Troubleshooting (4) - Problem Description

11. Rough edges, blisters, voids, or skips. Features: This problem (rough edges) is often confused with jagged and jagged edges. Looking through the microscope, you can see that the edges of the image are complete, but the image contains many small bubbles. This phenomenon often occurs in PTFs (Polymer Films) and in major graphics, also known as blank spots and off-prints. It often appears in the curved part of the graphic. Bubbles are often on the edge of the copper plate (or on previously printed conductors), which is parallel to the length of the squeegee. Defects in this category can easily be distinguished from other problems because: a) They always occur on the edge parallel to the length of the squeegee. b) They often occur at the bottom of the edge and at the top of the upper edge of the ink (the direction of the doctor blade is from top to bottom). The primary reason: Usually the straight edge is parallel to the length of the squeegee and parallel to the mesh line. As the squeegee approaches the edge of the stencil, air is pushed in. Secondary reasons: a) Ink viscosity is too high. b) The proportion of open mesh areas is low. c) The printing speed is too high. d) The clearance or separation speed is set too high. e) The edge of the squeegee is too hard. f) Printed edge stencil is too thick. 12. Smear Features: Sputum refers to undesired ink dots that extend beyond the stencil during printing. Many of the problems so far are "unwanted ink over the template," but the cause of this problem seems to be broader. However, when you want to solve the previously discussed problems, there are only two types of edge defects that fall into this category: movements that cause defects or excessive pressure on the squeegee. The most telling feature is the hue gradient or ink thickness. The first reason: a) When printing, the screen and the substrate move relatively. b) The scraper pressure is too high. Secondary causes: a) Tension is too low and the gap is too large, compensated with high scraper pressure. b) The pressure of the ink bar is too high. Regional defects 1. Ghost images Features: Previously printed images leave a faint shadow on new prints. The primary reason: poor mesh cleaning, emulsions and inks remain in the silk fabric. Secondary reason: If the screen is used for a long time, the mesh line in the ink-pervious area is worn out with the ink and the squeegee alternately pressed. The shadows that appear in this case cannot be eliminated by cleaning. Only new screens are replaced. 2. Mesh marks Feature: The surface of the ink layer leaves a screen texture. Net print also refers to the obvious webbing lines on the substrate. The primary reason (texture): For the mesh model, the ink volatilizes too quickly and it is too late to level. (ink line): Tension tension is too low, ink viscosity is too high. Minor reasons: a) The time between printing and drying is too short, causing the leveling time to be too short. b) Before the ink is printed, the ink will evaporate. c) Offset clearance and separation speed settings are incorrect for a given screen, ink, and substrate. d) There are too few places where the image is transilluminated. 3. Moire patterns Feature: There are no interference patterns in the image. A typical example is the interference between the halftone dots and the mesh, between the halftone dots and the halftone dots, between the lines. The first reason: The ratio of the number of screen lines to the number of screen lines is too low, and it is an integer. (If it is 2, 3 instead of 4.1 or 5.3) Secondary reasons: a) Stretching tension is too low. b) The mesh line extends in all directions. c) The printed sheets have different angles when printed. d) The texture of the substrate and the texture of the print interfere. 4. Orange-peer/fish-eye Features: There is a black circle around the middle white (orange skin) or a white circle around the middle black (fish-eye). Orange skin-like circles are distributed over the surface of the print, while fish-eye circles are less. The primary reason: a) Chemical changes have occurred between the ink and the substrate. b) Substrate stains change the properties of the ink. c) Ink properties are changed by additives or solvents. 5. Streak Features: There are many different width lines in the direction of the squeegee. Some of these lines are lighter than the ink, and some are darker than the ink. The first reason: The blade of the scraper is not properly caused. Secondary causes: a) Dirty or dry ink sticks on the scraper. b) There are air bubbles inside the squeegee blade, leaving marks on the print, but they are not easily visible. c) leaving traces of ink ribbon on the ink coating. Unevent ink deposit Features: Different colors and ink thicknesses may look different in different positions of the same print. The first reason: the printing plate, screen and scraper are not parallel to each other. Secondary reasons: a) Squeegee pressure is not uniform. b) There is a gap in the plate. c) The thickness of the substrate is not uniform. d) The tension of the stretch net is not uniform. e) The stencil thickness is not uniform (significant in halftones and fine lines). 7. Enlarged image area Features: Printed images are larger than original images. The primary reason is that stretching tension is not enough to cause screen expansion. Note: The graphic on the screen may be the same as the original, but it will be expanded after printing. Secondary reasons: a) Insufficient exposure of the stencil. b) The screen has not been tensioned and the template has been exposed. After the exposure, the open area of ​​the mesh increases. c) The original is stretched before exposure. This happens when the screen temperature is increased by 10F or more when the screen is placed on the revision table or when the screen is near the exposure table. Note: In the second reason, the graphic on the screen is larger than the original before printing. 8. Reduced image area Characteristics: The printed graphic is smaller than the original graphic. Usually the details still exist. The first reason: a) The stencil is overexposed. b) After exposure, the screen will dry too long at high temperatures. Secondary reasons: The mesh became loose before printing, resulting in graphics tightening. 9. Dirt marks Feature: There are small, unprinted ideas in the graphic area. Unlike pinholes, pinholes only appear in non-graphic areas. The primary reasons are: a) Loose particles on the substrate (or in the air) stick to the screen, blocking the channel of the ink. b) Dust and condensed ink blocked the ink channel. These ideas sometimes appear and sometimes do not appear. Minor reasons: a) There are pinholes in the graphic area of ​​the original. b) The screen is prepared in an unclean environment. c) There are some flaws in the mesh. All secondary causes leave permanent, regular defects on each print. 10. Pinholes Features: There are small, solid ideas outside the graphic. In the name business, there are holes in the template that allow ink to leak into non-graphic areas. It is different from the dirty track. The dirty track only appears in the graphic area. The first reason: Before printing, the underexposed template is decomposed. Secondary causes: a) Dust on the glass of the exposure table. b) There is dust on the original. c) The sensitizer on the template is not well distributed. d) The quality of the template material is poor or expired. Note: The pinhole always appears in the same place as the sheet, but not on every sheet. Holes in the stencil may be blocked and may open. 11. Part of the image is incomplete - Incomplete image-screen related Features: Large or small graphics are missing. The missing images on each print always appear regularly in the same place. This problem often appears with "edge clogging." The primary reason: Reused screens are hollow or have residual emulsions. Secondary causes: a) The original was scratched or otherwise damaged. b) Before printing, oil, grease, or additives stick to the screen. 12. Part of the image is incomplete - process related (Incomplete image-process related) Features: Large or small graphics missing, you can see a little different with the previous issue, the missing part does not always appear in the same place There is also a change in size. The first reason is that the tension of the stretch net is not enough and the clearance from the print is too small. Secondary reasons: a) Relative to other factors (ink viscosity, gap, screen strength, etc.), the blade speed is too high. b) The screen separation speed is not set to c) The gap is too large and the screen strength is too high d) The image is too close to the edge of the blade e) The image is too close to the end of the blade movement Conclusion: 1. Knowing the problem requires experience. To gain experience requires: a. Observing the problematic area with a microscope b. Compare it with the problem in the table and classify it c. Record the problem as detailed as possible. Many issues are not very different on the surface. You must identify them and know their respective reasons. Pay special attention to several problems similar to "dirty": mesh prints, missing details, ink lines, bleeding, ghosting, blisters on the edges 3. Defects in the image area are better recognized and classified than defects in the edges of the image, but care should be taken not to confuse the terminology and concepts: mesh prints and cellulite, dirty prints and pinholes, and incomplete image problems regardless of the template or distance.