The buzz phrase that ``product designers should talk early with manufacturers'' is a given these days - but it's even more urgent in rotational molding than in more-common processes like injection molding, said designers and rotomolding leaders.
The reason is simple: Rotomolding is, well, different. Plastic pools in the mold as it spins inside an oven, gradually adhering to the hot mold walls. There's no pressurized plastic slamming into a mold. It can be labor-intensive. But it's a great process that can produce large, double-wall parts with low internal stresses, industry boosters said.
Designers typically don't understand rotomolding. That was the reason the Society of Plastics Engineers held its Rotational Molding by Design Conference in June - to bring designers and industry officials together. Industrial designer Warren Ginn said more cross-pollination is needed.
``There's a lot of misconception by designers ... that all the parts have to look like toys [and have] big radiuses. That there was limited design flexibility,'' Ginn said in a presentation. ``Remember, most of these designers are used to injection molding, because largely, that's all they've been taught in school.''
He quipped: ``I think what surprises designers when they first see this is, `Where's the clamp? Where's all the steel?' ''
Rotomolding, which also competes against blow molding and twin-sheet thermoforming, won praise from Ginn, a designer at Integrated Design Systems Inc. of Great Neck, N.Y., and other designers at the conference. ``You can't get this kind of complexity and sophistication out of any other process, for the price,'' Ginn said.
Some others were not so positive.
Rotomolding has improved so it can make parts that were impossible 10-15 years ago, but products with many tight tolerances drive up the cost of production, said Stephen Osborn, president of Trilogy Plastics Inc. of Louisville, Ohio.
``Think of a rotomolded part as a pile of Jell-O. If you hold one tolerance, it's going to bulge out in another area.''
Osborn ran down the major cost factors for rotomolding. Resin is No. 1, commonly accounting for 30-60 percent of the total selling price for custom molders. Machine time is the second-largest cost area, followed by secondary costs such as assembly, scrap, packaging and shipping.
Bright colors are the most expensive. Granite color hides a lot of appearance problems, but you have to watch for streaking.
``As any molder will tell you, if you have one little speck of a contaminant, it will migrate around and end up in the most visible area of the part,'' he said.
Kenneth Bather, engineering director at Diamond Plastics Inc. in Dunkirk, Ohio, called for designers to contact the rotomolder early and learn about the process. ``I'm tired of these bad molds - poor designs that we allow our customers to force upon us,'' he said. ``We need to change that.''
Designers should look at established guidelines for rotomolding.
``Please use them. But they're guidelines; they're not commandments. They are not hard and fast. If you are in a run for the roses, if you are trying to get a design done and you've got four weeks to do it in, you'd better follow the design guides. If you're not, you will be upset. You'll annoy your molder. And your project will be late,'' Bather said.
He outlined several red flags for designers. Color can add complexity to the design. ``Please, please do not try to match a color for roto with one from injection,'' he said. Also, a high number of molds and features, such as leak-proof fittings, increase the price, he said.
Paul Arato, who runs Arato Designs in Toronto, said his firm tries to head off problems. ``When we start a project, we always insist on a very detailed preliminary design phase not just a series of sketches,'' he said. His firm makes clients commit to a molder, and if the customer later wants to change molders, it has to pay the first vendor for its work.
Conference attendees also learned about some case studies:
* Arato designed a point-of-purchase cooler that holds bottled drinks and packaged sandwiches at the takeout counter. The customer already makes stainless-steel coolers, but was interested in plastic, Arato said. Rotomolding, with foam insulation between the double walls, made a rigid cooler that will not dent and scratch, like stainless, he said. Also there is no condensation. Trilogy was the molder, and Wheeler Boyce Co. of Stow, Ohio, made the mold. The checkout cooler has been in pilot production for several months.
* Ken Pawlak, a designer in Vernon Hills. Ill., who specializes in medical products, designed a kidney dialysis machine that patients can use at home to filter their blood. The machine required a very rigid 12-gallon tank that is bio-compatible and could stand up to daily hot-water sanitation. The solution: a rotomolded polypropylene tank with a molded-in shelf. The tank was then reinforced by filament winding. The dialysis machine is now on the market.
* Gregory Stout, who runs his own design house in Stow called Blue-Reed LLC, helped create some fun projects, including a 6-foot-high outdoor advertising kiosk that plays a tape loop, has back-lit graphics and even can spray cooling mist on passersby. The kiosk has three rotomolded parts. The base, which holds 80 gallons of water and the pump mechanism, looks like a mound of rocks.
Stout also designed a foam-filled, portable cooler that looks like a catamaran racing boat with wheels. The lid was a challenge, a big flat space. ``The customer was adamant that the surfaces be smooth,'' he said.
And last, Stout talked about the DogiPot. When you walk your dog in the park, you grab a plastic bag out of the DogiPot, clean up the ... material and drop it in the container. Stout recommended adding ribs and rounding the radii to replace sharp corners. Think about it and be grateful, next time you take a walk in the park.