Plastic foam forming is divided into three processes: formation of bubble nuclei, expansion of bubble nuclei, and solidification of bubbles. For a PVC foamed sheet to which a chemical foaming agent is added, the expansion of the bubble nucleus has a decisive influence on the quality of the foamed sheet. PVC is a straight-chain molecule with a short molecular chain and a low melt strength. During the expansion of a bubble nucleus into a bubble, the melt is insufficient to encapsulate the bubble, and the gas easily overflows and merges into a large bubble to reduce the product quality of the foamed sheet. .
The key factor to improve the quality of PVC foamed sheet is to increase the melt strength of PVC. From the analysis of the processing characteristics of polymer materials, there are many ways to improve the melt strength of PVC, but the most effective way is to add additives to improve melt strength and reduce the processing temperature. PVC is an amorphous material. With the increase of melt temperature, the melt strength decreases. On the contrary, the melt strength increases with the decrease of melt temperature. However, the effect of cooling is only limited. ACR-based processing agents have the effect of increasing melt strength, with foaming modifiers being the most effective. The melt strength increases as the foam modifier content increases. In general, as long as the screw has sufficient dispersive mixing capability, adding high viscosity foaming modifiers is more effective in improving melt strength.