Reading a tubing spec: what actually drives cost
Two drawings can look similar and quote very differently. It is worth knowing which lines are doing the work, because relaxing the right one often costs nothing in performance.
Tolerance, not dimension
The nominal outer diameter barely affects price. The tolerance on it does. Tightening a diameter tolerance narrows the process window, raises scrap and adds inspection. If a dimension is not a fit or seal surface, a wider tolerance on it is usually free performance.
Lumen count and geometry
A round single lumen is the cheapest thing to extrude. Every additional lumen adds tooling complexity and makes concentricity harder to hold. Non-round lumens — D-shaped, crescent, keyed — add more. These are often worth it, but they should be there for a reason rather than carried over from a previous drawing.
Reinforcement
Adding a braid or coil moves the part from a single extrusion into a multi-step build: liner, reinforcement, jacket, reflow. It is the single biggest step change in a quote. If a plain extrusion meets the mechanical requirement, it will always be cheaper.
Segment count
Each durometer transition along the shaft is a separate operation. Three segments is routine; a dozen is a different kind of part. Ask whether every transition is earning its place.
Finishing operations
Printing, insert molding, tipping, flaring, hole punching and forming each add a step. They are individually inexpensive, but they accumulate — and when they are spread across several suppliers the logistics and the risk of handling damage cost more than the operations themselves. Consolidating them at one place is usually where the saving is.
Volume and tooling
Tooling is amortised. At prototype volumes it dominates the unit price; at production volumes it disappears into it. It is worth quoting both so the comparison is honest.
If you are early in a design and not sure which constraints are real, send the drawing anyway and say which dimensions are functional. That conversation usually removes cost without removing performance.
