Gold Ink Printing: A Practical Guide to Mixing, Two-Pass Overprinting and Quality Control

Gold-toned graphics are a familiar sight on product packaging and labels, lending a polished, luxurious and premium feel to the finished item. Gold ink printing, however, is a specialised process: both the materials it consumes and the way the press is run set it apart from standard ink printing.

1. Blending the gold ink

A gold ink is built from gold powder, gold varnish and a small quantity of additives. Unlike ordinary colour inks, which arrive as a paste in a can, gold ink normally reaches the press as two separate components — powder and varnish — held in different containers and combined only shortly before the run. The usual ratio is 1:1 gold powder to varnish. Pre-mixed gold inks are available for immediate use, but a printing house still has to weigh how much ink the layout demands, how long the run will take and how many pieces are involved. The reason is shelf life once opened: within one to two hours the ink's printing behaviour shifts and its metallic sheen starts to fade, so blending small batches just in time is the safer route.

1. Running gold ink on press

Neutral or semi-rigid packing is the usual choice, and the inking rollers should not bear down too heavily on the plate — excessive pressure invites plate blinding.

Transferring the gold layer from plate to paper or another substrate frequently leaves the ink looking thin, sometimes with the base showing through. To reach a fully saturated film and release the metallic effect properly, the job is normally printed in two passes. Two approaches dominate: laying down a faux gold base first and then overprinting with gold ink, or simply running the gold ink twice. The faux gold in the first approach is usually built from transparent yellow or medium yellow. Where the sheet is loose and its surface uneven, medium yellow should carry the mix, with a measured addition of gold ink.

Where the sheet is dense and smooth, transparent yellow with a touch of gold ink makes the better base. Keep the base coat light — just enough to avoid show-through or edge bleed. Overprint the gold while that base is still tacky, and the ink film can be slightly heavier; that is what delivers full saturation. With the double-pass route there is one further rule: the second pass should go down before the first has dried completely. Gold powder is a metallic pigment, and once it is fully dry its particles no longer key firmly to the surface.

1. What to watch for in gold ink printing

(1) Paper characteristics

Substrate choice shapes the outcome more than almost anything else. The smoother and denser the sheet, the stronger the metallic result; porous, coarse papers deliver noticeably less. Watch the surface pH too, which should sit above 5.5 — below that, sulphate residues in the paper attack the gold powder while printed stacks sit in storage, turning it black or even corroding it away altogether.

(2) Line and detail in gold

Keep gold artwork away from extremely fine lines and small type: plate blinding becomes likely and the reflective quality suffers.

(3) Scuff and abrasion resistance

How well the gold film resists rubbing is a key measure of print quality. Two remedies are in common use — blending with a slightly more viscous gold varnish, or applying a gloss varnish once printing is complete.

(4) Tarnishing of the gold layer

Left in storage, gold-printed work gradually meets sulphur, oxygen and moisture from the air. The printed surface loses brilliance and can blacken, dragging product quality down. This is the familiar tarnishing problem, and it deserves attention at the warehousing stage.

(5) Overprinting gold with other colour inks

Gold ink performs best over dark solid areas. Red, blue and black grounds all give good results; purple, black and deep blue are even better.

Going the other way — laying ordinary colour inks over gold — is best avoided. If a large gold area must be overprinted with process colour, cut the viscosity of the colour ink first and wait until the gold has dried completely. Print too early and the gold graphics and text will turn patchy.

—Reprinted from China Packaging Network (pack.cn). Original link: http://news.pack.cn/show-362280.html