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Rotational Molding - Seamless Manufacturing Method for Millions of Products

Jun 20, 2008
There are so many common products we take for granted in everyday life. While it's true that many urban kids don't know that milk comes from cows or what a real wheat field looks like, the same is true for manufactured products that are mainstream to life in America.

How about the garbage bin outside your home, the bright orange road cones used for road safety, bicycle helmets and giant tanks used for water storage? Chances are these disparate products were made using a popular manufacturing method called Rotational Molding. What they have in common is a hard, continuous outer shell and a hollow space inside.

The Process
Rotational molding was an innovation of the 1940s, but wasn't widely used until the development of better technology that made the process faster. In addition, new polymer and plastic products entered the marketplace and were well suited for rotational molds. This method of molding plastics has several advantages over some other techniques which result in higher prices for products and a less environmentally friendly process.

The first step in producing a lightweight and affordable polyethylene (plastic) product is to create a mold. This is usually done using a computer software program capable of creating three dimensional images. The mold is most often made of aluminum because the lightweight material is easier to handle than some other metals although it may be a bit more costly. Machinists get to work tooling the mold from the design. Once the mold is done, the rest of the process is in the hands of the plastics manufacturer where the rotational molding is done.

Here's where some imagination is required. Picture the ride at the State Fair where a person is strapped into a kind of gyroscope that turns them up, down, sideways and around. That's what will happen to the mold. But first, the manufacturer measures polymer resin, a granular powder, and pours it into the mold. The mold is fitted into place in an oven that's heated to an appropriate temperature. Inside the oven, the mold makes its axial turns, spreading the grains of polymer evenly over the inner surface of the mold.

As the aluminum quickly heats up, the resin melts and continues to coat the inside of the mold. The time the mold spends spinning and heating is critical to the quality of the product. In the past it was up to the rotational mold experts to judge when a mold was ready to remove from the oven and cool. Today, sensitive instruments gauge the air temperature in the mold, improving quality control.

Fans are often used to help the mold cool. Unlike some other mold methods - such as injection molds - the cooling of the aluminum causes the mold to shrink slightly away from the interior of the mold. This allows easy removal of the largest products, such as water tanks. The product is then carefully inspected by the rotational manufacturer and, if it meets all specifications, is shipped to the customer.

Molding Benefits
For many products, rotational molding makes a big difference in the price of a product and in its durability. The manufacturing process does not require lots of interlocking and moving parts. Therefore, maintenance is minimal as is replacement of parts. In addition, there's little waste of material, a factor that makes the process more environmentally sound.

Durability of products is also very important to manufacturers, particularly those marketing items that will serve customers over extended periods of time. For example, a water tank produced with the rotational molding process will provide much longer service than a comparable steel tank. Rotational molds can be made to any specification or shape, and can be manufactured in less time than steel. They are also very lightweight, cost less to transport and are not as challenging or awkward to install onsite. Of course, a big advantage to molded tanks is that, unlike steel, they will never rust.

Next time you see a one-piece, hollow product made of polymer plastic (maybe a kayak or the face of a doll) astound your friends with the fact that you know just how it was done - with the rotational molding process.
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