3D Printing Chapter 3

STL Format

• consists of an unordered list of triangular facets representing the outside skin of an object

2 STL Formats

1. binary format

2. ASCII format

• larger

• human readable

X, Y, Z coordinates for the 3 vertices

unit normal vector for whats inside and outside the object

Tessellation

• process of approximating original surfaces you want with tesselate (create a surface to become triangles or quads, so that there are no overlaps)

• surfaces can be smooth or precise, faceted or crude

  • (one can never print an object smoother than the STL model)

Chord height

maximum distance between the original surface and the resulting polygonal mesh (triangular mesh)

• (the smaller the chord height, the smoother the approximated surface)

Chord length

distance between two polygon vertices (intersection points with a surface)

Chord ratio height

1 - chord height/chord length

(the closer to 1 the chord height ratio, the smoother the surface)

STL File/Tessellation advantages

• simple method

• used by most CAD + AM systems

• small and accurate files

STL File/Tessellation disadvantages

• repeats information

• geometry flaws

  • requires a “repair software”

• slicing of large STL can take hours

STL File problems

• STL files only sees the triangles, not how they connect (no topological data)

• unreliable

• errors can occur

  • converting complex curved surfaced into triangles is difficult

• fixing broken stl models is difficult

Common STL errors

goalies dont own no man

• gaps in STL file

• degenerative facet

  • (when a triangle is collapsed)

  • colinear

• overlapping facets

  • due to rounding errors

• non-manifold conditions

  • where 4 facets share a common edge

  • one edge shares 2 facets only

Non-Manifold Edges

When more then two facets are connected to the same edge

Valid Tessellated Model

• valid when there are no errors or abnormalities

• sliced into 2D layers and converted into 1D scan lines

• the scan lines act as on/off points for the laser beam controller

Invalid Tessellated Model

• gap error?

  • system has no way of knowing its boundaries on a particular slide

• must repair first before building

  • done by generating ‘suitable’ triangular surfaces that fill the gap

STL format then vs. now

them

• single material

• homogenous shape

now

• multi-material and color

• lattice structure, random shape

• rough and textured surface

• microstructures

• better resolution

Why did we need to change STL file format?

• a growing need to support these new features

  • multi-material and color

  • lattice structure, random shape

  • rough and textured surface

  • microstructures

  • better resolution

AMF (additive manufacturing file) format

Official ISO/ASTM is…

• XML-based format

  • allows any CAD software to describe the shape and composition of any 3D object to be fabricated on any printer

What causes missing facets or gaps to occur?

• tessellation of surfaces with large curvatures