For free foaming, after the melt leaving the Die mouth which containing the foaming agent, it begins to expand freely. Within a certain distance from the die, the foamed product with the required shape and foaming density is obtained through the forming/sizing unit. The density of the section of this foamed 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 3000mm2, mainly thin-walled parts.
For the skinning foaming method, the calibrator is directly connected to the die, and its outer shape is the same as the die. The result of this setting form is that once the melt containing the foaming agent leaves out the die, the entire product surface is rapidly cooled, which preventing the formation of cells on the surface of the product. It also prevents any swelling of the external dimensions on the cross-section of the extrudate. 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 calibration process. The density of this foamed product varies greatly in the cross-section, the core density is lower, and the outer surface layer density is high. This method can also produce pipes, panels 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 skinning foaming allows the cross-sectional area of PVC products >3000mm2, some over 30000mm2.