Non-manifold body with Offset surface


I am designing a heat exchanger for 3D printing. For that i made a heat exchanger core containing out of multiple elliptic pipes.
I now have encountered a problem with converting the surfaces to solids. I managed to add thickness to the pipes and the outer body by using the “offset surface”. However the fluid collector is not manifold/watertight with that method. I somehoe get cracks into the collector pipes (picture).
Can anyone help me with this problem? Many thanks.

Best regards
Benjamin (90.3 KB)

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