Use of ink doctor blades in flexible packaging gravure printing

Many faults are likely to occur, such as mottled printing and loss of gloss; ghosting with uneven density; blade lines; and so on. These faults may come from the film itself, or from mechanical rotation, operator skill, or indoor temperature and humidity, and most of these faults are closely related to the

ink. Therefore, correctly selecting, installing and using the doctor blade will greatly reduce the failure rate.

The doctor blade plays a very important role in gravure printing. Correct use of the doctor blade not only makes the printed ink color bright and saturated, but also reduces printing quality accidents to a minimum.

When a gravure press runs at high speed driven by a motor, the doctor blade also oscillates accordingly. Gravure presses produced by different manufacturers have different structures for the doctor blade oscillating mechanism. For example, the

rotary gravure press produced by Guangdong Shanzhang Light Industry Machinery Factory that our company uses,

has its oscillation driven by a dedicated micro-motor, while the model gravure press produced by Zhejiang Haining People's Machinery Factory

model gravure press, the blade oscillation is driven by transmission gears. Whichever form of oscillating structure is used, the same goal must be achieved: scraping off excess ink while reducing the friction between the doctor blade edge and the printing plate, so as to extend the service life of both the doctor blade and the printing plate.

I. Selection of the doctor blade

If the doctor blade has no elasticity, it cannot quickly return to its original position, and it will also leave blade streaks, blade powder and copper plate powder produced during oscillation on the printing plate, greatly affecting the color of the printed product.

A doctor blade containing a certain proportion of

should be selected; the blade thickness is usually chosen as

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0.18mm

. A blade that is too thick has no elasticity and cannot scrape the ink clean; on the printed product this often shows a foggy appearance, which not only affects the registration of the next color but also causes the quality of the same batch of printed products to differ greatly from start to finish.

If the blade is too thin, it will easily leave blade marks on the printing plate, and under the action of friction the blade will gradually wear, which inevitably affects print quality. We have noticed that when printing a large solid white background, longitudinal streaks often appear when too thin a blade is used.

The hardness of the doctor blade also affects print quality. The blade is made of iron, while the gravure plate is a copper cylinder with a chrome-plated surface; since the hardness of copper is

, and the hardness of chrome is

, it is best to choose a doctor blade with a hardness of

so that it neither damages the printing plate nor easily leaves blade marks, and it can also scrape off excess ink from the plate surface.

The edge of the blade should be arc-shaped, which helps protect the printing plate. Since the plate surface is arc-shaped, if the blade edge is also arc-shaped, it will not easily damage the plate and can easily scrape the ink clean, ensuring that the non-image areas of the plate are clean and free of ink retention. If the blade tip is flat, since a flat edge is very sharp, it will both easily damage the plate and affect the scraping effect, thereby affecting print quality.

II. Installation of the doctor blade

When installing the doctor blade, a backing blade is often used for support. The backing blade is generally made of hard steel sheet; the backing blade thickness selected by our company is

. The backing blade is generally

clamped with a nut at the center of the doctor blade, at a distance from the back of the doctor blade of

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approximately, and from the doctor blade edge

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approximately (the specific position should be determined through tests).

If the blade is installed protruding too far, it will easily deform and become wavy; during high-speed printing, some areas are scraped clean while others are not, which not only affects print quality but also causes the blade to wear easily. If the blade protrudes too little, it lacks the necessary elasticity, and the pressure must be increased to scrape the ink off the non-image areas (without cells), which will also damage the printing plate. Therefore, installing the blade correctly is crucial and must be adjusted according to actual conditions.

The installation position of the doctor blade (the distance from the contact point between the doctor blade and the plate cylinder to the nip point between the impression cylinder and the plate cylinder) has a great influence on print quality.

Most gravure presses allow the blade to be adjusted up and down and back and forth by adjusting the adjusting nut of the backing blade. If the adjustment is poor, the printed product will be dull and unclear, and faults such as plate drying, plate blocking, no ink pickup, feathered edges and poor ink layer adhesion will appear during printing. The installation position of the doctor blade is related to printing speed and ink drying speed. Since the ink drying speed can be adjusted according to the printing speed, the installation position of the doctor blade should be adjusted according to the printing speed. Generally, a plate cylinder with a diameter of

approximately is used; when the machine speed is

60m/min

approximately, the distance between the doctor blade and the nip point is

. When the printing speed is high, a longer distance is better; when the printing speed is low, the distance should be shorter.

III. Use of the doctor blade

Before installing the doctor blade, whether it is a new blade or an old blade, the edge should be ground first to ensure there are no nicks or burrs; the edge angle should be

is appropriate. Our factory usually first adds a few drops of machine oil on an oilstone for rough grinding, and then uses

waterproof abrasive cloth, also with a few drops of machine oil, for fine grinding. Adding a few drops of machine oil provides lubrication and reduces friction; the grinding result is generally good, and fellow practitioners may wish to try it.

Most printing plants often encounter the phenomenon of printed ink being either too light or too dark, especially when printing halftone products. Apart from the ink transfer properties, the angle between the doctor blade and the plate cylinder should not be ignored either. Most gravure presses can be set to a

degree angle. A small angle gives a darker ink layer and good gloss, but the scraping effect is poor, and incomplete scraping and blurred images easily occur. If the angle is too large, the ink may not be transferred in the highlight areas, so that, for example, the main pattern with a shadow pattern is printed while its shadow cannot be printed. The solution is to adjust the blade position and raise the blade holder position so that as much ink as possible is transferred to the substrate film.

To adjust to the optimal angle, factors such as cell depth, blade position and printing speed must also be considered. If the cells are too deep, the angle should be larger.

Doctor blade pressure also has a very important influence on print quality; the pressure can be adjusted according to cell depth and image gradation. If the plate cylinder is very long and it is an etched gravure plate, the pressure applied should be greater. This is because etched gravure plates have deep cells, but burrs tend to appear at the edges of the pattern. However, too much pressure will easily scrape the ink out of the shallow cells, so that the highlight areas cannot be printed; moreover, excessive pressure will reduce the contact angle, making it difficult to scrape off excess ink from the plate surface, causing ink to accumulate under the blade and producing blade lines. Normally, for a doctor blade system in good working condition, the contact angle should be

Blade pressure that is too light is also not acceptable, as it will cause faults such as smearing. The solution is to blow air appropriately toward the blade edge, that is, the interface between the blade and the printing plate; a dedicated air-blowing device can be used for this.

After the doctor blade has been used for a period of time, burrs and cracks will appear, and the blade can no longer work effectively; at this point it is necessary to sharpen the blade or replace it with a new one. In actual work, sometimes the blade cracks, and its service life can be extended by grinding or adding shims, which saves the cost of using blades and also solves the corresponding quality faults.

After the doctor blade has been used, it should be kept

and lubricated; the usual practice is:

wipe the used doctor blade with a cloth dipped in

after drying, coat it with machine oil, put it into a

bag, and store it in a ventilated place. Usually, polyester blades are coated with

and stored separately by category for future use.

In summary, the correct selection and use of the doctor blade is crucial to improving the quality of gravure printed products.

This article is reprinted from China Packaging Network (pack.cn). Original link: http://news.pack.cn/show-376870.html. All rights belong to the original author; this article is for industry exchange and learning only.