Correct meshing of design and nondesign parts

Hi,

I'm a new user of HyperWorks and have not much experience with the software. I want to optimize the structure of an aluminum cast part. The geometry (see picture) consists of the design part (black) and the nondesign part (white). The white parts are the holes for the screws and die white part in the middle is for bearings. So, all forces and constraints are attached to the nondesign parts.

I've created the geometry for the design and the nondesign part with a CAD program and import both in HyperMesh (as an assembly). By that I have two components in HyperMesh, consisting of 9 solid (1 black and 8 white). Then I meshed the volume with 'Create -> Tetra Mesh ->Volume Tetra' and select all solids. And optically it looks as if the meshes of the two components are connected completely. But when I start OptiStruct I get the following error:

*** ERROR # 312 ***
In static load case 1
the compliance is negative or large 1.452366E+14
Optimization/buckling analysis cannot be performed due to possible rigid body mode

Therefore I think that the meshes of the design part and the nondesign part aren’t correctly connected.

I hope you understand my problem and can tell me how to mesh the volume correctly. Tanks!!