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ADDITIVE MANUFACTURING PROCESSES

FOR MATERIALS FORMING

Part 1 : Basic principles and examples

Claire Barrès, Jean-Yves Charmeau,Stéphane Dupin, Amir Msakni

INSA-Lyon

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Additive manufacturing technologies

❶ Basic principles

❷ 3D Printing

❸ @manufacturing

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Different types of material forming processes

By ablation

SculptureSculpture

MachiningMachining

By deformation

PotteryPottery

StampingStamping

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By addition

Rapid Manufacturing /

Additive processes

Building

❶ Basic principles

❷ 3D printing

❸ @manufacturing

Additive manufacturing technologies

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Creating a 3D physical model directly fromdigital data

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3D model from a computer

(CAD file)

S.T.L. file

Machine software

fabrication

• STL file :– Description of the surfaces of the part by

triangular facets

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SLICING

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The model is placedin the virtual spaceof the machine

Slicing into thin layers(thickness depends on the process)

Layer by layer building

LAYER ADDITION

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LAYER ADDITION AND SURFACE FINISHING

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Physical object out of the machine (final surface quality depends on the layer thickness ).

Object afterfinishing

Additive manufacturing technologies

❶ Basic principles

❷ 3D printing

❸ @manufacturing

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3D PrintingThere are a number of 3D printing technologies. They differ in the way

layers are deposited to create parts, in the materials that can be used,

and they all work differently.

Inkjet 3D-printing systems create the model one layer at a time by

spreading a layer of powder (plaster, or resins) and printing a binder in

the cross-section of the part using an inkjet-like process. This

technology allows the printing of full color prototypes. The strength of

bonded powder prints can be enhanced with wax or thermoset polymer

impregnation.

Some methods melt or soften material to produce the layers, e.g.

selective laser melting (SLM), selective laser sintering (SLS), fused

deposition modeling (FDM), while others cure liquid materials using

different technologies, e.g. stereolithography (SLA).11

PRINTER RESOLUTION

• It describes layer thickness and X-Y resolution in dpi(dots per inch) or micrometers.

• Typical layer thickness is around 100 micrometers (µm),although some machines such as the Objet Connexseries and 3D Systems' ProJet series can print layers asthin as 16 µm.

• X-Y resolution is comparable to that of laser printers.

• The particles (3D dots) are around 50 to 100 µm indiameter.

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Source Ex-one

3D PrintingPrinciple : using a printhead .

Advantage :- Can be used in engineering and design depts,

in drawing offices, at homeDrawback :- Fragile parts

Applications :- Prototyping, design office

Materials :- Light sensitive resins, epoxy- Polymer melt- Polymer binder on polymer, metal, sand,

ceramic powders.

Some machine manufacturers :- Molten polymer thread: Stratasys- Light sensitive systems : Objet- Powder + binder system : Prometal.Dimensions : up to 4 x 2 x 1 metres (Voxeljet, binder jetting)

FUSED DEPOSITION MODELING (FDM)

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Examples Fused Deposition Modelling (FDM)

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Stratasys

• Materials :

– ABS

– Polycarbonate

– PC-ABS

– Elastomer

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Fused deposition modeling (FDM)

Machine manufacturer :Stratasys, USA

Advantages :– Functional and flexible

models.– Soluble supports– Simple system, possibility

of desktop use.– Non-toxic materials

Drawbacks : − Extrudable materials only− Layer thickness (0.2 mm mini) and

wall thickness (0.3 mm mini)− Precision : +/- 0.15 mm

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BINDER JETTING ON A POWDER

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STEREOLITHOGRAPHY (SLA)

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STEREOLITHOGRAPHY (SLA)

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Laser scanning ���� point-by-point solidification of resin.

Alternative light source : Digital Light Processing or

DLP® projectorsto project voxel data (volumetric pixels).

Each voxel dataset is made up of tiny voxels with dimensions

as small as 16µm x 16 µm x 15 µm in X, Y and Z direction

Stereolithography

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Envisiontec

3D Systems

Examples of models for investment (« waste

wax ») casting

Objet

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STEREOLITHOGRAHY

• Advantages :+ Surface aspect, precision (esp.

DLP systems)+ Well-known, mature technology+ Large volume machines+ …

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• Drawbacks : – Only light-sensitive resins– Supports necessary– Quite fragile parts, UV-sensitive– Uncontrolled shrinkage

• Materials :– light-sensitive epoxy resins .– UV-curable flexible or high-T°resins…

LASER SINTERING

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Selective Laser Melting : equivalent of SLS for metal p owders

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Classification of additive processes

Materials

to betransformed

Principles of transformation

Technologies Acronyms Pictures Main groups

Photo-

sensitive resin

Photo polymerization

Laser or UV flashing

SLA STL DLP Stereolithography

Printhead Polyjet 3D printing

Powder : polymeric, metal, ceramic, sand

Binding Printhead 3DP

Sintering / fusion

Laser, IR flashing, or electronbeam

SLS SLM DMLS EBM SMS

Laser Sintering/Melting

Projection DMD Deposition

Polymerfilament

Liquid state « welding »

Deposition FDM

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La fabrication Rapide peut être la 1 ère étape pour la fabrication par réplication : moule s able, ou pièce modèle.

Additive manufacturing technologies

❶ Basic principles

❷ 3D Printing

❸ @manufacturing

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Industrial targets• Reduction of marketing times• Decrease of costs• Quality control (and standards)

Trends : • Evolution towards small / medium size series,

mass customization, increased complexity• Product lifetime �

• Delocalization of large series manufacturing, of mouldmaking industry …

• Need for flexibility and reactivity of production

Important economical stakes

PRODUCT DEVELOPMENT : CONTEXT AND STAKES

COMPARISON OF COSTS

Cost/part as a function of production volume

⊕ of AM development :� Independant on part complexity� Fast� No specific tooling

BUT :� Limited choice of materials� Materials cost� Production time� …

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SLS : on the way to

industrial production

Laser sintering

principle:• Powder preheating below melting T°• Spreading of powder layer• Scanning with CO2 laser (infra-red),• Particles fusion and « sintering »• Build tank going down by a layer

thichness• Next layer…

Advantages :• Self-supporting powder• Great freedom of shapes,• Possible assembling and functional

systems

Drawbacks :• Few commercial materials available• Anisotropy of parts.• Limited part size

Application field :• Prototyping, direct part manufacturing,

rapid manufacturing.

Main current materials available : Polyamide 11 or 12 : plain, filled : glass beads, aluminium, carbon, and/or flame retardantPolystyrenePEEK (specific machine, 1 in Germany)

Main machine manufacturers :EOS GmbH, 3D Systems.

Dimensions : 700 X 380 X 60027

Frittage Laser Polymère

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EOS – Systèmes d’aide à

l’assemblage

EOS – chambre à air pour

hélicoptère

EOS – Maquettage

EOS – centrifugeuseEOS – prothèse

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DEVELOPMENT OF ADDITIVE MANUFACTURING

FDM machines at the production facility of RedEyeOn Demand, a business unit of Stratasys in Eden Prairie, Minnesota

ADDITIVE MANUFACTURING PROCESSES FOR

POLYMERIC MATERIALS FORMING

Part 2 : Selective sintering processes for polymer powders

- Analysis of SLS physical mechanisms- Introducing SMS

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Prior to sintering, 2 conditions are very important for the process:

Powder flowabilityPowder bed density

Main influent parameters

Particle size►►

►►

►►

Size distribution

Sphericity

IDENTIFICATION OF THE KEY MATERIALS PARAMETERS

DIFFERENT POWDER MORPHOLOGIES

InnovPA : Exceltec

Duraform PA : 3D Systems

PA2200 : EOS

All of them need SiO2 as flowability agent

(< 1 wt%)

►►

►►

►►

Innov PA

50µm

Duraform PA

50µm

PA 2200

50µm

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Warpage due to shinkage throughout the crystallization is a key issue

Sintering of semi-crystalline polymers

IDENTIFICATION OF THE KEY MATERIALS PARAMETERS

►►

Re-crystallization must be controlled

Tf = 181°C

Tc = 151°C

Powder bed T° maintained in the processing window w : Tm - Tc

Part warpage (« curling ») avoided

Polyamide 12 mostly used thanks to very wide w range

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The energy provided to the polymer material by the laser

Process conditions of first order influence are :

The heat provided to the powder during the whole

fabrication cycle

►►

MOST INFLUENT PROCESS PARAMETERS

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135°C 135°C

150°C

173°C

Right feed heater Left feed heaterPart heater

Piston heater

Cylinderheater

Process parametersThermal cycle

■ Different temperatures of preheating35

Process parameters

The energy supply depends on the:

■ Scan spacing (S) via laser beam superimposition

■ Laser beam celerity (v)

■ Laser radius (r)

■ Laser power (P)

Energy Density ED : a single parameter

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Power supply by

surface unit

Time of exposure

Number of exposures

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By application of Archimedes’ principle on samples infiltrated with CH2I2

Calculation of open and closed porosities

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CHARACTERIZATION OF MICROSTRUCTURE1- POROSITY

Closedporosity

Open porosityfilled by CH 2I2

Evolution of the global porosity (vs. ED) :

■ Porosity decreases when ED increases

■ Porosity reaches a minimum value about 2%37

By 3D X-Ray tomography

Computation of closed porosity fraction, information

on size and spatial distribution of pores

Analogy with

medical

scanner

After image analysis, 3D

reconstruction of porosity

distribution

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ENHANCED CHARACTERIZATION OF POROUS MICROSTRUCTURE

Succession of 2D

sections of the

sample

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Characterization of porosity

by Xray tomography

Example for Innov PA powder

Position of the part in the

build tankPosition of the part

during analysis

Tomographic sections

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LOW ED : bad weldingbetween layers

Observation of the 2 types of porosity : open and closed

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INFLUENCE OF ENERGY DENSITY ED ON POROSITY

Open porosity

reaches core of

parts

Example for Innov PA powder

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INTERPRETATION OF PART ANISOTROPY

Porosity is concentrated at the interface between successive layers

This is mostly noticeable at low ED, but still present at high ED

Mechanical properties are anisotropic

►►

►►

InnovPA parts, ED = 0.024J/mm²

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Final microstructure is governed by:

DENSIFICATION PARAMETERS

During sintering 2 stages occur : Coalescence and melt densification

■ Evolution of the particles during coalescence : Frenkel’s model

With η : viscosityΓ : surface tension

■ Melt densification is due to diffusion/dissolution of gases from pores

� Granular characteristics which impact powder bed density

� Melt viscosity

� Crystallization temperature (and build tank T°during proce ss)

a0

x

a

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XCR: recrystallized phase crystal weightfraction (30%)

Source: D. Jauffres et al, Polymer 48, 6375-6383, 2007

With ∆HR: recrystallized phase enthalpy of fusion

∆HN: nascent phase enthalpy

XCN: nascent crystal weight fraction (50%)

Recrystallised fraction (fr) can becalculated by a deconvolution

method:DSC shows the presence of both

recrystallized and nascent fractions

CHARACTERIZATION OF MICROSTRUCTURE –2) CRYSTAL WEIGHT FRACTION AND RECRYSTALLIZED

PHASE

∆HR

∆HN

Objective:measure the evolution of fr and Xc with ED

Recrystallizedphase

Nascentphase

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Evolution of fr with ED

Occurrence of nascent particles between successive layers

From ED ≈ 2J/cm², no more nascent particles in the core of parts, but still present at the surface

INFLUENCE OF THE ENERGY PROVIDED BY THE LASER ON PARTICLE MELTING AND CONSOLIDATION

►►

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Strong increase of fr up to ≈ 2.5J/cm² then stabilizes

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Eρ�

INFLUENCE OF THE PROCESSING WINDOW

Crystalline fraction measured by DSC:

Xc when ED

because more nascent

polymer is melted

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Comparison between InnovPA and DuraformPABuild tank T° = 150°C

Crystallization T° measured by DSC at 10°C/min :

Innov PA Tc = 151°C / Duraform PA Tc = 147°C

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►► Xc lower for Innov PA samples

lower Tc for Duraform PA

slower crystallization,larger crystalline ratio

Comparison between InnovPA and DuraformPABuild tank T°= 150°CCrystallization T°measured by DSC at 10°C/min :Innov PA Tc = 151°C / Duraform PA Tc = 147°C

Nascent powders

Injection molded parts

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CONCLUSION ON FORMATION OF

POROUS AND CRYSTALLINE MICROSTRUCTURE

Powder bed density (granulometry and morphology of powders) strong impact on porosity formation

Time spent in molten state (also depends on T° of build tank) strong influence on porosity and on final crystallinemicrostructure

Eρ < 2 J/cm² Eρ > 2 J/cm²

Duraform PA/PA2200 : Innov PA :

►►

►►

Porosity can be also influenced by viscosity (coalescence )

low ED high ED low ED high ED

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RELATIONS BETWEEN MECHANICALPROPERTIES (TENSILE) AND MICROSTRUCTURE

Elongation at break is oneof the most critical featuresof sintered polymer parts

P2

P1

General trend : when ED � ,ductility � because Xc �

But relations are more complex

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Material parameters

■ Melt viscosity

■ Powder size/morphology

■ Tm

■ Tc

Process parameters

■ Laser features (ED)

■ T° of build tank

■ T° of powder bed surface

Microstructure

■ Porosity

Pore size

■ Fr

■ Xc

Mechanical properties

■ Eab

■ Stiffness

CONCLUSION

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SMS – Selective Mask Sintering by IR flashing

Principales différences avec SLS :

• Each section (slice) is sintered as a whole by IR flashing through a mask which is regenerated for each layer

• Potentially faster than SLS

• Size part less limited

• Present technological issue : mask generation

• Process still in development

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powder spreading printing flashing repeat cyclePrinciples of sintering by IR flashing through a mask

Patent owned by Sintermask GmbH, Parsberg, Germany

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SMS-IR flashing :Lab-built prototype machine at INSA

� Manually operated, but automatization in project� Can be equipped with thermocouples for T°monitoring

during sintering

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Temperature monitoring in lab-IR machine(here 3 thermocouples inserted)

Comparison with numerical simulation

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A few orders of magnitude :

Maximum build velocities : SLS : ~ 25 mm/h (small area )IR-SMS : 35 mm/h , 10 à 20 s / layer, 5s target

Cooling time ~ fabrication timeNb of powder re-uses : ~ 7 times, mixed with « fresher » powder (e.g. 50% from feed tanks, 50% from build tank, or 75% used + 25% fresh)

Layer thickness : ~ 100 µmMinimum wall thickness : ~ 300 – 500 µm

Diameter of laser beam : 250 µm

Material cost : 50 - 150 Euros/kg

Some features of polymer sintering processes

SOME CONDITIONS FOR THE INDUSTRIALDEVELOPMENT OF ADDITIVE MANUFACTURING

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- Reliability of the production :- Availability of adequate materials- Good control over the process- Good understanding of the physical

phenomena, and of the relations betweenprocess parameters, material featuresand final part properties

- Development of quality control- Development of standardization and

qualification of AM systems and materials- Increase production speed- Improve powder re-usability

… still much work to be done !