Problems in the extrusion process of PVC foam board
Mainly classified into 4 categories, one is the stability problem; the second is the melt strength problem; the third is the lubrication problem; the fourth is the dispersion problem. These four types of problems, especially the first three types of problems, will restrict each other and cross-impact. From the surface phenomenon, it is sometimes difficult to distinguish them immediately. To observe and analyze the analysis, the root cause of the problem can be solved.
1. Insufficient stability will affect the entire board surface, the board surface will be yellow, and the foamed sheet will be brittle.
Solution, reduce the processing temperature, if you do not see the improvement formula, increase the amount of stabilizer and lubricant, you can change one by one, easy to find the problem quickly, solve the problem at the fastest speed.
2. Insufficient melt strength results in a large foam cell and a long longitudinal bubble.
To determine whether the melt strength is insufficient, the most direct way is to press the sheet wrapped on the middle roll with a finger after the three rolls, and the melt strength can be felt when pressed. If it is difficult to bounce after pressing, the melt strength is poor. Because the screw structure and the cooling method are quite different, it is difficult to judge whether the temperature is reasonable. Generally speaking, in the load allowed by the extruder, the temperature in the 3-5 zone is preferably low. In order to obtain a foamed uniform product in a foamed sheet, it is also necessary to ensure that the PVC material has a good melt strength.
Therefore, the quality and type of the foaming regulator are essential, of course, the amount of yellow-white foaming agent should also be well matched.
Lubricants are classified into external lubricants and internal lubricants.
External slip is beneficial to demoulding, which is good for the smoothness of the surface of the sheet. The outer slip is too small. The temperature in the 5 zone of the extruder is not easy to control, and it is easy to heat up. This will result in high temperature of the combined core, large bubbles and string bubbles in the middle of the sheet. Yellowing and other problems, the surface of the board is not smooth;
If there is a lot of slippage, the precipitation will become serious. The structure inside the mold and the slippage outside the surface of the sheet will also appear as some individual phenomena moving back and forth irregularly on the board surface.
The internal slip is beneficial to the plasticization and the fluidity of the melt. The inner slip is not enough to control the thickness of the plate surface, and the thickness is thin on both sides of the plate;
There is a lot of internal slippage, and it is easy to have a high temperature of the combined core.
4, poor dispersion will bring the surface of the sheet is not smooth
Process temperature control issues: The four issues mentioned above are fundamental issues, basic and deep-seated. Compared with the above four problems, the process temperature control is much more intuitive and is a surface problem, but the temperature control is not good, which will induce the emergence of fundamental problems. Increasing the processing temperature, the material stabilization time will be reduced, and the stability problem will occur; the original lubrication balance will be broken, generally showing insufficient external lubrication, especially the later external lubrication, the need to increase the amount of external lubrication; temperature increase will also lead to The melt strength is reduced, the cells of the foamed sheet are enlarged, the number of cells is reduced, the sheet is brittle and easy to break; the temperature is increased, the melt strength is lowered, the melt viscosity is also lowered, the viscosity is reduced, the shearing ability is reduced, and the dispersion is dispersed. In the case of a screw that is not strong, uneven dispersion sometimes occurs.
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