The dim report (dimensional inspection report) ticked green. The parts still do not assemble the same way in October as they did in June.
That is nylon doing what nylon does. PA (polyamide / nylon) takes up moisture from the air. Dimensions grow. Stiffness and toughness shift. A part measured dry, straight out of the tool, is not the part the customer will clip together after a UK (United Kingdom) warehouse summer, or a damp autumn on the shelf.
If you bless the dry numbers and ship, the inspection happened too early. The expensive inspection happens later, on an assembly line, when snap-fits go sloppy or start to bind.
What moisture actually changes
As-moulded nylon is often dry relative to service. In ambient air it absorbs water until it approaches equilibrium with the local RH (relative humidity). Growth is not uniform folklore, it depends on grade, wall thickness, time and conditions, but the direction is consistent enough that ignoring it is a programme decision, not an oversight.
Fits move. A boss that was tight at T1 (first tool trial) can free up. A bearing bore that was free can close. Seals that sat correctly on a dry land can weep once the land has grown. None of that is a tool fault if the tool was never set up for the conditioned size.
Mechanical behaviour moves with it. Dry nylon can feel brittle relative to the conditioned state many data sheets quote for toughness. Designing clips to dry tensile numbers, then expecting conditioned toughness in the field, is two different products wearing one material name.
Colour and cosmetics can shift slightly with moisture and with the drying discipline before moulding. That is a separate process subject. The dimensional point remains: state the condition, or the green tick is incomplete.
Glass-filled nylon is not a moisture exemption
GF (glass-filled) PA still takes up moisture in the matrix. Fibre reduces the absolute dimensional swing compared with unfilled grades, but it does not remove anisotropy, knit-line risk or the need to know which state you are measuring. Stiffness goes up. Warp along flow goes with it. Moisture still belongs in the inspection plan.
If you are using GF nylon as a metal replacement, ask where the load path sits relative to fibre orientation, and when the critical dimensions will be checked. A dry CMM (coordinate measuring machine) loop that ignores conditioning is still a dry loop.
A part measured dry is not the part assembled after a UK warehouse.
When the inspection usually goes wrong
T1 is booked as a visit, not a hard pass or fail. Parts are measured within hours of moulding because the press window is short and the travel itinerary is fixed. The report looks complete. The balloons are filled. Nobody wrote “dry / as-moulded” as a limitation.
Goods-in repeats the same dry method weeks later on a different lot, or skips moisture state entirely. The numbers disagree with the try-out pack for reasons that get blamed on the supplier, the tool or “batch variation” before anyone asks about conditioning.
Assembly finds the truth. Clips that passed a dry push test fail a conditioned one, or the reverse. By then the launch date has already been sold, and the concession conversation starts with the wrong question: who signed the report, not what state the part was in.
The quiet failure mode is accepting a concession “just this once” on a dry size that will never be dry again in service. The next batch is made the same way. The exception has become the process.
What to put on the drawing and in the try-out plan
Name the production grade, not “nylon”. Viscosity, fibre level and impact modifiers change both process and moisture response.
State the inspection condition for critical dimensions: as-moulded dry, conditioned to a stated method, or both. If the product lives in a warehouse or in humid service, dry-only acceptance is a gamble written in calliper ink.
Allow time. Conditioning is not a footnote you squeeze between the last shot and the airport. If the schedule cannot wait, say so on the report and treat dry numbers as provisional, not as a green light for volume.
Design the fits for the state the customer will see. If the clip must work after moisture uptake, prototype and tolerance that state. Do not discover it on a returns bench.
Align purchasing with the same truth. A cheaper PA grade that moulds differently and conditions differently is not a drop-in because the family name matches.
Write dry, conditioned, or both. If you do not, the green tick is incomplete.
This is not anti-nylon
Nylon earns its place: toughness, wear, chemical resistance in the right grades, and a path out of metal for many brackets and housings. The failure mode is not the polymer. It is treating a dry laboratory tick as a field dimension.
Piece price on PA still looks attractive next to some alternatives (and piece price alone is a weak polymer filter). The programme cost of a fit that moves after packing does not. Moisture normalisation is not a materials trivia point. It is the size you ship.
If a nylon part is heading to T1 with a dry-only inspection list, or a snap-fit that was only ever checked on fresh mouldings, we can walk grade, critical fits and conditioning into a short Manufacturing Snapshot before the report becomes the argument.