Process Problems and Solutions of PVC Transparent Sheet Extrusion Production

PVC sheet extrusion line for transparency is one of the main varieties of rigid PVC. It is usually produced by PVC conical twin-screw extruder. Common problems encountered in the production process are: bubbles, more crystal points, and yellowish color of the sheet. Based on the actual operation experience of the company’s machine debugging personnel, I have sorted out some information, hoping to be helpful to my peers.

If there is any inappropriateness, please feel free to enlighten me.
1.Yellow color problem

The causes and solutions
First, the stability of PVC itself is poor
If it is a new PVC resin manufacturer, it may be that the stability of the PVC itself is poor, you can try changing the PVC. However, even if it is the brand PVC that has been used all the time, it may be caused by the PVC resin itself, because even if the PVC produced by the same manufacturer, the quality may vary in different batches.
Second, insufficient stabilizer
Third, there is a problem with the temperature controller
The displayed temperature is lower than the actual temperature, and the PVC will decompose and turn yellow if the temperature is too high. At this time, it is necessary to recheck the calibration temperature controller. The method is very simple, and the mercury thermometer is used for calibration. In actual production, due to reasons such as loose thermocouples and sticking materials, there will be a gap between the temperature displayed by the temperature controller and the actual temperature. In addition, it is recommended to use a high-quality temperature controller.

2.The sheet has small air bubbles

①.Excessive moisture in PVC raw materials: If the raw materials are damp in summer rainy days, the water may not be fully discharged in the barrel, and the heat will expand, and then air bubbles will be formed in the sheet.
②.In addition, the additives may contain too much water.To sum up, the solution can be adopted: Prolong the mixing time of the hot mixer appropriately, and use the cooling mixer. Can effectively remove moisture from raw materials.
3.The number of crystal points in the sheet exceeds the standard, also known as fisheye

The common reason for the generation of crystal points is: some PVC resin particles are not plasticized, or the plasticization is not good. The reasons for the formation are: the quality of PVC resin itself is not good. Under the condition of not replacing the PVC resin, the solution is as following:
First, appropriately prolong the mixing time and increase the mixing temperature to promote the absorption of additives. In addition, high-quality mixing units are used, and the pre-plasticizing effect of the mixing units on materials is irreplaceable.
Second, the mixed raw materials can be left for half a day or a day, let them mature, and then used.
Third, the set temperature of the barrel can be appropriately increased, the screw speed can be increased, and the head pressure can be increased to further improve the plasticizing performance.
Fourth, in terms of formula, we must carefully choose the appropriate additives and the amount of additives. At present, the additives market is chaotic, and the product quality varies greatly. We must choose carefully and find a better balance between price and performance.

In summary, the above reasons should be comprehensively considered to choose the optimal solution. Some solutions have contradictory relationships, such as increasing the temperature may cause the sheet to turn yellow, adding more stabilizers may cause cost increases, etc. Only by groping the way forward in practice.