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Développement rapide de produit ( N° spécial de la revue CFAO & d'informatique graphique 1998, volume 13, numéro 4-5-6)

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292 pages
Les articles publiés dans cet ouvrage ont été sélectionnés par les présidents des sessions de conférences des 7e Assises Européennes de Prototypage Rapide. Ils sont relatifs à la fois à des aspects techniques, logiciels, organisationnels, etc. Les articles de la première partie Procédés de fabrication rapide par couches et processus d'outillage rapide mettent en évidence des progrès technologiques relatifs à toute la chaÎne de fabrication de pièces fonctionnelles. Cela concerne des avancées relatives aux procédés de fabrication par couches, à des processus aval et enfin à des sources d'énergie. Ces contributions sont pour la quasi-totalité issues de laboratoires de recherche et de centres techniques, travaillant à partir de demandes industrielles. La deuxième partie de l'ouvrage traite des aspects méthodologiques, logiciels et de rétro-conception. On constate en effet des efforts importants dont l'objectif principal est de structurer la phase de développement du produit, en essayant de maÎtriser à la fois les aspects technique, organisationnel, financier et temporel. Cette maÎtrise est favorisée par des progrès significatifs dans le domaine de l'aide à la préparation de la fabrication sur machines fonctionnant par couches mais également par une meilleure intégration des environnements d'acquisition et de numérisation des formes tridimensionnelles (rétro-conception), avec une extension vers le contrôle des pièces complexes.
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Développement
rapide
de produit
coordonnateur
Alain Bernard
HERME S
ISSN 0298-0924
Revue internationale de CFAO et fl*iiift'ormati€|iie graphique
International Journal of CADCAM and Computer Graphics
Volume 13 • n° 4-5-6 — décembre/1998 bimestriel Revue internationale de CFAO
et d'informatique graphique Revue publiée avec le concours du ministère
de l'Enseignement supérieur et de la Recherche
(DIST)
Revue répertoriée dans la base PASCAL de l'INIST-CNRS
Revue parrainée par MICADO
© HERMES Science Publications et Yvon Gardan, 1998
ISBN 2-86601-747-1
ISSN 0298-0924
Commission paritaire n° 68665
Directeur de publication : Chantai Ménascé
HERME S Science Publications
8, quai du Marché-Neuf, 75004 Paris, France
Téléphone : (33) 01.53.10.15.20
Télécopie : (33)1
E-mail : hermes@iway.fr
Serveur web : http://www.hermes-science.com
Catalogage Electre-Bibliographie
ASSISES EUROPÉENNES DE PROTOTYPAGE RAPIDE (7 ; 1999 ; Paris)
Développement rapide de produit. Avancées méthodologiques techniques et logicielles :
ede la rétro-conception à l'outillage de pré-série : 7 Assises européennes de prototypage
rapide ; éd. sous la dir. de Alain Bernard. — Paris : HERMES Science Publications. 1999
ISBN 2-86601-747-1
RAMEAU : CFAO, systèmes de rétro-ingénierie (informatique)
prototypage (informatique)
DEWEY : 629.5 : Autres branches de l'art de l'ingénieur.
Commande automatique. Robotique
Le Code de la propriété intellectuelle n'autorisant, aux termes de l'article L. 122-5, d'une part, que les
« copies ou reproductions strictement réservées à l'usage privé du copiste et non destinées à une
utilisation collective » et, d'autre part, que les analyses et les courtes citations dans un but d'exemple et
d'illustration, « toute représentation ou reproduction intégrale, ou partielle, faite sans le consentement
d e l'auteur ou de ses ayants droit ou ayants cause, est illicite » (article L. 122-4).
Cett e représentation ou reproduction, par quelque procédé que ce soit, constituerait donc une
contrefaço n sanctionnée par les articles L. 335-2 et suivants du Code de la propriété intellectuelle. Revue internationale de CFAO
et d'informatique graphique
Rédacteur en chef
Yvon GARDAN
IFTS/LRIM
Comité de lecture
C. ALÉONARD T. DOKKEN
Centre de recherches industrielles Digital Equipment
Oslo, Norvège
B . ARNALDI
E. DOMBRE IRISA
LIRMM, Montpellier Rennes
J. FRANÇON A. BERNARD
Université Louis Pasteur CRAN
Strasbourg Vandœuvre-lès-Nancy
G. GUILBERT P. BÉZIER
Président de MICADO Consultant
J.-P. GlLLET B . BOURDET
RNUR, Direction des systèmes de CFAO Ecole Normale Supérieure de Cachan
et d'informations techniques LURPA
P. BRUNET G. HÉGRON
Université de Catalogne, Espagne Université de Rennes
M. KIMURA P. de FAGET DE CASTELJAU
Université de Tokyo PS A
Japon
R. CAUBET
M. LUCAS IRIT
Université de Nantes Toulouse
A. LUCIANI M. CHINA
IMAG Consultant
G. MAZARE P. COIFFET
ENSIMAG LRP , Vélizy 1
4 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
C . MINICH C. SAGUEZ
Université de Metz SIMULO G
M. NEUVE EGLISE J. SEQUEIRA
Matra Datavision Faculté des Sciences de Luminy
Marseille
J. PECHAUD
AMD-BA R. SOENEN
Université de Valenciennes
B. PÉROCHE
Ecole des Mines de Saint-Etienne D . VANDORPE
Université Claude Bernard, Lyon
G. PIERRA
ENSMA, Poitiers M. VERON
CRAN
V. PIAZZINI Vandœuvre-lès-Nancy
Consultant en CAO
J.-C. SABONNADŒRE
INPG, ENSIEG
Grenoble
NOTE DE LA RÉDACTION
La revue internationale de CFAO et d'informatique graphique, soucieuse de
rendre compte des développements récents de ce vaste domaine scientifique,
publie des articles de synthèse, de recherche et d'état de l'art sur :
• les aspects mathématiques (courbes, surfaces, volumes) ;
• les problèmes de normalisation, les échanges entre systèmes ;
• le développement des bases de données ;
• les systèmes existants et leur utilisation ;
• les interactions de la CFAO avec l'IA, le CIM, la robotique, l'automatique, etc. ;
• les nouvelles applications industrielles de la CFAO et de l'infographie ;
• les informations relatives au marché et aux manifestations.
i Revue internationale de CFAO
et d'informatique graphique
Volume 13 -n ° 4-5-6/1998 Sommaire
DÉVELOPPEMEN T RAPIDE DE PRODUIT
AVANCÉE S MÉTHODOLOGIQUES, TECHNIQUES ET LOGICIELLES :
D E LA RÉTRO-CONCEPTION À L'OUTILLAGE D E PRÉ-SÉRIE
• Préface — Alain BERNARD 7
Parti e I. Procédés de fabrication rapide par couches et processus
d'outillag e rapide
• Laser Engineered Net Shaping (LENS™): beyond rapid prototyping to direct
fabrication. Laser Engineered Net Shaping (LENS™) : au-delà du prototypage rapide
vers la fabrication directe — David M. KEICHER, W. Doyle MILLER 11
• Principe de la fabrication rapide d'outillages de mise en forme par l'emploi d'un
procédé de projection thermique amélioré. Principles of rapid tooling using an
improved thermal spray process — Guillaume JANDIN, Hanlin LIAO, Christian CODDET 17
• Microstéréolithographie utilisant un écran générateur de masques. Microstereo-
lithography using a mask-generator display — Virginie LOUBERE, Serge MONNERET,
Serge CORBE L 31
• Long-fibre reinforced composite material models using a deposition-based RP
process. Modèles en matériau composite renforcé par fibres longues fabriqués à l'aide
d'un procédé de prototypage rapide à base de dépôt de matière — R. lan CAMPBELL,
Tom J. CORDON, Andrea Nov AR A 45
• Stéréolithographie pour la fabrication de pièces en céramique. Stereolithography for
the manufacturing of ceramic parts — Serge CORBEL , Catherine HINCZEWSKJ,
Thierry CHARTIER 5 3
• Rapid prototyping advances employing ceramic composite materials. Avancées du
prototypage rapide dans le domaine de l'emploi des matériaux composites céramiques
Allan LIGHTMAN, Richard CHARTOFF, Don KLOSTERMAN 63
• Application of the RapidTtool process. Applications du procédé RapidTool
Franck LACAN, Duc Truong PHAM, Stephan DIMOV 71
• Two new approaches to tool making using stereolithography models. Deux nouvelles
techniques pour la fabrication de moules métalliques utilisant des modèles en
stéréolithographie — Marc PIECKO 85 6 Revue de CFAO et d'informatique graphique. Volume 13 - n" 4-5-6/1998
• Comparison between C0 and Nd: YAG lasers for use with selective laser sintering 2
of steel-copper powders. Comparaison entre laser C0 et Nd:YAG pour le frittage
2
sélectif par laser de poudre acier-cuivre — Jean-Pierre KRUTH, Parcifal PEETERS,
Thomas SMOLDEREN, Johannes BONSE, Tahar LAOUI, Ludo FROYE N 95
Partie II. Aspects méthodologiques, logiciels et de rétro-conception
• Formalization of the rapid prototyping process: from requirements to tool choice.
Formalisation du processus en prototypage rapide : du besoin au choix des outils
Patrice DUBOIS, Améziane AOUSSAT, Robert DUCHAMP 113
• Times savings in product development. Quantification des gains de temps dans
le développement de produits — Jouko KARJALAINEN, Juhani SEPPÀNEN,
Jukka TUOMI , Hannu KAJKONEN 131
• Procédé de Stratoconception®. Intégration des outils CAO et prototypage rapide
en vue de la reproduction d'oeuvres d'art. Integration of Stratoconception® process,
CAD and rapid prototyping tools for work of art reproduction — Eric HUGUENIN,
Claude BARLIER, Edith DURAN D7
• Aspects of shape decomposition for thick layered object manufacturing of large
sized prototypes. Aspects liés à la décomposition de formes pour la fabrication
d'objets de grande taille à partir de couches épaisses — Johan J. BROEK,
Imre HORVATH , Bramn J. DE SMIT, Alex F. LENNINGS , Joris S.M. VERGEES T 153
• Intelligent decision support for part orientation in stereolithography. Outil intelligent
d'aide à la décision pour l'orientation des pièces en stéréolithographie
Duc Truong PHAM, Stephan DIMOV, Rosemary GAUL T 173
• Error correction and prevention of rapid prototyped parts. A preliminary result.
Prévention et correction d'erreurs de pièces obtenues par couches. Un premier résultat
Millan K. YEUNG , Shouse Xu, Fengfeng Xi 18
• Définition des paramètres essentiels pour la génération automatique de processus
d'acquisition de forme par capteur laser. Definition of essential parameters for the
automatic generation of laser sensor scanning process — Alain BERNARD,
Benoît SIDOT, Stéphane DAVILLERD, Gabriel Ris 191
• Visual system for fas t and automated inspection of 3D parts. Système visuel
pour l'inspection rapide et automatisée de pièces 3D — Flavio PRIETO,
Tanneguy REDARCE, Richard LEPAGE, Pierre BOULANGER 21
• Tomographic industrielle à rayons X : contrôle et numérisation. X-ray Computed
Tomography: control and digitizing — David DASTARA C 229
• Outil de base pour l'extraction de caractéristiques de surfaces numérisées.
Basic tools for the feature extraction of digitized surfaces — Philippe VÉRON,
David LESAGE, Jean-Claude LÉO N 241
• Partition of 3D digitized data for CAD modelling. Partitionnement de données
issues d'une numérisation 3D pour la CAO — Guillaume OSTY, Claire LARTIGUE ... 263
• Reverse engineering in micro systems technology. Rétro-conception pour la
technologie des microsystèmes — Hans GRABOWSKI , Stephan RUDE, Gunther STORZ 273 Préface
eLes 7 Assises Européennes du Prototypage Rapide, organisées les 19 et
20 novembre 1998 par l'AFPR (Association Française de Prototypage Rapide) à
l'Ecole Centrale Paris, ont été l'occasion de faire le point sur les avancées
significatives dans le domaine du développement rapide de produit. Cet événement,
majeur au plan national, s'est appuyé sur une contribution internationale
représentative du savoir-faire industriel actuel et sur des avancées de recherche
ouvrant des perspectives à court et à moyen terme. Les nombreux participants ont pu
apprécier cet apport à la fois au niveau des conférences, de l'exposition et du Comité
International de Programme. Ce dernier, formé d'une trentaine des plus grands
experts internationaux du domaine, a décerné deux « Trophées de l'AFPR », l'un
pour la meilleure pièce, l'autre pour la meilleure étude de développement de produit,
récompenses remises par Monsieur le Ministre de l'Industrie. Ce dernier a souligné
l'aspect stratégique de ce domaine (une des technologies clés) et a assuré les
industriels du soutien de ses services, au travers en particulier de différentes aides
financières.
En effet, il s'agit bien de favoriser la compétitivité de nos industries et de leur
permettre de disposer de moyens efficaces d'aide à l'innovation, au développement
et à l'industrialisation de nouveaux produits, sur un marché concurrentiel
international.
Plus largement que la fabrication rapide par couches, au-delà de l'appellation
« prototypage rapide », terme traduit initialement de Rapid Prototyping, les filières
numériques et technologiques dont il est question dans cet ouvrage permettent
d'entrevoir des évolutions majeures vers la fabrication rapide de pré-séries, voire de
séries industrielles.
Ce nouveau challenge doit apporter une contribution majeure dans le
raccourcissement du temps de mise sur le marché d'un nouveau produit (time to
market), phénomène qui réduit les possibilités techniques, économique mais surtout
temporelle d'utiliser des filières traditionnelles, et oblige les entreprises à se tourner
vers des solutions alternatives que sont les processus de fabrication rapide et
économique de pièces fonctionnelles. 8 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
Dans ce contexte, les procédés de fabrication rapide par couches ne donnent à
eux seuls que des réponses partielles à ce besoin, car les caractéristiques des pièces
obtenues ne correspondent que partiellement aux exigences fonctionnelles. Afin
d'e n améliorer encore l'efficacité et étendre les domaines d'application, les
challenges techniques pour les dix prochaines années sont donc tournés à la fois sur
la diversification des principes physiques mis en jeu et sur l'amélioration de la
rapidité des procédés de fabrication rapide par couches, qui devront permettre en
particulier la mise en œuvre de matériaux techniques de meilleures qualités
fonctionnelles. Par contre, dès aujourd'hui, l'intégration de ces procédés, grâce en
particulier à des interfaces STL robustes et des logiciels de préparation à la
fabrication toujours plus efficaces et conviviaux, favorise la diminution du temps
d'obtention de modèles ou d'outillages qui servent de base à des processus « aval »
(filières industrielles de fabrication par coulée sous vide, injection plastique,
fonderie,...).
Les articles publiés dans cet ouvrage ont été sélectionnés par les présidents des
esessions de conférences des 7 Assises Européennes de Prototypage Rapide, tous
membres du Comité International de Programme de cet événement. Ils sont relatifs à
la fois à des aspects techniques, logiciels, organisationnels,...
Les articles de la première partie mettent en évidence des progrès technologiques
relatifs à toute la chaîne de fabrication de pièces fonctionnelles. Cela concerne des
avancées relatives aux procédés de fabrication par couches (à la fois au niveau des
principes physiques et des matériaux), à des processus « aval » et enfin à des sources
d'énergie. Ces contributions sont pour la quasi-totalité issues de laboratoires de
recherche et de centres techniques, travaillant à partir de demandes industrielles.
La deuxième partie de l'ouvrage traite des aspects méthodologiques, logiciels et
de rétro-conception. On constate en effet des efforts importants dont l'objectif
principal est de structurer la phase de développement du produit, en essayant de
maîtriser à la fois les aspects technique, organisationnel, financier et temporel. Cette
maîtrise est favorisée par des progrès significatifs dans le domaine de l'aide à la
préparation de la fabrication sur machines fonctionnant par couches mais également
par une meilleure intégration des environnements d'acquisition et de numérisation
des formes tridimensionnelles (rétro-conception), avec une extension vers le
contrôle des pièces complexes. Si les capteurs se démocratisent, les applications
logicielles fournies par les constructeurs et les travaux de recherche dans le domaine
ne sont pas en reste, diminuant considérablement les phases de mise en œuvre des
capteurs et de traitement des informations digitalisées en vue de la modélisation
surfacique ou le contrôle par comparaison à un modèle numérique de référence.
Pour plus d'informations, le lecteur peut contacter l'AFPR (Association
Française de Prototypage Rapide) qui entreprend en permanence une action de veille
et de dynamisation auprès des entreprises, et qui contribue également à la diffusion
de l'information dans ce domaine au travers d'un bulletin de veille trimestriel. II
peut également se référer à l'ouvrage Le prototypage rapide publié chez ce même
éditeur.
Alain BERNARD Partie I
Procédés de fabrication rapide par
couches et processus d'outillage rapide Laser Engineered Net Shaping (LENS™):
Beyond Rapid Prototyping to Direct
Fabrication
David M. Keicher — W. Doyle Miller
Optomec Design Company
Albuquerque, New Mexico 87107, USA
dkeicher® optomec. com
The Laser Engineered Net Shaping (LENS™) process has the promise to
revolutionize the manufacturing of mechanical hardware for a variety of
applications. This technology, a layer-additive process, has the ability to produce
functional metallic hardware to near-net shape directly from a computer aided
design (CAD) solid model rendering of the component. This method is capable of
processing a broad range of materials from standard materials, such as stainless steel
and copper, to high performance materials, such as nickel-based and titanium alloys.
Due to the unique operating conditions, this process is able to produce hardware
whose mechanical properties can far exceed those produced from conventionally
processed materials. In addition, the layer additive approach provides the ability to
produce structures that can not be directly fabricated using other methods.
Over the last decade Rapid Prototyping (RP) technologies such as Selective
Laser Sintering (SLS) or Stereolithography (SLA) have caused a paradigm shift to
occur in solving manufacturing problems. To fabricate conceptual models
representative of a final shape, these RP methods start with a three-dimensional
CAD solid model rendering and use this information to fabricate the object directly.
The CAD solid model is sliced into layers electronically within a computer and this
layer data is then used to drive these processes to fabricate the object a layer at a
time until the final shape is complete. These RP technologies provided a novel way
to fabricate these conceptual models to allow designers to more quickly realize their
designs and provide a method to produce a pattern that could be used for metal
casting processes. Since, however, the components that could be produced using
these processes are typically plastics they do not provide a method in which metallic
components can be fabricated in a single processing step.
Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998, pages 11 à 16 12 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
The LENS™ process now moves beyond the limitations of these RP processes
and provides an enabling technology for the direct fabrication of mechanical
hardware that has exceptional material properties and can be used for form, fit and
function testing directly from the CAD solid model rendering. Similar to RP
technologies, the LENS™ process begins with a .stl file representative of the desired
object. This file is then sliced electronically into a series of layers that define the
regions that compose the object and this layer information is then used to drive the
LENS™ process to build the object a layer at a time. A schematic representation of
the LENS™ process is shown in Figure 1. To begin the fabrication process, a metal
substrate is used as a base onto which new material is deposited. A high power laser
is focused onto the substrate to create a molten puddle and metal powder is injected
into the puddle. Thee is moved relative to the laser beam in a controlled
fashion to deposit thin metallic lines of a finite width and height. These lines are
deposited side by side in the desired regions to create the pattern for each layer. In
this fashion, each layer is built up line by line while the entire object evolves, layer
by layer.
Z-axis Positioning
Laser Beam
of Focusing Lens and
Powder Delivery Nozzle
Powder Delivery
Nozzle
Beam/Powder
Interaction Region
X-Y Positioning Stages
Figure 1. Schematic representation of the LENS process LENS™: Beyond Rapid Prototyping to Direct Fabrication 13
Aside from the most obvious advantage of forming a near net shape metallic
component directly from a CAD solid model, the LENS™ process has several other
significant advantages that provide uniqueness in the direct fabricated of
components over conventional processing as well as RP technologies. These
advantages include the ability to create buried structures, material properties that
exceed the properties for a comparable wrought material, and a minimum waste.
Several benchmark studies have now been performed suggesting that conformai
cooling in injection mold tooling will provide a significant cost reduction over
conventional tooling in the form of reduced cycle time. One such study performed
1 1by Express Tool ' suggests that the cycle time may be decreased by as much as 40%
through the use of conformai cooling. This reduction in time can reduce the total
number of tools that are required to complete a production run for plastic parts or
allow the manufacturer to get their products to market faster. An example of a
triangular passage through a solid piece is shown in Figure 2. Although this is a
relatively simple piece, it does demonstrate conceptually how the LENS™ process
can used to move beyond RP and other existing process to directly fabricate unique
components. Since this geometry was built without a support structure, only a
modest amount of post processing is required to achieve a finished piece of
hardware for functional evaluation and, ultimately, end use.
Since the LENS™ process uses a high power laser beam to fuse the metal
powder to a previously deposited layer, very unique material properties have been
obtained in a wide range of metallic structures. A partial list of the material that able
to be processed using the LENS™ method include: stainless steel, H-13 tool steel,
Ti-6A1-4V, Inconel 625, 690 and 718, and copper. A list of the tensile properties
obtained for three of these materials is included in Table 1. This data shows that the
strength of these materials can be significantly greater than that of the
conventionally processed and annealed materials. In addition, as shown in this data,
the increased strength can be accompanied by increased ductility as well. This is a
significant result. Typically, as the strength of a metal is increased through various
processes (i.e., heat treatment) the ductility will decrease. This effect is due
primarily to the reduced grain size obtained in metals using the LENS™ process.
Based on these results, as well as other experimental data, it is quite conceivable that
use of the LENS™ process will someday allow the properties within a monolithic
structure to tailored in a localized fashion.
For the LENS™ process any of the materials that are not consumed in the initial
deposition process are gathered and recycled for subsequent part building sequences.
Although there has not been a lot of effort focused on increasing the efficiency of
the deposition, preliminary material utilization studies have shown that any
of approximately 30% can be achieved with only a modest amount of laser power.
Some of the future efforts will focus on both increasing the process deposition
efficiency and also on producing a method to directly feed the unused materials back
to powder feed unit where they can automatically be used again. 14 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
Material Type Ultimate Yield Strength Elongation
Strength (ksi) (ksi) (% in one inch)
Optomec LENS™ 115 72 50
316 Stainless Steel
316 SS Anneal bar 85 35 50
Optomec LENS™ 135 84 38
Inconel 625
625 Annealed bar 121 58 30
Optomec LENS™ Ti-6A1-4V 170 155 11
TI-6AI-4V Annealed Bar 130 120 10
Table 1. Mechanical test data from LENS ™ manufactured tensile specimens
Figure 2. Triangular cross-section embedded cooling passage
Of course for every process, the real proof lies in seeing actual parts that may be
fabricated. A few sample components are included in Figures 3 and 4. In Figure.
3 (a), two parts of the same geometry are shown. The top part is a component that
was fabricated using the LENS™ process and is made of 316 stainless steel whereas LENS™: Beyond Rapid Prototyping to Direct Fabrication 15
the bottom part is the geometric shape but made using RP methods. The total
processing time for RP part is not known; however, the processing time to fabricate
the 316 stainless steel component was approximately 3 hours. In Figure 3 (b) the
same 316s steel part is shown after about 15 minutes of post process
finishing. The component shown in Figure 4 shows a more complex component that
was fabricated using the LENS™ process. Figure 4 (a) shows a piece of hardware as
it came out of the LENS™ system and Figure 4 (b) shows the same component after
it was hand finished. The final finished component is to be used in actual testing.
(a) (b)
Figure 3. Photographs of LENS™ fabricated engine components: (a) Unfinished
LENS™ fabricate d component shown with RP fabricated component, (b) same part
with some post process finishing performed on one side
As this technology moves from the National Laboratories to the Industrial
Sector, it is expected that many process improvements and innovations will ensue.
As demonstrated, this technology already has the capability to fabricate functional
metallic components directly from a CAD solid model with only a modest amount
of post-process finishing required. For tooling applications, the ability to embed
features such as conformai cooling passages is expected to significantly reduce cycle
time for injection molding of plastic components. This value added feature will
allow manufacturers to increase production without adding new tool sets. This is
exciting. Finally, as the LENS™ technology is further advanced beyond a single
material system to a system that has the ability to deposit multiple materials within a
given structure other application areas are expected to emerge. This technology truly
has the promise to revolutionize manufacturing and enable new classes of products
to be developed. 16 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
(a) (b)
Figure 4. Photographs of a more geometrically complex component (a) after LENS
fabrication with no post process finishing (b) samet with some post
process finishing Principles of Rapid Tooling Using
an Improved Thermal Spray Process
Guillaume Jandin — Hanlin Liao — Christian Coddet
Laboratoire d'Etudes et de Recherches sur les Matériaux
et les Propriétés de Surface (LERMPS), IPSé
BP 449, F-90010 Belfort cedex
christian, coddet@utbm.fr
ABSTRACT. For a number of years now, the direct manufacture of moulds is performed using
the thermal spray technique which allows to build directly metallic or ceramic shells on a
model. In principle, no intermediate step between the fabrication of the model and that of the
mould is needed and therefore fabrication limes are drastically shortened. Meanwhile, the
necessity of not damaging the model and to keep dimensional control requires to work near
the ambient temperature which generally leads to low mechanical properties of sprayed
shells, mainly due to high levels of porosity and oxides. Moreover, geometric distortions or
cracks can occur when residual stresses, related to spray conditions, are not controlled.
Therefore, a technique allowing to reduce these drawbacks has been developed. It is
presented here and some examples of actual realisations are given.
RÉSUMÉ. Cet article présente une description de la fabrication directe de moules par
l'utilisation du procédé de projection thermique. Cette technique permet la fabrication
directe de coquilles métalliques ou céramiques sur des modèles. En principe, aucune étape
intermédiaire n'est nécessaire entre la fabrication du modèle et celle du moule. C'est la
raison pour laquelle les délais den et la qualité de replication sont
considérablement améliorés. Cependant, la nécessité de ne pas endommager le modèle et de
garder un bon contrôle dimensionnel nécessitent de travailler à température proche de
l'ambiante. Ceci conduit généralement à de mauvaises propriétés mécaniques des coquilles
principalement à cause d'une forte proportion d'oxydes. De plus, des distorsions
géométriques et des fissures apparaissent lorsque les contraintes résiduelles, liées aux
conditions de projection, ne sont pas maîtrisées. Une technique permettant de résoudre ces
problèmes est présentée ainsi que des exemples de réalisations actuelles.
KEY WORDS: wire arc spray, rapid tooling, steel, residual stresses, Heatcool®.
MOTS-CLÉS: projection thermique à l'arc-fil, outillage rapide, acier, contraintes résiduelles,
Heatcool®.
Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998, pages 17 à 30 18 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
1. Introduction
1.1. Principle and objectives
Thermal spraying is a technique by which small molten material droplets are
deposited on a substrate. The successive deposition of these particles builds up a
coating. Once the proper thickness is reached, a shell is obtained after the removal
of the substrate. This shell can then be transformed into a mould.
Such a process drastically reduces times of mould production and allows to
produce pieces with acceptable dimensional reproduction.
1.2. State of Art
Several methods for rapid tooling already exist: lost wax casting, KelTool
process, ceramic paste agglomeration, rapid milling, etc., [STI 97]. Processes using
thermal spray systems are also known since a long time, with the use of low melting
temperature metals such as zinc or aluminium [MOG 63] or alloys. Meanwhile, the
performances of corresponding moulds, in terms of life cycle and wear resistance
are generally low. An improvement of the capabilities of moulds created by the
thermal spray route could be reached with the use of materials having higher
mechanical properties like steels but in that case structures often suffer of cracks or
distortions due to the build up of thermal stresses.
2. The Spray Forming Process
2.1. Thermal Spraying
Thermal spraying consists in processing materials through an enthalpic source
where they are partially or totally molten. A gas stream atomises and/or carry these
particles and give them velocity. Their flight is then interrupted when impacting a
substrate, where they rapidly cool and solidify to form a deposit (Figure 1 ). Deposits
are built up layer by layer and their properties depend on their composition and on
spray conditions. Nowadays, thermal spray processes are widely used to enhance
corrosion resistance, wear resistance, mechanical resistance, etc. of a large variety of
substrates.
Different spraying sources are available which can be divided into two main
categories: one for which energy comes from an electric discharge and the other one
for which energy comes from the combustion of a fuel. The first category comprises
the simple arc discharge (between two wires) and also the more sophisticated
plasma torches. The second category comprises the very simple flame gun (similar
to a welding torch) and the more efficient high velocity oxy-fuel systems (HVOF)
implementing a combustion chamber. RapidTooling through Thermal Spray Process 19
Construction of tooling implies the spraying of large quantities of material on a
very economical basis and therefore requires an efficient and low cost system. Thus,
the wire-arc system seems to be the most adapted in this case.
2.2. The Wire-Arc Process
In this process, two consumable wires are fed automatically to meet at a point in
an atomising gas stream. An electrical potential difference (18-40 Volts), with
amperage 35 to 150 Amps, is applied across the wire electrodes and melts the tips of
both wires. The atomising gas, often compressed air, is directed across the arc zone,
shearing the molten part of the wires to form the atomised spray, [WAN 94]
(Figure 1).
The arc process has generally higher spray rates than other spray processes
reaching easily 15 kg/hr. This range of spray rate mostly depends on the nature of
the sprayed material and wire diameter. Thick coatings can rapidly and
economically be built up, rending this process is a good issue for moulds spray
forming [THO 93], [FUS94]. Materials that can be sprayed are metallic wires (iron,
copper, zinc,...) or cored wires containing powders (for example WC/Co) in order to
produce composite materials.
Electric Arc Contact Tip
Metal Wire
Spray Gas\
Metal Wire
Spray Plume
Substrate Spray Nozzle
Figure 1. Wire-Arc spray process
2.3. Moulds Spray Forming
Three main steps are involved in this process. The first one is the spray
deposition of a thick coating on a prototype model. The second one is the separation
between the so builtg and the model. The last step concerns the fabrication of
a complete mould incorporating the sprayed shell. The success of the spray 20 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
deposition step depends mainly on the following constraints: the robotic
programming of the movements of the spray gun in order to ensure a good
replication of the model; the control of the residual stresses within the deposit in
order to keep the accuracy of the shape, which depends mainly on the spray
parameters and finally the separation of the deposit from the core.
2.3.1. Shape replication
Spray deposition is an incremental process with successive layers of deposit
creating the coating. One of the main parameters to reach during this step is an
homogeneous thickness repartition of the deposited material all over the shape
together with a sufficient toughness of the material for supporting mould production
conditions (high pressures), (Figure 2).
Spray Gun
Coating
Model
Figure 2. Spra y deposition onto a model
In this work, the first step was optimised with a computer simulation of the
deposition process onto the model. A software development [BON 99] permits to
create programs for building up coatings with an homogeneous material thickness
repartition, even though models that have to be coated have complex geometries.
The software can read I.G.E.S., Model and S.T.L. files giving the precise shape of
the mandrel. Before spraying the software permits to simulate the coating thickness
repartition with various spray and robotic parameters. When optimisation is reached,
a directly usable robotic program is issued and transferred to the robot controller.
2.3.2. Residual Stresses
In order to keep a good accuracy of the shape, a control of residual stresses
within coatings must be achieved. Such stresses have various origins [GIL 91] but
they can be categorised into three groups: quenching stresses, differential
contraction stresses and stresses produced by solid state transformations. RapidTooling through Thermal Spray Process 21
Solid state transformations appear only when phase changes occur during the
cooling down of the system.
The differential thermal contraction effect is due to differences between the
coefficient of thermal expansion of the substrate and that of the sprayed material.
Quenching stresses are produced when splats are cooled from their melting point
to the substrate temperature when impacting onto the substrate. The importance of
this stress factor is generally higher than that of the two other ones and is often
greater than the uniaxial yield stress of most materials; so mechanisms of stress
relaxation are activated. In the case of wire arc spray onto models having complex
geometries, the most frequent mechanisms are cracking and edge relaxation
illustrated by coating lifting up from the substrate (Figure 3).
Spray Gun
Coating
Model
Figure 3. Coating lifting up due to residual stresses
2.3.3. Core Removal
Once proper thickness has been reached, the core has to be separated from the
sprayed shell. Various techniques can be used, most of them being destructive ones.
These techniques can be divided into two groups: mechanical removal techniques
and methods transforming the material of the core into a liquid or a gas by
dissolving or burning the model. Mechanical methods are conventional or ultrasonic
machining or sand blasting. The choice of one of those depends on the composition
of the material of the model and also of that of the coating.
Another technique consisting in building a thin soluble interface between the
core and the deposit can be used for preserving the model from damages. It can be
easily observed that this technique has some limitations mainly due to geometric
situations or to the fact that the model is porous or not. 22 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
3. The HeatCool® process
In order to minimise tensile stresses different methods have been proposed in the
literature such as shot peening or high pressure sintering [FUS 94]. When such
processes are well controlled, interesting results are obtained but the corresponding
equipments are not easy to use. Therefore, a new process based on the thermal
control of the system was developed. This process, named HeatCool®, is described
below [COD 96].
3.1. Principle
The HeatCool® process comprises two simultaneous steps in addition to the
deposition step:
- In the first step, a surface pre-heating occurs immediately before spray
deposition . This preheating activates the surface and permits to improve the
interlamellar bonding [MEL 94] and to reduce quench stresses. When thermal spray
is made without such a control, interlamellar delaminations are present. Several
tools may be used for pre-heating the substrate such as oxy-acetylenic flame, laser,
or induction coil.
- In the second step, immediately after the deposition of the layer in progress,
cryogenic cooling is performed in order to limit thermal gradients [CHA 95]. This
cooling is made using a gas, C0 for example, in the liquid state. Nozzles are placed
2
so as to make the extremity of the C0 liquid gas plume to meet the end of the spray 2
plume: the coating is then immediately cooled and thermal stresses are reduced, the
whole system being kept near the ambient temperature.
3.2. Experimental set up
In this work, the wire arc spray gun, a TAFA 9000, was fixed onto a 6-Axis
ABB IRB2400 Robot.
High carbon (0.80C) steel wires were used as sprayed material.
In order to determine the structure of the deposits and the level of residual
stresses, thin steel plate substrates (80 x 20 x 1 mm) were fixed onto a rotating
wheel. (Figures 4 and 5) and sprayed. RapidTooling through Thermal Spray Process 23
Substrat e
Coolin g
Gu n
Heating ^
Gun M
Spray Gun
Figure 4. Experimental arrangement fo r HeatCool® process
Bending of the
Plate
Plate & Plate + Coating
Its Mounting & Mounting
Figure 5. Bending of plates during / after coating deposition
From the bending of these plates under the influence of stresses, it is possible to
determine the level of stresses within the coating. [MEL 94], [RIL 95]. 24 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
4. Experimental results
4.1. Structure of Deposits
As indicated before, the oxide content in the sprayed metal is an important
feature which governs the mechanical properties of the deposit. Therefore
experiments have been first carried out to determine thermal spray parameters that
could contribute to reduce the oxide content of the coatings. The following results
(Table 1 ) show that the oxide rate can be increased or decreased according to the
spraying gas rate and nature.
Ref. Amperage Primary Secondary Oxide
Spray Spray Content
Pressure Pressure %
A-l 100 A 350 kPa 275 kPa 18%
±1%
A-2 50 A 350 kPa 480 kPa 35%
±2%
N-l 100 A 275 kPa 350 kPa 14%
±1%
N-2 50 A 275 kPa 350 kPa 20%
±1%
Table 1. Spray experiments fo r optimising the oxide rate in the deposit
The gas flow which varies according to the gas pressure determines the inflight
speed of atomised particles. The Arc Jet® system is a secondary gas flow on the
TAFA 9000 Arc Spray Gun which reduces the plume diameter giving a less
diverging particles flow.
Most of time, for economical purpose, the spraying gas is air. In that case,
deposits have a classical lamellar structure showing the presence of fairly large
amounts of oxides.
In the Figures 6 and 7, the darkest zones are porosities (often associated to
unmolten particle or oxide tearings when polishing), the grey ones are oxides and
the white parts are steel.
Of course, the oxide rate can be reduced by using nitrogen as atomising gas as
shown in Figure 8. RapidTooling through Thermal Spray Process
Figure 6. Carbon steel deposit with about 18% oxide content (réf. A-l)
Figure 7. Carbon steel deposit with about 35% oxide content (refA-2)
Figure 8. Carbon steel deposit with about 14% oxide content (refN-1) 26 Revue de CFAO et d'informatique graphique. Volume 13 - n° 4-5-6/1998
4.2. Stress measurements
Measurements of the bending of steel plates specimens after spraying have been
achieved with a comparator. Plate deformations are given in millimetres in Table 2.
It clearly appears that the HeatCool® process permits to reduce notably residual
stresses within coatings. Of course, final result will depend on the setting up of the
parameters for the process.
Implementing Without
HeatCool® HeatCool®
Average 0.690 ±0.02 0.971 ±0.02
Max. Value 1.014 ±0.05 1.283 ±0.05
Min. Value 0.405 ±0.05 0.780 ±0.05
Table 2. Bending Measurements on sprayed steel plates (average of ten values
in mm)
4.3. Examples of Tooling
The previously described process was used here to spray steel onto different
models in order to obtain durable mould surfaces. Experiments have been conducted
onto polymer substrates such as SLA models in order to really achieve time
compression on the realisation of functional moulds
Figure 9 displays a made of steel with approximate dimensions 300x150x
100 mm. This shell has a thickness of 2-2.5 mm. Processing time was about
1.5 hour. The actual spray time was about 0.3 hours (the weight of the part being
0.4 kg) while the preliminary preparation step including robot programming took
about 1 day. Figure 10 shows the as-sprayed surface quality of the mould, the
average roughness (Ra) of which is about 5 um (±0.3um). RapidTooling through Thermal Spray Process 27
Figure 9. Upper butter dish sprayed shell
Figure 10. As-sprayed shell
Figure 11 shows a soapdish (dimensions 120x80x25 mm) placed in a frame to
obtain the closing faces of the mould. The roughness (Ra) has been established to be
about 6um (±0.3um)