There are a number of reasons to coat the cover paper of a perfect-bound print book, or the dust jacket of a case bound book, or even a poster, but the primary ones involve appearance and durability. If you want the print book, for instance, to endure heavy use or last a long time (or if you want to protect heavy ink coverage from fingerprinting), consider coating the sheet. Or, if you want to contrast various dull or gloss effects against one another to highlight the printed images, you may also want to add an additional coating.
Here are four options to consider when choosing a protective coating. (Remember that this is in addition to the gloss or dull surface of a coated sheet. Protective coatings go on top of the printed, dried press sheets.)
Press Varnish
The simplest and least expensive paper coating is a varnish. Essentially varnish is ink without its colorant (or the ink vehicle with no pigment). The custom printing supplier adds this coating by using one of the ink units on his press (let’s say a fifth or sixth unit on a six-color press, after the cyan, magenta, yellow, and black inks have been laid down).
In fact, if you’re printing your 4-color job on a six-color press and you’re not using a PMS match color in one of the remaining ink units, you might want to add both a dull and a gloss varnish.
Perhaps you could coat the photos with the gloss varnish to make them stand out, and coat the background with a dull varnish to make it recede. Using both varnishes together would make the contrast more striking, and would cause anything covered with gloss varnish to “jump” off the page.
An alternate use for varnish is to completely coat the press sheet. This is called a flood application, in contrast to a varnish laid down in a limited area, which is called a spot application.
An alternative to a clear coating of varnish is a tinted coat. You may want to use this inside a magazine, for instance, for a subtle, ghost-like image or type treatment that can only barely be seen.
Varnish is the least durable coating, and it may yellow over time, so it’s wise to consider how long your printed product will be in use. It also can darken the inks over which it is printed. And it is not particularly useful when printed on an uncoated sheet, since it will be absorbed into the paper fibers like any other ink, potentially rendering it useless for both protection and any aesthetic effect.
Aqueous Coating
Aqueous coating comes in dull, gloss, and satin (in between dull and gloss). Like varnish, aqueous coating is applied in-line. But unlike varnish, aqueous coating is applied using a separate aqueous coating tower, which immediately follows the four or six press inking units.
Aqueous coating is a water soluble polymer, so it dries to a hard surface. Therefore, it is very durable as well as attractive. However, aqueous coating is more suited to a flood application (over the entire press sheet) than a spot coating.
Not every custom printing vendor has equipment for aqueous coating. If you request this service, your printer may need to subcontract the work, adding to the cost and schedule of the job.
Ultraviolet Coating
UV (ultraviolet) coating “cures” under ultraviolet light. It is more expensive than either varnish or aqueous coating. Unlike aqueous coating, it can be easily applied as either a spot coating or a flood coating. Usually the process is completed off-line (as a separate finishing step), in contrast to the in-line nature of applying varnish or an aqueous coating.
Since UV coating “cures” instantly when exposed to light (rather than drying when exposed to heat), no solvents are necessary and no VOCs (volatile organic compounds) are released into the atmosphere during its application.
As with aqueous coating, not every printer can apply a UV coating. Your printer may need to subcontract this work.
Film Laminates and Liquid Laminates
Even more durable than UV coating is lamination. This comes as a film or as a liquid coating. Since it seals the press sheet completely, you might wind up with book covers that curl. In this case, the uncoated interior of the book cover absorbs moisture (humidity) and expands, while the coated side does not. You can avoid this problem by specifying “lay-flat laminate,” which is permeable and allows air to pass through the polyester coating.
Things to Remember
If you will need to write on a portion of your print job with a ballpoint pen, or if you will need to inkjet information (like addresses) onto the printed press sheet, you will need to leave an unprinted area with no protective coating. Otherwise, the ink (particularly ballpoint pen ink) will smear.
That said, there are always exceptions. I have seen inkjet addressing applied directly over some coatings. Therefore, unless you play it safe and omit the coating over such an area, you will need to discuss this with your printer to make sure his equipment will accommodate your needs.
This entry was posted
on Friday, November 7th, 2014 at 9:09 pm and is filed under Paper and finishing.
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