Key element control in the rotational molding process of polyethylene

1. Release agent
During the heating stage of the rotational molding process, chemical or physical bonding will occur at the interface between the 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, the polyethylene melt will flow into these defects and form local embedding. This will make it difficult to remove the product from the mold after cooling. In order to avoid the above situation, it is necessary to apply a layer of heat-stable material on the inner surface of the mold to prevent adhesion. This type of material is called a release agent. There are many kinds of industrial release agents. The rotational molding process of polyethylene has high requirements for release agents, mainly heat resistance. Oils, waxes and silicone oils are commonly used release agents, but they need to be applied once before each feeding, so they are called disposable release agents. This type of release agent has a low cost and a good demolding effect, but it is easy to migrate to the surface of the product and affect its surface properties. Cross-linked siloxane is a semi-permanent release agent. It does not require frequent application, will not migrate, will not be affected by temperature changes, and has a good demolding effect, but the cost is high.
Compounding a thin layer of polytetrafluoroethylene on the surface of the mold cavity (like a commercial non-stick pan) can achieve a permanent demolding effect. Polytetrafluoroethylene is a permanent demolding agent.
2. Temperature control
There is a special phenomenon in the polyethylene rotational molding process: during the powder melting process, the air trapped between the powder particles forms bubbles, and as the heating process continues, these bubbles disappear. Further research shows that the disappearance of these bubbles is not due to their movement to the free surface of the melt under the action 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 not enough to push the bubbles to the free surface. When the temperature rises to 200°C, all the bubbles disappear. Therefore, for the rotational molding of polyethylene, scientifically controlling the heating process is of great significance to eliminating bubbles in polyethylene products and improving product quality. Because the heating time of rotational molding is sometimes longer, especially when the wall of the product is thicker. It may last from half an hour to more than an hour. At this time, measures are required to prevent the thermal oxidation of the material and the reduction of material properties during the heating process. Usually, antioxidants are added to polyethylene plastics to achieve the purpose of prevention. However, when the polyethylene material is heated to too high a temperature or the heating time is too long, the antioxidant cannot prevent the oxidation of the material. When the product thickness is large and needs to be heated for a long time, the heating temperature must be lowered. If the heating time is shortened by increasing the temperature, the bubbles may be retained because the air in the bubbles does not have time to disappear. When the polyethylene plastic is heated to a molten state, the material will undergo a process of transformation from a crystalline state to a melt, which is exactly what happens when the polyethylene particles begin to melt and soften. It appears in a layer of material that contacts the inner wall of the mold, forming a uniform layer of molten material. Then, it gradually expands to the inner layer until the entire cross-section is completely transformed into a plastic melt. The next step is to continue heating to make the bubbles disappear gradually. 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 molecules of the polyethylene will change from a disordered state to a more ordered crystalline state. The crystallization process takes a certain amount of time, and the speed of crystallization is related to the viscosity of the polyethylene melt. When the polyethylene melt is cooled rapidly, the viscosity of the polyethylene melt increases rapidly, which hinders the growth of its crystals and affects the crystallinity of the polyethylene. When the crystallinity is different, the density of the polyethylene product is different, and the physical properties will also be different. Therefore, the rapidly cooled polyethylene rotomolded products have a lower density, while the slowly cooled products have a higher density. Of course, the slower the product cools, the longer its production cycle and the higher the cost. The polyethylene powder used for rotomolding production itself has a certain density, which is determined by the manufacturer of the material. However, after rotomolding production, due to different cooling rates, the density of polyethylene rotomolded products will change to a certain extent.