Rigid PVC microcellular foaming materials generally have three extrusion foaming processes.
(1) Free foaming: Free foaming is the free expansion of the melt as it exits the die without restriction, and after a short period of time, it enters the larger sizing device. Free foaming results in the formation of cells in the cross section of the extrudate, through cooling The growth of the surface cells is limited to a certain extent, resulting in a continuous density of hard, moderately smooth, smooth products. The advantage of this method is that the process is simple, suitable for producing a thickness of 2 ~ 6mm, a simple geometric shape, matte surface Products (such as pipes, sheets, and simple shapes with geometric shapes, etc.).
(2) Inward-foaming: The crust foaming method or the Celuka method. Using a special, core-in-core die, the plasticized material is diverted, the shaping device is connected to the die, and its outer contour is the same as that of the die. When the material is sent to the sizing jacket before demoulding, the melt containing foaming agent enters the cooling jacket as soon as it exits the oral membrane, undergoes rapid cooling over the entire surface, thereby preventing the formation of the surface cells and extrusion Any expansion on the cross-section of the material, thereby cooling the surface to form the skin. At the same time, the core in the die causes the cavity created in the semi-finished product to be filled with the foam formed by the rest of the melt, ie to foam inside. By controlling the cooling strength, it is possible to obtain products with a surface layer thickness of 0.1 to 10 mm and a product wall thickness of more than 6 mm. This method can produce profiled profiles with complex cross-section shapes, which are characterized by high surface smoothness and hardness and low core area density. Combining the method with Method 1, one side of the skin can be obtained, and the other side is a self-free product.
(3) Coextrusion: Through a combination head, the two extruders are used to separate the non-foamed surface layer and the foamed core layer, and the type or formula of the two plastics can be adjusted according to the requirements, so that the products meet the density and size required by the standard. Most of the core foam tubes produced are produced using this process.
Although the above three processing methods have their own characteristics in formula composition, head structure, and processing technology, how to control the foaming behavior of the melt and obtain a satisfactory cell structure in the extrusion process is common in the extrusion process. The core issue.
The final foaming process of the gas dissolved in the melt actually occurs 'suddenly' after the melt exits the die of the machine head. After the melt exits the die, the melt disintegrates due to a sudden drop in ambient pressure and a change in temperature. The gas is in a supersaturated state. The gas-liquid two phases are separated, and a large number of micro-cell pores are formed on the nucleation sites. The micro-cell pores continuously absorb the gas evolved from the surrounding melt and swell, and finally are shaped due to the cooling of the melt. The size of bubble growth depends on the saturated vapor pressure of the decomposed gas and the ductility and strength of the melt itself. On the one hand, the cells continue to grow under the influence of gas pressure. On the other hand, the strength and ductility of the melt The growth of air bubbles will be limited. It will be decided whether breakage or foaming will occur. Once the expanding force of gas outwards and the viscoelasticity of the melt increased due to cooling, it should be immediately cooled and fixed to maintain the cell structure and prevent collapse. Foaming. In the actual extrusion foaming process, the key factor in the quality of the ring-shaped foamed products is to control the formation and growth of bubbles, so that they form a small and uniform and mutually independent cell structure.