Step Wedge Test — A High Quality Print Requires A Good Stencil
Many screen printing beginners (or sometimes, even seasoned) are having a hard time knowing the right exposure time for the particular emulsion they use. It is really hard to tell what exposure time we should use because the type of emulsion is not the only factor that should be considered.
In determining the ideal exposure time, the two main factors that greatly affect the outcome of our stencils are the type of emulsion and of course, our exposure units.
Here in the Philippines, most screen printers use DIY exposure units, and this means that almost everybody will have different exposure unit setups. These exposure units can be different in dimension, number of light source, and the type of light source being used. With that being said, the only way to be able to find the ideal exposure time for the particular exposure unit setup you have is by conducting a “step wedge” test.
Step Wedge Test
This test is common, but I find it quite unpopular. The main purpose of this test is to actually determine the ideal exposure time for our stencils using of course, our own materials and exposure unit. As mentioned above, the emulsion and exposure unit are the biggest factors in determining the exposure time, so it is important to use our own.
Basically, the test will let you know the correct exposure time by doing a series of exposures of one screen. By using a time where the emulsion is sure to be under-exposed — and by adding small time increments and exposing the same screen again and again — until you reach a time where the screen is sure to be over-exposed, you will be able to determine what is the correct exposure time you should use.
To help you understand that last line, let us use this simple drawing:
Step 1: Choose Your Materials
The results of your step wedge test will largely depend on the materials you will use, particularly the emulsion, screen mesh count, and exposure unit. So, to properly conduct this test, you first need to choose what materials you will use. Note that if you are using different emulsions or screen meshes, it is recommended that you conduct different tests on each of them.
Also, another thing you need is a step wedge calculator. In the image below, we were able to download a free step-wedge calculator from SMR Software.
Once you have chosen your materials, make your screen to prepare for the actual test.
Step 2: Choose Your Time and Increments
Once you have your screen ready, choose the range of time that you will use during the test. Through experience, you can probably know the time when your emulsion is under exposed and over exposed. But if you do not know these times, you can probably start at 1 minute for the under exposed time and 8 minutes for the over exposed time.
Once you know these times, choose what time increment you will use (note that this time increment is what will determine the ideal exposure time for your particular set up). If you choose the 1 minute and 8 minute times, use a 1 minute time increment for each line on the step wedge calculator.
Step 3: Start The Test
Start the test by exposing the first line on the step wedge calculator by 1 minute (or the time you chose when your emulsion is under exposed). Note that you should only expose the first line, so you need to use some sort of card board as a cover to prevent the other lines from being exposed.
After exposing the first line, move the cover down to the second line and expose both the first and second line for another 1 minute (or the time increment you chose to use). Repeat this process until you reach the last line of the step wedge calculator.
NOTE: On the image above, we considered a 1 minute under exposed time and 1 minute time increment. Note that you are free to choose these times based from your own experience. Just make sure that you use equal time increments for each line on the step wedge calculator.
Step 4: Wash Out And Examine The Stencil
Once you are done exposing your screen, wash out the stencil using a power wash. After washing out the stencil, examine the stencil and see on which line has the best detail (the line where the stencil is properly burnt and all the details are seen on the stencil).
At the same time, further examine the stencil by wiping each line (on the ink side) using a tissue paper. As you wipe each line, see if there are small amounts of emulsion that are being wiped off. If there is no emulsion on the tissue paper, it means that that line is over exposed. And if there is too much emulsion on the tissue paper, it means that that line is under exposed.
The line in which just a small amount of emulsion is wiped off is the line where the emulsion is said to be exposed properly. If you have correctly conducted this test, that line should also have the best detail in it.
Once you have determined the line where the emulsion is properly exposed, the time in which that line is exposed is the ideal time for your particular set up.
Doing this test is very important because as you may already know, a good screen and stencil is one of the most important things you need in order to produce a high quality print on your shirt. Thank you so much for reading! If you have any thoughts or questions, please let us know by leaving a comment below!
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Regards, Alwin (Instagram: @alwin_rys)