Screen parameters and plate characteristics
Plate thickness and imprinting accuracy
The thickness of the plate is divided into two parts, one is EOM(emulsion over mesh), the other is mesh thickness Ft.Eom-hs (total thickness) one Ft, the larger d, the thicker the screen.
In general, the larger the Hs, the greater the ink adhesion thickness.In the process of printing, EOM is in contact with the substrate, and its nature is directly related to the quality of printing.If the thickness of EOM is too small, the ink cannot be released completely, resulting in imprinting imprinting. When the width of the copied line W≤ 2O + D,EOM directly affects the dot replication.W and EOM,
T (screen tension), P () pressure, to make the wire under the action of Р hit a printing surface, require P>T, deformation resistance of screen plate due to EOM:
PW=4T*EOM/W
The unit of T is N/cm. The wire contact width between the ink knife and the screen is about 0.25cm, so the tension of the contact part of the ink knife is only 0.25t, and P is generally 1.8n /cm.Calculation shows that under the condition of normal T and Р, W = 0.127 cm, that is when the line of the coarse W 0.127 cm, the ink thickness vary with the change of the EOM.This is very important in fine printing, especially in line printing, where EOM error should be controlled within ±2 m.The use of automatic coating equipment coating screen, film thickness is relatively uniform, good quality.
2. The roughness
Surface roughness Rz is used to describe the smooth condition of screen and plate surface, mainly related to the coating of D and photosensitive adhesive.The value of R can be measured by a surface photometer (curve profilometer), which calculates the average sum of the five highest points and the five lowest points over a length.
The surface profile of screen can be divided into n equal interval discrete element, in the outline of the transverse since О increased displacement of each ordinate value multiplied by the overlap range overlap with the ordinate value, corresponding to the product and divided by coincidence unit number, calculate the displacement of each selected from the relative value of, make a series of related values on the relationship between the displacement curve, is available on the surface of the screen autocorrelation function curve of ACF.
The ACF represents the length characteristics of the contour, while the PSD represents the frequency characteristics of the contour.PSD provides the frequency component of the screen surface profile waveform, the proportion of the different frequency components and other information.For a given screen, the above information can be calculated, and the screen can be further studied.
When the plate surface is rough, in the printing process, the plate can not be in full contact with the substrate, ink under the action of pressure will flow into the gap between the two, so that the impression produced sawtooth, burr, resulting in blotting expansion.For the higher smoothness of the plate, the imprint is smaller.The roughness also includes the smooth condition of the film edge. When the film edge is rough, the ink edge will become rough, which reduces the printing accuracy.
The expansion of the imprint is also related to the size of the substrate surface absorption, for the absorbent substrate, in a relatively short period of time, the ink will be quickly absorbed, because of its cohesion, too late to enter the gap between the plate and the substrate, the imprint is small.For the substrate surface rough, smooth plate, will also produce imprinting.
Rz is usually less than 10 um, considering the screen and substrate between the rough situation, desirable R=12~15 um or less.
3. Improvement of roughness
The surface coating method can reduce the roughness of screen printing plate.First use wet coating process coating screen, dry, and then surface coating.A relatively smooth plate can be obtained, while the thickness of the plate will not increase too much and affect the printing quality.
Experiments show that the initial screen with the edge radius of 0.75mm scraper for 2-2 coating.Then, with the edge radius of 0.25mm scraper, respectively, on both sides of the plate coated.The coating of the screen plate on one side of the scraper has formed a smooth contact surface, which makes the scraping plate smooth in the printing process, the ink filling is even, the imprinted edge is clear, in addition, it can also make the scraper wear less.One by one, in the plate to form a 2- 2/0, 2- 2/1-1, 2-2/2 one, 2- 2/3 one part.After measurement, the plate and substrate surface smoothness obviously improved.