Analysis of drying problem of sheetfed offset ink

First, the ink composition and auxiliary agent on the ink film drying

Diluter: The main component of the diluting agent is aluminum hydroxide and the binder. Aluminum hydroxide has strong water absorption and dissociates in water, which promotes ink emulsification and slow drying. It also adsorbs driers and disables them.

White oil: The white oil itself is a white transparent emulsion, and the texture is easy to emulsify the ink*.

Remover: Its main component is wax material (such as aluminum stearate), which has a small specific gravity and can float on the surface of the ink film, hindering the contact between the ink film and oxygen, and affecting the oxidative drying of the ink.

Desiccant: Excessive use will make the ink ink thicker and the ink emulsifies*. Imprinting is difficult to dry. (It will also cause the plates to become dirty and stencil. The dots will increase in size, hairiness, and thorn marks will be difficult to dry.)

<* Water droplets dispersed in the emulsified ink will affect the drying speed of the ink, and will form oil-in-water emulsified ink, affecting the drying of the imprint. 〉

Table 1 provides the amount of different ink dry oil, for reference only.
Table I

Do not use white dry oil% Red dry oil amount% BK5 ~ 102 ~ 5M52 ~ 5C3 ~ 41 ~ 3Y3 ~ 41 ~ 3

Nature of the ink: organic inks dry slowly, thin inks dry slower than thick ones, oily ones are much slower than oily ones. The metal ions in the ink composition can promote the oxidation and drying of the ink, and their ability to promote drying is as follows:

Co Mn Pb Ce Cr Fe Zn Ca

(For example, the chrome yellow pigment is a mixture of lead sulfate and lead chromate, and the iron blue pigment is potassium ferrocyanide. Since they all contain a lot of metal ions that can promote the oxidation and drying of the ink binder, they are used as pigment inks. The drying speed is relatively fast.Carbon black has no drying effect, instead it will adsorb desiccant to delay drying, so the black ink with carbon black as pigment will dry longer. Tungsten molybdenum salt pigment will also delay ink drying.)

Most of the organic pigments have the effect of delaying the drying of the ink, inhibit the oxidative polymerization of the binder, and act as antioxidants.

(For example, Jin Guanghong's pigments contain naphthol groups, and the chemical composition of the light blue has multiple aniline groups, so their drying is relatively slow. In production, we often use the light blue to tune in the black ink. We have to At the same time, the drying factor is taken into account. In addition, the oily nature of the light blue is not recommended for individual use.)

The proportion of dry oils used in modern offset printing tends to be smaller, the solidification characteristics of the resin are more obvious, and the influence of the pigment on the drying speed of the ink is relatively reduced.

Through the above analysis, we will find that the use of good ink in printing is the key to ensure that the ink film on the surface of the substrate is dried in time. At the same time, attention should also be paid to controlling the amount of ink adjuvant used in printing. When there is a problem of slow drying of the ink, the amount of drying oil can be increased in an appropriate amount, but it should not exceed the amount in Table 1 or it may delay the drying of the ink.

Second, the nature and structure of the substrate

1. If printed paper becomes acidic, it dries slowly. (For oxidized conjunctival inks, if the pH drops from 5.6 to 2.5, the ink drying time is extended by a factor of four!)

2. The surface of the paper with smooth surface, dense structure, poor permeability, and low degree of sizing is drying slowly. The dosage is shown in Table 1.

This is because the mineral oil in the ink cannot penetrate into the interior of the paper, which directly hinders the ink's osmotic drying process.

Conventional prints (coating paper, offset paper, newsprint paper, book paper, gravure paper, etc.) can be printed with ordinary osmotic drying, oxidative conjunctiva drying, and a combination of both drying. Printing on a non-coated paper with good permeability generally requires a general offset resin ink that is mainly osmotic drying for cost considerations. Light fast-drying ink is used for printing on coated paper. Do not use ordinary offset resin inks on coated paper, otherwise the prints will be difficult to dry.

Substrate with poor surface permeability can be properly added to the ink during the printing process.
〈 If surface absorption is poor (such as PVC, gold and silver cards, glass cards), try to use UV, IR, microwave and other drying methods. 〉

3. The moisture content of the paper is too large: First, it affects the speed of penetration of the binder in the ink, and then the evaporation of moisture in the paper affects the drying of the ink. (to be continued)