The development of modern cold stamping foils originated with the simple cold foils devised as a dieless technique for applying a metallic finish to self adhesive labels produced on reel to reel printing presses. There are many benefits of this process, and some disadvantages, although cold foiling has progressed a lot since then.
The benefits in reel to reel printing are obvious. If hot foiling is carried out in-line at the end of the printing process, then the press speed is limited by the foiling speed which is relatively slow. If the printing was maintained and the foil applied as a separate pass, there is a corresponding cost impact. Furthermore, the cost of hot foil dies is relatively high compared to a printing plate or sleeve and energy is needed to heat the die.
Reel to reel cold foiling involves printing a UV curable adhesive in the foil design needed, laminating the foil to the adhesive through a nip, curing the adhesive with UV lamps and separating the foil from the printed web. The foil that is removed from its carrier film is in the design of the printed adhesive, since that has been UV cured and so adheres to the foil.
By using a cold foil, the speed is limited only by the strength of the UV lamps or the optical density of the foil. The aluminium layer in the foil has to be thinner than that normally used in a hot stamping foil to allow sufficient transparency to the UV light. True, this is irrelevant when the foiling is carried out directly to transparent films since the light can be directed through the back of the film onto the adhesive, but this is frequently not the case. The foiling is often carried out over an opaque white ink which can be silk screened onto to the label prior to the adhesive and then the foil applied. This places an opaque layer between the film and the foil, so that the beam has to pass through the lacquer layer or top coat of the cold foil.
The UV beam passes through the lacquer and aluminium layers to reach the adhesive, so the aluminium must be as transparent as possible. The more opaque it is, the more lamps are needed or the slower the running speed. Since printers want the lamps set in a fixed position, and not changed around for each type of job, all curing is generally carried out through the foil than through the film being printed.
Given that this can be done, then cold foiling is much faster than hot foiling, and of course there is no need for a die, thus saving not only on die-cutting costs but also on energy. A very appropriate saving these days. So why are all reel to reel jobs not blocked using cold stamping foils? Mainly due to the quality. Cold foiling is generally not as brilliant as hot foiling, and also most stamping foil manufacturers offer a very limited range.
They have also tended not be very print receptive. If the overprint is small enough, it can adhere, though the cold foil surface tends to be fairly low in surface energy (low dyne level). Larger printed areas tend to have poor scratch resistance and tape adhesion, though many customers accept this in return for the lower costs. Modern developments, however, are tending to overcome these problems though truly overprintable cold foils are very difficult to find and depend very much on the type of ink being used.
Modern cold foil developments include sheet fed offset litho applications, whereby the adhesive is applied by traditional oxidation drying offset, and then the silver foil overprinted with oxidation drying offset transparent inks to produce an attractive coloured metallic effect. This process is currently patented. Further developments involve offset UV adhesives and inks, though the adhesion of the UV ink to the foil surface is still an issue here.
The volumes of cold foil being used world wide are increasing at a rapid rate, and much of this is in the offset field, where the major European printing machine suppliers are offering purpose build equipment, and add-ons that can be retrofitted to a printer's existing equipment. The printing quality is improving as more development work is carried by the hot stamping foil producers, and the quality modern cold stamping foils is rapidly approaching that of hot foiling.