Influence of screen geometry characteristics on precision of fine screen printing
2.1 Reproduction of screen parameters and minimum mesh point
2.1.1 Influence of mesh number and wire diameter on resolution
Screen is the basis of printing, choose a good screen, to improve the accuracy and quality of printing.Screen parameters mainly include mesh number M and wire diameter D.The size of the smallest dot that screen printing can reproduce is mainly determined by M and D.The size of M reflects the number of printing and reproducing dots per unit length. The larger M is, the smaller the printing dots will be.
According to the relation between diameter D and aperture U, screen can be divided into three categories:
(1) When the diameter of the wire mesh is larger than the wire diameter W>D,
(2) In the case of little or equal difference between them, W = d,
(3) The aperture is less than the line diameter w<D.
According to the size of A, in the printing process of screen whose aperture is much larger than the diameter of the line, the dot is more likely to form A complete ink point through the mesh, that is, the printing resolution is higher than that of screen whose diameter is larger than the diameter of the aperture, and it is also more suitable for reproducing fine dot in screen printing.
2.1.2 Calculation of the minimum node
For a given screen, the theoretical calculation formula of printing resolution is:
Square root of 2 x (o + d) x d/w
Is the smallest dot that can be copied theoretically.
Of course, there are other formulas for calculating printing resolution:
D, 2 x =)/M (1 + M, D)
D = 2 x (D + w)
D,=2d+w
D,=3d
Me and D both affect discrimination. In the above formula, d,= 2(d+ O)=2, d,= 3D, only one aspect of M and D is taken into account, and both have deficiencies in logic.Although formula D takes both M and D into account, it has the lowest printing area rate. Such a small missing printing area will cause dot loss and make it difficult to guarantee the integrity of the printing.D is the largest printable area among the four formulae, and relatively speaking, it can print complete dot, which is suitable for the calculation of dimming minimum dot and line width.At the time of printing we can be based on the printing accuracy and the range of tone, calculation, select the appropriate screen.Pay attention to M.The ratio of and the number of lines should be as high as possible to reduce the appearance of moire.
As can be seen from Figure 2-1, when the printing dot diameter D= 2D and the dot falls at the intersection point of the silk thread, the loss of the dot is the most serious. At this point, the dot area S=3.14xd’, and the opening area of the mesh taken up by the dot is about:
D3-3.14 (2 x 2 DXD – d) = 0.14 d2
The proportion of mesh points in the total area of screen openings is:
0.14 d2
W2 = 4.5%
Such a small ink transmittance will lead to the loss of small dot.
When the minimum dot diameter D≥3d is required to be printed, the dot can cover at least one opening even if the dot falls on the most unfavorable position on the screen, so that the screen can reproduce the dot.
When S is less than W, its dot reproducibility is the best, so when D≥ 3D, its dot reproduction must be better.Based on the above analysis, the following conclusions can be drawn: in fine screen printing, no matter what the relationship between D and O is, the minimum dot diameter printed has the following relationship with the wire diameter of the screen, namely:
D >= 3d
Under the action of tension, o changes significantly while D changes little.D ‘= d ×t, then the above equation becomes:
D or 3 D = 3 D * t
T is the elongation of the screen, which can be obtained by referring to the relevant screen specifications or tests.
2.1.4 Dot calculation of step adjustment screen printing
1. The number of lines in cm ·101 D=1.1284×√ Adjust percentage/number of lines ×1000
2.Number of lines in inches: D=1.1284× square root percentage/number of lines ×1000×2.54
D is related to mesh number M, mesh diameter D and opening O.The size of the minimum printable dimming dot is mainly related to O, and the dimming dot cannot be smaller than O, otherwise it is difficult to copy.The size of the printable minimum highlight dot is limited by D, the highlight dot with diameter less than D will be blocked by the mesh and lost.As mentioned earlier, D<O screen, ink is easier to pass through.The formula for determining the printable minimum dimming dot is:
SD mar – d + w
The formula for determining the printable minimum highlight dot is:
HDm. The in – d + a
When D ≥ O, the mesh will hinder the passage of ink and enlarge the dot. Using this screen, the formula for determining the printable minimum highlight dot is:
SD mar – d + w
The formula for determining the printable minimum highlight dot is:
HDmir – d + w
When D ≥ O, the mesh will hinder the passage of ink and enlarge the dot. Using this screen, the formula for determining the printable minimum highlight dot is:
Dmir=2(d+w)
To determine the percentage of highlights and dimming, the following formula can be used when calculating the printable order range of various lines and mesh sizes:
The settlement 8 s = 1 – x (t1D..XLC) x PI
Dark color % = 100 – x (SD) mi) (LC) x PI
LC= Number of network lines (cm)
As long as the screen printing plate can be determined the size of the minimum dark and highlight dot can be calculated for a particular number of screen wire screen will produce the minimum and maximum tone value.