Retail Hinged Lid Cigarette Box Overprint accuracy analysis of color printing

Retail Hinged Lid Cigarette Box Overprint accuracy analysis of color printing

 

Color plate making is a color decomposition process, and color printing is a color combination process, and this color combination process is called overprinting. One of the minimum quality requirements of color printing is that the overprint is accurate, that is, each monochrome picture and text is superplaced in the same position correspondingly, and finally the batch of printed matter becomes a faithful copy of the original manuscript. However, in the production process, due to various reasons, it is easy to cause product overprinting errors, so that a large number of products become defective or waste products, so that enterprises suffer great losses.

 

Therefore, the majority of printing workers make unremitting efforts to this end, summed up some valuable experience. In order to better exchange and study, this paper discusses the issue of printing overprint accuracy.

 

1. Factors affecting the accuracy of overprinting

There are many factors affecting the accuracy of overprinting. In addition to the film error caused by plate making factors, several main aspects of printing are described as follows:

1, the impact of the printing plate on the overprinting accuracy

Lithographic printing plates are mostly thin and light aluminum. During the printing process, changes in its shape or volume will affect the overprint accuracy.

 

The deformation of the plate is roughly caused by two aspects, the first is the stretching deformation, the second is the baking deformation. Stretching deformation refers to the overprinting inaccuracy caused by the artificial extension of the metal plate around the cylinder due to the improper force of the operator in the loading and drawing of the plate during the printing process. Baking deformation refers to the change in the linear size of the printing plate due to improper baking in order to improve the printing resistance of the PS plate. As we all know, most metals have the characteristics of thermal expansion, and when the plate is heated in the oven to reach 250 degrees Celsius, the plate base will be baked soft, affecting the stability of the layout size. The temperature in the oven is uneven or excessive baking, which will cause waste.

 

2, the impact of drum lining on the overprinting accuracy can ensure that the plate, rubber, imprinting 3 drum line speed is consistent, in order to effectively transfer under a certain pressure. If the plate drum lining is too thick, the drum radius is increased accordingly. In the case of the same angular velocity, the surface linear velocity of the cylinder with an increased radius will be accelerated. The resulting speed difference will not only cause the wear of the printing plate graphics, but also cause the layout graphics size to "compress" in the front and back direction. The drum lining is too thin, the radius is reduced correspondingly, the drum surface line speed is slow, the dot will be stretched in the transfer process, that is, it will cause the image area to "enlarge", resulting in inaccurate overprinting. On the two-color machine, this phenomenon is particularly obvious on the surface.

 

3, the impact of paper on overprinting accuracy

In lithography, the greatest impact on overprinting is paper, especially offset paper. This kind of misprinting caused by paper is difficult to control.

 

Paper is mainly composed of fibers. The characteristics of fiber are water absorption and expansion, dehydration and contraction. In the offset printing process, when the first color is printed and then the second color is set, the overprinting caused by the expansion of the paper is not very obvious. The paper has high structural strength, good smoothness, thick coating and small water absorption, which is especially serious for some papers with loose structure, light sizing and strong water absorption.

 

The paper expansion deformation due to the paper itself is affected by the temperature and humidity changes of the surrounding environment, in the printing process, but also by the extrusion of the drum. Extrusion makes the paper from the mouth part outward to the sector extension, the more to the two sides of the drag tip, the greater the overprinting error. This phenomenon is often referred to as "flinging."

 

4, the mechanical impact on overprinting

The printed paper must be effectively controlled by the mechanical components throughout the machine, and its relative position must be stable during transmission and handover. Only in this way can the requirements of overprinting accuracy be met, otherwise the overprinting accuracy will be greatly affected. The biggest impact mainly has the following situations: First, the paper positioning, that is, the problem of mechanical standard parts caused by overprinting is not allowed; The second is that in the process of transfer and handover of paper, the overprinting is not allowed due to the unstable handover, resulting in the change of the relative position of the paper.

 

In addition, when the paper is removed from the roller, it is subjected to the adhesion force and the peeling tension, and if the holding force of the paper teeth can not overcome the peeling force, the paper will twitch and there will be a small displacement. This kind of image appears as inaccurate overprinting on a single machine, and double shadow (ghost) on a multicolor machine.

 

Second, the control method of overprinting accuracy

According to the factors affecting the overprinting accuracy, the corresponding control methods are adopted to achieve the purpose of accurate overprinting.

 

1, the baking of the printing plate, must be done in the oven temperature to...

When correcting the version, we must standardize the operation, the clamping position is centered, the printing plate is right, and the clamping screw is fastened. When pulling the plate, the top screw should be loosened. As long as the force is moderate, the printing plate will generally avoid deformation.

 

2, the center distance of the drum is measured strictly according to the standard, and the axis line of the drum must be horizontal to each other, and the calculation of the envelope is accurate. Once the bag is lined, do a stress test. Generally, "screen", "map", "press bar" and other methods are used to ensure that the "pressure is flat" to meet the printing accuracy requirements.

 

3, to solve the impact of paper on overprinting accuracy, it is difficult, and there are many related factors. In order to improve the printability of paper and enhance its stability, the temperature and humidity of the printing environment should be kept relatively constant as far as possible. At the same time, according to the "hysteresis effect" of paper, the paper is processed before press. The usual method is hanging, the paper through hanging, so that its own water content and air humidity relative balance, can reduce the sensitivity of the paper to the environment.

 

For printing pieces requiring higher precision, you can also use the method of printing water on the paper first, so that the paper is fully absorbent and pressurized, so that the paper shape is before overprinting. In this way, the "swinging Angle" and the expansion effect are relatively zou, but the cost is higher.

 

4, the printing process requirements, in the case of ensuring the use of minimum water.

 

5, the overprinting caused by machinery is not allowed, mainly the teeth of the regular parts. The requirements of the front gauge are: according to the thickness of the paper, the height position is correctly adjusted, and the line of the front gauge stop paper point is parallel to the axis of the roller; The requirement for pulling foresight is: pulling paper is reliable. Each parting and feeding part shall ensure that the paper runs in a straight line. The holding force of the paper teeth should be uniform, the paper is accurate, and the two teeth should have enough time to hold the paper together to ensure the effective control of the paper, and the roller and the tooth shaft should ensure that there is no movement.

 

Meet the above requirements, under normal circumstances can improve the overprinting accuracy, to achieve the purpose of control.