Coating and Varnishing for Inkjet

By Ralf Schlozer / Published:

Coating and varnishing have a long tradition in print. From a functional view, there are two types of coating: protective and visual effect.

Protective coatings are required to prevent scuffing, rub-off and similar in finishing or transport, for example in inserting lines. In offset printing a protective coat is used as well to allow for a faster finishing when the ink is not sufficiently dried. This is less of an issue in inkjet. Protective coating is applied to the full print area.

Coatings for visual effect can be gloss or matte coating and creates a different visual impact compared to the uncoated print. There can be special effect coatings like drip-off, textured, pearlescent and more. The coating can be applied as spot or flood coating.

There are toner devices that can produce a varnish-like effect by using clear toner. This is not an option for aqueous inkjet. Accordingly, a separate coating/varnishing module would need to be added to the press.

Coating for Continuous-feed Inkjet

Continuous feed inkjet started its success story with transaction print applications. There was no need to varnish for visual effects for this application range – being able to add colour was already a big boon. However, sometimes a protective (silicon) coat was applied to the print to prevent smearing and rub-off in finishing – especially in inserting. The progress in inkjet inks and drying lessened the need for a protective coat somewhat. Also, modern inserting equipment tends to put less strain on the print. A number of sites continue to apply a protective coat to make sure that there are no failures, especially when older inserting equipment is used or to make sure the mailing arrives in an unblemished condition.

The application range of continuous feed inkjet expanded in recent years however, for example into direct mail and commercial print. Here visual effects as made possible by varnishing are very appreciated. Coating can improve the visual and tactile properties of lower cost papers. The adoption rate has been moderate so far, however with only occasional in-line coating units installed. A front-runner is probably HP. In 2020 the company launched an inline coating system for its 2-up high-speed inkjet webs, developed by Harris & Bruno, as part of its drive into the commercial market. The ExcelCoat ZPW has been designed for HP presses and is exclusive to HP. It can switch between aqueous and UV on the fly. Uptake has been good with about 40% of all presses now sold into direct mail equipped with a coater. Installations in commercial are rising as well.

The slow uptake is not due to a lack of coating unit suppliers. There are several providers of in-line coating solutions, such as: Contiweb, Hunkeler , Harris & Bruno, Matti, Takubo and Tresu. In many cases a priming unit can be modified and repurposed as a post-print coating solution.

If there is not sufficient volume to justify an in-line solution, off-line coating remains an option. Possibilities range from dedicated near-line coaters to sheeting the print and varnishing them in an offset press. Obviously, these solutions only make sense for small volumes. High end embellishment devices from MGI, Scodix and Steinemann fall into the off-line range as well

Coating hardware is only part of the equation, however. The coating liquid needs to be selected according to the requirements and adapted to the printer and finishing set-up. While off-the-shelf varnishes might work as well for inkjet in-line coating, they could require a much higher laydown. This increases the cost again and can even negatively impact the print and finishing performance. In the end not the price per litre but the cost of a product produced is important.

Coating for Cut-sheet Inkjet

While the demand for coating in continuous feed inkjet is still somewhat low based on the application range, the applications produced on cut-sheet inkjet devices should be more conducive to having an in-line coater.

Still most cut-sheet inkjet presses do not have an in-line coating option. The Fujifilm Jet Press does not have a coating unit built in, however a bridge to coating equipment is available. Yet coating is mostly done off-line. The most popular solution for Jet Press users is from Harris & Bruno. Some customers will also use the coating unit of their offset press for sealing if required.

Recently Canon and Plockmatic launched an inline integration of Canon’s VarioPrint iX-series digital inkjet sheetfed press with Plockmatic’s DigiCoater Pro 400 HD LED coating unit. This allows for UV flood coated prints at full press speed, broadening the application range of the iX. This integration not only eliminates process steps, it allows for immediate reprints if anything goes wrong.

Upcoming sheet-fed inkjet presses for commercial markets are likely to have inline coating options from the start. Although not confirmed the Ricoh Z75 is likely to have such an option.

Coating in Packaging Presses

While inline coating units on commercial inkjet lines are still uncommon, in folding carton inkjet presses it is a given. A considerable portion of folding cartons is varnished, accordingly all inkjet folding carton presses have a coating unit. The technologies applied in the coating units differ, however. The Landa S10 (with a Komori offset press base) and the Koenig & Bauer VariJET 106 use conventional coating units also deployed in offset presses. Spot coating is possible on these units, although a conventional (flexo) plate as used in analogue presses is required. The proposed Screen/Inca inkjet press for folding carton, the SpeedSet 1060, has an option for an inkjet coater. This press is scheduled for delivery in 2023, however.

Coating can be important in inkjet for corrugated as well. The gloss effect is less of importance however high quality graphics on high value products are getting a protective coat to prevent rub-off before the box reaches the consumer.

Label presses do benefit from varnishing units as well. All hybrid presses – combining a conventional press base and inkjet printing unit – offer varnishing options. Several pure digital inkjet label presses offer in-line coating as well, however coating is not always available.

Moving Forward

Outside of packaging coating/varnishing is slowly gaining traction. Some is due to the application mix with applications like transaction and books requiring a protective coat at best. With the move into commercial print (including book covers) the need for coating will become more pressing as coating on offset presses is fairly widespread. Direct mail is an established application for inkjet and here growth is expected as well. Inkjet reached a point where customer expect more quality from inkjet mailings (with press manufacturers providing the solutions already) and this will encompass coating and other value added enhancements as well.

HP T250 HD with Brilliant Inks and Harris & Bruno UV and aqueous coater

Coating is a good starting point for adding value to print as it can be quite cost effective. HP states an aqueous coating for an A4 page would cost around US$ 0.0012, while for a UV coating it will be around US $0.0045. That is a small premium to make a product stand out and differentiate. It is in line with the trend of demand moving away from commodity print towards high value print.

A challenge will be handling spot coating. In many cases a flood coating is sufficient, however the most striking effects are achieved by spot coating. In conventional presses this is achieved by special spot colour flexo plates, which are changed when the press needs to stop for the printing plate change anyway. In digital print changing a flexo coating plate would cause unwanted interruptions. An offline coater could be a work-around, however this requires extra process steps. In-line digital coating units using dedicated inkjet heads for coating could be a solution. Inkjet spot coaters are available today however they need to reach running costs, process speeds and better laydown quality to match today’s high-quality inkjet presses. Given the constant improvements in inkjet technology I would expect to see theses solutions coming up in the future.

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About the Author

Ralf Schlozer

Ralf Schlozer is Independent Print Analyst. Ralf provides analysis, sizing and forecasting the market for digital printing technologies and associated applications and business processes. Connect with Ralf on LinkedIn

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