Model Viewer Preferences
You can modify your system resource allocation to increase the capacity available for loading models by using any of the System resources settings shown in the Model Viewer Preferences dialog. You open this from the Preferences icon above the viewing area.
Showing back faces
By default the model viewer does not show the back-faces of surfaces as this allows you to navigate more smoothly around a model by not drawing surfaces that are not normally visible.
As some exporters are not consistent with how their front and back faces are shown, some parts of a model may be incomplete or missing. If this is happens, you can select Show back faces so that front and back faces are always shown. You select this in the Model Viewer Preferences dialog, see above.
Enabling Show back faces may reduce how quickly you can navigate around a model in the viewer.
This image shows a model in the viewer with and without Show back faces selected.