Method and apparatus for transposing extruded materials to fabricate an object surface

In an additive manufacturing system for fabricating an object according to a design description of the object describing which portions of a build space are to be occupied by the object once it has been completely fabricated, and including a machine having a first extrusion nozzle for depositing a first extruded trace having a first nominal trace width, at least one second extrusion nozzle for depositing a second extruded trace having a second nominal trace width and a motion controller for controlling motors of the machine that cause relative motion between the nozzles and a build plate, a method for controlling the motion of said extrusion nozzles to form a surface of the object, comprising:

• upon at least a portion of a surface of the object, determining a multiple extrusion surface region that is to be substantially formed by using both the first extruded trace and the second extruded trace in alternating intervals;
• along a plane that intersects the object according to the design description, determining at least one designed surface contour of the object that lies within the plane and is included within the multiple extrusion surface region;
• defining a first candidate tool path in the plane, paralleling the contour, that will cause either the first or second nozzle to deposit an extruded trace substantially conforming to the designed surface contour;
• defining a second candidate tool path in the plane, paralleling the contour and displaced from the first candidate tool path by a distance greater than or equal to the lesser of the first or second nominal trace widths;
• at a first interval along the designed surface contour, controlling the motion of the first extrusion nozzle to follow the first candidate tool path while depositing the first extruded trace and controlling the motion of the second extrusion nozzle to follow the second candidate path while depositing the second extruded trace, wherein the first extruded trace is interposed between the designed surface contour and the second extruded trace over the first interval;
• at a second interval along the designed surface contour, adjacent to the first interval, controlling the motion of the first extrusion nozzle to follow the second candidate tool path while depositing the first extruded trace and controlling the motion of the second extrusion nozzle to follow the first candidate tool path while depositing the second extruded trace, wherein the second extruded trace is interposed between the designed surface contour and the first extruded trace over the second interval;
• and
• at a third interval along the designed surface contour, adjacent to the second interval, controlling the motion of the first extrusion nozzle to follow the first candidate tool path while depositing the first extruded trace and controlling the motion of the second extrusion nozzle to follow the second candidate path while depositing the second extruded trace.