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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