Second, the wetness of the ink roller and its changes
The ink roller is mainly used to transport ink on the printing machine. In order to spread the ink quickly and evenly between the ink rollers, the printer is generally configured by alternately using soft and hard ink rollers so that adjacent ink rollers can make good contact.
1. The condition of the ink wets the ink roller. Each ink roller with good oleophilicity accepts ink from the previous ink roller and then transfers the received ink to the next ink roller. In this process, the ink wets the surface of the ink roller and then adheres to the roller. If the specific surface energy of the ink roller surface is Î³SG, the surface tension of the ink is Î³LG, and the surface tension between the ink roller and the ink is Î³SL, then the adhesion work of the ink roller to the ink Wa is:
The cohesive work Wc of the ink itself is
When the cohesive power Wc of the ink is less than the adhesion force Wa of the ink roller to the ink Wa, that is Wc<Wa, the ink wets the surface of the ink roller and adheres to the ink roller; when the cohesive power of the ink is greater than the viscosity of the ink roller to the ink When attaching power, that is, Wc>Wa, the ink cannot wet the surface of the ink roller and fall off from the ink roller. Taking into account the (1-23) and (1-24) equations, the conditions for the adhesion and shedding of the ink on the ink roller surface are:
Î³SG>Î³LG+Î³SL (ink adhesion)
Î³SG<Î³LG+Î³SL (ink shedding)
The soft ink roller used in the printing press is generally made of a high polymer material such as vulcanized natural rubber, synthetic rubber, gelatin and polyurethane. Although the roller surface of the soft ink roller is a low-energy surface, the ink roller material has good oleophilicity, so the ink can wet the roller surface and adhere to it.
Hard ink rollers used in printing machines are generally made of metal materials such as iron and copper. The roller surface of the metallic ink roller is a high-energy surface, Î³SG>>Î³LG+Î³SL, and it is apparent that the ink can wet the roller surface and attach to it.
2. Roller wettability changes. Most of the soft ink rollers are made of rubber material, and the wettability and other properties are basically the same as those of the blanket. However, the diameter of the ink roller is several times smaller than the diameter of the plate cylinder. Therefore, the angular velocity of the ink roller is approximately several times larger than the angular velocity of the plate cylinder. Therefore, under the condition of high frequency rolling friction, the phenomenon of heat rise occurs. Significantly, thermal aging occurs first on the surface of the roller, causing the surface to harden, crack, and even fall off in small pieces. Only by removing the heat aging layer that has been formed can the original oleo-ink transfer performance of the ink roller be restored. In order to slow down the thermal aging of the rubber, the tandem rollers in the ink inking unit of the printing press should be equipped with cooling and cooling facilities. During the printing process, it is also necessary to prevent the ink from forming dry film on the surface of the ink roller and the early drying of the ink on the ink roller, and to remove the remaining ink on the ink roller in time. At the end of printing, the ink roller must be cleaned.
Hard ink roller, due to the different materials, the change in wetting performance during the printing process is not the same.
Certain ink rollers of offset presses are made of metallic iron, such as the J2101 offset press. The iron itself has poor lipophilicity. When the amount of wetting fluid is large and the ink emulsification is severe, the electrolyte in the wetting fluid is Or hydrophilic colloid, with the wetting liquid dispersed in the fine beads in the ink is absorbed by the iron, the formation of a hydrophilic film on the ink roller, the ability of lipophilic drop, the ink is difficult to adhere to the ink roller surface, often appear "off "Ink" phenomenon, only the physical or chemical method to remove the hydrophilic film, ink roller can restore the original adsorption ink, transfer ink performance.
A layer of metallic copper is plated on the surface of the iron ink roller to make a metallic copper roller instead of the iron ink roller. For example, the J4102 Offset Press uses a copper tandem roller. Obviously, the ink transfer performance is better than that of an iron tandem roller. However, copper is a metal that is easily oxidized. Particularly, in an emulsified ink, an oxidant is contained in the droplets of the wetting liquid, and the copper roll surface is oxidized to form CuO (copper oxide). On the other hand, the soft rubber in the printing machine's inking system slowly desulphurizes during the transfer of ink. The copper ink roller is in contact with it for a long period of time, and black streaks are generated on the contacted parts. This is the occurrence of copper and sulfur. The chemical reaction produces Cu2S (cuprous sulfide) and CuS (copper sulfide). Copper surfaces covered with CuO, Cu2S, and CuS suffer from a decrease in the oleophilic ink-absorption property, resulting in the occurrence of a "deinking" phenomenon. Usually, 10% dilute nitric acid is used to remove this layer on the surface of the copper ink roller.
Polymers such as synthetic resins and plastics are non-polar and have good lipophilic properties. If a strong, wear-resistant polymer is cast onto the core of a metal ink roller to form a hard rubber roller, not only the ink transfer performance is enhanced, but also no hydrophilic inorganic salt layer or adsorption is deposited on the roller surface. Hydrocolloids, ink roller rarely occurs deinking phenomenon. Nowadays, some new types of printers compete with high polymer materials to form ink rollers. However, the mechanical strength of this kind of ink roller is not as good as that of metal. In use, striking collision and ink knife scraping should be avoided.
Polymer hard ink roller and rubber soft ink roller, rubber blanket, poor solvent resistance, cleaning ink roller, we must choose the right solvent formulated as a cleaning agent, or the ink roller surface will swell and dissolve, adhesive layer Loss and can not be used.
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