We will explore the subject of ink metalization and the new options that are now available.
HISTORY - THE INTRODUCTION OF METALLIC INKS
Evolving from the production of gold bronze and aluminum metallic powders for bronzing, Eckart, along with other companies, moved into the development and then production of cutting edge metallic inks with manufacturing facilities in Europe and North America.
These new generations of metallic inks were then used in the production of high-class labels, wrappers, box tops, covers, greeting cards and other work where bright, high impact effects were appropriate.
Unlike bronzing where a base size color was first printed and then dusted with a metallic powder, the metallic powders were incorporated into a liquid carrying medium (vehicle) to provide a press-ready metallic ink. Some of the early metallic ink firms supplied the powder and vehicle separately for mixing in small quantities fresh at the press, with improved results.
Metallic ink printing was more challenging for the press operator to control than conventional ink. One reason for this was that the metallic powder blended into the ink mixture and could not be ground as fine as other pigments, because the metallic ink would lose its lustre. The larger particles created problems on the press, especially with the offset process.
Metallic inks consist of powder-like metallic flakes, such as aluminum and copper alloys, which are mixed with a suitable varnish or pigment carrier. The varnish or carrier dries rapidly and binds the flakes to the surface.
Coated papers give the best results, while gravure, due to the ink film thickness, produces the best optical effects with gold and silver metallic inks. On rough surface papers, a base ink is usually printed first, allowed to dry and then overprinted with gold.
Metallic gold and aluminum inks can be printed by the offset, gravure or letterpress printing processes, although care should be taken to use a neutral fountain solution when printing offset to avoid tarnishing (see Figure 6.1).
Figure 6.1 - Metallic effects can be replicated by using inks
VACUUM METALIZED PIGMENTS
As mentioned, metallic inks contain metallic flakes. Ideally these should be flat platelets which when printed, all lie down flat to assemble a mosaic-like mirror. In reality however, the particles are not platelets and they don't all lie flat and so most metallic inks only give a very dull imitation of the effect they try to mimic.
The development of inks based on vacuum metalized pigments (VMP) exhibiting mirror finishes, effectively solves this problem (Figure 6.2).
Figure 6.2 - Vacuum metalized pigments (VMP) exhibiting mirror finishes
VMP PIGMENTS AND THEIR PRODUCTION
VMP pigments are produced by vacuum evaporation (physical vapor deposition) of a microscopically thin aluminum film onto a carrier foil. The aluminum layer is subsequently removed to be broken into particles for incorporation into various ink media. This process makes it possible to produce very thin and light pigments with a homogenous particle thickness hundreds of times less than that of conventional metal pigments.
Because they are lighter and thinner these pigments can align themselves parallel to the substrate surface faster than conventional metal pigments. When formulated correctly, these properties result in the formation of a smooth and even surface with very few edges and corners to scatter light. Figure 6.3 clearly shows the improved structure and alignment that can be achieved with a VMP flake versus a conventional pigment structure.
Figure 6.3 - The improved structure and alignment that can be achieved with a VMP flake versus a conventional pigment structure
CONVENTIONAL ALUMINIUM PIGMENT
Electron-micrograph of conventional aluminium flake
Electron-micrograph of VMP flake
The steps involved in the VMP manufacturing process are illustrated in Figure 6.4.
Figure 6.4 - VMP Manufacturing Process
PRINTING VMP METALIZED INKS
Press ready VMP metallic inks can be applied by all the major printing processes and are available in oil, solvent or water-based formats.
Whilst there are water passivated grades of VMP, which are specially treated to ensure stability of the aluminum for use in water-based coatings, solvent based systems tend to generate the best results.
The benefits of using VMP inks versus other types of metallic decorating technology are mostly in cost savings and ease of use.
STEP 1: RELEASE COAT
A release coating is applied to a carrier substrate e.g. polyester
STEP 2: PVD
Aluminum is then applied to the carrier substrate by physical vapor deposition (PVD)
STEP 3: STRIPPING
The metallized carrier substrate is then passed through a solvent bath. The release coat dissolves & the metallization breaks off into the bath to be collected
STEP 4: PARTICLE SIZING
This step involves breaking the metallization layer into particles of similar geometry.
Improved production speeds associated with printed metallics form an important part of the relative cost equation.
VMP inks are likely to be cost effective versus other types of decorating when they are printed selectively, but this varies by application and depends on many factors.
VMP is used extensively for wet glue applied beer label applications where gravure printing is commonly employed. Metallic inks are understood to have positive benefits when it comes to pack recyclability. The discreet ink particles make for easier penetration of the caustic solution and therefore allow for easy stripping of the label from the glass bottles.
Inks containing VMP’s are also experiencing growth for ‘no-label look’ self-adhesive applications. Reverse printed gravure onto a clear substrate such as polypropylene, means that VMPs are capable of achieving a bright and highly reflective appearance that resembles that of foil blocking.
The thick ink coverage possible with the rotary screen printing process enables the larger particle sizes associated with metallic inks to be more easily applied to the surface of the label or substrate.
As with the gravure printing process, solvented screen printing is able to float VMP pigments on a layer of solvent that is then dissipated by evaporation. The flakes align better to a clear substrate when reverse printed, thereby producing a high degree of reflectivity and brilliance, which is further enhanced (and protected) by the natural gloss of the filmic material.
The printing of metallics using both the rotary screen and gravure printing processes are illustrated in Figure 6.5.
Figure 6.5 - Printing VMP Rotary Screen versus Gravure
Both gravure and solvented screen systems require investment in extraction and a controlled explosion-proof environment, but for short runs screen printing can offer a number of advantages over its gravure counterpart.
A rotary screen printing system allows the re-use and re-imaging of screens. The ability to re-use a screen up to 15 times significantly reduces the costs per job, since the initial screen cost is apportioned to the number of jobs it is used for.
For longer runs and where screen wear can be an issue or for achieving subtle vignettes and halftones, the gravure cylinder can be a better solution.
The flexibility of screen modules can also be advantageous. The rotary screen head typically will slot into fixed positions on most printing presses and therefore can be used to deliver a wider range of ink systems and coating weights, which can be important in achieving a desirable metallic design feature (see Figure 6.6).
Figure 6.6 - Examples of VMP rotary screen printed metallics
ROTARY SCREEN VMP - SUMMARY OF BENEFITS
Solvented screen floats VMP particles on layer of solvent
Solvent offers optimum alignment of VMP platelets
Screen can be re-used up to 15 times
Flexible system for narrow web printing
MORE VMP METALLIC DESIGN OPTIONS
As designers become more familiar with the capabilities of VMP inks, more design options are emerging such as printing screens and vignettes and trapping colors over silver to create new effects.
Metallic inks for digital printing are now also available. These solvent-based inks are suitable for many standard piezo inkjet print heads.
Low migration metallic pigments, specially designed to comply with the strict EU legislation for food packaging, are also available.
New ink systems are emerging that are able to create a holographic or prismatic effect using a printing plate.
The addition of micro embossing to the VMP (vacuum metalized pigment) manufacturing process makes it possible to create a rainbow-effect metallic ink (Figure 6.7). These special refractive pigment particles have been formulated into solvent and UV base inks for most printing processes.
Figure 6.7 - Example of prismatic effect inks
Typically available as UV or solvent-based flexo inks, these ink systems eliminate the need for costly holographic substrates. The technology uses the flexographic process in combination with photopolymer printing plates to transfer the desired image to the substrate. UV curable ink or adhesive is used, with the holographic image laminated over the wet uncured ink or adhesive. The structure passes through a UV lamp and the ink or adhesive cures through and bonds the holographic material to the base substrate.
Applications include any that require foiling or the appearance of a holographic effect, such as cosmetics, drinks labeling and antifraud use.
METALLIC DOMING VIA ROTARY SCREEN (GALLUS SCREENY)
Metallic doming is a reflective, metallic raised structure manufactured using a relief in rotary screen printing (Figure 6.8).
Figure 6.8 - Relief effect using metallic doming
The lasting metal-relief effect can be achieved on paper or plastic film, such as tube laminates or transparent self-adhesive material.
The metallic doming method uses rotary printing to apply the printed image.
A metallic foil is then laminated over the relief. The process allows printed images with metallic reliefs (embossed effects) to be produced on plastic.
During metallic doming , a thermo-reactive glue is applied using screen printing, then dried with UV light. The applied print image is then laminated with a thermo-active metallic foil. A soft, heated laminating cylinder presses the foil onto the glue. The foil wraps around the glue and, together with the glue, is heated by the laminating cylinder.
This laminating process reactivates the glue and activates the foil.
This thermal activation means the foil sticks to the glue. After this, the printed image cools to reveal a metallic relief effect – this is the metallic doming effect (see Figure 6.9).
Figure 6.9 - The metallic doming process. Source- Gallus
OVER-PRINTING METALIZED SUBSTRATES
Over-printing a translucent wash or varnish onto a metalized substrate is a good, low cost way to give a colored reflective effect similar to foiling or ink metalization.
The areas left unprinted on the label allow background metalized substrate to show through.
Using a tinted translucent wash the metallic color can be easily adjusted. For example a yellow wash over a silver metalized substrate will give the appearance of gold or a translucent blue over silver will give a metallic blue (see Figure 6.10).
Figure 6.10 - Example of overprinting on metalized substrates
Overprinting ink onto hot and cold foil is also a trend worthy of note. In the past, foiling tended to be the last process, and on web presses the foil unit would have been at the end of the press. Now it is not uncommon for hot foil or cold foil units to be in the middle of the press in order to allow ink overprinting.
This allows nice effects to be achieved, but can pose a problem for the ink manufacturers and foil suppliers due to the poor ink receptivity of many grades of printed foil.
An interesting process from Color-Logic enables printers to achieve a ‘hot foil look’ by simply printing white and conventional CMYK inks over silver or holographic silver foils (Figure 6.11).
Figure 6.11 - Example of Color-Logic printed label
Using what is known as a Process Metallic Color System more than 250 metallic hues and special effects can be created. This technique eliminates the time-consuming and expensive hot stamping process, as well as the need to hold different foil colors in stock.
The Color-Logic system is compatible with offset, inkjet, flexography, digital presses, screen printing, and gravure processes.