Стандарты печати золотой краской и меры предосторожности

In commodity packaging and label printing, gold ink is often used to print gold-coloured graphics and text, giving people a refined, gorgeous and noble impression. As a special printing method, gold ink printing differs slightly from ordinary ink printing in terms of the printing materials used and the process operations involved.

1. Preparation of gold ink

Gold ink is made by mixing and blending gold powder, gold varnish and a small amount of auxiliary materials. Ordinary colour inks are all paste-like and stored in cans, whereas gold ink is usually stored as two separate substances, gold powder and gold varnish, in two different containers, and the gold ink is then blended in proportion (usually the ratio of gold powder to gold varnish is 1:1) when gold ink printing is to be carried out. At present, ready-blended gold ink is also available on the market and can be used for printing directly. However, printing houses must take factors such as the amount of ink required for the layout, the printing cycle and the number of products into account when using it, because one to two hours after the gold ink is opened its printing performance changes and it also loses its original metallic lustre. Therefore, when gold ink printing is used, it is best to blend it just before use.

1. Gold ink printing process

Gold ink printing generally uses neutral or semi-rigid packing, and the pressure of the inking rollers on the plate must not be too great, otherwise plate blinding will occur.

During printing, as the gold ink layer on the plate is transferred to the surface of the paper or other substrate, insufficient ink and even show-through of the base often occur. Therefore, in order to bring the gold ink to a saturated state and fully bring out its distinctive metallic lustre, a two-pass printing method is generally adopted. There are basically two two-pass printing methods in use today: one is to first print a faux gold ink as a base colour and then overprint the gold ink, and the other is to print the gold ink directly twice. The faux gold ink used in the former method is generally prepared mainly from two kinds of yellow ink, transparent yellow or medium yellow. If the paper is loosely structured and its surface flatness is poor, the faux gold ink should be based mainly on medium yellow with an appropriate amount of gold ink added.

If the paper is tightly structured and smooth, the faux gold ink is prepared from transparent yellow with a small amount of gold ink. During printing, the amount of base-colour ink should be as small as possible, following the principle of no show-through of the base and no bleeding at the edges. While the base-colour ink is not yet completely dry, the gold ink is then overprinted, and the ink quantity may be slightly larger; only in this way can the saturated state of the gold ink be achieved. When the latter method is used, one point must also be stressed: it is best to overprint the second pass of gold ink while the gold ink of the printing pass is not yet completely dry, because the gold powder in gold ink is a metallic pigment, and if it is left to dry thoroughly before printing, its particles do not adhere firmly.

1. Points to note in gold ink printing

(1) Properties of the paper

The properties of the paper have a great influence on the quality of gold ink printing: the higher the smoothness and the better the tightness of the paper, the better the metallic effect of the print; conversely, printing on paper with a porous surface and a coarse texture gives a poorer effect. In addition, the pH value of the paper surface should be greater than 5.5, otherwise, when printed products are stacked for storage, the sulphate radicals contained in the paper will cause the gold powder to discolour and turn black, and may even corrode the gold powder away until it disappears.

(2) Graphics and text printed in gold ink

The graphics and text on a gold ink printing layout should not be too fine, otherwise plate blinding is easily caused and the reflective effect is also poorer.

(3) Abrasion resistance

The abrasion resistance of the gold ink layer after printing is an important indicator for measuring the quality of gold ink printing. There are currently two measures for dealing with the abrasion resistance of gold ink: one is to use a slightly more viscous gold varnish to blend the gold ink, and the other is to apply a gloss varnish after printing is finished.

(4) Discolouration of gold ink

After gold ink printed products have been left for a period of time, the gold powder is attacked by sulphur, oxygen, moisture and the like in the air, which dulls the lustre of the printed surface and may even blacken it, affecting product quality. This is what is commonly called the problem of gold ink discolouration, and attention must be paid to it during the storage of printed products.

(5) Overprinting gold ink with other colour inks

Gold ink should as far as possible be printed on dark solid areas; for example, printing gold on red, blue or black gives a relatively good effect, and printing gold ink on colours such as purple, black and dark blue gives an even better effect.

In addition, printing ordinary colour inks over gold ink should also be avoided as far as possible. If ordinary colour ink must be printed over a large area of gold ink, the viscosity of the ordinary ink should first be reduced, and printing must be carried out only after the gold ink has completely dried, otherwise the gold ink graphics and text will become mottled.

——This article is reprinted from China Packaging Network (pack.cn), original link: http://news.pack.cn/show-362280.html