Additive Manufacturing Techniques
J.Ramkumar
Dept of Mechanical Engineering
IIT Kanpur
Advanced Manufacturing Choices
Table of Contents
2
1. Introduction: What is Additive Manufacturing
2. Historical development
3. From Rapid Prototyping to Additive Manufacturing (AM) – Where are we today?
4. Overview of current AM technologies
1. Laminated Object Manufacturing (LOM)
2. Fused Deposition Modeling (FDM)
3. 3D Printing (3DP)
4. Selected Laser Sintering (SLS)
5. Electron Beam Melting (EBM)
6. Multijet Modeling (MJM)
7. Stereolithography (SLA)
5. Modeling challenges in AM
6. Additive manufacturing of architected materials
7. Conclusions
From Rapid Prototyping to Additive
Manufacturing
3
What is Rapid Prototyping
- From 3D model to physical object, with a “click”
- The part is produced by “printing” multiple slices (cross
sections) of the object and fusing them together in situ
- A variety of technologies exists, employing different
physical principles and working on different materials
- The object is manufactured in its final shape, with no
need for subtractive processing
How is Rapid Prototyping different from Additive Manufacturing?
The difference is in the use and scalability, not in the technology itself:
Rapid Prototyping: used to generate non-structural and non-functional demo pieces or
batch-of-one components for proof of concept.
Additive Manufacturing: used as a real, scalable manufacturing process, to generate fully
functional final components in high-tech materials for low-batch, high-value manufacturing.
Why is Additive Manufacturing the Next
Frontier?
4
EBF3 = Electron Beam Freeform Fabrication (Developed by NASA LaRC)
Rapid Prototyping vs Additive
Manufacturing today
5
AM breakdown by industry today
Wohlers Report 2011 ~ ISBN 0-9754429-6-1
From Rapid Prototyping to Additive
Manufacturing
Rapid Prototyping in a nutshell1. 3D CAD model of the desired object is generated
2. The CAD file is typically translated into STL* format
3. The object described by the STL file is sliced along
one direction (the ‘z’ or ‘printing’ direction)
4. Each slice is manufactured and layers are fused
together (a variety of techniques exist). The
material can be deposited by dots (0D), lines (1D)
or sheets (2D)
6
A voxel (volumetric pixel or, more
correctly, Volumetric Picture
Element) is a volume element,
representing a value on a regular
grid in three dimensional space.
This is analogous to a pixel,
which represents 2D image data
in a bitmap.
*The STL (stereo lithography) file format is
supported by most CAD packages, and is is
widely used in most rapid prototyping / additive
manufacturing technologies.
STL files describe only the surface geometry of
a three dimensional object without any
representation of color, texture or other common
CAD model attributes. The STL file describes a
discretized triangulated surface by the unit
normal and vertices coordinates for each
triangle (ordered by the right-hand rule).
A limitation or an opportunity?
Compromises in Additive ManufacturingAnother key compromise is among process speed, volume and tolerances.
• Laminated Object Modeling (LOM)
• Fused Deposition Modeling (FDM)
• 3D Printing (3DP)
• Selective Laser Sintering (SLS)
• Electron Beam Melting (EBM)
• Multijet Modeling (MJM)
• Stereolithography (SLA, STL)
• Micro-stereolithography
(serial and projected)
• Two photon lithography
7
Laminated Object Manufacturing (LOM)
8
1. Sheets of material (paper, plastic,
ceramic, or composite) are either
precut or rolled.
2. A new sheet is loaded on the build
platform and glued to the layer
underneath.
3. A laser beam is used to cut the desired
contour on the top layer.
4. The sections to be removed are diced
in cross-hatched squares; the diced
scrap remains in place to support the
build.
5. The platform is lowered and another
sheet is loaded. The process is
repeated.
6. The product comes out as a
rectangular block of laminated material
containing the prototype and the scrap
cubes. The scrap/support material is
separated from the prototype part.
Laminated Object Manufacturing (LOM)
Laminated Object Manufacturing (LOM)
was developed by Helisys of Torrance, CA,
in the 1990s. Helisys went out of business
in 2000 and their LOM equipment is now
serviced by Cubic Technologies.
9
Equipment picture
Current market leaders
- Mcor Technologies (Ireland)
- Solido (Israel)
- Strataconception (France)
- Kira Corporation (Japan)
Mcor Technologies Matrix 300+
(uses A4 paper and water-based adhesive)Courtesy, Cubic Technologies
Laminated Object Manufacturing (LOM)
10
KEY APPLICATION AREAS
Maximum build size 40in x 40in x 20in
Resolution in (x,y) +/- .004 in
Resolution in z Variable
Speed Medium
Cost Low
Available materials Paper, Plastic
Sheet
KEY METRICS ADVANTAGES
DISADVANTAGES
• Relatively high-speed process
• Low cost (readily available materials)
• Large builds possible (no chemical
reactions)
• Parts can be used immediately after the
process (no need for post-curing)
• No additional support structure is
required (the part is self-supported)
• Removal of the scrap material is laborious
• The ‘z’ resolution is not as high as for other
technologies
• Limited material set
• Need for sealing step to keep moisture out
• Pattern Making
• Decorative Objects
Fused Deposition Modeling (FDM)
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1. A spool of themoplastic wire (typically
acrylonitrile butadiene styrene (ABS)) with
a 0.012 in (300 μm) diameter is
continuously supplied to a nozzle
2. The nozzle heats up the wire and extrudes
a hot, viscos strand (like squeezing
toothpaste of of a tube).
3. A computer controls the nozzle movement
along the x- and y-axes, and each cross-
section of the prototype is produced by
melting the plastic wire that solidifies on
cooling.
4. In the newest models, a second nozzle
carries a support wax that can easily be
removed afterward, allowing construction
of more complex parts. The most common
support material is marketed by Stratasys
under the name WaterWorks
5. The sacrificial support material (if available)
is dissolved in a heated sodium hydroxide
(NaOH) solution with the assistance of
ultrasonic agitation.
Fused Deposition Modeling (FDM)
The fused deposition modeling (FDM) technologywas developed by S. Scott Crump in the late 1980sand was commercialized in 1990. The doublematerial approach was developed by Stratasys in1999.
12
Current market leaders
- Stratasys, Inc.
Stratasys Dimension SST 1200
"Ribbon Tetrus" (Carlo Séquin)
Courtesy, Dr. Robin Richards,
University College London, UK
www.nybro.com.au
FDM process parameters
13
Fused Deposition Modeling (FDM)
14
KEY APPLICATION AREAS
Maximum build size 20” x 20” x 20”
Resolution in (x,y) +/- (0.002” - 0.005”)
Resolution in z +/- (0.002” - 0.01”)
Speed Slow
Cost Medium
Available materials Thermoplastics
(ABS, PC,
ULTEM…)
KEY METRICSADVANTAGES
DISADVANTAGES
• Economical (inexpensive materials)
• Enables multiple colors
• Easy to build DIY kits (one of the most
common technologies for home 3D
printing)
• A wide range of materials possible by
loading the polymer
• Materials suite currently limited to
thermoplastics (may be resolved by loading)
• Conceptual Models
• Engineering Models
• Functional Testing Prototypes
www.redeyeondemand.com
Fused Deposition Modeling (FDM)
FAB@Home• First multi-material printer available to the public
• Open-source system
• Project goal: open-source mass-collaboration developing personal fabrication technology aimed at bringing personal fabrication to your home (project started by H. Lipson and E. Malone at Cornell in 2006).
• Popular Mechanics Breakthrough Award 2007
RepRap• Open-source system
• Founded in 2005 by Dr. A. Bowyer at the University of Bath (UK)
• Project goal: Deliver a 3D printer that can print itself!
• 1st machine in 2007 (Darwin)
• Replication achieved in 2008
15
Do it Yourself FDM rapid prototyping systems
Fused Deposition Modeling (FDM)
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Do it Yourself FDM rapid prototyping systems
Cubify Cube• Commercially available fully built for $1,200
• Resolution 0.2mm
• 16 colors
• Prints in ABS and PLA
• Awarded 2012 Popular Mechanics Breakthrough Award
3D Printing (3DP)
1. A layer of powder (plaster,
ceramic) is spread across the
build area
2. Inkjet-like printing of binder over
the top layer densifies and
compacts the powder locally
3. The platform is lowered and the
next layer of dry powder is
spread on top of the previous
layer
4. Upon extraction from the
machine, the dry powder is
brushed off and recycled
17
3D Printing (3DP)
Z Corporation first introduced high-
resolution, 24-color, 3DP (HD3DP™) in
2005 (600 dpi). Z Corp was later bought by
3D Systems.
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Current market leaders
- Z Corporation
- Exone
- Voxeljet
Zcorp Z510
Olaf Diegel Atom 3D printed guitar
3D Printing (3DP)
19
KEY APPLICATION AREAS
• Widely used to print colorful and complex
parts for demonstration purposes
• Molds for sand casting of metals
Maximum build size 14 in x 10 in x 8 in
Resolution in (x,y) 640 dpi
Resolution in z Variable
Speed Fast
Cost Low
Available materials Plaster, sand, oxide
ceramics, sugar
and starch for food
printing
KEY METRICSADVANTAGES
DISADVANTAGES
• Can create extremely
realistic multi-color
parts (24-bit color)
using inkjet technology
• Can generate complex
components with
internal degrees of
freedom
• Economical
• Versatile
• Very limited materials suite
• Low resolution (lowest of all AM technologies)
• Negligible mechanical properties (unusable
for any structural application)
Printed with Z Corp 650
3D Printing (3DP)
Selective Laser Sintering (SLS)
1. A continuous layer of powder is
deposited on the fabrication
platform
2. A focused laser beam is used to
fuse/sinter powder particles in a
small volume within the layer
3. The laser beam is scanned to
define a 2D slice of the object
within the layer
4. The fabrication piston is
lowered, the powder delivery
piston is raised and a new layer
is deposited
5. After removal from the machine,
the unsintered dry powder is
brushed off and recycled
20
Selective Laser Sintering (SLS)
• SLS technology invented at UT Austin in the‘80s by Joe Beaman, Carl Deckard and DaveBourell.
• First successful machine: DTM Sinterstation2000, in late 1990s
• DTM later acquired by 3D Systems
21
Current market leaders
- 3D Systems
3D Systems Sinterstation
Important note:
SLS patent runs out in Feb 2014!
A huge influx of players and
technologies is anticipated. Metal Technology Co.
3D Systems
Bulatov Abstract Creations
Selective Laser Sintering (SLS)
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KEY APPLICATION AREAS
• Structural components
Maximum build size 700 mm x 380 mm x 560
mm
Resolution in (x,y) High (Spot Dependant)
Resolution in z 0.005”
Speed Medium
Cost Medium
Available materials Powdered plastics
(nylon), metals (steel,
titanium, tungsten),
ceramics (silicon
carbide) and fiber-
reinforced PMCs
KEY METRICSADVANTAGES
DISADVANTAGES
• Wide array of structural materials beyond
polymers
• No need for support materials
• Cheaper than EBM
• One of two technologies that allow
complex parts in metals
• Expensive relative to FDM, 3DP
• The quality of metal parts is not as high as
with EBM
Electron Beam Melting (EBM)
1. The fabrication chamber is
maintained at high vacuum and high
temperature
2. A layer of metal powder is deposited
on the fabrication platform
3. A focused electron beam is used to
melt the powder particles in a small
volume within the layer
4. The electron beam is scanned to
define a 2D slice of the object within
the layer
5. The build table is lowered, and a
new layer of dry powder is deposited
on top of the previous layer
6. After removal from the machine, the
unmelted powder is brushed off and
recycled
23
Electron Beam Melting (EBM)
24
Current market leaders
- Arcam AB (Sweden)
Arcam A2 machine
EBM process developed by
Arcam AB (Sweden) in 1997
Electron Beam Melting (EBM)
25
KEY APPLICATION AREAS
• Structural components for aerospace
(Ti6Al4V, gammaTiAl, Ni superalloys)
• Custom-made bio-implants (Ti6Al4V)
Maximum build
size
200mm x 200mm x
350mm
Resolution in (x,y) +/- 0.2mm
Resolution in z 0.002” (0.05 mm)
Speed Medium
Cost High
Available materials Metals: titanium,
tungsten, stainless
steel, cobalt chrome,
Ni-based superalloys.
KEY METRICSADVANTAGES
DISADVANTAGES
• Method of choice for high-quality metal
parts
• Wide range of metals
• Fully dense parts with very homogeneous
microstructures
• Vacuum operation allows building of highly
reactive metals (e.g., Titanium)
• High temperature operation (700-1000C)
results in structures free of internal stresses
• EBM allows even better microstructural
control than many conventional processes.
• Extremely expensive (more than SLS)
• Conventional machining may be required
to finish the goods (rough surface)
• Requires vacuum operation
Multijet Modeling (MJM)1. A piezoelectric print head with
thousands of nozzles is used to jet 16
micron droplets of photopolymer on
the printing structure. An additional set
of nozzles deposits a sacrificial
support material to fill the rest of the
layer.
2. A UV curing lamp is scanned across
the build to immediately cross-link the
photopolymer droplets.
3. The elevator is lowered by one layer
thickness and the process is repeated
layer-by-layer until the model is built.
4. The sacrificial material is removed:▫ The Objet system uses a photopolymer as
support material; the support material is
designed to crosslink less than the model
material and is washed away with pressurized
water.
▫ The 3D Systems InVision uses wax as
support material, which can be melted away.
The method of building each layer is similar to
Inkjet Printing, in that it uses an array of inkjet
print heads to deposit tiny drops of build material
and support material to form each layer of a part.
However, as in Stereolithography (see following
slides), the build material is a liquid acrylate-
based photopolymer that is cured by a UV lamp
after each layer is deposited.
For this reason, Multijet Modeling is sometimes
referred to as Photopolymer Inkjet Printing.
26
Multijet Modeling (MJM)
27
Current market leaders
- Objet
- 3D Systems
Multijet modeling (MJM) was
introduced by 3D Systems in 1996 as
a cheaper alternative to industrial-
grade Stereolithography machines.
Objet Desktop 30 Pro
3D Systems
Thermojet
Multijet Modeling (MJM)
28
KEY APPLICATION AREAS
• Automotive
• Defense
• Aerospace
• Consumer goods
• Household appliances
• Medical applications
Maximum build size 1000mm x 800mm
x 500mm
Resolution in (x,y) 450 dpi
Resolution in z 16 microns
Speed Fast
Cost High
Available materials Acrylate-based
photopolymer
KEY METRICS
ADVANTAGES
DISADVANTAGES
• Fast process
• Complex parts via sacrificial support
materials
• Accuracy is not as good as SLA
Stereolithography (SLA)
1. A structure support base is positioned
on an elevator structure and immersed
in a tank of liquid photosensitive
monomer, with only a thin liquid film
above it
2. A UV laser locally cross-links the
monomer on the thin liquid film above
the structure support base
3. The elevator plate is lowered by a small
prescribed step, exposing a fresh layer
of liquid monomer, and the process is
repeated
4. At the end of the job, the whole part is
cured once more after excess resin and
support structures are removed
29
A suitable photosensitive polymer
must be very transparent to UV light
in uncured liquid form and very
absorbent in cured solid form, to
avoid bleeding solid features into
the layers underneath the current
one being printed.
Stereolithography (SLA)Solidification of the monomer can occur intwo different modalities:
Free surface mode: Solidification occursat the resin/air interface. In this mode, caremust be taken to avoid waves or a slant ofthe liquid surface, which wouldcompromise the final dimensionalresolution. The elevator moves down ateach step (top-down build).
Fixed surface mode: The resin is storedin a container with a transparent windowplate for exposure, and solidification occursat the stable window/resin interface. In thismode, the elevator moves up at each step(bottom-up build).
30
H-W Kang et al 2012 J. Micromech. Microeng. 22 115021
Stereolithography (SLA)
Two fundamental process variations
exist:
▫ Scanning stereolithography. The laser
beam is rastered onto the surface. Parts
are constructed in a point-by-point and line-
by-line fashion, with the sliced shapes
written directly from a computerized design
of the cross-sectional shapes.
▫ Projection stereolithography. A parallel
fabrication process in which all the voxels in
a layer are exposed at the same time; the
topology to be printed on each layer is
defined by 2D shapes (masks). These 2D
shapes are either a set of real photomasks
or digital masks defined on a DLP projector.
31
Stereolithography (SLA)SLA was pioneered by Chuck Hull inthe mid-1980s (see picture below).Hull founded 3D Systems tocommercialize its new manufacturingprocess.
32
Current market leaders
- 3D Systems
- Sony
3D Systems iPro 9000 XL
Stereolithography (SLA)
33
KEY APPLICATION AREAS
• Patterns for metal processing (e.g.,
molding)
• Prototypes for demonstrational purposes
Maximum build size 1500mm x 750mm
x 550mm
Resolution in (x,y) Spot Dependent
Resolution in z 0.004”
Speed Medium
Cost High
Available materials Thermoset
polymers:
photosensitive
resins
KEY METRICS
ADVANTAGES
DISADVANTAGES
• Fast
• Good resolution
• No need for support material
• Photosensitive polymers have acceptable
mechanical properties
• Expensive equipment ($100-$500K)
• Expensive materials (photosensitive resins
are ~$100-200 /kg)
• Material suite limited to resins
Stereolithography (SLA)
• The application of rapid prototyping (RP)
techniques to MEMS and NEMS requires
higher accuracy than what is normally
achievable with commercial RP equipment.
• Laminated object manufacturing (LOM),
fused deposition modeling (FDM), and
selective laser sintering (SLS) all must be
excluded as microfabrication candidates on
that basis.
• Only stereolithography has the potential to
achieve the fabrication tolerances required
to qualify as a MEMS or NEMS tool.
• Latest enhancements that have made it an
attractive option are high-resolution micro-
and nanofabrication methods.
34
APPLICATION TO MEMS AND NEMS
EPFL, Lausanne, Switzerland
Stereolithography (SLA)
• Microstereolithography, derived from conventional
stereolithography, was introduced by Ikuta in 1993.
• Whereas in conventional stereolithography the laser
spot size and layer thickness are both in the 100-μm
range, in microstereolithography a UV laser beam is
focused to a 1–2-μm spot size to solidify material in
a thin layer of 1–10 μm.
• The monomers used in RP and micro-
stereolithography are both UV-curable systems, but
the viscosity in the latter case is much lower (e.g., 6
cPs vs. 2000 cPs), because high surface tension
hinders both efficient crevice filling and flat surface
formation at the microscale.
• In microstereolithography the solidified polymer is
light enough so that it does not require a support as
is required for the heavier pieces made in RP.
35
MICROSTEREOLITHOGRAPHY
www.miicraft.com
Stereolithography (SLA)
• Two-photon lithography provides a furtherenhancement of the SLA resolution.
• Special initiator molecules in the monomer onlystart the polymerization reactions if activated bytwo photons simultaneously. The laser intensityfield can be tuned so that this event only happensin a very small region near the focus. The result isextremely local polymerization, with resolutions inthe tens of nanometers range.
• Two-photon polymerization can occur everywherein the monomer bath, as opposed to only at the toplayer, simplifying the hardware requirementsconsiderably.
36
TWO-PHOTON LITHOGRAPHY
www.laser-zentrum-hannover.de
Current materials in Additive
Manufacturing
Materials in AM today- Thermoplastics (FDM, SLS)- Thermosets (SLA)- Powder based composites (3DP)- Metals (EBM, SLS) - Sealant tapes, paper (LOM)- Starch and sugar (3DP)• Functional/structural parts
▫ FDM (ABS and Nylon)
▫ SLS (thermoplastics, metals)
▫ EBM (high strength alloys, Ti, stainless steel, CoCr)
• Non-functional/structural parts▫ SLA (resins): smoothest surface, good for casting
▫ LOM (paper), 3D Printing (plaster, sand): marketing and concept prototypes, sand casting molds
• As new materials are introduced, more functional components will be manufactured (perhaps 30-40% by 2020).
• Importantly AM is one of the best approaches for complex architected materials.
37
Challenges in AM materials properties
predictions
• Most AM processes introduce anisotropy in mechanical properties (z different from x,y)
• Local differences in laser/EB power (e.g., perimeter vs center) introduce heterogeneity in
mechanical properties
• Laser fluctuations might result in embedded defects that are difficult to identify
• All existing machines are open-loop: temperature sensors have been introduced in some
processes, but the readings are not used to optimize the processing parameters on the fly.
38
Micro-Architected MaterialsOverarching vision
39
How can we fill unclaimed regions?
- Optimal topology
- Optimal geometry
- Base material optimization (nm-features)
- Hierarchical design
What do we need?
- Understand multi-scale mechanical behavior (deformation and failure modes)
- Understand processing -> microstructure -> mechanical properties (including size effects)
- Developing new models for FE analysis and optimal design
IMPROVED STRENGTH
AT THE FILM LEVEL
SIZE EFFECTS
IN PLASTICITY
AND FRACTURE
UNIQUE DEFORMATION
MECHANISMS
IMPROVED STRENGTH
AT THE MACROSCALE
A word of caution
Tech Consultancy Puts 3D Printing at Peak of "Hype Cycle"
40
PARAMETERS INVOLVED
DEFECTSDensity Problem• Scan speed has a significant effect on density .
• At sufficiently low scan speeds, the relative density is almost independent of the layer thickness for the selected range of the layer thickness, and a maximum of 99% relative density is achievable.
• At higher scan speed values, a higher layer thickness results in less density.
Residual Stress• Due to localized heating, complex thermal and phase
transformation stresses are generated during the process.• In addition, frequent thermal expansion and contraction of the
previously solidified layers during the process generates considerable thermal stresses and stress gradients that can exceed the yield strength of the material.
• Residual stresses can lead to part distortion, initiate fracture, and unwanted decrease in strength.
Surface finish
• Parts often require post‐processing operations such as surface machining, polishing and shot peening to attain final part surface finish.
• Surface roughness is heavily dependent on laser processing parameters.
PARAMETERS INVOLVED
LAY PATTERN
• Printing of layers in FDM has different types. Each type is used for different types of loading.
• The angle in which the layers are printed is called raster angle.
• The raster angle has a direct bearing on the resulting structure and plays a significant role in influencing the mechanical characteristics of parts produced.
INFILL PATTERN
• In FDM, the printed part will have a structure inside instead of being a solid. This is called infill pattern.
• This infill pattern provides high strength while reducing the total weight of the part produced. Also it reduces the printing time.
• There are many types of infill. Rectangular, triangular, wiggle and hexagonal or honeycomb are the widely used structures. Each structure offers different properties.
• We can also change the quantity of infill to be filled. 0% infill gives hollow part, and 100% infill gives solid part. Generally, 20-50% of infill is used.
SHELL• The top, the bottom, and the sides of the part are filled with
solid layers. This outside shape is called shell.
• Shells are the outer layers of a print which make the walls of an object, prior to the various infill levels being printed within. The number of shell layers can be varied.
ORIENTATION• Spending time optimizing the 3d model before printing can
greatly improve overall quality and reduce print time. It can be done by orienting the model on the print bed to minimize the amount of support needed.
• When the printer recognizes overhangs or features floating in mid-air, it starts printing supporting material alongside the model so that the printer has something to print on.
• One simple way to avoid support material is to rotate the model so that overhangs become bases.
• Another important aspect to consider when orienting the part is to start with a flat area that can adhere to the platform.
• While printing parts with overhangs, the orientation of the overhangs should be considered. Because, printing the support material increases the overall printing time.
• By choosing the appropriate orientation, the build time for support materials can be reduced.
DEFECTS
• Surface defects like staircase error can come from curve-approximation errors in the originating STL file.
• Internal defects include voids just inside the perimeter (at the contour-raster intersection) as well as within rasters. Voids around the perimeter occur either due to normal raster curvature or are attributable to raster discontinuities.
• Also parts produced using FDM are anisotropic. Their properties depend on the building direction as well as the tool path definition.
DISADVANTAGES
• Small features and thin walls cannot be made accurately.
• Layers are visible and surface finish is not good.
• The process is very slow.
• The built part is weak in build axis direction.
• Support structures are required for some shapes and support structure removal is a difficult process.
Stl format
Additive manufacturing
• Additive manufacturing refers to a process by which digital 3D design is used to build up a component in layers by depositing material
Steps in Generic Am process
Source: Gibson, Rosen, Additive Manufacturing
Stl format
• CAD model prepared in the first step is converted to STL (STerioLithography) format, a common language to almost all additive manufacturing machinary.
• Two types of formats are used for STL file
▫ ASCII format
▫ Binary format
• ASCII STL file is larger than that of binary but is human readable and hence is used widely
Stl format
• The STL format is the tessellated representation of the CAD model in which the CAD surface is approximated to a series of triangular facets.
Source: Gibson, Rosen, Additive Manufacturing
STL file information
• It stores information of the triangular facets that describes the surface to be built
• Each triangle is described as three points with their coordinates and a outward directed normal which is obtained when we move in a counterclockwise direction on the facet loop.
Source: Steriolithography_Materials, Process and Applications
The structure of an ASCII Stl format
Source: Steriolithography_Materials, Process and Applications
STL format rules• The generation of STL file follows two important rules
• Facet Orientation rule: The orientation of the facet involves the definition of the vertices of each triangle in a counterclockwise order.
• Adjacency rule: Each triangular facet must share two vertices with each of its adjacent triangles.
• Mobius rule: Since the vertices are ordered, the direction on one facet’s edge is exactly opposite
to that of another facet sharing the same edge.
Disadvantages of stl format• STL file is many times larger than the original
CAD data file
• STL file carries much redundancy information such as duplicate vertices and edges.
• Commercial tessellation algorithms are not robust and may give rise to errors which need to be repaired before proceeding for further steps
Rapid Prototyping_ Chua Chee Kai, Leong Kah Fai, Lim Chi Sing
Errors in stl format
• Gaps or missing facets
• Degenerate facets
• Overlapping facets
• Non-manifold topology conditions
Missing facets or gaps
• Tessellation of surfaces with large curvature can result in errors at the intersection between such surfaces, leaving gaps or holes along edges of the part model
Rapid Prototyping_ Chua Chee Kai, Leong Kah Fai, Lim Chi Sing
Degenerate facets
• A geometrical degeneracy will occur when all the facets’ edges are collinear even though all its vertices are distinct.
• Degenerate facets are less critical in STL and they seldom cause serious build failures
Overlapping facets
• These are generated due to numerical round-off errors occurred during tessellation
Rapid Prototyping_ Chua Chee Kai, Leong Kah Fai, Lim Chi Sing
Non-manifold errors
• There are three types of non-manifold errors
▫ Non-manifold edge
▫ Non-manifold point
▫ Non-manifold face
• These may be generated because generation of fine features is susceptible to round-off errors.
non-manifold edge
Rapid Prototyping_ Chua Chee Kai, Leong Kah Fai, Lim Chi Sing
Non-manifold point and non-
manifold face
Rapid Prototyping_ Chua Chee Kai, Leong Kah Fai, Lim Chi Sing
Valid and invalid models
• Valid model: A model is said to be valid if it is free of all types of errors.
• Invalid model: A model is said to be invalid if it has atleast one of the above abnormalities
• However if the model is invalid and not corrected and proceeded forward, then error in the geometric model would cause the system to have no predetermined boundary on the particular slice and the building process would continue right to the physical limit of the AM machinery.
• Hence invalid model is to be repaired before proceeding to next step.
Generic stl repair
• The basic approach is to detect and identify the boundaries of all the gaps in the model.
• Once the boundaries of the gap are identified, suitable facets would then be generated to repair these gaps.
• Two conditions are ensured in generating the facets.• First condition: The orientation of the generated facet is correct
and compatible with the rest of the model• Second condition: Any contoured surface of the model would be
followed closely by the generated facets due to the smaller facet generated
Missing facets problem
Rapid Prototyping_ Chua Chee Kai, Leong Kah Fai, Lim Chi Sing
Missing facets problem
• Detection of gap
• Number the vertices of the gap and the vertex of facet sharing an edge with it
• Numbering is done following the face orientation rule
• Representing the edges adjacent to the gap
Missing facets problem
• Sort the erroneous edges into a closed loop
• Representation of gap with all the edges forming a sorted closed loop
Missing facets repair• Generation of facets for the repair of the
gaps