You can add multiple part files to an existing build. Select Home > Import. In the Open dialog, hold down the Ctrl key and click on multiple files to select them. When finished, click Open. 3DPrint positions each subsequent part in the Build Bed for the fastest print times.
If a Part file opened in 3DPrint is rendered dark and the slices are not visible in the cross-sectional (2D) view, it often means the back facets of the file are showing. If this is the case, the part will not print properly. To correct the file in 3DPrint, select the part (see next section) and then select Settings > Normals > Invert All Normals.
Normals Inverted Normals Corrected