Main advantages of rotational molding

Main advantages of rotational molding

Main advantages of rotational molding

(1) Suitable for molding large and extra-large parts

In most plastic molding processes, such as widely used injection molding, compression molding, extrusion, blow molding, etc., the plastic and mold are under a very high pressure (stress) during the molding process. Therefore, when using these molding processes to produce large plastic parts, not only must molds that can withstand a lot of pressure be used, making the molds bulky and complicated, but the plastic molding equipment must also be designed and manufactured to be very strong, and the difficulty of processing and manufacturing the machine mold increases accordingly, and the cost increases.

In contrast, since the rotational molding process only requires the strength of the frame to be sufficient to support the weight of the material, mold and frame itself, as well as the closing force to prevent material leakage, even when roto-molding large and extra-large plastic parts, there is no need to use very bulky equipment and molds. The processing and manufacturing of the machine mold is very convenient, with a short manufacturing cycle and low cost.

In theory, there is almost no upper limit on the size of parts formed by rotational molding. For example, if the roto-molding process is used to prepare a polyethylene cylindrical roto-molded container with a diameter of 1.2m and a height of 1.2m, the mold is made of only 3mm thick thin steel plate through cold welding, and the roto-molding frame is welded with No. 8 channel steel. If blow molding is used, it is impossible to produce such a large plastic container without very expensive and large blow molding equipment.

(2) Suitable for the production of multiple varieties and small batches of plastic products

Since the mold for roto-molding is not subject to external forces, the mold is simple, low-priced, and easy to manufacture, so it is very convenient to exchange products; in addition, roto-molding equipment also has greater mobility. A roto-molding machine can be equipped with both a large mold and multiple small molds; it can not only mold parts of different sizes at the same time, but also mold products of very different sizes and shapes at the same time. As long as the roto-molded products use the same raw materials and have the same thickness, they can all be roto-molded at the same time. Therefore, the roto-molding process has greater mobility than other molding methods.

(3) Rotational molding can easily change the color of the product

During rotational molding, the material is directly added to the mold each time, so that all the material enters the product. After the product is taken out of the mold, the material required for the next molding is added. Therefore, when we need to change the color of the product, we will not waste any raw materials, nor do we need to spend time cleaning the machine and mold. When we use multiple molds to rotationally mold the same plastic product, we can also add materials of different colors to different molds and simultaneously rotate out plastic products of different colors.

(4) Suitable for molding hollow parts of various complex shapes

During the rotational molding process, the material is gradually coated and deposited on the inner surface of the mold. The product has a strong ability to replicate the fine structures such as the patterns on the mold cavity; at the same time, since the mold is not subject to external pressure during the molding process, precision casting and other methods can be used to directly produce molds with fine structures and complex shapes, such as toys and animal molds.

(5) Saving raw materials

The wall thickness of roto-molded products is relatively uniform and slightly thicker at the chamfers, so the efficiency of the materials can be fully utilized, which is conducive to saving raw materials. In addition, there is no waste material such as runners and gates during the roto-molding process. Once debugged, there is almost no recycled material in the production process, so the process has a very high utilization rate of materials.

(6) Convenient for producing multi-layer plastic products

Using the roto-molding process, you only need to load reasonably matched materials with different melting temperatures into the mold for roto-molding. The plastic with a lower melting temperature is first heated and melted, adhered to the mold, and formed the outer layer of the product. Then the material with a higher melting temperature is melted on it to form the inner layer of the product. Alternatively, first load the outer layer of plastic into the mold, roto-mold the outer layer, then add the inner layer material, and then roto-mold it to make a multi-layer roto-molded product. No matter which method is used, it can be achieved without complex equipment. If blow molding or injection molding is used to make multi-layer plastic products, special multi-layer molding machines and complex molds are required.

Limitations of Roto-molding

(1) Roto-molding is usually only suitable for producing hollow parts or shell parts (the latter are often obtained by cutting open hollow parts). This is because roto-molding relies on the gradual melting of the material loaded into the mold and adhesion to the surface of the mold cavity. The apparent density of plastics (especially powdered plastics) before molding is usually smaller than that after molding. Therefore, except for foamed products, solid parts cannot be made by roto-molding.

(2) Roto-molding cannot produce products with very different wall thicknesses or sudden changes in wall thickness. This feature is also directly related to the fact that roto-molding relies on the gradual melting of the material and adhesion 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 of the various parts of the mold (strengthening or reducing), but due to the good thermal conductivity of the metal mold, this adjustment effect is quite limited.

(3) Roto-molding is difficult to produce 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.

Main disadvantages of roto-molding process

High energy consumption

In each roto-molding cycle, the mold and mold frame must repeatedly undergo alternating high and low temperatures, so the roto-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 roto-molding machine was developed. The cold and hot media are pumped into the jacket of the roto-molding mold through a special circulation system to directly heat and cool the mold. This equipment has a significant effect on reducing the energy consumption of the mold frame, but the situation of the mold repeatedly being subjected to cold and hot conditions still exists, so the energy loss is still very large.

In contrast to the high energy consumption of roto-molding, a special case is the use of nylon monomer caprolactam to roto-mold nylon parts. For example, nylon 6 parts are directly roto-molded using caprolactam. Roto-molding and polymerization processes are carried out simultaneously. Roto-molding is carried out at a temperature lower than the melting point of nylon 6, and the products can be taken out at a higher temperature. Therefore, the temperature of the roto-molding mold does not need to be repeatedly heated and cooled within a large temperature range. It is said that the energy consumption of roto-molding nylon 6 parts using caprolactam is much lower than that of nylon 6 parts by blow molding or injection molding.

Long molding cycle

During the rotational molding process, the material is not subjected to strong external forces, and the material does not move violently like a turbulent state. It only relies on the gradual heat and melting during the contact with the mold cavity surface and adheres to the mold cavity surface. After the mold cavity surface is fully coated with molten plastic, the heat required for the plastic inside to heat up and melt needs to be conducted through the molten plastic layer. The thermal conductivity of plastic is generally poor, so the rotational molding heating time is quite long, usually more than 10 minutes, sometimes even more than 20 minutes, so the entire rotational molding cycle is also relatively long.

High labor intensity

During the rotational molding process, loading, demolding and other processes are not easy to mechanize and automate, and are usually operated manually, so its labor intensity is greater than that of blow molding, injection molding and other molding processes. The size of the product is poor. The size of the rotational molded product is not only affected by the type of plastic, but also by many factors such as cooling speed, type and amount of release agent (total demoulding effect). Therefore, the dimensional accuracy of the rotational molded product is difficult to control. Therefore, rotational molding is only suitable for plastic products that have no special requirements for dimensional accuracy, such as containers, toys and other parts.