Emulsion Exposure Times: How to Find the Right One
Why emulsion exposure times vary, the variables that change them, and how to run a step test to find the right exposure for your setup.
Brand free method for screen exposure: the variables that change exposure time, a step test, and signs of under and over exposure.
Short answer: how long to expose emulsion depends on your light source, emulsion type, mesh, coating thickness and lamp distance, so there is no single correct emulsion exposure time. Find yours with a step test: expose one coated test screen at several times, wash it out, and keep the time that holds fine detail with a fully hardened stencil.
Rule: expose a test screen through an exposure calculator or step wedge, wash it out, pick the exposure that holds fine detail with a fully hardened stencil, record it, and retest whenever any variable changes.
Part of the screen printing guides in the DAX Learning Hub. New to the process? Start with screen printing for beginners.
Also see Screen Reclaiming and Ghost Image Removal: A Shop Guide.
Why Emulsion Exposure Times Vary
Exposure is a dose of UV light, and the dose a stencil needs depends on everything between the lamp and the emulsion. A time that works in one shop can underexpose or overexpose in the next, even with the same emulsion. That is why emulsion makers publish starting guidance and then point you to a test, and why this page gives a method instead of a number.
Trade guidance is blunt about the stakes: underexposure is the most common cause of stencil trouble, from pinholes to stencils that break down on press.
Variables That Change Exposure Time
| Variable | How it shifts exposure | What to do |
|---|---|---|
| Light source (UV LED, metal halide, fluorescent) | Each puts out different intensity and wavelengths, so times do not carry over between units | Run a new test when you change or relamp a unit |
| Emulsion type (diazo, dual cure, photopolymer) | Sensitivity differs widely by chemistry | Test each emulsion separately |
| Coating thickness | More coats mean more emulsion to harden, so more exposure | Standardize your coating method and count |
| Mesh count and mesh color | Mesh changes stencil thickness; white mesh scatters light, dyed mesh limits scatter | Retest when you change mesh count or color |
| Lamp distance | Exposure rises sharply as the lamp moves farther away | Fix the distance and do not move it after testing |
| Lamp age and film positive | Aging lamps lose output; weak positives let light through | Track results over time; use dense positives |
How to Run an Exposure Step Test
There are two common tools. A factor style exposure calculator uses filters that pass different fractions of light, so one long exposure shows several effective exposures at once. A step method exposes strips of the screen for increasing times. Either works; follow the instructions that come with your tool.
- Start from an estimate: the emulsion maker's guidance or your past notes.
- Coat and fully dry a test screen exactly the way you coat production screens.
- Place the calculator or test film emulsion to emulsion, in tight contact, on a clean surface.
- For a factor calculator, expose for about twice your estimate. For a step method, add exposure in equal increments, covering one more section each time.
- Wash out as you normally would, then let the screen dry fully. Wet stencils swell and mislead.
- Pick the section that holds your finest needed detail with a hard, non-slimy stencil. Multiply by its factor, or read its time, to get your exposure.
- If every section washes away, increase the base time and retest. If every section is closed up, decrease it and retest.
- Record the time next to the emulsion, mesh, coating method and unit it belongs to.
Many shops also expose a small step wedge outside the image on production screens as a quick daily check. The target step depends on the wedge and the emulsion maker's guidance.
Signs of Underexposure and Overexposure
| Underexposure | Overexposure |
|---|---|
| Slimy or soft emulsion on the squeegee side after washout | Fine lines and small halftone dots close up or will not wash open |
| Pinholes and early stencil breakdown on press | Small gaps between elements bridge over |
| Stencil thins out or washes away during washout | Edges lose sharpness as fine detail fills in |
When in doubt, emulsion makers lean toward full exposure for durability and choose the detail you actually need, rather than underexposing to hold lines finer than the job requires.
Emulsion Types and How They Differ
Diazo Emulsions
Two part emulsions sensitized with diazo. They tend to be forgiving and economical but generally need more exposure than photopolymer types, and the sensitizer gives a visible color change that helps you judge exposure.
Dual Cure Emulsions
Diazo plus photopolymer. A common all round choice that sits between diazo and pure photopolymer in exposure speed and resolution.
Photopolymer Emulsions
Single part, presensitized emulsions. They usually expose fastest and pair well with UV LED units, but do not show the same color change, so judge them by detail and stencil hardness instead.
Your ink system matters too. Water based and discharge inks call for water resistant stencils, so match the emulsion to the ink. See plastisol vs water based ink, and pair mesh choices with the screen printing mesh count chart.
Frequently Asked Questions About Emulsion Exposure Times (FAQ)
How long do you expose screen printing emulsion?
As long as your own step test says. The right time depends on your light source, emulsion, mesh, coating and lamp distance, so a number from another shop or chart is only a starting estimate. Run the test, record the result and retest when anything changes.
How do I know if my screen is underexposed?
Look for slimy or soft emulsion on the squeegee side after washout, a stencil that thins or washes away, pinholes, or breakdown on press. Each is a sign to add exposure and retest.
Does mesh color affect exposure time?
Yes. White mesh scatters light and narrows your exposure margin, which can close fine detail. Dyed mesh limits that scatter. Run a new test whenever you change mesh color or mesh count.
Do LED exposure units expose faster?
Often, yes, especially with photopolymer emulsions. Compared with a modern metal halide unit the gap can be smaller. Either way, times from another unit do not transfer, so step test on the LED unit itself.
Ranges and methods on this page are typical starting points compiled from multiple supplier and trade sources. Figures vary by product, so always check your supplier spec and test before production.
Want to see exposure units, emulsions and screen room setups up close? Browse more guides in the DAX Learning Hub, and check cities and dates on the DAX show schedule when you are ready to compare screen making workflows with suppliers in person.