Granulators convert raw materials into particles of a specified size and shape. Their working principles vary according to the material being processed and the machine design. Common mechanisms include screw extrusion, mechanical compaction, and the removal of entrained air before or during granule formation.
For plastic pelletizing equipment, the main process typically involves conveying, melting, mixing, and extruding the polymer through a die. Other granulators use forming rollers or compaction rollers to consolidate powder into granules.
Screw extrusion granulation
Screw extrusion granulators use a rotating screw inside a heated barrel to convey, compress, and process the material before forcing it through a die.
As the screw rotates, it moves the feed material along the barrel. In plastic processing, the material is heated by the barrel's heating system and by mechanical energy generated through shear. The screw compresses and mixes the material, helping produce a uniform melt before it reaches the die.
The die openings determine the cross-sectional shape of the extruded material. Depending on the pelletizing system, the extrudate may be cooled and cut into pellets, or cut directly at the die face.
Screw design and plasticization
Screw geometry affects conveying, compression, melting, mixing, and output stability. Important design factors include screw diameter, channel depth, pitch, compression ratio, length-to-diameter ratio (L/D), and rotational speed.
The screw typically contains feeding, compression, and metering sections. The feeding section transports the material, the compression section increases pressure and supports melting, and the metering section helps deliver a more uniform melt to the die.
Increasing screw pitch alone does not guarantee more complete plasticization or better pellet quality. The screw geometry must be matched to the polymer's processing characteristics, the required output, and the operating conditions.
Granulation using forming and degassing rollers
Some specialized granulators use a forming roller together with a degassing or air-removal component. The precise arrangement depends on the machine design.
The degassing component is intended to help remove air entrained in the feed material. This can reduce the amount of trapped air that may interfere with compaction or contribute to defects in the resulting granules.
The forming component applies pressure to consolidate the prepared material into a defined shape. Depending on the machine, the material may be pressed through a die or formed by a shaped roller surface.
The effectiveness of this process depends on the material's compressibility, moisture content, particle characteristics, and the applied pressure. The specific functions of the forming and degassing components should be confirmed against the equipment drawings and manufacturer's documentation.
This type of mechanical compaction differs from plastic melt extrusion. It should not be treated as a standard component arrangement for every granulator.
Key design features
Screw geometry
Some extrusion machines use variable-diameter screws or screw channels that change in depth along the processing length. These design features can help control material conveying, compression, and plasticization.
The effect on service life depends on the screw geometry, processed material, operating load, and wear protection. Screw design should therefore be evaluated in relation to both processing performance and expected wear.
Barrel material and surface treatment
The barrel must withstand the operating temperature, internal pressure, and mechanical loads associated with the process. Depending on the application, manufacturers may use alloy steels or other suitable materials with appropriate heat treatment and surface finishing.
Hardening can improve surface hardness and wear resistance when the selected material and treatment are appropriate. Corrosion resistance, however, depends on the alloy composition, surface condition, and process environment. Heat treatment alone does not guarantee resistance to corrosion.
For abrasive or corrosive feed materials, material selection and surface treatment should be based on the actual operating conditions.
Heating system
Plastic extrusion pelletizers commonly use electric heater bands or other heating elements arranged in separate temperature-control zones along the barrel.
Zoned heating allows operators to establish different temperature settings along the processing length. This helps control the material's transition from a solid feed to a processed melt.
The required temperature profile depends on the polymer grade, screw design, throughput, and screw speed. Temperature control must also account for heat generated by mechanical shear during operation.
Factors that affect granule quality
Granule quality depends on the relationship between material properties, machine design, and operating conditions.
For plastic extrusion pelletizing, important factors include polymer type, feed consistency, screw geometry, barrel temperature, screw speed, melt pressure, die condition, and the pelletizing method.
For powder compaction granulation, relevant factors include particle size, moisture content, feed uniformity, compaction pressure, and the condition of the forming surfaces.
A stable process requires these factors to work together. Changing one setting without considering the other operating conditions may affect particle shape, size consistency, mechanical strength, or production output.
Custom machining for granulation equipment
Beyond granulation equipment components, Hansheng Automation provides custom precision machining for non-standard industrial parts based on customer-supplied drawings. Our services also cover parts for filling machinery and tobacco packaging equipment, precision components for yacht equipment, and parts for remote control racing cars. Each component is manufactured according to the specified dimensions, structural requirements, and drawing details.
