Silicone, Rubber and PVC Labels: Sorting Look-Alikes From PET Flake
A washing line does an exceptional job of removing things that behave differently from plastic: soil that dissolves, glue that softens, caps that float, metal that sticks to a magnet. But a bottle carries contamination that behaves exactly like the plastic itself — silicone valve seals, rubber gaskets, PVC shrink labels. They come out of the wash bath as clean as the flake they sit in, and from that point on density and water cannot touch them. This guide explains why the wet route stops working here, and how a dry line of air classification plus electrostatic separation handles the fractions that look like plastic because they are plastic.
The Contamination That Washing Cannot Reach
Look at what a post-consumer PET bale actually contains. The PET bottle washing line product page lists paper and film labels, glue residue, caps and rings, sand, organic residue, metal — and PVC labels. Every item on that list except PVC is separated by a mechanism the line already has: glue softens in heated wash liquor, caps and rings float off in density separation, sand sinks, metal is caught upstream.
Silicone and rubber are different. They do not dissolve, they do not soften at wash temperature, and their density sits close to the plastics around them. A silicone valve seal that goes into the crusher comes out as a silicone fragment of roughly the same size as the flake, and every downstream stage treats it as product. It passes density separation because it is not dense enough to sink and not light enough to float away. It passes the dewatering press. It reaches the drying stage and it reaches the bag.
The cost shows up twice. First in the specification: a buyer who tests for foreign polymer content finds silicone and rubber pieces in a stream that is supposed to be one material. Second downstream: silicone and rubber do not melt at the processing temperatures of PET or polyolefins, so they appear as gels, black specks and surface defects in the finished pellet or sheet — at a rate that is completely out of proportion to their share of the feed.
Why Float-Sink, Optical and Magnetic Stages All Miss It
It is worth being precise about the gap, because the fix is not a bigger version of a machine you already have:
- Float-sink (density separation) — works on a density difference. Silicone and rubber have densities close to the plastics they contaminate, so they do not separate reliably in water. This is the single reason the problem survives a wet line.
- Magnetic and eddy-current stages — work on conductivity. Silicone, rubber and PVC are all non-conductive, so nothing in the metal-recovery chain reacts to them.
- Optical and colour sorting — works on appearance. Black gaskets and clear seals in a mixed-colour flake stream are not distinguishable by colour, and the PET line product page is explicit that colour sorting is a separate stage configured to the target output — it is not a contamination-removal stage.
- Washing — works on surface soil. Silicone in the feed is not soil; it is a second polymer. Washing can clean it, but it cannot take it out.
What is left is a stage that separates on two properties the wet line ignores: aerodynamic behaviour and surface charge. That is exactly the design scope published for the silicone and electrostatic sorting line: dry separation of look-alike materials whose density and colour are too close for float-sink and optical sorting.
The Dry Line, Stage by Stage
The published process is a five-step dry sequence. There is no water treatment and no drying cost in it, which is also why it can sit after a washed-and-dried flake line without disturbing its water balance:
Published Dry Separation Sequence
- Feeding — material is metered in a single layer, so each particle meets the air stream and the field on its own.
- Air classification — light film, paper labels and fines are lifted out of the stream by aerodynamic behaviour.
- Electrostatic separation — charged silicone and rubber are deflected away from the plastic path.
- Silicone sorting — remaining seals and gaskets are identified and ejected.
- Product collection — separated fractions discharge to individual outlets.
The order matters more than it looks. Single-layer metering is not a convenience detail: a particle buried inside a cluster never sees the air stream or the field properly, and its charge is affected by its neighbours. Air classification runs before electrostatic separation because film and paper labels would otherwise carry charge into the field and confuse the signal — and because removing the light fraction first is what makes the remaining stream uniform enough for the electrostatic stage to work at the published accuracy.
The final mechanical stage is a dedicated sorter for what survived: seals and gaskets that are too similar in size and behaviour to be taken out by air or charge alone. Between them, the three separation principles — aerodynamic, electrostatic, recognition — cover three different reasons a piece of contamination is hard to remove.
Published Equipment Data
The line is specified as two units — an air separator and a silicone sorter — combined for mixed contamination. Both are also available standalone for integration into an existing line. The published figures:
Air separation (WX series)
| Model | Capacity | Separation efficiency | Duct width | Particle size |
|---|---|---|---|---|
| WX-600 | 800-1,500 kg/h | ≥92% | 600 mm | 5-50 mm |
| WX-800 | 1,500-2,500 kg/h | ≥92% | 800 mm | 5-60 mm |
| WX-1000 | 2,500-3,000 kg/h | ≥92% | 1,000 mm | 5-80 mm |
Silicone sorting (SS series)
| Model | Capacity | Recognition accuracy | Channels | Power |
|---|---|---|---|---|
| SS-500 | 500-800 kg/h | ≥99.5% | 6 | 42.5 kW |
| SS-800 | 800-1,500 kg/h | ≥99.5% | 9 | 63.5 kW |
| SS-1000 | 1,500-2,000 kg/h | ≥99.5% | 12 | 85.0 kW |
Two notes on reading these numbers. First, capacity figures are per unit: a 2,000 kg/h flake output needs a matched pair, not the largest single model in either table. Second, the separator's particle-size window is a specification input, not a footnote — a WX-600 tops out where a WX-1000 keeps going, so the PS-series crusher screen choice upstream is part of the sorting decision, not just a washing decision. That crusher offers 3, 5, 8, 10 and 15 mm screens and an output typically in the 5-20 mm range.
Where the Dry Line Belongs in Your Process
The air separator's product page describes it as designed for inline operation between crushing and further sorting or washing. That is the flexible position, and which side of the washing line you put this stage on depends on your feed:
- Before washing — suitable when the silicone and rubber load is heavy and you would rather not carry it through the wash bath at all. Air classification also lifts film, paper labels and fines before they reach the water, which reduces what the friction washer and the water treatment have to handle.
- After washing and drying — suitable when the contamination that survives the wet route is the smaller problem. Electrostatic separation requires dry, free-flowing material, and a washed-and-dried flake stream is already in exactly that condition. On the film line the published outlet moisture after drying is 2% or lower, which is the kind of feed this stage is designed around.
Moisture is the constraint that decides the layout, and it is worth quoting the published limit rather than guessing. On the air separator, material moisture above roughly 10% causes particles to stick and reduces separation quality; on the electrostatic side, high moisture makes particles stick and lowers separation quality generally. A dry stage placed straight off a dewatering press will not perform to the published figures — it needs dried feed.
Planning Data You Should Have Before Asking for a Quote
The published note on this equipment is unusually direct: actual results depend on material form, moisture content and feed uniformity, and a material test is required before a performance figure is committed. That is not hedging — it is a list of the four inputs that decide your line. Prepare them before the conversation:
- Feed description — which plastic stream, and what contamination classes are present: silicone seals, rubber gaskets, PVC labels, film and paper labels, dust and fines. The published material fit is PET flake with silicone seals, rubber gaskets and PVC labels, plus mixed rigid plastic fractions.
- Particle size — your current flake size distribution, because it sets which model's working window you fall inside.
- Moisture — measured, at the point where the dry stage would sit, not at the infeed of the plant.
- Target — what your buyer tests and at what limit. Recognition accuracy on the sorter and separation efficiency on the separator are published as minimum figures, and your achievable result is confirmed on your own sample.
A dry test on a representative sample settles all four at once, which is why the honest answer to "what purity will I get" is a test date rather than a number. For a plant already running a washing line, the practical question is narrower: how many kilograms per hour of silicone and rubber are currently leaving in your flake, and what is that worth at your buyer's price for contamination?
Frequently Asked Questions
Why does a washing line not remove silicone seals and rubber gaskets?
Because washing separates by density and removes surface soil, not by material identity. Silicone and rubber have densities close to the plastics they contaminate, so they float or sink with the plastic fraction instead of separating in water. They also do not dissolve or soften in the wash bath the way glue does, so they leave the line as full pieces inside otherwise clean flake.
How does a dry electrostatic line separate silicone from PET flake?
The line combines two different physical principles. Air classification first lifts light film, paper and fines out of the stream by aerodynamic behaviour; electrostatic separation then deflects particles by surface charge, so charged silicone and rubber are pushed off the plastic path. The silicone sorter identifies and ejects the remaining seals and gaskets, and each fraction discharges to its own outlet.
What purity or recognition figure is published for this equipment?
The published separation efficiency for the WX air separator series is 92% or higher, and published recognition accuracy for the SS silicone sorter series is 99.5% or higher. Actual results depend on material form, moisture content and feed uniformity, so a material test is required before a performance figure is committed for your plant.
Does moisture affect electrostatic separation?
Yes, moisture is the main constraint. High moisture causes particles to stick together and reduces separation quality, so the feed should be dry and free-flowing. On the air separator side the published guidance is that material moisture should generally stay below roughly 10%, because above that particles stick and separation drops.
Can the same line handle PVC labels?
PVC labels are addressed by the electrostatic stage rather than by air classification, and the exact split depends on the PVC content and particle size in your material. PVC is a separate contamination class from silicone and rubber and is confirmed in a dry separation test on your sample.
Have Silicone or Rubber Showing Up in Your Flake?
Send a sample of your flake and your target contamination limit. LVKESORT's engineering team will run a dry separation test and size the air classification and silicone sorting stages to your throughput.