The molding of polyurethane foam involves two stages: the foaming process and the curing process. The process from the mixing of raw materials to the cessation of foam volume expansion is known as the foaming process. In the foaming process of polyurethane foam machines, each bubble is subjected to two forces: atmospheric pressure (external pressure) and the pressure generated by the blowing agent (internal pressure). When the internal pressure exceeds the external pressure, the volume of the foam system continues to expand. When the foam reaches a point where internal and external pressures are equal, the foam ceases to enlarge.
After the foam stops expanding, the internal chemical reactions are not completely finished, and a slower cross-linking reaction continues until the foam reaches its final strength. This process is known as the curing process of the foam. The cessation of foam volume expansion marks the end of the foaming process and the beginning of the curing process. During curing, the foam no longer releases a significant amount of heat, and the temperature gradually decreases to match the ambient temperature.
At this point, the pressure inside the cells also decreases, leading to a pressure difference between the inside and outside. The pressure difference has a certain impact on the foam structure: if the strength of the cell walls is sufficient to support the pressure difference, the foam does not undergo significant deformation; if the strength of the cell walls is weaker, shrinkage of the foam occurs under the action of the pressure difference.
In theory, achieving immediate stability of the cell structure after the foam expansion stops is the best way to address foam shrinkage. In practice, it is currently impossible to achieve in the industry, meaning that the molding shrinkage rate is not zero. However, by adjusting the foaming materials with a scientifically formulated production process, the molding shrinkage rate can be controlled to within 1%.
