If I understand correctly, you are using Aspire to make an extrusion of a 2D drawing and export it as a stl to print it in 3D. Right?
Follow the previous advices and set a high resolution when you export the model.
But Aspire may not be the best tool for that, with its pixel based principle. I'd use a more conventional CAD system to extrude a 2D curve then export to STL.
I think there are small apps that can extrude a 2D for 3D printing, but I don't remember their names.
W7 - Aspire 8.517