The Production Tips of PE pipe floaters

1. Release agent

During the heating stage of the rotational molding process, chemical or physical bonding occurs 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 to form local embedding. This will make it difficult to remove the cooled article from the mold. In order to avoid the above situation, it is necessary to coat a layer of thermally 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 release agents for industrial use. The rotational molding process of polyethylene has higher 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 one-time release agents. This type of release agent has low cost and good release 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 mold release agent, which does not require frequent application, will not migrate, will not be affected by temperature changes, and has a good mold release effect, but the cost is relatively high.

Compounding a thin layer of polytetrafluoroethylene on the surface of the mold cavity (like a commercially available non-stick pan) can achieve a permanent release effect, and polytetrafluoroethylene is a permanent 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 air bubbles, and these bubbles disappear as the heating process continues. Further research showed that these bubbles disappeared not because they moved to the free surface of the melt under the action of buoyancy, but because the air in the bubbles gradually merged into the molten plastic melt. Experiments have shown 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 was raised to 200°C, all the bubbles disappeared. Therefore, for rotational molding of polyethylene, scientifically controlling 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 wall of the product is thicker. 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 reduction of the material performance. Usually, adding antioxidants to polyethylene plastics can 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 air bubbles may remain because the air in the bubbles does not have time to disappear. When polyethylene plastic is heated to a molten state, the material undergoes a transition from a crystalline state to a melt, which is exactly what happens when polyethylene particles begin to melt and soften. 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, it gradually expands to the inner layer until the full section becomes a plastic melt. The next step is to continue 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 close to 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 amount of time, and the crystallization speed 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 grains, thereby affecting the crystallinity of polyethylene. When the crystallinity is different, the density of polyethylene products is different, and the physical properties will also be different. As a result, rapidly cooled polyethylene rotomolded products have a lower density, while slowly cooled products have a higher density. Of course, the slower the product is cooled, the longer its production cycle and the higher the cost. The polyethylene powder used for rotational molding has a certain density, which depends on the manufacturer of the material. However, after rotational molding production, due to the different cooling rates, the density of polyethylene rotational molding products will change to a certain extent.

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