1,000 Offset Printing (248-256)

248. What is paper elasticity? What does it mean for offset printing?

A: Usually the paper has both elasticity and plasticity. Elasto-plasticity of paper refers to the elastic and plastic deformation of the paper when it is subjected to a certain external force. During the offset printing process, a certain pressure must be applied, and the graphic ink film on the printing plate can be transferred to the paper indirectly through the blanket. Paper will produce compression deformation at the moment of pressure action. When the force disappears, it can be restored to the original state of deformation, that is, elastic deformation; and the deformation still exists after the pressure disappears, that is, plastic deformation.

In multi-color, high-speed offset printing, the elastic deformation of paper will be repeated repeatedly. Therefore, the elasto-plasticity of paper is very important for offset printing, and the clarity and fullness of the graphic print are all related to the elasto-plasticity of the paper.

249. What is the surface strength of the paper? What is the reason for falling powder?

A: The paper surface strength refers to the binding fastness between the fiber, the filler and the rubber, or between the paint layer and the base paper.

In offset printing, there is a more complex mechanical force relationship between paper and ink. In order to ensure that the ink is transferred to the paper normally and the structure of the printed sheet is not damaged when it is peeled off, the following conditions should be met:

The binding force between the fibers, the filler and the rubber (or coating layer) in the paper ≥ the adhesion of the ink to the paper + the peeling tension + the adhesiveness of the blanket. Otherwise, paper lint and powder fallout will occur during the printing process.

250. How to avoid paper lint and powder loss?

A: Generally speaking, the paper binding force is a constant, but due to improper storage and storage, it will be reduced due to changes in water content. Therefore, if the above condition cannot be satisfied during the offset printing process, any one of the right side of the inequality will cause the paper to have different degrees of powder dropping and hair loss. Peeling or stratified peeling. Therefore, according to the actual situation of offset printing, make appropriate adjustments to ensure the quality of printed products. Specific adjustment method:

1 Reduce the adhesion of the ink to the paper surface. Under the premise of ensuring the ink color saturation, the viscosity of the ink is reduced and the thickness of the ink layer is reduced.

2 According to the size of the impression cylinder radius and printing speed to adjust to reduce the stripping tension. When the printing speed is constant, the smaller the radius of the impression cylinder is, the larger the peeling tension is, and vice versa; when the radius of the impression cylinder is constant, the higher the printing speed is, the larger the peeling tension is, and vice versa.

3 reduce the stickiness of the blanket. Under normal circumstances, generally do not need to consider the viscosity of the blanket, but if the blanket appears improper maintenance or desulfurization and sticky situation, it should consider reducing the viscosity of the blanket.

In actual offset printing, the method of reducing the printing speed or the viscosity of the ink is usually used to make up for the lack of paper strength, and to reduce the loss of paper and powder.

251. What is paper absorption and what does it mean for offset printing?

A: Paper absorbency refers to the paper's ability to absorb ink. In offset printing, the process of blotting the paper is accomplished under the combined effect of printing pressure, inter-fiber voids, and capillary tubes. The size of ink absorption properties of offset printing paper is mainly determined by the compactness and firmness of the fibrous tissue structure of the paper, and the strength of the ink penetrating the paper. In general, loose paper structure, the fiber gap is large, the capillary effect is strong, the ability to absorb ink is strong; paper structure is tight, the fiber gap is small, the capillary effect is weak, the ink absorption ability is weak.

If the ink absorption capacity of the offset printing paper is too large or too small, it will cause the imprint of the print to be dull, or the phenomena of overprinting, chalking and dripping will occur. Therefore, the ink absorption of the offset printing paper is crucial to the offset printing process.

252. What is paper smoothness and what does it mean for offset printing?

A: Paper smoothness refers to the smoothness or roughness of the surface state of the paper. It depends on the structure of the paper itself, the evenness of the tissue, and the compressibility. The paper smoothness plays a decisive role in the uniformity of ink transfer and penetration. Paper with high smoothness can be uniformly inked, and it is easy to obtain a clear graphic effect. If not, it is easy to cause the graphic imprinting to be unclear.

In the actual offset printing, when the paper smoothness can not meet the printing requirements, it is often adjusted by adjusting printing pressure or ink viscosity.

253. What is paper gloss and what is the difference between it and paper smoothness?

A: Paper gloss refers to the ability of the paper surface to reflect light, which is used to evaluate paper properties. Paper with high smoothness has good gloss, but glossy paper does not necessarily have high smoothness.

254. What is paper opacity and what does it mean for offset printing?

A: The paper opacity refers to the degree of opacity of the paper when the incident light is irradiated, which is used to describe the light blocking ability of the printing paper. Paper opacity describes the imprinting properties of the paper during the printing process. If the paper has low opacity, different degrees of print-through will occur during printing, affecting the clarity of the text. Therefore, for offset printing paper, especially for low-volume double-sided printing paper, the higher the paper opacity, the better.

255. What is the pH of paper and what is its effect on the drying of printing inks?

A: Paper pH refers to the degree of acidity or alkalinity of the base paper and coating layer. The acidity of the paper inhibits the curing of the ink film and affects the drying speed of the ink film on the surface. Offset papers are mostly weakly acidic because the offset fountain solution is acidic. If the paper is also acidic, it will significantly affect the drying of the blotting ink film. The paper's alkalinity has a certain squeezing effect on the ink, and at the same time it also affects the dosage control of the fountain solution. If it is too high, it will promote the emulsification of the ink and cause printing failure.

256. What are the common offset printing papers, and what are their characteristics and printability?

A: Common offset printing papers include offset newsprint, offset book, offset printing, writing paper, and cover paper. Offset printing coating paper, white paperboard, etc.

1 Offset newsprint. The paper is soft, uniform in texture, soft in color, uniform in quantity, moisture, stiffness, good opacity, moderate smoothness, high tensile strength, and high tearing. It can be used with web offset presses and resin type non-heat-set web offset printing inks with a printing speed of 10,000 prints/hour or more. However, when the roll is not tight, it is easy to produce wrinkles, color misregistration and other faults.

2 offset printing paper. The paper is soft, uniform in texture, uniform in hue, quantity, and moisture, good in opacity, high in smoothness, high in tensile strength, and high in tear. It can be used with web offset printing machines with a printing speed of 10,000 or more/hr and resin type non-heat-set web offset printing inks, or high-speed multi-color single-sheet offset printing machines and ordinary resin type offset printing inks.

3 Offset printing paper. The fiber structure is uniform, the texture is tight, white and smooth, the water resistance is strong, the stretchability is small, the opacity is large, the surface strength is high, and the ink absorption is moderate.

4 writing paper. The fibrous tissue is uniform, tight, smooth and has good water resistance. Compatible with monochrome or multicolor sheetfed offset and resin offset inks with print speeds up to 10,000 prints/hour.

5 book cover paper. The fibrous tissue is uniform, tight, smooth, consistent in color tone, and has good water resistance, folding resistance and abrasion resistance. It can be matched with single-color or multi-color sheet-fed offset presses and resin-type offset printing inks with a printing speed of 10,000 or less/hr.

6 offset printing coating paper. The surface is white, tight, smooth, with low stretchability, good resistance to wet friction, moderate ink absorption, and high surface strength. It can be used with sheet-fed offset presses and resin-based light fast-drying offset printing inks with printing speeds up to 10,000 prints/hour.

7 white cardboard. The paper is tight, tough, uniform in thickness, white and smooth, with low stretchability, good water resistance, moderate ink absorption, and high surface strength. It can be used with single-color or two-color offset printing machines and resin-based bright fast-drying offset printing inks with a printing speed of 10,000 prints/hour.