Impression trays: the stock tray flexes, and that flexure ends up in the model

The tray is treated as an accessory, whereas it is the support on which the material either deforms or stays stable. An excellent material in a tray that flexes produces an inaccurate impression.

Flexure occurs at removal, when force is applied to overcome undercuts. A thin plastic tray opens by a few tenths of a millimetre, and the material follows that deformation.

The result is an impression wider than reality, uniformly so across broad arches. The resulting crown comes out wide and does not seat, and the fault is attributed to the material rather than the support.

Metal trays are rigid by construction and remain the reference for fixed prosthodontics. Reinforced plastic trays with ribbing work well for limited sectors, less so across full arches.

The custom tray solves a different problem: material thickness. A stock tray leaves uneven thicknesses, thin where the arch is broad and generous where it is narrow.

Impression material contracts in proportion to its own thickness, so uneven thickness produces uneven contraction. A custom tray maintains constant thickness, and contraction becomes uniform and predictable.

The space to leave depends on the material. Addition silicones work well with 2 to 3 millimetres; polyethers, being stiffer, require less, because excessive thickness makes removal difficult.

Adhesive is the component applied in haste that determines whether the impression stays bonded to the tray. Material that detaches during removal produces a distortion no visual check reveals.

Adhesive is specific to the material family: silicone adhesive does not work on polyethers and vice versa. It must be spread over the whole inner surface and the borders, and left to dry for the stated time — usually several minutes.

Applying it and loading the material immediately amounts to not having applied it: the solvent has not evaporated and no bond forms.

Perforations are a mechanical alternative to adhesive, suited to alginates but not to fixed-prosthodontic materials. Material extruding through the holes forms retentions, but retention is punctate and does not prevent detachment between holes.

For functional impressions in complete denture work the custom tray is not an option but a requirement. Border moulding needs a support reaching close to the vestibular depth without exceeding it, and no stock tray does that.

In summary: rigidity so as not to deform at removal, uniform thickness for predictable contraction, and material-specific adhesive spread and allowed to dry. Three conditions worth more than the choice between one silicone and another.