Plastic grinding machine is used for size reduction, an essential step in plastic recycling. Plastic grinding machine have the ability to quickly break down plastic products such as plastic bottles, braket, 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, recycling the plastic need much more processing in sorting and separation, size reduction, washing, and pelletizing.
In plastic grinding machine, cutting knives are install on a high-speed rotating rotor driven by an electric motor. This rotor is encased in a cutting chamber where stationary knives are installed. As the plastic scrap enters this cutting chamber, the rotating knives come into contact with the stationary knives cutting the plastic into small pieces. A large screen mesh 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 the mesh holes. So, through adjusting the size of the holes, can control the size of the output.
Plastic grinder machine, grinding machine video:
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Plastic grinding machine Speed and Capacity
The speed or capacity of plastic grinding machine 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 mesh. A combination of these factors will decide how quickly the plastic grinding machine can produce regrinds of plastic scrap.
The factors include:
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 grinding compared to engineering plastics such as ABS, Nylon, and PC that are much tougher to fracture and will shatter when exposed to high force. But as the polyethylene (PE) and polypropylene (PP) mostly is soft film status, so the density is very low, so the capacity to grinding them will low too. And other common plastics, polyethylene terephthalate(PET), mostly is bottle, so same grinding machine to process PET bottle will have higher capacity than PE PP film.
Size of Cutting Chamber & Knives – A large cutting chamber combined with longer rotor and stationary knives results in higher volumes of plastic scrap the grinding machine can process at one time. So bigger size will have bigger capacity.
Quality of Kniges – 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. As the knife is quick-wear part, so better prepare more.
Quantity of Knives – The number of rotor and stationary knives depend on how the plastic grinding machine is designed. On the most part, grinding machine have 3-6 rotor knives with 2 stationary or fixed knives. While some models of grinding machine may have more than 30 rotor knives. A combination of the size of cutting chamber, the type of grinding machine, and the number of knives, and speed of motor will affect the speed of a grinding machine.
Screen Size – At the bottom of a gringing machine’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. So if you require the regrinds to be very fine, you need use a screen with very small holes. The grinding time will also increase as the plastic material must stay in the cutting chamber for longer periods of time. In general, screen hole sizes range from 12mm to 18mm in diameter.
Electric Motor – Grinding machine 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 grinding machine usually spin between 200-800 rotations per minute depending on the power of the electric motor.
Wet Plastic Grinding Machine
A wet grinding machine with added water jets or hoses aimed inside the cutting chamber. The water is spray into the cutting chanmber while the rotors cutting. To prevent rust, the rotor and grinding machine housing are plated with chrome nickel alloy or are galvanized.
A wet grinding machine 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 grinding machine slightly difficult as melted plastic must be scraped off. With wet grinding machine, 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 grinding machine is commonly used in PET bottle plastic recycling machine, and PE PP film plastic recycling machine.
Grinding machine are designed to be robust and long lasting. However, it’s very important to keep a grinding machine 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 grinding 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 grinding machine by chipping knives, damaging the rotor, and even breaking through a grinding machine’s casing.
Always keep an eye on the grinding machine screen making sure it is not clogged during operation. A clogged grinding machine screen blocks cut pieces of plastic from exiting the grinding 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.