PVC forming processing performance and characteristics
1. The viscous flow temperature of polyvinyl chloride is 136 ° C, and the decomposition temperature is 140 ° C, which is very close to each other. Therefore, PVC plastic processing is difficult. To this end, it is necessary to add a plasticizer to lower the viscous flow temperature, or to add a stabilizer to increase the decomposition temperature, or both. Regardless of the substance to be added, the purpose is to increase the range between the viscous flow temperature and the decomposition temperature to facilitate the molding process. In addition, the addition of stabilizers to polyvinyl chloride also extends the life of the product.
2. Whether it is hard or soft polyvinyl chloride, the rheological behavior of the melt is non-Newtonian, which means that its viscosity change is mainly related to the shear rate. In particular, heat-sensitive polyvinyl chloride is not only unhelpful but also harmful in order to increase fluidity and lower viscosity. Further, polyvinyl chloride has a large melt viscosity and poor fluidity. Therefore, pay special attention to the pressure control in the production process. At the same time, the flow path of the injection mold is required to be thick and short.
3. Polyvinyl chloride contains about 5% of a crystallite structure with a melting point of 200 °C. This is particularly harmful to the processing of polyvinyl chloride, but it cannot be removed. To this end, a plasticizer is added to lower the melting point and facilitate molding.
4. Polyvinyl chloride is a heat-sensitive resin, which is easy to remove hydrogen chloride under the action of heat or other factors, thereby causing degradation, cross-linking, etc., and the performance of the resin is deteriorated. Therefore, in addition to the addition of suitable stabilizers, long-term or multiple heats should be avoided during molding. For example, in the extrusion molding process, the aspect ratio of the screw should be smaller.
The deep groove screw should be used, and the gap between the screw and the barrel should not be too large. In the injection molding process, the rapid plasticization and high speed should be used. In the injection process, a yellow decolorizer or a colorant which can be masked by the degradation thereof is added, and a vent hole or the like is provided on the mold.
5. Polyvinyl chloride is a brittle resin. If a plasticizer is added thereto, a tough polyvinyl chloride product can be obtained. In addition, an impact modifier such as MBS, ABS, CPE or the like may also be added thereto. Their addition will introduce plastic components into the plastic to enhance the impact strength of the material.
6. The melt strength of polyvinyl chloride is low, and the adhesion between resins is not high, so that unstable flow phenomena such as melt fracture or shark skin disease are liable to occur. If the processing modifier ACR or M80 is added thereto, these disadvantages can be overcome, and at the same time, the gelation speed of the resin in the plasticizing process can be accelerated, which is advantageous for improving the fluidity of the melt and improving the quality of the product.
7. Polyvinyl chloride is an amorphous polymer, so its molding shrinkage is not large, and the rigid polyvinyl chloride is 0.1 to 0.4%. Soft products range from 1 to 5%.
8. The powdered material for molding is preferably dried or preheated before molding to remove moisture and improve the plasticizing effect. Otherwise, the surface of the product may be poorly finished, bubbles may appear, and even mechanical properties and electrical properties may be lowered.