Compared with other plastic extrusion products, the production of crusted low-foaming PVC plastic products requires good plasticization and molding under a certain melting pressure. At the same time, it also determines the size, number, and uniformity of foamed cells, and the thickness and hardness of crust , Brightness, finish, etc. have corresponding technical requirements. The quality of the foam is not only related to the formula and raw materials, but also directly affected by the shear performance of the extruder screw, melt pressure, temperature, extrusion (traction) speed, die lip gap, mold template gap, cooling water temperature, etc. Influence and constraints of factors. Therefore, compared with other extruded plastic products, crusted foamed plastic products have certain technical difficulties, and there are many unsolved topics in practice and theory that deserve research and solution by industry technicians.
1. Production technology and characteristics of skinned foamed PVC plastic sheet
The technical difficulty in the production of skinned foamed PVC plastic products is foaming. Foaming is the result of the melt viscosity rising and expanding, and the combined effect of gas pressure and atmospheric pressure produced by the foaming agent.
The foaming process generally goes through 3 stages:
(1) The foaming gas is dissolved into the plasticized homogeneous mixture melt under a certain temperature and pressure in the extruder and reaches a saturated state to form a bubble core;
(2). The melt is extruded from the die. With the release of pressure, the supersaturated dissolved gas nucleates, expands, and precipitates, and uniform and dense cells are generated in the gap between the template.
(3) Under the cooling effect of the setting device, the foamed product is solidified and shaped. Skinned foamed PVC plastic products with excellent quality have the characteristics of many and dense cells, small and uniform, density and thickness in line with requirements, flat appearance, high hardness and brightness.
2, Factors affecting the quality of skinned foamed PVC plastic sheet
The main raw materials that directly affect the quality of plastic cell structures are foaming agents and foaming coordinators. Stabilizers, lubricants, calcium carbonate, and other raw materials and extrusion process control can promote and coordinate foaming quality. Since the melt plasticizing temperature of the foamed product during the extrusion production process must be slightly lower than the decomposition temperature of the foaming agent, the lower the average degree of polymerization of the PVC resin, the lower the processing temperature required for melt plasticizing. To obtain fully foamed plastic products, the viscosity of the resin should not be too high. At the same time, compared with other foamed products, the skinned foamed plastic sheet has a wider cross-section. Therefore, it must be distributed to a width of about 1.3 m from the extrusion head into the die. The pressure of the entire cross-section of the plate should be coordinated, and the melt fluidity is relatively high. The molecular weight of the resin used is generally small, such as SC-7 or SC-8 suspension method loose resin.
