Knowledge Base
FAQ and Granulator Terminology
Answers to common questions about granulator knives, knife replacement, and the terminology used across the plastics size-reduction industry.
Frequently Asked Questions
Granulator Terminology
Bed Knife
A stationary cutting knife mounted in the granulator housing. Also called a stator knife or counter knife. Works in opposition to the rotating fly knives to shear plastic material.
Fly Knife
A cutting knife mounted on the rotating drum (rotor) of a granulator. Fly knives perform the primary cutting action as they pass the stationary bed knives.
Rotor
The rotating drum inside a granulator that carries the fly knives. The rotor spins at high speed to drive the cutting action.
Stator
The fixed (non-rotating) component of the granulator that holds the bed knives. The stator provides the stationary cutting edge against which the rotor knives shear material.
Knife Gap
The clearance between the fly knife edge and the bed knife edge at their closest point. Proper gap setting is essential for clean cuts, consistent particle size, and knife longevity.
Regrind
Granulated plastic particles produced by a granulator. Regrind is reintroduced into the production process as a substitute for or blend with virgin resin.
HRC (Rockwell Hardness C Scale)
A standard measure of material hardness. Granulator knives are typically hardened to 58-62 HRC, which provides the balance of hardness for edge retention and toughness to resist chipping.
D2 Tool Steel
A high-carbon, high-chromium cold-work tool steel commonly used for granulator knives. D2 offers excellent wear resistance, good edge retention, and dimensional stability after heat treatment.
Beside-the-Press Granulator
A compact granulator positioned directly beside an injection molding or blow molding machine to granulate runners, sprues, and rejected parts immediately at the press.
Central Granulator
A larger granulator positioned centrally in a facility to process scrap from multiple machines or production lines. Central granulators typically handle higher throughput than beside-the-press units.
Low Speed Granulator
A granulator that operates at lower rotor speeds (typically 20-100 RPM) to reduce noise, dust, and fines. Commonly used for bulky or thick-walled parts.
Screen
A perforated plate at the bottom of the granulator cutting chamber. Material is retained in the chamber until it is small enough to pass through the screen openings, which controls the maximum particle size of the regrind.
Fines
Undersized particles or dust produced during granulation, typically caused by dull knives, incorrect knife gap, or improper screen selection. Excessive fines reduce regrind quality and can indicate knife wear.
Duralife
A knife designation used for extended-life cutting tools engineered for longer service intervals. The Cumberland 50B Duralife Fly Knife is designed for higher wear resistance in demanding granulation applications.
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