The cold foiling process, as the name implies, involves foil blocking without the use of heat. Traditional hot stamping foils require heat and pressure to transfer the foil from the carrier film to the substrate being printed, whereas cold foiling does not.
A hot foil generally consists of a polyester carrier, or, more rarely, polypropylene or cellulose film, a release layer, a lacquered layer, the colour which determines the shade of gold or coloured metallic effect, a layer of vacuum deposited aluminium and finally a sized layer to give adhesion to the printed stock. A cold foil has basically the same structure, though cold foiling usually requires a thinner aluminium layer, and the adhesive layer is not always necessary.
The hot stamping foil is applied to the stock by means of a heated die, set at a temperature of between around 100 Celsius to over 200C, depending on the type of stamping press being used and the area being blocked. The heat activates both the adhesive layer and the release layer, and allows transfer of the foil from the carrier film according to the design of the die.
A cold foil, on the other hand, requires no heat and adhesion is achieved by means of a completely different mechanism. In cold foiling the design is not applied by means of a heated die, but is printed in the form a UV activated adhesive. Any printing technique can theoretically be used in cold foiling, though offset litho and flexo are the most common. The run is started, the design applied using UV adhesive from printing plate or blanket, web and cold foil are brought together through a nip then the sandwich passed under UV lamps which cure the adhesive. The cold foil and print webs are separated and rewound. Since the UV has to penetrated the aluminium layer, this is thinner in cold foiling than with hot foiling. A cold foil must have an optical density low enough for it to sufficiently transparent to UV light to allow it to cure the adhesive in the short time available.
So, in a nutshell, cold foiling involves printing an adhesive, nipping the cold foil sandwich to it, curing, then separation, leaving the foil adhered to the stock according the printed design of the adhesive.
Traditional cold foiling is normally reel-to-reel and used in the self-adhesive labelling industry, but recent developments have allowed a viable sheet-fed process to be used.
Whatever method is used, cold foiling has yet to give the high shine of traditional hot foil stamping. The nip, composed of a metal roller and a rubber impression roller, has to spotlessly clean or pinholes will be seen in the finished result. Cold foil is prone to pin-holing and other visual defects. The separation in cold foiling is crucial, as is flatness of the web. Cold foiling is particularly prone to creasing if the webs are not completely flat when brought together,
The advantages of cold foiling are:
1. In-line foiling. The cold foiling unit can be placed in-inline after the printing processes, and if required, between printing processes. In cold foiling, the process has been converted from a slow hot stamping process to a fast printing process using a cold foil. The printer therefore bypasses an additional relatively slow process through using a cold foil rather than a hot foil.
2. Fast set-up. The use of conventional printing methods such as offset allows fast set up of cold foil as against hot foil. There is no packing required in cold foiling to ensure even pressure to the substrate since the cold foil is applied to a wet adhesive.
3. The expense of having metal dies cut is eliminated with cold foil. The cold foiling process becomes more attractive for short runs, where previously dies costs could prohibit this.
4. Since we are using a printing process, there are fewer registration problems with cold foiling. The hot foil process frequently present problems in registration which require time to put right.
1. Cold foil print quality is not as good as that from hot foil. The brightness is generally lacking and pinholes are frequently a problem.
2. Cold foiling requires complete cleanliness if surface defects are to be kept to a minimum, and the hardness, cleanliness, alignment and pressure of the nip roller is extremely important.
3. The energy provided by the UV lamps must be properly maintained or the cold foil will not adhere to the partially cured adhesive. In cold foiling, the transmission of the UV energy through the foil is critical. Problems can be experienced with deep cold foil colours which act as UV absorbers, and foiling to black and deep blue substrates can be difficult in cold foiling since these colours contain pigments which act as UV absorbers and thus leaves less energy to cure the adhesive. For these reasons many suppliers do not offer cold foil in deep shades.
All in all, though, cold foiling is growing in popularity due to the increased profitability it gives, especially in areas where the very highest quality of foiling is not necessary. The newer offset cold foiling techniques are rapidly gaining ground on the quality of traditional hot foils, which is why most hot stamping foil manufacturers now include cold foil in their portfolio.
Copyright 2006 Peter Nisbet