Research on pre-compensation processing technology of flexographic prepress (2)

2. I Calculation of bending deformation compensation size of flexure

When pre-compensating pre-compensation for a flexo plate, if the former two methods are taken into consideration when designing the original or color separation before plate making, the amount of elongation of the plate is taken into account, that is, the corresponding value is subtracted from the circumferential length of the original. In order to compensate or reduce the size of the corresponding graphic text on the negative plate for compensation, it is necessary to first obtain the bending deformation compensation size of the flexographic plate so that the printed product will be relatively conformed to the dimensional requirement.

2. I. I Formula Method (DuPont Experience Formula)

The printing plate compensation coefficient (reduction ratio) is related to the thickness of the printing plate, in addition to the thickness of the plate cylinder and the thickness of the double-sided tape. In the flat exposure of the flexo plate, DuPont's empirical formula is generally used to calculate the plate compensation coefficient (reduce ratio) on the original or negative film.

Compensation coefficient (reduce ratio) = K/RX 100%

In the formula: R is the final copy length of the printed product.

R = 2Ï€(r+b+c)

Where: r is the radius of the plate cylinder; b is the thickness of the liner; c is the thickness of the plate. Wherein, is the coefficient, which depends on the thickness of the plate used, for example, when the thickness of the plate is 1.70mm, the value is 9.89mm; when the thickness of the plate is 2.29mm, the value is 13.56mm. The K value can be obtained by referring to relevant data. The calculation constant for various thicknesses of the flexographic plate can also be calculated using a simple formula for the thickness of the photosensitive resin. The thickness of the photosensitive resin is equal to the total thickness of the plate minus the thickness of the polyester support film, ie, k=2π×(plate thickness−support film thickness).

Therefore, the size of the original design or negative film in the direction of the redirection dimension is shortened to 100% as a compensation factor.

The printing plate elongation calculation formula is obtained when the plate cylinder is in a normal state. But in fact, not only the layout distortion but also many other factors affect the actual length of the printed matter.

1) The tensile deformation of the substrate. During printing, the web or web film under printing is also stretched under the action of feeding tension, which is related to the nature of the material, the structure of the printing press, the status of the powerful control system, and other auxiliary equipment. The use of the relevant;

2) At the time of loading, additional force is applied to the printing plate in order to apply force to flatten the printing plate;

3) During the printing process, due to mechanical squeezing of the dots, liquid spreading of the ink, and double reflectance of the paper, the area of ​​the dot-printed dot of the substrate is increased compared with the dot area of ​​the corresponding portion of the plate.

Therefore, compensating with DuPont's formula may not necessarily achieve the desired printing accuracy. For printed products that require precise overprinting, comprehensive shrinkage and bending calculations are required to make precise layouts. When it is difficult to calculate compensation, experimental methods or sampling methods can be used to determine.

2. I. 2 experimental methods

The necessary compensation data is obtained by making an experimental version of the scale marked full-length and full-width of the printing plate. The specific method is:

I) When the printing plate is removed from the mold, the length of the scale is measured immediately. Before the printing plate is pasted on the roller, it is measured once after being pasted to obtain the variation of the plate size before and after the plate is attached.
2) Use texts with different sizes, fonts, and thicknesses to check the status of text copying.
3) Determine the skewness of the pattern before and after the cooling of the mold by measuring the size of the round and square patterns.
4) After the experimental version of the sticker is printed, necessary compensation data can be obtained according to the extension, shortening, and distortion of the layout graphic.

2. I. 3 sampling method

During the printing process, the sample inspection is directly taken and the data before, after and after the printing plate are measured, and the compensation data for the color registration accuracy, bending and overprinting accuracy of the printing plate are obtained. Of course, the measurement value of this method is not accurate enough.

In practical production, the use of a single method is generally limited, and often the above three methods are combined to obtain a more accurate flexographic deformation compensation size. After the deformation compensation size is obtained, the compensation size is subtracted from the design original in the computer, or the output of the film size is reduced by calibrating the laser graphic imager to obtain a film suitable for the flexographic feature to obtain a fine print.

2.2 Adjusting the separation characteristics of the flexographic plate

The increase in the number of dots in printing is unavoidable. The reason for the expansion of dots is mainly physical reasons (mechanical reasons). That is, at the instant of imprinting, the ink on the printing plate dots will be deformed due to the effect of extrusion. , resulting in the expansion of outlets. In the prepress stage, the size of the dot is reduced by adjusting the color separation characteristic curve of the flexographic plate, so as to compensate for the enlargement of the flexographic dot.

2.2.1 Selection of Dot Shapes

Generally, before adjusting the color separation characteristic curve of the flexible plate, a suitable dot shape should be selected. Flexographic printing belongs to letterpress printing, the height of the printing surface must be high, and the flexo printing plate must be soft. If the printing dots are asymmetric, when the pressure is applied to the printing dots during printing, the dot area of ​​the narrower part in the cross section is The increase will be greater, while the wider part will increase less. Therefore, elliptical dots and square dots are not suitable for flexographic printing, and circular dots are commonly used in flexographic printing. Due to the weakness of the flexographic printing itself. The range of gradation that can be reproduced when using the amplitude modulation dots for step adjustment printing is narrowed, so now when the flexo separation sheet is generally used, amplitude modulation dots are used for the tone range of 10% to 85%, and for 10% The following or more than 85% of the areas use FM sites. This can control the dot gain rate to a minimum.

2.2.2 Drawing the Dividing Characteristic Curve of a Flexible Plate

Generally, a test version is used to conduct a printing experiment, and the dot gain curve of the printing process is obtained, and then a computer network application software is used to make a corresponding dot gain compensation curve, and then a job such as screen printing is performed. The specific method is:

1) Using a transmission densitometer to measure the percentage of grayscale on the negative film and the actual percentage of dots after printing, thus obtaining the dot gain curve of this printing process;

2) Use a reflection densitometer to measure the color density of some continuously adjusted images, as far as possible within the recommended range of ink density;

3) Observe and measure the registration mark on the printed sheet to determine the overprinting accuracy and determine the size adjustment at the time of copying.

4) Obtain the gray balance data by determining the density value of the overprint color. After obtaining a series of æ­¹0 test data, it is plotted and analyzed to obtain a color separation characteristic curve of a flexographic plate. Then use software to preprocess the dot gain. Because of the large deformation of the flexographic plate, it is required to make a series of tests periodically in order to ensure the accuracy of the data.

2.2.3 Adjustment of color separation characteristic curve

When performing gradation compression, it is necessary to adjust according to the color separation characteristic curve. Generally can not be linear and equal average compression, high and middle tone level should be less compressed, dark tone levels should be more compressed, so that high-level levels of the level difference between the open, the middle tone of the main tone level deeper, and dark tone end level The density is flat. In order to obtain the gray balance, when the flexo plate is made, there is a difference between the screening curve of the cyan plate and the screening curve of the magenta plate and the yellow plate in order to ensure the color balance of the screen. This value is generally 3% in the highlight area. ~4%, intermediate tone and dark tone 10% to 15%.

3 Conclusion

The factors affecting the quality of flexographic printing products are far from the two types of distortion and dot gain. However, in order to obtain fine prints, strict control must be exercised at every step and every process. In the pre-printing pre-compensation process, the bending size of the flexible plate is compensated and the color separation characteristic curve of the flexible plate is drawn, and the reduction in deformation size is combined with the adjustment of the tone adjustment, and the appropriate dot shape, screen angle and line number are selected. It is possible to control flexure deformation and dot gain in a relatively small range, and to make good preparations for obtaining a fine flexographic print.

Author / Wu Li Shenzhen Vocational and Technical College

Source: Packaging Engineering, 4th Issue, 2005