1. Mold release agent
In the heating stage of the rotational molding process, chemical or physical bonding will occur at the interface between polyethylene powder or melt and the inner surface of the mold due to surface oxidation. When there are local defects on the inner surface of the mold, polyethylene melt will flow into these defects and form local embedment. This will make it difficult to take the cooled product out of the mold. In order to avoid the above situation, it is necessary to coat a layer of heat stable material on the inner surface of the mold to prevent adhesion, which is called release agent. There are many kinds of release agents for industrial use. The rotational molding process of polyethylene has high requirements for release agents, mainly for heat resistance. Oil, wax and silicone oil are commonly used mold release agents, but they need to be coated once before each feeding, so they are called disposable mold release agents. This kind of release agent has low cost and good demoulding effect, but it is easy to migrate to the surface of the product to affect its surface performance. Cross linked siloxane is a semi permanent mold release agent. It does not need to be coated frequently, does not migrate, and is not affected by temperature changes. It has good mold release effect, but its cost is high.
The permanent demoulding effect can be obtained by adding a thin layer of polytetrafluoroethylene on the surface of the mold cavity (just like the non stick pot sold in the market), and polytetrafluoroethylene is a permanent demoulding agent.
2. Temperature control
There is a special phenomenon in the rotational molding process of polyethylene: during the powder melting process, the air trapped between the powder particles forms bubbles, which disappear with the continuous heating process. Further research shows that the disappearance of these bubbles is not because they move to the free surface of the melt under the effect of buoyancy, but because the air in the bubbles gradually melts into the molten plastic melt. The experiment shows that when the temperature rises to 150 ℃, bubbles of different sizes are formed in the polyethylene melt. Due to the high viscosity of polyethylene melt, the buoyancy of bubbles is not enough to push bubbles to the free surface. When the temperature rises to 200 ℃, all bubbles disappear. Therefore, for polyethylene roll molding, scientific control of the heating process is of great significance to eliminate bubbles in polyethylene products and improve product quality. The heating time of rotational molding 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 materials and the degradation of material properties during heating. Usually, the purpose of prevention can be achieved by adding antioxidants to polyethylene plastics. However, when the polyethylene material is heated to a high temperature or for a long time, the antioxidant cannot prevent the oxidation of the material. When the thickness of the product is large and 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 air in the bubbles may not disappear in time to retain them. When polyethylene plastic is heated to the molten state, the material will undergo a process of transformation from the crystalline state to the melt, which is exactly the process that occurs when polyethylene particles begin to melt and become soft. It first appears in a layer of material in contact with the inner wall of the mold, forming a uniform layer of molten material. Then, gradually expand to the inner layer until the full section is completely turned into plastic melt. The next step is to continue heating so that 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 polyethylene melt will drop from 200 ℃ to near room temperature, and the molecules of polyethylene will change from a disordered state to a more ordered crystalline state. The crystallization process takes a certain time, and the crystallization rate is related to the viscosity of polyethylene melt. When the polyethylene melt is rapidly cooled, the viscosity of the polyethylene melt increases rapidly, which hinders the growth of its grains, thus affecting the crystallinity of polyethylene. When the crystallinity is different, the density and physical properties of polyethylene products will be different. As a result, the rapidly cooled polyethylene roll molded products have a lower density, while the slowly cooled products have a higher density. Of course, the slower the product cools, the longer the production cycle, and the higher the cost. Polyethylene powder used in rotational molding production has a certain density, which depends on the manufacturer of the material. However, after rotational molding production, the density of polyethylene rotational molding products will change due to different cooling rates.
