The number of knife blades, their arrangement, tip angle, speed, and sharpness all have an important impact on granulate quality and granulator efficiency. Knives may be mounted on a stationary bed of a granulator or on the rotor, and most often on both.
The clearance between rotating and bed knives is crucial to size reduction performance. Some manufacturers have adjustable bed knives, adjustable rotor knives or both. Adjustable rotor knife machines use a jig to set knives outside of the machine. Fixed rotor knives with adjustable bed knives are set inside the machine.
The advantage of adjustable rotor knives is that you can maintain a constant cutting circle. The disadvantage is that it may wear a screen quicker by using it as a cutting surface. Advantages of an adjustable bed knife is solid rotor seat to mount the knives and potentially reduces screen wear as knives are sharpened. The disadvantage is it could remove the constant cutting circle after resharpening.
Adjustable bed knife pros are that it has a solid seat to mount the knives and potentially reduces screen wear as knives are sharpened; potential con is no constant cutting circle after sharpening. Adjustable rotor knife machines use a jig to set knives outside of the machine. Fixed rotor knives with adjustable bed knives are set inside the machine.
Two important factors in how well knives are able to cleanly and repeatably cut plastic materials are the profile of the knife tip and the cutting angle of the knife edges relative to one another. Knife tip will impact how cleanly a material can be sheered, how durable the tip is to impact and how it wears over time. Cutting angle also affects the quality of the sheering action plus how precisely the cutting gap can be maintained across the width of the rotor, power requirements and the noise level of the size reduction process.
There is a wide variety of knife configurations from which to choose, each of which lend themselves best to certain product forms and materials. They include:
Knife Tip Profiles
Also known as “short cord” the high shear knife design maximizes cutting strength where high impact cutting forces are required for tough materials.
Thick section pipe, profiles, moldings, sheet and light purgings up to 2” wall thickness.
Steep-Angle, Keen Edge
Sharp knives intersecting at a steep angle maximizes the cutting function where less tip strength is needed for higher energy absorbing elastic materials.
Thermoforming skeleton/scrap, sheet, film and fiber PE, and rubber type materials with elastic properties.
Steep-Angle, 55 Degree Landed
This configuration combines good cutting function with additional strength at the cutting edge for medium energy absorbing, high impact materials.
Large bulky thin walled parts, sprues and runners, thermoformed parts and skeleton sheet up to 3/8” wall thickness.
With rotor and bed knives mounted parallel to each other the straight cut design is limited to applications that require uniform knife gaps across the length of the knife or cutting chamber. It requires more power and can be noisy.
Slanted Rotor Knife, Straight Bed Knife
Slanting the rotor knife provides a sheering action like scissors that requires less cutting force and potentially generates less noise. It is generally used for applications where holding a precise knife gap across the entire knife length is not critical.
Slanted Rotor Knife, Slanted Bed Knife
Similar to slanted rotor knife, this configuration also provides a scissors-like sheering action, but with better gap control across the entire length of the knives.
With a Twinshear cross-cutting configuration both rotor and bed knives have an equal and opposite slant angle extending from the center of the chamber. It creates two shearing action points that converge at the center. Twinshear provides a very uniform knife gap and improved bite, with lower power requirements and noise.
The hog rotor configuration uses multiple short rotor knives in a helical or staggered pattern to create small, separate cutting segments. This limits the bite on the material to “nibble” away in order to process thick and difficult materials.It’s effective on energy absorbing process materials, thick blocks or purgings.