Reversed Normals

In 3D modeling a reversed normal is the direction that a face is pointed at. If a normal is pointed toward the outside of a model there is no problem for 3D printing. But if a normal is pointed toward the inside of the model it will make the geometry appear to be blacked out and when 3D printing nothing will be printed as a result of a reversed normal.

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To correct these problems simply select the object in your CAD software and reverse the normals.