PVC Common Problems During The Foaming Process

In the extrusion process of PVC foaming sheet, the problems encountered can be classified into 4 categories
1. Is a stability issue
2. It is the melt strength problem
3. It's a lubrication problem
4. Is the problem of decentralization
These four types of problems, especially the first three types of problems, will restrict each other and cross affect each other. It is sometimes difficult to distinguish the problems immediately. Only by observing and analyzing the root cause of the problem can they be fundamentally solved.
Stability problem: insufficient stability, will affect the whole plate surface, plate surface yellow, foaming sheet brittle.
Melt strength problem: insufficient melt strength will lead to large foaming sheet bubble hole and very long longitudinal cut surface bubble. To judge whether the melt strength is insufficient, the most direct way is to press the sheet on the roll wrapped in the middle roll with fingers, and the melt can feel the elasticity when the melt strength is well pressed. If it is difficult to bounce up after pressing, it indicates that the melt strength is poor. Because of the large difference between the screw structure and the cooling mode, it is difficult to judge whether the temperature is reasonable. Generally speaking, within the allowable load of the extruder, the temperature of 3-5 zones is better than low. In order to obtain uniform foam products, it is also necessary to ensure that PVC material has a good melting strength.
Lubrication problem: lubricant is divided into external lubricant and internal lubricant, external slip is beneficial to release, on the plate surface smoothness is too little, the temperature of the extruder area 5 is not easy to control, easy to heat, which leads to the core temperature is high, the plate bubble, series bubble, yellow, the surface of the plate is not smooth, external slip, precipitation will become serious, in the structure of the mold and plate surface slip, also for some individual phenomena on the plate surface is not regularly moving back and forth.The inner slide is beneficial to plasticizing and the melt fluidity. It is difficult to control the thickness, which is more thin; the inner slip is prone to the high temperature of the confluence core. Poor dispersion will bring about the phenomenon that the plate surface is not smooth.
Process temperature control problem: the four problems belong to the fundamental problem, is the foundation, is a deep-seated problem. Compared with the above four problems, the process temperature control is much more intuitive, is a surface problem, but the temperature control is not good, will induce the fundamental problems.Improve the processing process temperature, reduce the material stability time and stability problems; the original lubrication balance will be broken, generally manifested by insufficient external lubrication, especially the late external lubrication, increase the external lubrication amount; the temperature increase also reduces the melt strength, increases the foam bubble hole, the number of bubble hole, sheet brittle and easily fracture; the temperature increase reduces the melt strength, reduces the melt viscosity, reduces the viscosity, shear and dispersion ability, sometimes uneven dispersion.