Posted it here originally.

There are keys (push buttons) on the front panel of an avionics device that fits inside the aircraft cockpit. I need to know the material of which those key-tops are made from.

enter image description here

I can guess the material is some type of clear transparent plastic and then its painted with black color. The text on it is un-painted key surface. There is green light under the keys (buttons). That green light glows the text (due to transparent plastic) on the key-top as green in color.

This cockpit device is from a very old aircraft model of 1950's so we can say it is not using any modern high performance thermoplastic or composite material. How can I best identify the material of these key-tops?

Edit: How do I know if the key-top material is compatible with NVIS (Night Vision Imaging Systems)?

  • 2
    $\begingroup$ Can't put my finger on it, but I have serious doubts about these being an original item from a 50's plane... $\endgroup$ – Jpe61 Sep 15 '20 at 14:19
  • 1
    $\begingroup$ I’m with the guess for double shot molding from Nguyen, with colored plastic. This is also used for some keyboard switches. Painting or even coating switches lead to horrible results after some time and these look still fine ;) $\endgroup$ – Peter Sep 15 '20 at 19:34
  • $\begingroup$ What plane are these from? The keypad is similar to ones seen on IRS (or INS) panels, and a rather close match to F-18 Hornet's UFC keypad... $\endgroup$ – Jpe61 Sep 15 '20 at 21:25
  • $\begingroup$ @Jpe61It's a mirage plane. $\endgroup$ – scico111 Sep 16 '20 at 4:12
  • $\begingroup$ @scico111 original earliest 50's Mirages did not have panels with such keypads, they will be from a later cockpit update. $\endgroup$ – Jpe61 Sep 29 '20 at 12:41

The part looks injection-molded. The choice of transparent, moldable polymers available in the 1950s is manageable. The most likely candidates are:

  • PVC (back then the most widely used polymer, but normally not used in injection molding)
  • Celluloid (unlikely due to its high flammability)
  • PMMA (plexiglass)
  • Polystyrene

PVC can be detected when a heated copper wire is pressed into it and then again held into a gas flame. The copper will react with the chlorine of the PVC and color the flame greenish (Beilstein test). The last two candidates can be easily identified by the smell they produce when molten. Styrene in the case of polystyrene and derivates of benzene in case of PMMA. Since PMMA is rather soft, it can easily be notched but also has low mechanical strength. When dropped on a table, PMMA parts will create a more muffled, lower frequency sound than parts made of polystyrene.

Both are similarly well suited for being painted. Some plastics like polypropylene and polyethylene are unsuitable for painting because of their low surface energy. But while already known in the early Fifties, those materials were not yet used for parts production. The best substrate for painting would be ABS, but similarly while being known back then, transparent ABS types suitable for molding were not yet used.

My guess would be polystyrene, but given the few hints I am not really sure.

  • $\begingroup$ Can Polycarbonate (PC) be used for the keys? The original keys may or may not be made of PC but what ever material those were made from will it be a safe choice to use PC material for the keys? $\endgroup$ – scico111 Sep 16 '20 at 4:10
  • $\begingroup$ @scico111 While PC would be a good candidate from the late 1960s on, in the 1950s it was still a lab curiosity. $\endgroup$ – Peter Kämpf Sep 16 '20 at 7:00
  • $\begingroup$ so if i am free to choose from PVC, PMMA, ABS, Polystyrene and PC then which one is the best candidate for the front panel buttons? $\endgroup$ – scico111 Sep 16 '20 at 8:37
  • $\begingroup$ @scico111. Clearly, that would be ABS because it shows the best adhesion with the paint. Transparent ABS is now well established (while ABS was first made in 1946, I very much doubt that it would had been available in a transparent grade in the 1950s). $\endgroup$ – Peter Kämpf Sep 16 '20 at 9:54
  • $\begingroup$ How do I know if the key-top material is NVIS material? $\endgroup$ – scico111 Sep 29 '20 at 12:25

Give the part to a machinist who works with plastics. He will touch the part to a grinding wheel, melting the plastic and making smoke. By smelling the smoke and looking at how the plastic melts, he will be able to tell you which plastic compound the part is made of.

  • $\begingroup$ Sometimes dropping it on a table is enough, because the different moduli of elasticity of different plastics will result in a clicking sound typical for that plastic. $\endgroup$ – Peter Kämpf Sep 15 '20 at 19:18
  • $\begingroup$ These must be blue Cherry switches...oh wait ;) $\endgroup$ – Peter Sep 15 '20 at 19:36

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