Blanket and Liner Application Questions in Offset Printing (30-39)

30. What is the harm to the printing plate caused by excessive printing in the printing?

The friction of the rollers during the printing process can damage the plate and reduce the resistance to printing. Due to the speed difference on the surface of the drum, the cumulative friction amount causes the printing plate base layer to be damaged due to the frictional shearing force, so that the graphic basis gradually falls off and the plate is off, the blank grit is accelerated and flattened, and the printing force is reduced. Especially rigid tree mats, the friction is more concentrated on the surface, the printing plate is particularly easy to wear. Therefore, the three drums are relatively unequal in radius, and the surface speed of the three drums is unequivocal. Then, in addition to the compression flag running on the three drums, friction, pushing, and sliding are positive. Therefore, according to different models, different papers, different machine speeds, and product quality, we constantly change the thickness of the splicing liner so that the friction force develops in a direction that is conducive to the improvement of product quality. This is to explain the friction force. The purpose.

31. What is the relationship between excessive friction and dot distortion and dot quality?

The dot deformation is due to the speed difference exists in the three roller contact imprinting, which will inevitably produce a relatively large relative friction. Since the beginning of the new version, the dot gradually develops into an ellipse shape, causing directional expansion, unclear contour level, and finally forming a dirty version, paste version Seriously spread.

Ghosting is the process of dots in the printing process. Because the roller line speed error is too large and the dot overlaps, there is a serious slippage. This causes the dot to produce two imprints, one is a ghost image, and the other is a real image, which is called a ghost image. This is due to the speed difference caused by the friction of the blanket, and can not be reset instantaneously; sometimes friction is not large, but the poor tightness of the blanket will also produce ghosting.

Of course, the quality of ghosts and blankets also has a definite relationship. A good blanket can be tightened and can be reset instantaneously. A bad blanket stretches out a ghost image. When it is too tight, it can't be reset quickly. Slowly to find the feeling, generally no measurement of elastic gauges, this is a place where many offset machines have similar problems and are neglected. It is thought that the tighter the blankets are, the better. This is not the correct way.

32. What is "smooth hair" and "down hair"? How did it come about?

The plate surface is subject to excessive friction, and the ink is strong in polarity and the pigment particle group is hard, playing the role of abrasive, and the imprinting (especially hollow words or lines) can occur in a large area or in a small area evenly. "Flipping" phenomenon. Sometimes when the two rollers come into contact with each other, they come into contact with each other and then slip or slide. Then, the contact occurs. In the direction of friction, there is a point of falling hair. If the blanket cylinder has a large speed, the surface of the printing plate is subject to a large frictional force that points toward the mouth (in the direction of rotation), resulting in inverted hair (the direction of the generated hair pointing to the mouth is called inverted hair), and vice versa.

33. How did the "escape" and the paper escape phenomenon occur in printing?

The back of the blanket has a large amount of friction, so that the pad and felt underneath the blanket are displaced. When the displacement accumulates to a certain distance, it will cause a failure to escape the paper and escape; if you run forward (to the direction of the bite) ), indicating that the printing plate rolls quickly, that is, the linear speed of the surface of the plate cylinder is greater than the linear speed of the surface of the rubber cylinder, then the rubbing direction of the rubber cylinder is the running direction, so it escapes in the running direction; if it escapes backwards (dragging) It shows that the rubber roller is fast, and the direction of the rubbing force of the rubber roller is reversed, so it flees backwards. Escape the paper to escape it, so that the blanket is seriously uneven, and sometimes the paper will smash crushed rubber blankets.

When the rubber blanket escapes from the paper, traces of water ripples can be observed on the surface of the rubber and on the surface of the printed matter. When the surface of the rubber roller is touched by hand, it can be handled in time.

In order to prevent escape of paper, it is necessary to make the radius of the three drums. In addition, the blanket material (except for the blanket and the paper) can be sewn with a needle at the mouthpiece to make it a whole. In this way, the phenomenon of escaping from fleeing paper, which is generally directed toward dragging, can be considerably alleviated, because after one whole, at the same time, fleeing backwards and making moves is less likely, of course, only as an expedient measure.

34. What is the relationship between excessive ink friction and bearing wear?

There are quite a few reasons for ink sticks. Excessive friction force, under the effect of friction force, the layout can appear intermittent wear and imprint extrusion, resulting in similar creeping traces of tooth marks, known as pressure ink bars. This is mainly the alternating appearance of the blanket surface and the back gap friction.

Because the roller pressure is too large, the bearings and journals are subject to excessive friction, so that their wear is accelerated and the service life of the machine is reduced. Particularly, the wear resistance of the material is poor, and the damage is particularly serious.

35. The superiority of blanket transfer point?

Lithographic offset blankets are used as intermediate media for graphic transfer. The main reason is that the blanket is a kind of elastomer and lipophilic hydrophobic substance. Lithography is used for indirect media because it has: small pressure per unit area, Buffering effect, too much pressure can disappear in the elasticity, so that the pressure and softness of the printing plate is small, the amount of friction is small, the graphic ink is transferred clearly, and the network is also due to the characteristics of real reasons. Therefore, the transfer characteristics of the blanket is to use its elasticity, elasticity of the blanket can be divided into: high elasticity (or called balanced high elasticity) and viscoelasticity.

36. What is the high elasticity and viscoelasticity of the blanket?

(1) High elasticity of the blanket. The deformation of the blanket after the force can keep up with the speed of the external force, that is, the strain and the stress instantly reach balance, which is called high elasticity, or balance high elasticity.

(2) The dry elasticity of the blanket. Viscoelasticity is also called hysteresis elasticity. Some macromolecules exhibit both viscous and elastic properties under external forces. If the external force on it is slow and takes a long time, the deformation that it produces, when the external force is removed, the deformation does not disappear completely instantaneously, but gradually returns to its original shape after a period of time, it is like a highly viscous liquid, and its main performance For plastic deformation; if the external force is very fast and the action time is very short, they are like elastic solids, and they mainly show high elastic deformation. With this speed of action, the relationship between the action time and the deformation is called viscoelasticity.

37. What are the specific performances of viscoelasticity of rubber blankets in lithographic offset printing?

There are three issues:

(1) The degree of vulcanization of the rubber is poor, and the high elasticity is unsatisfactory. When the number of printings increases, the imprinting pattern produces a low concave deformation after the blanket is imprinted. Or sand, paper, etc., causing the blanket slight indentation deformation, etc., can not be restored within a short time, to achieve the plastic deformation of the blanket from the viscoelastic deformation, which is a great harm to the quality of the copy .

(2) The rubber blanket is resistant to heat, oil, and solvent. When the gasoline solvent or plant, mineral oil is brewed and the temperature is higher, the rubber surface will swell; on the one hand, the surface is sticky, and on the other hand, the external force After the action, the deformation recovers slowly.

(3) The aging stage of the blanket causes the blanket to completely lose its high elastic balance, so that the surface is hardened. Under the action of the accumulation of hard objects or ink, there is a trend of viscoelastic deformation and plastic deformation.

A good quality blanket can print about 2 dry squares. If it is 30,000 a day, a blanket can be used for about 3 years. In fact, a blanket used in the first half does not work, most of which is crushing, crushing, crushing; there is also a bad protection, such as the solvent is not wiped clean, long-term tension and so on. In order to extend the life of the blanket, the blanket should be loosened during the holidays. It is best to use two blankets in turn to alternately store and restore to maintain good elastic deformation.

38. What is the phenomenon of blanket aging?

The process in which the properties of a polymer compound gradually deteriorate is called aging. After the blanket is printed on the machine, there is a problem of the aging period.

During the printing process, the aging of the blanket generally shows:

(1) Surface smooth hardening;

(2) Loss of the balance of high elasticity;

(3) The transfer of outlets is not complete, and sometimes it is not true.

(4) Decrease in lipophilicity;

(5) The wear plate is severe and the resistance to printing is reduced.

39. What is the reason for the aging of the suede cloth?

There are many reasons for the blanket to go. There are probably:

(1) The rubber blanket is damaged due to the influence of ambient conditions for a long time, such as the effects of air, light, and heat, etc. The chemical structure is destroyed, the physical properties are changed, the mechanical properties are reduced, and it becomes hard, brittle, cracked or soft Change, no longer apply;

(2) Under the action of ozone and certain metal salt impurities, the aging of the blanket is promoted;

(3) Various reasons such as the structure of the blanket and the printing process conditions, such as the stress of the long-term fixing of the blanket warping and tightening on the drum, and under the effect of the pressure, with the cumulative increase of the number of printing, the rubber molecular chain Stiffness, loss of balance, high elasticity, peristalsis, stress relaxation, and other lags in the alternating force field and internal friction, etc., then promote the aging of the blanket.

(4) In the daily printing production, the ink containing a small amount of insoluble resin accumulates on the surface of the rubber and solidifies, so that the surface of the blanket is bright and smooth, hard and brittle, and the surface is cracked when serious, so that the aging period comes earlier.

(5) When the residual ink on the surface of the blanket is used with gasoline, it will be exposed to gasoline for a long time, and it will swell and become sticky and gradually lose its elasticity. In addition, long-term contact with acidic substances, electromagnetic effects, etc., can also accelerate the aging of the blanket.

Source: Guanhua Screen Printing