When it comes to exposing artwork (burning a screen) there are several variables that determine how much time (amount of light) should be used. The variables include: the exposure unit itself and type of bulb (LED much faster), emulsion used, amount of emulsion coated on the screen, and mesh count. When dialing in the perfect exposure time for different mesh counts it is very important to only change one variable at a time. So if you are trying to find the right amount of time to burn a 110 mesh screen, then make sure to always use the same type and amount of emulsion on each test screen. Changing any of the variables will affect the amount of time it takes to burn a screen and thus they all should be respected.
To find the right exposure time for different mesh counts it is important to have a 21 Step Sensitivity Guide and maybe even an “Exposure Calculator”. A 21 step sensitivity guide is a small strip that contains 21 small boxes with different amounts of dots (detail). How it works is the 21 step guide is exposed on a screen and then when it is washed out a screen printer can see how much detail is possible to hold based on the exposure time. An exposure calculator is like the 21 step sensitivity guide on steroids as it can really show you how much detail can be held when burning the mesh for that amount of time using the same type and amount of emulsion.
If a design is “underexposed” it means it wasn’t burned long enough and during the process of washing out the stencil the emulsion is not strong enough to take the pressure from a power washer. This causes too much of the design to wash out and thus ruins the stencil. Tip: An underexposed screen’s emulsion will have a slimy feel to it. An overexposed screen is one that was burned for too long and thus the design is hard to wash out / not possible to create the necessary stencil to print with. So it is important that each mesh count with the same type and amount of emulsion be dialed in from the beginning so screens can be burned without problems and hold as much detail as possible within a design.