Hot stamping foil adhesion is measured in a number of different ways, and there is a lot of misconception about what each test really measures. Take the simple scratch test for example. Is that a definitive adhesion test or simply a measure of the hardness of one's fingernail?
In fact, most adhesion tests carried out are comparative rather than a true measure. If you have two foils on the same substrate, both blocked at the same time, then you can test their comparative resistance to your fingernail, assuming that you do not subconsciously scratch harder on that you want to fail. This is more common than you might think, which is why a true scientific measure of adhesion is preferred.
However, such measurements are rare, and there is nothing that could actually be routinely taken out by a trade blocker or printer. Even a rub or abrasion tester is not testing adhesion as much as the hardness of the foil surface. The so-called 'Scotch tape' test is generally the most widely used, but there are so many variables involved, that again this is normally not definitive without exactly the same tape is used each time.
Apart from the adhesion of the foil itself, the amount of wax release applied can significantly affect the adhesion and abrasion resistance of a hot stamping foil as it is tested. Hot foil can be made using a wide variety of waxes, and is not present in the form of a coherent separate layer. The wax layer is generally applied by direct gravure, and is only around 0.005 gsm (grams per square meter) when dry.
The solvents used in the consequent lacquer or top coat are generally fairly aggressive, and even though tend not to dissolve the wax when cold, will invariable strike through the wax layer immediately on coating, and also in the drying tunnel. The result is a wax rich resin layer at the carrier film interface (usually polyester) that becomes progressively more wax deficient the deeper into the lacquer layer – ie the further the distance from the polyester where the wax layer was originally applied, the less wax will be intermixed with the resin.
After blocking, the wax in the resin tenders to rise to the surface. Although some wax will be at the surface of the foil immediately on hot stamping, this will increase with time as it migrates through the lacquer layer to the surface. Here, abrasion resistance improves with time and your test results will therefore vary according to how long after blocking you test the foil.
The same could be true of the Scotch tape test: the surface will become increasing less receptive to the tape as the wax concentration increases. What you are measuring, then, is the adhesion of the tape to the wax-rich surface relative to the adhesion of the hot foil to the substrate it is blocked to.
There is in fact a lot more to it than just that, but it indicates that what you might have thought was a simple test is, in fact, not so straightforward. There are several other properties of a hot stamping that affect its perceived adhesion, too many to discuss in this article. However the principles involved are interesting and worth finding out because they can shed light on some of the problems you might have encountered on your press.
Sometimes understanding why a problem occurs can give you an idea of the solution. However, I will finish of with a few questions that have little to do with solutions but more to do with the testing itself. How many people go around testing their greetings cards or wine labels with a piece of sticky tape? Is your testing related to the end use of the foiled article, or carried out out only because it is in the testing specification?
Good hot stamping foil adhesion is frequently needed, but would you sacrifice performance and speed in order to use a hot foil that stands up well to a piece of sticky tape after foil blocking?