In recycling, “efficiency” is not only tons per hour. It’s the ability to produce a consistent output grade without constant downtime caused by contamination. Sorting machinery is where most plants win or lose that battle—because it decides what enters the wash line and what reaches the extruder.
This guide explains the major sorting stages used in modern plastic recycling lines and how each stage affects purity, yield, and equipment protection.
Conclusioni rapide
- Sorting has two jobs: protect equipment and raise product value through purity.
- Mechanical sorting prepares the stream; sensor sorting refines it by polymer/color.
- PVC removal is a critical quality task for PET recyclers because it can damage rPET quality.
A Practical “Sorting Stack” Map
Use this table as a quick reference for what each module actually removes. It’s also a useful RFQ checklist when you compare suppliers.
| Sorting stage | Main purpose | Removes / separates | Attenzione per l'acquirente |
|---|---|---|---|
| Pre-sort + inspection | Keep “line killers” out | Oversize metal, wood, bricks, non-plastics | Define acceptance rules; otherwise sorting becomes endless manual work |
| Fines screening | Reduce grit load | Sand, glass fines, small debris | Fines carry moisture and contamination; plan dust/waste handling |
| 2D/3D separation | Improve presentation | Film/paper (2D) vs bottles/rigids (3D) | Presentation quality drives optical sorting performance |
| Magnet / eddy current | Protect cutting and extrusion | Ferrous and non-ferrous metals | Place stages where metal is liberated; plan cleaning routine |
| Optical sorting (NIR/color) | Separate polymers and grades | PET vs PVC vs PP/PE; colors; certain objects | Dirt/labels/moisture can reduce detection; maintenance matters |
| Robotics (vision picking) | Polish and recover | Targeted contaminants or valuable items | Works best after bulk separation; needs stable belt loading |
What Sorting Is Trying to Achieve
Most recycling projects sort for three outcomes:
- Remove non-plastics (metals, glass, stones, wood) that damage machinery
- Separate by polymer (PET vs PVC vs PP/PE) to meet quality specs
- Separate by grade (color, form factor, product type) to improve end market access
The exact stack depends on feedstock (MRF bales, industrial scrap, automotive plastics, film) and your output goal (flake vs pellet, packaging vs non-packaging markets).
The Main Sorting Technologies (In the Order They Usually Appear)
1) Mechanical screening and form separation
These stages control how well sensor sorting works later by: – removing fines – separating 2D (film/paper) from 3D (bottles/rigids) – stabilizing the belt presentation for optical sorting
Ballistic separators and trommels are common tools in this step.
2) Magnetic and eddy-current separation (metal removal)
Metal is one of the highest-risk contaminants for size reduction and extrusion equipment.
Typical approach: – magnets for ferrous removal – eddy-current systems for non-ferrous removal (where justified by the stream)
3) Optical sorting (polymer and color separation)
Near-infrared (NIR) and related sensor technologies can sort many polymers when the presentation is consistent. Air-jet ejection creates separated fractions for washing or direct processing.
In practical terms, optical sorting performance depends on: – belt loading and singulation (objects must be visible to the sensor) – surface condition (dirt, labels, and moisture can reduce detection quality) – calibration and maintenance discipline
4) Robotics and vision systems (targeted picking)
Robotic picking systems can help remove specific contaminants or recover valuable items from mixed streams. They are often used as a polishing step (after bulk separation) rather than as the only sorting method.
How to Write a Sorting Acceptance Test (So Performance Is Measurable)
If you are buying sorting equipment, define performance as purity and recovery on a defined input condition. Otherwise, you will get a demo that looks good on a small sample and then drifts in production.
| Acceptance Item | What to Define | Perché è importante |
|---|---|---|
| Input condition | Photos + contamination window + moisture condition + 2D/3D ratio | Sorting results depend heavily on presentation and contamination. |
| Purity target | Target polymer purity (and color limits where relevant) | Purity drives selling price and buyer acceptance. |
| Recovery target | % of the target polymer captured in the accept fraction | High purity with low recovery can still be a bad business outcome. |
| Sampling plan | How often, how many samples, and how you measure purity/recovery | Prevents “cherry-picked” results and makes tuning repeatable. |
| Cleaning interval | Hours between lens cleaning, belt cleaning, and sensor maintenance | Real uptime depends on maintenance access and routine discipline. |
Maintenance Reality: Sorting Performance Is a Maintenance Plan
Optical sorting does not fail only because of sensors—it fails because of dirty lenses, unstable belt loading, and inconsistent upstream preparation. When you evaluate a sorting proposal, ask:
- where cleaning access points are (lenses, hoods, air jets, chutes)
- what happens when belts carry dust and fines (dust collection and clean-out)
- whether calibration can be done by operators with clear procedures
Those details decide whether you get stable performance in month 6, not just week 1.
Critical Application: PVC Management in PET Recycling
PVC contamination in PET streams is a common quality risk. When heated, PVC can release hydrogen chloride (HCl), which can corrode equipment and affect polymer quality.
Practical actions: – reduce PVC at the sorting stage (optical sorting and QC checks) – remove PVC sleeves and labels where possible – build an inspection plan for incoming bales and flake output
Energycle integrates sorting modules and complete line designs for projects that require higher purity output; see its recycling solutions overview.
How to Choose the Right Sorting Stack
Before you buy, define: – feedstock source and variability (how often composition changes) – target polymer and quality spec – throughput target and operating hours – available labor for manual QC points – space and power constraints
If you share photos of your input stream and your target product spec, Energycle can recommend a sorting layout and line configuration via its pagina dei contatti.
FAQ (Domande reali sugli appalti)
Do I need optical sorting if I already have a wash line?
Often yes, if your output spec requires polymer purity. Washing removes dirt and some surface contamination, but it does not reliably separate polymers like PVC from PET or PP/PE caps from PET without the right upstream separation strategy. Optical sorting can create cleaner fractions before washing so the wash line is cleaning the right material instead of mixing problems together. The decision depends on your feedstock variability and your buyer’s rejection criteria. If customers penalize PVC and mixed polymers, optical sorting usually pays back by reducing scrap and protecting downstream equipment.
Why is PVC contamination such a big deal in PET recycling?
Because PVC behaves differently in heat and can damage product quality. When heated, PVC can release hydrogen chloride (HCl), which is corrosive and can contribute to equipment and quality issues if PVC enters hot processes. That’s why PET lines often treat PVC removal as a critical quality-control task. Your best risk control is to reduce PVC as early as possible with sorting and inspection checkpoints and to define a measurable acceptance limit. (Source: NIOSH Pocket Guide — hydrogen chloride)
How do I decide between more mechanical sorting vs more sensor sorting?
Start with what causes the most downtime or rejects. Mechanical sorting is usually the cost-effective way to remove bulk contaminants, improve 2D/3D separation, and present material cleanly to sensors. Sensor sorting is where you separate polymers and grades to hit higher-value specs. If sensors are missing targets, the fix is often upstream: better fines removal, better singulation, and less overlap on belts. Ask suppliers to show how they will manage presentation (belt speed, spreader design, and cleaning access), not only sensor brand names.
Are sorting robots worth it, or is it marketing?
Robots can be worth it when they are used for targeted tasks: polishing a stream after optical sorting, removing a small set of recurring contaminants, or reducing labor in repetitive zones. They usually fail when a plant expects robots to replace all upstream preparation. The key is to define the target objects and measure success with output purity and recovery rates at your throughput. Require a trial plan, a cleaning/calibration routine, and a downtime fallback mode. If a proposal doesn’t include those, it’s not a serious production design.
What should I provide suppliers so they can design the right sorting stack?
Provide an input spec (photos, typical and worst-case contamination list, moisture condition), your target product spec (polymer, color limits, contamination limits), and your throughput and duty cycle. Also include how you will measure acceptance (sampling plan) and what you will do with rejects. If you buy bales, align language with market specs; APR’s model bale specifications are a common reference for how bales and contamination are described. (Source: Specifiche del modello APR Bale)
How do I verify sorting performance before I buy?
Ask for a reference case on a similar stream and then require a trial or acceptance test plan. The test should define purity and recovery targets, sampling frequency, and what happens when performance drops (cleaning, calibration, and maintenance actions). Also confirm what “success” means operationally: hours between cleaning, time to change belts or cameras, and spare parts on-site. Sorting equipment that performs well in a demo can underperform in production if presentation and maintenance are not designed into the system.
