Problems and solutions caused by PVC profile degradation

Degradation

It is well known that PVC (polyvinyl chloride) is a heat-sensitive plastic with poor light stability. Under the action of heat and light, it is easy to undergo HCl removal reaction, which is usually called degradation. The result of degradation is that the strength of plastic products decreases, discoloration, and black lines appear. In severe cases, the products lose their use value. Factors affecting PVC degradation include polymer structure, polymer quality, stabilization system, molding temperature, etc. According to experience, the yellowing of PVC profiles is mostly due to the appearance of paste at the die. The reason is that the die flow channel is unreasonable or the local polishing in the flow channel is not good, and there is a stagnant area. The yellow line of PVC profiles is mostly caused by the appearance of paste in the barrel. The main reason is that there is a dead angle between the sieve plate (or transition sleeve) and the material flow is not smooth. If the yellow line is a vertical straight line on the PVC profile, the stagnant material is at the die outlet; if the yellow line is not straight, it is mainly in the transition sleeve. If the yellow line appears when the formula and raw materials remain unchanged, the cause should be mainly found from the mechanical structure, and the starting point of decomposition should be found and eliminated. If the cause cannot be found from the mechanical structure, it should be considered that there is a problem with the formula or process. The measures to avoid degradation include the following aspects:

(1) Strictly control the technical indicators of raw materials and use qualified raw materials;

(2) Establish reasonable molding process conditions under which PVC materials are not easy to degrade;

(3) The molding equipment and molds should have good structures, and the dead corners or gaps that may exist between the equipment and the material should be eliminated; the flow channel should be streamlined and of appropriate length; the heating device should be improved to increase the sensitivity of the temperature display device and the efficiency of the cooling system.

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