Layered Manufacturing of Thin-Walled Parts

1. Layered Manufacturing of Thin-Walled Parts Sara McMains, Jordan Smith, Jianlin Wang, Carlo Séquin UC Berkeley

2. 3.5”, 20hrs 2.5”, 15hrs 3.0”, 25 hrs Is Layered Manufacturing really Rapid Prototyping? • How can we speed up these manufacturing technologies?

3. Raster Scan Technologies • Example: 3D Printing • Speed of roller limits the process • Build time =z-height • Speed up: pack build volume in xy with many parts

4. Vector Scan Technologies • Example: FDM (Fused Deposition Modeling) • Build time = volume scanned (material used) • Our Goal: create a sturdy part that is visually equivalent but uses less material, so that it builds faster

5. QuickSlice FDM 3D B-Rep STL SSL SML Slicer Support Roads Building Solid Parts with QuickSlice • Software interface to Stratasys 1650 FDM Machine • Input: STL boundary representation • Slices model into z-layer contours (SSL) • Builds support structure • Builds roads (nozzle fill path) (SML)

6. QuickSlice Fast Build QuickSlice FDM 3D B-Rep SML STL SSL Slicer Support Fast Roads • Builds a semi-hollow version of the solid • n solid offset rings • Center filled with a loose crosshatch pattern

7. z Fast Build Limitations • Structurally conservative • Only applied to slice layers whose center area is completely covered by slices above and below it • Gradually sloping surfaces prevent its application • Worst case example

8. Can Approach Be More Aggressive? FDM 3D B-Rep Automated Process? SML • Our Goal: • Create an automated process • Input: the boundary representation of a desired solid geometry • Output: a sturdy, physical part that is visually equivalent while using less material • Benefits: faster build times and material conservation • Our Assets: • QuickSlice software as a black box • Specifically the loose fill crosshatched roads option

9. z Idea #1: 3D Offset Pipeline FDM 3D B-Rep Polyhedron Offset STL Quick Slice SML • Solid-fill the volume between the input and the offset surfaces • Crosshatch-fill the volume within the offset surface Assume we have true 3D offset surface at the desired distance inward Unfortunately, the 3D offset is • Difficult to implement robustly • Too aggressive: slicing can produce gaps near gradually sloping walls

10. Idea #2: Approximate 3D Offset QuickSlice FDM 3D B-Rep Slices Slicer SSL SML Slicer Support Roads • Key ideas: • Offsetting is much simpler in 2D than in 3D • The manufacturing process eventually represents the part as a stack in z of layers of 2D contours • Start: slice polyhedron into desired set of 2D contours • End: input SSL to QuickSlice to build support and roads

11. 2D Contour Offset FDM 3D B-Rep QuickSlice Slices Offsets Slicer Contour Offset SML S S R • Data: layers of 2D contours • Offset the 2D contours inward by a specified distance = n layer thicknesses • Near vertical walls, this is the correct 3D offset • Approximation degrades as the walls approach horizontal SSL

12. 2½D Polyhedron Offset FDM 3D B-Rep QuickSlice Slices Offsets Slicer Contour Offset 2½D CSG SML S S R • Data: layers of 2D contours and offsets • Adjust the loose fill areas in regions where the vertical coverage above or below is less than n layers thick • Perform 2D boolean (CSG) combinations of the contours and offsets of the ith layer with the n layers above and below it • We use OpenGL for the 2D booleans SSL

13. Regularized Boolean Operations • Unregularized:op  { , , - } • Regularized:op*  { *, *, -* } • A op* B = Closure( Interior( A op B ) ) • If A & B are 2D areas and C = A op* B then C is a non-degenerate 2D area or  A B A  B A * B

14. z 1-Layer Thick 2½D Offset

15. z 1-Layer Thick 2½D Offset

16. z 1-Layer Thick 2½D Offset

17. z n-Layer Thick 2½D Offset

18. z n-Layer Thick 2½D Offset

19. z n-Layer Thick 2½D Offset

20. QuickSlice Fast Build Time: 504 min (8:24) Filament used: 22.1 m 2½D Offset Method Time: 232 min (3:52) Filament used: 7.6 m Results: the Bolt Part QuickSlice took 2.71 times as long and used 2.9 times as much filament

21. Conclusion • We have implemented a robust 2D contour offsetting program. • We have conservatively approximated the 3D polyhedron offset using 2D contour slices, 2D offsets, and 2½D boolean operations. • We have demonstrated a novel approach to speeding up FDM manufacturing. • Our approach decomposes the desired geometry into a thin sturdy outer shell with a loosely filled center volume. • Our approach saves time and material as compared to the built-in QuickSlice solution.

22. Thanks to our Sponsors • NSF • CyberCut • CADRE: • MOSIS++: A Distributed Manufacturing Resource (EIA-9905140) • Ford Motor Co.

23. 2D Contour Offset Implementation Input Offset 0.1 Offset 0.2 • Difficulties arise from global interactions • Robust approach based on Voronoi diagram • Generalization of the approach described by M. Held 1991

24. Voronoi Diagram of a Contour • Input sites are both Vertices and directed Edge Segments • VD divides the plane into zones s.t. every point in a zone is closest to the corresponding input site than to any other site • Vertices of VD have an associated signed distance • VD is a signed distance function

25. z Voronoi Mountain z = 0 • Create a height field by raising the vertices of VD in z by their signed distance • Offsetting by n is the same as slicing the mountain with the plane z = n

26. Offset Slicing z-monotone parabolic VD edges for each unvisited VD edge if VD edge  z = n Crawl VD CCW around peak CW around each VD face

27. Dragon Curve Example Input Voronoi Diagram Offset