ROTATIONAL MOLDING: Big machines, small hassles
Canadian PlasticsCanadian Plastics Plastics Processes
What’s it like working in a plastics processing sector where short-run production is the norm and foreign competition is almost nonexistent? Just ask a rotational molder.
Where would we be without rotational molding? There would be no pink plastic flamingos on front lawns to celebrate milestone birthdays and new babies, for one thing. But it goes beyond that. Storage tanks, playground equipment, furniture, toys, garbage cans, airplane parts, helmets, road cones, canoes and kayaks — they’re all made by rotomolding, as are almost all other hollow parts. But despite its ubiquitousness on the consumer goods market, rotomolding is still considered the simplest of the primary plastics processing methods, in large part because, as a casting process, it uses low pressure compared to injection or blow molding.
In fact, the whole industry can seem a little bit low pressure — remarkably free, in particular, of cutthroat competition, both domestic and foreign. Welcome to the unique, relatively placid world of rotational molding in Canada.
MACHINES, MOLDS, MATERIALS
The principle of rotational molding of plastics isn’t complicated. Basically the process consists of introducing a known amount of plastic in powder, granular, or viscous liquid form into a hollow, shell-like mold, and then rotating that tool biaxially in an oven until the resin melts and coats the inside of the mold cavity. The tool is then cooled, and the part is removed from the mold. And that’s it. If you’re counting on your fingers, that’s a mere four steps: mold charging, mold heating, mold cooling, and part removal.
The equipment is a bit more complicated, and rotational molding machines themselves are made in a wide range of sizes. Rotomolding units normally consist of molds, an oven, a cooling chamber, and mold spindles. The spindles are mounted on an arm biaxially rotating the mold or molds, which provides a uniform coating of the plastic inside each mold as it’s heated. There are two types of turret rotational molding machines commonly used: independent-arm and fixed-arm. In fixed-arm turret machines, all of the arms — typically three — index at the same time. This requires that heating, cooling, and servicing operations must be done at the same length of time for each arm. Independent-arm machines, by contrast, provide process flexibility by allowing one arm to index while the other arms can remain stationary.
There are other popular machine configurations, as well. Carousel machines, which require the largest amount of floor space, consist of three or four arms on which the molds are mounted; these arms rotate the molds biaxially and move from station to station on the machine being loaded, heated, cooled, unloaded, and loaded again ready for another cycle. Shuttle machines, which generally require a smaller amount of floor space, may have one or two carts on either side of the oven on which molds are mounted. These carts will move into and out of the oven in turn. If two carts are used, one cart is in the oven heating while the other cart can be unloaded and loaded again for the next cycle. Clamshell machines, which are smaller still, are usually single-arm units that run one part at a time, and are not as productive as multi-arm and two-cart rotomolders. A final design is the rocking oven — also called the rock-and-roll rotomolding machine — which is used primarily for making long, thin parts like kayaks.
The industry was semi-revolutionized about 15 years ago when Italian machinery maker Persico introduced the Leonardo, the world’s first completely automated rotomolding system; but in North America, only a handful of the expensive machines, with highly engineered molds, have been sold, and those to companies producing high-volume parts. In 2013, Persico unveiled the Smart system, which it billed as a more flexible, but still automatic, rotomolder.
The most common types of molds used in rotomolding are cast aluminum, fabricated aluminum, and steel and stainless steel. “Historically, almost all rotomolds were cast aluminum made in a foundry or sheet-metal molds,” said Bruce Muller, president of Plastics Consulting Inc., a rotomolding specialist based in Palm City, Fla. “The acceptance of CNC molds has grown lately, especially in Europe. These molds are stronger, have a longer life, conduct the heat faster, and potentially offer more precision.”
And the most common type of material to be rotomolded is PE, hands-down. “Ninety per cent of all rotomolded parts are made from PE,” Muller said. “The rotomolding process is relatively long and therefore aggressive on polymers, and only PE and a few other resins — including PVC (plastisol), nylon, PP, Hytrel, PC, and cross-linked PE — are suitable.”
Since most rotomolding parts are generally not mechanically conveyed away from the mold, there is very little post-molding equipment necessary. Before loading the mold is a different story, however. “Since rotomolding primarily uses powder instead of pellets, it requires an additional piece of equipment to pulverize the resin,” Muller said. “This can be done by the resin supplier or in the rotomolding plant.” On a related note, a new automatic weigh powder dispense system with PLC was developed for the rotomolding industry by Wittmann Canada Inc. just last year. Called RotoLoad, the system is available in 50-, 150- and 300-lb models, with identical controls for each.
OFFSHORING? OFF LIMITS!
We mentioned above that rotomolding doesn’t have much in the way of competition, but that’s not entirely true. Blow molding and twin-sheet thermoforming are the competing technologies since all three make hollow parts. Choosing between these processes usually comes down to product volume. “Rotomolding is very slow, which makes it ideal for low-volume production and product testing,” said Dave Carter, president of Newmarket, Ont.-based custom rotational molder M.B.C. RotoMould Inc. “Achieving high volumes in rotomolding requires a large capital investment in tooling.” Part size is a second determining factor. “Blow molding and twin-sheet thermoforming can only make parts up to a certain size, whereas rotomolding goes much larger — even as large as 22,000-gallon tanks, which are about the size of a semi-trailer truck,” said Bruce Muller.
The ability to make extremely large parts is a definite advantage of rotomolding, but not the only one. “Since rotomolding is a casting process that doesn’t use pressure, the tooling costs are low, which means molds are inexpensive,” said Dave Carter. “Rotomolding also offers tremendous design flexibility, the ability to do short runs economically and in multiple custom colours, and it makes complicated shapes easily. It also accommodates production complexities such as stiffening ribs, molded inserts, and different surface textures; and achieves consistent wall thickness with corners tending to be thicker, which increases product strength and integrity.”
And from a business perspective, it’s definitely nice to work in a product sector that isn’t directly threatened by overseas competition. “Competing products do come in from Asia — mainly goods that can be stacked or nested — but it’s not as big a concern for us as for the injection molders, since it generally doesn’t make economic sense to ship hollow parts across the ocean,” Carter said.
A FEW DRAWBACKS
That’s not to say rotomolding doesn’t have its downsides. “The industry is constantly trying to reduce cycle times, but the physics of the process and the molecular structure of the materials involved limit how fast we can go,” Carter said. “Raising oven temperatures to try to bond the materials faster, for example, doesn’t work.”
Another limitation lies in the molds themselves. “Unlike other processes where only the product needs to be cooled before being removed, with rotomolding the entire mold must be cooled,” Bruce Muller said. “While water-cooling processes are possible, there is still a significant downtime of the mold.”
The process is also labour-intensive, and it can be difficult to find good workers. “Most people don’t have the skill set that we require, even if they have experience in other kinds of plastics processing,” said Dave Carter. “Our workers don’t just stand at the end of processing lines breaking off bits of plastic; we change a mold every 45 minutes on average, and they’re all involved with that.”
At the end of the day, though, it’s still a good time to be a rotational molder in Canada. “It’s a challenging, interesting industry,” Carter said. “Also, startup shops are few and far between — mainly because the Canadian market isn’t big enough to lure in many entrepreneurs — so there’s enough work for everybody, which keeps it from getting cutthroat.”
Just the environment you’d expect from the makers of pink flamingos.