pvc direct foaming method (also called directional foaming), this kind of extrusion foaming is inseparable from free foaming process and controllable foaming process (also called skinning foaming method, inward foaming method)
For free foaming, after the melt containing the foaming agent leaves the die, it begins to expand freely within a certain distance from the die, and a foamed product with the required shape and foaming density is obtained through the shaping device. The density of the cross section of the foam product is approximately the same, and the periphery is surrounded by a dense but thin outer skin. Tubes, plates, and profiles can be formed by free foaming under suitable process conditions. The current consensus view is that the application of the free foaming method is often limited to products with smaller cross-sections, the cross-sectional area is generally less than 3000 square mm, mainly thin-walled parts.
For the controllable foaming method, the shaping device is directly connected to the die, and its outer contour is the same as that of the die. The result of this styling form is that once the melt containing the blowing agent leaves the die, the entire surface of the extrudate is rapidly cooled, preventing the formation of cells on the surface of the extrudate and the cross-section of the extrudate. Any swelling phenomenon on the external dimensions. At the same time, the cavity generated by the mold core in the die is filled with the cells formed by the melt during the shaping process. The cross-sectional density of this foamed product varies greatly, the core density is low, and the outer surface layer density is high. This method can also produce pipes, plates and profiles, and the strength structure is better than free foaming, so it is often used to manufacture PVC low-foaming structural materials that meet different requirements, and controllable foaming allows the cross-sectional area of PVC products >3000m square millimeter, and some more than 30,000 square millimeters.