Offset Ink Pigment & Resin Selection: Tack, Viscosity & Colour Strength Data

Overview #

Offset ink formulation is one of the most consequential decisions in a packaging print run — and one of the least visible to brand partners until something goes wrong. The wrong pigment-resin combination shows up as ink set-off on the delivery pile, colour shift between press sheets, or a varnish that won’t bond cleanly over a high-tack ink film. We specify offset inks across our sheet-fed and web offset lines for folding cartons, rigid box wraps, and premium label stock, and the selection criteria we apply — tack range, viscosity at press temperature, pigment loading, and resin compatibility — directly determine whether a job holds colour across a 50,000-sheet run. This article lays out the full technical framework we use internally, so brand partners understand why we ask the questions we do during the pre-press briefing.

Pigment Selection: Colour Strength, Lightfastness and Substrate Interaction #

Pigment choice drives three production outcomes simultaneously: colour strength (which determines ink film thickness and therefore dry time), lightfastness (critical for retail shelf life), and substrate interaction (particularly relevant on coated SBS board and cast-coated papers).

We work primarily with organic pigments for process colours — Pigment Yellow 13 and 83 for yellows, Pigment Red 57:1 for warm magentas, Pigment Blue 15:3 for cyan — and carbon black (Pigment Black 7) for key. For brand spot colours, we frequently specify Pigment Red 254 (DPP-based) and Pigment Violet 23 where high chroma and lightfastness above Blue Wool Scale 6 are required by the brand brief.

Pigment loading in a standard process ink runs 12–18% by weight for organic pigments and 8–12% for carbon black. Higher loading increases colour strength but raises viscosity and can compromise ink transfer on fine halftone screens below 10% dot. On our Heidelberg XL 106 sheet-fed line, we target an ink film thickness of 0.8–1.2 µm for process colours on 350 gsm SBS board — above 1.5 µm we see mottle on solid coverage areas, particularly in cyan.

Lightfastness is tested against ISO 105-B02 (Blue Wool Scale), and for any packaging destined for outdoor retail or extended shelf display, we require a minimum rating of 6 on that scale. Pigment Yellow 13, while cost-effective, rates only 4–5 and is unsuitable for packaging with more than 6 months of direct light exposure. We flag this to brand partners during the ink specification review.

Resin Systems: Tack, Viscosity and Press Compatibility #

The resin system determines how the ink behaves on press — specifically its tack (resistance to film splitting), viscosity (flow under shear), and how quickly it sets on the substrate. For sheet-fed offset on coated board, we use alkyd-modified rosin ester resins as the primary binder, with mineral oil and vegetable oil (typically soy or linseed) as the vehicle system.

Tack is measured on a Tack-O-Scope at 30°C and 400 rpm. Our standard process ink sequence runs:

• Black (K): tack 14–16 units
• Cyan (C): tack 12–14 units
• Magenta (M): tack 10–12 units
• Yellow (Y): tack 8–10 units

This descending tack sequence is critical in wet-on-wet printing. If the second-down ink has higher tack than the first, it pulls the previously printed film off the substrate — a trapping failure that shows up as colour contamination in the shadow areas. We verify tack sequence on every new ink batch before it goes to press.

Viscosity at press temperature (typically 28–32°C on our press room floor) is measured with a rotational viscometer. We target 40–60 Pa·s for sheet-fed process inks. Below 40 Pa·s, the ink becomes too fluid and we see dot gain above our acceptable threshold of 15% at 50% screen tone (measured per ISO 12647-2). Above 60 Pa·s, ink feed becomes inconsistent on the ductor roller and we see starvation streaks in large solid areas.

For UV-curable offset inks — which we use on our UV coating line for premium folding cartons — the resin system shifts to epoxy acrylate or urethane acrylate oligomers. These require a minimum UV cure energy of 120–160 mJ/cm² (measured with a UV radiometer) to achieve full cross-linking. Under-cured UV ink is the most common cause of adhesion failure when a subsequent aqueous flood coat is applied.

Ink System Comparison: Sheet-Fed Offset, UV Offset and Waterborne Flexo #

For brand partners evaluating print process options across their packaging range, the table below summarises the key formulation and performance parameters we work with across the three most common ink systems in our facility.

The UV offset column is particularly relevant for brand partners specifying metallic foil laminate wraps or high-gloss folding cartons — conventional oxidative inks do not cure reliably on non-absorbent substrates, and we will always recommend UV offset or UV flexo in those cases.

Compliance and Food-Contact Ink Specification #

For any packaging that contacts food directly or indirectly (including outer cartons for food products), ink formulation compliance is non-negotiable. The key frameworks we work within are:

• EU Regulation 10/2011 (plastic food contact materials — relevant for laminated structures where the ink layer is adjacent to a food-contact film)
• FDA 21 CFR 176.170 (components of paper and paperboard in contact with aqueous and fatty foods)
• Swiss Ordinance SR 817.023.21 (the most comprehensive positive list for printing inks in food packaging, widely adopted as a global benchmark)
• REACH Regulation (EC) No 1907/2006 — we require full SDS documentation from all ink suppliers confirming SVHC (Substances of Very High Concern) content below 0.1% w/w

For indirect food contact packaging (e.g. a folding carton around a sealed inner pouch), we specify low-migration UV inks with photoinitiator packages that comply with the Swiss Ordinance positive list. Migration testing per EN 1186 or equivalent is available on request for new product launches where the brand’s regulatory team requires documented evidence.

Mineral oil contamination (MOSH/MOAH) from conventional offset inks migrating through paperboard into food is an active regulatory concern in the EU. We have transitioned our food-adjacent carton lines to mineral-oil-free ink formulations and use functional barriers (PE or PET laminate inner liners) where migration risk cannot be eliminated by ink selection alone.

Specification Notes for Brand Partners #

When you brief us on a new folding carton or rigid box wrap project, the ink specification conversation starts with three questions: What is the substrate? What is the end-use environment (retail shelf, e-commerce, food-adjacent)? And do you have a Pantone reference or an existing approved colour standard?

The most common brief mistake we see is brands supplying only a Pantone number without specifying whether the match is required on coated or uncoated stock. Pantone 485 C and 485 U are visually very different on press — the coated version is significantly more saturated. We always confirm the substrate and viewing condition before we mix or approve a spot colour.

Our standard sampling process: digital colour proof (G7-calibrated, ISO 12647-7 compliant) in 3–5 working days; press-pass physical sample on production substrate in 10–15 working days; production lead time 20–30 working days after colour approval sign-off. For UV ink jobs on foil laminate, we add 3–5 working days for ink adhesion and migration testing before we release to full production.

Frequently Asked Questions #

Q1: What tack range do you specify for the yellow ink in a 4-colour process sequence, and why does it matter?
A: We specify tack 8–10 units for yellow, which runs last in our KCMY sequence. Yellow must have the lowest tack in the sequence — if it were higher than the magenta already on the sheet, it would pull back the wet ink film and cause trapping failure in the shadow areas. Getting this sequence right is one of the first things we verify when qualifying a new ink batch.

Q2: What is your minimum order quantity and lead time for a folding carton job using UV offset inks?
A: Our standard MOQ for UV offset folding cartons is 5,000 units, with a production lead time of 23–35 working days depending on structural complexity and the number of spot colours. UV jobs require an additional 3–5 working days for ink adhesion and cure verification before we release to production, which is built into that timeline.

Q3: How do you ensure your inks comply with food packaging regulations for EU market products?
A: For EU food-adjacent packaging, we specify low-migration UV inks with photoinitiator packages listed under the Swiss Ordinance SR 817.023.21 positive list, and we require full REACH compliance documentation confirming SVHC content below 0.1% w/w. Where migration risk cannot be controlled by ink selection alone, we specify a functional barrier laminate (PE or PET inner liner) and can provide EN 1186 migration test reports on request.

Q4: Can you match a Pantone spot colour to within a visible tolerance on coated SBS board?
A: Yes — on our sheet-fed offset line, we target a Delta E (CIE 2000) of ≤1.5 for spot colour matches on coated SBS, measured against a G7-calibrated press proof under D50 illumination per ISO 3664. For brand colours where the tolerance is tighter than ΔE 1.5, we recommend a press-pass approval on production substrate before committing to full run quantities.

Q5: What causes ink set-off on the delivery pile, and how do you prevent it?
A: Set-off is almost always caused by ink film thickness above 1.5 µm on coated board combined with insufficient anti-set-off powder or a delivery pile that builds too fast. On our Heidelberg XL 106, we run anti-set-off powder at 0.3–0.5 g/m² and limit pile height to 800 sheets between jogger cycles on heavy coverage jobs. If we see set-off during makeready, we reduce ink feed before adjusting powder — excess powder causes adhesion problems for any subsequent varnish or lamination.

Planning a packaging project? Contact our team to request a complimentary specification review and sample quote.