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What is the principle and classification of rotational molding?

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

What is the principle and classification of rotational molding?

Rotational molding is a kind of hollow molding method of thermoplastic. Its principle: the resin is added into the mold, the mold is closed, the mold is heated, and two right angle intersecting rotating shafts are used at the same time to make the mold rotate / roll in three dimensions. The resin is evenly covered with the mold cavity by its own gravity and gradually melted until the resin is completely melted and uniformly adhered to the inner wall of the mold cavity, and then the heating is stopped and the mold is cooled After cooling and solidifying, the product is demoulded to obtain the required seamless hollow product.

The rotational speed of the mold is not high during rotational molding, and the molding of the product depends on the natural flow of resin, so the product has almost no internal stress and is not easy to deform and sag.

Rotational molding can be divided into three types according to the form of resin used.

1. Dry powder rotational molding represented by polyethylene resin, which is the most widely used type in rotational molding process. Because the resin is extruded from powder to melt without external force, the air bubbles formed in the melt between the original powder particles can only grow up by itself, and then get rid of the melt. Whether the bubbles can be completely discharged directly affects the physical and mechanical properties of the products.

2. Rotational molding of PVC paste resin is commonly known as plastic lining in the industry. The main production of soft PVC hollow products.

3. The monomers of rotational molding resin polymerized by monomers enter the mold cavity, complete the polymerization and flow molding under the appropriate temperature conditions and additives. Comparatively speaking, the practical application of this process is relatively less at present.

Rotational molding process has wide applicability, which can form seamless hollow products such as table tennis, yacht or submachine boat, or chemical storage tank with a volume of more than 100 cubic meters; the wall thickness of products can be adjusted from less than 2mm to several centimeters without changing the mold; complex structure products can be manufactured in one step; inserts or other assemblies can be added to the products according to the design needs.

Rotational mold is easy to make and its cost is low, generally only 1 / 4 ~ 1 / 3 of the cost of other molds of the same size; rotational mold is especially suitable for the production of large products, which can realize edge thickening to ensure the edge strength of large diameter products; in addition, rotational parts have no flash and less material waste. The shortcomings of rotational molding process are: the production cycle is relatively long, the materials need to be grinded in advance, many manual operations, high labor intensity and so on.

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Taking rotational molding of polyethylene powder as an example, the whole process can be divided into four parts.

1. In the feeding process, clean the inner wall of the mold, measure the added materials as required, and then close the mold to prepare for the subsequent rotational molding operation.

2. In the forming process, the mold and the rotating system enter the designated hot forming station, and turn on the vertical rotation to make the mold do three-dimensional rotation and rolling, and then heat it from the outside of the mold (hot air circulation, electric heating radiation, or open fire baking can be used) until the process set temperature (260 ~ 320 ℃) is maintained for a certain time to ensure that the melt adheres completely to the mold cavity wall, has uniform thickness, and exhausts all bubbles.

3. In the cooling process, keep the three-dimensional rotation and rolling state of the mold, turn to the cooling station, and use cold air. Spray and spray are used to cool the mould and products, so that the product is completely consolidated and finalized.

4. In the demoulding process, stop the mold rotation, turn to the loading / unloading station, open the mold and take out the products. Clean the inner wall of the mold and prepare for the next cycle.

Pigment products used in rotational molding products should pay close attention to heat resistance, dispersion, light resistance, weather resistance, safety and other characteristics.

1. Heat resistant rotational molding is a non forced molding process. It needs the melt to have very good flowability;

For the resin, the heat conduction mode is unidirectional natural transfer, and there is no friction and shear heat generation; the bubbles in the melt need enough time to naturally accumulate and break. Therefore, the processing temperature is much higher than other molding processes of the same kind of resin. Similarly, the operation time at this high temperature is much longer than other processes. If nitrogen is not used as protection in the mold cavity during rotational molding, the oxidation at high temperature must also be considered. Therefore, the pigment must have excellent heat resistance, otherwise the quality of the product can not be guaranteed.

2. The dispersion can be seen from the rotational molding process, if the resin system is not pre mixed, but simply mixed with dry powder (at present, quite a number of manufacturers use this method). Then, the pigment powder particles have not been effectively dispersed in the whole process. If the powder particles are coarse and not easy to disperse, it will cause color spots and other defects, which will affect the product quality.

3. Light and weather resistant rotational molding products are widely used outdoors, especially for yachts, anti impact piers and large toys. Therefore, the pigments used in these products must have excellent light and weather resistance.

4. A large number of civil products and toys in safety rotational molding products are in direct contact with human body, especially children's toys. Therefore, we must pay attention to the safety of pigment products.

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