Foil stamping is one of the most impressive, eye-catching finishing techniques available today. Designers have a large number of foils and stamping options at their fingertips and the process has grown in popularity over the years. One process, referred to as overprinting foil, however, remains somewhat untouched and can add value to a finishing job and provide customers with expanded printing options.

Overprinting foil is just that—the process of foil stamping a substrate and then printing over it. For the purposes of this article, I will categorize this technique into two categories.

First, it can be used to incorporate the striking effects of foil with printed matter that contains very fine or detailed lines. This can be done in a one-color or a four-color process. We once did a job that was originally to be a two-color foil stamp with a gold background and the image of a black building over it. The image was very detailed and simply did not look sharp enough when we ran the sample. When we suggested printing the image over the foil, the end result was a rich, high-quality piece. Another example of this type of work was for a customer that wanted to print a four-color process on a black sheet of paper for an annual report. Rather than printing it on a white sheet of paper and adhering it to the substrate, they asked us to stamp a solid white area on the black paper. They then used that foil as their background and printed the colors over it. The result was stunning.

The second, more common, category for overprinting foil involves printing over foil to create an effect. This technique is often used on items such as Christmas cards and other specialty pieces where refraction dies (dies that contain very fine patterns and refract light) are used on metallic backgrounds and then a screen is printed over it. When the final product is moved under the light, the result is a blurry effect because the light is refracted in many directions rather than reflected back to the eye and the background appears to be moving.

There are several issues to consider when planning to overprint foil. First and foremost, plan ahead! I realize I say this frequently, but for these jobs in particular, it is imperative you communicate with and, if necessary, meet with your finisher before the job is run. Both the ink and the foil manufacturers must be contacted to ensure the two elements are compatible. Not all inks are appropriate for overprinting and not all foils will receive the ink. Once the inks and foils are chosen, the finisher needs to run a sample of the foil and send it to the printer for testing. This is one of the few times we at the postpress end have the luxury of receiving the job prior to its completion…it’s great!

If the appropriate foil is not used, problems can occur. First, your finisher should be careful to choose a foil with the appropriate amount of release. Tight release foils are used for very fine areas to achieve a clean sharp look (such as the examples I mentioned early in this article); heavy release foils are used for large areas; and medium release foils are for anything in between. Selecting the wrong release can result in pick-off (where the foil comes off in small pieces either during the stamping or printing process), flaking, and/or an excess of foil around the edge of the stamp.

Selecting the appropriate ink is also critical in overprinting foil. We consulted API Foils, Inc. based out of Lawrence, Kansas, one of the leading foil suppliers, for tips on ink selection. They told us several problems can occur in this process: pick-off during the offset printing process, poor drying, easy smearing, and very lengthy drying time for certain inks. To avoid these problems, they offer the following advice:

Use ordinary transparent and semitransparent litho inks as long as they are quick-drying solvent or waterbased inks and they dry by evaporation or oxidation, not absorption. (These inks are often used on metallic paper and board.)
Thin out the ink to speed up the drying process and ease print action.
Add transparent white to extend the ink and/or increase the thinner content.
Use a harder than normal offset blanket and a “tack reducer” (such as linseed oil) to decrease the amount of ink laid down during each pass. This will reduce the chance of pick-off.
Raise the water temperature and, if necessary, reduce the speed of the printing press.
Compensate for the thinner ink at the color separation stage. A dot gain of around 10% is usually sufficient.
For further information on your hot stamping needs, API's Technical Educator, Eric Layson, will be happy to work with you, "We are here to offer support and technical assistance to our customers in any area of the hot stamping process.” He and his support team can be reached at 1-800-255-4605.

These tips provide general guidelines only. It is important to consult with your individual ink manufacturer for specific direction in this process.

A final issue that relates to this topic concerns paper selection and surface tension. Surface tension is the structure of the outermost layer of the substrate and is measured in dynes per centimeter. For foil stamping, a dyne level of 38 dynes per centimeter is recommended. Anything less than this makes foil adhesion very difficult. This also explains why it is nearly impossible to foil stamp on UV coated materials. UV varnish has a dyne level of approximately 30. Foil stamping on such a low dyne level is like trying to write on wax paper with a ballpoint pen. Aqueous coatings and press varnishes, on the other hand, have relatively high dyne levels and work well in foil stamping jobs. For maximum gloss, we highly recommend foil stamping first and then applying UV coatings.

For those of us who are constantly searching for new ways to market our products and add value to our jobs, overprinting foil offers a new twist on some old techniques— and another finishing option your customers may want to consider.