Heat and Humidity Ageing for Stored Inventory: A Silicone Weight Test Protocol
A silicone ankle weight that cleared every studio check can still reach a customer looking tired after nine months in a warehouse. Stored stock rarely sits in the conditions your test report was written for: a container roof on a July dock, a logistics mezzanine warmed by skylights, a stockroom beside a steel roller door. Heat and humidity are the two variables that most often turn a sound batch into a complaint about tacky surfaces, white bloom, lifted print and a steel core quietly corroding under the skin.
Why storage decides the claim more often than training does
Purchase specifications describe use: drop heights, cleaning cycles, classes per week. Almost none describe the ten to twenty weeks between the factory gate and the retailer shelf, which is where a range usually loses its appearance. A studio weight sees two hours of handling a day and a wipe with mild cleaner. A carton in a warm warehouse sees 35 to 45 °C for months, with humidity swinging between a dry aisle and a damp loading bay. Silicone is chemically stable, but stable is not immune. The network keeps curing slowly, volatile residuals migrate outward, and every bonded interface sits under continuous load from heat and moisture.
The four ageing routes worth measuring
When stored stock returns discoloured or sticky, the cause is usually one of four routes. Post-cure continues inside the box: crosslink density rises and the surface feels harder and drier than the approved sample. Volatiles and short chains migrate out as a film that reads as haze, holds dust and photographs badly. Moisture reaches the bond line between silicone and a metal insert, where hydrolysis and differential expansion weaken adhesion until the core can be twisted inside the shell. Print and coating layers absorb water, swell and lose key, so edges lift first and then flake. Each route needs its own test, which is why one "ageing" line in a specification says very little about what happens on a pallet.
Running the oven and damp-heat protocol
Take the thermal side to ISO 188 or ASTM D573 in an air oven, and run moisture as a separate damp-heat cell. A screening matrix that suits accessories is 70 °C for 7 days, 70 °C for 14 days, and 40 °C at high relative humidity for 21 days, with at least five parts per cell plus an unaged control from the same lot. Measure before and after: Shore A at three points per part, tensile strength and elongation at break on a dumb-bell cut from a production part rather than a lab plaque, mass to 0.01 g, dimension at the widest section, colour in CIELAB units, and a peel test on any bonded insert. Photograph each part under one light source with the control beside it. A lab plaque will pass a cycle that a moulded part with a parting line and a thick rib fails, so sampling from production matters more than the oven model.
Pass and fail lines you can write into a purchase order
Acceptance should be numbers the factory can measure and the buyer can audit. Workable lines: hardness shift within five points Shore A of the control; tensile retention of at least four fifths of the control; mass change within 0.3 g on a 500 g part; colour shift within three CIELAB units; no visible bloom after 24 hours of recovery at room temperature; no lifting at print edges under a fingernail check; and no corrosion, staining or movement at the insert when twisted by hand. Record lot code, mould number, cure system and post-cure schedule beside the result. A report without those four fields cannot be extended to another batch, and buyers who request them at the enquiry stage get data they can reuse for a second season. Our capabilities page lists which checks run in house and which go to an external laboratory.
Carton and warehouse measures, and what to send before the first run
Ageing cannot be stopped, but it can be slowed at carton level. Specify a barrier bag with a stated water vapour transmission rate instead of a generic polybag, add desiccant sized to the carton volume and the storage period, and keep the bag sealed until the goods reach the studio. Keep silicone away from PVC and freshly printed film, because plasticiser migration marks the surface permanently. In the warehouse, place pallets away from skylights, roller doors and container walls, leave a gap for air movement, and ask the logistics provider for the recorded temperature range of the aisle where stock will sit. For sea freight, request a moisture barrier liner and a loading photo set. If a range is seasonal and held for more than one season, repeat the 14-day cell on the first batch each year against the first approved control. Before the first run, send four things: a spec sheet with hardness, cure system and post-cure schedule; the storage climate and months of cover you expect; the packaging format; and the claim you intend to publish. The matrix is then matched to the claim rather than the reverse. The weights this protocol covers are on our silicone weights page.
Frequently asked questions
How long should a heat and humidity ageing test run before bulk production?
Plan for a 14-day oven cell at 70 °C plus a 21-day damp-heat cell, with five parts per cell and an unaged control from the same lot. That fits inside a normal sampling window if the mould is already signed, but it adds roughly five weeks, so start the run in parallel with artwork and packaging approval rather than after it.
Can an ageing report be reused for a new colour or a new mould?
Only partly. Pigment changes affect surface appearance and sometimes cure, and a new mould changes wall thickness and bond geometry, so both need their own parts in the oven. The test conditions, sample size and acceptance lines can be carried over, and that is usually the larger part of the work.
What storage conditions should we write into a container shipment?
Name the barrier bag specification, the desiccant quantity, the container liner, the stowage position away from the roof and doors, and the maximum recorded temperature you will accept from the forwarder. Ask for a loading photo set and the temperature log at destination, so a future claim can be traced to a leg of the journey rather than blamed on the material.