Three factories, one finished set: the silicone shell from one plant, the cast iron core from a foundry two provinces away, and the pad printing from a shop that also decorates bottle caps. The first shipment matched the golden sample. The second arrived with a wall 0.3 mm thicker, a grey that reads warm under a D50 lamp, and a logo sitting 1.8 mm lower than the approved artwork. No single part is defective. The set simply stops feeling like one product, and your buyer notices before your inspection report does.
Consistency fails at the interface, not inside each factory
Each supplier can hold its own drawing and still deliver an assembly that does not fit, because the tolerance stack at the interface belongs to nobody. Write the stack into the purchase specification before you split the order: core outside diameter 34.80 mm plus or minus 0.10, shell inside diameter 34.55 mm plus or minus 0.08, wall 2.6 mm plus or minus 0.15, which yields an interference fit between 0.00 and 0.35 mm. Measure at three heights and two axes, four hours after demoulding, at 20 to 25 degrees C on a granite plate. Then name the owner of the stack, usually the assembly plant, and give it the right to reject a core lot or a shell lot against those interface limits rather than against each supplier's own drawing. A quality management system only helps if the limits reach the contract: certification across three plants shows process discipline, not that the three processes talk to each other. See the ISO 9001 standard page for what such a system does and does not cover.
Colour and hardness drift by material lot, not by operator mood
Hardness read on a finished shell comes out 3 to 5 Shore A points above the value on a 6 mm plaque, because the geometry constrains the indentor. Decide which value is contractual before the first batch, then keep one reference plaque per colour under the same conditions as the production floor. Ask for a pigment lot number on every delivery and a cure window of plus or minus 5 degrees C and plus or minus 3 minutes; silicone that is under-cured feels softer on day one and harder after a week, which is exactly how a same-specification complaint starts. For colour, measure on a spectrophotometer at D65/10 degrees against a master plaque no older than 12 months, with a limit of 1.5 Delta E for shell-to-shell comparison and 2.0 for shell-to-print. Above that, the difference is visible in a studio with mixed lighting even when every part passes its own check.
First article approval that survives a handover between plants
An approval signed by one factory is worth little at the second. Build a single pack: five parts per cavity, a full dimensional report, a five-point hardness map, print position measured to a stated datum within 0.5 mm, cross-hatch adhesion on the print, press-fit force between 180 and 260 N, piece weight, and a 1.2 m drop on three faces. Photograph the pack under fixed lighting and store the images with the golden sample. When you add a cavity or move production, the pack is re-run in full rather than partially; partial re-approval is how a new cavity enters the chain with a different wall thickness and nobody notices until pallet three. The same discipline applies to the weights themselves, which is why our custom silicone weight programme starts with an interface drawing rather than a sample photo.
Incoming inspection rules that catch drift before assembly
Sample lot by lot following ISO 2859-1, with critical, major and minor classes written on the drawing. Place hardness outside the band, core weight, print misregistration beyond 1.5 mm and interference outside the agreed stack in the critical class with zero acceptance, and leave cosmetic issues in the major class. Inspect at the receiving plant rather than at the supplier, and keep a three-lot trend chart on shell weight and press-fit force: a drift of 0.1 mm per lot is invisible in a single report and obvious on a trend. The sampling logic behind those classes is set out in this guide to the ISO 2859 sampling plans. Quarantine any failing lot, retain samples for 12 months and require a written disposition before release to the line.
Change management is the only document that keeps three plants aligned
Most consistency failures arrive as unannounced improvements: a new pigment that is the same, a repaired mould, a different sub-supplier for the valve or the insert. Require a product change notification 30 days before implementation, covering resin and pigment lots, cavity and mould repairs, plant relocation and sub-tier changes, with a re-run of the full approval pack and a revised drawing number. Mark the revision on the cavity and on the carton, and use one traceability code that links shell lot, core lot and print lot, so a complaint can be traced to a single combination within one working day. Deviations are acceptable when they are written and time limited; they are fatal when they are discovered in the warehouse.
Before the next purchase order leaves your desk, confirm six points: the interface stack with tolerances, the hardness and colour reference method, one approval pack accepted by all three plants, the defect classes for receiving inspection, a named owner for change notification, and a traceability code format. Send us your current set and we will map it against those six points before you commit to a production slot.
Frequently asked questions
Can one factory produce the whole silicone set?
Only when that plant controls core sourcing and printing in house. Otherwise the assembly plant must own the interface tolerances and hold a single approval pack signed against all three processes.
How do we keep colour and hardness stable between batches?
Fix the measurement method first: hardness on an agreed plaque or on the part, colour at D65/10 degrees against a master plaque under 12 months old, with limits of 1.5 and 2.0 Delta E, and record the pigment lot and cure window on every delivery.
What must a change notification cover?
Resin and pigment lots, cavity and mould repairs, plant relocation and sub-tier supplier changes, announced 30 days ahead, followed by a full re-approval and a new drawing revision marked on the part and the carton.