Polymer Extrusion and Pelletization
The final phase of mechanical recycling involves thermal compounding. Washed plastic flakes are transformed back into homogeneous, easy-to-transport pellets. This requires precise management of heat, pressure, and volatile gasses to prevent the molecular breakdown of the polymer chains.
Extruder Architectures
Single-Screw Extruders
The workhorse of plastic recycling. A single rotating screw inside a heated barrel conveys, compresses, and melts the flakes. Frictional heat from the screw rotation provides the majority of the melting energy, while external heater bands maintain exact temperature profiles across different barrel zones.
Co-Rotating Twin-Screw Systems
Used for heavily contaminated or highly blended materials. Twin screws provide superior mixing and devolatilization (removal of gases). They are essential when compounding recycled plastics with virgin additives, impact modifiers, or colorants to elevate the final product specifications.
Vacuum Degassing
Post-consumer plastics often retain moisture and absorbed chemicals. When melted, these expand into gasses. Extruder barrels are equipped with high-vacuum vent zones that extract these volatile organic compounds (VOCs) before the plastic is pushed through the die, preventing weak, porous final pellets.
Melt Filtration Mechanics
Even after rigorous washing and sorting, trace impurities (paper, wood, higher-melting plastics like Teflon or Silicone) remain in the melt stream.
Continuous Screen Changers
To avoid halting production to change clogged filters, continuous melt filters use a hydraulic system to slowly index fresh laser-drilled steel mesh across the melt flow. Impurities are scraped off by rotating blades and purged via an extraction auger, ensuring pressure within the extruder remains constant.
Underwater Pelletization
The filtered molten plastic is pushed through a die plate featuring dozens of small holes. Immediately as it exits the die, high-speed rotary knives cut the plastic strands underwater. The water instantly cools and solidifies the plastic into spherical pellets, which are then conveyed away via slurry lines to centrifugal dryers.