Join / boolean union

I have some closed solids but when I want to join it by using a boolean union, it failes although the solids overlap. I want to make a mesh out of it in a later stadium for a 3d printer. Does someone know how to fix this? Thanks in regards!

Your file is a bit of a mess to be honest. Many of your closed polysurfaces are extremely thin. Thickness of one of I checked is just 0.000554 centimeters. You’ll have to clean everything before you can join anything reliably.