Three-dimensional printing with multi-material support

1. A method for fabricating an external structure for a multi-material object, the method comprising:

receiving a digital model of an object, the digital model specifying two or more different build materials;

prioritizing the two or more different build materials with a ranking;

identifying exclusive locations in a plurality of layers where a surface layer of the object has one of the two or more different build materials;

generating an external structure for the object according to one or more design rules;

matching a first build material of the external structure to a second build material of the object at each of the exclusive locations, the matching based on at least one shared property of the first build material and the second build material;

generating tool instructions for fabricating the object and the external structure using at least the first build material and the second build material; and

fabricating the object using at least the second build material and the external structure using at least the first build material according to at least the matching and prioritizing of the first build material and the second build material.

2. The method of claim 1 wherein the external structure includes a raft.

3. The method of claim 1 wherein the external structure includes a support structure.

4. The method of claim 1 further comprising projecting each exclusive location outward in a number of projections from a corresponding surface location into the external structure.

5. The method of claim 4 further comprising selecting a build material for the external structure along the number of projections according to the ranking of build material at a number of corresponding locations on the surface layer of the object.

6. The method of claim 4 wherein the number of projections includes a projection vertically downward from each exclusive location.

7. The method of claim 4 wherein the number of projections includes a projection horizontally outward from each exclusive location.

8. A method for fabricating a support structure for a multi-material object, the method comprising:

receiving a digital model of an object, the digital model specifying two or more different build materials;

prioritizing the two or more different build materials with two or more priorities having a ranking;

generating a support structure for the object according to one or more design rules;

identifying a number of locations on a surface of the object contacting the support structure, and identifying one of the two or more different build materials of the object at each of the number of locations;

matching a build material of the surface to a build material of the support structure at any immediately adjacent locations, the matching based on at least one shared property of the build material of the surface and the build material of the support structure;

projecting each of the number of locations on the surface into the support structure in a number of projections;

selecting a build material for the support structure at a point where two projections from two locations on the surface intersect based upon the ranking of the build material at each of the two locations on the surface;

generating tool instructions for fabrication of the object and the support structure using a three-dimensional printer; and

fabricating the object and the support structure according to at least the matching and prioritizing of the two or more different build materials.

9. The method of claim 8 wherein the two or more build materials include three or more build materials each having a different priority.

10. The method of claim 8 wherein the number of projections includes a projection vertically downward from each exclusive location.

11. The method of claim 8 wherein the number of projections includes a projection horizontally outward from each exclusive location.

12. The method of claim 11 wherein the projection horizontally outward is tangential to a surface layer at each exclusive location.

13. The method of claim 8 wherein generating the support structure includes applying a sweep line algorithm to identify regions of support.

14. The method of claim 8 wherein the two or more different build materials include two or more build materials having different colors.

15. The method of claim 8 wherein slicing the digital model includes generating independent paths in each of a plurality of layers for each of the two or more different build materials.

16. The method of claim 15 further comprising creating a bridge for at least one portion of the object before slicing the digital model into independent paths for each of the two or more different build materials.

17. The method of claim 8 further comprising creating a raft for the object and projecting each exclusive location on a bottom surface of the object downward into the raft.

18. The method of claim 8 wherein the at least one shared property includes one or more of a color, a material type, a structural property, a mechanical property, a polymer structure, and a functional property.

19. The method of claim 1 wherein the at least one shared property includes one or more of a color, a material type, a structural property, a mechanical property, a polymer structure, and a functional property.