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Question and Answer

Meeting the Challenge of UV Coatings & Hot Stamping Foil

by Staff

May-June, 1998
For stamping over UV coatings has long been thought of as virtually impossible. Many graphic finishers will not accept jobs that have been UV coated if the client is asking to have hot stamping foil applied over it. The belief that any job that has been UV coated will not accept foil is becoming much more of a myth than reality. InsideFinishing will explore the subject of foil stamping UV coatings and provide a few helpful guidelines for working with UV and foil.

The Make-Up of UV Coatings

It is true that not all UV coatings are overstampable. In today's environment, most UV coating suppliers have specific formulations that are labeled "foil stampable." These overstampable UV coatings are available in a number of different viscosities (thicknesses) to support coating applications on a variety of coating/varnishing equipment. Low viscosity UV coatings are usually applied by roll coaters and gravure. Medium viscosity UV coatings are applied through screen printing, flexo printing, or blanket coaters. Very high viscosity UV varnishes are applied through the last inker of a sheet-fed or web litho printing press in-line with the printing process. These high viscosity coatings are also commonly used off-line, where sheets can be UV coated at very high speeds. Applying the UV coating off-line is recommended when overstamping a UV coated sheet to allow the ink to properly dry. When the coating is applied in-line, the ink can be "wet-trapped" between the paper and the coating, creating real problems when trying to apply foil in a separate pass.

The very highest gloss UV coatings are formulated with silicone oil additives that yield a very high gloss and an extremely clear UV coating. Proper formulating can also produce low COF or slip properties, and good rub resistance. These coatings are usually typified by a greasy feel to the touch and will not accept hot stamping foil or glue for folding/gluing applications. A stampable UV formulation eliminates the high surface tension, reducing silicone oil additives and using selected polyethylene waxes to provide slip and rub and abrasion resistance. These waxes do slightly reduce the clarity of the UV coating and lower the gloss minutely. The gloss reduction is in the order of 1% to 2%, an amount undetectable by the naked eye. The surface tension of these UV formulations is in the area of 40-43 dynes, which we will discuss next.

Surface Tension and Dyne Count

The rule of thumb for applying hot stamping foil to a coated sheet is that the dyne count must be 38 or higher to allow the foil to adhere properly. If the dyne count is below 38 (high surface tension,) the foil cannot create a proper bond, and will begin blistering or gassing. To test the surface tension of a UV coated sheet, it is recommended to utilize a dyne pen set. Most of these sets are available with different dyne levels between 30 and 60. UV Process Supply offers a dyne pen test set with five dyne levels of 36, 38, 42, 44, and 48, which should suffice for testing most all applications. Simply mark the sheet with a straight line with the dyne pen. If the liquid from the pen beads up on the sheet, the dyne level is lower and the surface tension is higher than the dyne pen you are using. For instance, if you begin testing the sheet with a 36 dyne pen and the marked area beads up, then use the 38 dyne pen. If it still beads up, go to the 42 dyne pen. If the 42 dyne pen lays down a solid mark, the dyne level of the sheet is 42 + or - 1 dyne/cm. Below is a chart outlining common coatings and laminates with the normal dyne counts of each.

Coating/Laminate Surface Tension Dyne Level
Water-based varnish (acrylic)
Press varnish (alkyd)
Soybean oil alkyd varnish
UV varnish (high gloss)
UV varnish (foil stampable)
Polypropylene laminate
Acetate laminate
Polyester laminate
42 dyne/cm
38 - 40
36 - 38
38 - 40

Other Key Considerations

1. Choosing the Correct Foil Product The key to being able to foil stamp over UV is to first, choose an overstampable UV (as we discussed) and to second, use a foil that has been tested over UV coatings and is advertised as a UV hot stamping foil. Until recently, foil manufacturers have stayed away from publicizing a UV foil product. However, there are now hot stamping foils available specifically formulated for use over UV coatings. One of those foil products, the Kurz Hastings new Ultra Foil, has shown real promise for stamping UV coatings. Recent tests have demonstrated successful foil coverage at dyne counts as low as 35. Check with your current foil supplier to find out what type of foils they recommend for UV coated surfaces.

2. Corona Treatment "Surface Preparation" Corona treatment is a process that treats the surface of a coated sheet, raising the critical surface tension to the desired level. The corona treatment process uses an electrical discharge to break the molecular bonds on the surface of the coated sheet, creating a higher dyne count to accept hot stamping foil. This, of course, can become a necessary step when a sheet has been coated or laminated with a high surface tension where foil is not adhering. Remember, if the UV coating has a high silicone level, treating the sheet through a corona treatment process will have no impact on the ability to apply hot stamping foil to it.

3. Block Sand the Stamping Surface Another option to help break up the tension of the UV coated sheet is to block sand the image area. Attach a piece of #400 - #600 emery paper over the engraving with the grit facing the sheet, then simply run a complete pass without foil and a second pass (without the emery paper) with hot stamping foil. The disadvantages of this are that it will take another pass on press, and it can create a somewhat gritty look to the foil stamped image.

4. Texture the Engraving If the high surface tension of the sheet is causing the foil stamped image to bubble and leave voids, using a textured engraving can be an option. A textured line pattern can be etched through the image, meaning that no solid areas are stamped. This option works best for medium to large type or small graphic images. Larger images can lose their impact with the use of a textured engraving.

Through continued improvements with hot stamping foil products and foil stampable UV coatings, real headway has been made toward the compatibility of foil and UV. As we stated, there are still UV coatings with high silicone additives that will not accept foil. However, even coated and laminated sheets with dyne counts as low as 34 - 36 are now being foil stamped successfully. If you are asked to foil stamp a UV coated sheet, don't be so quick to say no. If the right precautions and the right tools are used, it may not be as difficult as you think.

InsideFinishing would like to thank Melanie Hobbs of Tantec Inc., (847) 524-5506, Jim Kingsby of JK Hot Stamping, (913) 764-4947, and Elmer Griese of Cork Industries, (216) 353-0980, for their assistance with this article.

Dyne Pen sets for testing the surface tension of UV coated sheets can be obtained by contacting UV Process Supply, Inc. at (773) 248-0099, or Tantect, Inc. at (847) 524-5506.