History of Plastic Injection Moulding
The injection moulding has seen steady growth since its beginnings in the late 1800's. 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 it was quite suitable for their purposes. It contained a basic plunger to inject the plastic into a mould through a heated cylinder.
Revolutionizing the plastics industry in 1946, James Hendry built the first screw injection moulding machine with an auger design to replace Hyatt's plunger. The auger is placed inside the cylinder and mixes the injection material before pushing forward and injecting the material into the mould. Today, almost all injection moulding machines use this same technique.
History of Rotational Moulding
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.
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, houses 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.
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).
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 coalescence 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 mold pressurization has not yet been widely adopted.
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