Demystifying Metallic Inks for Screen Printing
Find out what metallic inks are composed of, which areas of production require attention when working with these materials, and how you can sucessfully use them to boost the visibility of the products you print
Metallic gold and silver inks are very popular among buyers of printed goods, but these inks remain an enigma--and occasionally a nuisance--to many of the screen printers who are asked to work with them. Before we can take advantage of the perceived value metallics add to screen-printed products, we need to clarify the enigma and understand how they differ from conventional pigmented ink systems. Here, we will review the make-up of metallic screen-printing inks, the special requirements they bring to the printing process, and applications for which they are and are not well suited.
What is a metallic ink?
Metallic screen-printing inks contain two main components: metallic pigment and a liquid resin system (sometimes called a varnish) that serves as a carrier for the pigment and protects it after drying. These components are sold separately or together.
In metallic inks with a gold appearance, the pigment is made of a mixture of copper and zinc in various proportions. Larger amounts of zinc (around 70% copper and 30% zinc) will give the ink a greenish shade of gold called rich gold. More copper will give the ink a reddish appearance, often called pale gold (90% copper, 10% zinc). In between these two extremes, we find rich pale gold, which is made up roughly of 85% copper and 15% zinc. These shades may be tinted, usually with a transparent orange colorant, to create colors that match those identified in common color-matching systems, such as PANTONE. Silver ink pigments typically comprise aluminum particles in various proportions.
The raw materials for metallic pigments come from copper, zinc, or bauxite mines. The materials are processed into pigments by metallic pigment manufacturers, who atomize the ores into fine particles, grinding and polishing the pigment flakes using special mills.
The final pigment is typically classified according to its end usage. The coarsest grades, which are used in industrial paints and coatings, feature pigment flakes that typically measure around 25 microns in size. For flexo and gravure printing, pigments measuring around 10-12 microns are generally used, and in offset printing the finest metallic pigments measure about 3-7 microns. For screen printing, pigment sizes vary from 3-15 microns. The specific particle size required for screen printing is typically determined by the application for which the metallic ink is destined.
An important factor to remember when working with metallic inks is that, as a general rule, the coarser the particle size, the more reflectance or brilliance the printed ink film will display. With larger particles, more of the pigment surface is exposed, and more light reflects from it. However, larger pigments also lead to more difficulty in transferring the ink effectively during printing and require larger mesh openings. Finer metallic pigment particles will transfer much more reliably than larger ones through higher mesh-count screens, but they sacrifice brilliance. With these characteristics in mind, it's clear that for lower-resolution applications like T-shirt printing, coarser pigment flakes are ideal, while for CDs and other high-resolution graphics (Figure 1), the smallest pigment particles are most desirable.
In reality, however, no metallic ink contains only the ideal pigment size--the inks typically comprise a mixture of pigment particles, most of which fall within a target range. While new pigment processing technologies have improved the ability to separate particles according to size, coarse and fine particles still make their way into the same inks. This means that some of the pigment flakes simply won't be able to penetrate the mesh openings and will remain on the surface of the screen. Nevertheless, improved pigment processing has resulted in metallic screen inks with lithograde pigment particles, the majority of which will transfer through meshes as fine as 560 threads/in. and are suitable for applications that require high detail.
Although you sacrifice some of the brilliance by using inks with finer pigment particles, you also get better coverage (for equal weights of pigment, finer particles can be distributed over a broader coverage area than coarser pigment particles). The challenge for the screen-printing ink manufacturer is to make inks using the finest possible pigments that will deliver high coverage, brilliance, and a tight particle distribution.
Properties of metallic inks
To predict the kinds of results we can achieve with metallic inks, we must probe the chemistry of these inks more closely. Metallic pigments are usually coated with some type of fatty acid during processing, which allows the pigment to rise to the surface of the printed ink film and the pigment flakes to orient themselves into a flat and reflective surface. Metallic inks that perform in this way are said to be "leafing" inks.
The downside to leafing metallic inks is that because the pigment rises to the surface of the printed ink film, it flakes and rubs off easily. The problem is most pronounced with larger flake sizes--smaller flakes have less surface area exposed and tend to hold more securely to the clear resin. To get around this problem, non-leafing metallic inks are available. Non-leafing pigments sink to the bottom of the ink film, which improves their adhesion, but sacrifices brilliance because the particles don't align and because light has to penetrate the varnish before reflecting from the pigments. Non-leafing pigments are used extensively in the silver family of inks. They are also found in some gold varieties, but are not as popular in these inks.
A question frequently heard from screen printers who are unfamiliar with metallic inks is whether the inks can be used for outdoor applications. In some cases they can. Metallic gold and silver pigments can be specifically treated to resist tarnishing and oxidation from outdoor exposure. When these pigments, which are similar to the pigments found in automotive paints, are used in screen-printing inks, the resulting prints resist not only UV exposure, but will even withstand salt spray and other extreme conditions. However, inks that feature these treated pigments tend to be more expensive than those using normal metallic pigments. Also note that treated golds are not as tarnish resistant as treated silver metallics.
The chemistry of both gold and silver pigments makes it difficult for manufacturers to provide them as part of single-component, ready-to-use ink systems. In most cases, metallics are still sold as two-part screen inks. Because of the chemical composition of metallic pigments, they tend to react and emulsify when mixed with the clear base. Depending on various factors, once the pigment and the clear are mixed, the shelf life of the ink can range from just a few hours to a few months. Pigment and ink manufacturers are continually working to overcome this limited shelf life, and several one-part metallic screen inks are now offered with a minimum shelf life of six months. Within the next five years, such single-component inks are expected to dominate the market.
When using screen-printable metallic inks, substrate selection is critical. A porous substrate will absorb the clear resin, especially if the ink has a low viscosity, and leave the pigment on the surface of the substrate. The diminished clear coating around the pigment particles will make silver metallics appear more gray and gold metallics appear more brown. If we have no alternative but to print metallics on an absorbent substrate, the best solution is to print a base coat of ink (such as a conventional clear coat) to seal the substrate surface, then print the metallic ink on top. For added protection against flaking and rubbing with any metallic ink, a final clear topcoat can also be applied. We just need to remember that the more clearcoat that lies on top of the metallic pigment particles, the less reflectivity and brilliance they will exhibit.
Because metallic pigments are made of metals, the pigments are prone to oxidation. In the case of gold metallics, this oxidation occurs with the copper and zinc particles, resulting in diminished brilliance of the ink layer. In some cases, the ink will even begin to exhibit a greenish cast. In silver metallics, the aluminum pigments tend to become dull and whiter with oxidation. Oxidation is one reason why color consistency is hard to achieve with metallic inks.
Another reason why color consistency is difficult to maintain has to do with the way pigments align, especially in leafing inks. In leafing inks, the metal flakes align at the top of the ink film with small gaps between one another. The coarser the pigments, the larger these gaps.
After printing, the presence of these little gaps can allow the substrate color or underlying ink colors to show through, creating a visual shift in the apparent color of the metallic layer. For example, a blue background beneath a silver metallic ink will give a bluish cast to the silver, and a yellow or green underneath a gold will change the shade of the gold. Underbasing with a light, neutral color (white, light gray, etc.) may help alleviate the problem.
While silver metallics are normally offered in only one shade, the golds not only come in the rich, pale, and rich pale varieties, but are also available in a range of oxidized colors, where the pigments have been allowed to react with oxygen in a controlled fashion. The results can include shades of gold with a lemon-yellow or reddish-orange cast.
Gold metallics also come in tinted shades, like green or purple. These shades are made by adding various colored pigments to the gold ink. However, they tend to be more unstable and prone to inconsistencies than pure metallic colors. They also are less likely to be colorfast.
Transparent colorants are often used to tint silvers so that they appear as gold metallics. This approach helps users avoid the oxidation problems associated with golds made from copper and zinc particles. These tinted silvers are available in non-leafing form, but the colors usually do not look as clear as standard leafing silvers.
Besides conventional colors, most color-matching systems also identify a range of metallics (Figure 2). These metallics are made by using rich, rich pale, pale gold, and silver metallics as the base color, then tinting them (usually with red, yellow, or orange ink). The challenge in manufacturing such inks is that some of the red, yellow, and orange pigments used for matching may attack the metallic pigments and create a negative reaction that ruins the printed image. Ink manufacturers take great care in formulating their metallic colors to avoid such problems.
The use of color-matching systems offers benefits and drawbacks. With a color-matched metallic ink, we increase our coverage area and may be able to more accurately match specific customer color requirements. However, the other components added to create the match also diminish the brilliance of the metallic pigments. The tradeoff may be too great for some customers to accept. Whenever maximum reflectivity is specified by the customer, we should recommend the use of the rich, rich pale, and pale golds, as well as silver metallics, in their purest forms.
Another factor influencing the brilliance of the printed ink is the purity or clarity of the resin used by the ink manufacturer in the ink. The clearer the resin, the greater the amount of light that is reflected and the more brilliant the metallic effect.
While we're discussing the appearance of the metallic ink, we should probably note a phenomena that is very prevalent with these formulations. Because of the way the pigments are oriented in most metallic inks, the appearance of the printed ink film will change with the viewing angle. This is why traditional graphic arts spectrophotometers are usually not recommended for measuring metallic prints. The only accurate measurement tool is a type of spectrophotometer used in the automotive industry. This device reads the colors at multiple angles for an average value, but the cost of such a unit is usually prohibitive for screen-printing operations.
If we purchase metallic inks in two parts, there are other variables we need to take into consideration. First, mixing the two parts should be done carefully and slowly. A high speed blender or mixer will create heat, which will promote oxidation in gold metallics and cause the metallic flakes to break down, turning the silver to gray and the gold to brown, and diminishing both reflectivity and brilliance.
The mixing proportion or ratio of pigment to clear varies according to the size of the pigment flakes and the properties desired from the printed ink film. Generally, it's safe to start with as little as 8% silver pigment (by weight) or 15% gold pigment. The ratio with both pigment varieties can go as high as 50/50 in many cases. Follow the ink manufacturer's recommendations to ensure that ink adhesion and appearance aren't sacrificed.
In general, using the lowest possible amount of pigment is the most desirable approach because it results in better ink-transfer characteristics, greater durability and adhesion, and lower pigment costs. And despite what many printers think, adding more pigment powder or paste to the mix does not make the ink more brilliant. The fact is that the smaller the amount of pigment, the better the adhesion and the more uniform the ink film we can achieve. Greater uniformity means a smoother, more reflective surface, and consequently, better brilliance. The best approach is to use the minimum amount of pigment that allows us to achieve the target color and coverage area. Anyone who uses these inks quickly discovers that a little pigment goes a long way.
One additional note about two-component metallic ink systems. If we are mixing silver inks and our aluminum pigment concentrate is in a powdered form, we must be extremely careful. Powdered aluminum reacts with water/humidity and releases hydrogen, which can explode (that's why similar aluminum flakes are used in explosives for slurry mines, rocket fuel, and fireworks). So if we keep powdered aluminum to produce silver metallic inks, we must make sure to store the aluminum in a dry, temperature-controlled location. Fortunately, aluminum is safer in a paste form, and all producers of aluminum powders also offer their pigments as a paste in varieties compatible with solvent-based, water-based, and UV clears.
The future for screen-printable metallics
Screen-printing ink manufacturers continue working to refine metallic inks so that they are easier to use and provide more stunning results. Among recent developments, the list of metallic-effect inks has grown to include new systems that provide a chrome-like appearance (Figure 3). However, to get a mirror finish that accentuates the chrome effect, the printer must carefully test to determine how much pigment is needed. Here again, the less pigment used, the better the results typically are.
In general, the needs of the screen-printing industry have been overlooked by metallic pigment suppliers, and ink manufacturers in our industry have been confined to a limited range of raw ingredients for their formulations. One good example of missed opportunity relates to the use of metallic inks as barrier coats against light transmission. Metallic inks, especially silver, are completely opaque to transmission of light, making them ideal for many screen-printing applications, such as selective blocking of light transmission in backlit instruments. But when a highly reflective silver is used as a barrier coat, it may alter the appearance of colors printed over it. This problem is most pronounced with black inks, which tend to be somewhat transparent.
The solution seems very simple: Pigment manufactures could create a non-reflective metallic ink specifically for use as a barrier coat, one that would not affect the appearance of subsequent colors that are printed over it. In fact, such pigments would actually involve one less step in manufacturing than the reflective varieties since the pigment flakes would not require polishing. However, as long as pigment manufacturers don't realize that this application exists, it's unlikely that pigments for such a metallic ink will ever be developed. Better communication between printers, ink makers, and pigment manufacturers is clearly needed.
With metallic inks, users can add value to their printed goods for a variety of markets, including packaging, labeling, container decorating, display graphics, and more. And just as with any other product a screen printer might use, understanding what makes up metallic inks, how they are manufactured, and what their capabilities and limitations are will make it easier to pass their benefits on to customers. For those printers who do their homework, metallic inks represent golden opportunities with silver linings.