Products
UV and LED Letterpress Inks
Relief-plate inks for narrow web label presses and for tube decoration, cured by LED or mercury UV at the thin film letterpress lays down.
Ink range
A hard dot, a thin film, an instant cure
Letterpress prints from a raised image, and it has not gone away. Two places keep it working. The first is narrow web self-adhesive label printing, where a large installed base of rotary letterpress presses runs across Asia. The second is the decoration of lamitubes and other cylindrical or seamed bodies, where the same raised plate inks a blanket in the arrangement the tube industry calls dry offset, or letterset. In both, the image stands proud on a hard photopolymer plate, a roller train inks the relief, and the plate meets the substrate at kiss impression. There is no anilox metering the film and no dampening solution to balance.
What follows from that geometry is a thin ink film and a hard dot. Small text stays sharp, hairlines stay open, and work that UV flexo struggles to hold — four-point pharmaceutical dosage text, guilloche, fine reversed type, the clean end of a vignette — prints at the small formats that labels and narrow web work is laid out in. UV or LED cure fixes the film as it leaves the station, so the next colour, the die cutter, the laminator or the seaming and heading tooling can follow at once. The trade is coverage: at this film weight a dense solid or an opaque white on film usually wants a heavier lay, a second hit or a screen station.
That sets the specification for the ink. It has to transfer cleanly from a hard relief plate at low film weight, with enough body to sit on the dot and enough tack release to split at press speed. It has to hold the dot, neither gaining at the edge under over-impression nor filling in the shadows where the plate’s relief depth runs out. It has to cure fully in a film of a few microns, where the ratio of surface to volume is high and oxygen inhibition bites hardest. And it has to adhere to whatever is underneath: coated label paper on one job, PP or PET film on the next, the polyethylene surface of an ABL or PBL tube after that.
The range is built around Unipress 30 LED, a dual-cure letterpress ink for narrow web labels and tube decoration: it is formulated to cure both under medium-pressure mercury lamps and under LED at 385–405 nm, so a house converting its fleet a press at a time carries one ink code rather than two. The name marks the LED window the initiator package is built to reach; it does not mean the series is LED-only. The cure window for each lamp type — peak irradiance and dose — is to be confirmed, and it should be established on your own press and lamp rather than carried across from another site. All grades are almost 100 % solids: no solvent, no drying tunnel, and nothing to evaporate at the duct. TPO is excluded throughout, having been reclassified as reprotoxic category 1B under CLP by the 19th ATP, applying from September 2025, and sitting on the EuPIA exclusion list. Low-migration versions for food, pharmaceutical and personal-care packaging are built on photoinitiators chosen for low mobility and carry no ITX and no benzophenone. Every grade ships with a Technical Data Sheet, a Safety Data Sheet and a Regulatory Summary.
The range
One flagship grade and the sub-ranges built around it. Colour lists and grade codes are issued with the technical documentation.
Unipress 30 LED — UNIPRESS-30 series
The flagship. A dual-cure letterpress ink for narrow-web labels and for tube decoration, formulated to transfer from a hard photopolymer relief at low film weight and to cure both at 385–405 nm under LED and under medium-pressure mercury lamps, so a house converting its fleet a press at a time carries one shade standard and one stock code. Cure window: under mercury at 120–200 W/cm, 80–140 mJ/cm² UV-A at the substrate for process shades and 150–250 mJ/cm² for opaque white and dense black; under LED at 8–16 W/cm² peak irradiance at the substrate plane, 250–450 mJ/cm² in band for process shades and 400–700 mJ/cm² for opaque white. Process shades UNIPRESS-30PY / 30PM / 30PC / 30PK, dense black 30DK, mixing white 30MW, extender 30EX, stiffening and reducing pastes 30SP and 30RP. TPO-free. The name marks the LED window the initiator package is built to reach; it does not mean the series is LED-only.
Low-migration builds — the -LM suffix, e.g. UNIPRESS-30PC-LM
For food-adjacent, pharmaceutical and personal-care packaging. Raw materials selected under the EuPIA Charter; photoinitiators chosen for low mobility — high-molecular-weight and multifunctional acylphosphine oxides with polymeric amine synergists — and no ITX and no benzophenone. Runs in the same cure window as the rest of the series, and full cure at the stated dose is part of the specification, because the species that migrate are unreacted monomer and free photoinitiator. Migration assessed on the finished pack, on request.
Opaque white and dense black — UNIPRESS-30OW, 30OWH and 30DK
Higher pigment loadings for film labels and tube bodies, where one thin letterpress film will not cover. The white runs at 2.0–2.5 g/m² per pass for a contrast ratio of 0.70–0.80 — a ground, not a full hide — with a pin-cured double hit at 4.0–5.0 g/m² giving 0.85–0.92, or the high-hide grade 30OWH, or a screen station; re-check cure at the increased film weight. White scatters the UV and rutile absorbs below its band edge near 405–410 nm, and black absorbs it, so these two set the lamp requirement for the whole job — 150–250 mJ/cm² UV-A under mercury and 400–700 mJ/cm² in band under LED, at 395 or 405 nm in preference to 385 nm. Set the lamp for the hardest colour first.
Dry offset work for tubes — the same UNIPRESS-30 shades
In dry offset every colour traps wet on wet on a common blanket before a single transfer and a single cure, so the set has to run in a descending tack sequence and the colour order is fixed before the job is laid out. Bench tack 11 ± 2 as supplied for a single ink (IGT-type tack meter, 400 rpm, 30 °C, 1.3 cm³ charge, read at 60 s); the ladder is a separate specification, graduated 12 → 8 across the set with the opaque white first down at 12 ± 1. Sequence and tack graduation are set with the job.
Spot and brand shades — UNIPRESS-30MB01 to 30MB12 mixing bases
Blended from the twelve-base house mixing system on heavy-metal-free pigments, with mixing white UNIPRESS-30MW for tinting and transparent extender 30EX for strength reduction. A brand shade is judged as it prints, on the customer’s own substrate and cure, rather than as it looks in the can: shades are blended and verified in the laboratory at ΔE*ab ≤ 2.0 as the agreed match tolerance, CIELAB D50/2°, ISO 13655 M1, at the agreed film weight, and corrected there rather than on press, where an addition dilutes the photoinitiator package in proportion.
Matched overprint coatings — UNIPRESS-30OG gloss and 30OM matt
In-line UV and LED overprint coatings in the same series, applied at 1.5–3.0 g/m², to finish the surface, raise rub resistance before die cutting, matrix stripping, seaming or capping, and carry the top of the ink film past oxygen inhibition. Gloss read at 60°, ISO 2813, on the printed solid: typically 60–75 GU on coated label stock unvarnished and 82–92 GU under UNIPRESS-30OG, with 30OM at 10–20 GU. They carry the cap-torque, scuff and product-resistance duty that the thin ink film on its own is not asked to provide; those results are typical on the stated construction and are qualified on the pack.
Product data
| Process | Rotary and flat-bed letterpress, narrow web, and dry offset (letterset) for tubes and other cylindrical or seamed bodies. |
|---|---|
| Cure | LED at 385–405 nm and medium-pressure mercury, on the dual-cure grades. Almost 100 % solids free-radical acrylate: no solvent, no water, no drying tunnel and nothing to evaporate at the duct. |
| Plate | Hard photopolymer relief, Shore D 62–72 (ISO 868), inked by a roller train at kiss impression. Narrow-web label plates 0.73–0.95 mm; dry offset tube plates 0.95–1.14 mm, which is a different machine rather than a different specification. No anilox metering the film and no dampening system. |
| Film weight | Thin by class: 1.0–1.5 g/m² at solid, about 1.0–1.4 µm — the same region as UV offset and below UV flexo, determined by weighing a cut area of printed against unprinted stock. Opaque white runs heavier at 2.0–2.5 g/m² per pass, or 4.0–5.0 g/m² as a pin-cured double hit; dense black runs at the top of the standard window. |
| Substrates | Coated and uncoated label papers, PE, PP and PET films, ABL and PBL tube laminate and extruded bodies. Films and PE surfaces require corona, flame or plasma treatment, typically 38–42 mN/m measured at the press with test inks rather than taken from the film certificate. Adhesion is qualified per substrate by cross-cut and tape 24 hours after the run: typically classification 0 with no removal on ABL and PBL laminate treated to ≥ 38 mN/m, and fibre tear on coated label stock. Treatment level, ageing and slip additive differ between laminate suppliers and between reels, so the result is confirmed on the customer’s own material. |
| Indicative cure energy | Mercury: medium-pressure lamp rated 120–200 W/cm (about 300–500 W/inch) at a peak UV-A irradiance of 0.6–1.2 W/cm², delivering a typical 80–140 mJ/cm² UV-A at the substrate for process shades and 150–250 mJ/cm² for opaque white and dense black. LED: 385–405 nm array at a peak irradiance of 8–16 W/cm² measured at the substrate plane and not at the emitting window, head gap 2–5 mm, delivering a typical 250–450 mJ/cm² in band for process shades and 400–700 mJ/cm² for opaque white. The LED figures are three to four times the mercury figures, and that is the right way round: at a 1.0–1.5 g/m² letterpress film the limit is oxygen inhibition at the last-curing surface rather than depth, and a 385–405 nm array carries no short-wave output with which to beat it. Dose is measured with a radiometer carried through the press at running speed and read on the heaviest, darkest area, with the head calibrated for the band the source actually emits — a UV-A 320–390 nm head at a mercury lamp, a 385–405 nm LED-band head at an LED station, since a UV-A head is calibrated below most of a 395 or 405 nm array and under-reports it. These figures are indicative for the class: establish the working window on your own press and lamp, record it at set-up, and re-check it as lamp hours accumulate. |
| Low-migration option | Available across the range under the -LM suffix, for example UNIPRESS-30PC-LM and UNIPRESS-30OW-LM. TPO excluded throughout — reprotoxic category 1B under CLP by the 19th ATP, applying from September 2025, and on the EuPIA exclusion list — with ITX and benzophenone excluded from the low-migration builds, high-molecular-weight and multifunctional acylphosphine oxides with polymeric amine synergists, heavy-metal-free pigments and raw materials selected under the EuPIA Charter. No migration figure is quoted for the ink on its own; migration is assessed on the finished pack, on request. |
| Pack sizes | 1 kg and 2.5 kg tubs and 5 kg pails for process shades; opaque white and mixing bases also in 10 kg pails; auxiliaries in 1 kg tubs. |
| Shelf life and storage | 12 months from the date of manufacture in unopened original containers at 15–25 °C, and 9 months on the opaque white, which is shorter because rutile settles. Store closed and in the dark, away from daylight, stray UV, heat and ignition sources; do not exceed 30 °C and do not store below 5 °C. Shelf life on a 100 % solids acrylate is limited by slow thermal and light-initiated polymerisation rather than by evaporation, so light and heat are what shorten it. Not frost-sensitive: bring to room temperature and stir back before use, stirring the white through the full depth of the container. Keep part-used ink in a closed opaque container and use it first rather than returning it to the original pack. |
| Documentation | Technical Data Sheet, Safety Data Sheet and Regulatory Summary with every grade. |
Compliance and standards
- Raw materials selected to the EuPIA Charter
- TPO-free
- ITX- and benzophenone-free in low-migration grades
- Heavy-metal-free pigments
- Low-migration grade available
- Migration assessment on request
- REACH-registered raw materials
White paper
Oxygen inhibition at low film weight
Every UV and LED ink competes with atmospheric oxygen. Radicals released by the photoinitiator are scavenged by dissolved and in-diffusing oxygen to give peroxy radicals, which terminate rather than propagate the acrylate chain. The effect scales with the ratio of surface to volume, so it is worst in exactly the films letterpress lays down: a few microns, indicatively 1.0–1.5 g/m². The inhibited layer is only a fraction of a micron deep, but it is the fraction the converter rubs, rewinds and stacks.
Where the problem sits depends on the lamp. At 385–405 nm the output passes through the film and cure builds from the substrate up, so the surface gels last and it gels in air. A medium-pressure mercury lamp carries short-wavelength output that is absorbed in the top of the film, so it cures surface down and closes the surface early; through-cure becomes the risk instead. An ink proven under mercury lamps will not necessarily behave the same way after an LED conversion, and surface cure is the first thing to check, not the last. This is the reason a dual-cure grade still has a cure window to establish for each lamp type rather than one number for both.
We treat it in the formula rather than by asking the press to slow down: photoinitiator packages weighted for surface response without TPO; high-molecular-weight and multifunctional acylphosphine oxides in the low-migration grades, where the mobility of a fragment matters as much as the speed of the cure; amine synergy from polymeric and oligomeric aminated acrylates rather than free amines; and oligomer functionality raised so that gelation arrives at lower conversion. On press, peak irradiance does more for a thin film than a long dose — a shorter, more intense exposure gets radicals ahead of oxygen diffusion. Judge cure by solvent rub and by set-off on the rewound roll, not by feel. For migration-sensitive work a passing rub test is a starting point; the evidence is a migration test on the finished pack.
White paper
Two different jobs: interstation cure on labels, wet-on-wet on tubes
On a narrow web rotary letterpress the stations cure as they go. Each colour is gelled or fully cured before the next lays down, so trapping is dry on dry and the live variables are impression, ink train setting and dot behaviour on the plate. Over-impression is the common fault: press a hard relief plate beyond kiss and ink squashes past the dot edge, which reads as a halo around type and as filled shadows once the relief depth can no longer clear the surplus. The remedy is on the press, but the ink can help by being short — enough body to stand on the dot, enough release to split cleanly at speed.
Tube decoration works the other way round. In dry offset the plates ink up without dampening, each colour transfers to a common blanket, and all colours lie wet on wet on that blanket before one transfer to the body and one cure. Every ink therefore has to trap on an uncured film. That needs a descending tack sequence, commonly a point or two lower at each successive station, and a colour order fixed before the job is laid out. A later colour with the higher tack will lift the earlier film off the blanket and show as mottle or a weak shade. Trapping is judged on the printed body, not on a drawdown.
Adhesion on tubes is a surface question before it is an ink question. ABL and PBL laminates and extruded bodies present a polyethylene surface, which needs flame, corona or plasma treatment in line, typically to 38–42 mN/m. Treatment decays in storage, so a body treated last week is not the body the trial was run on. After decoration the film still has work to do: it passes the seaming and heading tooling, meets the heat of head injection, then has to live with the filled product and the user’s hands. Check adhesion after heading, not straight off the decorator.
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