Offset Printing 1000 Questions (129-141)

129. What is the alkali resistance of ink? What is the relationship with offset printing quality?

A: The resistance of the ink to alkaline substances is called alkali resistance of the ink. The alkali resistance of inks depends mainly on the alkali resistance of the pigments and binders used. In the offset printing process, the effect of alkali resistance on the quality of offset printing is mainly manifested in the following aspects:

1 In the offset printing process, when the ink comes in contact with a weakly alkaline coating paper and is severely affected by the alkaline surface of the printing plate, the structure of the ink will be damaged, and the pigment and the binder will be decomposed, resulting in the appearance of discoloration or discoloration of the offset printing.

2 In the offset printing process, if an alkali-resistant ink is blended with another non-alkali-resistant ink, the transferred ink will not be alkali-resistant, resulting in offsets such as offset printing.

130. How to evaluate the alkali resistance of ink?

Answer: Assessing the alkali resistance of inks is usually carried out by hanging leaching or osmotic dyeing. It is also possible to take 20mm x 100mm squeegee samples. Under normal temperature conditions, rinse with 1% sodium hydroxide solution for 5 minutes. Remove and rinse with distilled water. The samples were then dried at 50°C and observed for changes. Alkali-resistant inks generally do not change.

131. What is the solvent resistance of the ink? How does it affect the quality of offset printing?

A: The resistance of inks to organic solvents, such as alcohols, esters, ketones, benzene, etc., is called the solvent resistance of inks. The solvent resistance of an ink depends mainly on the solvent resistance of the pigment. Some of the major lake pigments are generally not resistant to alcoholic solvents.

The wetting liquid itself does not contain alcohols, but in offset printing, in order to reduce the amount of wetting fluid and improve the wetting effect, some printers add ethanol or isopropyl alcohol to the wetting fluid. In addition, most packaging prints also need to be coated with calendering or laminating with a coating oil or adhesive containing alcohol base, nitro or acrylic resin and ester, ketone, benzene solvent, in order to improve the gloss and durability of the printed matter. Therefore, when the solvent resistance of the offset printing ink is not suitable, it will affect the offset printing product.

Quality, offset printing should pay special attention.

132. How to determine the solvent resistance of ink?

Answer: The hang-leaching method can be used to determine the solvent resistance of inks. If alcohol resistance is measured, 95% ethanol is usually used as a test solution.

133. What is the wax resistance of the ink? How does it affect the quality of offset printing?

Answer: The ink resistance to wax melt is called wax resistance. In the packaging and printing of foods such as bread, candy, ice lolly, and ice cream, most of them need to be coated with a wax melt of 80° C. or higher to protect the food. If the wax resistance of the ink used is relatively weak, after the coating process, the imprinting of the ink will cause discoloration and bleeding, which will affect the appearance of the food packaging quality and inherent quality.

134. How to evaluate the wax resistance of offset printing ink?

Answer: The wax resistance of ink is mainly determined by hanging immersion method. The specific operation steps are: scrape the ink on the paper (or take a sample), and wait for it to dry to take 15mm x 100mm as a sample. Half of the sample was immersed in a food wax melt (heated by melting in an oven at (115 ± 2) °C). At the same time, the timer was started. After 5 minutes, the sample was taken out of the oven, and then the sample was clamped with tweezers. After cooling, the wax was examined. The degree of bleeding and the degree of discoloration of the ink film are judged according to the following criteria:

Level 1: The food wax is heavily eroded and the ink film is severely discolored or discolored.
Level 2: Food waxes are visibly oozing, and the ink film is noticeably discolored or discolored.
Level 3: The food wax slightly bleeds, and the ink film slightly discolors or fades.
Level 4: The food wax does not bleed and the ink film does not change color.
Level 5: The food wax does not bleed and the ink film does not change color.

135. What is the lightfastness of ink? What are the factors affecting the lightfastness of inks?

A: The lightfastness of ink refers to the resistance of the ink film to sunlight.

In general, fully lightfast pigments are not available under current technical conditions, but only have different light fastnesses. The lightfastness of the ink is mainly determined by the pigment used and its lightfastness and its concentration in the ink system. Generally, the greater the concentration of the pigment in the ink, the higher its lightfastness. Therefore, when the color of the ink is evacuated, due to the change of the pigment concentration, the change of the light resistance of the ink will be caused correspondingly, and the degree of change will depend on the level of the ink being evacuated.

136. What is the significance of the lightfastness of offset inks for offset printing?

A: For prints that require long-term posting and display of advertisements, pictures, and product packaging, it is of great significance to reasonably choose the lightfastness of printing inks. Because the general ink ink film under the action of sunlight, the color of the pigment and the linker will change, for example: the color ink made of organic pigments, easy to become gray white; and the color ink made of inorganic pigments, it is variable dark. The fading or discoloration of prints will cause them to lose their artistic value and use value.

137. How to determine the lightfastness of the ink?

A: At present, the lightfastness of the ink is measured by using a rapid light fastness tester. The specific method is: the ink sample (scrape or printed sample) is cut into small pieces according to the specified size, together with the standard blue wool fabric, half covered with white paper, the other half exposed, clamped with the sample clip , mounted on a drum. The drum is driven by a motor and is subjected to a discoloration test under the irradiation of a UV lamp. The degree of fading can be divided into 8 grades. The lightfastness of grade 1 is the worst, and the grade 8 lightfastness is the best. The specific judgment criteria can be checked against the relevant rating standards.

138. What is the printability of ink?

A: The printability of ink, generally speaking, refers to the ink used for a certain printing method should have to adapt to its process and printing conditions (such as model, working speed, type, blanket, dampening fluid, paper, etc. The various necessary properties of the ink make it possible to balance the ink with various related factors to obtain the best printing effect and print quality.

In 1953, the second session of the International Institute of Printing (Association) institute defined the meaning of the suitability of printing inks as follows: After printing inks are used in printing operations, the printed products obtained after the drying process can fully match the expected results. Therefore, the full meaning of the printability of the ink refers to the sum of its applicability on the printing machine, the drying process, and the print quality.

In the actual printing research, for the sake of simplicity, it is usually divided into two aspects of process technology adaptability and print quality suitability.

139. What are the basic theories of printing ink suitability? What are the printing suitability of ink?
A: The printability theory of ink is relatively complex and involves a wide range of aspects, mainly based on the following three basic theories.

1 ink colorimetry theory: Ink colorology simply put, is to study the tricolor (or four-color) ink film overlay or juxtaposed on the surface of printing materials (such as paper), the color of the phenomenon and nature of the color, And its discipline in the relationship between various factors in the process of color reduction. In the printing process, the application of color light theory is mainly to explore, guide and research the suitability of ink color printing, focusing on the study of ink coloration efficiency and color density, glossiness, transparency, etc. to obtain the best color effect and realization. Gray balance, to achieve the printing process and people's visual requirements.

2 Rheology Theory of Ink: Ink rheology is simply to study the rheological phenomena and properties of inks in the process of printing, especially in the transfer, transfer and separation of the fractures on the printing press, and the resulting flow. A discipline that changes the relationship between various related factors. In printing, it focuses on the study of the mechanical behavior of the ink to explore and guide the printability of the ink.

3 Ink interface chemistry theory: In the printing process, ink flow and deformation at the same time, especially in the transfer process, accompanied by some interface chemistry. The interfacial chemical theory of ink is mainly used to study and process the transfer of ink, the separation and breakage of the ink film, the ink and the wetting fluid coexist at the interface of the printing plate and the blanket, and the ink penetrates into the fiber pores through the surface tension. Problems that arise during the process.

140. What is the interface chemistry of the ink?

A: The interface chemistry of inks refers to the characteristics of the interfacial chemical phenomena that occur during the flow, deformation, and transfer of the ink. It depends on the dispersion and wetting of the pigment in the binder. The wetting ability of the ink film on the surface of the printing plate, blanket, and paper in the transfer process mainly depends on the interfacial tension and surface tension of the ink, and the contact angle formed with the interface of each related body.

141. What is the technical applicability of printing ink?

A: The process technology suitability of printing ink refers to various properties that should be possessed during the printing process, especially when printing normally. The process technology adaptability of ink is also called operational applicability. It mainly relates to the ink characteristics of ink flowability, ink down, transferability and transferability.

Source: "Printing Technology"