Rotational molding products belong to hollow plastic products. Therefore, hollow products with double walls have become a common form. For example, the roll plastic incubator is a typical double-layer wall hollow product. Usually, the incubator will be filled and foamed between the double-layer walls to play the role of heat preservation and enhancement. So, when designing rotational plastic products, how much distance should be left between the two walls, especially between the parallel walls? This involves the problem of rotational molding process and raw materials.

The raw materials of rotational molding are mostly in the shape of powder, and the apparent density of powder is relatively low, and its volume is about three times larger than that after crystallization. Therefore, in order to make the raw material powder fully reach each embedded surface during rotational molding, it is necessary to leave sufficient space between the walls of the rotational molding product with at least three times the wall thickness. In other words, even if the spacing distance between the walls of the rotational molding products reaches three times of the average wall thickness, the powder filled in between will just become two walls after melting. This space filled with powder is easy to cause powder bridging during rotational molding, which hinders the free flow of powder to other parts of the mold cavity. In this way, some parts of the product will have uneven wall thickness, and the bridging area will also cause local thickening. The thickened part will cause internal pores and external shrinkage marks. These thickened parts are heated for a long time and shrink more than the surrounding thin-walled parts, which will lead to increased internal stress and deformation. Therefore, in order to avoid these adverse conditions, the larger the distance between the two walls of rotational molding products, the more conducive to the flow of raw materials. Generally, it should reach at least four to five times the average wall thickness.