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Making Measurements that Matter

(May 2008) posted on Tue May 27, 2008

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.


By Tamas S. Frecska

click an image below to view slideshow

Measuring devices are becoming easier to use and more affordable. As a result, measuring microscopes and electronic linear scopes are becoming commonplace in many screen-printing companies. The simplicity of these devices often leads to untrained employees measuring critical data on products or processing tools. Here, we’ll take a look at some of the issues your employees should be aware of when measuring misregistration on any material and when assessing dimensional changes with soft substrates.

 

Image reversal

While most employees are vaguely aware that simple lenses reverse an image (upside-down and left-to-right), many do not take this image reversal into account when working with a measuring microscope. As long as the measurements are simply recorded as absolute values, this reversal is unimportant. However, if the direction of the measurement is critical (for example, adjusting for misregistration), image reversal becomes a problem.

As Figure 1 illustrates, aligning two registration targets through the microscope is tricky, unless you pay attention to the reversed image. The required adjustment to the left and down (as viewed on the microscope in Figure 1) would actually need a movement to the right and up on the printing press.

Employees should be trained to recognize this image reversal. It can be quite important when misregistration data are collected for statistical purposes (as in statistical process control). Misregistration values should carry a minus sign if the corrective action requires movement in the left or downward directions. Of course, under the microscope, these values will be observed to the right and upwards.

 

Measuring misregistration

Image alignment (registration) is accomplished without any visual feedback on 99% of screen-printing presses. Unfortunately, without a video-optical system, the press operator doesn’t know his adjustment results until he prints with the new adjusted settings. This being the case, the only accurate and quick way to make the registration changes is to measure the misregistration on the printed piece and adjust the press-registration devices accordingly (using dial gauges for feedback).

While this sounds relatively simple, it requires operator insight and skill when taking the measurements. If adjustments of less than 15 mil have to be made, it becomes increasingly important to measure misregistration accurately. The same is true if the data are collected for statistical purposes.


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