210. What are the requirements for the performance of offset materials for offset printing?
1 For woolen felt and felt, it requires uniform thickness, good sensitivity and small plastic deformation, and has a certain tensile strength, and it has low elongation in the process of tension.
2 For the backing paper used for the lining of the plate cylinder and the rubber cylinder, especially for the backing paper used for the hard lining, the thickness is uniform, the structure is tight, the texture is hard, and the plastic deformation is small under the strong pressure; Can afford a larger pull, not torn or thinned during the tightening process. Ordinary paper is difficult to meet these requirements, and Manila standard paper is generally used.
211. What problems should you pay attention to when blankets are cut?
1 Identify the warp and weft directions of the bottom cloth of the blanket. The bottom cloth of the blanket is formed by interweaving warp and weft threads. When the same tension is applied, the elongation in the warp direction is far smaller than the elongation in the weft direction. When the blanket is stretched, the more it stretches, the more severe the thinning of the adhesive layer will result in the lower elasticity of the blanket and the more uneven the pressure. Generally, the base cloth of the blanket has a mark indicating the direction of tightening, and it can also be tried by hand. The person who does not easily stretch is the longitudinal direction and should be used as the direction of tightening, especially in the pair that will be scrapped due to hard damage. When full-length blankets are cut into 4 open blankets, attention should be paid to warp and weft directions.
2 The two sides of the blanket must be parallel in the axial direction, otherwise the blanket may be twisted due to the different degrees of tightness. Before you cut the blanket, carefully draw the parallel lines and cut them with a sharp cutting knife.
212. What is the composition of the compression deformation of the blanket cylinder liner?
The compression deformation of the blanket cylinder liner is composed of three parts: sensitive elastic deformation, hysteretic elastic deformation and plastic deformation.
213. What is the sensitive elastic deformation? What are the characteristics of sensitive elastic deformation in the lining?
Sensitive elastic deformation refers to the effect that the deformation energy that the object undergoes after the force can keep up with the external force, that is, the stress and the strain energy reach equilibrium momentarily. After a long period of use, the lining of the lining is changed because of the conformation of the rubber molecules, and the molecular chain gradually becomes rigid and loses its sensitivity and elastic deformation.
214. What is called elastic deformation? What are the characteristics of the elastic deformation of the lining?
Hysteresis is the speed at which the deformation of an object does not keep up with external forces. It existed at the beginning of the lining and disappeared after printing a large amount of printed matter. Even if it is a new liner, because the elastic deformation recovery time is much longer than the second printing interval, it can be treated as a plastic deformation.
215. What is plastic deformation? What are the characteristics of plastic deformation in lining use?
Plastic deformation means that the deformation that occurs after an object is stressed cannot be restored after the external force is removed. The new package lining in its early use, the greater the amount of plastic deformation, but in each printing cycle, the occurrence of plastic deformation is smaller than the previous cycle, the overall lining deformation gradually stabilized. However, very little plastic deformation is always present. After a large number of printings are combined, the stability of the blanket deformation disappears.
216. Why does the newly changed liner have sufficient pressure when it is used, and may be under pressure after printing for a period of time?
Because of the large plastic deformation of the new liner during its initial use, the thickness of the liner decreases and the pressure decreases. With the use of a soft liner, the thickness of the lining is reduced more, and approximately 4000 sheets need to be printed in order to achieve approximately the stability of the lining deformation. At this time, it is necessary to add a liner, and the amount of additional lining is large. The use of a hard lining reduces the thickness of the lining and requires approximately 1000 sheets. At this time, it is also necessary to make up the lining, but the amount of lining is less.
217. Why is the original web position prone to underpressure after the printing format has increased?
When printing a certain sheet of paper, the pressure between the blanket cylinder and the plate cylinder is uniform, but the pressure between the blanket cylinder and the impression cylinder is not uniform. Within the width of the paper web, the blanket cylinder is compressed The amount is larger, the pressure is greater, and the amount of plastic deformation that occurs is also greater; at locations other than the paper web, the blanket cylinder has less compression or no compression. When reprinting paper with a larger format, the thickness of the original inner liner is different due to the different amount of plastic deformation caused last time. The pressure generated by this printing will also be different. The printed product will have ink color. Uniform phenomenon. The severity of this phenomenon is related to the following factors.
1 printing quantity. The more the number of prints for the previous product, the more severe the heterogeneity.
2 printing pressure. The greater the printing pressure of the previous product, the more severe the resulting non-uniformity.
3 Blanket performance. In general, the elasticity of air-cushion blankets is better than that of ordinary blankets, and pressure unevenness is not likely to occur.
218. What is the tensile deformation of the blanket?
The stretch deformation of the blanket means that in order to tighten the blanket on the blanket cylinder, tension must be applied to the plates at both ends of the blanket, and the resulting blanket is deformed.
219. Why can't the blanket stretch too tight?
1 Because of the incompressible nature of the rubber, elongation of the rubber will inevitably lead to thinning of the adhesive layer, decreasing the thickness of the lining, and reducing the elasticity. The tighter the tension, the more the elasticity is reduced. For the air-cushion blanket, even the microbubble sponge layer will fail.
2 The tensile force and tension of the platen at both ends of the blanket are relatively large. In the middle, it also receives the positive pressure of the cylinder body against the blanket, the tension is small, and the tension in the entire circumferential direction is not even, which makes the thickness of the blanket in the circumferential direction decrease. Uneven levels can cause uneven printing pressure. The tighter the tension, the worse the pressure unevenness.
3 is too tight and will accelerate the stiffening of the blanket.
4 Stretching too tightly will change the rolling ratio of the blanket cylinder to the plate cylinder and the impression cylinder, which may cause an increase in the slippage of the nip gap.
----"Printing Technology - Packaging and Printing"
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