Factors affecting the quality of PVC foam boardFactors affecting the quality of PVC foam board
In the extrusion process of PVC foam board, the problems encountered can be basically classified into four categories. One is stability problems; the other is melt strength problems; the third is lubrication problems; and the fourth is dispersion problems. These four types of problems, especially the first three types of problems, will be mutually restrictive and cross-influential. It is sometimes difficult to distinguish immediately from the surface. It is necessary to observe and analyze the situation and find the source of the problem to solve it.
1. Insufficient stability will affect the entire board surface, the board surface is yellow, and the foamed sheet is brittle
2. Insufficient melt strength will cause foaming sheet to have large cells 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 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 to 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.
Lubricants are divided into external lubricants and internal lubricants. External sliding is beneficial to demoulding, which is good for the smoothness of the surface of the sheet. Too little external sliding is not easy to control the temperature of zone 5 of the extruder and it is easy to heat up, which will cause the confluence core High temperature, large bubbles in the middle of the sheet, stringing, yellowing, etc., and the surface of the sheet is not smooth; too much slippage, precipitation will become serious, the structure in the mold and the precipitation of the slippage on the surface of the sheet will also show 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 board 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 In the case of a screw with insufficient capacity, uneven dispersion sometimes occurs.
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