Rotational moulding has had a long history of development dating back to the Egyptians who used rotational casting processes for creating ceramics. Moulding processes were used hundreds of years ago by the Swiss to make hollow chocolate eggs.
In more recent times, somewhere between 1940 and 1950 in the USA, the rotational moulding process was developed for a small number of plastics but its popularity did not take off because it was regarded as a slow process. In the past few decades, however, process control improvements and plastic powder developments have resulted in a very large increase in its use.
According to Noel Mansfield Ward’s article called “A History of Rotational Moulding” [plastiquarian - no. 19 - Winter 1997] the original application of rotational moulding was in the creation of dolls’ heads during the 1940s. The mould was created from electroformed nickel-copper and the polymer used was PVC (polyvinyl chloride) plastisol. This new process caught on in the industry and quickly replaced the papier-mâché method of the time. Soon many other toys for children were being manufactured by this process, such as squeaky toys and play balls.
By the late 1950s, when the process was better understood, applications for other industries were developed including road cones, marine buoys, and car armrests. The Engel process was developed in Europe in the early 1960s that enabled large hollow containers to be created in low density polyethylene (LDPE) by rotating (or rocking) a mould on a chassis containing open gas jets, through 30 degrees which coated the inside of the mould with the polymer. The cooling method was simply switching off the burners. This allowed the rocking to continue until the moulding could be extracted.
The Association of Rotational Moulders
Formed in Chicago in 1976, the Association of Rotational Moulders (ARM) is a worldwide trade association that is currently representing companies in 58 countries. The goal of the association is to promote and champion rotational moulding and is actively involved in research and development. ARM is largely responsible for the development and market awareness of the process.
Popularity of the Process
By the 1980s the rotational moulding process was streamlined by larger material suppliers around the world. And in response to increasing demand, new materials and grades became available. Storage tanks of all sizes and shapes typically dominated the industry in various compositions including polycarbonates, polyesters, polypropylene, LDPE, nylon and ABS (acrylonitrile butadiene styrene) complemented LDPE and high density polyethylene (HDPE). And in 1984, rotomoulding became the process of choice for manufacturing inexpensive whitewater and sea kayaks. Studies at Queen’s University, Belfast, led to much improved scientific understanding of the rotomoulding process including the development of a system for continuous in-mould temperature measurement called the “Rotolog” system.
Today, the primary aspects attracting current research involve lowering the cycle times and improving the quality of parts. Pressurization techniques offer some hope in lowering the cycle times as applying a small pressure at the right point in the heating phase may speed the coalescing of polymer particles. This will produce parts with less bubbles in a shorter amount of time than at atmospheric pressure. Critics of the pressurization technique justify their position by pointing out the danger of explosion of pressurized parts; this is the reason mould pressurization has not yet been widely adopted.
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