3D Printing is a bastardization of the process called stereolithography which includes a variety of processes, including Selective Laser Sintering (SLS) which can fuse metal powder to create objects much more durable than plastics. There’s also Selective Laser Melting and E-Beam Melting. The whole topic of 3D Printing is mangled by non-technical writers who don’t take the time to learn what they’re talking about, but the idea of creating metal parts onboard a spacecraft is entirely feasible, requiring only a stock of raw material rather than hundreds of already-manufactured parts. That’s what the OP referred to asteroids as a source of raw material.
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