Printer for printing a 3D object based on a computer model

A method of controlling a 3D printer for printing a 3D object based on a computer model, the printer comprising:

• a stage defining a slicing plane,
• an extruder configured to extrude material, and
• a controller configured to define the 3D object with an outer contour determined by the computer model and with an inner structure defined by a pattern of lines extending in a lengthwise direction and having a cross-sectional shape and a line width,
• wherein the controller is configured to control relative movement between the extruder and the stage in an X-direction, a Y-direction, and a Z-direction, the Z-direction being perpendicular to the slicing plane, the Y-direction being perpendicular to the Z-direction, and the X-direction being perpendicular to the Z-direction and being perpendicular to the Y-direction, and
• wherein the line width is a dimension perpendicular to the lengthwise direction of the line and perpendicular to the Z-direction,
• the method comprising:
• defining at least two portions of the 3D object and a desired engineering property of each of the at least two portions,
• define an inner structure with lines extending continuously in a transition from one of the at least two portions into another of the at least two portions, and
• defining a change of the cross-section in the transition, wherein the change of cross-section is defined in accordance with the desired engineering properties, and
• controlling the printer to extrude material and control relative movement between the extruder and the stage to form the 3D object, wherein the controller is configured to change a path of the line in the transition, and
• changing the path from a single pass section in which the line contains only one segment extending in one direction, to a multiple-pass section in which the line contains multiple segments extending back and forth in opposite directions, and
• wherein the controller is configured to create the multiple-pass section with an uneven number of segments, and to create the multiple-pass section between two single-pass sections to create a continued line structure, or
• wherein the controller is configured to create the multiple-pass section with an even number of segments, and where the multiple-pass section terminates the line structure.