Anatomy of the problems of offset printing and dirty oil (1)

Dirty plate is a common fault, but in a sense is also a typical failure. In the production process, it is necessary to carry out a detailed analysis according to signs and parts of the layout with dirt, find out the cause of the malfunction, and take effective measures to prevent and solve them. In general, the layout with dirty can be divided into floating dirty, dirty oil, paste version and so on.

Dirty

Dirty refers to the phenomenon that a non-graphic part with a relatively large range and a relatively low density is dirty. Its dirty parts have no fat basis. Because of this, the non-graphical part of the filth, visible on the printed sheet, may be visible on the plate, and may not be visible. Observed from the surface of the printed matter, the dirty spots sometimes float everywhere. It is possible that the locations of the previous and the next dirty spots are different. In addition, generally with a dirty printed matter, the ink is usually dark and faint.

The main cause of buoyancy is ink emulsification caused by ink imbalance. In addition, there are oil paper quality, water, ink roller and printing pressure and other factors.

The specific causes and processing methods are as follows:

(1) Insufficient moisture resistance of the ink is emulsified in the wetting liquid.

Treatment method: To increase the ink consistency for soft or thin ink, add moisture-resistant varnish to the short-ink ink that is easy to soak.

(2) Precipitating the emulsifier from the paper coating and causing it to float.

Treatment method: change paper or harden ink.

(3) In multi-color printing, if the ink of each color is dirty, it may be a paper problem. If only one or two colors of ink are dirty, and other inks are printed clearly, it is an ink or plate problem. Monochromator also has a similar phenomenon.

Processing method: change paper, harden ink or change ink.

[Case 8-6]

A factory uses a four-color offset press to process a batch of 250g/m2 color box. From the point of view of the field prints, the four colors are all dirty and the black spots are the most. After the printer shuts down the plate, it disappears after cleaning the plate, but it appears again after starting the machine.

[case analysis]

Similar to the above example, can it be considered a paper problem? Although all four colors are filthy, but the degree is not equal, so we cannot conclude that it is a paper problem. How should we check it?

(1) Check the viscosity, drawability, and ink volume of the ink.
(2) Check the water roller and ink roller pressure.
(3) Check whether the ink balance is in the best condition (observe whether the water in the layout is too large)
(4) Check whether the parameters of the tank are normal, including temperature, alcohol concentration, conductivity value, and PH value.
(5) Clean the water tank and replace the wetting fluid.

Follow the above procedure carefully and then draw a conclusion.

Fouling is often caused by large quantities of printed materials. This type of quality accident happens in packaging and printing, and the operator must pay attention.

[Case 8-7]

The sticky box of a batch of cigarettes printed by a certain factory is printed in red, and the Chinese pinyin is a hollowed out word. In order to highlight the blank after indentation, the white ink is overprinted in the word. When the operator prints red, the layout is printed. The overall production of dingy white words so that the hollow red, and finally make the batch of one-third of the live.

[Expert Tips]: This kind of example is strictly speaking should not happen, this is mainly due to the operator's lack of awareness of quality, that the red version is a little dirty, white ink can cover, but the results prove that mistakes.

Two oil dirty

Dirty oil refers to the blank part of the printing plate that is not fat-sensitive, so that the ink adheres to it and forms a dirty plate. The main causes of fat in the blank part of the printing plate are: insufficient amount of water in the blank part or weak acidity of the single wet liquid, or other reasons that damage the hydrophilic salt layer in the blank part of the printing plate.

The biggest feature of the dirty oil is that the dirty parts are relatively fixed, and the dirty parts are not easily wiped off. In addition, the water can be removed by rubbing the syrup and peach gum, but the oil dirt with serious damage to the sand eyes cannot be eradicated.

There was oil in the printed matter. If you look for the reason, it is because the relevant parts of the printing plate and the friction are in doubt. example:
(1) The plate to blanket roller pressure is too high.
(2) The ink roller pressure is too high.
(3) The plate is not tightened during loading.
(4) Insufficient lipids and additives.
(5) Wetting fluid PH is too large.
(6) Blanket aging, lack of elasticity.