Feed presentation is the condition material is in when it reaches a sensor-based sorter. That condition is how fast the material arrives and how deep it is piled. The condition is also how far across the belt the material is spread and whether individual pieces are separated from each other. Published throughput numbers for optical sorters assume good feed presentation. If your plant does not deliver it, you are not running the sorter you bought. The fix is upstream, on the conveyor.
The Sorter Is Probably Not Your Bottleneck
When a sort line underperforms, the usual response is to specify a better sorter. The optical and AI sorting on the market now is good. The sorter itself is not the point. The point is what those throughput numbers assume: material arriving singulated, evenly metered, and spread at a steady depth across the belt.
What often arrives instead is slugs. The flow is a surge from the feeder, a bare patch, and another surge. The sorter sees a pile and guesses.
You bought a recovery rate. The results you get depend on your feed presentation.
Before you specify the next sorter, stand at the belt that feeds the sorter you already own. Watch the belt for ten minutes. If the material comes in waves, the sorter is not the limiting factor. The conveyor in front of it is.
What a Sorter's Throughput Rating Assumes
A sensor-based sorter classifies material piece by piece. Near-infrared sorters shine light on the stream and read which wavelengths each material reflects back. Camera and AI systems read color, shape, and texture. Either way, the sorter maps what is on the belt. Then it fires a bank of precisely timed air jets at the discharge to knock the targets off their trajectory.
Three things have to be true for the sorter to work:
- The sensor has to see each item. Overlapping or stacked pieces hide each other. The sensor cannot classify an item that it cannot see.
- The controller has to know where each item will be a fraction of a second later. The future position of the item is a prediction based on belt speed. If the item tumbles, or a pile shifts on the way to the discharge, the prediction is wrong and the jet fires at empty air.
- The jet has to move that item alone. Every valve that fires into a clump moves the neighboring pieces too.
Industry guidance on preparing material for sensor-based sorting states the requirement plainly. The goal is to get material down to a single layer, so the separation device has a chance to succeed. That single-layer condition is what the rated throughput was measured under. If you feed the same machine a pile, the sensor sees a pile.
Feed Presentation, Broken Into Parts
Feed presentation sounds vague until you split it into variables you can change. Each variable is set by a machine upstream of the sorter.
| Feed variable | What good looks like | What controls it |
|---|---|---|
| Feed rate | Steady tons per hour, no waves, no dry spots | Storage bin, feeder, and a metered feed conveyor with variable speed |
| Burden depth | Thin, approaching a single layer | Belt width and belt speed for the tonnage being run |
| Lateral spread | Material across the full belt width, not in a narrow strip down the center | Chute and hood geometry, feeder pan width, spreaders |
| Singulation | Pieces separated from each other, not touching or stacked | A speed step up from the feed belt to the acceleration belt |
| Stability at speed | Items riding flat, not floating, tumbling, or drifting sideways | Belt speed limits, drop height, a wind tunnel over the acceleration belt |
| Size range | Oversize and fines already removed | Screens ahead of the sorter |
Feed Rate: Surge Control Belongs at the Bin
A bucket loader dumping onto a floor hopper creates surges. Nothing downstream can average out these surges by itself. So plants that run well put a storage-and-metering step between the loader and the line. A live floor bin absorbs the dump and discharges at a controlled rate instead of all at once. A vibratory feeder under the bin makes the flow more even, moving material forward in short strokes at a rate set by the drive.
From there the infeed conveyor sets the pace for the whole line. We build these with variable speed drive as an option, plus an automated feed control. The automated feed control monitors how much material is on the belt. It varies belt speed from a low of 20 percent of normal up to 200 percent of normal, to smooth out surges and dry spots. The same logic applies to the sort line feed conveyor ahead of a manual pick station. A steady belt makes the labor worth paying for.
Burden Depth: Width and Speed
Burden depth is the thickness of the material layer riding on the belt. For a given tonnage, burden depth, belt width, and belt speed change together. The same tons per hour spread over a wider belt, or over a faster belt, ride thinner. There is no way to hold tonnage constant, keep width and speed the same, and get a thinner layer.
That arithmetic is why a sorter installed on an existing narrow feed belt often does not meet expectations. The sorter is rated for a near-single-layer presentation. That belt cannot produce one at the tonnage you want to run. The same limit applies to the people on a quality control line. Nothing buried gets identified, whether the thing looking at it is a camera or a person.
Incline matters here too. A steep feed section meters better than a shallow one, because gravity works against the pile and releases it in small amounts. A dual-angle incline applies the same reasoning: it starts steep to help with metering and then flattens to prevent rollback. Our guide to maximum conveyor incline angle covers what those limits are.
Lateral Spread: Use the Whole Belt
If material forms a narrow strip down the middle of a wide belt, the belt works as a narrow belt. You paid extra for width that you do not use. Material lands where the chute puts it. So chute and hood geometry are part of the sorter specification, whether or not anyone quoted them that way. The standard fix is a vibratory feeder or a disc spreader that drops material across the full width of the acceleration conveyor. That fix is a smaller project than reworking the sorter itself.
The Speed-Ratio Principle
Here is the mechanism behind an acceleration belt. Put two conveyors in series. Run the second conveyor faster than the first. As each item transfers, it accelerates away from the item behind it. The center-to-center spacing between pieces grows in proportion to the ratio of the two belt speeds. If you run the second belt twice as fast, the spacing roughly doubles.
The speed ratio is the whole principle. The acceleration conveyor does not sort anything. It changes a dense, slow, touching stream into a fast, thin stream with gaps, because the sensor and the valve bank need gaps.
There is a limit on how fast you can run a bare belt. Trade guidance on preparing material for sensor-based sorting puts the limit around 550 to 600 feet per minute. That limit is the speed at which light sheet paper and film start to fly up and drift. The solution is a wind tunnel over the acceleration conveyor. The wind tunnel moves air along the belt at the same speed as the belt, so light items stay down. With a wind tunnel fitted, the same guidance puts acceleration conveyor speeds at 800 to 1,000 feet per minute.
Why Slugs Cost You Twice
A surge does not cause only misses. It also causes wrong ejections. Buried target material rides through into the residue. The jets that do fire into a clump eject good material with the bad. Both errors have the same result: a recirculation load, a downgraded bale, or both.
Then the problem moves downstream. Whatever the optical sorter misses reaches the quality control sorting conveyor, where people catch it by hand at whatever speed the surge dictates. A sort line paced by slugs alternates between overwhelmed and idle, which is the worst way to buy labor.
Mechanical separators have the same problem. A disc screen fed in surges stops separating by size, because a slug buries the discs and everything rides over the top, including the unders. Screening is presentation work too. A machine that is fed badly makes the feed to the next machine worse.
Ten Minutes at the Belt
Before anyone writes a capital request, watch the feed conveyor with a stopwatch. Look for these six things:
- Waves. Time the peaks. If the flow has a rhythm instead of a steady stream, the metering upstream is the problem.
- Bare belt. Dry spots between surges are throughput you already paid for and did not use.
- Depth. Check if you can see belt between pieces. Check if the material is two and three items deep in the heavy patches.
- Edges. Check if the outer six inches of a wide belt carry any material.
- Tumbling. Look for items that roll or flip on the way to the discharge. These items make the position prediction wrong.
- The residue. Sample the reject stream. If good material comes out in clusters and not as single pieces, the jets are firing into clumps.
If the answers show that the problem is upstream, a new sorter gives you nothing except a faster way to guess.
Fixing It Is a Conveyor Problem
Feed presentation is built from ordinary equipment that is specified deliberately. The belt width is right for the tonnage. The speed is right. The drive holds a set rate under load. The chute geometry puts material across the full width. There is enough metering upstream that the belt receives a stream instead of a dump.
Remcon has built heavy-duty recycling equipment since 1977. We build every conveyor to order, with belt widths from 12" to 72". Variable frequency drive is optional. Automated feed control is available on infeed conveyors. We also select and integrate equipment that we do not build. Examples are disc screens, vibratory screens, and vibratory feeders, plus the support structures, transition chutes, and platforms that connect them. Everything is quoted per job, because the right width and speed depend on your tonnage and your building, not on a catalog page.
If you are already requesting quotes on a sorter, ask the vendor what feed presentation their throughput number assumes. Then ask whether your existing conveyor can produce it. Our guide to buying a conveyor covers the rest of the questions worth asking.
Optical Sorter Feed Presentation FAQ
Why is my optical sorter underperforming its rated throughput?
Most often, the cause is how material is presented to the sorter, not the sorter itself. Sensor-based sorters are rated on the assumption that material arrives at a steady rate. The material is spread across the belt in close to a single layer, with pieces separated from each other. If material arrives in surges and piles, the sensor cannot see buried items and the air jets fire into clumps. The fix is on the conveyor and metering equipment upstream.
What is singulation in recycling, and why does it matter?
Singulation means separating a dense mass flow of material into individual pieces with gaps between them, rather than a touching or stacked stream. Singulation matters because an optical sorter classifies material piece by piece and aims its air jets at individual targets. Without gaps, the sensor sees overlapping items, and each burst of air moves whatever is nearby. So good material leaves with the reject, and target material rides through.
What is burden depth on a conveyor?
Burden depth is the thickness of the material layer riding on the belt. For a given tonnage, belt width and belt speed set it. The same tons per hour spread across a wider belt, or carried on a faster belt, ride thinner. Sensor-based sorters and screens both work better at low burden depth. Low burden depth is why feed belts ahead of them are often wider and faster than the conveyors that feed them.
How does an acceleration conveyor spread material out?
The acceleration conveyor runs faster than the belt that feeds it. When an item transfers from a slower belt to a faster one, it accelerates away from the item behind it. So the spacing between pieces grows in proportion to the ratio of the two belt speeds. Placing conveyors in series at different speeds is the standard way to turn a dense stream into a thin, gapped one ahead of a sorter.
How fast does the conveyor feeding an optical sorter run?
The conveyor feeding an optical sorter runs faster than most plant conveyors. Trade guidance on preparing material for sensor-based sorting puts the bare-belt limit around 550 to 600 feet per minute. That limit is the speed at which light sheet paper and film start to fly up and drift. A wind tunnel over the acceleration conveyor blows air along the belt at belt speed. The moving air holds the sheet paper and film down and lets the belt run at 800 to 1,000 feet per minute.
Should I upgrade the sorter or fix the feed conveyor first?
Before you decide, watch the existing feed belt. If material arrives in waves, with bare patches between surges and pieces piled two or three deep, the sorter is not the limiting factor. A new sorter will inherit the same problem. Metering equipment, belt width, belt speed, and chute geometry set the limit that the sorter works under. Changing them is normally a smaller project than replacing the sorter.