FAQ & Knife Terminology

Granulator

A machine that reduces plastic parts, sprues, runners, and scrap into small, uniform particles (regrind) for reuse in the production process. Granulators use rotating and stationary knives to cut material to a defined particle size.

Shredder

A machine that performs primary size-reduction of large, bulky, or thick-walled material. Shredders use heavy-duty rotating blades to tear and cut material into smaller pieces, typically before granulation.

Regrind

The granulated plastic material produced by a granulator. Regrind can be blended back into the production process as a percentage of virgin resin, reducing raw material costs.

Rotor Knife

The rotating cutting blade mounted on the granulator or shredder rotor. Rotor knives work in conjunction with stationary bed knives to create a scissor-like cutting action.

Bed Knife (Stator Knife)

The stationary cutting blade mounted in the machine frame. The gap between the bed knife and rotor knife determines cutting efficiency and particle size.

Knife Gap

The clearance between the rotor knife and bed knife. Proper knife gap setting is critical for cutting efficiency, particle quality, and blade longevity. Consult your machine manual for the recommended gap specification.

Throughput

The volume of material a granulator or shredder can process per hour, typically measured in kg/hr or lbs/hr. Dull or improperly set knives reduce throughput and increase energy consumption.

Purge

The material purged from an injection molding or extrusion machine during color or material changes. Purge is typically hard, dense, and abrasive, requiring heavy-duty knives for effective size reduction.

Two-Stage Size Reduction

A process that uses a shredder for primary size reduction followed by a granulator for final particle sizing. Common for bulky or large-format material that cannot be fed directly into a granulator.

OEM Tolerance

The dimensional specifications set by the original equipment manufacturer. ZERMA America knives from Virtus Equipment Direct are manufactured to OEM tolerances, ensuring proper fit and performance in your machine.

Wedge Knife

A knife with a tapered wedge-shaped cross-section. The geometry creates a splitting action as the blade contacts material, making it effective for thick or tough feedstock in shredder applications.

Hardness (HRC)

A measure of a knife's resistance to surface deformation, rated on the Rockwell C scale. Higher HRC values indicate a harder, more wear-resistant blade. ZERMA knives are heat-treated to specific HRC ranges matched to each application.

Knife Holder

The mounting block or clamp that secures the knife to the rotor or stator. Proper seating of the knife in the holder is critical for accurate knife gap and safe operation.

Screen (Sizing Screen)

A perforated plate inside the granulator that controls the final particle size of regrind. Material recirculates in the cutting chamber until it is small enough to pass through the screen openings.

Tangential Feed

A granulator design where material enters the cutting chamber tangentially to the rotor, allowing for a gentler, more controlled feed. Common in beside-the-press granulators processing sprues and runners.