High-resolution screen printing demystified

today on Applied Science we're going to talk about screen printing this is a well known process for transferring graphics on to a variety of different substrates and so you'd think there would be a lot of great info on the web about getting started perhaps even a kit of all the equipment you need to do this but surprisingly there is almost no information available for printing stuff other than t-shirts and so in this video I'm going to describe all the equipment that I used after trying a bunch of different things and you'll be able to save a lot of time if you want to get into this okay so the basic process goes like this we take an empty screen and coat it with a chemical that is photosensitive and let it dry and then we take a transparent or translucent printout from an ordinary desktop printer and put it in good contact with the layer with the photosensitive layer and shine light on it and wherever light goes through the transparency and hits the photosensitive layer it will become insoluble in water and all the areas that were kept dark will be soluble in water so after the exposure is over we take this to an ordinary sink just plain water will rinse away the photosensitive layer where we don't want it so then we're left with a screen that is patterned and there's open areas wherever we had a dark area in the printout then we suspend this above the object we want to print and squeeze ink through all those open areas pretty straightforward but going to the moon is also pretty straightforward if you just say we blast off and then we land it's really the details that determine whether a process is tricky or not and so let's step through the process kind of sequentially and I'll point out all the different choices you have and all the best things you should buy for high resolution screen printing on substrates like metal and glass let's start off by talking about the screen itself if you're like me you're probably pretty concerned with getting the highest resolution possible and when you start searching for screens you'll notice that the thread counts are one of the main parameters there so this is a 160 screen it has 160 openings per inch and this is a 350 screen 350 openings per inch and you might assume that if you want really high-resolution printing you should just get the highest mesh count screen you can find and that's sort of true but there's actually more going on here then you might think the photosensitive layer actually has some thickness to it I can actually catch my finger on the edge of this photosensitive material that we coated on to the screen and the thing that actually determines the shape that you're going to print is in fact this layer so basically what I'm saying is that this photosensitive layer can partially occlude one of the openings in the screen so the screen resolution is not really like a pixel count don't think of it like digital imaging where if you have more pixels that's great because your image won't be as pixelated it really doesn't work that way it's much more of an analog process especially because it's really the photosensitive layer that determines your ultimate resolution having said that generally yeah you need to have a really fine support structure because the strength of this photosensitive layer is not that high so if you have really fine details you kind of need a fine screen but don't think that it's like a one to one pixel relationship when this process was first invented the screens were all made of silk hence the term silk screening but oh that's all polyester these days even though we still use the term silk screen in a lot of cases just keep in mind it's an antique term even though everything is polyester now you can buy polyester screen on rolls and stretch it over a frame yourself or you can buy these pre stretch frames which I would strongly recommend if you're getting started another thing you might find surprising is that these pre stretched frames are all very large like common sizes are like 20 by 30 inches about and if you only want to print small stuff you might think well I don't need that I only need a small frame like this however you can only use the central about 70% of the screen even this is about as close to the edge as you can get because in in the process of printing you actually have to stretch the screen that's really a normal part of the printing process and you can't stretch the screen if printing all the way to the edge so really it's important to only use the center 70 or 80 percent of the screen you'll also notice that some of the screens are white and some of them are yellow what's the deal with that the yellow is the dye that they add to the polyester that they call an anti halation technique and so if you want high resolution when we're shining ultraviolet light in here to expose the photopolymer what we don't want to happen is the light to go through the layer and then bounce off of the white screen and then expose the neighboring areas so the yellow dye basically absorbs the ultraviolet light so that we don't get reflections and hence lower resolution you'll pretty much find that all of the screens above 200 or 250 openings per inch automatically come stained yellow like this and it's just a the only reason that these are not yellow that all screens aren't yellow is just a safe money so in the lower resolution screens it's white just to save money one reason not to use the highest mesh count screen you can get is that it passes less ink than larger screens so these two were printed same process same artwork same everything but the print on the right was made with the 350 count screen and the print on the left was made with the 250 count screen and you can see that there's not quite as much coverage here but I was able to do it here so it's one of these things where the higher mesh count screen is more sensitive to technique and loading and the type of ink and the viscosity and everything and so the reason that screens above 305 or 350 are super uncommon is because they're just more difficult to use so you generally want to stick to the 250 to 300 range and you get more consistent results okay so we've got our screen picked out I happen to like 250 to 300 openings per inch and this is like an 11 by 17 inch pre-stretched frame that I've had pretty good luck with so the next step is to coat this with the photosensitive layer and traditionally this was done with something called a scoop coder and what you do is pour this photosensitive liquid into the scoop coder and then tip it up here and distribute it on to the screen and there's all kinds of different techniques you know from one side and then the other and then you turn it around and wipe it again and there's quite a bit of skill involved with managing the scoop coder depending how viscous your stuff is and how often you wipe it and then you have to let it dry of course and the orientation in which the screen is held is important is it face down as it face up this way and to be honest the whole thing is actually really difficult and I don't recommend doing this there's a much easier way instead of pouring this liquid photosensitive layer into the scoop coder they sell photo emulsion already in a sheet on like a plastic backing and what we do is wet down the screen with some water and then unfurl this layer of photosensitive material onto the front of the screen and then just quickly use a squeegee to brush off the excess water and give it a little bit of pressure the screen can be dried in any orientation and it will dry more quickly than this old-fashioned liquid photo polymer and you will have a much more consistent layer so remember I was saying what the resolution is a largely determined by the consistency of your photo polymer and as you can imagine trying to get like a perfectly you know within 10 micron consistent layer of stuff on there with this hand process it's kind of iffy so it's really much better to use a premade film and just be done with it and you get this perfectly smooth photo polymer layer one thing is that there's about 10 or 15 different common kinds of photo polymer this is called the emulsion in screen printing lingo and you'll be overwhelmed with the number of options available to you there's a lot of terminology some of these are called dyes Oh some of them are who knows what else dual-action or whatever in my experience with my limited experience it doesn't make as huge of a difference as you might think they're really sort of squeezing out the last bit of performance from a screen printing process and if you're just getting started pretty much any of them will work just to get your feet wet with the process I'll of course put my favorite list of things that worked for me in the description well it's true that all of these chemicals are photosensitive you don't want to work in sunlight of course they're really not that sensitive don't think of them like photographic papers or films your safe light conditions really don't have to be that amazing and what I'm using is generally just a an LED like warm white LED bulb in my garage here and I don't cover the windows up or anything it doesn't make any difference when you see later on the exposure times are on the order of like 10 minutes being full blasted by an ultraviolet lamp and so a little bit of room light isn't going to hurt anything even like standard you know computer monitors spraying out a little stray light it's no big deal however if the screen has to dry for a day or two even that can't happen in the room because that's too many hours of exposure even with a dim light so you need to have somewhere dark to dry these things out and I'm using this metal filing cabinet and it's not ventilated of course so what I do is put all the screens in the upper drawer and then open the lower drawer and aim a fan in there to get some fresh air in and the way the metal drawers are constructed there's a pretty good light baffling it's pretty cold and rainy here right now so the drying times are quite long I was noticing that it takes almost a full 24 hours for this green film to dry and even longer for the pink liquid that I applied with the scoop coder again I don't recommend doing that but I was trying a whole bunch of different things out while the screens are drying you want to get your artwork ready for printing and at first you might think you have to use transparencies like this but this actually doesn't work that well because the ink doesn't stick to this smooth surface a better solution is to use this translucent vellum paper and you might think that it has to be transparent for the light to get through but actually this passes more ultraviolet light than this clear plastic does I've measured it myself and so there's basically no reason you ever want to use a transparency like this just go with the thick vellum paper you can use plain old tracing paper the only downside is that it tends to jam in the printer it will make little creases and wrinkles and those will actually affect you in a bad way so the best thing that I've found is relatively thick and heavy vellum paper regarding the printer itself I'm using a 600 dpi laser printer nothing too special about it you can use an inkjet printer of course but don't be don't assume that that magic 4800 dpi number they always throw around really means anything the black resolution of a inkjet printer is probably 600 or maybe 1200 but probably not higher than that and as you can imagine the manufacturers really like to stretch the numbers as much as they can so for the most part 600 dpi is what you're going to be printing the main thing that you want to focus on is contrast you really want the dark areas to be it's absolutely dark as possible so that when you expose your screen you get the biggest contrast between the dark and the light areas so if there's a density adjustment on your printing software you want to crank it up as high as possible okay so now that we've got our artwork printed and our screen coated with the photopolymer and dry it it's time to make the exposure so I am using this ultraviolet light source and this works pretty well but it's not ideal which I'll explain why in a minute anyway the way this works is to put the artwork print side up and then take the screen that has the photopolymer on the surface and put it facedown and this will in fact create a mirror image on there but that's fine because then when we print this back onto the substrate it mirrors it again so you end up with the correct image and you know if you don't have one of these ultraviolet light sources it's no big deal it's really just a bunch of fluorescent tubes in there and the tubes are in fact special but they aren't that hard to get it's just ultraviolet fluorescent tubes and so you could build one of these but if you don't want to build one and don't have one you can actually use the Sun itself the Sun is a great ultraviolet light source and in that case what you want to do is have a compliant backing like this cube of foam and put your screen on top of that and then the artwork face down onto the photosensitive layer and then critically you have to use a piece of glass to force the the printed layer the artwork flat down onto the surface and you have to apply a lot of pressure I mean really build a fixture to hold this down by far the most critical part of the entire screen printing process is getting the artwork in really good contact with the photosensitive layer if you want to capture details that are a hundred microns or a couple hundred microns if you're off the surface by a couple hundred microns the shadow caused by that it's going to swamp out your detail really this really is the most critical part of the whole process the sunlight is pretty variable of course and so if you want to do this indoors with more repeatability you can use a mercury vapour or metal halide lamp you can probably hack one out of an old computer projector or just buy a mercury vapour lamp from the hardware store if they still have them but the point is that the the nice thing about that sort of a light source is that it's basically almost a point light source and so if you set up the lamp four feet away from this setup when the light shines down here even if there is a slight air gap or whatever in there the shadow will be very sharp and so all of the really professional screen printing machines in fact do use metal halide or mercury vapour lamps for this reason the problem with the fluorescent tubes is that the light is coming from this extended source so that if there is a slight gap between the artwork and the photosensitive layer it really will cause a much bigger problem so to get around this what I had been doing is put this down here you don't need a piece of glass for this method of course the glass is already on the surface here we put the artwork here and then I put the foam layer here and put some weight on here and I found out that I was using a drill press vise and that was in fact not heavy enough so I upgraded my weight so that this foam was really compressed down can't stress enough how important it is to make sure everything is super compressed together if it isn't you end up with problems like this where you might think there was some other issue here but the problem was that the edge contrast was lost because there was in fact a slight air gap with this set up with the fluorescent tubes my exposures were about 10 minutes for this setup that includes the attenuation through that translucent vellum paper and the sensitivity of the photopolymer everything when I was using that pink photopolymer that I applied with the scoop coated the exposure times were on the order of two minutes so your exact setup your exact chemicals the attenuation of your paper that you're using or the transparency that you're using wall effect the exposure time so it's kind of tough to give an exact value luckily it's pretty easy to figure out and also if your exposure is 10 minutes being plus or minus you know 20 seconds doesn't make a big of a deal so you don't need a timer or as a fancy darkroom timer because these exposure times are so long after the exposure is complete you want to wash away all the parts of the photopolymer that we're not cured by the UV light and you still have to do this under you know quote darkroom conditions but I did it in my kitchen a few times and it's fine even with normal room lighting it's not that big of a deal just because it isn't that sensitive and normal room lighting doesn't output that much UV light you can also do it outdoors at dusk or something with a garden hose that works fine too you don't need a pressure washer to do this step at all especially if you're using a film emulsion that doesn't really soak into the mesh as much as the scoop KOTOR method does later on it's helpful to have a pressure washer to clean a screen that is very stubborn especially ace Green that has been used with these epoxy inks or epoxy paints that are very sticky and hard to get off but the clean out of the of the photopolymer layers is relatively easy it's okay to use a brush to clean out stubborn areas and you might be surprised at how much of a manual process this is use cool water you don't have to add anything to the water to clean out the photopolymer but it is true that using a soft bristle brush can make the process happen more quickly and also you can kind of focus on areas that need a little bit more cleaning now that the screen is washed out and dried it's time to print this process is pretty straightforward the only thing to note is that the screen should not come down into contact with the substrate that's a big problem you have to suspend the screen above the substrate and then when you come through with the squeegee it actually pushes the screen down into momentary contact this is super critical if the screen is allowed to come into contact with the substrate for too long the ink will wick out under the covered areas on the screen and you'll end up with a really poor resolution image the standoff distance doesn't appear to be that critical I was just using a couple pieces that may be you know five or six millimeter thick acrylic and as long as you can easily push the screen down without feeling like you're stressing anything it's probably fine the choice of ink though is another interesting thing the fact that you can use any ink you want with screen printing is one of its main benefits right I mean you can use conductive inks or electroluminescent inks or anything under the Sun but if you use a purpose-built screen printing ink you'll definitely get the best results and again if you're just starting out it's kind of nice to start with something that works so well one thing that was new to me was the use of these epoxy paints so you can see here on the right this is a black two-part epoxy paint and the performance is just absolutely incredible you can see the density these were all printed with the same screen the same process the same substrate glass this is sort of a medium quality black enamel and oil-based paint this is actually an undiluted artist's oil paint so this is not really the same thing because this is a carried in a solvent base this is actually drying by oxidation that it's very dense but the epoxy paint is still better it leaves a really nice finish the adhesion is super high the density is really nice the only downside is that cleaning up this epoxy paint is a super nightmare there's a special cleaner that you can buy from the screen print places that will help you clean it up but it's really a whole other level of pain like if you think that cleaning up oil-based paints is difficult just wait until you clean up an epoxy based paint the only reason you use them is because the performance is really that amazing once they're dried they're solvent resistant and fade resistant I mean they're really really nice paints this cleaner is pretty aggressive and if you let it set on the emulsion for too long it will actually hurt the emulsion so you have to be pretty quick at cleaning the ink out with this if you're using an oil base or a water-based paint just use water or naphtha to clean your screen if you're using the epoxy based paints you pretty much have to use this because I haven't seen anything else that will clean up those epoxy paints when you're completely done with a design and want to print something else it's not really economically feasible to throw the whole screen out so then you use this chemical to loosen the emulsion and spray it away with a garden hose or in a sink and this works pretty well I've never had much of a problem getting rid of the film coating cleaning off the pink stuff that I applied with the scoop coder again was more difficult so you saved time on both the front and the back end when using this film process and then as a final cleaning process after the emulsion has been removed you can use this stain remover and degreaser which is basically soapy water with maybe like pumice powder in it or something it does roughing up the screen a little bit which is great because that provides adhesion for the next emulsion you're going to put on there and the degreaser is important so that you get a nice clean sheet of water on there which helps the emulsion stick as well so with everything I've shown in the video you can easily get down to 400 degrees and 300 micron pitch eight-point font looks pretty good Sixpoint is marginal and anything less than that doesn't appear to work and you can see with a side by side with the original artwork my printer can print 200 micron pitch and easily get down to six point and maybe even four point font so there's a little bit of loss going from the print to the or from the printed artwork to the screen printed but with a little bit of tweaking and a little bit more care I think it can actually make it as good as a laser print in terms of resolution but anyway they're really the strength of screen printing of course is that you don't have to use ink from a printer you can use all these other weird things and you can print on substrates that you could never feed through a printer anyway I hope that was helpful and as always feel free to put your questions in the comments and I will see you next time bye

Channel: Applied Science

Views: 284,031

Rating: 4.9607301 out of 5

Keywords: screen printing, screenprinting, silkscreen, silkscreening, applied science, printing, epoxy ink, epoxy paint, high-res, high resolution, dpi, lpi, detail, glass, metal, technical, panel, front panel, labels, mesh

Id: NS8Q9LUIKA8

Length: 23min 26sec (1406 seconds)

Published: Mon Jan 21 2019