Precautions for the production process of PVC foam board
During the extrusion process of PVC foam board, the problems encountered can be basically classified into four categories,
One is the stability issue
Secondly, the issue of melt strength
The third issue is lubrication
The fourth issue is decentralization
These four types of problems, especially the first three, are mutually constrained and cross influenced, and sometimes it is difficult to distinguish them immediately from surface phenomena. It is necessary to observe and analyze the root cause of the problem in order to fundamentally solve it.
Insufficient stability can affect the entire board surface, resulting in yellowing and high brittleness of the foam sheet.
Insufficient melt strength can lead to large foam pores and long longitudinal bubbles in foam sheets.
The direct way to determine 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 elasticity when pressing. If it is difficult to bounce after pressing, it indicates poor melt strength. Due to significant differences in screw structure and cooling methods, it is difficult to determine whether the temperature is reasonable.
Generally speaking, within the allowable load of the extruder, it is better to have a lower temperature in the 3-5 zone.
In order to obtain uniformly foamed products in foamed pipes, it is also necessary to ensure that PVC materials have good melt strength. The foaming agent developed by the company is designed to solve the problem of foaming products, which can effectively improve the quality of foaming pipes.
Lubricants are divided into external lubricants and internal lubricants. External sliding is beneficial for demolding and is beneficial for the smoothness of the sheet surface. If there is too little external sliding, the temperature in zone 5 of the extruder is difficult to control and easy to rise, which can lead to high temperature of the confluence core, large bubbles, string bubbles, yellowing and other problems in the middle of the sheet, and the sheet surface is also not smooth; If there is too much external sliding, the precipitation will become severe, manifested in the structure inside the mold and the precipitation on the surface of the board. It can also be manifested as some individual phenomena that move back and forth irregularly on the board surface. Internal sliding is beneficial for plasticization and the fluidity of the melt. Insufficient internal sliding makes it difficult to control the thickness of the plate surface, manifested as the thickness in the middle and thin on both sides of the plate; The phenomenon of high temperature of the confluence core is prone to occur due to excessive internal sliding. Poor dispersion can lead to uneven surface of the board.
Process temperature control issues:
The four issues mentioned above are fundamental, fundamental, and deep-seated issues.
Compared to the above four issues, process temperature control is much more intuitive and is a surface issue, but poor temperature control can trigger the emergence of fundamental problems. Increasing the processing temperature will reduce the material stability time and result in stability issues; The original lubrication balance will be disrupted.
Generally manifested as insufficient external lubrication, especially in the later stage, it is necessary to increase the amount of external lubrication added; An increase in temperature can also lead to a decrease in melt strength, an increase in foam pores and a decrease in the number of foam pores in the foam sheet, making the sheet brittle and prone to fracture; The increase in temperature reduces the strength of the melt and also reduces the viscosity of the melt. The viscosity decreases and the shear dispersion ability decreases. For screws with weak dispersion ability, uneven dispersion may sometimes occur.
