Baggy Webs: Part II Measuring

I'm not usually a big fan of management adages, but this is one I often can support: “You can't manage what you don't measure.” How do you expect a defect to go away if you don't measure it?

The minimum measurement is binary (“got it vs. don't got it”). True measurement begins with quantifying something (the defect was this big, or there are this number of defects per a population). The next step in measurement is to add trending to it, either by location or by time.

Regarding baggy webs, what should you measure? Bagginess by itself isn't usually a problem until 1) you try to run the baggy web through a nip, or 2) it is so bad the web wrinkles or your laminates are curly on one side and flat on the other.

The goal of measuring baggy webs is not to get a number but to correlate bagginess to real, permanent defects and to work to reduce the source of your bagginess. The trouble with unmeasured baggy webs is you can't decide if a particular roll of material will cause problems or not. Also, if you aren't measuring it, it is difficult to convince a supplier that the web they are sending you is too baggy or baggier that it was before.

Here's a fairly exhaustive list of options to measure your baggy web. Tests 1 through 6 measure bagginess off-line with a sample in lab. Tests 7 through 12 are on-line measurements to check a moving web.

  1. Sweep out a length of web and measure skew from a straight line.
  2. Lightly tension a web sample in a horizontal span between two aligned rollers and measure crossweb droop variations.
  3. Inspect a web sample from the top of a wound roll and qualitatively judge the force to pull the web taut (0 is a perfect web, 1-5 requires finger, wrist, elbow, shoulder, or full-body strength to pull out the bagginess.
  4. Combine 2 and 3 above, measuring tension to pull horizontal span up to a target sag dimension, for example less than 0.5 in.
  5. Mark parallel crossweb lines on the flat tensioned web, then cut it into machine- direction strips and measure the length differential of the untensioned strips.
  6. Place a sheet sample on a flat plate and measure ripple or curl deviations from planar.
  7. Perform Test 2 online with a scanning distance measuring device, such as a laser triangulation micrometer or an ultrasonic sensor.
  8. Perform Test 2 online and compare the lines displayed on a web from perpendicular and low-angle light sources. The low-angle light will appear wavy when compared to the perpendicular line if there are any crossweb sag variations. Combine this with a camera and vision system to quantify bagginess.
  9. Measure crossweb tension variations with a segmented tension roller.
  10. Measure crossweb tension variations with a segmented tension beam. This is similar to Test 9 but uses narrower, non-roller elements (a service provided by PAPRICAN, Canada's paper research institute in Montreal).
  11. Measure crossweb tension variations while the web is pneumatically conveyed over an air turn bar.
  12. Measure crossweb tension variations in the speed of sound through the web (a device developed in the lab of Dr. Richard Lowery at the Oklahoma State Univ. Web Handling Research Center).

I don't want to leave the topic of measurement without the necessary warning: Before you go crazy measuring web bagginess, invest some time in finding out whether your measurements actually correlate to defects.

Tim Walker has 20+ years of experience in web handling processes, education, development, and production problem solving. Reach him at 651/686-5400; tjwalker@tjwa.com; www.webhandling.com.


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