Closed Polysurface become open mesh

I am preparing a 3d print file, so I tried to create mesh from my 3d object. It was a closed polysurface but when I ‘mesh’, the result became an open mesh. Why would this happen?

Thanks.

Could be a number of reasons, but it generally indicates one or more hidden problems with your closed polysurface that caused the Rhino mesher to create an open object. Often it is due to edges that are actually out of tolerance that have been “forced” to join, or some other geometric anomaly in the surfaces themselves.

If you can post the object here, someone can look. Otherwise, using ShowEdges>Open Edges will show you where the openings are. You can try using the MeshRepair function in Rhino to repair and close the mesh itself, otherwise you will need to investigate the original polysurface to find out why the open mesh is getting created in the first place.