[MITaylor]

In my engineering class, we used sodium hydroxide (brand WaterWorks Soluble Concentrate P400SC) to melt some plastic on a clarinet reed container I printed from the school's 3D printer.  Before putting any reeds in the container, I'd like to make absolutely sure there isn't sodium hydroxide left on the printed container's surface (I've heard water and NaOH are a bad mix).
Thanks!

[Mitch]

Mixing water and sodium hydroxide is not bad, but it can generate a lot of heat. So it is advisable to always add base to water, and not water to base.

Your question regarding if all the sodium hydroxide is gone makes no sense. What do you mean?

[MITaylor]

Here, I'll do some quick editing...

[Borek]

Flushing with excess water should work. You may test pH of the water to see if it goes up after contact with the container - as long as it does, there was still NaOH on the surface. Being easily soluble it should disappear quite fast.