What is underwater granulator?

An underwater granulator is a specialized machine used in the processing of polymers or plastics. It functions by cutting molten plastic material into uniform pellets while submerged in water. Underwater granulators are widely utilized in various industries to create spherical and precisely shaped plastic pellets, especially with sticky polymers. They play a vital role in producing high-quality plastic materials for a wide range of products and applications.

Pelletizing principle of the underwater granulator

In this process, molten plastic material is pressurized and conveyed to a pelletizing die via an extruder or melt pump. The pelletizing die and cutter are situated within a chamber filled with flowing water. As the molten plastic emerges, the cutter swiftly slices it, and the water circulating through the die surface cools and shapes the plastic instantly. The resulting pellets are then efficiently separated into solid and liquid components using a rotating drum centrifugal dryer, with uniform, qualified particles discharged from the centrifugal dehydrator, completing the pelletizing process.

This method boasts low kinetic energy consumption during cutting, ensuring the plastic particles are uniform, spherical, and devoid of sharp edges. Furthermore, the system’s fluid dilute phase transport mechanism minimizes the likelihood of plastic particles re-bonding during transportation and cooling, preventing the adhesion of cutting debris onto the particle surfaces.

Main components of underwater granulator

1. Diverter: The diverter plays a critical role in ensuring complete melting of the polymer melt within the extruder or reactor. It can be shifted to the discharge position, enabling operators to examine the polymer melt. Once the melt reaches full melting, the operator uses a hydraulic station to switch the diverter to the working position, guiding the polymer melt toward the underwater die head for extrusion.
2. Underwater Die Head: The underwater pelletizer’s core component is the die head. It is meticulously designed by manufacturer, employing unique technology tailored to different polymers and applications. The die head features smooth flow channels and holes without any dead corners. Special insulation technology ensures that the melt from upstream is adequately heat-insulated, preventing freezing at the die head, which could obstruct the die hole and affect the cutting process. Various heating methods, such as cartridge heaters, oil heating, and electromagnetic heating, are employed based on the raw materials and production capacity.
3. Pelletizer: The pelletizer’s primary function is to swiftly cut the material emerging from the die head while immersed in water, ensuring efficient and precise cutting. The resulting pellets or granules are carried away by the water flow. The knife, powered by compressed air or a hydraulic station, makes precise contact with the die head’s surface, and its durability is assured as both ends are constructed from alloyed steel. 
4. Tempered Water System: The tempered water system plays a crucial role by providing precisely heated water to the die head chamber, enabling accurate polymer cutting. The heating temperature can be preset via the Human-Machine Interface (HMI), and the system’s efficiency is upheld through an effective heat exchanger and filtering system, ensuring uninterrupted and smooth cutting processes.
5. By-Pass Pipe: The by-pass pipe unit is a crucial component of the underwater granulator, with the pipe diameter determining the required water flow rate for the material and production capacity. Different circulation designs optimize the entire system’s operation.
6. Centrifuge Dryer: The centrifuge dryer plays a central role in separating water from pellets or granules. Drawing from European expertise and technology, this system has undergone enhancements to ensure improved stability and performance during its operation.
7. Smart Controlling System: The entire system is controlled through PLC (Programmable Logic Controller) and HMI (Human-Machine Interface). It offers one-key start/stop functionality, interlocking with upstream equipment, and displays critical parameters like temperature, rotation speed, pressure, flow rate, data storage, alarm analysis, and one-key reset.

Advantages of underwater granulator

1. Consistent Quality: The adavanced underwater pelletizing system technology ensures that the polymer melt is consistently cooled and cut in water, resulting in pellets with consistent shape and size. In addition to size, the granules produced by the underwater granulator also have a spherical shape, which enhances the flow into the hopper and feed port and increases the packing density.
2. Versatile Application: Advanced underwater pelletizing technology can be employed with a wide range of melt materials, enhancing its versatility and applicability. They can effectively manage various resin types, each with its distinct viscosity, rendering them well-suited for a diverse array of industries, including automotive, packaging, and construction.
3. Environmentally Friendly: The enclosed pelletizing process promotes environmentally friendly production practices, minimizing waste and emissions.
4. Simplified Design: With its simplified structure, the underwater pelletizer occupies minimal production site space, optimizing workspace efficiency.
5. Enhanced Efficiency: The underwater pelletizing system boasts higher production efficiency and cost-effectiveness, making it a valuable asset in manufacturing processes.
6. Automation: The underwater granulator allows extensive automation, with PLC control managing various functions like blade sharpening and die-plate grinding, reducing downtime.
7. Blade Life: Extended through cutter pressure control.
8. Wide Pellet Range: Produces a variety of pellet diameters, including micropellets used in masterbatches and other applications.
9. Low Dust: Minimal dust and fines due to cutting in molten form.

Key factors of choosing underwater pelletizer

When choosing the right underwater pelletizer for your needs, it’s essential to follow a systematic approach:

1. Production Requirements

Begin by thoroughly assessing your current production needs and envisioning future demands. Develop a five-year projection for materials and required capacities to ensure the system meets your nominal capacity.

2. Production Setup and Conditions

• Standard Compounding Plants: For setups using a straightforward water bath for strand cooling, factors like throughput, flexibility, and system integration play a crucial role. Layouts can vary significantly based on these demands.
• Continuous Compounding Plants: In cases with fewer product changes and higher capacities, automation can be advantageous. It helps reduce costs while enhancing product quality.
• Material Sensitivity: Consider the sensitivity of your materials, especially regarding moisture. For moisture-sensitive compounds or ingredients, a belt-conveyor strand pelletizer is the preferred choice.

3. Pellet Shape

Define the desired pellet shape. If a more spherical shape is preferred over cylindrical, an underwater hot-face cutter is the best option. Simple grinding operations may offer a cost-effective means of size reduction, but they often result in inconsistent particle shapes and sizes.

4. Thermodynamic Equilibrium

Understand the temperature equilibrium within the cutting chamber and die plate of the underwater pelletizing system. Pellet temperature and residual moisture have an inverse relationship. Sticky compounds, especially at elevated temperatures, may lead to agglomeration issues.

5. Maneuverability and Maintenance

Consider the ease of operating, maintaining, and automating the equipment. User-friendliness and automation can greatly impact efficiency and productivity.

6. Material Compatibility

If your raw materials are corrosive, pay close attention to the material and service life of critical contact parts, such as spinning blades and filter screens in the centrifugal dryer. Corrosion can lead to significant wear and tear.

Choosing the right underwater pelletizer involves a comprehensive evaluation of production requirements, material characteristics, pellet shape preferences, thermodynamic equilibrium, equipment maneuverability, and material compatibility. This thoughtful approach ensures that the selected pelletizer aligns with your specific needs and optimizes your production process.