Size reduction: Earning respect

Size reduction equipment tends to draw mild interest and at worst is greeted with overt indifference. Despite continuing upgrades, granulators often generate less attention than they deserve in the plastics processing community. For plastics processors, granulators and other size reduction equipment such as shredders remain a necessary element in the production cycle.

Scrap, whether from flash, runners, purgings, or defective parts, is an inevitable by-product of processing, and granulating that scrap for disposal, resin reclamation, or the protection of proprietary product designs is a necessity.

Any granulator that is tackling jobs outside its capabilities will create regrind only suitable for the dumpster. With conservative estimates placing processors? scrap rates from 2 to 5 percent, the money lost annually on unusable regrind could probably pay for a new, more efficient granulator.

For some parts in medical injection molding and other markets, use of regrind is forbidden or impractical. In other markets and applications that have lower cosmetic requirements or where regrind could serve as a coinjected material, granulated scrap can constitute as much as 25 percent of a new part. In some instances with the crosslinking of polymers, regrind can even serve to augment a part?s overall strength.

The Right Match for Your Needs
The engineering behind a standard granulator is fairly straightforward. A rotor fitted with three to five rows of rotating knives revolves inside a cutting chamber with a screen situated below it. The rotating knives move across two, or in some cases, three stationary knives, which then cut materials that are hand or vacuum fed from above. Through gravity or suction, the screen at the bottom of the granulator only allows specific-sized granules to pass through, keeping larger regrind in the chamber until it?s cut down further.

Processors interested in a new granulator need to consider the size, material, and temperature of the scrap. Secondly, is the ultimate goal to reclaim the resin or simply to reduce its size? Plant layout also plays a role. Will the granulator operate beside the press in an automated production cell or will it be centrally located? What material and/or color changes will the granulator be asked to handle? These considerations influence the choice of blade, rotor, screen, and feed design as well as motor size and physical location of the granulator within the facility.

Taking Material into Account
Brittle or high-impact materials such as polystyrene require a lower-power granulator than other resins since the materials shatter upon impact with the blade, generating some fines. Regrinding these materials after they?ve entirely cooled or with a high-rpm rotor can greatly increase the amount of fines. Ideally, brittle materials are granulated while still relatively warm and in a low-rpm unit. Processing the materials while still warm can also reduce the amount of power needed.

Softer, energy-absorbing, or elastic materials such as flexible PVC or polyethylene require a cutting action from sharp-angle knives, instead of the blunt-edged knives used for engineering thermoplastics or other hard resins reinforced with talc or glass.

An analogy involving two pencils, one sharp and one dull, helps explain this variable. If you tried to puncture a common clear household food-wrapping film (polyvinylidene chloride) with a sharp pencil, it would puncture easily. If you tried to puncture the same film with a dull pencil, it would break through eventually but only after expending much more energy. Such is the case with flexible materials and granulators. Sharp-angle knives are necessary to slice more elastic materials into smaller pieces.

Rotor, Feed Design are Critical
Rotors are designed in three basic styles?open, solid, and staggered. Open rotors consist of a rotor shaft with knife carriers welded to it at equidistant lengths. This creates a small opening between the knives and shaft?hence the moniker ?open.? Ideally suited for granulating warm materials like high-temperature runners or blowmolding flash, the gaps create air pockets for heat release and expedite material cooling. For very hot materials, it may be necessary to suction out of the cutting chamber or install water cooling lines around it. The three-blade open rotor is probably the most standard, most versatile rotor, especially for very warm material.

A solid rotor, as its name implies, is constructed from a solid piece of steel with no openings between the knives and the rotor shaft. Less popular now than in the past, solid rotors offer more inertia given their sheer weight, but they struggle to granulate warmer materials and create a heat sink that can melt scrap.

A staggered rotor is essentially a solid rotor but usually operates on machines in the range of 150 hp to process purgings and other similar materials by taking small bites. Larger or bulkier materials can bounce off the blades, making the staggered design inefficient for some jobs.

There are two basic feed design options. Straight-drop units position the scrap feed above the rotor with the screen directly below the cutting chamber to allow smaller pieces to be quickly processed, and then dropped through by gravity or vacuum.

A tangential feed granulator takes material at an angle from the side. This allows larger pieces to be broken up more easily since the feed accommodates more of the part at once. Materials can have a harder time falling through the screen since it?s no longer taking regrind from a straight drop, but larger screen sizes are available to help compensate.

A frequent misconception holds that higher rpms produce greater granulation speed and efficiency. Throughputs can actually go down as the rpms increase. Peak efficiency occurs at about 400 rpm. Higher speeds can sweep regrind past the screen and cut it again and again.

In addition to running quieter, a low-rpm motor also means one with less horsepower, which has both a lower purchase price and lower energy prices.

No Lack of Equipment Options
For processors with long and wide trim scrap, CMB Wittmann (Torrington,CT) has introduced the MS 1435 central granulator. The 14-by-35-inch feed sends material into a fabricated steel cutting chamber with two bed knives. Material can be simultaneously fed into a rear chute to further increase throughput in the 400-rpm unit.

Conair (Pittsburgh, PA) offers the CR Series, with operating rates from 190 to 225 rpm. The CR Series uses a staggered rotor to create throughputs of up to 140 lb/hr. The feed chamber is tangential and contains slant knives for a scissor cutting action.

Weima America (Fort Mill, SC) has developed a grinder with reversible counterplates and a hydraulically controlled ram. The machine is said to produce uniform chips with no strips. The screens can be equipped with a quick changeover device to enable faster cleaning. A timer shuts the machine off when the hopper is empty.

The A Series 1600X line of beside-the-press granulators from Cumberland (South Attleboro, MA) deliver horsepower ranges from 20 to 40, allowing throughputs of 900 to 1500 lb/hr. The 16.5-inch cutting circle is big enough to take on a wide variety of scrap, and it houses an open rotor with three knives and scooped wings for improved material feed and cooling.

From Ball & Jewell (South Attleboro, MA) comes the 612 Series granulators, compact units designed to take on small molded or extruded parts, as well as profiles, sprues, and runners. The multifeed hopper accepts scrap into the cutting chamber with a rotor operating at 1800 rpm. Vacuum discharge can deliver regrind directly into the plant?s material handling system.

The Seven Series from AEC/Nelmor (South Attleboro, MA) is another beside-the-press size reduction line. Three different cutting chamber sizes range from 6.5 by 8 inches to 6.5 by 16 inches. The open, slant-cut rotor uses three knives to cut scrap against the two counter-slanted adjustable bed knives. The hopper is double-walled to reduce sound emissions.

For really big jobs, Hosokawa Polymer Systems (Berlin, CT) offers the Rotoplex line of heavy-duty granulators. Built for film recycling, the Rotoplex weighs 27 tons and uses a 36-by-114-inch cutting chamber to process 15,400 lb/hr of film at 1640 ft/min. A 600-hp motor rotates 10 rotor knives against seven rows of stator knives.

The latest from Rapid Granulator (Rockford, IL) is the 1514-CD, a compact, specialized unit developed specifically to meet the need of the optical disk manufacturing industry for direct recycling of sprues. Optical disk producers demand extremely high-quality granulate and reliable mechanical equipment. Special adjustments have been made to the machine, which are essential for achieving a low level of waste, i.e., a high yield.

As the granulator is part of a closed loop system, it can run over long periods without shutting down for servicing. Rapid guarantees 12 months of uninterrupted machine uptime between services.

The Rapid 1514-CD granulator operates with a very low rotor speed, using a completely sealed collection bin to ensure high-quality regrind and guarantee no contamination of the material or the plant atmosphere. A small motor and special knife material reportedly yield low operating costs.

The ReTech heavy-duty series of single-shaft rotary plastics scrap grinders from Vecoplan (High Point, NC) are low-speed, high-torque, ram-fed grinders. The K-Model Series, including Model RG52K, uses oversized components, including hydraulics, gear reducer, rotor shafts and outboard double-row roller bearings, reinforced ram and frame, cushioned ram cylinders with hinged access cover, close tolerance rotor and screens, quick-disconnect screens for easy cleaning, and three-program control panels. The units are designed for processors and recyclers needing to process purgings up to 1000 lb in one pass. The grinders can readily handle film and fiber, whole bales, reject parts, and pipe. Complete systems with separation and conveying can be designed. The units may be leased or purchased.

Hamilton Avtec (Mississauga, ON) has introduced the AH Vectra series of quiet, low-speed granulators for beside-the-press operation. They are well suited for the molder who requires regrind virtually free of fines and dust. The cutting chamber consists of two double-edged jaw cutters that first break up the sprues and parts. This presizing allows the coarse feed to be pelletized by toothed rollers, each of which produces an equal-sized pellet. This requires a high torque, motor-driven gear box to drive the cutting rotor at 25 to 30 rpm. Standard features include the low-speed toothed rotor to virtually eliminate dust and fines, double-edged hardened pre-cutters, and dual electromechanical safety devices. No screen is required and no tools are needed to obtain access for cleaning. The sound-proof hopper uses a clam-shell tilt-back design.

Getecha Inc. (Kalamazoo, MI) has introduced the GRS angled rotor series of beside-the-press, low-speed granulators. The GRS Series uses either seven- or 10-blade rotors. The units are said to be practically jam-proof, in part due to the angled rotor and the precutter design.

The angled rotor design, multiple cuts per revolution, 200-rpm rotor speed, and continuous cutting circle combine to ensure an excellent-quality regrind with low fines and a highly consistent particle size, even with highly filled engineered materials, according to Getecha. A divided, drop-down cutting chamber provides immediate, tool-free access to the rotor, hopper, screen and entire cutting chamber, allowing clean-out times of just 5 to 10 -minutes.

Eurohansa Inc. (High Point, NC) has brought to market the Zeno ZTH horizontal knife mill designed to process profile sections and thin-walled pipe.

Dual-nip roll feeders draw the material into the cutting chamber. To prevent overfeeding, the infeed rollers are driven by individual gear motors.

Zeno can provide the mills in more than 30 standard sizes with infeed openings ranging from 10 to 80 inches. Throughputs vary from 200 to 10,000 lb/hr.

Gloucester CMG ( Newport, NH) is a North American joint venture established between Gloucester of the U.S. and CMG of Budrio, Italy. Their products include stand-alone granulators, blowers, cyclones, conveyors, and complete turnkey packages. Granulator and shredder equipment uses motors ranging from 3 to 300 hp.

Granutec (East Douglas, MA) constructs the Models TFG2030, TFG2530, TFG2548, TFG2060, and TFG3600?all of which offer scrap reclamation at up to 4000 lb/hr. The 2000 and 3000 Series tangential cutting chamber ingests such items as 3-liter PET bottles and preforms, 1-gal HDPE bottles, polycarbonate water jugs, blowmolded parts up to 25 sq in, 55-gal drums, and 5-gal pails.

With throat sizes ranging from 20 by 30 to 36 by 48 inches, the heavy-duty, central granulator line offers optional equipment that allows for the easy feeding of continuous film scrap, sheet, long items up to 10 inches high like automotive door panels, thin-wall pipe, fascia, and profiles.

Screenless Technology
Screenless granulator technology is often overlooked, according to Don Maynard of Size Reduction Specialists Corp. (East Lansing, MI). It was first introduced in the U.S. in 1988 and continues to grow in the plastics industry. Screenless granulators operate at less than 50 rpm, using .5 to 3 hp. The no-screen technology combined with low speed allows screenless granulators to produce premium granules with no dust or longs and minimal fines. Screenless granulators do not destroy or alter the integrity of fillers such as glass. Manufacturers can immediately reprocess the regrind.

There are two styles of screenless granulators in use today?single shaft and dual shaft. The single-shaft technology has the S-shaped breaking cutters and nibbler cutters all on one shaft. The nibbler cutters cut against a stationary blade to form the particles.

The two-shaft technology has the S-shaped cutters and nibbler cutters on separate shafts. Since the nibblers are on a separate shaft, they can be set at a close-tolerance setting, which allows processing of thermoplastic elastomer (TPE) materials.

Editor?s note: PA&M wishes to thank Bob Harrison, product sales manager with CMB Wittmann, for his assistance with this story.