Layered Manufacturing of Thin-Walled Parts

1
Layered Manufacturing of Thin-Walled Parts

• Sara McMains, Jordan Smith,
• Jianlin Wang, Carlo Séquin
• UC Berkeley

2
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
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)

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QuickSlice Fast Build
FDM
QuickSlice
3D B-Rep
SML
STL
SSL
Roads
Support
Slicer
Fast

• Builds a semi-hollow version of the solid
• n solid offset rings
• Center filled with a loose crosshatch pattern

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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

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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

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Idea 1 3D Offset Pipeline
FDM
3D B-Rep
Polyhedron Offset
Quick Slice
STL
SML
Assume we have true 3D offset surface at the
desired distance inward

• Solid-fill the volume between the input and the
  offset surfaces
• Crosshatch-fill the volume within the offset
  surface

Unfortunately, the 3D offset is

• Difficult to implement robustly
• Too aggressive slicing can produce gaps near
  gradually sloping walls

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Idea 2 Approximate 3D Offset
FDM
QuickSlice
3D B-Rep
Slices
Slicer
SSL
SML
Roads
Support
Slicer

• 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

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2D Contour Offset
FDM
3D B-Rep
QuickSlice
Slices
Offsets
Slicer
Contour Offset
SML
R
S
S
SSL

• 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

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2½D Polyhedron Offset
FDM
3D B-Rep
QuickSlice
Slices
Offsets
2½D CSG
Slicer
Contour Offset
SML
R
S
S
SSL

• 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

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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 ?

B
A ? B
A
A ? B
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1-Layer Thick 2½D Offset
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1-Layer Thick 2½D Offset
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1-Layer Thick 2½D Offset
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n-Layer Thick 2½D Offset
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n-Layer Thick 2½D Offset
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n-Layer Thick 2½D Offset
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Results the Bolt Part

• QuickSlice Fast Build
• Time 504 min (824)
• Filament used 22.1 m

• 2½D Offset Method
• Time 232 min (352)
• Filament used 7.6 m

QuickSlice took 2.71 times as long and used 2.9
times as much filament
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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.

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Thanks to our Sponsors

• NSF
• CyberCut
• CADRE
• MOSIS A Distributed Manufacturing Resource
  (EIA-9905140)
• Ford Motor Co.

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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

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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

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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

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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
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Dragon Curve Example
Input
Voronoi Diagram
Offset