STOW, OHIO — Molds with moving sections — already used in other plastics processes — have come to rotational molding, literally putting a squeeze on parts to add living hinges, solid flanges, very thin tabs and sharp corners.
Those features cannot be made by traditional rotomolding, known for its hollow parts with smooth, rounded-off corners. Without moving sections, the plastic never would flow into those tight sections, according to backers of the technology, called transfer rotational injection process, or Trip.
"The potential is pretty much unlimited," said John Fawcett, who designs rotomolded products at his Kent, Ohio-based Fawcett Designs Inc. "This allows you to add injection molded and blow molded details and put it on a rotomolded part."
Don Schraegle, a rotomolding consultant in Medina, Ohio, is marketing Trip through his firm, SJS Industries Inc. The moving mold was invented by Craig Wallin, owner of a small molder in Logan, Utah, called Sirtec North America Ltd. Inc. A U.S. patent is pending.
Sirtec has used the molds to rotomold bumpers for city buses, taking market share from structural foam molded parts. The special feature: A section of the mold squeezes down on melted plastic to form a long, narrow flange that fits into a metal mounting bracket on the bus. Sections of the mold squeeze down to press the plastic into a flange made of solid plastic.
Sirtec recently added a thin tab on the end of the bus bumper to cover a parting seam.
Wheeler Boyce Co. is making the molds, complete with spring-loaded mechanisms that move to compress the melted plastic. Schraegle, Fawcett and Julie Stout, marketing manager at the mold company, described the molds during an interview at Wheeler Boyce in Stow.
Schraegle carries around samples that combine, all on one part, a hollow section with two separate sections, a living hinge, a solid handle, another thin piece that could serve as a fastening fin and even a delicate plastic screen.
It does not look like a rotomolded part.
Stout said Wheeler Boyce displayed Trip-molded parts in October at the Association of Rotational Molders' fall meeting.
"There's a lot of interest. ... We had parts in our booth, and people just were coming up and raving about it," she said.
Schraegle will present a paper on the molds at ARM's spring meeting, set for March 18-21 in San Diego.
After years of use, moving sections are fairly common in blow molding and injection molding. But rotomolding faces unique challenges. In injection and blow molding, the job of the mold is to cool down the melted plastic. But molds for rotomolding are required to both heat and cool the material, all while spinning, cycle after cycle, through an oven and a cooling station. Consequently, molds get banged around.
Typically, Wheeler Boyce designs any mechanism it puts inside a mold to withstand temperatures of 650° F. Spring-loaded moving sections are the simplest way to do Trip molding, but Stout said Wheeler Boyce also is looking at pneumatic power, using air from existing lines to pressurize molds. The seals and moving parts would have to be insulated heavily, adding to the cost of the mold, she said.
Higher mold cost is one reason SJS is not a drop-in replacement.
"It's not going to be just: Put the part in the machine, turn it on and go," Schraegle said.
SJS is licensing the process to rotomolders, but Schraegle is recommending a processor get expert advice before adopting Trip molds. Special care is required to maintain and clean the molds.
Trip changes impact design, because Trip-molded parts do not shrink as much as traditional rotomolding. A molder also has to examine where in the part the excess plastic flows after it gets squeezed out, according to Fawcett.