Hi Mik, I'm not entirely sure if I understand properly but I think you can get what you want by using Edit > Separate on the pieces that you want to flex, and only using Flow on those pieces and then for the straight ones only use regular rotation to move those around...

There are various CAD programs that have detailed special mechanisms for doing sheet metal design, you will probably be better off using one of those programs for this type of thing rather than Moi which is not designed specifically for sheet metal models.

You might take a look at Geomagic Design (formerly called Alibre Design), I think it has some special sheet metal tools in it, check out the "Sheet Metal Design" section of this page: http://support1.geomagic.com/link/portal/5605/5668/Article/2285/Geomagic-Design-Support-Video-Learning-Center

- Michael