Jump to Plastic Injection Moulding or Rotational Moulding.
History of Plastic Injection Moulding
Injection moulding has seen steady growth since its beginnings in the late 1800s. The technique has evolved from the production of combs and buttons to major consumer, industrial, medical and aerospace products.
In 1868, perhaps in response to a request by billiard ball maker Phelan and Collander, John Wesley Hyatt invented a way to make billiard balls by injecting celluloid into a mould. By 1872, John and his brother Isaiah Hyatt patented the injection moulding machine. The machine was primitive yet quite suitable for their purposes: it contained a basic plunger to inject the plastic into a mould through a heated cylinder.
Revolutionising the plastics industry in 1946, James Hendry built the first screw injection moulding machine, with an auger design replacing Hyatt's plunger. The auger sits inside the cylinder and mixes the injection material before pushing forward and injecting it into the mould. Today, almost all injection moulding machines use this same technique.
History of Rotational Moulding
Rotational moulding has 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.
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, enabling large hollow containers to be created in low density polyethylene (LDPE) by rotating (or rocking) a mould on a chassis, housing 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, allowing the rocking to continue until the moulding could be extracted.
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 compositions including polycarbonates, polyesters, polypropylene, LDPE, nylon and ABS (acrylonitrile butadiene styrene), complementing LDPE and high density polyethylene (HDPE).
Today, the primary aspects attracting research involve lowering cycle times and improving the quality of parts. Pressurisation techniques offer some hope of lowering cycle times, as applying a small pressure at the right point in the heating phase may speed the coalescence of polymer particles. This produces parts with fewer bubbles in a shorter time than at atmospheric pressure. Critics of the technique point out the danger of explosion of pressurised parts, which is why mould pressurisation has not yet been widely adopted.