1. Release agent
In the heating stage of the rotomolding process, at the interface between the polyethylene powder or melt and the inner surface of the mold, chemical or physical bonding occurs due to the surface oxidation. When there are local defects on the inner surface of the mold, the polyethylene melt will flow into these defects and form local embedding. This will make it difficult to remove the cooled product from the mold. In order to avoid the above situation, a layer of thermally stable material needs to be coated on the inner surface of the mold to prevent sticking. This type of material is called a mold release agent. There are many types of mold release agents for industrial use. The rotational molding process of polyethylene has higher requirements for mold release agents, mainly heat resistance. Oils, waxes and silicone oils are commonly used mold release agents, but they need to be applied once before each feeding, so they are called disposable mold release agents. This type of mold release agent has low cost and good mold release effect, but it is easy to migrate to the surface of the product and affect its surface performance. Cross-linked siloxane is a semi-permanent mold release agent. It does not require frequent application, will not migrate, and will not be affected by temperature changes. It has good mold release effect, but the cost is higher.
A thin layer of polytetrafluoroethylene (like a commercially available non-stick pan) is compounded on the surface of the mold cavity to obtain a permanent mold release effect. PTFE is a permanent mold release agent.
2. Temperature control
There is a special phenomenon in the rotomolding process of polyethylene: during the powder melting process, the air trapped between the powder particles forms bubbles, and these bubbles disappear as the heating process continues. Further research shows that the disappearance of these bubbles is not due to their movement to the free surface of the melt under the effect of buoyancy, but because the air in the bubbles gradually merges into the molten plastic melt. Experiments show that when the temperature rises to 150 ° C, bubbles of different sizes are formed in the polyethylene melt. Due to the high viscosity of the polyethylene melt, the buoyancy of the bubbles is insufficient to push the bubbles to the free surface. When the temperature rose to 200 ℃, all the bubbles disappeared. Therefore, for the rotomolding of polyethylene, scientifically controlling the heating process is very important to eliminate bubbles in polyethylene products and improve product quality. Because the heating time of rotomolding is sometimes longer, especially when the product wall is thick. It may last from half an hour to more than an hour. At this time, it is required to take measures to prevent the thermal oxidation of the material during the heating process and the degradation of the material performance. Usually, the antioxidant can be added to the polyethylene plastic to achieve the purpose of prevention. However, when the polyethylene material is heated to an excessively high temperature or the heating time is too long, the antioxidant cannot prevent the oxidation of the material. When the thickness of the product needs to be heated for a long time, the heating temperature must be reduced. If the heating time is shortened by increasing the temperature, the bubbles may remain because the air in the bubbles is too late to disappear. When the polyethylene plastic is heated to a molten state, the material will undergo a process of transformation from the crystalline state to the melt, which is exactly what happens when the polyethylene particles begin to melt and soften. It first appears in a layer of material in contact with the inner wall of the mold to form a uniform layer of molten material. Then, gradually expand to the inner layer until the full cross-section becomes completely plastic melt. Next is the process of continuing heating to make the bubbles gradually disappear. The temperature control and time control of this process need to be adjusted.
3. Cooling process
During the cooling process, the temperature of the polyethylene melt will drop from 200 ° C to near room temperature, and the polyethylene molecules will change from a disordered state to a more ordered crystalline state. The crystallization process requires a certain amount of time. The speed of crystallization is related to the viscosity of the polyethylene melt. When the polyethylene melt is rapidly cooled, the viscosity of the polyethylene melt increases rapidly, so that the growth of its crystal grains is hindered, thereby affecting the crystallinity of the polyethylene. When the crystallinity is different, the density of polyethylene products is different, and the physical properties will also be different. Thus, rapidly cooled polyethylene rotomoulded products have a lower density, while slowly cooled products have a higher density. Of course, the slower the product cools, the longer the production cycle and the higher the cost. The polyethylene powder used for rotomolding itself has a certain density, which depends on the material manufacturer. However, after rotomolding production, due to the different cooling rates, the density of polyethylene rotomolding products will change to some extent.