Compared with other plastic extrusion products, the production of low foaming PVC plastic products with crust not only requires good plasticization and molding under a certain melting pressure, but also has corresponding technical requirements for the size, quantity and uniformity of foam pores, as well as the thickness, hardness, brightness, and smoothness of the crust. The quality of foam is not only related to the formula and raw materials, but also directly influenced and restricted by factors such as the shear performance of the extruder screw, melt pressure, temperature, extrusion (traction) speed, die lip gap, setting template gap, and cooling water temperature. Therefore, compared with other extruded plastic products, skinned foam plastic products have certain technical difficulties, and there are many unsolved issues in practice and theory that are worth researching and solving by industry technicians.

        1. Production Technology and Characteristics of Skinning Foamed PVC Plastic Sheet

        The technical difficulty in the production of skinned and foamed PVC plastic products is foaming. Foaming is the result of the increase and expansion of melt viscosity, as well as the combined action of gas pressure and atmospheric pressure generated by foaming agents.

        The foaming process generally goes through three stages:

        (1) The foaming gas dissolves into the uniformly plasticized mixture melt at a certain temperature and pressure in the extruder and reaches a saturated state, forming bubble nuclei;

        (2) The melt is extruded from the mold, and with the release of pressure, the supersaturated dissolved gas nucleates, expands, and precipitates, generating uniform and dense bubbles in the gap of the molding template;

        (3) Under the cooling effect of the molding device, the foam product solidifies and shapes. Excellent quality skinned and foamed PVC plastic products have the characteristics of multiple and dense foam pores, small and uniform foam, density and thickness meeting requirements, smooth appearance, high hardness and brightness.

        2. Factors affecting the quality of skinned and foamed PVC plastic sheets

        The main raw materials that directly affect the quality of plastic foam structure are foaming agents and foaming coordinators. Raw materials such as stabilizers, lubricants, calcium carbonate, and extrusion process control have a certain promoting and coordinating effect on foam quality. Due to the fact that the plasticization temperature of the melt in the extrusion production process of foam products must be slightly lower than the decomposition temperature of the foaming agent, the lower the average polymerization degree of PVC resin, the lower the processing temperature required for melt plasticization. To obtain fully foamed plastic products, the viscosity of the resin used should not be too high. At the same time, compared with other foam products, the skinned foam plastic sheet has a wider cross-sectional area for molding, and it needs to be distributed around a width of 1.3 meters from the extruder head to the mouth mold. In order to facilitate the dispersion and expansion of bubbles, the pressure across the entire cross-sectional area of the sheet should be coordinated and consistent, with higher requirements for melt fluidity. The resin molecular weight used is generally smaller, such as SC-7 or SC-8 suspension method loose resin, which is more suitable.