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Exploring the Past of Rotationally Molded Plastics
Sep 22, 2017

Other than standard extrusion and injection molding, there is another molding method in which plastic is molded, rotational molding. The process is entirely different from conventional molding and involves a rotating mold. For making rotationally molded plastics, a hollow mold is heated to a predetermined temperature and raw material is put into the mold. Then, the filled mold is rotated along two perpendicular axes slowly, and as the centrifugal force starts to act on the softened raw material, the material moves towards the wall and sticks. Under this action, plastic is molded various applications, including plastic tanks, plastic containers, and roto molded tanks.

This method may look new; however, it has its roots in the 19th century, when it was used for making artillery shells. Let us explore the history of rotational molding to know how it has evolved over time.

Like many things, rotational molding also originated in Europe; it was the idea of a British man, who in 1855, used rotation and heat to make metal shells for artillery. He modified his techniques to make other hollow materials, too. With this method, it was observed that the wall thickness and density of hollow objects could be made consistent. However, the method did not become popular and remained in the shadows of alternative molding methods.

The next development in this method came from an American in 1905, when the method was used for manufacturing hollow wax objects, and soon, the method was applied for making hollow chocolate-it was the beginning of chocolate eggs. The method for a long time was applied in the confectionery industry and was not used in large-scale industrial manufacturing.

The precursor of the present day roto-molding was the plaster of Paris molding, which started in the second decade of the 20th century. Gradually, in the 1940s, rotational molding of plastic found traction, and toy makers started to make hollow toys using the process. The machines used for manufacturing was quite crude which had parts from automobile, like the rear axle of a vehicle. The machine was driven by an electric motor, and gas burners were used for heating. For making molds, nickel and copper were used, and polyvinyl chloride was the raw material. Cold-water baths were used to cool down the molds.

Since the method produced toys with consistent thickness and density, it got further traction, and industries started to employ the technique to produce other items such traffic safety equipment, armrests, and buoys. As a number of manufacturers embraced the method, more efficient and bigger designs started to become available on the market, which had an indirect heating system.

The next stimulus to the method came in Europe, where a new raw material, polyethylene, and a new cooling system were used for making large hollow containers. The cooling system had burners that could be switched off during the process, so the hardening process could start while the mold was still in motion.

Soon, fabricators began using other raw materials, such as nylon and polycarbonate, which were molded into plastic fuel tanks, water tanks, and industrial molds.