Optical and Mechanical Sorting Technologies
The fundamental barrier to high-grade plastic recycling is cross-contamination. Mechanical recycling processes dictate that polymers must be separated by chemical composition, color, and prior use (food vs. non-food). Industrial sorting utilizes cascading stages of mechanical and optical intervention.
Pre-Sorting: Dimensional Separation
Before sensors can analyze material composition, the waste stream must be physically regulated to prevent sensor blinding and mechanical jamming.
Trommel Screens
Massive rotating cylindrical screens that utilize gravity and angular momentum. As the cylinder rotates, materials smaller than the screen apertures fall through. This removes fine debris, shattered glass, and non-target small organics before the main sorting lines.
Ballistic Separators
Inclined oscillating paddles that separate materials based on density and shape rigidity. Rigid 3D items (like PET bottles) bounce backward down the incline, while flexible 2D items (like LDPE films) "walk" up the paddles. This prevents film wrappers from clogging high-speed optical sorters.
Velocity and Precision
Modern conveyor belts in optical sorting zones operate at speeds exceeding 3 meters per second. At these velocities, sensory data processing must occur in milliseconds to accurately trigger pneumatic ejection valves.
Advanced Optical Arrays
The core of polymer identification relies on light spectrum analysis.
NIR (Near-Infrared) Sensors
Different molecular structures in plastics absorb specific wavelengths of near-infrared light. By bombarding the waste stream with halogen light and reading the reflected signature, NIR units can map a belt of mixed plastics and fire compressed air jets to eject target polymers into separate hoppers.
Visible Spectrum Sorting
Once a stream of PET is isolated via NIR, it must often be sorted by color (Clear, Light Blue, Green, Mixed) to ensure the final recycled resin maintains a predictable color profile. High-speed RGB line-scan cameras achieve this secondary purification.