Photographic Screen Exposure
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Mixing the Emulsion and Sensitizer
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Do this in a dimly lit room, the less light the better. It's not going to make a huge difference but the sensitizer is light sensitive and the less light it gets the better. The emulsion I used here is our FLAT FOX DIAZO EMULSION which has a diazo sensitizing powder.
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Please note: if you are using our FLAT FOX ONE STEP EMULSION simply skip the first step of these instructions (One Step emulsion is pre-sensitized and doesn't need diazo powder added).
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When you mix the diazo sensitizer powder (which comes in a foil pouch) into the emulsion, use a plastic or wooden spatula or mixing stick or anything that won't scrape the sides of the container the emulsion is in. Give it a good mixing and the colour of the emulsion should change a lot, if it starts out blue, after mixing, it should be a more turquoise colour. Different emulsions will be different colours.
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Once you mix the sensitizer into the emulsion it will have a shelf life of 3-4 weeks, if using a different emulsion brand check your supplier's instructions.
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Emulsion with diazo sensitizer powder added
Flat Fox Premium Emulsion
Applying Emulsion to the screen
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Before applying your emulsion to the screen it's very important to make sure that your screen is degreased. If you don't have a degreaser then using a little bit of dishwashing liquid and water works as well. When you degrease the screen, only use a little degreaser, apply with a sponge and get it to make a little lather. Wash the degreaser off with copious amounts of water. You need to make sure that the degreaser is completely cleaned out of the mesh. Running the water over the mesh instead of through the mesh is good, that way you can see if there are any areas that are repelling the water.
Now dry the screen - you want to make sure not to get dust or dirt on the screen mesh. A good way to dry it, is to use a hair dryer to speed up the drying time. Make sure the screen is completely dry before coating your screen.
Use a SCOOP COATER for applying the emulsion to the mesh. Put enough emulsion into the scoop coater to coat the whole screen. Start with the side of the screen that will go onto the fabric you are printing onto, hold the screen vertically and tilt the screen at an angle, tilt the scoop coater towards the mesh, it will pool against the mesh, pull the scoop coater upwards and coat the mesh. Do this to the other side of the screen. Once both sides are coated pull the coater along the screen again and scoop off excess emulsion. The final side that you scoop emulsion off should be the side that gets the ink, this way you push emulsion towards the print side of the mesh.
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To make it easier to follow , I've included a video and some pictures below.
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Mixed emulsion
Mixing them together with a wooden spatula
Drying coated screens
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Once coated, your screens need to dry flat in a dark, dust free, warm and dry place.
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Away from light - The emulsion (once mixed with the diazo sensitiser) becomes light sensitive and if exposed to light will start to harden (meaning it starts to expose itself) . Therefore it's important to coat your screen in a dimly lit room and to let the screen dry in a dark room or cupboard.
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Temperature - room temperature or slightly warmer is ideal for drying your coated screens (although make sure the temperature is below 30°C - anything hotter than that can cause the emulsion to start exposing itself)
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Dust and dirt - Our homes are filled with dust, dirt, pet hairs etc often not that visible to the eye. Make sure the area you are coating and drying your screen in, is as clean as possible. Dirt/dust/hairs etc appear as fish eyes and other blemishes on the emulsion, but don't stress too much if you do end up with some imperfections caused this way, you can always place your design around the blemishes, and if there is an imperfection that needs fixed, you can use correction fluid (eg Tippex) or masking tape to stop ink from going through that area.
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Humidity - If a coated screen has been left to dry in a humid environment, it can cause you problems later on. You might think that your screen is dry, but it might actually not be completely dry, and then once it is exposed and you start printing, your stencil can start to break down. A de-humidifier is ideal to avoid this happening.
Place your screens flat to dry, print side down. If you have coated several screens, you can use spacers to stack them flat on top of one another (see picture below - I use my daughter's lego blocks as stackers).
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Drying time - I cannot emphasise enough how important it is to make sure that your screens are completely dry before exposing. That's why I recommend that you leave your screens for at least 24 hours to dry (in humid or cold conditions I would leave them for longer).
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Signs of the emulsion not being dried for long enough - If you find that after exposure, while you wash out your exposed screen, the emulsion is soft, rubbery or pulls off the screen, then this is a sign of the emulsion not having dried enough before exposure. If you rub your screen with a cloth and the emulsion starts to come off the screen, it's kind of like washing a print that hasn't been cured properly.​
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To further explain why this is so important, I've written a separate blog about Why Emulsion needs to be dry which goes into detail about cross linking and what happens during exposure.
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Positive
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Your positive should be as dark as possible and should not have any grey areas. Grey areas let light through during exposure and in these places where the light gets through, the emulsion hardens which means you lose the detail in the grey areas. If you have a dark positive you can expose the screen for the right amount of time. If your positive is light, it will let light through making washout difficult, you might have to underexpose your screen to get the detail needed. The better your positive/artwork, the better the result you will get. Making your design CMYK and rich black will help (C-75/ M-67/ Y-68/ K-90).
Rubylith positives work really well as the red blocks UV light so you can expose for the right amount of time. Have a look at our shop for Rubylith.
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Light Source and Exposure
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In order to ensure a successful exposure, it's really important that your light source emits enough UV and that your exposure time is long enough . Here are some examples of types of lights and exposure times that our customers have shared with us that have worked for them - you can use this as a guide to see what will work for your set up.
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It is good to do a step test in the beginning so that you can work out the right time for exposure for your setup. A step test is where you take a screen ready for exposure and place a positive like the following Step Test Page taped to the screen. You work out what you think your exposure time will be and cover the positive (except the first row) with cardboard or something opaque to not let light through.
Rubylith Positive
Positive taped to the print side of screen
The screen with positive is put onto glass above 500w halogen lights, weighted down with a book
Once exposed, wet the screen on both sides to stop the emulsion exposing. Let the screen sit for a minute or two before cleaning out the soft unexposed emulsion. A hose works but for finer detail and longer exposures a small pressure washer will make your life a lot easier.
Washing out the softened emulsion using a hose
After exposure, take you screen and wet the print side and then wet the ink side and turn it around and wet the print side again. When you wet the screen it stops the emulsion from exposing. You can leave it for a minute or two to let the emulsion soften which will make the cleaning out of the screen easier. Clean the emulsion out of the design with a hose or pressure washer if you have one from the print side. Once you have cleaned out the design feel the ink side of the screen, if is feels soapy you probably have under exposed your screen, that soapy feel is apparently emulsion. This side needs washed or rinsed too. You then shake off excess water and leave your screen flat to dry with the print side down. If you get what looks like a slime trail from a snail in your design which blocks the mesh you have underexposed your screen and not washed it out enough, this is excess underexposed emulsion. We started off exposing for 2.5 minutes at the place I worked and the screens exposed well but the emulsion would break down while printing and then sometimes when cleaning of the screens with water, the edges of the squeegee would wear through the emulsion too. If this happens then you have under exposed your screen or exposed it while the emulsion wasn't completely dried. We used to get that soapy feeling on the ink side of the screen and while drying the sliming would happen. I started exposing for longer and using a dehumidifier and these problems went away.
Once your screen dries, you check for pinholes which can be fixed by dotting the holes with emulsion and left to dry, once dry you can put your screen back under your light to expose the blocked dots of emulsion. With the Diazo + Sensitizer emulsion it isn't that important to re expose your screen, it apparently can help harden the emulsion a bit but if it was under exposed you would have washed the unexposed emulsion away while cleaning the screen out so if it's grossly under exposed the second exposure won't make a big difference.
I haven't used other emulsions and again it has all been learn by doing. My initial instructions that I was given were very basic and not even right. I learnt by phoning suppliers and by doing loads of exposures. I eventually got it right and had beautiful smooth coatings and managed to expose some curve balls from customers. We exposed everything from 32T up to 120T in different sizes and so many different designs. If you have anything that you could add to this or have any tips you know, please share and I can update this guide.
Things can be frustrating when you get them wrong but you have to learn from your mistakes and it's really rewarding to get things right. Photographic exposures are pretty straight forward and you can get good results with a little practice. If you're thinking of giving it a try I hope this inspires you to go for it and if you have problems mail me and I'll try help you if I can. I've made a lot of mistakes and maybe I've made the same ones !
So to recap :
- degrease your screen
- dry your screen
- coat your screen
- dry your screen again
- once completely dry - expose
- wash out
- dry
- tape up and print
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To clean your emulsion off the screen once you need to clean it, try our EMULSION REMOVER in our shop. Conveniently sold in a powder to make it easy to mix up as much as you need.
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