Silicone tubing dimensions look simple until a supplier receives an RFQ that mixes inside diameter, outside diameter and wall thickness without explaining which value controls. A drawing may show two dimensions, an email may add a third, and a sample may not match either after storage or handling. That ambiguity can slow supplier review and make quotations difficult to compare.
For industrial buyers, the goal is not to force every dimension into one universal tolerance. The goal is to create a clear dimension convention, identify the functional features, and agree on how the tubing will be measured. This guide explains the questions to settle before requesting a quote for custom silicone tubing.
Start with three related dimensions
The three basic dimensions are:
- Inside diameter (ID): the opening through the tube.
- Outside diameter (OD): the overall diameter across the tube.
- Wall thickness: the material thickness between the inside and outside surfaces.
For an ideal, perfectly round tube, these values are mathematically related. Real flexible tubing, however, can become slightly oval under gravity, packaging pressure, cutting, or the force of a measuring tool. The two wall readings on opposite sides may also differ. That means an RFQ should not assume that three independently stated nominal values will always reconcile without a precedence rule.
State which two dimensions define the requested section and whether the third is a reference value. If all three are inspection requirements, say so explicitly and ask the supplier to review whether the requirement set is internally consistent.
Identify the dimension that controls the application
Different assemblies care about different features. ID may control fit over a barb, flow passage, or clearance for an inserted component. OD may control routing through a clamp, sleeve, guide, or housing. Wall thickness may influence flexibility, handling, collapse behavior, and how the tube responds during assembly.
Describe the mating components instead of sending dimensions alone. Include relevant fitting diameters, insertion length, clamp type, available envelope, and whether the tube is pushed, stretched, bent, or compressed during installation. A supplier can then review the dimension set in the context of the actual interface rather than treating every number as equally critical.
Where possible, mark critical-to-function dimensions on the drawing. Do not rely only on a general title-block tolerance when one interface deserves specific discussion.
Define diameter and wall conventions clearly
Use unambiguous labels such as “ID,” “OD,” and “wall thickness.” Avoid a bare expression such as “6 × 10 mm” unless the drawing or specification defines the order. Suppliers and buyers may interpret paired dimensions differently.
A useful RFQ note can state:
- the nominal ID and OD;
- whether wall thickness is calculated or independently inspected;
- the controlling dimension;
- units and decimal format;
- whether dimensions apply to the free, unassembled tube;
- any location-specific requirements near an end, mark, joint, or formed feature.
If tubing is supplied in several sizes, give each size a unique part number or line item. This reduces the chance that a quotation, sample label, or purchase document separates the numbers from their intended dimension convention.
Align the measurement method before approval
Flexible silicone can deform during measurement. A contact tool that applies too much force may reduce the apparent OD or wall thickness. An ID gauge may produce a different practical result from a visual scale or a method based on calculated values. Temperature, conditioning time, coil memory, and the distance from a cut end can also influence readings.
The RFQ should therefore ask how the supplier proposes to measure each controlled dimension. Topics to align include:
- measuring instrument or gauge type;
- contact force or non-compressive approach;
- number and orientation of readings;
- sample conditioning before inspection;
- measurement location along the tube;
- treatment of ovality and local surface variation;
- rounding and recording rules.
The appropriate method depends on the part geometry, application, and agreed drawing. A reference sample can help the discussion, but it should be identified by revision and should not silently replace the written requirements.
Handle tolerances as a project review
Avoid copying a tolerance from an unrelated tube or assuming one value applies to every diameter and wall combination. Extrusion behavior, material selection, section proportions, measurement method, and the length being inspected can all affect what a supplier needs to review.
Instead, separate functional needs from preferred targets. Explain what happens if ID becomes smaller, OD becomes larger, or the wall varies around the section. If only one side of a tolerance creates an assembly risk, make that clear. Ask the supplier to respond to each controlled feature and to flag conflicts before sampling.
This approach also makes supplier comparisons more meaningful. Two quotations are not equivalent if one supplier interprets OD as controlling while another assumes wall thickness controls the section.
Use samples to confirm the drawing, not repair it
Samples are valuable for checking fit, handling, routing, and the proposed inspection method. They are less useful when the drawing remains ambiguous. Before approving a sample, record the part number, drawing revision, material description, sample date or lot identification, and the agreed measurement results.
If the sample leads to a dimension change, revise the drawing and approval record. Do not leave the accepted geometry only in an email or an unlabeled physical sample. Production review should reference the same controlled documents used during quotation and sampling.
Build a cleaner silicone tubing RFQ
Before sending the request, prepare:
- A drawing with ID, OD and wall conventions defined.
- A note identifying the controlling and reference dimensions.
- Mating-component and installation information.
- The proposed measurement and conditioning method.
- Critical-to-function features and project-specific tolerance needs.
- Required supply form, cut length or coil arrangement, and labeling needs.
- Sample and revision-control expectations.
- Application, environment, quantity stage, and documentation questions for supplier review.
Dimension clarity at the RFQ stage helps engineering, procurement, quality, and the supplier evaluate the same tube. It also creates a better basis for sample approval and receiving inspection without turning an early quotation into an unsupported performance promise.
To request a project-specific review, submit the drawing, dimension convention, assembly context, quantity stage, and inspection questions through the Forvard Tech RFQ form.