Photocopying empirical formulas for flexographic printing

With the progress of various technologies, flexographic printing has also made considerable progress in recent years. Especially in the field of packaging and printing, flexo printing has become the fastest growing printing method. Flexographic printing has been able to achieve such a great success. The flexible photosensitive resin version launched in the 1970s was indispensable.

We know that the most obvious feature of flexographic printing plates is their elasticity. After being mounted on a plate cylinder, the plate will be stretched and deformed along the surface of the cylinder. If this is not done during the front-end process, Compensation processing will result in deviations and distortions in the size of the original text from the printed image. For example, a label that is very small in size can have a significant effect if it is deformed. This is unacceptable in today's era when customers are demanding more and more print quality. Therefore, measures must be taken to compensate for this change in size. The method of compensation can be roughly divided into the following two categories:
1. Distortion compensation for photo size

2. Compensation by roller type printing method

Among them, the second method uses a printing plate cylinder of the same size as the printing roller, so that the original design is hardly considered for any size compensation. This method is used for sleeve printing in flexo printing.

Since not all flexographic plate making departments have roller plate-making equipment, and flat plate-making equipment applications are still very popular, this article mainly discusses the first method. In order to compensate the deformation of the image, it is necessary to reduce the corresponding graphic size of the printing negative image in the circumferential direction of the printing roller. At present, most empirical formulas are summarized through experimental methods. Here we describe the most widely used empirical formula for the reduction of the size of the photosensitive resin plate, and on this basis, an empirical formula derived from itself, supplemented by calculation examples, for flexo platemaking

1. Photographic resin plate size miniature empirical formula

Photographic resin plate version of the empirical formula. The formula is as follows:
Compensation coefficient β = K / L × 100 %
Deformation rate disr = ( 1-β)
Negative film size = L ×disr

In the above formula: L is the printed repeat length of the printed matter, and K is the elongation of the printing plate. Normally, the L value is given with the printed product. If it is not given, it can also be calculated by: Let L = 2 π (r + b + cp ), where r is the radius of the plate cylinder, b It is the thickness of the double-sided adhesive tape, cp is the thickness of the printing plate, and the deformation rate isr is the reduction ratio.

The K value is a constant, different plates can be pre-calculated their respective K value, the empirical formula for calculating K value is: K = 2 π cs = 2 π (cp - cb), cs is the soft thickness of the printing plate, ie The thickness of the photosensitive resin layer of the printing plate; cb refers to the thickness of the printing plate support film, usually taken as 0.005 in., but there are also other special plates that have a value of 0.007 in. cb.

The relationship between commonly used different thickness plates and K values ​​is shown in Table 1.

Table 1 Commonly Used Different Thickness Plates and K Values

2. Derive the empirical formula of the plate size of the photosensitive resin plate

In fact, after derivation of the empirical formula, we can easily derive the following empirical formula:

Deformation rate disr = <print diameter - plate soft thickness × 2> / print diameter × 100% = 1 - plate thickness / print radius

Among them, the printing radius is the radius of the printing plate roller, the thickness of the double-sided tape and the total thickness of the printing plate. The soft thickness of the printing plate is the same as the soft thickness cs of the printing plate in the empirical formula, and the size of the negative picture is finally obtained as L×disr.

Let's take a look at an example and use the above two formulas to calculate:

Example: The print repeat length of a print is 15.0 in., the paste tape thickness is 0.015 in., and the thickness of the flexible photosensitive resin plate is 0.112 in., and it is mounted on a printing roller with a diameter of 4.5 in. Yin picture deformation rate and size?

Empirical formula calculation:

K = 2 π × ( 0.112 - 0.005 ) = 0.672 in.
Compensation coefficient β = K / L × 100 % = 0.672 / 15.0 = 4.48 %
Deformation rate disr = ( 1-β) = 95.52 %
The size of the negative picture = L ×disr = 15.0 × 95.52 % = 14.328 in.
Derivation of empirical formula calculations:
Deformation rate disr = <print diameter - plate soft thickness × 2> / printing diameter × 100% = 1 – (0.112-0.005) / (4.5 / 2 + 0.015 + 0.112) = 95.93 %
The size of the negative picture = L ×disr = 15.0 × 95.53 % = 14.329 in

From the above calculations, we can see that for the same set of data, the calculated compensation coefficient β values ​​are not exactly the same (thereby the calculated deformation rate disr is not exactly the same), but it is very close. In fact, due to the very complex deformation characteristics of the flexible photosensitive resin plate, the inner side of the printing plate is compressed and deformed, and the size is reduced, while the outer side of the printing plate is tensile deformation and increases in size. When we make compensation for deformation in the foregoing, we only assume that the outer side has undergone tensile deformation, neglecting the influence of the inner side deformation; in addition, in addition to the influencing factors in the above formula, the graphic structure on the printing plate is not only the graphic part but also the non-graphic part. The area ratio also affects the deformation of the printing plate. Therefore, we use the above empirical formula to calculate the compensation coefficient β <or deformation rate disr>

It is only an approximation. In spite of this, the calculated value of β or disr can still give a good guide for actual production. It can make us have evidence in the experiment.

The flexible and elastic properties of the flexo-printing photosensitive resin make the flexibility of flexo printing extremely wide, and at the same time, it also raises the issue of offset compensation for flexo printing. If roller-based plate making is used, this problem can be basically solved, but if plate-based plate making is still used, special attention must be paid to the issue of the miniature ratio discussed in this article. It is believed that taking the empirical formula of this article as a reference, together with the experimental summary, the flexo printing platemaking department will be able to form a set of data that suits its own production methods, solve the problem of deformation compensation for flexographic printing plates, and ultimately achieve high quality. Flexo printing provides assurance.

Long Chen Printing Technology Network