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What is the principle and classification of rotational molding?

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

Rotational molding is a kind of hollow molding processing method of thermoplastic. Its principle: the resin is added to the mold, the mold is closed, according to the heating of the mold, and in addition, two diagonal cross transmission shafts are used to rotate continuously to make the mold rotate / flip in three dimensions. The resin evenly spreads over the inner wall of the mold depending on its own gravity effect, and gradually melts until the resin is completely melted and evenly adhered to the inner cavity of the mold core, and then the heating is terminated and the cooling is switched to full cooling In the process, after the product is cooled and consolidated, the seamless hollow product is obtained.

The rotational speed ratio of the mold is not high during rotational molding, and the molding of the product depends on the natural flow of resin. Therefore, the commodity basically has no internal stress and is not easy to have deformation, depression and other defects.

Rotational molding can be divided into three types according to the shapes of common resins.

1. Powder rotational molding with polyethylene resin as its meaning is the most widely used type in rotational molding process. Because of the forced extrusion of the resin from powder to melt without external force, the gas bubbles between the particles of the original powder equipment in the solution can only grow up by itself, and then get rid of the solution. Whether the air bubble can be completely emptied immediately endangers the physical and mechanical properties of the product.

2. Rotational molding of PVC viscous resin. Key production of soft plastic PVC hollow products.

3. The rotational molding resin of single aggregation molding enters into the mold core, and the aggregation and flow molding are carried out under the appropriate temperature standard and auxiliary effect. Relatively speaking, the specific application of this process is relatively less at this stage.

Rotational molding process has a wide range of applications. It can form seamless hollow products such as table tennis, cruise ship or submachine boat, or chemical tank with several hundred cubic meters of capacity. It can adjust the wall thickness of products from less than 2 mm to several centimeters without changing the mold. It can produce complex products in one step. It can add inlays or other components to the products according to the design requirements And so on.

Rotational mold is easy to make, and its engineering cost is cheap, which is generally only 1 / 4 ~ 1 / 3 of the engineering cost of other molds of the same specification; rotational mold is especially suitable for large and medium-sized products, which can complete the edge thickening to ensure the edge compressive strength of large-diameter products; in addition, rotational products have no burr and consume less raw materials. The shortcomings of rotational molding process depend on: long production cycle, materials need to be ground into powder in advance, many manual operations, high labor efficiency 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 cavity of the mold, measure and add materials as required, then close the mold and prepare for subsequent rotational molding operation.

2. The mold and rotation system software of the forming process enter the specific hot forming process, and turn on the vertical rotation to make the mold do three-dimensional rotation and turnover. The mold is heated from the outside (hot air circulation, electric heat transfer, or fire burning can be selected) until the process setting temperature (260 ~ 320 ℃), which is maintained for a certain time to ensure the quality of the mold The solution adheres completely in the mold wall, the thickness is uniform, and the bubbles are emptied.

3. In the refrigeration process, maintain the three-dimensional turning state of the mold, turn to the refrigeration process, and select the cold air. Spray and spray methods are used to refrigerate moulds and products, and finally make the products completely consolidated.

4. When the mold is out, stop the mold rotation, turn to the loading / unloading process, open the mold and remove the product. Clean the mold cavity and prepare for the next cycle.

The color paste used in rotational molding products should be highly concerned about the temperature resistance, dispersion, light resistance, weather resistance, safety and other characteristic index values.

1. Temperature resistance rotational molding is a non external force mandatory molding process. It needs the solution to have good fluidity;

For the resin, the way of heat conduction is the way of unidirectional natural transmission, without friction and cutting heat generation; the bubbles in the solution need sufficient time, of course, to accumulate and break bubbles. Therefore, the processing temperature is much higher than that of other molding processes. Similarly, the operation time at this high temperature is much longer than that of other processes. In rotational molding, if N2 is not required for mold cavity maintenance, the oxidation effect at high temperature must be considered. Therefore, all need to use the color paste must have excellent temperature resistance, otherwise can not ensure the quality of goods.

2. The dispersity is known from rotational molding process. If the resin system is not mixed in advance, it can be simply mixed with powder (many manufacturers choose this method at this stage). So, the particles of color paste powder equipment have not experienced reasonable dispersion effect in the whole process. Because the particles of powder equipment are thick and not easy to disperse, it will lead to defects such as color spots and endanger product quality.

3. Light and weather resistant rotational molding products are widely used outdoors, especially for cruise ships, digging piers along the air, large toys, etc. Therefore, the color paste used for this product must have excellent light resistance and weather resistance.

4. Many civil products and toys in safety rotational molding products touch the body immediately, especially children's toys. Therefore, we must be concerned about the safety of color paste products.

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