is much more intuitive. It is a superficial problem, but poor temperature control will induce the emergence of fundamental problems. If the processing temperature is increased, the stability time of the material will be reduced, and stability problems will occur; the original lubrication balance will be broken, which is generally manifested as insufficient external lubrication, especially in the later period, and the amount of external lubrication needs to be increased; The melt strength decreases, the cells of the foamed sheet increase, the number of cells decreases, and the sheet becomes brittle and easy to break; the increase in temperature reduces the melt strength and also reduces the melt viscosity, and the viscosity decreases, and the shear dispersing ability decreases, and the dispersing ability is reduced. For a screw with low capacity, sometimes uneven dispersion occurs.