A custom sleeve is a five-dimension part that people usually describe with three dimensions and a photograph. The gap between the two is where quotations go wrong and samples get rejected. This is what a supplier needs to quote accurately the first time.
Why a drawing beats a description
“Same as this sample but 5 mm longer” is a complete specification only if both sides agree on what “this sample” is. They rarely do: the sample has been in a drawer, the bore has relaxed, and nobody knows whether the 5 mm is added to the overall length or to the tail. A single A4 sheet with five dimensions removes the argument before it starts.
The drawing does not need to be a formal engineering release. A sectional sketch with dimensions, an arrow for the direction of insertion, and a material note is enough. Hand-drawn is fine. Unlabelled is not.
The five dimensions that define the part
- Bore — the internal diameter at the entry. Specify it as the diameter the terminal or wire must pass, and state whether it is a free-state or fitted measurement. These differ by the amount the part stretches.
- Wall thickness — not outside diameter. OD is a result; wall is a decision. If you specify OD, the moulder cannot tell you whether the wall will hold up.
- Length — overall, and the split between the body and the tail. Where a sleeve covers a crimp and then grips insulation, those are two lengths, not one.
- Tail opening — the bore at the wire end, which sets grip. If the tail tapers, give both ends.
- Flange or step — anything that stops the sleeve or that the operator feels when seating it. If there is no positive stop, say so explicitly, otherwise a moulder may add one.
Tolerance: decide what actually matters
Moulded elastomer parts cannot hold machined tolerances, and asking for them raises cost without improving the assembly. Split the dimensions into two groups. Dimensions that affect fit — bore at the terminal, tail bore, the length that has to cover the crimp — deserve a tight callout. Dimensions that affect appearance — overall length on a part with a rounded end, flange thickness — can carry a loose one.
A practical split on a small sleeve is roughly: fit-critical bores called out tightly, lengths moderately, everything else by general tolerance. The moulder will tell you what the tool can hold; if a requested tolerance is tighter than the process, believe the process number rather than the drawing, and change the drawing.
Material and colour need a reference, not an adjective
“Soft PVC” and “good silicone” are not specifications. State the polymer, the hardness in Shore A, and the colour against a reference — a Pantone number, an RAL number, or a physical chip. Colour by photograph is unreliable: phone cameras shift reds badly, and the two sides of a WeChat conversation are looking at different screens.
If the part must meet a substance requirement — RoHS, REACH, a customer's restricted substance list — say so at quotation stage, not at shipment. The grade that meets the requirement may be a different compound with a different minimum order and a different price, and finding that out after tooling is expensive.
The sampling sequence, and what each step is for
- Step 1 — drawing review. The supplier confirms what is ambiguous and what the tool can hold. Expect two or three questions; a supplier who asks none has not read the drawing.
- Step 2 — first sample from production tooling. Not a 3D print, not a machined prototype. A printed part proves the shape and hides every problem that matters: shrinkage, wall uniformity, how the part releases from the tool.
- Step 3 — fit trial on your assembly. This is the only test that validates the design. Push the sample onto the real terminal with the real wire, in the real order of operations, and check the three fit points: bore, coverage, tail grip.
- Step 4 — dimensional report against the drawing. Do this after the fit trial passes, not before; a part that measures perfectly and does not fit is still wrong.
- Step 5 — batch approval. Keep the approved sample, signed and dated, with the drawing revision on it. Incoming inspection needs something to compare against that is not a memory.
What to put in the RFQ
- Drawing with the five dimensions and the material note.
- Annual and per-order quantity — this decides whether a stock size, a modified stock size, or a new tool is the right answer.
- Application: what the assembly is, where it is installed, what it touches.
- Any substance or documentation requirement, stated up front.
- Required sample date and required production date, as separate dates.
- For repeat orders: the previous part number and the approved sample reference.
Four mistakes that cost the most
Specifying outside diameter instead of wall thickness, so nobody can judge whether the part will hold its shape. Omitting the insertion direction, so the draft runs the wrong way and the part fights the operator. Quoting a colour from a photo. And approving a sample without the drawing revision written on it, which makes every later batch arguable.
None of these is a moulding problem. They are specification problems, and they are all avoidable with one extra line on a drawing.