How to Choose a Granulator
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.
The number of knife blades, their arrangement, tip angle, speed, and sharpness all have an important impact on granulate quality and granulator efficiency. Knives may be mounted on a stationary bed or on the rotor. The clearance between rotating and bed knives is crucial to size reduction performance. Smaller clearances produce cleaner, more efficient cuts and are important when grinding softer, more ductile polymers.
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.
Low knife angles have blunter tips and are suited for more brittle materials. High knife angles with sharp knife edges are used to cut softer, energy-absorbing materials. Lower knife tip speeds can increase output of uniform granulate, reduce noise and lessen knife and cutting chamber wear. With a high-speed rotor, the particles are more susceptible to being carried around the machine or frayed into fines by repeated cuts. A slower rotor speed prevents this by giving each granule a greater chance to pass through the screen.
Frequency of knife sharpening will depend on the abrasiveness of the material (glass-filled and reinforced compounds are the worst on knives), and on the steel alloy composition of the blades. Knife steels are generally designed for either toughness or wear resistance. For soft, non-abrasive materials, knives made of chromevanadium steel (CVS) are tough and resistant to chipping but wear faster than other tool steels. D2 steel, the most common choice for granulator knives, is harder and provides more wear resistance, but may be more susceptible to chipping, which can be a problem if tramp metal is present. A regular periodic maintenance program should be established to check knife wear and sharpness. Sharpened blades are important to produce quality regrind with minimal fines.
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.
The goal in a process environment is to produce the highest quality part 100% of the time with a 0% scrap rate and 0% waste in the process. This goal should be accomplished using the fastest possible machine cycle time and a minimum amount of human contact. To maximize profitability, this should all be performed at the least cost to the processor. Unfortunately, not all processors are able to achieve this goal. Technology and experience has led to significant improvements in the design and proper implementation of the correct auxiliary equipment to match the process. Selection of the proper auxiliary process equipment is critical to successful processing. By taking the time to properly select the right equipment to match the process, you will be able to maximize the efficiency of the process and help to obtain the goal and expectations of your business. Do not always look at the initial cost of the equipment. Besides fit, form, function and compatibility with your other equipment; look at the "value" that the equipment can add to your process. Evaluate the productivity gains that can be achieved (short and long term) through proper implementation of this equipment in your process. Utilizing the latest in today's technology allows you the opportunity to match and integrate the right auxiliaries into your process. This match will optimize your operations and most importantly will increase your bottom line.