A form of rotational molding was employed by ancient Egyptians who used this technique to create ceramics. Between 1940 and 1950, an updated rotational molding was developed for the plastic industry in the US but didn’t become popular because it was so slow. However, in the past few decades, improvements in the plastic powders used in the process have made it more viable.
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Rotational molding involves a lot of heat but low pressures. A hollow mold is filled with powdered plastic resin and then rotated or spun. It is transferred into an oven where the resin melts and ultimately coats the walls of the mold. This is then followed by a cooling step. The mold keeps rotating until the resin fully hardens into its final shape. The mold can then be opened and the part removed.
Rotomolding is best suited for large, single-piece hollow parts or double-walled open containers like kayaks, oil tanks, and coolers. Unlike blow molding, rotational molding produces no weld lines or pinch-off seams. The product thus doesn’t require secondary finishing processes. Rotomolding is more efficient at lower production volumes – fewer than 3,000 per machine annually. This makes it ideal for start-ups, inventors, and small businesses.
To learn more, see our guide on What is Rotational Molding.
Rotomolding has the following advantages over blow molding:
The following are considered disadvantages of rotational molding compared to blow molding:
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Injection molding machines work by injecting molten thermoplastic into a two-part mold. The first injection molding machine was developed in 1872 by John Wesley Hyatt. This machine was used to mass produce items like buttons. The first screw injection molding machine, which is what modern machines are based on, was built in 1946 by James Watson Hendry.
The process works by melting plastic resin pellets inside a barrel. The barrel contains a screw with a shaft whose diameter increases along its length. As the screw rotates, the plastic is forced into an incrementally smaller area. This compression is the primary mechanism responsible for melting the plastic. The barrel is also heated to further aid the melting process. Once enough plastic to fill the mold has melted, the screw retracts. The screw is then pushed forward to force the plastic into the mold at very high pressures.
Injection molds are made from at least two parts. They are held together with hydraulic rams so that the high-pressure plastic injected into the mold cannot escape from the parting line. The mold has cooling channels that cool the part enough so that it can be ejected from the mold without damaging it.
Injection molding can achieve high production volumes unmatched by any other plastic processing technique. It can easily produce millions of parts per machine annually. Injection molding can create complex parts. It is used for everything from gears to garden chairs. Injection molding is better than blow molding for the high-volume production of complex plastic parts. To learn more, see our guide on the Molding Manufacturing Process.
A typical injection molding setup is shown in Figure 2:
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