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Additive Manufacturing Techniques

J.Ramkumar

Dept of Mechanical Engineering

IIT Kanpur

[email protected]

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?

http://en.wikipedia.org/wiki/File:Rapid_prototyping_slicing.jpg

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.



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


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)

11

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.

http://www.custompartnet.com/wu/images/rapid-prototyping/fdm.png


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


- 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


14


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


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


16

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.

18


- Z Corporation

- Exone

- Voxeljet

Zcorp Z510

Olaf Diegel Atom 3D printed guitar

3D Printing (3DP)

19


• 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


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


• 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


- 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


22


• 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


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


24


- Arcam AB (Sweden)

Arcam A2 machine

EBM process developed by

Arcam AB (Sweden) in 1997


25


• 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.


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


- 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


• 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


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


- 3D Systems

- Sony

3D Systems iPro 9000 XL


33


• 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


• 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


• 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


• 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


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


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


Non-manifold point and non-

manifold face


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



• 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


• 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

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