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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

When misregistration is observed with a measuring microscope, the image will look similar to Figure 2. The question is, what is the amount of misregistration and how can it be measured?

The typical approach to the problem is to measure the difference between the edges of two registration targets (shown as Rx and Ry in Figure 2). The approach is fine if the two targets are identical in size.

In practice, however, the target widths are often different. The differences may be in the original art work, which may have different line widths or target sizes, or in the screen, where one target is more or less exposed and washed out than the other. Misregistration is the deviation between the center of the two targets, so measuring the edges of the targets is useless if their widths are not the same.

In this case, three measurements are needed to establish misregistration value (Figure 3) in one direction. You need to know the total width of the overlapping—or non-touching—targets (Dx), as well as the individual widths of the two targets you are aligning (dx1 and dx2). The amount of misregistration between the two is the difference between the total width and the average width of the two targets. The formula for this is Rx = Dx – ((dx1+ dx2) ÷ 2). The formula would look the same in the vertical direction, except the “x” would be replaced by “y.”

To prove this point, let’s assume the Dx=23 mil and two targets have widths of 10 and 15 mils. Measuring by conventional methods—target edge to target edge--you get either 8 or 13 mil misregistration depending on which color was printed last (Figure 4). These values are obviously wrong, because the misregistration value should be the same no matter which color is on top. Using the formula, on the other hand, will give you a single, correct value that reflects the distance between the center of the targets: Rx = 23 – ((10+15) ÷ 2) = 10.5 mil.

 

Measuring dimensional changes on soft substrates

Woven fabric, foam rubber, corrugated plastic, and paper all change size due to temperature and humidity variations. Measuring and evaluating this variation is not as easy as in the case of hard, non-absorbent substrates, but it is possible.


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