Choosing a granulator depends upon the material to be cut as well as its size, shape and thickness. The amount of material that needs to be granulated and how the regrind will be processed are also key considerations. The feed hopper must be evaluated for operator safety. It should be sized to receive the largest part being granulated without requiring labor-intensive and potentially dangerous precutting. Most safety devices in today’s granulators involve electrical interlocks that prevent access to the cutting chamber until the rotor has stopped turning. Older granulators may not have these built-in safeguards.Cutting Chamber Design
There are three basic cutting-chamber designs: tangential, conventional and straight-drop, plus a hybrid model. The popular tangential design positions the rotor at an offset from the feed opening so that the feedstock is directed into the downward cut of the rotating knives at a tangent to the cutting circle. This cutting chamber provides a large “bite” radius, which is the most efficient way to achieve high throughput and clean regrind with bulky, thin-walled parts.
A straight-drop design feeds material perpendicular to the cutting circle and is recommended for thick-walled parts. Its “nibbling” bites are less likely to take too much material and perhaps stall the rotor. The hybrid model presents material to the knives at an angle somewhere between the first two and allows custom molders with a wide range of part sizes and shapes to process both thick- and thin-walled materials in the same machine.
The four main types of rotors are open, closed or solid, staggered and staggered/segmented.Open rotors provide unrestricted air-flow through the cutting chamber, so they are more often used for granulating heat-sensitive resins or feedstocks that are still warm from processing. Closed or solid rotors have no open spaces between the rotating knives and the center of the shaft, providing a much stronger knife mounting arrangement and adding inertia to cut through the thickest feedstocks, such as large cold purgings. Staggered rotors are offered in closed or semi-closed configurations and are excellent for cutting most heavy, thick-walled parts. In heavy-duty applications, flywheel-type pulleys increase rotor inertia and are often a more practical, cost-effective means to achieve greater cutting force without adding horsepower. Staggered/segmented rotors are usually offered in a helical pattern, which provides more cuts per revolution than traditional rotor designs. The cutting circle stays constant after sharpening, minimizing screen plugging, heat build-up and fines. This configuration is excellent for cutting most heavy, thick-walled parts.
Many granulator designs offer only two stationary or bed knives. Check with the granulator manufacturer for proper knife clearance recommendations based on materials being granulated. Increasing the number of knife blades on the bed, or the rotor, can significantly increase throughput by increasing the number of cuts per revolution. A “slant-knife” arrangement that produces a scissors cutting action provides higher throughputs with less horsepower, less noise, and reduced fines and dust than the traditional straight-cut knife.
Screen hole size is heavily dependent on material composition and machine size. When running at slower rotor speeds, smaller screen holes and a thinner screen will produce more uniform particle sizes and highest throughput. On the other hand, at higher speeds, larger screen holes are needed to achieve throughput and reduce fines, although they may increase the range of particle sizes, particularly with brittle materials. Reversible screens have a longer useful life.