Mesh transfer screen printing ink layer thickness control

In screen printing, the control of ink layer thickness is very important. The thickness of the ink layer on the lithographic and letterpress prints is only a few microns, and the gravure printing is only 12 to 15 μm, while the screen printing ink layer thickness is generally 20 μm, special thick film printing can reach 100 μm, and the thinnest ink layer is 6 μm. It can be seen that the screen printing ink layer is not only thick, but also has a wide range of adjustment. Therefore, how to control the ink layer thickness of the printed matter has become an important topic for the screen printing industry.

This paper starts with the theoretical analysis of ink penetration and seeks specific solutions for controlling the ink layer thickness of printed products. Through printing tests, it provides the basis for obtaining the mesh number that matches the ink layer thickness.

. Theoretical analysis of ink penetration

1. Throughput

Ink penetration refers to the total amount of ink permeating through the mesh per unit area of ​​the screen, usually measured in cm3/m2. The amount of ink transmitted is one of the main parameters strictly controlled in screen printing, and is the key factor in determining the thickness of the printed ink layer, and is also the basis for selecting the number of screen meshes.

In accordance with the different meaning of the amount of ink through, the amount of ink can be divided into two kinds, that is, the theoretical amount of ink and the actual amount of ink through.

(1) Theoretical ink penetration Theoretical ink penetration is the total amount of ink permeated through the screen mesh within the unit area determined by the screen technology parameters. The technical parameters of the screen generally include the mesh number of the screen, the screen wire diameter, the screen opening, the screen opening ratio, and the thickness of the screen. Once the technical parameters of the screen are determined, the theoretical ink penetration of the screen is a fixed value.

(2) There are many factors that affect the amount of ink through the actual ink penetration. In addition to the technical parameters of the screen, other factors will also have a greater impact on the amount of ink that passes through, so it is usually obtained under certain printing conditions in printing. The amount of ink penetrated is referred to as actual ink penetration. Certain printing conditions mainly include the following aspects.
1 scraper angle and scraper hardness. The smaller the angle of the squeegee, the greater the amount of ink to be transmitted, and vice versa, the smaller the amount of ink to be penetrated; the greater the hardness of the squeegee, the smaller the amount of ink to be penetrated, and vice versa, the greater the amount of ink to be squirted.
2 ink viscosity. The greater the viscosity of the ink, the smaller the amount of ink passing through, and vice versa, the greater the amount of ink passing through.
3 printing materials. The greater the surface roughness of the printing material, the better the ink absorbability and the greater the amount of ink penetration, and vice versa, the smaller the amount of ink penetration.
4 printing speed. The higher the printing speed, the smaller the amount of ink transmitted, and vice versa, the greater the amount of ink passing through.
5 printing environment. The higher the printing environment temperature, the greater the amount of ink passing through, and vice versa, the smaller the amount of ink passing through.
It can be seen that the change of printing conditions has a great influence on the amount of ink penetration, and maintaining the stability of the printing conditions is very important for the control of the amount of ink penetration.

2. Calculation of theoretical ink penetration

Since theoretical ink penetration is determined by the technical parameters of the screen, theoretical ink penetration is a function of the screen's technical parameters, ie:
Thv=f(L,M0,d,δ) (1)
Where: Thv-theoretical ink penetration (cm3/m2), L-mesh mesh (mesh/cm), M0-mesh opening (cm),
Δ-mesh thickness (cm), d-wire diameter (cm)
The formula for theoretical ink penetration can be obtained, ie: Thv=M02×L2×δ×10000 (2)
In practical work, when the technical parameters of the screen are determined, the theoretical ink penetration can be easily calculated.

3. The relationship between the theoretical amount of ink penetration and the number of screen meshes From the above analysis, it can be seen that the screen mesh number is the main factor that determines the theoretical amount of ink penetration. If we can find out the correspondence between the screen mesh number and theoretical ink penetration amount. Relationships can basically determine the approximate range of theoretical ink penetration. For this purpose, we specially analyzed the technical data of the polyester screen provided by Thal Screen in Switzerland and found out the theoretical amount of ink permeation corresponding to the number of screen meshes (5-495 meshes/inch). There is a good correlation between them, and draw their relationship curve. Analyzing this curve, you can find the following rules.

(1) With the increase in the number of screen meshes, the theoretical amount of ink penetration is correspondingly reduced.
(2) When the number of screen meshes is low (5 to 30 mesh/inch), that is, at the AB section of the curve, the theoretical ink penetration decreases sharply with the increase in the number of screen meshes, and its theoretical ink penetration Decrease from 1216cm3/m2 to 224m3/m2, indicating that the mesh size has a great influence on the theoretical ink penetration. In this case, special care should be taken when selecting the mesh size.
(3) When the number of meshes is a medium to low mesh (30 to 130 mesh/inch), that is, in the BC section of the curve, the theoretical amount of ink decreases correspondingly with the increase in the number of meshes, and the theoretical amount of ink is decreased from The decrease of 224m3/m2 to 45m3/m2 indicates that the influence of the mesh size on theoretical ink penetration has been alleviated.
(4) When the mesh is a mid-height mesh (130-495 mesh/inch), that is, in the CD segment of the curve, the theoretical ink penetration decreases slightly with the increase in the number of mesh meshes, and its theoretical ink penetration is from The drop of 45m3/m2 to 7.5m3/m2 indicates that the influence of the screen mesh on theoretical ink penetration is even more gradual. Therefore, when selecting a screen within this range, a lower mesh screen can be selected as much as possible.
(5) In practical work, according to the user's requirement for the thickness of the ink layer, the relationship between the theoretical ink penetration amount and the number of screen meshes can be used to initially determine the range of the screen mesh number, which will greatly simplify the screen mesh number. The selection step has good operability.

Printing ink layer thickness control

Some printing products have higher requirements on the ink layer thickness, and some manufacturers also use the ink layer thickness as one of the important indicators for the quality inspection of printed products. In order to reasonably control the ink layer thickness, we should focus on the following two links.
(1) Analyze the intrinsic relationship between the thickness of the ink layer and theoretical ink penetration, and convert the thickness of the ink layer (known) into the theoretical ink penetration of the screen.
(2) The printing conditions are limited to a certain range in response to the influence of changes in printing conditions on the amount of ink transferred.

In order to unify these two points, a combination of printing test and calculation is used to solve this problem. Since the dot diameters of the printed dots are relatively large, the ink layer thickness will be greatly different, so it is discussed in two cases.

1. Calculate the amount of ink printed in large dots (dot diameter D>2.5mm)

Due to the large dot area, the thickness of the photosensitive emulsion on the screen is negligible. Therefore, the required thickness of the ink layer should be ensured by the number of screen meshes. In this case, the following steps can be used to determine the required screen mesh number. To meet the basic requirements of ink thickness.

(1) Test printing. Select one kind of screen mesh number, and find the corresponding theoretical ink permeability from the technical manual provided by the wire mesh manufacturer, and use X (unit cm3/m2) as the test mark.
In the trial printing, in order to reduce the influence of the printing conditions on the amount of ink penetration, the printing conditions that are consistent with the actual printing should be adopted, that is, the printing conditions are limited within a reasonable range.
(2) Testing. The thickness of the ink layer of the test print was measured after the test and expressed in y (unit μm).
(3) Assuming that the required ink layer thickness is x (unit cm3/m2), the required theoretical ink penetration Thv is obtained. The following relationships exist between the parameters:
Thv/X=x/y (3)
which is:
Thv=X(x/y) (4)
(4) Determine the number of screen meshes required. From the desired theoretical ink penetration Thv, the required screen mesh number is retrieved from the screen technology data.

2. Calculate the amount of ink printed in small dots (dot diameter D≤2.5mm)

Because the printing dots are smaller and the thickness of the ink layer of the printed product is thinner, the effect of the thickness of the emulsion on the thickness of the ink layer cannot be ignored. Therefore, the ink layer thickness of the printing product should be controlled by the mesh size and the thickness of the emulsion. . Therefore, when calculating the required theoretical ink permeability, the necessary corrections to equation (4) must be made:
Thv=X[(xg)/(yg)] (5)
That is, in calculating the theoretical amount of ink penetration, the thickness g of the emulsion should be subtracted from the thickness y of the ink layer of the test print and the thickness x of the desired ink layer of the printing ink. In general, the thickness of the emulsion is usually 5 to 7 μm.

3. Experiment

In order to verify the rationality of the trial printing method, the actual printing was taken as an example to conduct a trial printing.
(1) Thickness of printed ink layer: x=70μm
(2) Experimental conditions 1 Experimental equipment: manual screen printing machine, pneumatic stretching machine 2 Experimental materials: * Cardboard, * monofilament polyester mesh, * scraper: hardness HV60, scraper angle of 45 °, * Ink: Viscosity 80Pa·s
3 printing speed: 3 meters / minute 4 ambient temperature: room temperature 15 °C
(3) Experimental procedure 1 Mesh with a mesh size of 110 mesh/inch (optional) was selected, and the theoretical ink penetration X was 56 cm3/m2 from the material supplied by the screen manufacturer.
2 Use the silk screen selected above to conduct trial printing. After the quality of the printed product is stable, take 10 sheets and measure the ink layer thickness separately. Then calculate the average value to obtain the average ink layer thickness y=43 μm.
3 According to formula (4), calculate the theoretical ink-permeation amount Thv of the desired screen=56×(70/43)=91(cm3/m2).
4 check the theoretical transmission Thv (for 91cm3/m2) corresponds to the number of screen mesh L = 71 mesh / inch.
5 Use screen mesh with a mesh size of 71 mesh/inch for printing. After the quality of the printed product is stable, take 10 sheets and measure the ink layer thickness. Then calculate the average value to obtain the average ink layer thickness x'=66μm. .
6 test knot