Relationship between protective glue and plate making quality
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The printed graphic part of the printing plate has the characteristics of oleophilic water-repellent; on the contrary, the non-image part is hydrophilic and oil-repellent, so the plate-making quality will directly affect the quality of the offset printing product, and the blank part of the positive-working PS version is Metal oxide Al2O3 and its crystal water are high-energy surfaces (0.7J/m2), which can be well wetted by dampening solution during printing. It is easy to react with O2, N2, CO2 in air, etc. At the point, the surface energy is lowered, the wettability is deteriorated, and impurities, oil stains, and the like are adsorbed, and it is difficult to clean and cause viscera during printing. Therefore, it is necessary to protect the blank portion, utilize the film forming property of the glue (the low energy surface of the polymer compound), isolate the layout from the air, and prevent the surface from being scratched, protected from light, and ensure the hydrophilicity of the blank portion itself and impart The blank part has better hydrophilic properties. This article provides some references for plate making by introducing the methods and precautions for the use of protective adhesives.
First, the composition and role of protective glue
1. Composition
A transparent glue composed of natural glue or synthetic glue, which can protect the graphic part and the non-image part of the printing plate, and prevent the plate from oxidizing a dirty liquid. Commonly used protective glues are gum arabic and new polymer glue (the protective glue used by the author is gum arabic, and this article takes this as an example).
2. Characteristics
2.1 hydrophilic. The molecular structure of gum arabic consists of -COOH and -OH attached to the same hydrocarbon bond. These two functional groups are hydrophilic groups, and the hydroxyl group (-OH) is more susceptible to moisture than the carboxyl group (-COOH). Therefore, when the gum arabic is dissolved in water, it will dissolve for about 72 hours according to the diffusion-water absorption-swelling process to form a molecular solution of the dispersion system, wherein the hydroxyl groups in each molecule are extremely hydrophilic and can be in water. The automatic dissolution is a single-phase stabilization system, which does not condense or precipitate, and can ensure the stability of the dispersion system for a long time.
2.2 weakly acidic. Since peach gum contains arabic acid (XCOOH), its aqueous solution is weakly acidic, neutralized by alkali, reacts with metal or metal oxide to form arabic acid salt, and emits hydrogen, which has good adsorption.
2.3 is easy to corrupt. In hot flashes (gum arabic use temperature is -5 ° C ~ 45 ° C), yeast is easy to propagate in the gum arabic fermentation, so that the arabic acid is free, changing the gelatinity. This is the generality of sugar substances, and it is also the reason why gum arabic foam and odor can be rotted when used for a long time. As the temperature increases, the acidity increases, the corrosion of the metal plate is enhanced, and the plate is easily lost. Al2O3 falls off. This should be paid special attention in the summer to keep the protective glue away from high temperatures.
3. Function
Improve the hydrophilicity of the non-image parts and prevent the print from getting dirty. Protect the layout and prevent scratches.
Second, Reasons for the use of protective glue in accordance with the prescribed ratio:
Take the school-based factory protective rubber as an example, dilute it with water in different proportions, and test its specific gravity separately, as shown in Table 1.
Figure 1 is drawn according to the test conditions in Table 1.
1. Relationship between specific gravity and dilution rate It can be seen that as the dilution ratio of the rubber increases, the specific gravity decreases, and after reaching a certain ratio, the specific gravity is close to 1 (the specific gravity of water).
2. The relationship between dilution rate and quality
We also do this experiment: according to different proportions, look at the quality of the plate after coating.
It can be seen from Fig. 2 that the quality of the PS plate is the best when the specific gravity of the protective rubber is between 1:1 and 1:3. It can be seen that the specific gravity of the protective rubber has a large change in the proportion, which should be strictly correct. The proportion uses the PS version of the protective glue.
Third, the failure of the use of protective glue caused by improper
1. The protective glue is too thick
1.1 When printing on the machine, the ink is not good
Figure 3 shows the state of the ink on the upper machine when the protective rubber is normal; B is the state of the ink on the upper machine when the protective rubber is too rich.
The protective glue is too thick, which causes the adhesive layer on the plate after the glue to be too thick. When the machine is on the machine, it is easy to cause the printing of the plate in the printing process.
Countermeasure: Dilute to the standard glue concentration; at the same time, use the water plate before the machine.
1.2 It is prone to cracking before printing on the machine
Figure 4A shows the state of the upper machine when the protective glue is normal; B is the state of the upper machine when the protective glue is too thick.
When the protective gel is too thick, it will cause cracking (as shown in Figure 4C). The black wavy line in the picture is the crack, where the blank space will be
Oxidation occurs directly, causing the upper machine to print dirty.
Countermeasure: It can be printed normally after being treated with Jiejie paste.
2. The protective glue is too thin
2.1 It is easy to rub the damaged layout before printing on the machine.
Figure 5A shows the state in which the protective gel is in the normal state of the machine; B is the state in which the protective gel is too thin.
This kind of situation often occurs during plate making, and attention should be paid during plate making. At the same time, the operator must pay attention to prevent scratches when taking the plate.
Countermeasure: Add padding between the two plates to adjust the concentration of the glue to avoid rubbing the surface. Our plate makers can reuse the paper in the newly purchased media.
2.2 Before brushing the machine, the spots are dirty.
Figure 6A shows the state in which the protective gel is in the normal state when the glue is normal; B is the inked state when the protective gel is too thin. Obviously, it has been oxidized before the machine is on, because the layout is not well protected, and the spots are dirty.
Countermeasure: Adjust the concentration of the glue, and print it with the cleaning paste.
3. When the glue layer is not dry, the overlap or storage causes the glue layer or the photosensitive layer to peel off, as shown in Fig. 7.
After the protective glue is dried
Countermeasure: After applying the glue, blow dry and stack.
4. Stop the dirty
The reason is that the screen is not coated with protective glue after the middle of printing, and the downtime is too long, so that the surface of the PS plate is oxidized and caused by dirt.
Countermeasure: Apply protective glue after shutdown, and print with the clean version of the paste.
5. Protective glue drying temperature is too high
The state in which the protective gel is in the normal state of the machine. When the gel drying temperature is too high, the state of the machine is inked.
Countermeasure: Reduce the drying temperature and control it below 70 °C. Our actual temperature control is about 65 ° C, not more than 70 ° C, otherwise the protective adhesive will cure.
Fourth, the use of protective glue:
Hand glue: Use collagen solution and water to dilute 1:1. Pour an appropriate amount of glue (about 45 ml of 1 m2) onto the plate, spread it with a cellulose sponge, wipe off the excess glue with a dry cloth, and dry it.
Machine gluing: use collagen solution and water diluted 1:3, drying temperature should be controlled below 70 ° C, generally 60 ° C ~ 65 ° C.
Note: The glue applicator without supplementary structure should be replaced every three days. Gluing machines with additional construction should be replaced once a week. The lower the concentration of the protective gel, the better the ink receptivity at the time of printing, but it is easy to be scratched and dirty. On the other hand, if the concentration of the protective gel is too high, it is likely to cause poor ink.
Five, protective glue in the use of management considerations
Schematic diagram of the conductivity, specific gravity, and pH of the protective rubber in various states (Fig. 9).
■ When the protective gel concentration is normal, the pH of the protective gel is weakly acidic.
■ The developer is backflowed, causing a sudden rise in pH.
■ Water reverses, reflecting a decrease in specific gravity, electrical conductivity, and an increase in pH.
Prevent the concentration of protective glue: Since the moisture in the protective rubber is volatile, the concentration of the glue will become larger, so the protective glue should be diluted or replaced in time.
The use of a deteriorated protective adhesive is difficult to remove during printing, and the adhesion of the photosensitive layer to the PS plate is reduced, the plate is lost, and the oxide layer is peeled off in severe cases. For the large-scale surface of the large-format glue, the protective glue concentration can be appropriately reduced.
When the protective glue is not dry, do not touch the layout in the printed area to prevent the fingerprint from being printed on the printed matter. The protective adhesive is harmful to the human body. Pay attention to safety when using it. Do not let the protective plastic touch the skin, clothing, or splash into the eye. Inside.