Avoiding Unforced Errors in Size Reduction
Because producing good regrind is hard enough already without you making it tougher.
February 1, 2013 by Mark Stephen, editor
Whether you’re on the golf course or in a plastics processing plant, it’s never fun to wind up in a trap. In the first scenario, you probably risk bogey or worse. In the second, it’s even grimmer: wasting time and material, including valuable regrind that should go back into the product. This really hurts.
Shredders and granulators are crucial in making effective use of reclaimed resin, but are you making effective use of your shredders and granulators? The answer might surprise you. According to the experts, the granulator is the single most misapplied piece of equipment in our industry, and shredders aren’t far behind.
This would seen to fly in the face of advances in cutting technology and machine design available for today’s size reduction systems — but maybe these developments are part of the problem. “Too many processors haven’t kept up with technological changes in shredders and granulators,” said Mike Cyr, vice president of Rotogran International Inc. “They’re operating just as they would with older machine designs from 20 years ago, which isn’t a good way to maximize efficiencies in modern units.”
The fact that, too often, size reduction equipment is treated as the lowliest face on the totem pole doesn’t help, either. “They’re often the last pieces of equipment to be considered for purchase in a plastics processing operation and the first to be ignored when it comes to proper care,” said Jeff Taylor, national sales manager for size reduction for The Conair Group.
Small wonder, then, that processors tend to fall into some common traps. But it doesn’t have to be this way. Here are a few guidelines to follow so you can avoid major shredder and granulator errors.
Shredders are capable of processing a large variety of materials to a multitude of fraction sizes and throughput rates. Placed ahead of a granulator, the theory is simple: pre-shredding material before granulation will typically increase the throughput of the granulator, and reduce wear of the granulator blades and screen by reducing the amount of recirculation. It should be the perfect system for size reduction. But beware the first trap: a shredder and/or granulator that is improperly configured for the task.
Machines bought used are a big cause of the problem here. “The likelihood that a machine purchased by a processor on the used market is going to be configured properly for the intended application is extremely rare,” said Madison Burt, vice president of sales for Weima America Inc. “The unit may well do the work, but not as quickly or as efficiently as will a tailored machine.”
Having a pre-conceived — and almost invariably incorrect — notion of the required size of a new shredder or granulator plays a role, too. “Processors will purchase a 20-hp unit, for example, and then discover it’s not powerful enough or the throat opening is too small,” said Brian Davis, general manager of Maguire Canada Inc. “Defining their own needs clearly before going out for tender is hugely important for the processor.”
However the mistake is made, undersizing the granulator tends to be the most common configuration problem…and it’s getting worse. “Undersizing is happening more often now than in the past, probably for the simple reason that processors are trying to save money,” Mike Cyr said. “Their mistake is in calculating their throughput need based only on their scrap rate, forgetting the fact that granulators are often surge-fed additional bad parts.”
The result? A jammed granulator or the sad sight of a product bouncing around in the cutting chamber for long periods before finally being ingested by the rotor. This leads to lower throughputs than expected from the machinery, not to mention very poor regrind quality.
In order to cut material efficiently, you’ll need a shredder or granulator configured specifically for your application. If not, expect higher energy consumption, excessive dust and noise, lower capacity, and increased knife wear. The good news is, most sophisticated size reduction machinery suppliers offer products that allow the builder to configure core machine components to match the specific application requirements. “Especially with granulation, you’ll need at least seven critical pieces of information to size the unit appropriately,” said Kirk Winstead, CEO of Rapid Granulator Inc. “These include what the application or process is going to be, what material is going to be used, what the method of feeding will be, a physical description of the intended parts, the part dimensions, how many pounds or kilograms per hour you expect to run, and the screen size.”
DOESN’T CUT LIKE A KNIFE
Poor knife design and positioning are another common trap that can bedevil your size reduction process, creating non-uniform regrind with high dust and fines content, and a high level of wear and tear on the shredder or granulator.
The number one culprit? Again, machinery bought on the used market. “It’s almost impossible for a processor to properly configure the knives themselves if a shredder has been bought on auction,” said Greg Parent, Canadian sales representative for Vecoplan LLC.
Another potential drawback of a used size reduction unit is that the blades might not be suited to the resin you’re planning to run through the machine. “Don’t use a nylon 66 resin purge in a shredder with carbide knives, for example, because it will very likely chip them,” said Madison Burt. “Also, don’t use knives made from low-grade tool steel in a PVC application because PVC is a high-wear material; for PVC, it’s better to use carbide knives.”
It helps to keep in mind what well-designed, well-positioned knives should do. “The goal is to have proper scissor-cutting action, good rotor design, optimized location of fixed knives, and optimal rotor knife speed,” said Kirk Winstead. Fortunately, pre-adjustable knives and cassette knives are features found on most modern machine designs. “Recognizing the importance of knife sharpness and gap and their relationship to final granulate quality, many machine manufactures have standardized on more maintenance-friendly designs for the knives,” Winstead continued.
Understanding bed knife angles and the different blade rotor styles available in the cutting chamber — and tailoring them to whatever it is you’re shredding or granulating — can go a long way to avoiding problems, particularly in granulation. “A granulator can have either high shear or low shear, the differences being the angle of approach and how much back-angle exists on the knife,” said Mike Cyr. “High shear is best suited for grabbing and grinding thin-wall parts or film, while low shear is better at handling thicker parts.” It gets trickier. “A ‘super-tangential’ cutting chamber with a three-blade open rotor is the preferred configuration for granulating bulky items such as bottles and containers, as well as for large quantities of runners and small parts and for film scrap,” said Kirk Winstead. “A ‘tangential’ cutting chamber with five-blade open rotor is good for sheet, thick-walled parts, and large quantities of shredded material.”
And do we really have to remind you to keep the blades sharp? Apparently so. “Dull blades are, far and away, the most common granulator problem,” said Jeff Taylor. “Check your blades regularly; a quick visual inspection to verify that the blades are sharp should be built into your regular preventive maintenance schedule.”
As with people, stuffing too much too quickly into size reduction machines usually leads to bad outcomes.
Before going any further, it should be noted that the trap of overfeeding is actually much more of a problem for granulators. With shredders, in fact, the opposite mistake tends to happen. “Many operators, afraid of jamming the shredder, don’t put in enough material,” said Greg Parent. “Actually, it’s better to put as much into a shredder as you can because the ram pushing the material into the rotor, and the weight of all the material pushing down, helps maximize throughput.”
With granulators, though, overfeeding usually arises when a unit sized for a particular application is wheeled across the shop floor to perform its duties for a different part or job in which the materials and throughput are different. “Overfeeding your granulator will obviously back it up, reducing productivity,” said Sebastien Diaz, sales and marketing manager for Mo-Di-Tec. “In some cases of overfeeding, the granulator approaches the maximum amp load capacity of the drive motor and simply jams or ceases working.”
It helps to understand how the granulator is sized in terms of throughput. First, there’s a big difference between instantaneous rate and intermittent rate, the latter generally being how the granulator is sized. “A granulator sized for 2,000 pounds per hour should be fed at a rate of approximately 33 pounds per minute,” said Kirk Winstead. “If a processor dumps 100 pounds of material in the hopper in a matter of seconds, don’t expect the granulator to perform. It’s a very common occurrence with hand-fed granulators.”
The best solution is to automatically meter-feed your granulator, either with a robot or a conveyor. And consider the more exotic solution of a second flywheel on the granulator. “The second flywheel adds more inertia to the rotor, giving it the extra torque to cut through a shock-load of material,” said Mike Cyr.
Similarly to shredders, it’s also important to avoid underfeeding the granulator. “If your granulator sits idle and the rotor spins without parts, the energy efficiency ratio is greatly decreased,” said Jeff Taylor. Plus, if your scrap is just sitting next to your machine in a drum, box or storage area, it’s not hard to imagine what can happen next: hoping to make up time, the over-eager operator dumps a whole box into the granulator. The result: downtime. “Regular, steady feeding of your granulator is best,” said Brian Davis.
So while staying out of that yawning bunker on the 15th fairway might still be beyond your power, avoiding the common traps in shredding and granulation doesn’t have to be.
The Conair Group (Cranberry Township, Pa.);
Dier International Plastics Inc.; (Unionville, Ont.);
Industries Laferrière (Mascouche, Que.);
Maguire Canada/Novatec Inc. (Vaughan, Ont.);
Barway Plastic Equipment Inc. (Vaudreuil-Dorian, Que.);
Mo-Di-Tec/Auxiplast Inc. (Ste-Julie, Que.);
Rapid Granulator Inc. (Cranberry Township, Pa.);
Dier International Plastics Inc. (Unionville, Ont.);
DCube (Montreal); www.dcube.ca; 514-272-0500
Rotogran International Inc. (Toronto); www.rotogran.com; 905-738-0101
Vecoplan LLC (High Point, N.C.);
www.vecoplan.com; 336-861-6070; Greg Parent; 416-678-0154
Weima America Inc. (Fort Mill, S.C.);