Low-Pressure Molding Option

When you talk about materials, you have to talk about process too. Beyond the potential of different materials and different uses for traditional materials themselves, there is plenty of potential in design for using different processing technologies. One interesting processing technology is rotational molding. Beall, founder of Glenn Beall Plastics Ltd., has written a book on the subject. The former chairman of five different Society of Plastics Engineers committees, he is a member of the Plastics Hall of Fame and the Rotational Molders Hall of Fame. Here are his thoughts on rotational molding.

What is rotational molding?

It really is a coating process, not a stretching process. It's a unique process, different from others. There's no pressure pushing plastic through the cavity. There's no pellets. In the process, you grind pellets to a powder, put the powder in a mold, rotate the mold in two directions simultaneously, then place the mold in an oven where the particles adhere to the hot inside of the mold. It's a slow rotation—1 to 12 rpm. When completed, this molding process produces hollow parts.

What are the markets?

The largest markets are tanks and shipping containers. Other applications include power boats with double-walled hulls. You can also use rotational molding for toys and medical products, such as drainage containers or housings for diagnostic equipment. Also, floor-scrubbing equipment and earth-moving equipment components. The process is great for molded-in metal inserts, and great for textured surfaces. The process produces a more uniform wall thickness than produced by blow molding, which unlike rotational molding, is a streching process.

What products is the process not good for?

It's not good for parts with square outside corners, but it's great for hollow parts with a radius on the outside corner.

How big are the parts you can make?

The biggest products so far are 21,000-gallon tanks and a 21-ft powerboat.

Are high temperatures and presures involved in the rotational molding process?

Yes, they are, but low pressure. Because of the low pressure, there is low residual stress and good impact strength. Also, since the pressures are low, the molding machines themselves are light-duty and relatively inexpensive. Tooling is also inexpensive. Those low costs mean that rotational molding works well for small volumes of both large and small parts.

What do engineers have to take into consideration when designing products for rotational molding?

Draft angles and corner radiuses are critical. Avoid thin ribs—the material won't flow into them. Also, engineers must leave space for material to penetrate into the nooks and crannies of the design. The material won't flow to 1-mm parts.

Reach Beall at [email protected]