Advanced Manufacturing Choices

Additive Manufacturing Techniques J.Ramkumar Dept of Mechanical Engineering IIT Kanpur [email protected] 2 Table of Contents

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 3 From Rapid Prototyping to Additive Manufacturing

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. 4 Why is Additive Manufacturing the Next Frontier?

EBF3 = Electron Beam Freeform Fabrication (Developed by NASA LaRC) 5

Rapid Prototyping vs Additive Manufacturing today

AM breakdown by industry today

Wohlers Report 2011 ~ ISBN 0-9754429-6-1 6 From Rapid Prototyping to Additive Manufacturing A limitation or an opportunity?

Rapid Prototyping in a nutshell 1. 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) *The STL (stereo lithography) file format is supported by most CAD packages, and is is widely used in most rapid prototyping / additive A voxel (volumetric pixel or, more manufacturing technologies. correctly, Volumetric Picture STL files describe only the surface geometry of Element) is a volume element, a three dimensional object without any representing a value on a regular representation of color, texture or other common grid in three dimensional space. CAD model attributes. The STL file describes a This is analogous to a pixel, discretized triangulated surface by the unit which represents 2D image data normal and vertices coordinates for each in a bitmap. triangle (ordered by the right-hand rule). 7 Compromises in Additive Manufacturing Another 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 8 Laminated Object Manufacturing (LOM)

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. 9 Laminated Object Manufacturing (LOM) Current market leaders - Mcor Technologies (Ireland) Laminated Object Manufacturing (LOM) - Solido (Israel) was developed by Helisys of Torrance, CA, - Strataconception (France) in the 1990s. Helisys went out of business - Kira Corporation (Japan) in 2000 and their LOM equipment is now serviced by Cubic Technologies.

Equipment picture

Mcor Technologies Matrix 300+ (uses A4 paper and water-based adhesive) Courtesy, Cubic Technologies 10 Laminated Object Manufacturing (LOM)

KEY METRICS ADVANTAGES

Maximum build size 40in x 40in x 20in • Relatively high-speed process • Low cost (readily available materials) Resolution in (x,y) +/- .004 in • Large builds possible (no chemical Resolution in z Variable reactions) • Parts can be used immediately after the Speed Medium process (no need for post-curing) Cost Low • No additional support structure is required (the part is self-supported) Available materials Paper, Plastic Sheet DISADVANTAGES

KEY APPLICATION AREAS • Removal of the scrap material is laborious • The ‘z’ resolution is not as high as for other • Pattern Making technologies • Decorative Objects • Limited material set • Need for sealing step to keep moisture out 11 Fused Deposition Modeling (FDM) 5. The sacrificial support material (if available) 1. A spool of themoplastic wire (typically is dissolved in a heated sodium hydroxide acrylonitrile butadiene styrene (ABS)) with (NaOH) solution with the assistance of a 0.012 in (300 μm) diameter is ultrasonic agitation. 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 12 Fused Deposition Modeling (FDM) Current market leaders The fused deposition modeling (FDM) technology - Stratasys, Inc. was developed by S. Scott Crump in the late 1980s and was commercialized in 1990. The double material approach was developed by Stratasys in 1999.

"Ribbon Tetrus" (Carlo Séquin)

www.nybro.com.au

Stratasys Dimension SST 1200

Courtesy, Dr. Robin Richards, University College London, UK 13

FDM process parameters 14 Fused Deposition Modeling (FDM)

KEY METRICS ADVANTAGES

Maximum build size 20” x 20” x 20” • Economical (inexpensive materials) • Enables multiple colors Resolution in (x,y) +/- (0.002” - 0.005”) • Easy to build DIY kits (one of the most Resolution in z +/- (0.002” - 0.01”) common technologies for home 3D printing) Speed Slow • A wide range of materials possible by Cost Medium loading the polymer Available materials Thermoplastics (ABS, PC, ULTEM…)

KEY APPLICATION AREAS

• Conceptual Models www.redeyeondemand.com • Engineering Models • Functional Testing Prototypes DISADVANTAGES • Materials suite currently limited to thermoplastics (may be resolved by loading) 15 Fused Deposition Modeling (FDM) Do it Yourself FDM rapid prototyping systems

FAB@Home RepRap • First multi-material printer available to the public • Open-source system • Open-source system • Founded in 2005 by Dr. A. Bowyer at the University of • Project goal: open-source mass-collaboration Bath (UK) developing personal fabrication technology aimed at bringing personal fabrication to your home (project • Project goal: Deliver a 3D printer that can print itself! started by H. Lipson and E. Malone at Cornell in 2006). • 1st machine in 2007 (Darwin) • Popular Mechanics Breakthrough Award 2007 • Replication achieved in 2008 16 Fused Deposition Modeling (FDM) 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 17 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 18 3D Printing (3DP) Current market leaders - Z Corporation Z Corporation first introduced high- - Exone resolution, 24-color, 3DP (HD3DP™) in - Voxeljet 2005 (600 dpi). Z Corp was later bought by 3D Systems.

Olaf Diegel Atom 3D printed guitar

Zcorp Z510 19 3D Printing (3DP)

KEY METRICS ADVANTAGES Maximum3D Printing build size 14 in x(3DP) 10 in x 8 in • Can create extremely Resolution in (x,y) 640 dpi realistic multi-color parts (24-bit color) Resolution in z Variable using inkjet technology Speed Fast • Can generate complex components with Cost Low internal degrees of Available materials Plaster, sand, oxide freedom ceramics, sugar • Economical and starch for food • Versatile Printed with Z Corp 650 printing

KEY APPLICATION AREAS DISADVANTAGES

• Widely used to print colorful and complex • Very limited materials suite parts for demonstration purposes • Low resolution (lowest of all AM technologies) • Molds for sand casting of metals • Negligible mechanical properties (unusable for any structural application) 20 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 21 Selective Laser Sintering (SLS) Current market leaders - 3D Systems • SLS technology invented at UT Austin in the ‘80s by Joe Beaman, Carl Deckard and Dave Bourell. • First successful machine: DTM Sinterstation 2000, in late 1990s • DTM later acquired by 3D Systems

Bulatov Abstract Creations 3D Systems Sinterstation

Important note: SLS patent runs out in Feb 2014! 3D Systems A huge influx of players and Metal Technology Co. technologies is anticipated. 22 Selective Laser Sintering (SLS)

KEY METRICS ADVANTAGES Maximum build size 700 mm x 380 mm x 560 • Wide array of structural materials beyond mm polymers Resolution in (x,y) High (Spot Dependant) • No need for support materials • Cheaper than EBM Resolution in z 0.005” • One of two technologies that allow Speed Medium complex parts in metals Cost Medium Available materials Powdered plastics (nylon), metals (steel, titanium, tungsten), ceramics (silicon carbide) and fiber- reinforced PMCs DISADVANTAGES

KEY APPLICATION AREAS • Expensive relative to FDM, 3DP • The quality of metal parts is not as high as • Structural components with EBM 23 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 24 Electron Beam Melting (EBM) Current market leaders EBM process developed by - Arcam AB (Sweden) Arcam AB (Sweden) in 1997

Arcam A2 machine 25 Electron Beam Melting (EBM) KEY METRICS ADVANTAGES Maximum build 200mm x 200mm x • Method of choice for high-quality metal size 350mm parts Resolution in (x,y) +/- 0.2mm • Wide range of metals • Fully dense parts with very homogeneous Resolution in z 0.002” (0.05 mm) microstructures Speed Medium • Vacuum operation allows building of highly reactive metals (e.g., Titanium) Cost High • High temperature operation (700-1000C) Available materials Metals: titanium, results in structures free of internal stresses tungsten, stainless • EBM allows even better microstructural steel, cobalt chrome, control than many conventional processes. Ni-based superalloys.

DISADVANTAGES KEY APPLICATION AREAS • Extremely expensive (more than SLS) • Conventional machining may be required • Structural components for aerospace to finish the goods (rough surface) (Ti6Al4V, gammaTiAl, Ni superalloys) • Requires vacuum operation • Custom-made bio-implants (Ti6Al4V) 26 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 The method of building each layer is similar to layer-by-layer until the model is built. Inkjet Printing, in that it uses an array of inkjet 4. The sacrificial material is removed: print heads to deposit tiny drops of build material and support material to form each layer of a part. ▫ The Objet system uses a photopolymer as However, as in Stereolithography (see following support material; the support material is slides), the build material is a liquid acrylate- designed to crosslink less than the model based photopolymer that is cured by a UV lamp material and is washed away with pressurized after each layer is deposited. water. For this reason, Multijet Modeling is sometimes ▫ The 3D Systems InVision uses wax as referred to as Photopolymer Inkjet Printing. support material, which can be melted away. 27 Multijet Modeling (MJM) Current market leaders Multijet modeling (MJM) was - Objet introduced by 3D Systems in 1996 as - 3D Systems a cheaper alternative to industrial- grade Stereolithography machines.

Objet Desktop 30 Pro

3D Systems Thermojet 28 Multijet Modeling (MJM) KEY METRICS Maximum build size 1000mm x 800mm x 500mm ADVANTAGES Resolution in (x,y) 450 dpi • Fast process Resolution in z 16 microns • Complex parts via sacrificial support materials Speed Fast Cost High Available materials Acrylate-based photopolymer DISADVANTAGES KEY APPLICATION AREAS • Accuracy is not as good as SLA • Automotive • Defense • Aerospace • Consumer goods • Household appliances • Medical applications 29 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 A suitable photosensitive polymer prescribed step, exposing a fresh layer must be very transparent to UV light of liquid monomer, and the process is in uncured liquid form and very repeated absorbent in cured solid form, to 4. At the end of the job, the whole part is avoid bleeding solid features into cured once more after excess resin and the layers underneath the current support structures are removed one being printed. 30 Stereolithography (SLA) Solidification of the monomer can occur in two different modalities:

Free surface mode: Solidification occurs at the resin/air interface. In this mode, care must be taken to avoid waves or a slant of the liquid surface, which would compromise the final dimensional resolution. The elevator moves down at each step (top-down build).

H-W Kang et al 2012 J. Micromech. Microeng. 22 115021

Fixed surface mode: The resin is stored in a container with a transparent window plate for exposure, and solidification occurs at the stable window/resin interface. In this mode, the elevator moves up at each step (bottom-up build). 31 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. 32 Stereolithography (SLA)

SLA was pioneered by Chuck Hull in Current market leaders the mid-1980s (see picture below). - 3D Systems Hull founded 3D Systems to - Sony commercialize its new manufacturing process.

3D Systems iPro 9000 XL 33 Stereolithography (SLA) KEY METRICS

Maximum build size 1500mm x 750mm ADVANTAGES x 550mm • Fast Resolution in (x,y) Spot Dependent • Good resolution Resolution in z 0.004” • No need for support material • Photosensitive polymers have acceptable Speed Medium mechanical properties Cost High Available materials Thermoset polymers: photosensitive resins DISADVANTAGES

• Expensive equipment ($100-$500K) KEY APPLICATION AREAS • Expensive materials (photosensitive resins are ~$100-200 /kg) • Patterns for metal processing (e.g., • Material suite limited to resins molding) • Prototypes for demonstrational purposes 34 Stereolithography (SLA) APPLICATION TO MEMS AND NEMS

• 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.

EPFL, Lausanne, Switzerland 35 Stereolithography (SLA) MICROSTEREOLITHOGRAPHY

• 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. www.miicraft.com • 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. 36 Stereolithography (SLA) TWO-PHOTON LITHOGRAPHY

• Two-photon lithography provides a further enhancement of the SLA resolution.

• Special initiator molecules in the monomer only start the polymerization reactions if activated by two photons simultaneously. The laser intensity field can be tuned so that this event only happens in a very small region near the focus. The result is extremely local polymerization, with resolutions in the tens of nanometers range.

• Two-photon polymerization can occur everywhere in the monomer bath, as opposed to only at the top layer, simplifying the hardware requirements considerably.

www.laser-zentrum-hannover.de 37 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. 38 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. 39 Micro-Architected Materials Overarching vision

UNIQUE DEFORMATION MECHANISMS SIZE EFFECTS IN PLASTICITY How can we fill unclaimed regions? AND FRACTURE - Optimal topology - Optimal geometry - Base material optimization (nm-features) - Hierarchical design IMPROVED STRENGTH IMPROVED STRENGTH What do we need? AT THE MACROSCALE AT THE FILM LEVEL - 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 40 A word of caution

Tech Consultancy Puts 3D Printing at Peak of "Hype Cycle" PARAMETERS INVOLVED DEFECTS Density 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