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Specular gloss values measured at 60° are useful for ranking products but misleading in absolute terms. A 720 GU mirror foil on a smooth coated board looks brighter than a 680 GU foil only when the substrate is genuinely smooth — once you move to uncoated stock or a textured varnish, surface roughness dominates and the spec gap closes or inverts. Always compare gloss values on a printed sample in the actual substrate and overprint stack you intend to produce.

For brushed or satin effects, do not chase peak gloss. The visual identity of a brushed foil depends on the directional micro-structure of the carrier and the diffusion of the metallised layer, not on the headline gloss number. Two foils with the same 60° reading can look completely different at 20° and 85°.

ASTM D3359 cross-hatch tape-pull is the most cited adhesion test and the right place to start. A 5B classification means no flaking; 4B means up to 5% flaking along the cuts. For prestige cartons and labels that will be handled extensively, target 5B on the worst-case substrate in your run. Do not accept a 3B or lower result for any production application — it will fail in transit.

Adhesion depends as much on the adhesive cure as on the foil itself. A foil with a strong adhesion track record will still fail a tape-pull if the UV lamp is under-dosed or the press is running too cold. Always validate adhesion at the printed speed and temperature you intend to run, not at a low-speed setup pass.

The maximum press speed quoted on each datasheet is the verified upper bound at which the foil delivers full transfer with no pinholing on a reference substrate. In practice converters run 60–80% of that ceiling for headroom. If your press qualification is borderline at 80% of spec, drop to 60% rather than relax the foil specification — the cost of a recall vastly exceeds the throughput lost.

Holographic and rainbow foils carry tighter speed windows than solid metallics because the micro-embossed surface is more sensitive to nip pressure and UV dose variation. If your press cannot hold pressure and dose within 5% across the web width, holographic foil is not the right specification for that line.

The over-printable column tells you whether a foil is broadly compatible with downstream inks and varnishes. The real questions are about specific systems: which ink chemistry (UV, EB, water-based, conventional offset), which varnish (UV gloss, soft-touch, matte OPV) and at what coverage. Tinting a base foil with a transparent overprint to hit a brand Pantone metallic is the most common use case; soft-touch varnish over the entire decoration is the most demanding because it changes the apparent colour temperature of the foil.

When in doubt, ask for a printed drawdown on the exact ink-foil-varnish stack you intend to produce, on the substrate you intend to produce on. We supply these drawdowns as part of any sample qualification.

Cold foil is sensitive to temperature, humidity and UV exposure during storage. The shelf-life value in the table assumes storage at 18–22°C and 45–55% relative humidity, away from direct sunlight, in the original sealed packaging. Storage outside that envelope shortens usable life, and after expiry the release layer can either tighten (causing pinholes) or loosen (causing dust and edge break-up).

FIFO inventory management is the single biggest lever a converter has to avoid foil-related defects. Tag every master roll with its receipt date, rotate stock strictly first-in-first-out, and quarantine any roll older than the datasheet shelf life for a re-qualification check before production use.