The direct foaming method used by PVC foam board manufacturers is mainly formed by two processes, one is a free foaming process and the other is a controllable foaming process. 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 obtains the desired shape and the foamed product required by the foaming density through the setting device. The density of the foam product is approximately the same in cross section, and the periphery is surrounded by a dense but thin outer skin. Pipes, plates, and profiles can be formed by the free foaming method under suitable process conditions. At present, it is more consistent that the application of the free foaming method is often limited to products with smaller cross sections, mainly thin-walled parts.
For the controllable foaming method, the shaping device is directly connected to the die mouth, and its outer contour is the same as the die. As a result of this type of setting, the melt containing the foaming agent, as soon as it leaves the die, causes the entire extrudate surface to be rapidly cooled, preventing the formation of surface cells in the extrudate and the extrudate cross section Swelling of any form factor. At the same time, the cavity generated by the core in the die mouth is filled by the cells formed by the melt during the shaping process. The density of this foam product varies greatly in cross section, the core density is low, and the density of the outer surface layer is high. This method can also produce pipes, sheets 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 the controlled foaming allows the cross-sectional area of PVC products Bigger.
