Pressurex pressure-sensitive film

Jon Titus -- Test & Measurement World, 4/1/2001

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Power transistors in your product continue to burn out, and you suspect poor contact between the transistor’s case and the heat sink, but you can’t be sure. You’ve positioned a test fixture, and it doesn’t seem to tighten into place properly, but you have no way to be certain. What can you do to examine the pressures between your surfaces?

Your best bet may be Pressurex film, a thin plastic material that indicates the magnitude and distribution of pressure between mating parts. You simply cut out holes for bolts, aligning pins, and other projections, place the film between the two surfaces, and mechanically fasten the parts in place as usual. The pressure the parts exert on one another translates into color intensity on the film, thus giving you a better understanding of where the parts meet with the proper pressure and where they don’t.

The film ranges in thickness from 4 to 8 mils (0.1 to 0.2 mm), so it shouldn’t interfere with the proper assembly of most mechanisms during testing. It’s particularly useful when you can’t place a load cell or strain gauge on mechanical equipment. Pressurex film measures pressure over ranges, from 2–20 psi (0.14–1.4 kg/cm²), up to 7100–18,500 psi (500–1300 kg/cm²). The film costs about $55 per ft. in small quantities, although you can purchase it in rolls.

How does it work?
The Pressurex material incorporates two polyester layers, a transfer sheet and a developer sheet (Figure 1). When something applies pressure to the film, microencapsulated chemicals in the inner surface of the transfer sheet press against the developer sheet. Under a specified pressure, the microcapsules break, and a chemical reaction takes place that leaves permanent magenta marks on the film.

The intensity of the mark in the color-developing layer relates directly to the pressure exerted on the sheet. The more pressure, the deeper or more intense the magenta color. To interpret the resulting color pattern, you compare the intensity of the color on the film with colors shown on a graph the company supplies (Figure 2). You can get an accuracy of within 10% of the actual pressure by using the comparison graph. In many cases, the intensity information alone may provide enough information—a simple indication of relative pressure rather than a quantitative analysis of the actual pressure between parts.

What if you need quantitative results? The company will analyze your “exposed” film and provide results with an accuracy of within 2%. The analysis scans the image and produces a pseudo-color image based on the intensity of the magenta color. The processed image shows high-pressure areas in red and lower-pressure areas in deep blue (Figure 3).