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Do you know the advantages and disadvantages of rotational molding process?

By admin In Art, News Posted 2023-03-29

In the case of investigating the advantages and disadvantages of rotational molding process, we should mainly pay attention to many essential features of the process, that is, in rotational molding, the material is immediately loaded into the mold, coated and adhered to the mold core by the mold rotation; in the whole molding process, the mold and the material are completely free of external force; in each molding cycle, the mold and the material are completely free of external force The tool and structure continuously endure heating and cooling, and go through a relatively large process of heating and cooling (no heating and cooling). This characteristic of rotational molding technology makes it show a series of advantages and disadvantages.

1. Key advantages of rotational molding process

(1) It is suitable for molding large and huge parts. Most of the plastic molding processes, such as injection molding, shrinkage molding, extrusion and injection molding, are widely used. In the molding process, the plastic and mold are under very high pressure (pressure). Therefore, when using this molding process to produce large plastic parts, not only must the mold which can bear a lot of pressure be used, The mold becomes heavy and complicated, and the plastic molding equipment must be designed and manufactured very solidly. The difficulty of processing and manufacturing the machine mold is relatively enlarged, and the cost is rising.

On the other hand, because the rotational molding process only requires that the strength of the frame is sufficient to support the net weight of the material, the mold and the frame itself, as well as the closing die force to avoid material leakage, even for large and giant rotational plastic parts, it does not need to use very heavy equipment and mold, so the processing and manufacturing of the mold is very convenient, the manufacturing cycle is short, and the cost is low.

Theoretically speaking, there is no limit on the size of the parts formed by rotational molding. We have used rotational molding technology to make polyethylene cylindrical rotational molding vessels with a diameter of 1.2m and a height of 1.2m. The mold is only made of 3mm thick thin steel plate by cold welding, and the rotational molding frame is made of 8 channel steel. It is impossible to produce such large plastic vessels without expensive and huge injection molding equipment if we choose injection molding.

(2) It is suitable for the production of many kinds and small batch of plastic products. Because the molds for rotational molding are not subject to external force, the molds are simple, high quality, low price and easy to manufacture, so it is very convenient to exchange goods. In addition, the rotational molding equipment also has great controllability. One rotational molding machine can install a large mold, and also can distribute multiple small and medium molds;

It can not only mold parts with different sizes, but also mold products with extremely different sizes and shapes. If the raw materials used for rotational molding products are the same, and the product is very thin and thick, it can be additionally rotational molded. Therefore, the rotational molding process has higher controllability than other molding methods.

(3) Rotational molding is very easy to change the color of products. In rotational molding, the materials are added to the mold immediately every time, which makes all the materials enter the product. After the product is removed from the mold, the materials necessary for the next molding are added. Therefore, when we have to change the color of products, it will not consume a bit of raw materials, and it will not take time to clear the equipment and mold. When we use multiple molds to roll mold the same plastic product, we can also add different colors of materials in different molds, and roll mold different colors of plastic products.

(4) It is suitable for molding various kinds of hollow parts with complex shapes. During the rotational molding process, the materials are slowly coated and accumulated on the inner surface of the mold. The product has strong copy ability for the fine structure such as the grain on the die cavity. In addition, because the mold is not subject to external pressure in the molding process, it can immediately select the methods such as fine casting to make the products with fine structure Molds with complicated shapes, such as toys, small animal molds, etc.

(5) The thickness of rotational molding products is relatively uniform and the chamfer is slightly thick, so the efficiency of materials can be fully used, which is beneficial to saving raw materials; in addition, in the rotational molding process, there is no runner, rubber inlet and other waste, once adjusted, there is basically no furnace feed in the production process, so the utilization rate of materials in this process is very high.

(6) It is easy to produce plastic products with double-layer materials by rotational molding process. It only needs to load the materials with different melting temperatures into the mold to carry out rotational molding. The plastic with lower melting temperature first meets the hot melt and adheres to the mold to produce the surface layer of the part, and then the material with higher melting temperature melts on it to produce the inner layer of the part.

Or the surface plastic is first put into the mold, then the surface layer is formed by dry rotational molding, and then the inner layer material is added, and then the double-layer rotational products are made by rotational molding. No matter which way, it can be completed without complicated equipment. If injection molding or injection molding method is used for double-layer plastic products, special double-layer molding machine level and complicated molds are required.


2. Limitations of rotational molding process

(1) Rotational molding process is generally only suitable for the production of hollow parts or shell parts (the latter is usually cut from the hollow parts). It is because rotational molding is formed by means of the material slowly melted into the structure and adhered to the surface of the die. However, the apparent density of plastics (especially powder plastics) before molding is generally smaller than that after molding. Therefore, in addition to developing foam products, solid parts can not be made by rotational molding.

(2) Rotational molding process can not produce products with very different wall thickness and abrupt wall thickness. This characteristic is also related to the fact that rotational molding is formed by means of material melting slowly and adhering to the surface of the core. To adjust the thickness of the part, we can adjust the thickness of the part appropriately according to the method of changing the heat (strengthening or reducing) of each component of the die. However, because the heat transfer of the metal die is very good, the effect of this kind of adjustment is very limited.

(3) It is difficult to produce flat side parts by rotational molding. When the die rotates, it is not easy for the material to stay at the flat position, which easily leads to the thin wall thickness of the part at the flat position, thus reducing the use effect of the part.

3. Key defects of rotational molding process

(1) Because of the high energy consumption of rotational molding and other plastic molding processes, it is necessary to change the rotational molding process. In order to reduce the kinetic energy loss of the mold when it is heated and cooled continuously, a jacket type rotational molding machine is developed. The cold and hot materials are pumped into the barrel of the rotational molding mold according to the special circulation pump, and the mold is heated and cooled immediately. This kind of equipment has a significant effect on reducing the energy consumption of the mold embryo, but the mold still bears the cold and hot conditions continuously, so the kinetic energy loss is still low It's big.

Different from the situation of high energy consumption in rotational molding process, it is the use of nylon monomer caprolactam, rotational molding nylon parts, such as the use of caprolactam immediately rotational molding nylon 6 parts, rotational molding and convergence process are carried out in addition, rotational molding is carried out at a temperature less than the melting point of nylon 6, and the product can be removed at a higher temperature, so the temperature of rotational molding mold is not high It is used to continuously raise and reduce temperature in the range of large temperature difference. It is reported that the use of caprolactam rotational molding nylon 6 parts, compared with nylon 6 injection molding or injection molding nylon 6 parts, energy consumption is much lower.

(2) The molding cycle is long. In the rotational molding process, the material does not bear the obvious effect of external force. The material does not have the same movement as violent turbulence. It only uses it to touch the surface layer of the die cavity. During the process, it slowly meets the heat and melts and adheres to the surface layer of the die core. After the surface layer of the die core is coated with molten plastic, the heat required for the temperature rise and melting of the plastic inside must be increased According to the heat conduction of the melted plastic layer, the heat transfer of the plastic is generally poor, so the heating time of rotational molding is very long, generally more than 10 minutes, sometimes even more than 20 minutes, so the entire rotational molding cycle is also relatively long.

(3) The labor intensity is very high. In the process of rotational molding, it is not easy to mechanization and automation technology in the process of discharging and discharging. Generally, manual operation is used, so the labor intensity is higher than that of injection molding and injection molding.

4. The dimensional accuracy of rotational molded products is poor. In addition to the harm of the types of plastics, the dimensions of rotational molded products are also affected by many factors, such as cooling rate, type and dosage of release agent (total mold effect), so the dimensional accuracy of rotational molded products is difficult to control. Therefore, rotational molded products are only suitable for plastic products without special requirements for dimensional accuracy, such as utensils, toys and other products.

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