Moving Coating Thickness Measurement System

Hi, I measure coatings with an instrument where I work, but what I am measuring is weld cladding. However the instrument that I use is designed for thin coatings such as paint thickness. Judging by the measuring device I use (FISCHER-dual-scope MP20) you are going to run into a couple problems...this device is a handheld model and it sends the frequency through 2 probe points that are approximately 1/4" in diameter since your product is corrugated you need to have accuracy when the instrument is placed on the part, if it is over the top of the ribs it will be inaccurate.

What is the design of the corrugated piece? What I am asking is that I have seen corrugated material with a perpendicular wall between the sheet metal and I have seen it with a zigzag diagonal path between the sheet metal. If the direction of the corrugated rib or wall is not perpendicular to the coated face of the part the instrument will not read right.

The other problem that I see is that you are going to try and measure it while it is moving. The company that makes this instrument may be able to offer you more options because they specialize in NDE equipment. Maybe they have a laser that can read the measurements as they pass by. From my experience in the painted and plated automobile trim industry and the NDE experience that I have measuring coatings I would suggest that this not be done as the part was moving, unless they can pause at a certain station and be measured by a stationary laser, but that can eat up time too. The best way to do this is for an inspector to randomly pick the test pieces and run studies to find your average, minimum and maximum thickness. Keep in mind that I am not up to date on automation technology and there may be a sensor out there that can do it but it doesn't seem like an accurate way to measure a coating to me. Also if you run into problems with it giving bad measurements, how much downtime will be created while waiting to recalibrate this system? Just a couple things to think about. This is very interesting and I would like to know the outcome of this. Please keep me posted. Here are a couple places to try:

www.helmut-fischer.com

www.panametrics-ndt.com

Greetings~ The Fischer unit is similar in concept to the Positector 6000 that we have, in that it can switch between eddy current mode (for measuring coating thickness on top of non-ferromagnetic materials) and magnetic mode (for measuring coating thickness on top of ferromagnetic materials). The KEY WORD just used is "thickness". These instruments are intended for use in the typical coatings industries, where accuracy is to within microns, when measuring "typical" coatings (to within mils, when measuring thicker coatings). The eddy current method is probably going to be the best bet, but most likely utilizing a custom set-up. A "Go/No-Go" acceptance test could be established using a differential coil arrangement (with a "scope-type", not digital read-out eddy current test instrument), with one coil establishing the Standard (over a known good sample). Contact an ECT specialty company (e.g., Zetec) and explain your application in detail, with photos and cad sketches. They'll be quick to answer what can be done. Best of luck....

One point that it looks like I did not make clear: What we are measuring is the thickness of the coating on a corrugated foil. The foil is corrugated, then sprayed. This corrugated foil is not sandwiched between flat layers of foil, it is processed as a single layer. We want to measure the thickness of the coating on the foil and the height/depth of the ridges and grooves. These measurements can be done separately. The more interesting measurement is the coating thickness, but we will eventually have to do both.

Thank you for your help.

Every measuring instrument has inherent capabilities and limitations. There is no "one instrument does-it-all", or the rest would be out of business(wink). From this comes the saying, "Purpose-built".

You mentioned 1-3 inch widths. Is the peak-to-valley (or crest-to-trough) measurement a quarter-inch? 1/32nd inch? Is the angle between them a sharp 90 degrees? 45 degrees?... or a curve? and if so, what radius?

We make several different types of foils. The width varies from 1" to 3". The peak-to-valley measurements vary and the spacing (peak-to-peak) varies. The peak to valley will be between .100" and .300". The spacing will be between .200" and .300" peak-to-peak. The bends are smooth curves with different radii depending on the height and spacing. Looking at the corrugated foil from the side it looks like a sine wave, we vary the "frequency" and "amplitude". Again, we are most interested in measuring the coating thickness on this foil as it moves down the line.

'Checking-back', here, I am sorry to see no further offerings to your dilemma. Have you come up with anything as yet? Having pondered this on several occasions, I highly suspect that the technology you seek is a rare commodity. In all likelihood, the person whose insight we need is one who works with Al Oxide deposits on semiconductors, or maybe anodized finishes for aerospace apps. Nonetheless, for discussion's sake, you might approach a company that is proficient with thermal imaging : if the line speed is steady enuf, an LED heat source underneath (not enough to cause any harm or even perceptible effect, other than being faintly detected by a thermal imaging sensor facing the topside, a foot or so down the line).

The corrugation poses a severe handicap for air-coupled ultrasonic transducers to work in conjunction with pulsed laser / ultrasonic combo. [poses difficulties for other technologies as well] But, having a known "calibration_coupon" of acceptably-coated foil would allow a "pass/fail" (too thick or too thin) thermographic system to be set-up with relative ease.

Wishing you success ~