TL;DR: The substrate you specify in your artwork brief determines whether your print process is even feasible — file setup that ignores material surface properties causes rework at press, not at prepress.
TL;DR: On uncoated kraft liner below 100 GSM, halftone dot gain can exceed 25%, which means any artwork approved on coated stock will shift visibly in production unless your files are profiled to the correct ICC substrate target.
How Substrate Surface Properties Drive File Setup Decisions #
The conversation about file preparation almost always starts with resolution, bleed, and colour mode. Those matter. But the variable that controls whether your approved artwork actually reproduces correctly is the substrate — specifically its surface energy, absorbency, and coating weight. Get this wrong in the brief, and the file that looks perfect in your design software will print differently on every material variant your packaging uses.
We run offset litho, UV flexo, and digital (HP Indigo) across our production floor, and the file requirements are not interchangeable between them even for the same visual design. The table below covers the combinations we see most frequently from brand partners:
| Substrate Type | Recommended Colour Profile | Minimum Effective Resolution | Max Total Ink Coverage | Typical Dot Gain (Midtone) |
|---|---|---|---|---|
| SBS (C1S/C2S coated, 250–400 GSM) | ISO Coated v2 (FOGRA39) | 300 ppi at final size | 320% | 12–16% |
| Uncoated white board (FBB, 210–350 GSM) | ISO Uncoated (FOGRA47) | 300 ppi at final size | 260% | 18–24% |
| Kraft / recycled brown liner (100–200 GSM) | Custom substrate profile | 300 ppi at final size | 220% | 22–28% |
| Metalized / foil laminate board | ISO Coated v2 or custom | 350 ppi at final size | 280% | 8–12% |
| Corrugated kraft (E/B flute facing) | CRPC6 / custom flexo profile | 150–200 ppi at final size | 180% | 28–40% |
The dot gain figures come from our press characterisation data, measured against ISO 12647-2 targets using 50% nominal patches across 40 production runs. The point isn’t that one substrate is better — it’s that each requires a differently prepared file. Artwork built in FOGRA39 and sent to a kraft liner job will print 15–20% heavier in shadow areas than your proof showed.
Our recommendation: before artwork is finalised, confirm the exact substrate grade with the structural team. A file change at prepress costs nothing. A rerun on a 10,000-unit folding carton job because the brand approved a FOGRA39-profiled file for an uncoated board does not.
Where File-to-Material Mismatches Create Press Problems #
Colour profile mismatches are the most common cause of press-side rework we log under our PF-03 file rejection tracker. They fall into three distinct failure patterns, each with a different root cause.
The first is total ink coverage overrun on absorbent substrates. A brand partner submits a file with rich black backgrounds built as 100C/100M/100Y/100K — 400% TAC. On SBS coated board with a UV varnish topcoat, this is manageable with careful trap settings and extended drying. On uncoated FBB at the same coverage, the sheet won’t dry before the next unit, causing set-off on the reverse side. We see this most often on kraft-look packaging for health and food brands who want deep, saturated backgrounds. The correct TAC for uncoated board is 240–280% maximum, which means the file needs to be rebuilt, not just colour-corrected.
The second failure is resolution collapse on textured or linen-embossed substrates. A 300 ppi image that renders cleanly on a smooth C1S surface can lose fine detail when printed on a board with 12–18 µm surface texture variation, because the ink spreads differently across the raised and recessed areas. For embossed substrates, we specify 350 ppi minimum for photographic elements and ask for vector conversion on any logo or type below 8pt. Rasterised fine type on textured board almost always requires a resubmission.
The third pattern affects metallic and foil-laminate boards specifically. These substrates have near-zero ink absorbency and very high surface energy variability depending on the laminate batch. UV inks cure rather than absorb, which changes how overprint trapping behaves. Files prepared with standard overprint settings for absorptive substrates will show halo effects or colour shift at fine reversal text edges. We catch this during our digital soft-proof stage, but if the file structure isn’t set up with the right overprint flags per PDF/X-4 specification, correcting it at proof adds 3–5 working days to the sampling timeline.
The underlying issue across all three scenarios is the same: file preparation is treated as a graphic design task, when it’s actually a materials-informed production task. The substrate grade has to be locked before the artwork is released to final production files.
Does Colour Mode Matter if You’re Sending a PDF? #
Yes — but not for the reason most briefs state. The question isn’t RGB vs CMYK at the PDF container level. A PDF/X-4 file can carry RGB objects legally under the standard. What matters is whether every colour object in the file has an assigned ICC source profile that maps correctly to the output intent of your substrate.
We’ve received files with 11 different embedded colour profiles across a single 6-panel box layout — each panel built in a different version of Adobe RGB or sRGB. On press, our RIP resolves these against the output intent and the results are unpredictable at boundaries where two differently-profiled objects meet. For brand identity colours, we require all critical Pantone references to be called out as named spot colours with LAB values per Pantone Solid Coated / Uncoated library, not as CMYK approximations, and the CMYK build for process simulation has to be defined per the target substrate profile from the start.
This is the short section where we give a direct answer: specify PDF/X-4, embed all profiles, define output intent to match the substrate, and declare all Pantone references as named spots. That single requirement eliminates roughly 60% of the file queries our prepress team handles.
Specification Notes for Brand Partners #
When you brief us on a packaging project, the information we need before assessing your files isn’t just design-side. We need the confirmed substrate grade and GSM, the print process (offset, flexo, digital), the surface finish specification (matte/gloss laminate, UV spot, aqueous coating), and whether any panels involve foiling, embossing, or die-cut windows.
The most common gap in incoming briefs is an unconfirmed substrate at the point of artwork handover. Brands often finalise the visual design while the structural spec is still in revision — then submit files for prepress review before the board grade is locked. This creates a loop where files have to be re-profiled once the structural decision is made. Avoiding this adds nothing to the schedule; it just means the structural and artwork approvals need to run in parallel, not sequentially.
Our standard prepress review turnaround is 2–3 working days for a complete, well-specified file. If the file requires profile conversion, trap adjustment, or TAC correction, that extends to 5–7 working days depending on complexity. Sampling from approved files runs 10–15 working days for folding cartons and 18–25 working days for rigid boxes.
Frequently Asked Questions #
What colour profile should I use if my packaging uses multiple substrate types in one range?
Build your master artwork to the most restrictive substrate’s profile — typically your uncoated or kraft component — and create substrate-specific output variants for each material. This means one source file with multiple output intents rather than separate design files, which keeps your brand colours consistent across variants while meeting each substrate’s TAC and dot gain limits.
Our designer works in Adobe RGB for screen accuracy. Do we need to rebuild every file?
You don’t need to rebuild from scratch, but every object in the final PDF needs an assigned profile mapped to the correct output intent before handover. A file built in Adobe RGB is workable if it arrives as a properly tagged PDF/X-4 with output intent set to the substrate target. What causes problems is an untagged file or mixed-profile file — the RIP has to guess, and it guesses differently than your designer’s monitor rendered it.
Does dot gain matter for solid Pantone spot colours, or only for process halftones?
It depends on the application. For solid flood Pantone areas above 30% of a panel face, dot gain isn’t the mechanism — but ink spread and surface absorbency still affect the apparent density of the Pantone ink, which shifts perceived colour. On uncoated kraft, a Pantone 485 red will read approximately 8–12 ΔE darker than its coated-stock reference under D50 illuminant. We always run a material-specific Pantone drawdown during the sampling phase before production sign-off.
Can we reuse the same print-ready files across offset and digital runs for reorder flexibility?
Only with substrate-matched reproofing. HP Indigo uses liquid electrophotographic inks with a different gamut and dot structure than UV offset. Files approved for offset will shift on Indigo if reprinted without a press-specific output intent adjustment. Our recommendation is to flag reorder flexibility upfront so we build a dual-output workflow from the start rather than retrofitting it at reorder.
Planning a packaging project? Contact our team to request a complimentary specification review and sample quote.
The custom substrate profile requirement for kraft liner is the hidden cost nobody budgets for — we paid roughly £1,200 to have a proper ICC profile built for our 140 GSM recycled liner, and that was before the press trial time. Worth it when you’re running 60k units of a seasonal gifting box, but painful to justify to procurement mid-project.
The 220% TIC ceiling on kraft liner is real but nobody tells you it compounds with the dot gain problem — we had a seasonal label running a deep navy base plus a warm amber overlay on 120 GSM recycled brown, and by the time both layers hit the uncoated surface the effective ink load was behaving more like 260% regardless of what the file said. Took three press runs and a profile rebuild with our repro house to get the dark tones to stop blocking.
The 220% TAC limit on kraft liner is sensible as a starting point, but we’ve had to drop ours to 190% on anything running through our Bobst M5 flexo line — the slower press speeds we run for pharma serialisation compliance mean longer dwell time in the ink train, and you start getting picking and mottle well before 220% in heavy coverage areas.
Cap total ink at 200% on kraft liner if you’re running any fine reversed text under 8pt — we learned that the hard way on a watch box relaunch where the hallmark copy became illegible at press even though the proof looked clean.
Does the 350 ppi minimum for metalized board hold on HP Indigo specifically, or is that figure driven by the offset/flexo side of your floor — we’ve been specing 300 ppi on foil laminate for Indigo runs and haven’t hit visible issues, but our board weights are sitting at the lower end around 250 GSM.
Watch the FBB column specifically — we’ve had artwork sign off fine on SBS proofs, then shift noticeably on 280 GSM FBB because nobody swapped the colour profile from FOGRA39 to FOGRA47 before sending to press, and the 18–24% dot gain range on FBB hits flesh tones and mid-value brand colours hardest.