What is a normal?

In 3D geometry a surface normal, often simplified as 'normal', is a vector that is perpendicular (right angled) to the tangent plane of a surface in a certain point. A vector always has a direction and a magnitude. 

Here you can see a curved surface (red), the tangent plane (blue) of a point of the curved surface (black dot) and the normal of the tangent plane (green). 

What is an inverted normal?

In 3D data sets the direction of a normal defines whether a surface is an inside or outside face (closed set of edges). An 'inverted normal' is a normal that is pointing in the wrong direction, telling the computer that an outside face is actually an inside face when it isn’t. 

An inverted normal is a data error which needs to be fixed before the 3D data can be used for printing. This example shows a full part (left) and a hollow part (right) with correct normals:

What can I do to fix the data error?

The first option to fix the error is to recheck the original data for topological issues. Once these issues are addressed, then reupload the fixed data. The second option is to reupload the data in a different data format. If the issues remain, you can contact our Customer Service Team for advice.