Plastic, Delrin, Brass, and Stainless din plugs have been around for a long time. They are made from/by a variety of both small and large companies. They have thier place however they do need to be used carefully, if at all.
That said. What is most critical of any DIN valve is that it meet CGA (Compressed Gas Association) specifications. (This is voluntary not law). The spec is designed to prevent the accidental pressurization of the valve body should a regulator, or DIN plug not be in place completely. This simple feature will let the valve bleed off and not let the full force of the gas be placed against the unseated device. Simply it is a small hole about a the size of a BIC pen tip that is machined into the side of the valve just below the female threads. Deeper than the hole is a small (1/16 of an inch) space between the final thread and the "seat" of the DIN connection.
In the event of pressurization gas flows out of the hole preventing the projectile. Those who have spec valves and have had a regulator lose an oring before use have seen how this is helpful. It's also helpful for stainless din caps as well.
This link shows the Thermo brand valve -- notice the little hole on the left of the valve
http://69.72.128.66/~scuba/tdl/store/images/tdl/Val-PN-3000.jpg
As for plastic caps. (I include injection mold plastic and delrin in the term plastic) The reason for use is to keep dirt and moisture out and to protect the threads. Make sure they have a decent size hole drilled right down the center of them so they can't "seal." But keep in mind that the threads on those non-metal objects are not designed to take the force of 3000 psi or more against them without the threads shearing off and turning that plug into a riot gun pellet.
If you use Stainless Plugs you had best be sure you have the CGA hole in those valves. Too many times I have seen a diver put a wrench on those plugs because they were "too tight" not realizing that the "tight" was pressure. If the valve has the saftey hole, then the moment the oring seal on the plug is broken there is enough room for the gas to get out the hole and make it "safe" again.
Check your DIN valves. If they have the hole you are a step safer. If you don't have the hole you had best be REALLY careful to make sure your regulators are seated completely before pressurizing.
Regards,
