Common problems in PVC foaming process

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

1. is the stability problem;

2. is the melt strength problem;

3. is the lubrication problem;

4. is the dispersion problem.

These four types of problems, especially the first three types of problems will be mutual constraints, cross-influence, from the surface of the phenomenon is sometimes difficult to immediately distinguish, to say party observation and analysis, to find the root cause of the problem in order to fundamentally solve it.

Stability problems: the lack of stability will affect the entire board, the board surface yellowing, foam sheet brittleness.

Melt strength problem: insufficient melt strength will lead to foam sheet bubble hole, longitudinal section of the bubble is very long. To judge whether the melt strength is insufficient, the most direct way is to go to the back of the three rollers and press the sheet wrapped in the middle rollers with your fingers, the melt strength is good and you can feel the elasticity when you press it. If it is difficult to spring up after pressing, it means that the melt strength is poor. Because of the screw structure and cooling methods differ greatly, it is difficult to judge whether the temperature is reasonable, in general, within the load allowed by the extruder, the temperature of the 3-5 zone to low for good. In the foamed pipe to get foaming uniform products, also need to ensure that the PVC material has good melt strength.

Lubrication problems: lubricant is divided into external lubricant and internal lubricant, external slip is conducive to demoulding, the surface of the plate has the benefit of smoothness, external slip is too little, the temperature of the extruder 5 area is not easy to control, easy to heat up, which will lead to the merging core temperature is high, the middle of the plate out of the large bubbles, string of bubbles, yellowing and so on, and the surface of the plate is not smooth; more than the external slip, the precipitation will become serious, the performance of the structure of the mould inside the surface of the plate and precipitation of the external slip The precipitation will become serious, manifested in the structure of the mould and the precipitation of the outer slip on the surface of the plate, which will also be manifested as some individual phenomena moving back and forth on the surface of the plate irregularly. Inside slip is conducive to plasticisation and melt fluidity, inside slip is not enough plate surface is difficult to control the thickness, manifested as the middle of the plate thick and thin; inside slip more, easy to appear merging core temperature high phenomenon. Poor dispersion will bring the phenomenon of plate surface is not smooth.

Process temperature control problems: the four problems mentioned above belong to the fundamental problem, is the foundation, is a deep-seated problem. Relative to the above four problems, process temperature control is much more intuitive, is a surface problem, but the temperature control is not good, will induce the emergence of fundamental problems. Increase the processing temperature, material stability time will be reduced, stability problems; the original lubrication balance will be broken, generally manifested as insufficient external lubrication, especially late external lubrication, the need to increase the amount of external lubrication additive; temperature will also lead to a reduction in the melt strength, foaming sheet vesicle increase, the number of vesicles is reduced, sheet brittle and easy to fracture; temperature increase to reduce the melt strength will reduce the viscosity of the melt, the viscosity of the melt to reduce shear dispersion ability to reduce the viscosity, the viscosity of the melt. Reduced shear dispersing ability to reduce the dispersing ability of the screw is not strong, sometimes uneven dispersion.