I have printed a few retainers. We use a Nextdent 5100 (we're an in-house lab) and OrthoFlex resin, which can go as thin as 1.5 mm. We haven't tried going any thinner. It is possible to design retainers in 3Shape; you just have to trick the software a little. In the step where you add the bar, just pick the bar option with the smallest dimensions (I can't remember which one it is off the top of my head) and make a short spline. I usually put mine on the incisal edge of the anteriors. Make the bar as narrow as possible and "bury" it in the shell. It should be small enough that you won't see it on either the interior or exterior of the shell. Then proceed through the rest of the steps, but don't adapt the retainer to the opposing teeth. I don't know if this method will work for you, but the dentists and the patients seem to be happy with the ones produced this way.
