Rotational molding, also known as rotomolding, rotary molding, etc., is a hollow molding method for thermoplastics. The method is to add plastic raw materials first into the mold, and then the mold is continuously rotated along two vertical axes and heated. Under the action of gravity and thermal energy, the plastic raw materials in the mold are gradually uniformly coated and melted and adhered to the mold cavity. The whole surface is formed into the required shape, and then cooled and shaped into a product.
The basic process of rotational molding is very simple. It is to put powdered or liquid polymer in a mold, heat it, and the mold rotates and revolves around a vertical axis, and then cools to form. At the beginning of the heating stage, if a powdery material is used, a porous layer is formed on the surface of the mold, and then gradually melts with the cycle process, and then a homogeneous layer of uniform thickness is formed; if a liquid material is used, it flows and flows first.
Coated on the surface of the mold, when the gel point is reached, the flow stops completely. The mold is then transferred to the cooling work area, cooled by forced ventilation or water spray, and then placed in the work area, where the mold is opened, the finished part is taken away, and then the next cycle.
Advantages of rotomolding products
In terms of formability
- The wall thickness of the product is uniform, and there is no corner waste, no welding seam; products with a large wall thickness range can be formed, such as polyethylene rotary forming parts, and the wall thickness can be in the range of 1-16mm. However, it is difficult to form thick-walled products due to viscosity. Rotational molding process is suitable for molding 2~5mm plastic products.
- Usually only hollow products or shell products can be produced, and it is difficult to process solid products. Moreover, the surface condition of the product is highly dependent on the surface of the mold cavity.
- The dimensional accuracy of rotational molding products is low, and its dimensional accuracy is usually ±5%.
In terms of molding process
- Suitable for molding large and super large parts. Because the rotary molding process only requires the strength of the frame to support the weight of the material, the mold and the frame itself, and the closing force to prevent material leakage, even if the processing of large and super large plastic parts, it does not need to use bulky equipment and molds. Therefore, theoretically, there is almost no upper limit on the size of products made by the rotational molding process.
- Suitable for the production of multiple varieties and small batches of plastic products-due to the simple structure and low price of the mold used for rotational molding, it is convenient to change products.
- It is suitable for processing large hollow products with complex shapes, which is unmatched by other molding processes;
- The color of plastic products is easy to change. When the color of the product needs to be changed, only the molding mold needs to be cleaned.
- The main disadvantage of rotational molding is: high energy consumption, because the mold and mold base need to withstand repeated heating and cooling in each molding cycle; the molding cycle is longer, because the plastic conducts heat mainly under static conditions , So the rotational molding heating time is long; the labor intensity is greater, and the product dimensional accuracy is poor.
The basic processing process of rotational molding is very simple, that is, the powder or liquid polymer is placed in the mold and heated while rotating around two vertical axes (rotation and revolution).
- Coating release agent: The release agent is coated on the mold to facilitate the removal of the product from the mold, and at the same time, it can effectively avoid the damage of the product during release due to the large adhesion between the product and the mold.
- Install inserts and related molding accessories: inserts mainly serve as local reinforcements. Molding accessories mainly point out ribs or modules for forming special parts. They must be properly installed in the new setting before adding materials to the mold Position
- Loading: Before adding materials, strict metering should be carried out. When additives need to be added, the relevant components should be premixed. Weigh the weight of the powder resin required for the final product, add it to the bottom of the separate mold, and then fasten the two mold halves together and install it on the supporting shaft;
- Closing the mold: Before closing the mold, pay attention to removing the remaining materials at the mold clamping position, and ensure that the two parting surfaces are tightly sealed to avoid material leakage during the processing;
- Heating: Put the mold with the material in the heating furnace (or use other heat sources such as gas flame to heat), the temperature of the heating furnace is set above the melting point of the resin, and the main and auxiliary shafts are perpendicular to each other along with the frame. ,rotation. During the rotation, the mold is heated to a given temperature and then kept warm. The material gradually melts under rotation and heating, sinks to the entire inner wall of the mold cavity and gradually removes the gas trapped in the material until the finished part is formed.
- Cooling and shaping: When the resin is fully melted, transfer the mold to the cooling chamber and cool it. While continuing to rotate there, it is cooled by forced ventilation or water spray.
- Demoulding and cleaning the mould: The former should avoid damaging the product during demoulding, and the latter should remove the remaining materials and debris on the mould to prepare for the next molding cycle.
Compared with other mold processes, the rotational molding process provides us with more design space. Under the correct design concept, we can combine several parts into a complete mold, which greatly reduces the high assembly cost.
The rotational molding process also includes a series of inherent design thinking methods, such as how to adjust the thickness of the side wall and how to strengthen the external settings. If some additional designs need to be added, then we can also add the strengthening ribs to the design.
The rotomolding process injects endless imagination of the designers into the product. Designers can choose the production materials during the production process, including various materials approved by the Food and Drug Administration. The additives placed in the production process can effectively resist external objective factors such as climate attack and static electricity interference. In the design process, the surface design of the insertion port, screw thread, handle, inverted device, and the surface design are among the highlights. Designers can also design a multi-wall mold, the interior of which can be hollow or filled with foam.
When cost has become one of our considerations, the rotomolding process has market advantages over other types of processes. When compared with the blow molding process and the injection molding process, the rotational molding process can easily produce parts of different sizes within the effective cost range. His mold is relatively cheaper, because it does not have some internal cores to be completed. And without an internal core, it can be made into another model with a little change.
Since the various parts in the production process are finally formed under the high temperature and rotating process, which is different from those formed under heavy pressure, the rotomolding mold does not need to be processed like the injection molding process. Withstood the test of heavy pressure.