Before applying a protective coating, substrates are typically prepared by abrasive or machine blasting. This process removes previous coatings and mill scale, while roughening the surface to promote coating adhesion. The resulting surface is comprised of a complex pattern of peaks and valleys that must be accurately measured to ensure long-term coating performance.
Industry standards typically require that the peak-to-valley height of the surface profile be measured prior to coating application. A surface profile that is too low may not provide sufficient coating adhesion, and negatively impact long-term performance. A surface profile that is too high requires more coating to cover the peaks, and may prevent the coating from fully penetrating into the valleys, potentially reducing corrosion resistance.
For decades, Testex replica tape has been the industry standard for measuring surface profile height. It is accurate, easy-to-use, inexpensive, and is recognized by international standards from ISO, ASTM, and AMPP. It consists of a layer of compressible foam affixed to an incompressible substrate. When burnished against a surface, the foam collapses and forms an impression of the surface. Placing the burnished tape between the anvils of a micrometer and subtracting the thickness of the incompressible substrate gives a measure of the distance between the peaks and valleys of the surface profile.
Multiple grades of replica tape are available for different surface profile heights, as indicated by the range printed on each piece of tape. Start by choosing a grade of tape appropriate for the expected profile height.
Before testing, it is important that the surface is free of contaminants. Cleaning putty can be used to remove dust, debris, or residual blast media from the surface. Firmly press the putty onto the surface, and remove.
To begin, pull a single adhesive-backed piece of replica tape free of its release paper, and apply it to the blasted surface. Press the adhesive-backed ends of the tape to hold it firmly in place during the burnishing process.
The new high accuracy burnishing tool simplifies the burnishing process. Simply press the ball at the bottom of the tool against the circular window in the replica tape, until the base of the tool rests on the replica tape. A built-in spring mechanism ensures that consistent force is used, regardless of how hard the tool is pressed against the surface. Move the tool in varying linear and circular motions, removing the tool periodically to inspect. Burnishing is complete when a grey stippled pattern appears consistently across the entire area, with no white areas remaining. When measuring convex surfaces or tight areas, the opposite end of the burnishing tool, or the legacy acrylic tool can be used. Care should be taken to use light, consistent force when using a burnishing tool without a spring mechanism.
Once burnishing is complete, remove the replica tape from the surface. The top-left corner has no adhesive, making removal easy.
Inspect the replica to ensure that no contamination from the surface is present (e.g. dust, debris, residual blast media). When held up to a light source, ‘points’ of light should be visible consistently across the replica where the foam has been compressed by the peaks of the surface profile.
Once the burnishing process is complete, a micrometer is used to measure its thickness. The Testex Digital Micrometer and PosiTector RTR are designed for use with replica tape, and make the measurement process easy. Before use, it is important to clean the anvils of the micrometer to remove any residue or debris. Place a cleaning card or piece of paper between the closed anvils, and gently move it back and forth.
To use the Testex Digital Micrometer, press the menu button to turn it on. By default, X-coarse grade replica tape is selected, and conversion mode is enabled. With these modes enabled, the thickness of the 2 mil/ 50 micron backing layer is subtracted, and a conversion factor is applied to maximize measurement accuracy over the entire range of each grade of replica tape, with no need to average measurements from different grades of replica tape. Using the conversion factor and high accuracy burnishing tool, replica tape has an accuracy of up to plus or minus 8 microns or zero point 3 mils.
If desired, these modes can be disabled so that the micrometer operates like a conventional micrometer- useful for measuring free films of coating or other measurement tasks. Simply press the menu button to switch between replica tape grades or conventional micrometer mode.
Alternatively, if using an analog micrometer, this process can be completed manually by subtracting the thickness of the backing layer and using the lookup table in the instructions to apply the conversion factor.
Press down on the lever to open the micrometer anvils and insert the piece of replica tape, centering the circular opening of the replica foam between the anvils. Slowly release the lever, allowing the anvils to gently close on the replica.
Once the measured value stabilizes, the displayed value will flash and be recorded if memory is enabled.
When the replica tape is removed, the gage should return to negative two mils or negative fifty microns. If a different number is displayed, clean the anvils as detailed previously.
At least two measurements should be taken at each location, and averaged. The Testex Digital Micrometer has a built-in averaging mode that prompts the user to take two readings and automatically computes the average
If the two initial replicas differ by more than 5 µm (0.2 mils), a third replica is recommended. The average of the two closest replicas should be recorded.
Once measurements have been saved, PosiSoft software makes it easy to manage data from multiple probes and create professional, custom reports. Simply connect the USB cable, import the readings, and quickly save a PDF report. Learn more about PosiSoft Software in the, “How to use PosiSoft Software for Better Coating Inspection” video.
Testex Replica Tape is the accurate, easy-to-use, and inexpensive method for measuring surface profile. When paired with the Testex Digital Micrometer or the PosiTector RTR Replica Tape Reader, measurements are digital and can be stored, saved, and reported with ease.
To learn more, visit testextape.com.