The six sides of the Poser box are not joined together.  This is why it can render the box with very sharp edges. I'm not familiar with your modeling program, but in mine, if I want to separate a single object into two or more objects, I select all the polygons of the part I wish to separate and select extract.   The object is now separated but no parts were moved.  I can now shift-select each object and export them out as one object.  When imported into Poser and rendered, the edges along the separate parts should now render sharp.  They should also render clean, without any dark smudges on the surface areas.  I believe this is caused by Poser's built in smoothing control which distorts or warps what should be flat facets.  Another way to lessen this rounding effect of edges is to split the polygons nearer the edges of a single object.  It all depends on the model and how sharp you want your edges to be.