Process and Solution for Extrusion Production of PVC Transparent Sheets

PVC calendered transparent sheet is one of the main varieties of rigid PVC, usually produced by conical twin screw extrusion. The common problems encountered in the production process are: bubbles, more crystal points, yellowing of sheet color and so on.

1. There are small bubbles in the sheet.

Reasons for this:

(1) The excessive moisture in PVC raw materials, such as in summer rainy days, when the raw materials are wet, it is possible that the moisture in the barrel can not be fully discharged, heated and expanded, and then formed bubbles in the sheet.

(2) Additional additives may also have excessive moisture content.

To sum up, the solution can be adopted: properly prolong the mixing time of the heat mixer, and use the cooling mixer. It can effectively remove water from raw materials.

2. The number of crystal points in the sheet exceeds the standard, which is also called fisheye.

The common causes of crystal point are: some PVC resin particles are not plasticized, or plasticized poorly. The reasons are: the quality of PVC resin itself is not good. Under the condition of not replacing PVC resin, the solution measures are as follows:

(1) Appropriately prolong the mixing time and increase the mixing temperature to promote the absorption of additives. And the use of high-quality mixing units, mixing units on the pre-plasticizing effect of materials, is irreplaceable.

(2) Mixed raw materials can be placed for half a day or one day, let them ripen and reuse.

(3) The setting temperature of the barrel can be properly increased, the screw speed can be increased, and the pressure of the nose can be increased to further improve the plasticizing performance.

(4) In the formulation, we must carefully select the appropriate additives and dosage of additives. At present, the market of additives is chaotic and the quality of products varies greatly. We should carefully select the additives and find a better balance point in price and performance.

3. Color yellowing

The causes and solutions are as follows:

(1) The stability of PVC itself is poor. If it is a new PVC resin manufacturer, it may be that the stability of PVC itself is poor, you can try changing PVC. However, even the brand of PVC that has been used for a long time may be caused by the PVC resin itself, because the quality of PVC produced by the same manufacturer may vary from batch to batch.

(2) Insufficient stabilizers

(3) There is a problem with the temperature controller, which shows that the temperature is lower than the actual temperature. Excessive temperature will cause the decomposition and yellowing of PVC. At this time, it is necessary to re-check the more accurate temperature controller. The method is simple and accurate with mercury thermometer. In actual production, because of the loosening of thermocouple and the sticking of materials, there will be a gap between the display temperature and the actual temperature of the thermostat. In addition, it is suggested to use high-quality temperature controllers. The usual foreign brands are RKC, OMRON and Shanghai Yatai.

In summary, we should consider the above reasons and choose the best solution. Some solutions are contradictory, such as increasing temperature may cause yellowing of sheets, and adding more stabilizers may increase costs. Only in practice can we explore the way forward.