132. Why should the use of longitudinally twisted paper be used for sheetfed offset printing?
The increase in the size of the paper caused by water absorption is different in the direction of the longitudinal threads and in the direction of the cross threads, and the elongation in the direction of the longitudinal threads is about 2-8 times the elongation in the direction of the cross threads. When printing with a sheet-fed offset press, the use of longitudinally oriented paper on the one hand can make the absolute elongation of the long side and the short side of the paper not too different from each other; on the other hand, because of the long side of the paper during printing. Parallel to the roller axis of the printing press, overprinting of the shorter edge due to the change in the paper size can be remedied by adjusting the pad of the printing plate cylinder, and the overprinting of the roller axis cannot be made irreparably.
133. What are tight edges and flounces?
Stacked paper baskets, when the humidity of the environment is less than the moisture content of the paper, the moisture is released on all four sides of the paper, and the water is not in contact with the air in the middle. The water content remains unchanged, and the entire sheet is tightly deformed due to uneven deformation. When the humidity of the environment is higher than the water content of the paper, the paper absorbs moisture from the air and stretches on all four sides. The moisture content in the middle of the paper remains constant, and the four sides are wavy and deformed, and are called flounced edges.
134. How does the tight edge and ruffle of the paper affect printing?
Deformation of the paper brings difficulties to the transport of paper and proper positioning, especially for thicker paperboards, which are not easily corrected by auxiliary equipment after deformation. Therefore, after the cardboard is opened and cut, it should be put into print production as soon as possible. For thinner paper, wrinkles and overprinting are not allowed due to tight edges or flounces during embossing. Ruffled paper is prone to small bows and wrinkles on the trailing edge, and the tightly edged paper tends to appear as an elongated bow.
135. What are the factors that affect the moisture content of paper?
1 The type and purity of the raw material pulp;
2 At ambient temperature, the moisture content of the paper decreases with increasing temperature;
3 Ambient humidity, the moisture content of paper increases with the increase of humidity.
Therefore, the printing shop should try to maintain a relatively stable temperature and humidity.
136. What is the pH of paper?
The pH of paper refers to the paper's acidic or alkaline nature.
137. How does the pH of the paper affect printing?
Paper should generally be neutral, acidic paper is not conducive to drying the oxidized conjunctiva. If the acid-alkaline substances in the paper fall off and are transferred to the bucket, the pH of the wetting fluid will be affected. Therefore, the pH of the offset wetting fluid should be adjusted according to the acidity and alkalinity of the paper.
138. What is the ink absorption of paper?
Paper absorbency refers to the paper's ability to absorb ink after it has been inked. If the structure of the paper is loose, the fiber gap is large, and the capillary action is strong, the ink absorption capacity is large, and vice versa.
139. What is the negative impact of ink absorption on printing? How to overcome?
Too much ink absorption capacity of the paper will result in dull images or dull printing and chalking. If the ink absorption capacity is too low, poor drying may occur. The paper with too small ink-absorbing capacity is required to control the thickness of the ink layer, and quick-drying ink, dusting and other measures are used to prevent back-scratching. Generally, offset paper has better ink absorption than coated paper, and coated paper has better ink absorption than white paper. Thin-printing offset paper is prone to print-through phenomena, while printing thicker ink layers on whiteboard paper is prone to back-scratching.
140. What is the smoothness of the paper?
The smoothness of the paper refers to the smoothness and smoothness of the surface condition of the paper.
141. How does the smoothness of the paper affect printing?
Poor smoothness of the paper can cause poor contact on the imprinting surface and affect the ink transfer effect. In order to make the ink transfer uniform and complete, it is often necessary to increase the printing pressure, increase the amount of ink and enhance the ink flowability, but this will easily cause the ink spread to expand the dots and affect the level reduction. The finer the printed cable, the higher the smoothness of the paper. The smoothness of the paper is an important prerequisite for determining the number of color separation screens.
142. What is the whiteness of paper?
The whiteness of paper refers to the degree of whiteness of the paper surface.
143. How does the whiteness of paper affect printing?
The whiteness of the paper directly affects the color rendering effect of the color printing ink. The whiteness of color printed paper should be higher, and the whiteness of the same batch of printed paper should be the same.
144. What is the surface strength of paper?
The surface strength of paper refers to the binding fastness of the fibers, fillers and compounds in the paper composition.
145. How does the surface strength of paper affect printing?
The difference in surface strength of the paper is a major factor that causes hair loss and powder fall off during printing. The surface strength of coated paper is high, and the surface strength of whiteboard paper, which is commonly used in packaging offset printing, is poor. In the printing process, blankets and printing plates need to be cleaned frequently, which has a great influence on the production efficiency, printing quality, and printing durability of printing plates. The surface strength of paper decreases with the increase of water content.
146. How does the thickness of the paper affect printing?
The paper thickness refers to the thickness of the paper. The adjustment of the printing pressure is achieved by adjusting the center distance of the drum according to the thickness of the paper. The same batch of printed paper requires a uniform thickness. If it is not uniform, different printing effects may occur due to different printing pressures.
147. What are the contents of paper cutting quality?
1 The dimensions are the same and meet the specifications. When printing thick cardboard, the steel used for the feeder is harder. If the front and back of the cardboard are not uniform, when the length is long, the paper nozzle cannot pick up the first paper from the paper. The phenomenon of vacancy occurs; when the length is short, the steel sheet can not extend into the pile of paper to scrape the second sheet sucked by the suction nozzle and double-sheet phenomenon occurs. Therefore, cutting thick cardboard has higher requirements for the unity of size.
2 whether the incision is neat. Because the fiber on the back of the board is rough, when the knife is not sharp enough, it is prone to flash. The raw paper hairs are easily peeled off during the printing process and adhere to the blanket to affect the print quality. The burrs at the mouth can also easily affect the normal detection of the divergence detection mechanism and cause a shutdown.
Printing Technology - Packaging Printing