Overprint accuracy is an important aspect of measuring product quality. However, the accuracy of overprinting is often degraded due to various reasons during work, which seriously affects the quality of printed products. Especially in offset printing, there are many reasons for overprinting faults. The main factors are printing materials, printing processes and equipment. This article attempts to make a more complete analysis and discussion on the formation and exclusion of overprinting faults from the aspects of influencing factors of overprinting accuracy, analysis methods and control of each link.
First, the factors affecting the offset printing machine is not allowed to color printing plate is a color decomposition process, and color printing is a color combination process, this color combination process is called overprinting. One of the minimum quality requirements for color printing is the accuracy of overprinting so that the print is faithful to the original. In the printing process, due to various reasons, it is easy to cause the product overprinting error to exceed the standard, causing the printing product to be scrapped. Here are some of the aspects involved in overprinting errors:

1. Effect of Printing Plate on Overprint Accuracy

Lithographic printing plates are now mostly thin and light aluminum. In the printing, the deformation of the plate or the change of the volume affects the overlay accuracy.

The deformation of the printing plate is generally caused by two aspects. The first is the tensile deformation and the second is the baking deformation. Tensile deformation refers to the overprint in the process of printing, due to the operator's improper force in the plate and pull the plate, artificially caused by the metal plate in the roller circumferential extension of the graphic changes caused by the overprint is not allowed. The baking deformation refers to the change in the linear size of the printing plate due to improper baking when the baking is performed to improve the printing resistance of the dried PS plate. When the temperature inside the baking box is too high or uneven, the plate base becomes soft and affects the dimensional stability of the layout.

2. Effect of roller lining on overlay accuracy

Only when the radii of the lining are equal, it is ensured that the linear speeds of the printing plate, the squeegee and the embossing roller are the same, and the transfer can be effectively performed under a certain pressure.

When the lining thickness is too large, the radius of the drum is increased accordingly. In the case of the same angular velocity, the linear speed of the drum surface is accelerated, and there is a speed difference between the drums, which will not only cause wear of the graphic images and texts, but also cause a layout image. The "shrinkage" of the drum pack is too thin, the radius is reduced accordingly, and the line speed is slowed down. The dots will be "stretched" during the transfer process, which will cause the image to "zoom in", resulting in inaccurate overprinting.

3. The effect of paper on overlay accuracy

In lithography, paper is an important factor that affects the precision of overprinting, especially offset paper. Due to the influence of paper, overlay accuracy is difficult to control.

The effect of paper on overprint accuracy mainly comes from the deformation of the paper. Deformation of paper is various, but it can be divided into two kinds, namely, natural deformation and pressure deformation.

Natural deformation is the characteristic that the paper absorbs water and expands and shrinks due to water absorption under the action of no external force. The moisture content of the paper changes with the change of temperature and humidity. When the moisture content in the air is greater than the moisture content of the paper, the paper absorbs moisture, and the paper edge Elongated, undulating, commonly known as "foliage"; if the air moisture content is less than the moisture content of the paper, the edge of the paper to release moisture, there "close the edge" phenomenon until the two reach equilibrium. Experiments have shown that for every 10% change in relative humidity, the moisture content of the paper will vary by about 1%. Figure 1 shows the relationship between relative humidity and paper stretch. In the offset printing process, when the first color is printed and then the second color is set, the overprint due to the stretching of the paper is inaccurate. In particular, those papers with loose structure, light sizing and strong water absorption are more serious.

The pressure deformation is the deformation of the straight line size and area caused by the paper being squeezed in the drum and the fibers being misaligned. In the printing process, due to the action of the roller, the paper extends from the mouth to the outside and becomes closer to the two sides of the trailing edge. The overprint error is greater. This phenomenon is often referred to as "sweep angle."

In addition, the cutting size of the paper varies greatly, and overprinting or wrinkling can result in overprinting.

4. The Influence of the Usage of Fountain Solution on Overprint Accuracy

Excessive amount of liquid during printing will increase the paper's elongation and deformation, and can easily cause various types of printing failures. When the moisture of the printing plate is too large, the transferred paper stretches due to excessive moisture absorption by the fibers. When chromatic printing is performed, the paper will be deformed irregularly, resulting in incorrect overprinting. In addition, when the paper is printed normally under a certain tension, if the water suddenly increases, the tension will change, and the rules will also be out of position, resulting in inaccurate overprinting.

In the offset printing process, whether the ink-moisture balance is right, not only has an impact on the accuracy of overprinting, but also has a very close relationship with the normal transfer of the print, the darkness of the ink, and the dryness of the printed matter. Therefore, whether or not we can correctly control and control the balance between water and ink is the key to ensuring the quality of printed products.

5. Effect of Ink Viscosity on Overprint Accuracy

When the viscosity of the ink is too large, the peeling tension of the paper on the blanket is increased due to the effect of the adhesive force. When the paper is peeled off from the drum, it is affected by adhesion and peeling tension. If the force of peeling cannot overcome the peeling force, paper whipping will occur and a small displacement will occur. This phenomenon appears to be misprinted on a monochrome machine and appears as a ghost image on a multicolor machine. Another situation is that the paper is peeled off the surface of the blanket under the action of the impression roller and the traction force of the impression cylinder, which causes the paper dragging section to be elongated. This phenomenon is more likely to occur under the following conditions: (1) The viscosity of the ink is more High; (2) Paper is printed on the ground; (3) printed on thin paper; (4) Excessive amount of fountain solution. For the wet-press printing method of multi-color offset printing presses, ghosting defects occur when the degree of stretching of each paper sheet is not uniform.

6. Effect of Printing Color Sequence on Overprint Accuracy

At this stage, four-color offset printing presses are mostly used, and four-color offset printing presses use wet-wet printing. The high speed makes the separation speed between the paper and the drum, the curvature of the drum, and the inertia force increase. In the production, problems such as peeling or picking, color mixing or color shifting, back sticking, and ghosting are prone to occur. Therefore, four-color offset printing machine wet wet printing requirements for the printing process is more stringent, in addition to the requirements of the printing ink layer is thinner, the less the better the layout moisture, ink and paper suitable for printing and printing pressure is reasonable, but also must Reasonably arrange the color registration sequence.

From the perspective of overprinting accuracy, the general order of color arrangement is: (1) Overprinting strict preprinting and less demanding postprinting; (2) Main colors first and secondaries behind; (3) Background colors first, After pressing in the color; (4) several adjacent sets of color printing should be one-color adapter, it is not appropriate to set with two colors at the same time; (5) line drawing network first, the field behind. Two-color or multi-color offset printing machine printing on the basis of the above-mentioned color, should also pay attention to the light color in front, dark color in the latter, the secondary picture and text in the front, the main picture in the post, the consumption of less ink before the consumption of ink After much volume, etc.

In addition, it should be noted that the number of overprints should not be too much, otherwise the possibility of errors is even greater due to multiple overprints.

7. Mechanical influence on overprinting

The printing paper must be effectively controlled by the machine throughout the machine, and the relative position must be stable during the transfer and handover process. Only in this way can the requirements for overprinting accuracy be met, otherwise overprinting accuracy will be greatly affected. The most influential factors are the following: First, the accuracy of the printing equipment itself is low and the error is large, that is, the mechanical wear is caused by the long-term use of the printing equipment or improper maintenance, which leads to misalignment of the various parts of the equipment and overprinting. This is mainly reflected in the problem of paper positioning, that is, the parts of the rules. Second, overprinting of the paper is not allowed due to the change in the relative position of the paper due to unsmooth transfer during the transfer and transfer of paper.
Second, the accuracy of overprinting control For factors affecting the accuracy of overprinting, take appropriate control methods to achieve the purpose of overprinting accuracy.

1. To avoid dimensional changes in the plates after baking. When baking the printing plate, it is necessary to make the temperature in the oven even, so that the plate is heated evenly, the temperature should not be too high, and the time should not be too long. For a short version that requires high accuracy, it is best not to bake.

When the school presses to standardize the operation, the position of the folder is centered, the printing plate is upright, and the clamping screws are fastened. When pulling the plate, tighten the top screw. As long as the force is moderate, deformation of the printing plate can be generally avoided.

2. When the offset printing of a sheet-fed offset printing press occurs, the disadvantages of inaccurate longitudinal overprinting of the inner chart often occur. Its characteristic is that the vertical and horizontal crosshairs are all registered, and the inner image of the mouth or the pinching position cannot be aligned in the vertical direction, which affects the product quality.

The reason for this disadvantage is that in addition to the factors such as paper stretching and gestures, the other important factor is the inaccuracy of the plate pad.

In some printing plants, the operator does not have a kilometre measurement when loading the plates. It depends on seeing, hand touching, and experience to determine how much the pad of the plate is. Although some machines are equipped with a thousand-point card but do not use them, the pad is “almost” lining, artificially causing large diameters of the plate cylinder, and the problem of being small in color, resulting in inaccurate overprinting of the inner and outer drawings. In particular, when the mouth is pinched, there are small hollow white hollow words on the tip, and the hollow words on both sides are difficult to register.

Some offset press operators do not know how much the pad will change the size of the graphics, although some know that there are changes, but how do you not understand? How much change? The following is a description:

The roller diameter of the offset press is equal to the pitch circle diameter of the roller gear. For example, the diameter of the J2108 machine roller is:

D pressure = D section = 300mm

The arc length of one revolution of the drum is:

D pressure · π = 300 × 3.1415926 = 942.447778mm

The arc length for each degree of operation is:

942.47778÷360=2.61799mm

If the plate is 0.2mm, the diameter of the cylinder increases by 0.4mm, the diameter of the plate cylinder reaches 300.4mm, the circumference of the cylinder increases to 943.73441mm, and the arc length per degree increases to 2.62148mm. Assume that the original length of the plate graphic is 520mm. That is, the arc length is 520mm, which is according to the standard pad lining of the printing plate. That is to say, when the plate cylinder diameter is 300mm, the degree of rotation of the plate cylinder when printing is:

520÷2.61779=198.64°

When the printing plate is more than 0.2mm and the plate cylinder diameter is 300.4mm, when the printing is finished, the degree of rotation of the printing plate cylinder is:

520÷2.62148=198.36°

0.28° less than the original, the arc length is:

2.62148×0.28=0.73mm

That is, due to the 0.2mm multi-pad, the surface linear velocity of the plate cylinder is faster than that of the surface of the blanket cylinder at a certain rotation angle. The specific length is 0.73mm, so the figure transferred on the blanket is The size of the text is 0.73mm smaller than the original size.

On the contrary, if the plate is less than 0.2mm, the final image size is 0.73mm larger than the original.

Therefore, depending on the look, hand model and experience, it is unreliable how much the plate liner is determined. Use a micrometric card to measure the thickness of each plate, and then pad with the appropriate backing paper based on standard pad data. In the event that the padding is not properly made due to the padding lining during production, the number of pads in the plate should be increased or decreased according to the above calculation method.

3. Pressure adjustment is also a very important factor. If pressure regulation is not good, premature printing plate wear, inaccurate overprinting, etc. may easily occur, affecting the quality of printed products. The ideal pressure enables the print to be firm, clear, and the minimum pressure at which the plate dots are transferred to the paper without distortion.

To achieve "ideal pressure," we must select and install the padding in strict accordance with the design requirements.

If other effects are not considered (such as ink roller pressure unevenness, improper pH of dampening solution, mechanical vibration, mechanical precision, etc.), the pressure formed by the above pad is very close to the ideal pressure, and the print under such pressure conditions Stable quality, firm and clear dots, multi-color overprinting accuracy, error within ±0.2mm, high printing plate resistance, and up to 3 screens