Using a loupe to check for errors makes for a good method of identifying flaws and making improvements in the printing process.
This is an issue for screen printers that are using high halftone line counts on higher mesh screens and it sometimes a problem with four-color-process printing. The mesh does not appear to have been stretched straight onto the screen frame. It may appear wavy or jagged and this can occasionally appear and cause frequency patterns in high-level printing because the thread lines may line up with some of the lines in the halftone dot patterns (one of the reasons that the dpi angles of halftones in screen printing do not use 45˚ or 90˚). If it is found that the screens have this issue, they should be re-stretched carefully or used for non-critical work.
Looking at coated screens
Several concerns can be noted by observing coated screens through magnification before they are exposed. The most common problems to look for at this point are the way the emulsion is wrapped around the mesh and the surface of the emulsion itself. Both of these issues are usually related to coating technique. If the emulsion is coated onto the screens too quickly, there will be a noticeable wave to the emulsion coating where the emulsion may be shallow in the middle of the mesh openings and heavy in the woven areas. Visualize this almost like buttering toast: If the knife moves too quickly, it doesn’t allow the butter to sink into the toast and it will clump and roll on the surface and leave uncovered holes. Slow, careful coating of screens can maintain a good layer of emulsion that will leave a much more consistent surface for exposure of films and printing.
The surface of the emulsion itself relates to how smoothly it covers the screen and if it varies from one side of the screen to the other or from the top to the bottom. Proper coating technique and good quality maintenance of the coating equipment ensure a level and clean edge to the coater.
Checking the exposed screens
The main reason to check exposed screens through a loupe is to see if the emulsion edges pass inspection. If the films started out as very clean and opaque positives and then a problem appears on the screen, it can be quickly narrowed down to an exposure or washout problem. Exposure issues can be either over- or under-exposure. Over exposure usually shows up as little ripped areas on the dots. Under exposure looks like dots may falling off or smeared on screens.
Advanced printers will occasionally view how the stencil holds the shape of the actual dots onto the mesh, since dots that are even slightly distorted may cause dot gain over large printing runs. The cleaner and clearer the emulsion is, the easier the print run will be.
Taking a close look at the final print
Observing a final printed piece can provide a wealth of information and enable the printer to keep improving results. Some of the things that can be noticed from a magnified perspective include: registration accuracy, trapping concerns, underbase opacity, printed dot shape, dot gain, and garment fibrillation (where the fibers in the garment push up through the ink), edge quality, and overall ink surface quality. These observations can be useful in documenting the printing standards and recording press settings that achieve a certain outcome. The better the variables are managed, the easier it is to keep improving printing standards.
Observe and record as a standard procedure each time issues come up. Every print doesn’t need to be magnified and inspected, but developing a regular check at established intervals will provide better quality in every step of preparation and printing. There is no need to avoid the loupe if you’re not sure what the problem is, just call your local representative to come in and look at your process. Odds are they will bring their own loupe with them.
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