01 Mar COATING THICKNESS TEST FOR PROPER EXPOSURE
How many times have you had to make two or more strokes to achieve opacity? How many times have you exposed a stencil only to realize that the squeegee side was slimy and had that sinking feeling that the screen was underexposed and would probably not make it through the run without breaking down? Are you tired of blocking out pinholes from underexposed stencils? Would you like to increase the tonal range in your halftone prints? This article will solve all those problems for you and more. Isn’t it time to take the guessing out of your game?
In 1964, I developed the Coating Thickness Test. I used it in several shops to assure the practicality of the test before I started teaching it to my students at the School of Screenprinting and to my consulting clients. This test produces nine different emulsion over mesh ratios on a single screen, for determining which emulsion over mesh ratio is best for a particular mesh count/thread diameter and ink. Be sure to tension the mesh to the mesh manufacturers recommended tension level.
The purpose of the Coating Thickness Test is to allow you to set standards that will provide consistent, predictable and repeatable (CPR) coating procedures so that you will never need to use an exposure calculator or step test again. You will gain knowledge of what mesh count is needed for each ink and ink color and substrate. You will know how to coat a screen to achieve the correct stencil thickness that will allow you to cease making multiple strokes.
Sound good? Then let’s get started!
The 5 Step Coating Process
You will need to do this test on each mesh count/thread diameter/mesh color used in your shop. You will also need an electronic thickness gauge, priced from USD$17 to USD$900. Just look up Paint Thickness Gauges on the Internet. You will want to assure that the one you purchase comes with a “metal puck” to place on the other side of the stencil and that it can measure thicknesses from 40 to 1,000-microns within plus or minus of 1-micron.
Step 1: Coat each screen once on each side to completely fill the mesh openings with emulsion, scraping off the excess, so there is no emulsion accumulation over the thickness of the mesh. Dry this stencil until the moisture content is below 6%. The thickness of the dry stencil should be equal to the thickness of the uncoated mesh. This is our base coat.
Step 2: Using the less radiused edge (thinner edge) of the coating trough, make a pass on the substrate side (bottom) of the screen that goes up 2/3 of the way, and let dry.
Step 3: Duplicate Step 2, but only go 1/3 of the way up the screen, and let dry. At this point, you will have three thicknesses of emulsion on a single stencil.
Step 4: Turn the screen sideways and make a pass that goes 2/3 of the way up the screen and let dry.
Step 5. Finish with a pass that goes only 1/3 of the way up the screen and let dry. You now have a screen that has 9 different thicknesses of the emulsion.
Coating Thickness Test Film
While you are waiting for the emulsion to dry, download our free resource, Coating Thickness Test Film, as shown below. Print it to a clear film positive without changing the size as it will change the point sizes and line thicknesses. You will cut each of the nine squares individually. These will be placed into the center of each nine squares on your screen.
Let’s take a moment to discuss the drying of the emulsion. In order to achieve the best exposure possible, in terms of detail, reduction of pinholes, and to hold up well on press, the screen must be dried to less than 6-percent moisture level, and preferably at 2-percent. This can be checked with a moisture meter, such as the TQM Moisture Meter.
Exposing the Stencil
Use an exposure that is the median between the different emulsion thicknesses and print the images. Note, that this will not produce a perfect exposure for each of the nine squares. It is only important to know at this time which emulsion thickness prints well. Choose the best print from the nine in terms of detail held and note the square that produced this image. Now you know how thick the emulsion needs to be for your particular situation. Keep the screen, as you will want to test it on different substrates, colors, and with different inks.
In case you were wondering about the statement made above, “Use an exposure that is the median between the different emulsion thicknesses,” let me explain. Exposing emulsion is similar to cooking. A thin coating of emulsion compared to a thick coating of emulsion is a lot like comparing crepes to pancakes. If you are on a gluten-free diet, then compare it to cooking a steak. The thicker they are the more heat is needed to cook them all the way through. When we expose emulsion, we need to cross-link the emulsion all the way through the thickness, otherwise, the emulsion on the squeegee side will not be fully exposed and wash away in the development of the stencil.
If you listen to others, particularly the salespersons, they will tell you that you need to buy an exposure calculator for about $65. If it were only that simple. Once you make the purchase, you realize that you have to coat and expose each mesh count/thread diameter / and stencil thickness (coating technique) combination used in your shop. And then you find out that it still doesn’t work all that well. That’s because as humans, we are not perfect and we cannot coat any two screens the same. So, one will be underexposed and the other may be overexposed, but they cannot both be exposed correctly with the same exposure if they are different thicknesses.
Eliminate Double Strokes
That is where the thickness gauge comes in. Once we know which of the nine squares of different emulsion thickness produced the best print for this particular mesh, ink, and printing conditions, we can measure this thickness as a standard thickness for our unique combination. Of course, you will find that any change made in the mesh, ink or printing conditions will mean that we have to change the thickness of the emulsion.
Knowing the Correct Exposure
Once you know that the correct exposure is for a particular emulsion is, for example, 32 seconds on a stencil that measures 100-microns, you have knowledge that is translatable into useful data. You record this time on a whiteboard above the exposure unit. Next, you have a stencil that is only 75-microns thick to expose. As long as they are both the same color of mesh, and were coated with the same emulsion, you only have to divide 75 by 100 to find that that our next exposure is 0.75% of the 32-seconds of the 100-micron stencil or 24 seconds, by using the formula of 32 / 0.75 = 24 seconds.
As an example, once you know that your particular ink needs to be printed through a 120-tpi, 80-micron mesh tensioned to 40N/cm2, with stencil thickness of 150-micron, all that is needed is to measure the thickness of the stencil before you expose it understand that meets the needed requirements to print an opaque deposit. If the stencil thickness is less you can always build the stencil up to the proper thickness before the exposure.