Magnify Your Process to Multiply the Results
Using a loupe to check for errors makes for a good method of identifying flaws and making improvements in the printing process.
There is hardly a step in the screen printing process that can’t be improved after a review under magnification. That’s because looking at film, mesh, screen, emulsion, or print under magnification will make little flaws just jump out. For some reason, using a loupe is looked upon as extreme in screen printing, as if using one signified an excessively nitpicky printer. This couldn’t be further from the truth, because a printer who regularly checks processes and prints under magnification will be able to correct numerous problems that are hard to detect.
Important issues to remember when reviewing your process under magnification are: Have a clear concept of what you are looking for; know how to adjust or improve what you have; and identify what the effects will be from changes that are made. It doesn’t make sense to do a close examination of an exposed screen unless you already know what a high-quality screen should look like. Understanding what you are looking for requires some education and trouble shooting, but the rewards of learning how to manage your printing stages on a small scale will pay back in a multitude of ways. Increasing reproduction accuracy, lowering print setup costs, and smoothing out printing runs are just a few possible advantages. The attention to detail is particularly beneficial to printers who want to move up a level in printing difficulty and start to handle more difficult images, higher dpi halftones, and higher mesh-count printing.
The easiest way to start with a focused review of the printing process is to work from the beginning and then move through reproduction stages, ending at the final print. During each stage, the magnified image can be reviewed for areas to be dealt with, as well as items that work well. In some cases it will become apparent that an entire process needs to be changed and new equipment or methods are the right answer. Even if that happens, and it isn’t possible to change the equipment immediately, it is always better to know what the variables are and what is causing the trouble. Then plan for a time in the near future when it will be fixed eventually.
Check the screen printing process in the same order the design would follow through the production process. The focus review would check films, blank screen mesh, coated mesh, exposed mesh, and final print. This allows for insight on how one issue can transfer into another area to compound a problem that eventually leads to major printing concerns. The simple equipment needed for this type of magnified check are loupes that you often get for free from mesh or screen-printing suppliers. A 10x loupe will work fine though higher powered ones are available for those who print on flat stock and at higher resolutions than typical garment printing.
Reviewing the films
Films are the most critical portion of the process to examine because screens and print will never be any better than the detail that is captured and transferred from the original film positives. The primary thing with close-up film examination is to understand what you’re looking for and what may be problems. Start by creating a simple file that will create a film with a variety of edge qualities to it and several halftone line counts so that these can be reviewed first (Figure 1). The reason for this is a random piece of artwork may not have the right value range to show the dots clearly depending on the design.
Improving the film process through investigation with a loupe requires knowing what to look for. A simple diagram shows some common trouble spots in films (Figure 2).
The biggest issues with films that can be seen clearly with a loupe are edge quality and opacity. These areas can be difficult because they affect the exposure directly and show up as screen problems. It is best to catch defects at the film stage and then make corrections before they appear as ink on a garment.
Edge-quality problems with films can indicate issues with several areas of film production. Additionally, it is important that the digital source is looked at to eliminate it as a root cause. Sometimes the problem starts from a low resolution scan or bad pattern in the values of the source file that is wrongly attributed to the RIP or the output device/printer. To avoid this, make certain that the original digital file used for comparison is a clean, high-resolution file that can be trusted to produce a quality image. This isolates one variable after first controlling other surrounding variables. Otherwise conclusions change in sync with variables. Areas in film production affecting halftone dot edge quality are primarily the RIP and the output device.
It is dangerous to assume that an RIP that produces dots from a grayscale value always does a great job. Some popular RIPs used for producing screen print films may even damage or degrade dots. The easiest way to check is to use the loupe and look at several different percentages of dots. Look for dots that appear misshapen, irregular, or infused with some kind of digital pattern. Sometimes an old RIP or software can interfere with the current one and produce strange or inconsistent results. If a film's dots appear to have multiple patterns across them instead of just the normal halftone quality, then there may be interference between software, and even hardware that runs the printing device. In this case, have an original, clean film that printed from the same RIP and printing process to compare with the damaged one.
Viewing the opacity of films initially doesn’t require a loupe because you can see if the darkest areas of the film positive are truly opaque by overlapping them with another film and seeing if you can see through the top one. The magnification process is helpful to make certain that the dots in a halftone pattern are completely opaque in nature, and they don’t start to break up in smaller dot sizes. This issue is more common on dry toner systems that may use a heat application to bond the toner to the film.
Laser printers will often create smaller dots that look like little piles of sand loosely grouped together as the toner particles are fused to the page. Obviously this type of dot will not create the best dot on an exposed screen because the image will not wash out very well. It will also contribute to dots that have a higher surface tension and a tendency to pull ink back off of the shirt rather than leave a clean print. This can snowball and create a situation where the printer must increase pressure to get the ink to clear out of the screen, causing dot gain in the halftone to gain significantly over a print run. Add this issue to a touchy burn time from vellum films and it can be a real challenge to get halftones on press that will behave the way that they should during volume printing of shirts.
A group of halftones produced inaccurately may have a secondary pattern or distortion and additional holes in the film’s black areas (Figure 3). In this case, software issue may be creating dots that have additional, incorrect, and unnecessary holes in the recreation of the halftone patterns. This problem may be invisible to the naked eye but will be readily apparent when viewed under a loupe. Once again, a true copy of the best reproduction your equipment can produce is necessary for a clear comparison.
Checking the screens before coating
Screen printers may look puzzled when asked if they examine uncoated mesh that has been stretched onto a screen though a loupe. They may think, “What could possibly be an issue with a screen that doesn’t have emulsion on it yet?” The truth is, there are many reasons to look at screen mesh before coating with emulsion. The most common are thread diameter vs. open area, mesh contamination, and weave angles.
Thread diameter vs. open area
A screen’s open area varies quite a bit, depending on the mesh count, thread diameter, and mesh tensioned. The way this affects the printing process can be dramatic if you consider that a poorly tensioned screen with a thick thread and a lower mesh count may have far less open area in the mesh than the thickness of the mesh itself (Figure 4). Though you may be printing through an 86-thread/in. mesh, the amount of ink that goes through the screen is less than typical and a lot of the ink will be stuck in the overlapping parts of the mesh or bounced back into the screen as the printing stroke passes. This is even more important when considering inks that are thicker or have special effect properties like high-density inks or glitters. Talking with your mesh provider can ensure that you get the best quality mesh that will have the appropriate open area ratio and provide the easiest printing quality.
Using a loupe to check used screens can determine quickly if a mesh is stained or clogged. Ink or emulsion particles will occasionally stick in the mesh and this will block areas. To the naked eye, a clogged screen is similar to one that has been stained. A stained screen is one where the actual mesh of the screen has taken on some of the ink color and there are little or no particles actually blocking the openings in the mesh; therefore, printing will not be impeded. While it is true that screens need to be cleaned as much as possible, it is a production fact that the show must go on, and it is useful to know if a screen is going to cause trouble without a cleaning, just look a little stained, or have to be replaced because it cannot be cleaned.
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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