Dossiers

What you should know when printing metallic inks

Ink pigments for colour production (Source: Siegwerk)

In order to create striking effects on substrates, metallic inks would be an ideal choice. These special inks are based on conventional printing inks enriched with metallic pigments and thereby create metallic effects. ­Solvent-based, water-based and UV metallic inks are available.

by Rolf Montag

The special characteristics of UV metallic inks

Printing UV metallic inks is considerably more difficult than using solvent- or water-based ones. Therefore, printing such inks may quickly lead to unsatisfactory results. So, what should we bear in mind to achieve optimal metallic effects with this type of ink?

Basically, metallic inks are made up of a dispersion of brass or aluminium particles in a liquid medium. On the face of it, there is nothing about them to reveal why UV metallic inks should be harder to print. But there are a number of important differences that make UV metallic inks more difficult to handle:

- Hardly any shrinkage while the ink dries
- Chemically reactive binder matrix
- Higher ink viscosity than solvent-based inks
- Competition between reflection (gloss) and curing

These characteristics may cause certain difficulties in printing. They can for example affect the stability, chemical resistance or curing properties of the ink. This means it is important to keep the peculiarities of such inks in mind – when formulating and when processing UV metallic inks – in order to achieve optimal print results.

How are metallic pigments manufactured?

The starting material for silver is aluminium at a purity of at least 99.5%; the starting material for shades of gold is brass, which is a copper-zinc alloy. The starting material for the copper shade is pure copper. Different alloy ratios of copper/zinc are employed to produce the standard shades of rich gold (70/30), rich pale gold (85/15) and pale gold (90/10).

To produce the pigment particles, the hot molten metal is atomized into tiny solidified beads by a spray method using compressed air or nitrogen. These particles are separated out by sifting and screening and classed according to the respective end product requirements. For aluminium pigments, the particles are ground in ball mills with a waxy lubricant and an inert solvent until the tiny spheres have been converted into more or less platelet-shaped, flaky pigments.
With gold-bronze pigments, the grinding step can be done dry, without any solvent. Pigments formed in this manner are called flakes. Spherical aluminium particles formed especially into round disk-shaped pigments are called silver dollars. These produce a higher, brighter gloss for a comparable particle size. The lubricant/additive coats the pigment homogeneously and protects it against oxidation or other chemical attack.

When producing aluminium pigments, there is another, much more elaborate process known as the VMP method (vacuum metallized pigments). In this case, the aluminium is vapour deposited onto a plastic foil in a vacuum and then separated off again. Pigment particles produced this way are much thinner compared to ground flakes, and have an ultra-smooth surface. Silver colours with VMP pigments offer a very high gloss or even an effect much like a stamping foil or a metallized surface.

How to affect the gloss of metallic inks

The purpose of every metallic ink is to achieve the highest possible gloss – regardless of the ink technology or printing process. The gloss depends on a number of factors:
- Pigment shape, size and alignment
- Floating behaviour of the pigments
- Viscosity and printing speed
- Ink coverage and type of coverage

In addition, the shrinkage of the ink is important in order to achieve the final brilliance of UV metallic inks. Basically, the aim is to achieve the flattest possible alignment of metallic pigments in the film of ink in order to achieve the greatest possible gloss in printing metallic inks. If the pigments lie flat against each other, then they form a smooth, flat surface that reflects the incident light symmetrically in one plane so the surface appears shiny and uniform. When the pigments do not rest flat enough against each other or lie parallel to the surface, then they do produce a metallic effect, but one with a grayer, darker character.

The alignment of the pigments depends on the wetting of the pigment particles. If they are well wetted from all sides, then they sink or float freely and are unaligned in the ink film. If the wetting is not so perfect, the binder tends to surround each pigment particle irregularly and there tends to be floating in the ink film, which is also called “leafing”. The positive effect of this phenomenon is that this floating leads to a very good alignment of the pigment particles on the surface and thus to a higher gloss. This is why no UV metallic inks are marketed whose pigments do not float, otherwise they could not be described as a silver or gold gloss.

For the sake of clarity, one disadvantage should be mentioned in this regard: Floating pigments that are not cleanly bound in the film can be scraped off the surface more easily, meaning such surfaces often have slightly poorer resistance to scuffing or poorer suitability for lamination or overprinting. It stands to reason that lower viscosity and lower printing speed favour the floating effect before the ink system is UV cured. Considering ink viscosity and printing speed, the best metallic surfaces are achieved with UV screen printing by using low-viscosity inks and the worst are achieved with UV offset/UV letterpress printing. The results of UV flexo metallic inks are somewhere in the middle. Furthermore, the greater ink coverage results in a stronger gloss of the substrate surface.

Comparing the different metallic inks it can be stated, that due to the mentioned “leafing effect”, the wetting and thus the alignment of the pigments on the surface, UV metallic inks basically have a disadvantage because their lack of solvent. This is due to the fact, that solvents cause appreciable shrinkage of the ink film upon drying (generally: >50%; for UV inks: just about 2–5%). Even in well wetted metallic pigments, this shrinkage of solvent-free and water-based printing inks results in good particle alignment and therefore beautiful gloss. In the case of solvent-based and water-based metallic inks – unlike for all UV metallic inks – the fact that the pigments in such non-leafing metallic inks (i.e. non-floating pigments) are well embedded in the ink film results in better mechanical resistance and good overprintability.

If all these above mentioned characteristics and peculiarities of metallic inks – especially of UV metallic inks – are taken into consideration, optimal print result with the best possible metallic effect can be achieved without any further obstacle.

Angela

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