Granulateurs industriels : principes et applications

A. Definition and Core Function

An industrial granulator, frequently referred to as a plastic granulator or grinder, is a specialized machine engineered to reduce materials—primarily plastics—into small, consistently sized particles known as granules, flakes, or regrind. The core function of a granulator is to produce a high-quality, uniform output material that is suitable for direct reprocessing, recycling, or other applications requiring precise particle characteristics. Unlike shredders that perform coarse reduction, granulators are designed for precision and finesse in size reduction, typically operating at high speeds and low torque to cut and slice materials.

B. Working Mechanism

1. Operating Principle: High Speed, Low Torque, Precision Cutting/Slicing

Granulators operate on a principle contrasting sharply with that of shredders: they utilize high rotational speeds with relatively low torque. Standard rotor speeds typically range from 400 to 500 RPM, but can extend up to 1460 RPM in some models. The size reduction is achieved through a distinct cutting or slicing action, often likened to the mechanism of scissors, where sharp knives repeatedly take small “bites” of the material. This high-speed cutting action is designed to produce clean-cut, uniform particles rather than the torn or crushed output characteristic of shredders. The reliance on speed rather than brute force means granulators are best suited for materials that are less bulky and tough than those processed by shredders.

2. Composants clés

The precision of granulators is derived from their specific component design:

• Rotors and Knives: Granulators feature a rotor equipped with multiple, sharp rotating knives. These knives work in close tolerance with one or more stationary (bed or dead) knives mounted within the cutting chamber. The quality and sharpness of these knives, often made from high-resistance, heat-treated steel, are crucial for efficient granulation and output quality.
• Chambre de coupe : The cutting chamber is designed to optimize the cutting action and facilitate the flow of material towards the screen. Some specialized granulators, particularly in pharmaceutical or chemical applications, might refer to this as a mixing bowl or granulation chamber, and may include impellers or choppers for different granulation processes like agglomeration. However, for plastic granulators, the rotor/knife/screen system is the standard configuration.
• Screens (Essential for Sizing): A perforated screen is an indispensable component of a granulator, positioned at the bottom or periphery of the cutting chamber. The precisely sized holes in this screen (typically ranging from 6mm to 12mm, or approximately 1/4 to 3/8 inch) determine the final particle size and ensure output uniformity. Material is continuously cut and re-cut within the chamber until it is small enough to pass through these screen openings. The screen is thus a critical quality control element. Unlike in shredders where screens provide coarse sizing or are optional, in granulators, the screen is integral to achieving the desired output. The material is retained and repeatedly processed by the knives until the particles meet the screen’s size specification. This meticulous process ensures the high uniformity and specific particle dimensions required for direct reuse in manufacturing processes such as injection molding or extrusion. Consequently, the choice of screen size directly dictates the properties of the regrind, making screen selection, integrity, and maintenance paramount for optimal granulator performance and the economic value of the output. This also implies that granulators are less tolerant of contaminants that could blind the screen or damage the knives.
• Flywheels: Due to their lower torque motors, many granulators incorporate heavy flywheels. These flywheels store kinetic energy, providing the necessary momentum and cutting power to slice through the material effectively, especially during peak loads.

3. Types of Granulators

Granulators are available in various configurations to suit different operational scales and specific application needs, primarily within the plastics industry:

• Granulateurs à côté de la presse : These are relatively small, compact units designed for inline operation directly alongside plastics processing machinery, such as injection molders or blow molders. They are used to grind small volumes of production scrap like sprues, runners, and minor defective parts, allowing the regrind to be immediately recycled back into the production process.
• Granulateurs centraux : Larger and more powerful than beside-the-press models, central granulators are designed to handle higher volumes of scrap material collected from multiple processing lines or molding cells. This category can include heavy-duty or “hog” granulators, which are built to process larger, thicker parts and purgings.
• Thermoforming Granulators: These are specialized units designed to handle the skeletal waste and off-cuts generated during thermoforming processes.
• Wet vs. Dry Granulators: While the primary focus of this comparison is on dry granulators used in plastics recycling, it is worth noting that “granulation” as a broader industrial process can involve wet methods. Wet granulators incorporate liquids (e.g., water, detergents) during operation, often to cleanse contaminated materials or cool the machinery. Dry granulators, which operate without liquids, are standard in most plastic molding, thermoforming, and extrusion facilities. The broader industrial context of granulation, as seen in pharmaceutical, food, and chemical sectors, may involve different mechanisms like agglomeration or fluid bed granulation to create specific particle forms. However, for the purpose of comparing with shredders in waste and recycling contexts, the focus remains on mechanical size reduction via cutting.

C. Output Characteristics

The material produced by an industrial granulator is distinguished by its quality and consistency:

• Particle Size Range: Granulators generate significantly smaller and more consistently sized particles than shredders. The typical output size is dictated by the screen and usually falls within the range of 6mm to 12mm, though other sizes can be achieved with different screens.
• Shape: The output consists of uniform granules, flakes, or regrind. The particles generally have a consistent shape and often closely resemble virgin plastic pellets in form, facilitating their use in subsequent processing.
• Uniformité: A high degree of uniformity in both particle size and shape is a hallmark of granulated material. This consistency is crucial for applications where the regrind is blended with virgin material or used directly in manufacturing processes, as it ensures predictable processing behavior and final product quality. This high-quality, uniform regrind can often be directly substituted for virgin plastic pellets, yielding substantial economic and environmental benefits. Economically, it reduces the reliance on and cost of virgin raw materials and often requires less energy to reprocess compared to producing new plastics from scratch. Environmentally, it curtails the amount of plastic waste sent to landfills and lessens the demand for virgin resource extraction. This positions granulation as a key enabling technology for achieving true closed-loop plastic recycling, effectively transforming waste into a valuable industrial commodity. The “finesse” attributed to granulators directly translates into higher value recovery from scrap materials.

D. Materials Processed and Typical Applications

Materials Processed:

The vast majority of materials processed by granulators are plastics. This includes a wide range of thermoplastics such as Polyethylene (PE), Polypropylene (PP), Polystyrene (PS), Polyvinyl Chloride (PVC), and engineering plastics like Polycarbonate (PC), Polyethylene Terephthalate (PET), and Acrylonitrile Butadiene Styrene (ABS). Forms of plastic scrap include offcuts, sprues, runners, rejected parts, bottles, films, and pipes. While plastics are their forte, granulators can also process certain other materials like rubber, some types of e-waste (e.g., cable insulation), copper cables, paper, and lightweight non-ferrous metals.

Applications typiques :

The predominant application for granulators is in the plastics recycling industry, where they are used to convert plastic scrap into reusable regrind. They are integral to in-house recycling operations within plastics manufacturing facilities, allowing for the immediate reprocessing of production waste. Granulators also find use in e-waste recycling for liberating and sizing non-metallic components and in the general recycling of other suitable non-plastic materials. While some sources mention broader applications in food, pharmaceutical, chemical, and metallurgical industries for creating granular forms of various substances, this report’s comparative focus is on their role in waste reduction and recycling alongside shredders.