how to prevent IR signal to go through 3d printed plastic? I'm working on two arduino (one a dock, the other one on a robot) communicating through a IR to let the latter know about its location.

I 3D printed a piece which goal was to channel the 3 IR led lights, to prevent them from overlapsing (exactly in the same manner as the Kobuki.

For a reason I don't know, the IR signals seem to go through  the 3D printed piece which doesn't seem to let visible light through.

I known that there is no 100% absorbant materials, and I know "black for visible lights" doesn't necessarily mean "black for IR lights".

I think my problem is more related to my lack of experience in the field. My question is : why do I get signals from a IR led channeled toward a location I am not in ? 

Patently, it means my light channels are not channeling light.

Therefore, better : How can I make a better (and cheap) channel for IR light ?

The piece I 3D printed is grey (silvery like I guess)

edit : to prevent incomprehension, i would like to clarify "channel". What I mean is to block all light "not going" in the right direction, whatever happens to that light. I apologize if it is not the word I should have used.
You can not use common modulation remote control receivers as they contain an ACG feature which will likely "turn up the volume" until it detects even a weak reflected signal.  Instead consider if this reflective sensor, which does not contain an AGC feature, is appropriate for your design.

Alternatively you might explore the response time of the AGC feature to see if you can defeat its intended purpose.  That is to say, rapidly switch between IR transmitters in order to keep the AGC level at low gain.  Essentially blinding it from weak reflected signals.

You might also transmit continuously on all IR sources and determine if the AGC will put the weak signals below the noise floor.