Which type of filament material(s) is safe to use as an in-wall box for regular, 120v wiring? For instance, an electrical outlet box.
In case it matters, location is the state of Washington, USA.
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Something might be perfectly safe, but it doesn't make it legal to do or allowed to use. Parking your car over double-yellow lines is one example that is perfectly safe but violates the traffic codes.
Any 3D printed box would violate for example WAC 296-46B-300, as it isn't in compliance with NEC Class 3 Standard.
(1) Cables and raceways for power limited, NEC Class 2 and Class 3 conductors must be installed in compliance with Chapter 3 NEC unless other methods are specifically required elsewhere in the NEC, chapter 19.28 RCW, or this chapter.
The NEC is also known as NFPA 70, and availeable at the Website of the National Fire Protection Agency. You will need to look in Article 725.3 for the exact, current specifications that a cable box would need to follow.
725.3 Other Articles. Circuits and equipment shall comply with the articles or sections listed in 725.3(A) through (N). Only those sections of Article 300 referenced in this article shall apply to Class 1, Class 2, and Class 3 circuits.
Also note, that mains wire work is usually regulated and you might not even be allowed to do it yourself!
I haven't tested the commercial "blue boxes" used to hold 120/240 V electrical outlets, switches, and splices to see how they behave when heated. As such, this argument is based on intuition, which is intrinsically flawed as a logic device. Never-the-less, I think the no extruded molten plastic (FFF) 3D printing filament will work.
The purpose of the junction box is to contain an overheating connection or switch and prevent it from causing a fire in the wall. Any FFF filament will have a melting point below the ignition point of wood, and would therefore flow away from the overheating point. It seems that any thermoplastic with a "normal" melting point would have this problem.
You might look at UV polymerized printing resins, such as are used in the Stratasys Objet, Form Labs, and Prusa SL1. These printing processes aren't constrained to use plastics that can be melted or heat softened. Because the polymerization can involve more aggressive crosslinking (polymerization) that FFF materials, they have the potential to be good for a higher temperature.
As an example of a high-temperature, non-melting plastic which could perhaps have an analogue in SLS resin, polyester "casting compound" is cross linked by a methyl-ethyl-ketone-peroxide catalyst to form clear solid. 24 hours after the polymerization starts, the solid does not melt under the influence of a hot air gun. I tried to melt it and it would not melt. It slightly softened, but the plastic cup I had cast it in was dripping away -- but the polyester was not melting.
I looked through the Stratasys materials and Form Labs materials and did not see a much higher temperature material.