Granulator FAQs

Cumberland Green and White Plastic Example by Cumberland Plastics

What is quality regrind?

Quality regrind may look different for different applications. But, a good rule of thumb is looking for consistent, uniform granulate size with minimal sharps and low dust. Material density will dictate if the regrind is a flake or granulate. Whereas screen size may dictate the particle size, the end goal is to reclaim as much quality, useable regrind to minimize additional virgin material and recycle back into the process and mitigate waste.

How many rotor knives should I use and why?

This typically depends on the size and density of the parts. The goal is to use enough knives to maximize throughput without sacrificing part ingestion into the cutting area. More knives means more cuts per minute, however, there are diminishing returns after a certain threshold where parts/material will bounce around in the chamber rather then allowing the knives to “bite” on the downstroke.

What determines how many bed knives to use?

Generally, the material characteristics and throughput requirements will dictate how many knives to use and in what configuration.

When to use steep-angle knife design?

Maximizes cutting function for higher energy-absorbing softer materials and stretchy resins. Also good for many custom-engineered compounds. For example, thermoforming skeleton/scrap, sheet, film and fiber PE, and rubber type materials with elastic properties.

When to use a radial knife design?

Used to maximize strength for high force wedge fracturing of the toughest high impact materials. Usually under-slung mounted in rotor providing additional strength. Mature technology, not always the most efficient/effective cutting technology. Currently, typical applications are limited to wire/metal granulating requirements.

When to use a straight-cut knife design?

This parallel configuration is typically limited to applications that require uniform, exact knife gaps across the length of the knife or cutting chamber. Requires more power to cut and tends to be noisy since the full length of knives can be cutting at any instant.

When to use a slanted rotor + straight bed knife design?

The slanted rotor knife provides a shear of scissors type cutting action that requires less cutting force and potentially creates less noise.  This configuration is generally used for applications where knife gap is not as critical.  The knife gap at the center of the knife or machine is usually slightly larger than the others.

When to use a slanted rotor + slanted bed knife design?

Similar to the Slanted Rotor knife configuration, this configuration provides for a shear or scissors cutting action.  By counter slanting the bed knife at an equal and opposite angle the larger gap at the center of the knife is minimized.

When to use high-shear knife design?

Also known as “short cord”, this is typically a high-strength design with a thicker knife profile. This is optimal for high-impact, cut-resistant applications, like materials with thick walls, hard resin, large mass, and/or rubber. Examples would be: section pipe, profiles, molding, sheet, and light purgings with up to 2” wall thickness.

What is a hog rotor design?

In this configuration the knives are short segments positioned on the rotor in a helical or staggered pattern providing small separate cutting segments which limits the bite on the process material. Ideal applications are usually energy absorbing process material and thick block or purgings.

When to use water-cooled rotor?

Used in high temperature applications, a water-cooled rotor protects the bearings from overheating. Heat isn’t allowed to build up in the chamber, which helps to create a heat sink.

What is the purpose of a rotor flywheel?

Rotor flywheels add additional inertia into the cutting system. Therefore it helps conserve energy by requiring less horsepower in the cutting system.

What is a cross-cutting rotor?

Both rotor and bed knives have an equal and opposite slant angle in a cross-cutting rotor.  As the rotor knife moves down with respect to the bed knife, two scissors cutting action points converge towards the center.  There is a uniform knife gap across the entire knife.  This configuration provides an improved bite on the process material, lower power requirements, lower noise levels, and a balanced load on the rotor bearings.

Tangential, Offset or Straight Drop Chamber?

Tangential allows processing larger parts through a smaller machine. A three-blade rotor compliments the tangential geometry for large part ingestion. Offset is intended for thicker walled parts and long profile extrusions. A straight drop chamber used in applications where parts have heavier cross sections because it minimizes the “bite.” There is also a combination chamber that allows for better ingestion while limiting bite.

Cumberland Cutting Chambers Image by Cumberland Plastics


When to use a screenless grinder?

Screenless granulators are ideal for sprue pieces, runners and small parts where low noise and low dust are desirable. They are low rpm systems and easy to move around with the goal of reclaiming more regrind for reuse back into the process.

Do I need a shredder before my granulator?

The shredder phase helps to protect the granulator by reducing the load during regrind once it is pre-shredded. It is best to use a shredder for heavy duty materials at a high volume. Shredder type may vary depending on the material type (e.g. single-shaft vs. multi-shaft). Most shredders can be used inline for continuous shredding.

How do I increase the longevity of my Size Reduction equipment?

Keeping you granulators and shredders maintained is very important. Be sure to regularly sharpen and replace knives when necessary. Dull knives produce less quality regrind and increase vibrations, which may cause more frequent maintenance. Check out “Service Tips” for more suggestions.

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