- Time of issue:2019-11-22 00:00:00
1.the bubble mechanism:
In the PVC foam  products, the purpose of adding ultra-high molecular weight polymer: First, in order to promote the plasticization of PVC; Second, in order to improve the PVC foam material melt strength, to prevent the merger of bubbles to get uniform hair And the third is to ensure that the melt has good fluidity to obtain a good appearance of the product. Due to the different products of different manufacturers of foam products, the equipment, processes, raw materials and lubrication systems are different, so we have developed a different performance of the foam regulator to meet the different needs of users.
1.Definition of foaming material
Foaming plastic, also known as foam, is a plastic as the basic component, containing a lot of bubbles, it can be said to be filled with gas-based composite materials.
2.Classification of foamed sheets
According to the different expansion ratio, can be divided into high foaming and low foaming, according to the soft texture of the foam, can be divided into hard, semi-rigid, flexible foam. According to the cell structure can be divided into closed-cell foam and open-cell foam. Common common PVC foam sheet is a rigid closed-cell low foam sheet.
3.PVC foam sheet application
PVC foam sheet has resistance to chemical corrosion, weatherability, flame retardant, etc., widely used in many areas, including the display panel, marking, billboards, partitions, building plates, furniture, sheet metal and so on.
4.A key factor in evaluating the quality of foamed sheets
For foamed materials, the size and uniformity of the cell is a key factor in the quality of the sheet. For low-volume foam sheet, the cells are small and uniform, foam sheet toughness is good, high strength, good surface quality. From the viewpoint of reducing the density of the foamed sheet, it is difficult to further reduce the density by making it possible to reduce the density even if the cells are small and uniform.
5. PVC foam process control key points
Plastic foam molding is divided into three processes: the formation of bubble nuclei, the expansion of bubble nuclei and the curing of the foam. For the PVC foamed sheet to which the chemical blowing agent is added, the expansion of the bubble nucleus has a decisive effect on the quality of the foamed sheet. PVC is a linear molecule, the molecular chain is short, the melt strength is low, the bubble in the bubble into the bubble process, the melt is not enough to cover the bubble, the gas easily overflow into a large bubble, reduce the foam sheet product quality
A key factor in improving the quality of PVC foam sheets is to increase the melt strength of PVC. There are several ways to improve the melt strength of PVC from the processing characteristics of polymer materials, but the most effective way is to add additives to improve the melt strength and reduce the processing temperature. PVC is an amorphous material, with the melt temperature increases the melt strength decreases, on the contrary with the melt temperature to reduce the melt strength, but the limited role of cooling only play a supporting role. ACR class of agents have to improve the role of melt strength, in which the most effective foam control agent. The melt strength increases as the suds suppressor content increases. In general, as long as the screw has enough dispersion and mixing capacity, add a high viscosity of the foam regulator to improve the melt strength effect is more obvious.
6.The role of processing aids in PVC foamed sheets
ACR class of processing aids to promote PVC melting, improve surface finish, improve the melt elasticity, enhance the melt elongation and strength of the role. Is conducive to covering bubbles, to prevent cell collapse. The molecular weight and amount of the foaming agent have a significant effect on the density of the foam: as the molecular weight increases, the melt strength of the PVC increases, the density of the foam sheet can be made lower, and the amount of the conditioner Have the same effect. But this effect does not have a linear relationship, continue to increase the molecular weight or increase the amount of the role of reducing the density is not very obvious, the density will tend to be constant.
There is an important relationship between the foaming agent and the foaming agent, and the density of the foamed sheet is in equilibrium with the foaming agent, and at this equilibrium point, the density of the foamed sheet is not affected by the blowing agent content , Maintaining a constant, that is, increasing the amount of blowing agent can not reduce the density, the reason for this phenomenon is due to a certain amount of foam regulator dosage, PVC melt strength is limited, excessive gas lead to cell collapse or merge.
In the PVC foam sheet extrusion process, the basic problems can be classified as four categories, one stability problem; the second is the melt strength problem; third is the lubrication problem; four is the dispersion problem. These four types of problems, especially the first three types of problems will be mutual constraints, cross-impact, from the surface phenomenon is sometimes difficult to immediately clear, to observe the analysis, find the root causes of the problem can be solved.
8.Lack of stability, will affect the entire board, the board is yellow, foam sheet brittle
Insufficient melt strength will cause the foam cells to be large, and the longitudinal cut vesicles are long. To determine whether the melt strength is insufficient, the most direct way is to press the back of the three rollers behind the package in the roll on the sheet, the melt strength can feel flexible when pressed. If it is difficult to bounce after pressing, indicating poor melt strength. Because the screw structure and cooling methods vary widely, it is difficult to determine whether the temperature is reasonable, in general, in the extruder to allow the load, 5-zone temperature is low as well.
In the foam pipe in order to obtain foam uniform products, but also need to ensure that PVC material has a good melt strength. The development of the company's foam additives is to solve the problem of foam products developed, and can effectively improve the quality of foam pipe.
Lubricants are divided into external lubricants and internal lubricants, slip is conducive to stripping, the smoothness of the surface of the board is good, too much outside the slide, the extruder 5 area temperature is not easy to control, easy to heat up, which will lead to confluence High temperature, the middle of the plate out of the big bubble, string bubble, yellow and other issues, the surface of the plate is not smooth; outside the slide, precipitation will become serious, the performance of the mold and the surface of the sheet outside the slippery, For some individual phenomenon in the board from time to time to move back and forth. The slippage is conducive to the plasticity and melt flow, the slippage is difficult to control the thickness of the plate, the performance of the middle plate thickness on both sides of the thin; within the slide more prone to high core temperature phenomenon.
9.Poor dispersion will bring the surface of the plate is not smooth
Process temperature control problem: the four issues mentioned above are fundamental issues, is the basis of deep-seated problems. Compared to the above four issues, the process temperature control will be much more intuitive, is the surface problem, but the temperature control is not good, will lead to the emergence of fundamental problems. Improve the processing temperature, material stability time will be reduced, the stability of the problem; the original lubrication balance will be broken, the general performance of the external lubrication is insufficient, especially after the late lubrication, need to increase the amount of external lubrication; The melt strength decreases, the foam sheet increases, the number of cells decreases, the sheet is brittle and easy to break; the temperature increase reduces the melt strength and reduces the melt viscosity, the viscosity reduces the shear dispersing ability, Ability is not strong in terms of screws, sometimes there will be scattered uneven.