What Is Rotational Molding?

Rotational molding, also known as rotomolding, rotary molding, rotary molding, etc., is a method of thermoplastic hollow molding. In this method, a plastic raw material is first added to a mold, and then the mold is continuously rotated and heated along two vertical axes. The plastic raw material in the mold is gradually and uniformly coated and melt-adhered to the cavity under the action of gravity and thermal energy. On the entire surface, it is formed into the required shape, and then cooled and shaped into products ^[1]

Chinese name: Rotomolding
Foreign name: rotational moulding
Definition: Thermoplastic hollow forming method

Material: PVC paste resin
produce: Toys, balls, bottles, etc.
Advantage: Reduced high assembly costs

Rotational molding principle

The basic process of rotomolding is that the powder or liquid polymer is placed in a mold and heated while the mold is rotated and revolved around a vertical axis, and then cooled. At the beginning of the heating stage, if a powdery material is used, a porous layer is formed on the mold surface, and then gradually melts with the cyclic process, and finally a homogeneous layer of uniform thickness is formed. It is applied on the surface of the mold and stops flowing completely when the gel point is reached. The mold is then transferred to the cooling area, cooled by forced ventilation or water spray, and then placed in the work area, where the mold is opened, the finished part is removed, and then the next cycle is performed. ^[2]

Rotational molding principle

Rotomolding Features

Rotomolding in terms of formability

1) The product has a uniform wall thickness, no corner waste, and no welds; products with a wide range of wall thickness can be molded, such as polyethylene rotary moldings, and the wall thickness can range from 1 to 16 mm. However, due to the viscosity, forming thick-walled products is more difficult. The rotary molding process is particularly suitable for molding 2-5mm plastic products.

2) Normally only hollow or shell products can be made, and it is difficult to process solid products. Moreover, the surface condition of the product has a large dependence on the surface of the mold cavity.

3) The dimensional accuracy of rotary molded products is low, and its dimensional accuracy is usually ± 5%.

In terms of rotomolding

(1) Suitable for molding large and extra large parts. Since the rotary molding process only requires the strength of the frame to be sufficient to support the weight of the material, mold and frame itself, and the mold closing force to prevent material leakage, even if processing large and extra large plastic parts, there is no need to use very bulky equipment and mold . Therefore, in theory, there is almost no upper limit on the size of the products made by the rotational molding process.

(2) Suitable for the production of multi-variety and small-batch plastic productsSince the mold structure for rotary molding is simple and the price is low, it is very convenient to change products.

(3) Suitable for processing large hollow products with complex shapes, which is unmatched by other molding processes;

(4) The color change of plastic products is easy. When the color of the product needs to be changed, it is only necessary to clean the molding mold.

(5) The main disadvantage of rotary molding is that the energy consumption is large, because the mold and the mold frame need to withstand repeated heating and cooling in each molding cycle; the molding cycle is longer, because plastic mainly conducts heat under static conditions Therefore, the rotational molding heating time is long; the labor intensity is large, and the dimensional accuracy of the product is poor. ^[1]

Rotomolding production process

The basic process of rotomolding is very simple, that is, placing a powder or liquid polymer in a mold and heating it to rotate around two vertical axes (rotation and revolution) at the same time.

Coating release agent: The release agent is applied to 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 mold release due to the large adhesion between the product and the mold.
Installation of inserts and related molding accessories: Inserts are mainly used to reinforce parts. Molding accessories are mainly used to indicate ribs or special parts for molding. They must be properly installed in the newly set position before the material is added to the mold. on;
Loading: Strict metering should be performed before adding materials. When additives need to be added, the relevant components need to be premixed. Accurately weigh the powder resin required for the final part, add it to the bottom of the separate mold, and then fasten the two mold halves together and install them on the bearing shaft;
Closing the mold: Before closing the mold, pay attention to removing the remaining materials at the mold clamping area, and ensure that the two parting surfaces are tightly intact to avoid material leakage during processing;
Heating: Put the mold containing the materials into the heating furnace (or use other heat sources such as gas flames). The temperature of the heating furnace is set above the melting point of the resin. . During the rotation, the mold is heated to a given temperature and then kept warm. The material gradually melts under the rotation and heating, sinks on the entire inner wall of the mold cavity, and gradually excludes the gas trapped in the material until the molded part is completed.
Cooling: When the resin is fully melted, the mold is transferred to a cooling chamber and cooled, where it continues to rotate while being cooled by forced ventilation or water spray.
Demolding and cleaning of the mold: the former should avoid damaging the product when demoulding, and the latter should remove the remaining materials and debris from the mold to prepare for the next molding cycle. ^[1]

Rotomolding applications

With the development of the rotary molding process, the application range of its products is continuously expanding, and the application of rotary molding products has been extensive. Some representative applications can be exemplified as follows.

1. Rotary molded parts of containers. These plastic parts are widely used in water tanks, storage tanks for various liquid chemicals (such as acids, alkalis, salts, chemical fertilizers, pesticides, etc.), and gasoline containers (gasoline storage tanks). And fuel tanks for automobiles and airplanes), battery casings, etc.

2. Rotary molded parts for automobiles, mainly using polyethylene and polyvinyl chloride paste resin, rotomolded various pipe fittings, such as air-conditioning bends, backrests, armrests, etc.

3. Sports equipment and various substitutes. There are various parts made of PVC paste rotomolded, such as water polo, float ball, bicycle seat cushion, boat, and shock absorber between boat and dock. Rotomolded cross-linked polyethylene barrels made with Philip's rotomolded grade cross-linked polyethylene tree "Maricxcl-100" can compete with metal barrels, and have good chemical resistance and low maintenance costs. Rotomolding pallets have been commercialized in the United States, Japan and other countries in the early 1970s; surfboards, boats, etc. are also rotomolded parts often mentioned in the literature.

4 Toys, models, crafts, etc. Rotary molding molds can be manufactured by precision casting, electroforming and other processes; the surface of rotomolded parts replicates the fine structure of the mold cavity surface is very effective, so the rotomolding method can make the product quite delicate and beautiful, so it is commonly used For the production of products with relatively large ornamental value, especially toys, models, and crafts.

In addition to the above, rotary box products are also widely used in various boxes, shells, large pipes and other parts, such as turnover boxes, trash cans, machine housings, protective covers, lamp covers, bathrooms, toilets, telephone booths, and yachts. and many more. Rotary molding products have been widely used in the storage and transportation of liquid chemicals, chemical companies, industrial coatings, washing tanks and reaction tanks in the preparation of rare earths, as well as river and sea buoys and domestic water tanks. ^[1]

Rotomolding advantages

Rotomolding design advantages

Compared with other mold processes, the rotomolding process provides us with more design space. With the correct design concept, we can combine several parts into a complete mold, which greatly reduces the high assembly cost.

The rotomolding process also includes a series of inherent design thinking, such as how to reconcile the thickness of the sidewalls and how to strengthen the external settings. If you need to add some additional designs, then we can also add the link of strengthening ribs to the design.

The rotomolding process infuses designers with endless imagination. Designers can choose the best production materials during the production process, including various materials approved by the General Food and Drug Administration. Additives added during the production process can effectively resist external environmental factors such as climate attack and static electricity interference. In the design process, the insertion port, the thread, the handle, the inverted device, and the perfect surface design are all among the highlights. Designers can also design multi-wall molds. The interior can be hollow or filled with foam.

Rotomolding cost advantage

When cost is also one of the factors we consider, the rotomolding process has a market advantage over other types of processes. In comparison with blow molding and injection molding processes, rotomolding processes can more easily produce parts of different sizes within the effective cost range. His mold is also relatively cheap, because it does not have some internal cores to complete. And without an internal core, you can make another model with only a small change.

Because the various parts in the manufacturing process are finally formed under high temperature and rotating process, different from those formed under heavy pressure, the rotomolding mold does not need special processing like the injection molding process. Withstand the test of stress. ^[3]