In the 3D modeling world when two surfaces lie directly on top of each other it’s what is known as planar surfaces. It is easy to tell that a model has planar surfaces when you rotate the model around you will see faces that flicker and distort.
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When you bring a model into Z print to 3D print you will notice the faces flickering inside the Z print software. When you go to look at the 2D view of each layer that is printed you will notice that some layers will be distorted and have jagged lines running through the print. When the printer goes to print this layer only a part or none of that layer will be printed causing problems for your model. To correct this problem, delete the unnecessary plains or faces that are causing planar surfaces or extrude the faces so that they do not rest on top of each other.