A plastic granulator is a machine used for size reduction, an essential step in plastic recycling. Plastic granulators have the ability to quickly break down plastic products such as plastic bottles, crates, drums, and films into small, uniform pieces called “regrinds” or “flakes”. In some cases, this maybe the only step required before it can be reused in manufacturing new plastic products. For the most part, however, recycling of plastic scrap requires much more resources in sorting and separation, size reduction, washing, and pelletizing.
In a plastic granulator, cutting knives are mounted on an open rotor spun to high speeds by an electric motor. This rotor is encased in a cutting chamber where stationary knives are mounted. As the plastic scrap enters this cutting chamber, the rotating knives come into contact with the stationary knives cutting the plastic into little pieces. A large screen with many holes is placed at the bottom. The plastic will continue to mix and be cut by the knives until it is small enough to fall through this screen. Hence, by adjusting the size of the holes, one can control the size of the cut shreds.
Types of Plastic Granulators
Although the basic concept of how granulators work is the same, you’ll find manufacturers offering granulators with various cutting chamber designs and knife arrangements. Generally speaking, there are three mainstream types including:
- Double Scissor Cut or Double Angle Cut – In a double scissor cut granulator, straight rotor knives are aligned in parallel at a slight angle; Slanted just several degrees from being completely horizontal. The stationary knives are also aligned in parallel at the same angle, however, in the opposite direction of the rotor blades. This ensures a constant cutting gap along the full length of the knives which results in low temperature increases during granulation and minimal power consumption. Most manufacturers recommend the double scissor cut as this blade geometry in combination with an open rotor design provides cleaner “true scissor” cuts with fewer fines, less noise, and higher capacities.
- Chevron or V-type Rotor – As in its name, Chevron style rotors have granulator knives arranged in a V shape (similar to the Chevron logo) that are mounted in parallel. The cutting edge is on the inside of the V with stationary knives that are either straight or also in the V design. As the rotor knives spin past the stationary knives, the plastic feed stock is cut along the two cutting edges that start initially on the outside edges and coming together into a single point. While most manufacturers recommend the double scissor cut, the V-shape rotor designs also have a large fan base. Double scissor cutting is indeed ideal when the blades are sharp, however, as the blades dull, much of the plastic is pushed to one side of the granulator’s cutting chamber. As the granulator continues to operate, one side of the knives will become more dull causing uneven wear, friction, increased electric consumption, and more fines. A V-shape granulator, on the other hand, continues to push plastic scrap inside the cutting chamber toward the center even as the blades dull. As the V-shape rotor knives pass the stationary knives, larger plastic pieces are clamped within the diminishing cutting area which solves another issue of double scissor cutting machines; plastic pieces often squirm or move about before they are cut.
- Helical Rotor – In a helical rotor granulator, knives are arranged in a helix or spiral design in a closed rotor. Unlike open rotors of the previously discussed granulator types which are well suited for injection or extrusion plastic scrap, a closed-rotor design is robust, durable, and can withstand greater impact load, perfect for granulating thick cross-components.
Granulator Speed & Granulation Rates
The speed or capacity of a granulator depends on several factors including the type of material, size of the cutting chamber and knives, the quality of knives used, the quantity of knives, the speed of the electric motor, and the size of the screen. A combination of these factors will determine how quickly the granulator can produce regrinds of plastic scrap. Let’s take a closer look at each of these factors:
Type of Material – The most common consumer plastics, polyethylene (PE) and polypropylene (PP), are soft plastics with high impact strength that can easily be cut. These are much faster to granulate compared to engineering plastics such as ABS, Nylon, and PC that are much tougher to fracture and will shatter when exposed to high force.
Size of Cutting Chamber & Knives – A large cutting chamber combined with longer rotor and stationary knives results in higher volumes of plastic scrap the granulator can process at one time.
Quality of Knives – While the design of the rotor and the arrangement of knives play an important role in cutting, all is loss if low quality knives are used. Low quality knives highly affect production as they require constant sharpening and dull easily which lowers granulation rates. In general, high quality knives are made from high-carbon, high-chromium D2 / SKD 11 steel.
Quantity of Knives – The number of rotor and stationary knives depend on how the granulator is designed. On the most part, granulators have 3-6 rotor knives with 2-3 stationary or fixed knives. While this is so, some models of granulators may have more than 30 rotor knives. A combination of the size of cutting chamber, the type of granulator, and the number of knives, and speed of motor will affect the speed of a granulator.
Electric Motor – Granulators work by a spinning rotor, the faster it spins to a certain degree, the higher the cutting action within the cutting chamber. Rotors within a granulator usually spin between 200-800 rotations per minute depending on the power of the electric motor which can range from less than 1 kw/h to something monstrous like 800 kw/h.
Screen Size – At the bottom of a granulator’s cutting chamber is a metal screen with circular holes. The regrinds remain in the cutting chamber until they are small enough to fall through. Hence, if you require the regrinds to be very fine, you’ll use a screen with very small holes. The granulation time will also increase as the plastic feed stock must stay in the cutting chamber for longer periods of time. In general, screen hole sizes range from 1 inch to 1/8 inch, or 25mm to 3mm in diameter.
A wet granulator is a granulator with added water jets or hoses aimed inside the cutting chamber. The water is contained within the cutting chamber via the addition of extra seals. To prevent rust, the rotor and granulator housing are plated with chrome nickel alloy or are galvanized.
A wet granulator has many benefits over drying processing primarily as it partially cleans the plastic during size reduction. The water also acts as a lubricant between the rotor and stationary knives. In the dry granulation process, the knives usually become hot enough to melt the plastic within the cutting chamber. This makes cleaning the granulator slightly difficult as melted plastic must be scraped off. With wet granulators, the added water significantly lowers friction between the knives lowering the temperature way below plastic’s melting point. It’s been proven to also reduce the dulling of the knives reducing up the three times the need for sharpening.
A wet granulator is commonly used in plastic film washing lines.
Proper Granulator Usage
A couple operating tips for properly operating a granulator:
- Never start the granulator with material still inside the cutting chamber.
- Keep foreign objects on or around the granulator away during operation.
- Granulators were not meant for large lumps or purgings.
- Always empty the collection bin on-time, do not let the collection bin overload into the cutting chamber.
- Do not batch feed a granulator. Shredders are much more adapt to batch feeding.
- Turn the granulator on before feeding the plastic into the machine.
- Do not overload the granulator by over feeding.
- Inspect knives for wear and clearance. Adjust, sharpen, and replace as necessary.
- Rotate the granulator screen routinely to extend screen life.
- Allow the granulator to purge before turning it off.
- If a blower is being used to evacuate the cut regrinds, make sure to stop the granulator before stoping the blower 30 seconds later.
Plastic granulators are designed to be robust and long lasting. However, it’s very important to keep a granulator properly maintained which will prevent costly repairs or parts exchanges in the future. First and foremost, it is essential to routinely sharpen and rotate both the rotor and stationary knives. This will ensure maximum granulation efficiency as well as putting less stress on the wear and tear of the machine. During operation, make sure the keep foreign, stiff objects such as metals away. Commonly overlooked items such as nuts and bolts have the potential to severely damage a granulator by chipping knives, damaging the rotor, and even breaking through a granulator’s casing. Always keep an eye on the granulator screen making sure it is not clogged during operation. A clogged granulator screen blocks cut pieces of plastic from exiting the granulation causing a buildup of material inside the cutting chamber which jams the rotor. A jammed rotor can lead to the premature wear of the drive belts, and in more serious cases, a burned out electric motor. Lastly, always keep the motor and bearings well oiled.