Digital Print Primers and Coatings

A digital press forms the image without a plate, without an anilox and without an ink train. The consequence is that the substrate carries much more of the burden. In conventional printing, the ink film is metered, tacky and worked onto the surface. In digital printing the imaging fluid or the toner particle arrives on its own terms, and whether it wets, keys, spreads or dries is decided largely by the top few micrometres of the stock. The Arcprime range supplies those few micrometres.

The mechanism differs by process, and the range is built around that difference. In liquid electrophotographic printing, that is liquid toner, the image is developed electrostatically and transferred to a heated intermediate blanket, where most of the carrier fluid is driven off and the particles form a coherent tacky film. That film is then laid onto the substrate by heat and pressure. It sits on the surface rather than soaking into it, so adhesion depends on the surface energy and the surface chemistry of the stock and not on absorption. Synthetic, filmic and metallised substrates almost always need a primer, and many uncoated and coated papers benefit from one. In inkjet, a receptive coating controls how the drop spreads and how quickly the carrier is taken away: too absorbent and the dot feathers along the fibre and the colour sinks into the sheet, too closed and the ink puddles and will not dry, so the coating is engineered for tone value increase and optical density and not simply for adhesion. In dry toner printing the toner is fused by heat and pressure at the fuser nip, so the primer has to hold its properties at fuser surface temperature without softening, blocking or transferring. Over the top, a varnish intended for digital print has to respect an ink film that sits differently from an offset film and that can be sensitive to co-solvents and to aggressive UV chemistry, because a varnish that behaves perfectly over offset can lift or craze a digital image. A white basecoat does a different job again: it gives an opaque ground so that a CMYK digital image reads correctly on a film or a metal surface that is not white.

No other part of the Arctic range supplies a receptive layer for a digital imaging process. There are two near neighbours and Arcprime is distinct from both. The adhesion primers inside Speciality Coatings and Metal Packaging Coatings are made to key conventional inks and laminating adhesives to a difficult surface, not to condition a surface for electrostatic transfer or for drop spread. The conventional overprint varnishes elsewhere in the range are formulated over wet offset, UV offset and flexographic ink films, which behave differently under a wet varnish from a fused toner or a cured inkjet film. Arcprime is specified from the imaging process outwards, and is therefore a new category rather than an extension of an existing one.

Products are described here by process only: liquid electrophotographic, dry toner, UV inkjet and aqueous inkjet. Arctic does not publish press compatibility lists, because the same process behaves differently from one machine and one substrate lot to the next. Primer selection is closed by a trial on the customer’s own press with the customer’s own substrate, and Arctic supplies trial quantities for that purpose. The figures in the table below are the typical operating window for the range, and confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

Primers for liquid toner presses, where the transferred ink film keys to the surface chemistry rather than into the sheet. One grade for paper and board, one for synthetic, filmic and metallised stock.

Receptive coatings that control drop spread, tone value increase and optical density for inkjet. A water-based grade for aqueous and pigment inkjet on paper and board, and a UV grade for UV inkjet on closed and non-absorbent surfaces.

Primers that promote adhesion of thermally fused toner and hold their properties through the fuser nip, without blocking, transferring or embrittling the sheet.

Varnishes formulated to go over digital ink films, chosen for low co-solvent aggression and controlled wetting so that the image is protected rather than lifted or crazed.

Opaque white grounds applied to film and metal so that a CMYK digital image reproduces to the intended colour on a substrate that is not white.

The figures below are the typical operating window for this range, not a measured result for any one batch. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

Where the coating sits in the line

Arcprime primers are applied before the digital press, either inline on a coating unit ahead of the imaging engine or offline on a separate coating machine, in reel or in sheet. Offline priming is the more common route for synthetic and metallised stock, because it allows web tension, corona treatment and drying to be set independently of the press. Arcprime DV overprint varnish is applied after printing, inline on a finishing or converting line or offline on a coater. Arcprime WB white basecoat is applied first of all, and is normally dried or stoved and then cooled fully before the primer or the image goes on top.

Application methods

Water-based grades run on flexographic anilox, gravure, roller coater or air knife. Anilox volume and cell geometry set the coat weight, and a change of anilox is the first thing to check when coverage moves. UV grades run on anilox or roller coater with UV cure at the stated dose. Coat weight is the single most important variable in this category: a primer laid too thin gives patchy adhesion and mottled solids, and one laid too thick closes the surface, slows drying and raises the risk of blocking in the stack or the reel.

Surface preparation

Film and metallised substrates need corona or flame treatment immediately before coating. Treatment decays with time and with storage temperature, so a reel treated at the film plant weeks earlier should be re-treated in line rather than trusted. Measure the wetting tension with test inks before coating, not after a failure. Metal and foil surfaces must be free of rolling oil and drawing lubricant.

Drying, cure and conditioning

Water-based grades need enough hot-air capacity to remove the carrier completely at line speed. Residual moisture in a primer is a common cause of poor toner fusion and of blocking. Dry-toner primers must be cooled to ambient before stacking or rewinding. Primed stock should be conditioned to the pressroom temperature and humidity before printing, because a cold reel brought straight onto a press will pick up condensation, and moisture on the surface changes how the image transfers.

Inkjet specific points

For inkjet, the coating is tuned for two things that pull against each other: control of tone value increase and optical density. The correct setting depends on the drop volume, the drop spacing and the ink chemistry of the process being used, so the target coat weight is established on the customer’s own equipment and then held. Once it is set, changes in anilox, drying profile or substrate lot will move the print result, and all three should be treated as controlled variables.

Overprinting digital ink

A digital ink film sits on the surface differently from an offset film and can be sensitive to co-solvents and to aggressive UV chemistry. Always test the varnish over the actual digital image, not over an offset proof of the same artwork, and allow the printed stock the dwell time it will have in production before varnishing. Lifting and crazing usually appear within minutes, but edge lift on a heavy solid can take a day to show.

Checking adhesion

Cross-cut testing to ISO 2409 is used where the substrate is film or metal and the blade can cut the coating without cutting through the base. On paper and board the blade cuts the sheet itself and the classification means nothing, so adhesion of the image over the primer is judged by a tape pull on the printed area, run the same way each time and against a retained reference.

Press trial

Arctic describes these products by process, that is liquid electrophotographic, dry toner, UV inkjet and aqueous inkjet, and does not publish press compatibility lists. Selection is closed by a trial on the customer’s own press with the customer’s own substrate and artwork. Trial quantities and a coating recommendation are supplied for that purpose, and the trial result is what goes into the specification.

Handling and storage

Store in closed original containers, clear of frost and of direct sun. Water-based grades must not be allowed to freeze. UV grades are kept away from light and from heat sources. Stir before use, do not thin beyond the recommendation on the Technical Data Sheet, and do not return part-used material from a running press to the bulk container. Safety Data Sheets are issued with every delivery and on request.

Digital primer selection cannot be made from a substrate name alone. The answers below are what Arctic needs in order to specify a grade and to design a press trial that will settle the choice.

The process

• Which digital process is used: liquid electrophotographic, that is liquid toner, dry toner, UV inkjet or aqueous inkjet?
• Is the press web-fed or sheet-fed, and at what running speed?
• Is there a coating unit on the press, or will priming be done offline?
• For dry toner, what surface temperature does the stock reach at the fuser, and for how long?

The substrate

• What exactly is the stock: paper, board, film, synthetic paper, metallised paper or film, foil, or metal sheet?
• What is its grammage or gauge, and is the surface coated, uncoated, varnished or already laminated?
• For film, what is the measured wetting tension, and is corona treatment available in line?
• Is the substrate white? If it is not, does the job need an opaque white ground?
• Is the supply single-source, or will several suppliers and lots be used against the same specification?

The print result required

• What optical density and what tone value increase does the job have to hold?
• Is the image full coverage, line and text, or solids over large areas?
• Is a spot or flood white required under the image, and at what contrast ratio?
• What finish is wanted over the print: matt, satin or high gloss?

Downstream processing and end use

• What follows printing: varnishing, lamination, foiling, die-cutting, creasing, gluing or sealing?
• Will the printed surface be laminated or adhesive-bonded, and with which adhesive system?
• What mechanical and chemical resistance does the finished item need: rub, scuff, water, condensation, alcohol, oils or cleaning agents?
• What temperature will the printed item see in filling, sterilisation, storage or transport?
• What is the intended end use, and which regulations must the finished pack satisfy in the market where it is sold? Arctic issues a compliance dossier or declaration on request with the Technical Data Sheet so that the customer can complete that assessment.

Commercial and line constraints

• What drying or curing capacity does the line have, and at what web width and speed?
• Which anilox volumes or coater settings are available?
• What is the annual volume, and in what pack size is delivery required?
• When can a trial be run on the customer’s own press, and who will assess the result?

With these answers Arctic proposes a grade and a coat weight, supplies trial material, and confirms the specification from the trial. Confirmed values are issued with the Technical Data Sheet and Certificate of Analysis by our laboratory.

The Technical Data Sheet, Safety Data Sheet and regulatory declaration for any grade in this range are issued by our laboratory against the substrate and process you are running. Tell us the job and we will specify against it.