Polyamide, polyurethane and polyethylene tubing all carry compressed air, but they fail in different ways and at different temperatures. Polyamide holds pressure and shape, polyurethane bends tightly and survives being trodden on, polyethylene is the cheap option for low pressure and chemical exposure. Choosing between them is a question of bend radius, temperature and what happens when someone drives a pallet truck over the line.
This guide covers the three materials, what the pressure rating on the reel actually means, why bore size decides cylinder speed, and the mistakes that cost pressure between the compressor and the machine.
What separates the three materials?
| Property | Polyamide (PA 11, PA 12) | Polyurethane (PU) | Polyethylene (PE) |
|---|---|---|---|
| Typical working pressure at 20 °C | 10 to 16 bar | 8 to 10 bar | 6 to 10 bar |
| Temperature range | −40 to +80 °C, briefly +100 °C | −35 to +60 °C | −20 to +60 °C |
| Minimum bend radius | Largest, roughly 5 to 8 × outside diameter | Smallest, roughly 2 to 4 × OD | Medium |
| Crush recovery | Poor, kinks stay kinked | Excellent, springs back | Moderate |
| Chemical resistance | Good to oils and greases | Weak against some solvents, hydrolysis in hot water | Very good, including many acids and alkalis |
| Typical use | Fixed machine plumbing | Moving arms, festoons, drops to hand tools | Water, low-pressure chemical lines |
Polyamide is the machine-frame material. It holds a straight run without sagging, tolerates the highest working pressure of the three and stands up to the oil mist that ends up on every surface near a compressor. DIN 73378 is the reference standard for PA tubing in pneumatic and automotive work; the PA 12 grades are the ones usually stocked for compressed air.
Polyurethane is the moving-part material. Where the line has to follow a cylinder, a robot arm or a drop to a hand tool, PU bends to a radius that would kink polyamide, and it recovers its shape after being crushed. Two cautions: hydrolysis degrades polyester-based PU in hot, damp conditions, so ether-based PU is the choice for wet environments, and PU is the softest of the three, so it abrades where it rubs a frame edge.
Polyethylene is the chemical and water material. It resists a wider range of media than either of the others and costs less, but it creeps under sustained pressure and its temperature window is the narrowest. For compressed air it belongs on low-pressure, static runs.
What does the pressure rating on the reel mean?
The figure printed on the tube is the working pressure at 20 °C. It falls as temperature rises, and on plastics it falls steeply.
A rough rule for polyamide and polyurethane: expect to lose in the region of half the rated pressure by the time you reach the top of the material's temperature range. A tube rated 10 bar at 20 °C is not a 10 bar tube at 60 °C.
This matters more than it looks, because the hot spots in a plant are exactly where the air lines run: above ovens, alongside drives, in cabinets with no ventilation. Specify on the temperature at the tube, not the temperature of the room.
Burst pressure is typically three to four times working pressure, and it is not a design figure. It is the number that tells you the safety margin exists, not a licence to use it.
Bore size decides how fast the cylinder moves
A cylinder cannot fill faster than the tubing lets air in. Undersize the bore and the cylinder will be slow no matter what the valve and the actuator are capable of, and no amount of raising the regulator will fix it.
| Outside diameter | Typical bore | Reasonable run for a small cylinder |
|---|---|---|
| 4 mm | 2.5 mm | Short drops, sensors, pilot lines |
| 6 mm | 4 mm | Mini cylinders up to 25 mm bore, short runs |
| 8 mm | 5.5 or 6 mm | Cylinders to 50 mm bore |
| 10 mm | 6.5 or 8 mm | Cylinders to 63 mm bore, longer runs |
| 12 mm | 8 or 9 mm | Larger cylinders, manifold feeds |
Two habits cause most of the slow machines seen in practice.
Sizing the tube to the port thread. The port on a mini cylinder is often M5 and accepts a 4 mm fitting, which does not mean 4 mm tubing should feed it over five metres. Size the run on flow and length, then step down at the cylinder if you must.
Ignoring the fittings. Every push-in fitting, elbow and tee restricts flow, and a right-angle elbow costs far more than a swept bend. A line assembled from six elbows and a metre of tube can flow less than a straight run twice as long.
Cutting and preparing the tube
Push-in fittings seal on the outside diameter of the tube and grip it with a toothed ring. Both depend on the cut.
- Cut square. An angled cut leaves part of the O-ring unsupported and the joint weeps, often intermittently, which is the hardest kind of leak to find.
- Cut clean, with a tube cutter rather than side cutters. A crushed end will not seat and a burr will score the seal.
- Push fully home. Most push-in fittings need the tube to pass the gripping ring and reach the internal shoulder. A tube that stops at the ring holds at rest and blows out under pressure.
- Mark the insertion depth on the tube before assembly if the fitting is deep. It is the quickest way to see at a glance whether a joint is properly made.
Where each material belongs
Fixed plumbing on a machine frame, oil mist present, 8 to 10 bar. Polyamide. It stays where it is put, tolerates the heat near drives and does not abrade against the frame.
Drop to a hand tool or an air gun. Polyurethane, or a coiled PU hose where the operator needs reach without a line on the floor. PU survives being stepped on and trodden across, which polyamide does not.
Line following a moving axis, festoon or drag chain. Polyurethane, ether-based if the environment is damp. The bend radius is the deciding property.
Low-pressure water or chemical transfer near the machine. Polyethylene, provided the medium is compatible and the temperature stays modest.
Anywhere near a heat source above 60 °C. None of the three without checking the derated pressure first. This is where a PTFE or metal line earns its cost.
A short specification checklist
- Medium and its compatibility with the tube material.
- Working pressure at the highest temperature the tube will see, not at 20 °C.
- Bore, sized on the flow the actuator needs over the actual run length.
- Minimum bend radius against the tightest bend in the route.
- Mechanical exposure: crushing, abrasion against edges, drag chain cycles.
- Fittings, counted and their restriction accounted for.
- Colour coding, if the plant distinguishes services by tube colour. Decide it once and stick to it; a plant where blue means air on one machine and water on the next is a plant waiting for an incident.