How to Specify Silicone Tubing Wall Thickness and Tolerance
ID, wall and concentricity are three separate promises. Confusing them is the most common reason a tube that “meets drawing” still leaks on the filling line.
Thicker is not simply stronger
A silicone tube is a spring and a hose at the same time. Wall thickness sets how much it resists collapsing under vacuum, how much force a barb needs, and how quickly a peristaltic pump fatigues it. Past a point, more wall just makes the part stiff and expensive.
The three numbers on a tube drawing
Most disputes come from treating these as one tolerance. They are measured differently and they fail differently.
| Dimension | Commercial | Precision | What drives it |
|---|---|---|---|
| Inside diameter | ±0.15 mm | ±0.08 mm | Mandrel size and draw speed |
| Wall thickness | ±0.15 mm | ±0.10 mm | Die gap and extrusion pressure |
| Concentricity | 0.20 mm TIR | 0.10 mm TIR | Mandrel centring in the die |
| Length | ±1.0 mm | ±0.5 mm | Cut method and cooling |
Tolerance stack-up, worked through
Take a 6.0 mm ID × 1.5 mm wall tube mated to a 6.0 mm barb. At commercial tolerance the ID can sit at 6.15 mm — that is already 2.5%% loose before the barb tolerance is considered. Add a soft 50 Shore A compound and the joint will weep under 1 bar.
The fix is rarely a tighter tube. It is usually a slightly undersized ID callout (−0.05 mm) so the interference is always positive.
Writing the callout
- Give ID and wall, never OD. OD is a result and stacks both tolerances.
- Put concentricity on the drawing. An off-centre tube passes ID and wall checks and still fails a pinch valve.
- State the hardness. Tolerance and hardness interact; 0.15 mm means different things at 40 and 70 Shore A.
- Say how you will measure it. Pin gauge, laser or optical comparator give different answers on a soft tube.
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Questions about your own part?
Send a drawing or a sample. We will tell you which compound and hardness we would run, and what it costs at your volume.