Plastic is sorted for recycling through a multi-stage process that combines mechanical separation, density-based water tanks, and high-speed optical scanners. Most of this happens at a Material Recovery Facility (MRF), where mixed recyclables arrive in bulk and get broken down into clean, single-polymer streams that can actually be reprocessed. The process moves from rough separation of obvious contaminants down to precise polymer-by-polymer identification.
The Seven Types of Plastic
Every plastic container carries a small triangular symbol with a number from 1 to 7. These resin identification codes tell recyclers what polymer the item is made from, and each behaves differently during sorting and reprocessing. The two most widely recycled are code 1 (PET), found in water bottles and food containers, and code 2 (HDPE), the opaque plastic used for detergent bottles and milk jugs. Most municipal programs accept both without question.
Code 3 (PVC) is harder to recycle and shows up in some bottles and toys. Code 4 (LDPE), the soft plastic in grocery bags, can clog sorting machines and is often collected separately at grocery stores rather than through curbside programs. Code 5 (polypropylene) appears in yogurt cups and straws. Code 6 (polystyrene, commonly called Styrofoam) is generally not accepted by recycling programs. Code 7 is a catch-all for everything else, including polycarbonate and bio-based plastics. In practice, codes 1 and 2 make up the vast majority of what actually gets recycled.
What Happens When Recyclables Arrive
At a MRF, trucks dump mixed recyclables onto a sunken conveyor lined with screws that break open collection bags and carry material up to an elevated sorting station. Workers standing along this conveyor pull out obvious contaminants by hand: wood, large metal objects, tangles of hose or rope, and anything else that would damage equipment downstream. This rough manual sort is fast and imprecise by design. Its only job is to protect the machines that follow.
From there, the stream passes through a disc screen with small holes that shakes out dirt, broken glass, and debris. What remains is a cleaner mix of containers, bottles, cans, cartons, and film plastic, all still jumbled together. The real sorting begins now.
Separating Plastics From Other Materials
Before plastics can be sorted by type, they need to be isolated from metals, glass, and paper. This happens through a sequence of physical separation steps, each exploiting a different material property.
A powerful electromagnet pulls ferrous metals (steel cans, tin lids) off the conveyor. Next, an air classifier blows lighter materials like plastic bottles, aluminum, and cartons upward while heavy items like glass fall down. The glass stream goes to its own optical sorter for color separation. The lighter stream continues to an eddy current separator, which handles aluminum. Alternating magnets beneath a spinning roller induce electrical currents in any conductive metal passing over it. Those currents create a repulsive force that physically flings aluminum cans away from the non-metal stream. Aluminum foil gets separated from cans by an air knife further down the line.
After cartons are pulled out using automated sorting technology, what’s left is a stream that’s predominantly plastic, ready for polymer-level sorting.
How NIR Scanners Identify Each Plastic
The primary technology for telling one plastic from another is near-infrared (NIR) spectroscopy. Sensors mounted above a fast-moving conveyor belt shine infrared light onto each piece of plastic. Different polymers absorb and reflect that light in distinct patterns based on the chemical bonds in their molecular structure. PET reflects differently from HDPE, which reflects differently from polypropylene, and so on.
The sensors read these light signatures at extraordinary speed. Sophisticated systems process tens of thousands of spectra per second, enough to identify individual items moving on a belt at industrial pace. Once a sensor identifies a piece, a precisely timed burst of compressed air fires from a nozzle below the belt, knocking that item into the correct collection bin. Items that don’t match any target polymer continue to a reject stream.
This technology works well for most plastics but has a notable weakness: black plastic. Carbon-black pigment absorbs infrared light across the spectrum instead of reflecting a readable signature, making black containers essentially invisible to standard NIR sensors. That’s one reason black plastic trays and containers often end up in landfill even when they’re technically recyclable.
Density Separation in Water
Some facilities use a simpler, older technique alongside or instead of optical sorting: sink-float tanks. This method exploits the fact that different plastics have different densities. When mixed plastic flakes or pieces are dropped into a tank of water, polyethylene (HDPE) and polypropylene float because their density is below 1.0 g/cm³. Heavier plastics like PET (density 1.33 to 1.37 g/cm³), PVC (1.32 to 1.37), and polystyrene sink.
To separate polymers that are close in density, facilities adjust the liquid. Adding sodium chloride (salt) to the water raises its density in controlled steps, from about 1.055 up to 1.175 g/cm³ or higher. At each density level, a different plastic floats while others sink. Ethanol solutions with a density around 0.935 g/cm³ can distinguish between HDPE and polypropylene, which are otherwise difficult to separate. This approach is especially common in reprocessing plants that handle pre-shredded plastic flakes rather than whole containers.
One persistent challenge is that PET and PVC have nearly identical densities, making them hard to separate in water alone. Facilities typically rely on NIR sorting to catch PVC before it reaches the sink-float stage, because even small amounts of PVC contamination can ruin an entire batch of recycled PET.
Ballistic Separators for Films and Rigids
Flexible plastic films (bags, wraps, pouches) behave completely differently from rigid containers on a conveyor. Ballistic separators handle this split early in the process. These machines use angled, vibrating paddles that bounce rigid three-dimensional objects like bottles and tubs in one direction while flat, two-dimensional materials like films and paper slide the other way. Operators adjust the paddle angle, speed, and airflow to fine-tune the separation. This step is critical because film plastic wraps around spinning parts and jams equipment if it moves further into the rigid-container sorting line.
AI and Robotic Sorting
A growing number of facilities are adding robotic arms equipped with cameras and machine-learning software to their sorting lines. These systems use visual recognition to identify items by shape, color, label, and texture, then pick and place them at high speed. One classification model called PlasticNet, developed at the University of Wisconsin-Madison, achieved over 87% accuracy across plastic types and 100% accuracy on certain specific polymers.
Robotic sorters are particularly useful as a quality-control step near the end of the line, catching items that NIR scanners missed or misidentified. They don’t get fatigued like human sorters, and their accuracy improves over time as they process more data. Most facilities today use them alongside optical sorters rather than as replacements.
Baling and Shipping
Once plastics are separated by resin type, the sorted PET, natural HDPE, and colored HDPE streams each go to storage containers. From there, the material is shredded, granulated, or perforated, then compressed into dense bales weighing roughly 1,000 to 1,500 pounds. These bales are sold to reprocessors who wash, melt, and pelletize the plastic into raw material for new products.
The quality of sorting directly determines the value of each bale. A bale of clean, single-polymer PET commands a significantly higher price than one contaminated with PVC, food residue, or mixed resins. This economic pressure is what drives facilities to invest in multiple overlapping sorting technologies rather than relying on any single method.
Why So Little Plastic Actually Gets Recycled
Despite all this technology, the U.S. recycling rate sits around 30 to 32%. The gap between what’s technically recyclable and what actually gets recycled comes down to several practical problems: contamination from food waste, mixed-material packaging that can’t be cleanly separated, limited local infrastructure, and the simple economics of virgin plastic often being cheaper than recycled. The EPA estimates that with better infrastructure, the national recycling rate could rise from 32% to roughly 61%, suggesting the bottleneck is capacity and collection, not just technology. Only about 15% of states even track how much recyclable material their curbside programs actually capture, making it difficult to measure progress.