Rotomolding products are simple and practical, which is a big advantage, but it also brings some deficiency to the technology. For example, the inner part of products is prone to produce bubbles, and the surface is easy to become empty; products are easy to bend, shrinkage and change color, etc. These phenomena not only influence the appearance of the product image but also cause serious damage to the mechanical performance of products. Therefore, the research of bubbles’ formation and disappearance, bending, the formation of shrinkage and disappearance, and impact strength of products is of great significance to the improvement the quality of molding products.
There are many factors affecting impact strength of products, such as heating temperature, heating time, cooling rate, particle size, mould materials and so on, among which heating temperature and heating time are the main factors affecting the impact strength. If the temperature is too low, molten material is not enough, leading to impact strength of products decreasing, otherwise, plastic would produce degradation, decreasing the impact strength of products, too. The melt flow rate MFI has direct relationship with its impact strength, MFI falls with the impact strength, thus MFI can be used as indication of the impact strength.
Shrinkage and deformation is also another big defect of
rotomolding products, which is usually caused by the formation of residual stress when cooling. It is proved by experiment that alternating cooling in different ways of cooling can reduce the residual stress, or fill the mold with compressed air during cooling to prevent it drop from the mold wall or deformation, which has the effect of cooling to finalize the design. And because this method cools the inner surface of the hardened plastic at the same time, having changed the cooling mechanism, so as to reduce the residual stress, restrain the shrinkage and deformation of products. Experiments prove that most of the plastic bubbled with 0.11 MPa compressed air when cooling, the phenomenon of shrinkage and deformation will basically eliminate.
