Influence of screen parameters on printing film thickness

In mesh screen printing, the control of ink layer thickness plays an important role.Lithography and letterpress prints have an ink layer only a few microns thick, and gravure prints can be as thin as 15 microns.Under general conditions, screen printing ink layer thickness of 10-20 microns, special thick film printing can reach 100 microns.It can be seen that the ink layer of screen printing is not only thick, but also wide adjustable range.Therefore, how to control the thickness of the ink layer has become an important issue that the screen printing industry should focus on.

Starting from the theoretical analysis of ink penetration, this section seeks for a specific scheme to control the thickness of printing ink layer, and through the printing test, provides the basis for obtaining the number of mesh matching the thickness of the ink layer.

Theoretical analysis of screen ink penetration

(1) Ink penetration

Ink penetration refers to the total amount of ink passing through mesh per unit area, usually measured in cm3/m2.The amount of ink penetration is one of the main parameters strictly controlled in screen printing. It is the key factor to determine the thickness of the ink layer and also the basis to choose the mesh number.

According to the content of ink penetration is different, ink penetration can be divided into two types, namely theoretical ink penetration and actual ink penetration.

(1) Theoretical ink penetration

Theoretical ink penetration is determined by the screen technical parameters of the unit area through the total ink mesh mesh.Screen technical parameters generally include screen mesh number, screen aperture, screen opening, screen opening rate and screen thickness.Once the screen’s technical parameters are determined, the screen’s theoretical ink penetration is a fixed value.

(2) Actual ink penetration

Due to the complex factors affecting the amount of ink penetration, in addition to screen technical parameters, there are other factors in the printing will also have a greater impact on the amount of ink penetration, so, usually the amount of ink penetration obtained in a certain printing condition is called the actual amount of ink penetration.The so-called certain printing conditions mainly include the following aspects.

Scraper Angle and scraper hardness: the smaller the scraper Angle, the greater the penetration of ink, and vice versa, the smaller the penetration of ink;The hardness of the scraper is larger, the ink penetration is smaller, on the contrary, the ink penetration is larger.

Ink viscosity: the greater the ink viscosity, the less ink penetration, on the other hand, the greater the ink penetration.

Substrate: the larger the surface roughness of the substrate, the better the ink absorption, the larger the ink penetration, and vice versa, the smaller the ink penetration.

Printing speed: the higher the printing speed, the less the ink penetration, on the contrary, the greater the ink penetration.

Printing environment: the higher the printing environment temperature, the larger the ink penetration, and vice versa, the smaller the ink penetration.

Thus it can be seen that the change of printing conditions has a great impact on the ink penetration, and it is very important to maintain the stability of printing conditions for the control of ink penetration.

Calculation of theoretical ink penetration

Because the theoretical ink penetration is determined by the screen’s technical parameters, the basic screen technical parameters including wire diameter, mesh number and screen thickness, other parameters can be calculated by these parameters 23.Therefore, the function expression of theoretical ink penetration can be written in the following form, namely

Thv = f( M,d,t)

Theoretical calculation formula of ink penetration,

Thv = (1 / M – d) ‘x M * t * 108

In practice, when the screen technical parameters are determined, it can be very convenient to calculate the theoretical ink penetration.

Theoretical relationship between ink penetration and mesh size

It can be seen from the above analysis that mesh size is the main factor determining the theoretical ink penetration. If the corresponding relationship between mesh size and the theoretical ink penetration can be found, the approximate range of the theoretical ink penetration can be basically determined.To this end, the technical data of polyester silk screen 2 provided by Thal Silk Screen Co., Switzerland was analyzed to find out the theoretical ink penetration amount corresponding to different mesh sizes (from 2T mesh /cm to 195T mesh /cm), and a good correlation was found between them.

Analysis of this curve reveals the following pattern.

With the increase of mesh size, the theoretical ink penetration decreases.

(2) When the mesh number is low (2 ~ 12T mesh /cm), that is, the AB segment of the curve, with the increase of mesh number, the theoretical ink penetration drops sharply, and the theoretical ink penetration drops from 1216cm/m’ to 224cm’/m2, indicating that the mesh number has a great impact on the theoretical ink penetration.In this case, the selection of mesh size should be particularly careful.

(3) When the mesh number is medium to low mesh (12T~51T mesh /cm), that is, the BC segment of the curve, the theoretical ink penetration decreases with the increase of mesh number, and the theoretical ink penetration decreases from 224cm’/m’ to 45cm’/m, indicating that the influence of mesh number on the theoretical ink penetration is somewhat relieved.

(4) When the mesh is medium and high mesh (51T~195T mesh /cm), that is, the CD segment of the curve, with the increase of mesh number, the theoretical ink penetration decreases slightly, from 45cm’m’ to 7.5cm’m’.This indicates that the influence of mesh size on the theoretical ink penetration is more gradual.Therefore, in this range of screen selection, should try to choose a lower mesh screen.

(5) In practical work, according to the user’s requirements on the thickness of the ink layer, the relation curve between the theoretical permeability and mesh number can be used to preliminarily determine the mesh number range, which will greatly simplify the determination of mesh purpose, and has good operability.

3 printing ink layer thickness control

Ink layer thickness is divided into wet film ink layer thickness and dry film ink layer thickness of two forms.

(I) Wet film ink layer thickness

(1) Factors affecting the thickness of wet film ink layer

There are many factors affecting the thickness of wet film ink, which mainly include the following aspects.

Screen theory of ink penetration

Theoretical ink penetration is an important technical parameter of screen and a determinant of wet film ink thickness, and its calculation formula

As the formula.

2) Photosensitive adhesive layer thickness

The thickness of the photosensitive adhesive layer can be selected in a wide range of 3-17 u m.

3) Ink transfer rate (R)

Ink transfer rate is mainly related to the Angle of the scraper and the net distance.Under the condition of ensuring good transfer of ink, the screen spacing should be small, not large, so that not only can improve the printing plate resistance rate, but also can improve the accuracy of overprinting.In addition, ink transfer rate and ink viscosity, printing speed, substrate material absorption and other factors.

As can be seen from the above analysis, the calculation of wet film ink thickness also has certain limitations, mainly in the following two aspects.First, ink transfer rate is a variable and influenced by many factors, to bring greater difficulty ink layer thickness calculation; second, in general, the customer requirements of the ink layer thickness is dry film ink layer thickness, due to the effect of penetration of the substrate, the volatilization of solvents in the ink and ink liquidity, the influence of factors such as viscosity, in general, the dry film and ink layer

Penetration, ink in the volatile solvent and ink fluidity, viscosity and other factors, in general, dry film ink layer thickness is less than the wet film ink layer thickness.In addition, wet film ink thickness is difficult to be measured by conventional methods, requiring special measuring instruments.Therefore, wet film ink layer thickness can not be used to control the ink layer thickness, so discuss the dry film ink layer thickness control has more practical significance.

(ii) Control of the thickness of printing ink

The ink layer thickness discussed here actually refers to the dry film ink layer thickness.

The thickness of ink layer is mainly determined by the theoretical ink penetration, the higher the theoretical ink penetration, the thicker the ink layer thickness;On the contrary, the smaller the theoretical ink penetration, the thinner the film thickness.

In order to control the reasonable thickness of ink layer, should focus on the following two links.

First: analyze the internal relationship between the ink thickness and the theoretical ink penetration, and convert the ink thickness (to know) into the screen’s theoretical ink penetration.

Second: according to the change of printing conditions on the impact of ink penetration, the printing conditions should be limited to a certain range.In order to unify the above two points, the method of pre-printing T and calculation is adopted to solve the problem.As a result of printing dot diameter difference is large, its ink thickness will have a large difference, so divided into two cases are discussed.

(1) Calculation of ink layer thickness for large-dot printing

Because the dot area is large, the thickness of the screen plate photosensitive latex can be ignored, so, the required ink layer thickness should be guaranteed by the mesh number, then can follow the following steps to determine the required mesh number, in order to meet the basic requirements of ink layer thickness.

1) printed.Select any mesh number and pre-print the corresponding theoretical ink penetration Thvt(cm”/m’) in the technical manual provided by the screen manufacturer.Preprinting, in order to reduce the impact of printing conditions on ink penetration, should be used in line with the actual printing conditions, such as scraper Angle, spacing, ink viscosity and substrate materials, will be limited to a reasonable range of printing conditions.

2) detection.After pre-printing, measure the thickness of the ink layer of the pre-printed product, represented by Z.

3) According to the required ink thickness Zz, obtain the required theoretical ink penetration Thvz.Because there is a linear relationship between the theoretical ink penetration and the ink thickness, the relationship between the two is as follows, that is

Thvq/ Thvz= ZlZz

(2-27)

Thv=Thvi(ZolZ)

(28) 2 -

4) Determine the required mesh size.According to the required theoretical ink penetration Thvz, the required mesh number is obtained from the screen technical data.

(2) Calculation of ink layer thickness for small dot printing

Because the printing dot is small, the thickness of the ink layer is relatively thin, the influence of the thickness of the emulsion on the thickness of the ink layer can not be ignored, so the thickness of the ink layer of the printing should be controlled by the mesh number and the thickness of the emulsion.For screen printing plate, due to the coating of the photosensitive film, the ink storage in the screen printing plate will change with the change of the thickness of the photosensitive film. Therefore, when calculating the required theoretical ink penetration, it is necessary to modify Formula 4, namely

Thv z= Thv[(Z1-sV(Z2-s)]

That is to say, when calculating the theoretical ink penetration, the thickness of photosensitive emulsion s should be subtracted from the thickness Z of the ink layer of the preprinted sample and the thickness Zz of the ink layer to be printed.

With the continuous expansion of market demand and the progress of printing technology, the quality of screen printing has been increasingly improved, and the ink layer thickness is an important part of the printing quality control, therefore, it is of great practical significance to study the control of the ink layer thickness., starting with silk screen and ink quantity, various influence factors of strike through quantity is briefly analyzed, the theory is derived through the ink quantity and silk screen technology parameters, the intrinsic relationship between ink thickness, and printing printed and calculation, the method of combining the theory of ink, ink layer thickness, number of wire mesh and printing conditions such as organically relates in together, thus forming a print ink layer thickness control system of work, to strictly control the printing quality of the ink layer thickness, choose the number of wire mesh provides a reliable basis.

4 summary

The choice of screen is the basis, its geometric characteristics directly affect the sequence process.In the process of printing and reproduction, after Mc has determined, 14 should select the screen with the minimum d, which is conducive to the replication of details. According to the printing requirements, select the screen with d ≥ 3D, so that the dot restoration can be better.D small screen surface undulation, relatively smooth, easy to coating Rz small plate, in the printing process is not easy to form imprinting sawtooth and other quality problems, is conducive to the improvement of printing accuracy.Select a fine screen to pay attention to the screen elongation to be small, high tension, so that there will be a better printing suitability., starting with silk screen and ink quantity, various influence factors of strike through quantity is briefly analyzed, the theory is derived through the ink quantity and silk screen technology parameters, the intrinsic relationship between ink thickness, and printing printed and calculation, the method of combining the theory of ink, ink layer thickness, number of wire mesh and printing conditions such as organically relates in together, thus forming a print ink layer thickness control system of work, to strictly control the printing quality of the ink layer thickness, choose the number of wire mesh provides a reliable basis.