Matters needing attention in the production process of PVC Celuka foam boardMatters needing attention in the production process of PVC Celuka foam board
During the extrusion process of PVC foam board, the problems encountered can basically be classified into 4 categories.
One is the stability problem
The second is the melt strength problem
The third is the lubrication problem
Four is the problem of dispersion
These four types of problems, especially the first three types of problems, will restrict each other and have cross-effects. From the surface, it is sometimes difficult to distinguish them immediately. It is necessary to observe and analyze the problems and find the root causes of the problems to solve them.
Insufficient stability will affect the entire board surface, the board surface will turn yellow, and the foamed sheet will be brittle.
Insufficient melt strength will result in large cells of the foamed sheet and long vertical sections.
To judge whether the melt strength is insufficient, the direct way is to press the sheet wrapped on the middle roll with your fingers behind the three rolls. When the melt strength is good, you can feel elasticity when pressed. If it is difficult to bounce up 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 should be low.
In order to obtain uniformly foamed products in the foamed pipe, it is also necessary to ensure that the PVC material has a good melt strength. The foaming additives developed by the company are developed to solve this problem of foamed products and can effectively improve the quality of foamed pipes.
Lubricants are divided into external lubricants and internal lubricants. The external slip is good for demoulding and the smoothness of the surface of the sheet. Too little external slip makes it difficult to control the temperature in zone 5 of the extruder, and it is easy to heat up, which will lead to a confluent core High temperature, large bubbles, string bubbles, yellowing and other problems in the middle of the plate, and the surface of the plate is not smooth; more slippage, precipitation will become serious, which is manifested in the structure of the mold and the precipitation on the surface of the plate. For some individual phenomena, it moves back and forth on the board irregularly. Internal slip is beneficial to plasticization and melt fluidity. Insufficient internal slip is difficult to control the thickness of the board surface, which is manifested by the thickness of the plate in the middle and thin sides; the internal slip is more, and the phenomenon of high confluence core temperature is easy to occur. Poor dispersion will cause the surface of the board to be uneven.
Process temperature control issues:
The four issues mentioned above are fundamental issues, foundations, and deep-seated issues.
Compared with the above four problems, the process temperature control is much more intuitive. It is a superficial problem, but poor temperature control can induce fundamental problems. Increasing the processing temperature will reduce the material stabilization time and cause stability problems; the original lubrication balance will be broken.
Generally manifested as insufficient external lubrication, especially for later external lubrication, the addition of external lubrication needs to be increased; the increase in temperature will also cause the melt strength to decrease, the foamed sheet's cells will increase, the number of cells will decrease, and the sheet will become brittle and easy to break. ; The increase in temperature reduces the melt strength and the melt viscosity. The viscosity decreases and the shear dispersion ability decreases. For the screw with weak dispersion ability, uneven dispersion sometimes occurs.