Granulators.

There's a lot to look for in a granulator: Easy cleaning and maintenance, safety and soundproofing, quality of regrind, and--most important--its ability to swallow whatever you plan to feed it.

A granulator's a granulator, right? As long as it chops up sprues, runners and rejected parts, one is just about the same as another, and you might as well go for the lowest-priced model. Right? Not if you want to be sure of getting the best design for your overall manufacturing needs, most savvy processors today would answer. You don't need a special or unique application to shop carefully for a granulator. For the great number of molders, factors like safety, soundproofing, quality of regrind and simplicity of maintenance--all at a good price--are determining factors for which granulator they ultimately buy.

"Buying granulators has become much like buying a car," says one leading granulator manufacturer. "It depends on what your needs are. There's the fast and sporty model or the slow and practical model." The "fast and sporty" model is for grinding small parts like sprues and runners from multicavity molds. The "slow and practical" version is better suited to reducing thick, dense, and usually large parts. And some say kinder to its cutting blades.

"I haven't had to sharpen a blade in three years," says Rex Roe, maintenance manager for Nypro Iowa, a 20-machine, custom injection molding shop in Mt. Pleasant, Iowa. "With a low-speed granulator, you don't have that high-speed beating of the blades, so they last longer."

While a low-rpm motor may be one way for a granulator to achieve a cleaner cut (i.e., less noise, fines, and dust), it's not the only way. "Low speed is not the key factor," in one supplier's opinion. "By using fewer knives you can accomplish the same thing."

Furthermore, custom molders would be unwise to limit themselves to single-speed granulating, say molders and suppliers alike. Because the custom molding shop processes dozens of materials into parts of innumerable shapes and sizes, variable-speed granulators provide needed flexibility to adapt to different applications.

A KEY COMPONENT

Most processors interviewed for this story consider their granulators to be integral parts of the production stream--a far cry from the old days when the lowly scrap grinder was treated like a stepchild of manufacturing. Consequently, processors say easy maintenance is a key to choosing a granulator. It must have a minimum of downtime, and that means cleaning and routine maintenance must be as easy and quick as possible. Simple access to blades and screen is vital. "What used to take us three or four hours now takes about an hour," says one processor who recently switched granulator suppliers.

As important as quick access to the blades is the quality of steel used in the blades. For soft and easy-to-cut materials like LDPE film, lower-quality steel blades will probably do. But if you're chopping glass-filled materials or other extremely strong plastics, cheap blades could result in poor regrind uniformity or, at worst, they could chip, contaminating the regrind with metal bits. To process tougher materials, high-quality steel alloys and even carbide coatings are becoming more in vogue.

Most processors say they prefer the double-angled cutting design that has been incorporated into most suppliers' offerings over the last 20 years. This design, they say, ensures good quality regrind with a minimal amount of fines. It also consumes less power and generates lower levels of heat and noise.

With OSHA standards dictating allowable noise levels in the workplace, this consideration is important to granulator users. Some processors insist that their suppliers soundproof the granulator enclosure. Others opt to build their own sound enclosures to save money.

SAFETY FIRST

Among the safety options processors look for are an automatic shutoff in case sprues or parts back up in the feed chute, an easily reachable emergency cut-off button, hands-off auger feeding, and an anti-flyback tangential-feed design. In many molding shops, automation is a key to efficient operation. Auger feeding fits right into that trend, users say, by allowing either robotic or conveyor feeding at the press. "We've tried to automate our lines as much as possible," says one Pennsylvania molder. "Auger feeding automates our line even more by not having people handle sprues or rejected parts."

A Grinder for Every Need

One processor with an unusually difficult granulating requirement illustrates the importance of consulting suppliers about your application before making a purchase, and possibly getting them to configure a granulator for your specific needs.

Thermedics, Inc., Woburn, Mass., compounds medical-grade polyurethanes. The company produces its materials in 2 x 4 ft slabs that are 2-3 in. thick, granulates them, then repelletizes the granulate for sale to medical processors. The slabs, which weigh 50-80 lb, tend to be extremely dense and difficult to chop, says Carl S. Johnson, Thermedics manufacturing engineer. Initially, Thermedics used a homemade guillotine to reduce the slabs to smaller pieces that were then hand-fed into a 100-hp granulator. "The slabs burned out the drive belts," Johnson explains. Thermedics wanted a less labor-intensive solution.

"Most suppliers tend to spec their machines based on grinding polyolefins," he says. "That didn't really help us. We needed something stronger than what's used for polyolefins."

After examining numerous suppliers' offerings and discussing with granulator manufacturers how to solve Thermedics' unique problem, Johnson found the solution in the form of an extremely heavy-duty, 150-hp granulator with a chain drive in place of the usual belts, a unique rotor design, and an oversized (10-hp) blower. "It cost a lot more than anything else out there," Johnson says. "We paid top dollar but we'd rather pay more up front and get the reliability and service. If that machine goes down, everything else here stops."