When investigating the advantages and disadvantages of the rotational molding process, we should pay attention to several basic characteristics of the process, that is, in rotational molding, the material is directly loaded into the mold, and the mold is rotated to coat and adhere to the cavity. Molded on; during the entire molding process, the mold and the material are almost completely in a state of not bearing external forces; in each molding cycle, the mold and the frame are repeatedly heated and cooled, and undergo a relatively large heating and cooling process (The mold base of the jacket rotomolding machine is not heated and cooled). These characteristics of the rotational molding process make it show a series of advantages and disadvantages.
1. The main advantages of the rotational molding process
(1) Suitable for molding large and super large parts. Most plastic molding processes, such as injection molding, compression molding plastics, extrusion, blow molding, etc., are widely used. In the molding process, the plastics and molds are in the same position. Under high pressure (pressure), when applying these molding processes to produce large plastic parts, not only must the mold that can withstand a lot of pressure be used, which makes the mold bulky and complicated, but also the plastic molding equipment must be designed and manufactured. Very solid, the processing and manufacturing difficulty of the machine mold increases correspondingly, and the cost increases. In contrast, because the rotational molding process only requires the strength of the frame to support the weight of the material, the mold and the frame itself, as well as the closing force to prevent material leakage, even if rotomolding large and super large plastic parts, there is no need Using very heavy equipment and molds, the processing and manufacturing of machine molds is very convenient, the manufacturing cycle is short, and the cost is low. In theory, there is almost no upper limit on the size of the parts formed by the rotational molding process. We have used the rotational molding process to prepare a polyethylene cylindrical rotomolding container with a diameter of 1.2m and a height of 1.2m. The mold is made of only 3mm thick thin steel plate by cold welding, and the rotomolding frame uses No. 8 slot. It is made of welded steel to produce such a large plastic container. If blow molding is used, it is impossible to use very expensive and huge blow molding equipment.
(2) Suitable for the production of multiple varieties and small batches of plastic products. Because the mold for rotational molding is not subject to external force, the mold is simple, low-priced, and convenient to manufacture, so it is very convenient to exchange products; in addition, the rotational molding equipment also has a large Mobility, a rotomolding machine can be installed with one large mold or multiple small molds; it can not only mold parts of different sizes at the same time, but also mold the same size and shape at the same time Rotomolding products can be rotomolded at the same time as long as they use the same raw materials and have the same thickness. Therefore, the rotomolding process has greater flexibility than other molding methods.
(3) Rotational molding is very easy to change the color of the product. Rotational molding adds the material directly to the mold each time, which makes all the materials enter the product. After the product is taken out of the mold, add the materials needed for the next molding. Therefore, when we need to change the color of the product, neither waste of raw materials nor time-consuming cleaning of machines and molds. When we are using multiple molds to rotomold the same plastic product, we can also add materials of different colors to different molds and rotomold plastic products of different colors at the same time.
(4) Suitable for forming various hollow parts with complex shapes. During the rotomolding process, materials are gradually coated and deposited on the inner surface of the mold. The product has a strong effect on the fine structure such as the pattern on the mold cavity. Reproduction ability; at the same time, since the mold is not subject to external pressure during the molding process, precision casting methods can be directly used to produce molds with fine structures and complex shapes, such as toys and animal molds.
(5) Saving raw materials. The wall thickness of rotomolding products is relatively uniform and the chamfers are slightly thicker, so the effectiveness of the materials can be fully exerted, which is beneficial to saving raw materials; in addition, there is no waste material such as runners and gates in the rotomolding process. Once the commissioning is done, there is almost no recycling material during the production process, so the process has a very high utilization rate of materials.
(6) It is convenient to produce plastic products with multi-layer materials. The rotational molding process only needs to load reasonably matched materials with different melting temperatures into the mold for rotational molding. The plastic with a lower melting temperature is first melted by heating and adheres to On the mold, the outer layer of the part is formed, and then the material with higher melting temperature is melted on it to form the inner layer of the part. Or first put the outer layer of plastic into the mold, after the outer layer is rotomolded, then add the inner layer material, and then rotomold to make a multi-layer rotomolded product. Either way, it can be realized without complicated equipment. If blow molding or injection molding is used to make multilayer plastic products, special multilayer molding machine level and complicated molds are required.
2. Limitations of rotational molding process
(1) The rotational molding process is usually only suitable for producing hollow parts or shell-like parts (the latter is often obtained by cutting hollow parts). This is because rotomolding relies on the gradual melting of the materials loaded into the mold and adhesion to the surface of the cavity of the mold. For plastics (especially powdered plastics), the apparent density before molding is usually lower than that after molding. Therefore, except for foamed products, it is impossible to make solid parts by rotomolding.
(2) The rotational molding process cannot produce products with very different wall thicknesses and sudden changes in wall thickness. This feature is also directly related to the fact that rotational molding relies on the gradual melting and adhesion of materials to the surface of the mold cavity. To adjust the wall thickness of the part, we can adjust the wall thickness of the part appropriately by changing the heating (strengthening or reducing) of each part of the mold. However, due to the good thermal conductivity of the metal mold, the effect of this adjustment is Quite limited.
(3) Rotational molding process is difficult to make parts with flat sides. When the mold rotates, the material is not easy to stay in the flat part, which easily causes the wall thickness of the flat part to be too thin, thereby reducing the use effect of the part.
3. The main disadvantages of the rotational molding process
(1) High energy consumption In each rotational molding cycle, the mold and the mold base have to repeatedly undergo high and low temperature alternate changes. Therefore, the rotational molding process usually consumes more energy than other plastic molding processes. In order to reduce the energy loss of repeated heating and cooling of the mold, a jacketed rotomolding machine was developed. The cold and hot media are pumped into the jacket of the rotomolding mold through a special circulation system with a pump to directly heat and cool the mold. The equipment has a significant effect on reducing the energy consumption of the mold base, but the mold is still subjected to cold and hot conditions repeatedly, so the energy loss is still very large.
A special case that is completely opposite to the high energy consumption of the rotational molding process is the use of nylon monomer caprolactam to rotationally mold nylon parts. For example, the use of caprolactam to directly rotationally mold nylon 6 parts. Rotational molding and polymerization are performed at the same time. Molding is performed at a temperature lower than the melting point of nylon 6, and the product can be taken out at a higher temperature, so the temperature of the rotomolding mold does not need to be repeatedly raised and lowered within a large temperature difference range. It is said that the use of caprolactam rotomolding nylon 6 parts, compared with nylon 6 blow molding or injection molding nylon 6 parts, energy consumption is much lower.
(2) In the process of rotomolding with a long molding cycle, the material is not subjected to the strong action of external force, and the material does not move like a violent turbulent state. It only relies on its gradual heat and melting during the contact with the mold cavity surface and adheres to the mold cavity. On the surface, after the surface of the mold cavity is coated with molten plastic, the heat required for the temperature and melting of the plastic on the inner surface also needs to be conducted through the molten plastic layer. The thermal conductivity of the plastic is generally poor, so the heating time for rotational molding Quite long, usually more than 10 minutes, sometimes even twenty minutes, so the entire rotomolding cycle is also relatively long.
(3) High labor intensity In the rotational molding process, the loading and demolding processes are not easy to mechanize and automate, and manual operations are usually used. Therefore, the labor intensity is greater than the molding processes such as blow molding and injection molding.
Product dimensional accuracy is poor. The size of rotomolded products is not only affected by the variety of plastics, but also restricted by many factors such as cooling rate, type and amount of release agent (total effect of mold release), so rotomolded products The dimensional accuracy is difficult to control, so rotational molding is only suitable for plastic products that have no special requirements for dimensional accuracy, such as containers, toys and other parts.
Post time: Jun-21-2021