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FAQ & Terminology
Answers to common ordering questions and a complete glossary of granulator, shredder, and plastic size-reduction terminology.
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Granulator
A machine that uses rotating cutting knives working against stationary bed knives to reduce plastic parts, sprues, runners, and scrap into uniform regrind particles. Granulators are classified by rotor type, cutting chamber size, and application — from compact beside-the-press units to large central systems.
Rotor Knife
The replaceable blades mounted on the granulator rotor that perform the primary cutting action. Rotor knives work in a scissor-like action against the stator (bed) knives. Knife geometry, steel grade, and edge condition are critical to cut quality and energy efficiency.
Stator Knife (Bed Knife)
The stationary cutting blades mounted in the granulator frame that work against the rotor knives to shear plastic material. Stator knife clearance — the gap between rotor and bed knives — is a key setup parameter that affects particle size and knife wear rate.
Block Knife
A thick, heavy-duty knife style used in granulators processing rigid plastics, purge material, and thick-walled parts. Block knives are designed for high-impact applications and are available in notched and smooth profiles depending on the machine's locating pin configuration.
Backing Plate
A precision-machined plate that supports and positions the stator knife in the granulator frame. Backing plates ensure correct knife alignment and a consistent cutting gap. Worn or damaged backing plates should be replaced at the same time as the knives to restore proper geometry.
D2 Tool Steel
A high-carbon, high-chromium tool steel widely used for granulator knives due to its excellent wear resistance and edge retention. D2 is heat-treated to 58–62 HRC for granulator knife applications, providing long service life in demanding plastic processing environments.
HRC (Rockwell Hardness C Scale)
The standard scale for measuring the hardness of hardened steel. Granulator knives are typically hardened to 58–62 HRC. Higher HRC values indicate greater hardness and wear resistance, but also increased brittleness — the 58–62 HRC range balances edge retention with toughness for plastic processing applications.
Knife Clearance
The gap between the rotor knife edge and the stator knife edge, measured in thousandths of an inch or millimeters. Proper knife clearance is critical — too wide produces fines and dust; too tight causes premature knife wear and potential rotor damage. Clearance is set during knife installation and checked during routine maintenance.
OEM Geometry
Knife dimensions, angles, and profiles manufactured to the exact specifications of the original equipment manufacturer. OEM geometry ensures the knife seats correctly in the machine without shimming or modification, maintains the designed cutting gap, and delivers the expected throughput and regrind quality.
CNC Precision Grinding
The manufacturing process used to finish granulator knife cutting edges and seating surfaces to tight dimensional tolerances. CNC grinding ensures consistent knife geometry across a production run, which is critical for maintaining correct clearance and balanced wear across all knife positions.
Regrind
The granulated output produced by a granulator — small, relatively uniform plastic particles that can be reintroduced into the production process, blended with virgin resin, or sold as secondary material. Regrind quality is directly affected by knife sharpness, clearance, and screen hole diameter.
Fines
Undersized regrind particles — dust and very small fragments — generated during granulation. Excessive fines indicate dull knives, incorrect knife clearance, or material that is too brittle. High fines content reduces regrind value and can cause processing problems when reintroduced into production.
Cutting Chamber
The enclosed area inside a granulator where size reduction takes place. The cutting chamber houses the rotor, rotor knives, stator knives, and screen. Chamber geometry — open rotor, closed rotor, tangential feed — determines material flow and regrind quality.
Rotor
The rotating shaft assembly inside a granulator that carries the cutting knives. Rotor designs include open (staggered knife), closed (solid disc), and tangential configurations. Rotor speed, diameter, and knife count directly affect throughput capacity and regrind particle size.
Screen
A perforated plate positioned below the cutting chamber that controls the maximum particle size of the regrind output. Material recirculates in the cutting chamber until it is small enough to pass through the screen holes. Screen hole diameter is selected based on the target regrind specification.
Purge Material
The plastic material extruded from a machine during a color or material changeover to flush the previous resin from the barrel and screw. Purge material is typically a dense, irregular slug that requires block knives or hardened rotor knives to process effectively before granulation.
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