Analysis of Drying Problems of Sheetfed Offset Ink

In the printing process, whether the ink can be fixed on the surface of the substrate in time, and whether the ink can be dried quickly after the printing is finished directly affects the quality and the production efficiency of the printed matter. If the ink is not dried in a timely manner, it will cause the back of the printed product to be stained, stick back, and cause reverse fault-printing or color mixing when entering the next set of printing, which will affect the duration of the postpress production. So we want the ink to fix and dry quickly after it is transferred to the surface of the substrate. Below we will analyze the problem of ink drying in the sheetfed offset press (hereinafter referred to as offset printing).

To solve this problem, we must first understand these issues:

Dry form of ink

There are three main types of dry ink used in offset printing:

Permeation drying (Rely on the penetration of the low-viscosity component in the ink during printing, leaving the highly viscous components <resin and pigment> on the surface of the paper)

Oxidative polymerization drying (reaction with oxygen in the air, is a three-dimensional spatial distribution of dry vegetable oil molecules into a network of macromolecular structures.)

UV drying (Ink linking material is added with a photosensitive material, which decomposes activated atoms or atom groups under the irradiation of a certain wavelength of ultraviolet light, and undergoes a chain reaction with an ink component containing an unsaturated bond, so that the molecules crosslink, and the ink is cured and formed into a film. ).

In fact, there are many ways to dry ink, such as volatile drying, infrared drying, microwave drying, hot air drying, but in the current use of sheetfed offset printing machine is still small, solid text is not described>

Ink composition

　　 The composition of the ink is mainly composed of a pigment, a binder, a filler, and an adjuvant, and the main ingredient that determines the drying method is a binder. The composition of the binder varies and the drying mechanism varies.

There are two main types of inks we use in sheetfed printing today: regular resin inks and fast-drying bright inks.

The main components of the binder resin in ordinary resin inks are resin, mineral oil, and a small amount of dry vegetable oil, which are mainly dependent on the penetration of the mineral oil components in the binder, so that the resin binder on the surface of the substrate continuously increases the viscosity and achieves Dry solid, which dry vegetable oil but also with the oxygen in the air oxidation polymerization reaction further drying. This also determines the scope of application of this ink is mainly for the printing of the surface of the absorbable substrate.

The content of dry vegetable oil in fast-drying bright ink is obviously higher, and the content of mineral oil is lower than ordinary resin ink. The drying of ink mainly depends on the two steps of mineral oil penetration and dry oil oxidation conjunctiva. The dry oil component contained in the adjuvant can promote the oxidation process of the dry oil in the ink (principle: adsorption of phospholipid antioxidants in the ink, and promote free radicals, free radicals with high activity Polymerization continues to occur). It is mainly used for substrates with poor smooth surface absorption (such as exquisite pictures and printed copperplate paper).

Transfer of ink from printer to substrate surface

The first is the transfer of ink from the ink fountain through the ink distribution system and the printing plate to the surface of the blanket.

High-viscosity inks are subject to splitting, shearing, and rubbing on the press, the viscosity decreases and the flowability increases (this is due to the thixotropic nature of the resin colloid in the ink.)

"The so-called thixotropy is the effect of the colloid under the action of the external force to reduce the viscosity of the fluid and improve the viscosity and fluidity of the colloid when the external force is removed."

The ink is then transferred from the blanket to the surface of the substrate.

The ink is absorbed by the surface of the substrate under pressure. At the same time, the low-viscosity phase (mineral oil <ink oil> and a portion of vegetable oil) in the ink begins to penetrate the interior of the paper. The viscosity of the ink begins to increase due to the loss of the low-viscosity phase. When the property is lowered, the ink changes from a liquid state to a semi-solid state and cannot be transferred again. This is the fixing stage of the ink.

Then the substrate leaves the embossing area and begins to enter the next color print (multicolor machine).

Due to the disappearance of pressure, the surface of the ink and the substrate are relatively stationary, and the viscosity of the ink is further increased (thixotropy of the ink). The strength of the semi-solid ink film formed at this time is to ensure that the ink is firmly fixed on the paper to ensure that the ink film is not damaged when the next overprinting and product stacking are carried out, and no pre-conditions for back-scratching occur.

Finally, the stationary process of the substrate after printing on the printing press. In this process, the remaining mineral oil in the ink on the substrate continues to penetrate into the paper (we refer to this penetration as free penetration) and the viscosity of the ink continues to increase; at the same time, the dry vegetable oil in the ink reacts with oxygen in the air. A huge network of molecular structures is formed. This process is the thoroughness of the ink.

The ink printed on the surface of the substrate until then has been completed. After understanding this process and its mechanism, we will find that it is very simple to analyze the problem of ink drying, because the problem of slow drying of ink in almost all printing is caused by the intervention of a step in the process. . Let's do some specific analysis of some factors that affect ink drying.

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.

Color Do not use white dry oil % % of red dry oil
BK 5 to 10 2 to 5
M 5 2 to 5
C 3 to 4 1 to 3
Y 3 to 4 1 to 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.