UV adhesives for cold stamping

In conventional offset printing, a dampening agent is used to separate the printing and non-printing sections on the printing plate. It comprises mainly water. The pH value should be between 4.8 and 5.5, the water hardness should be in the range 8 to 12°dH, and the water should be at a temperature of approx. 10°C to 15°C. Dampening agents normally contain plate protection agents, wetting agents, isopropanol (IPA) and other antimicrobially active substances. Gum arabic is used as the plate protection agent, and the wetting agents and IPA serve to reduce the surface tension. IPA-free dampening agents contain glycol as an alcohol substitute. Particular dampening agents may affect the properties of the adhesive. UV cold foil adhesives, in particular, are very sensitive to dampening agents and can emulsify large quantities of them. The amount of dampening agent used should therefore be kept as low as possible.

Particular care should be taken to ensure that the water rollers are decoupled from the plate cylinder during machine downtimes.

The minimum dampening agent level can be determined by reducing the dampening agent until the adhesive-dampening agent balance in the adhesive/ink unit becomes unstable and leads to smearing or scumming on the print sheet.

Now incrementally increase the dampening agent until no more smearing or scumming is visible.

The dampening unit can be checked using suitable test plates (e.g. from Fogra).

This effect can arise when the adhesive-dampening agent balance is exceeded. The adhesive can then no longer be applied evenly to the substrate, and color-intensive dots appear on a relatively weakly colored background. This defect in the print image can be caused by a non-uniform retro-reaction of the adhesive on the printing blanket, and a non-uniform penetration of the adhesive into the paper surface. If the technically necessary level of emulsion formation - water in adhesive - is significantly exceeded, this produces an unstable emulsion and a non-uniform coating on the substrate surface, a loss of dot sharpness, and an uneven reproduction of raster areas.

The water content of a stable emulsion should be approx. 20 %. The pH value of the dampening agent is also important, because a too low value can lead to increased emulsification.

Viscosity

Viscosity is a measure of the resistance of a medium to flow. The particles of a viscous medium are more tightly bound to one another and therefore less mobile. The bonding within an adhesive is very high due to the high level of internal cohesion. Adhesives can therefore easily detach the cold foil from the carrier film.

In offset printing, for example, tack can refer to the stickiness of the cold adhesive. The tackiness can be measured by means of a tack meter, or the so-called "finger test”.

The tack also refers to the separating strength of the adhesive film, e.g. between two ink rollers. The higher the tack, the higher the heat generation in the roller inking unit and the tendency for fibers or particles to be pulled out of the paper surface.

On the other hand, a higher tack improves the adhesion of the adhesive on the printing areas of the printing plate or the rubber blanket, and the print image becomes sharper.

The tackiness is a complex combination of viscosity, cohesion within the adhesive, adhesion to the roller, and other parameters. Due to this complexity, the tack value offers a technically relevant, practical and process-oriented measure for this printing phenomenon.

In offset printing, the rubber blanket plays a very important role as an intermediate carrier. The packing together with the rubber blanket can be used to produce rubber cylinder sleeves of various hardness. The two together - rubber blanket and rubber cylinder sleeve - contribute to the print quality, e.g. how precisely the raster dots are transferred, whether the raster is printed too heavily, and how flat the surfaces are.

Every rubber blanket consists of multiple layers, with intermediate woven fabric layers, and compressible layers. The woven fabric layers determine the stretch resistance. It should be noted, however, that these fabric layers possess a kind of "running direction”, because the fabric is only substantially stretch resistant in the direction of the so-called warp threads. The top rubber layer, the cover layer, is the principle determinant of the print quality. It must not be washed with an unsuitable washing agent as this can negatively impact on its properties.

The compressibility of the rubber blanket affects the pressure between the plate cylinder / rubber blanket cylinder and the rubber blanket cylinder / counter pressure cylinder in the so-called nip zone. For optimum ink transfer, the industry recommends a pressure of 70 - 100 N/cm² in the nip zone to allow the printing dot to be transferred from the printing plate to the rubber blanket and from there to the substrate.

As a rule, rubber blankets are produced with three different levels of surface roughness and structures. This, amongst other factors, can influence the quick release (QR) effect of a rubber blanket.

8 - 10 µm                   coarsely buffed printing blankets, e.g. for newspaper printing

3 - 5 µm                     finely buffed printing blankets, e.g. for commercial job printing

1 - 3 µm                     ultra-finely buffed printing blankets, e.g. for sheet printing

The following factors can also affect quick release in the printing process:

The following rubber blankets have proven most popular for cold foiling applications:

SUPER PRESS from Streb

This rubber blanket was specially developed for high-pressure rotary newspaper printing machines. Its Shore hardness value is around 88° and it is constructed from three layers (1.85 mm). Small glass beads in the surface (approx. 7000/cm²) ensure a clear, clean print and prevent the foil from being entrained in the printing unit. This rubber blanket is insensitive to the commonly used printing chemicals, benzene and oil, and is highly abrasion and temperature resistant.

IRIOBLANKET from Vulcan

This rubber blanket was developed for printing with pearl gloss inks. It is used for sheet-fed offset printing, packaging printing, and also for tin printing.

The blanket has a white-glazed surface, comprises 4 layers (1.96 mm) and has a surface roughness of 0.3 - 0.5 microns. It is resistant to swelling solvents and printing inks.