Quality Control Skills of Rotomolding Products

- Mar 13, 2021-

Rotational molding is a thermoplastic hollow molding method. Its main principle is to put powdered plastic raw materials into the mold. During the heating process, the mold rotates around two vertical axes for a certain period of time to melt the plastic raw materials in the mold and evenly coat it. Cover the surface of the inner cavity of the mold, and then cool and shape the mold to obtain a hollow plastic product. Rotational molding has fewer controllable process parameters, and its quality is more difficult to control. The following discusses how to control and improve the molding quality of rotomolding products from three aspects: surface bubbles, deformation and mechanical properties.

1. How to eliminate surface bubbles

Studies have shown that in rotational molding, the generation of surface bubbles is a necessary stage in the process. The existence of surface bubbles not only affects the appearance, but also deteriorates the performance of the product. There are two ways to eliminate surface bubbles:

(1) Use appropriate plastic powder particle size. The influence of plastic powder particle size on surface bubbles is related to the heating temperature in the mold. When the heating temperature is lower (such as lower than 190°C), the smaller the particle size, the more surface bubbles; when the heating temperature is higher (such as higher than 210°C), the smaller the particle size, the fewer surface bubbles, but The particle size should not be too small, otherwise it is not conducive to molding;

(2) Increase the heating temperature. Increasing the heating temperature is the first option to eliminate surface bubbles. Moreover, the smaller the particle size of the powder used, the lower the heating temperature required to eliminate surface bubbles. However, as mentioned earlier, too small powder particle size is not conducive to molding.

2. How to solve the deformation problem

Rotomolding is generally used to form large hollow plastic products, such as various large containers, sporting goods, toy models, kayaks and various boxes, shells, large pipes, etc. These large products are prone to deformation problems. To solve the deformation problem, we can start from the following three aspects.

(1) Adopt the preheating procedure. For slender rotomolding products, especially those with thin ends and thick middle, since the two ends are far away from the heating source, and the thickness is thin, the cooling speed is the fastest, which easily leads to a low temperature here. The low temperature will affect the feeding volume, resulting in uneven wall thickness, resulting in deformation. The two ends of the mold can be pre-heated before forming to increase the temperature here to ensure the amount of material at the two ends, reduce the difference in wall thickness, and slow down the deformation.

(2) Increase the heating temperature in the mold. In the rotomolding product, there is a difference between the bubble and the bubble-free area during cooling, resulting in uneven shrinkage and inducing deformation. As mentioned earlier, increasing the heating temperature is the preferred option to eliminate surface bubbles, which is also applicable to eliminating bubbles in the wall.

(3) Change the cooling method. During the cooling process, if the thickness of the product varies greatly, there will be significant differences in temperature at different parts, which is the main factor for deformation. Therefore, cooling methods that expand the temperature difference cannot be adopted, such as full forced air cooling, water cooling, and so on. Instead, use natural cooling as much as possible, adopt local air cooling for higher temperature parts, and adopt local reheating for lower temperature parts to adjust the temperature difference, so as to avoid major deformation of the product.