Teaching employees to use measurement tools correctly and assess the results accurately can significantly improve both production speed and product quality. Discover how proper measurement techniques can help you correct for registration errors and compensate for dimensional changes in substrates.
2. Print the six sample sheets and dry only five of them in your drying system. Let the sixth sheet, which we’ll call the control sheet, air dry, and store it as you would normally store in-process products.
3. Clean up the press without removing the screen or the squeegee and with out changing any setup parameters.
4. Because this is a test and not a production run, you must determine how long to wait before you proceed with successive printing steps and measuring. In general, try to make the waiting period as long as the waiting period during actual print runs.
5. Using a different ink color, repeat steps two and three on the original six sample sheets using the same setup on the press.
Performing the experiment with textile materials
Follow this procedure when conducting a dimensional change test on textile substrates:
1. Multicolor textile printing is normally done wet-on-wet, in which case material shrinkage plays no role, because the product is not cured until printing is complete. The only time shrinkage becomes important is when the product is flash-cured between successive colors. For this reason, your test-printing setup must include a flash-curing unit. You may use any ink for this test.
2. With the first color, print as many as pieces as the number of stations on your press allows. Cure all of them but one (the uncured piece is the control) with the flash-curing unit and leave all the pieces on the platens.
3. Clean up the press without removing the screen or squeegee and with out changing any setup parameters on press.
4. Add a different color of ink to the screen and print all the samples once again. Cure all except the control piece.
Measuring and recoding the results
Before measuring the samples, examine the control pieces. All the registration targets and images should cover each other perfectly on these pieces. If they do not, two possibilities exist:
a. The printing setup is inaccurate.
b. Applying ink to the substrate is enough to cause dimensional changes. (Note: This possibility is not applicable to textiles.)
If the center target (in the middle of the control sheet) is off register, the printing setup is probably inaccurate. If the center target is right on and one or more of the other targets are not, then repeat the experiment and don’t cure any of the substrates. If you see consistent misregistration among the peripheral registration marks, record the results as “dimensional changes due to ink coverage.”
If the printing is done accurately, the center target will always be perfectly aligned on both the control and test textile pieces. Center targets that are not aligned mean that you have a registration problem. On non-textile materials, only the control sheet center target will be aligned. The printed and cured test sheets, however, will show the center target off register in both the X and Y dimensions. This is normal because we are using three-point edge alignment, not center alignment.
Use a measuring microscope to measure the differences between the targets near the substrate edges of the test samples. Measure edge-to-edge differences as shown in Figure 7.
The total change in substrate size is the sum of the differences in both X and Y directions (Figure 8). For example, if X1=0.010 in. and X2 = 0.015 in., then the total change is 0.025 in. Assuming the distance between the two edge targets on the X axis was 10 in., then the rate of change per inch in the X direction would be 0.025/10 or 0.0025 in./in. for that substrate. Repeat the measurements in the Y direction.
Measure and record the rate of change for each sample. You can average the results to determine the approximate rate of change you can expect when using the same material in production and apply that rate to any image size you print. This method of evaluating dimensional change is applicable to any substrate and will allow you to differentiate between misregistration caused by setup, screen distortion, or dimensional changes in the substrate.
Tamas S. Frecska is former editor and associate publisher of Screen Printing magazine. With more than 30 years of experience in the industry, he has been involved in print production, equipment manufacturing, and consulting. Frecska is a member of the Academy of Screen Printing Technology and a five-time winner of the Swormstedt Award for technical writing.
Did you enjoy this article? Click here to subscribe to the magazine.