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November 6, 2002

8 Min Read
Technology Notebook: Improving granulator efficiency

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A three blade, open rotor offers good airflow for cooling, while the herringbone or twin shear design maximizes cutting efficiency over the wide cutting chamber. The knives are gapped outside of the granulator for accurate and easy replacement.

Editor?s note: Bob Harrison is product sales manager for Wittmann Inc. (Torrington, CT), a supplier of auxiliary equipment.

The last 10 years have brought significant improvements in granulator design, increasing safety, quality of regrind, ease of use, and energy efficiency.

One key to effective plastics processing is uniform quality of regrind. The bulk density of many virgin plastics is in the 30 to 35 lb/cu ft range. Since the bulk density is greatly influenced by the size and shape rather than the type of material, the bulk density of regrind may be considerably less, in the 15 to 25 lb/cu ft range. This may be due to the thickness of the scrap being ground, but also can be the result of fines, dust, and filaments known as ?angel hair? that are generated in the regrind process. When the mixture of virgin and regrind is introduced into the system, process requirements such as drying, blending, and melting can change significantly.

Uniformity of Regrind
The standard method of measuring regrind uniformity is the sieve test. Regrind is passed through a series of screens with progressively smaller hole sizes. Particles that do not pass through the top and largest screen (typically .2500 to .3125 inch) are considered ?longs? and may present material handling problems because of their large size. Particles that are captured by the .1875- and .0937-inch screens are considered good regrind, and those that pass through the .0937-inch screen are considered fines.

Many variables affect regrind quality, including the physical and mechanical properties of the material. Hard, brittle materials shatter upon impact with the knives. Soft, flexible, energy-absorbing materials must be cut by the knives. Influential operating conditions include feed temperature, feedrate, and method of evacuation. Granulator characteristics that have an impact include rotor design, drive design, cutting angle, knife tip design, knife material, screen location and design, and screen hole size.

As the processor may have little control over the choice of material for a particular job, the main focus must be on the operating conditions and capabilities of the granulator. However, there are certain elements, such as knife gap, that are more or less critical depending on the type of material. Soft, flexible materials that must be cut require a very tight clearance between the rotating and stationary knives. A general rule of thumb is that brittle, energy-impacting materials such as polystyrene or those filled with glass, mineral, or talc tend to fracture upon impact with the blades and require less horsepower to process. Since the material is fracturing upon impact with the blades instead of being cut, the granulate tends to lack uniformity and have a high fines percentage. Regrind quality generally improves when the parts can be processed while still warm and somewhat pliable.

On the other hand, soft, energy-absorbing materials such as flexible PVC, urethane, and styrene butadiene copolymer (like K-Resin) must be cut by the knife and generally require more horsepower. For these materials it is more critical that the knives be sharp, the knife tip angle steep, and the gap between the rotating and stationary knives as small as possible. In general, small clearances between the rotating and the stationary knives favor more energy-efficient knife cutting in contrast to the more energy-consuming, tearing-type cut that occurs when the gap is larger. An analogy would be to think about how a pair of household scissors work?or don?t work, for that matter?when the bolt holding the two blades together loosens and the blades are not held as tightly together.

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An extended rotor shaft allows for the addition of an extra flywheel to store energy. This gives more inertia when cutting thick cross-sectional parts. Reducing energy demands on the motor minimizes amperage spikes and increases motor life.

The Right Rotor
There is no single rotor design that is best for all applications. However, the three-blade, open-rotor design is the most versatile. An open-rotor design allows for good airflow through the cutting chamber, and hence good cooling, which can be critical when processing easily degradable materials such as styrene butadiene copolymer. On the other hand, the staggered rotor can be more energy efficient when processing thick cross-section parts. By design, it takes smaller bites. Less horsepower is required to cut the same thick cross-sectioned part and motor amperage spikes are reduced. A drawback is that a solid rotor restricts airflow through the cutting chamber. The solid rotor is not optimum for bulky, thin-walled parts that are becoming more common.

Several granulator manufacturers have designed their rotors with adjustable rotating knives instead of fixed knives. Three major features of adjustable rotating knives contribute to improved granulator efficiency and regrind quality:

  • The cutting circle remains constant even after the knives have been sharpened. With a constant cutting circle, the distance between the knife tips and the screen does not change, thus ensuring maximum cutting efficiency.

  • Rotating knives are adjusted individually, which allows for the smallest possible gap between each knife. With fixed knives, the gap must be set for the largest knife?the one with the largest cutting circle?as all of the other knives have the same or larger gap depending on their height.

  • Adjustable rotating knives offer extended knife life since the knives can be sharpened individually, requiring only the minimum amount of metal be removed from each knife. With fixed rotating knives, the knives must be sharpened as a set and all of the knives dressed down to the knife with the most wear. Herringbone or twin-shear rotors are used to minimize the knife gap over wide cutting chambers.

Various knife tip angles should be available for processing different types of materials. High angle knives (with a high free angle) are more efficient for processing soft, flexible materials. By decreasing the free angle while maintaining a constant cutting angle, knife tips can be strengthened to reduce chipping for processing hard, brittle materials. It is very important for high-quality regrind and energy efficiency to have a relief angle on the stationary knives as well.

Rotor Speed vs. Cutting Efficiency
Drive design features directly impact quality of regrind and energy efficiency. Excessive rotor speeds actually increase the time it takes to granulate most materials. Excessive speeds also lead to a less uniform particle size and a higher percentage of fines. High knife speeds do not allow correctly sized particles to easily pass through the classifying screen holes. Instead, the particles are swept around and around while being cut further into undesirable fines and dust. That same unnecessary cutting also leads to excessive noise, increased energy consumption, and excessive wear on the knives and the cutting chamber.

Rotor speed reduction using lower rpm and higher torque motors can actually increase knife tip force. A 20-hp, six-pole (1200-rpm) motor has the same torque as a 30-hp, four-pole (1800-rpm) motor. To help cut through tough materials using a low rotor speed on larger units, the rotor shaft can be extended to include an additional flywheel mass opposite the solid flywheel drive pulley. The additional flywheel uses stored mechanical energy to drive the blades through thick cross-sectioned parts. It does not require the motor to work harder. Minimizing motor amperage spikes also increases motor life.

If acquiring a granulator or replacing a motor on an existing unit, consider a high-efficiency motor. An example from a motor supplier catalog demonstrates how an additional investment of $178 up front on a higher-efficiency motor can save several thousand dollars over the average lifetime of a granulator motor, which is often seven to 10 years.

The single most important step a processor can take to maximize granulator efficiency is to be sure that the granulator knives are sharp. This is a commonly neglected task.

Some granulator manufacturers offer a knife preadjustment fixture that allows for the knives to be gapped outside of the machine where it is safer and easier to perform. The fixture ensures an accurate and consistent knife gap setting, allowing personnel to verify that the knives have been properly sharpened to maintain the correct knife tip angles. The fixture also allows for a spare set of knives to be gapped in advance, allowing for faster knife change and less downtime. By making it quicker and easier to change the knives, the system makes it more likely that knives are changed more often for maximum cutting efficiency.

Safety and Sound Requirements
A good point of reference, though not mandatory in North America, is the new European safety standard EN 12012-1, which requires granulators to have two sensors (for redundancy) that monitor the rotor motion and signal the electronic interlock to open and allow access only when the rotor has come to a complete stop. This is a far better safety than a time-delay limit switch and provides easier access to the cutting chamber. In addition, a mechanical rotor brake has been designed that is automatically applied when the cutting chamber is opened to prevent the rotor from spinning easily.

Increasingly strict guidelines for allowable noise levels and the increasing popularity of quiet all-electric injection molding machines have created the need for better sound control of granulators. This has been met with soundproofed hoppers, sound enclosures for the base of the machines, and reduced sound levels via higher-efficiency cutting designs. Reducing the time it takes to granulate a given quantity of material also reduces average decibel levels.

CONTACT INFORMATION
Wittmann Inc., Torrington, CT
Bob Harrison
(508) 422-9524; www.wittmann-ct.com

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