Screen printing involves pushing ink through a stencil made of mesh coated with emulsion. Maybe that’s everything you’ve ever wanted to know about emulsion’s role in screen printing, in which case, you’re welcome! The rest of us are going to get a little more detailed into the hows, whats, and whys.
What Even is Emulsion?
Emulsion is a general name for a solution made up of two liquids that normally do not mix together, but will in the presence of an additional ingredient called an emulsifier. You most commonly encounter emulsifiers in food, especially food that combines oil and water, like salad dressing, ice cream, and margarine.
In the case of the emulsion used in screen printing, it would be better named “UV light-sensitive crystal liquid,” in my opinion. The important thing about it is that it contains light-sensitive crystals that cause it to harden when exposed to UV light – that’s what makes it useful for screen printing. But no one asked me.
Oh, and from now on, whenever I say “emulsion,” assume I’m talking about the emulsions used in screen printing. Unless I say otherwise or start giving you a recipe for salad dressing.
How Emulsion is Used to Make a Screen Printing Stencil
Now we get to the bit where you find out how we actually use emulsion to make screen printing stencils.
Here it is in one-minute video form and also written out:
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Step 1:
Coat a nylon mesh screen in emulsion and let it dry somewhere a) dark, b) warm but not face-melting hot (75°-95°F), and c) dry-ish (30% to 50% humidity).
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Step 2:
Print your design on transparent film, e.g. acetate. Make sure to use a dark color for the bits you want to mark with ink when you screen print. For example, if we wanted to create a stencil of the QLP logo, the logo would be dark and the areas around it would be left clear. Fancy printers can also print directly on the emulsion.
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Step 3:
Lay your design in position on the set emulsion, and turn your UV lamp on. The exposed bits of emulsion – the clear parts of your design – will harden, while the covered bits stay soft. So in our example, the QLP logo will be soft, and everything around it will be hard. How long you leave the lamp on depends on the type of emulsion you’re using, but it’s important you get it right (you’ll see why in the next step.)
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Step 4:
Spray the emulsion with a hose. The bits that weren’t exposed (the logo) are still soft so they’ll wash away, leaving empty mesh for the paint to go through. That’s why you need to expose the emulsion long enough for the bits that should harden to harden – otherwise you’ll lose those too. But you also don’t want to do it for so long that the thinner areas on your design harden too, because then the paint won’t go through those parts and you’ll lose the fine detail.
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Step 5:
Lay your emulsion screen on the product you’re screen printing on, and scrape the ink over. The ink goes through the bare mesh where your design has been washed away. Ta-dah!
Three Different Types of Emulsion
You want to get really nerdy? OK, we appreciate that.
One of the most important factors that determines which emulsion screen printers choose is the type of ink they’re using – which itself is determined by the type of material they’re printing on. For example, some emulsions are more water resistant than others, which makes them good for water-based inks, and some are specifically designed for solvent- or UV-based inks.
Emulsion is often grouped into three types, based on how it’s prepared:
| Type | How it works | Exposure Time | Expiration Time | Cost |
|---|---|---|---|---|
| Diazo emulsion | Requires the user to mix in the light-sensitive diazo compound themselves before applying the emulsion to the screen. | Slow to expose, which gives you a wider margin of error than photopolymer emulsion, but it also takes longer to make each screen. | In dry, cool conditions, 4 to 6 weeks after mixing in the diazo – longer if stored in a fridge. | The least expensive type of emulsion. |
| Dual cure emulsion | Uses two sensitizers, with one typically being a diazo you have to mix in yourself. | A shorter exposure time than pure diazo emulsions, but longer (and so more forgiving) than photopolymer emulsions. | Same as diazo: once the diazo is mixed in, 4 to 6 weeks in the right conditions, and slightly longer in a fridge. | Usually between diazo and photopolymer emulsions. |
| Photopolymer emulsion | Already contains light sensitive compounds, so it’s ready to go when you take off the lid. | Much faster than diazo or dual cure emulsions, which means you have to be precise with your timing. | A few months to a year in dry, cool conditions (but not too cold.) | The most expensive type. |
The Pros and Cons of Emulsion in Screen Printing
Now that you know the role emulsion plays in the screen printing process, here’s a quick summary of why it’s so popular, as well as some potential issues screen printers have to look out for when working with it.
Pros
- You can get lots of detail into your design, especially with dual cure emulsion.
- You can add multiple colors.
- Emulsion can be used to create stencils for lots of products, including signs, prints, snowboards, balloons, and all kinds of textiles.
- Emulsion is relatively affordable, especially if you’re using the same design multiple times. Which leads me to…
- …You can reuse the same emulsion stencil many times, as long as you use the correct ink, and clean and store it properly.
- Once you’re done with the stencil, you can reuse (or reclaim) the screen by washing off the emulsion (this is harder the longer the emulsion has been on there.)
Cons
- Although mesh screens and emulsion are relatively affordable, the process requires access to a dark, dry room where you can dry the emulsion, and also a UV lamp.
- Stencils can break down over time, especially if you use the wrong ink. It’s relatively simple to repair small holes, but you can gradually lose some of the detail.
- Emulsion exposure times can either be so short you overexpose your design and lose detail (photopolymer emulsions) or long (diazo emulsions), which gives you a wider margin of error but slows down production.
- Once you’ve added diazo, you only have a few weeks to use any remaining emulsion.
The Bottom Line
Emulsion stencils remain a relatively simple, relatively affordable way to add detailed designs to a variety of products using screen printing.
Of course, it still takes practice, knowledge, and access to certain equipment. But once you’ve got the tricks and quirks smoothed out (screen printing pun), you can create great quality stencils to use over and over again. Now please sign my petition to change the name of the substance this article is about to “UV light-sensitive crystal liquid.”
Natasha Lavender
Natasha is an experienced writer and editor who has covered topics including technology, health and safety, business, and pop culture. Her work has been published by Good Housekeeping magazine, Forbes, Success, and more.
