For Free Foaming Method, melts containing foaming agent start expanding after leaving mould inlet. Needed foaming products with wanted shape and density come into being from a certain distance to mould inlet. Density on the cross section of this kind of foaming product is almost the same.they are surrounded by tight surfaces. PVC pipe, plates and profile can be moulded by free foaming moulding method with proper technology. It is a mainstream conclusion that application of free foaming method is limited to PVC products with cross section area of 3000mm2 maximum.
For Controllable Foaming Method, moulding device joints together with mould inlet. Its external outline is the same as inlet mould. This type of moulding form results in extruded products being cooled quickly once melts coming out of mould inlet. It will also help prevent extruded products from cell structure on the surface and expansion on shape and size. Meanwhile, cavities inside the mould inlet shall be filled up with cell from melts in the process of moulding. Density on the cross section of this kind of foaming products varies a lot. Density is low in the core, while high in surfaces. This kind of method can also be used for PVC pipe, plate and profile making. And foaming products produced with this method have higher strength than those by free foaming method. So this method is usually used for PVC low foaming products. Controllable foaming method will allow PVC products to have a bigger cross-sectional area, more than 3000mm2.