24 Dec Perfecting Exposure Times
Screens must be properly exposed to ensure maximum durability on the press. Underexposure is often the culprit when screens break down prematurely, especially with water-based inks. A unique exposure time is required for each mesh count, thread size, mesh color, tension level, emulsion type, and stencil thickness used in production. That sounds overwhelming until you realize that once you have a system in place it becomes simple.
Bill Hood, ASDPT Fellow
Most screens are underexposed only because there is no system in place. You will need three indispensable tools to create the Perfect Stencil Exposure System and give you a wall chart (Figure 1) for each stencil configuration in your shop.
Item 1: Non-Destructive Moisture Meter
All emulsions must be dry before exposure. Dry means that the moisture content of the emulsion must be less than 4%. The non-destructive moisture meter (Figure 2) is the only tool that can provide this measurement. Only the non-destr
uctive or pinless meters will work when measuring the moisture of stencils on the mesh. You cannot use the moisture meters meant for measuring wood as they have sharp points that will damage the mesh.
Item 1: Thickness Gauge
An electronic thickness gauge is used to measure the stencil thickness prior to exposure (Figure 2). Since the thickness of the stencil determines how much time is needed to completely cross-harden the entire thickness of emulsion this tool is indispensable. With the thickness gauge, you pre-determine the correct exposure time and never have a stencil that is not exposed properly.
Item 2: Stouffer T-2115 Transmission Gauge
The Stouffer T-2115 (Figure 3) is placed on the lamp side of the stencil. It has 21 neutral density filters to assure that you have the correct exposure every time. Your goal is to have a solid Step 7 on each exposure. If it is off by a step, you can use the guide printed on the packaging to give you the correct exposure time. The Stouffer T-2115 Transmission Guide will enable you to determine the proper exposure time.
Steps for Achieving Perfect Exposures
Step 1 – Use the Stouffer T2115 Transmission Gauge (Figure 4) to find the correct exposure for each mesh count, thread size, mesh color, tension level, emulsion type, and stencil thickness in your inventory. Assure that each test is conducted on a thoroughly dry stencil, i.e., the moisture level is below 4% as measured with the Pinless Moisture Meter (Figure 2), and check the stencil thickness with the Electronic Thickness Gauge (Figure 3).
Step 2 – Create a wall chart (Figure 1) denoting the parameters for each type of stencil. There may be more than one set of parameters for each mesh count.
Step 3 – When you bring a screen to the exposure unit, check the thickness with the Electronic Thickness Gauge to assure that it matches the chart. If it is a bit thinner or thicker, you will need to adjust the exposure time based on the actual thickness of the stencil
To find the correct exposure, you must divide the new thickness by the base thickness, which will give the percentage of difference. Then multiply the percentage of difference by the base exposure time to find the correct exposure for the new thickness.
For example, if your base calculation for a given mesh is a thickness of 150 microns and the exposure time is 28 seconds and if your new thickness is 140 the formula would be 140 / 150, which equals a percentage of the difference of 0.9333. You would then multiply the base exposure by the percent of difference and find that the new exposure for the slightly thinner stencil to be 26 seconds.
If the next screen is slightly thicker and measures 160 microns the new exposure time for the slightly thicker stencil would be 29.8 or 30 seconds.
Note, that all other parameters must match the base calculation. Only the thickness makes a difference. If your base calculations were for a clear (white) mesh, then the new stencil would have to be on a clear (white) mesh. That is correct, there is no white mesh. It is actually a milky transparent thread, which only appears white to the eye.
Keep adding the different thicknesses and the correct exposure times to your chart so that you do not have to use a calculator each time you expose a stencil.
- Extech Pinless Moisture Meter, http://www.extech.com/categories/moisture-meters/
- Highline Electronic Thickness Gauge, https://www.elcometerusa.com/HighLine-Meter-Paint-Thickness-Gauge-2.html
- Stouffer T-2115 Transmission Guide, https://www.stouffer.net/TransPage.htm