Although, it is an Internal Lubricant only that promotes plasticization, improves polymer fluidity and the surface finish of products. External Lubricant like PE wax is used to reduce the friction between the polymer and the surface of the machine to prevent it from adhering to the mechanical surface. High melt OPE waxes, provide good lubrication in middle and late processing. Lubricants should follow the principle of lubrication in the early, middle and final stages so that all materials can be protected at all stages, and the product must be stable for a long time without scaling. Internal and external lubrication systems are usually composed of PE and OPE waxes are used.
To obtain a uniform cell structure, the gas is either injected or evolved by heat and must be thoroughly dispersed in the polymer melt. Some of the essential factors on which the cell structure is depended are:
-Speed and pressure of the gas
-Dispersive properties of the machine
-Decomposition rate of foaming agent
-The melt viscosity of the PVC resin
Most foam profile is extruded using the celluka method also known as the inward foaming process. In this method the calibration unit is situated adjacent to the die and has the same dimensions as the die, thus controlling foam expansion. This setup gives very rapid cooling. Bubble formation is quenched in the surface of the product and solid outer skin is produced, usually about 0.5 mm thick, and a lower density core. The outer surfaces of celluka products are the same as those of solid extrusions.
This structure gives the material higher mechanical strength and lowers thermal conductivity. Thus, the two major differences between U-PVC and foamed PVC is that foamed PVC is much stronger and offers much better insulation (higher V.R. value) due to its cellular structure.
Foamed PVC is usually used for construction, furniture, signboards and outdoor advertising. It can also be used for window frames and guarantees better efficiency and performance. The solid foamed PVC construction provides the strength and durability of wood or non-porous plastic while maintaining the ease of maintenance of a standard PVC hollow chamber and lightness.
