Development of Build Strategies for Droplet-based Additive

Transcription

1 Development of Build Strategies for Droplet-based Additive Manufacturing

2 Outline 1. Institute of Micro Technology and Medical Device Technology 2. Motivation for additive manufacturing 3. Plastics in additive manufacturing 4. Droplet-based additive manufacturing 5. Process optimisations 6. Shrinkage and distortion 7. Summary 1. Institute of Micro Technology and Medical Device Technology (MiMed) Institute of Micro Technology and Medical Device Technology Technical University of Munich Department of Mechanical Engineering Garching Fields of research Medical robotics Kinematics Department organisation Prof. Dr. rer. nat. Tim C. Lüth 16 scientific staff 7 non-scientific staff p F Quality management Certified to ISO9001 and ISO AgeTech Micro technology Droplet-based Additive Manufacturing 2

3 2. Motivation for additive manufacturing Most of the manufacturing of plastic parts involves injection moulding and extrusion (approx. 80%) (Johannaber, 2001) The effort and cost associated with injection moulding increase with: The complexity of the part Lower unit volumes Suitable for medium volume and high-volume production Individualised parts increasingly require: Production of small-volume batches Prototypes Web configuration option (Zäh, 2006): Increasing relevance of additive manufacturing Unit costs Complexity/ individualisation Unit costs ARBURG Unit volume Small-volume batches: up to 20 Medium-volume batches: up to 1000 High-volume batches: over Plastics in additive manufacturing State-of-the-art For current technologies the build material often needs to be specially developed for the relevant process Restricted choice of materials and poorer component properties ARBURG is taking the approach of making commercial standard granulates used in injection moulding suitable for additive manufacturing as well Current development: semi-crystalline polymers Droplet-based Additive Manufacturing 3

4 3. Plastics in additive manufacturing Semi-crystalline plastics Approx. 50 % of plastic production in Germany in 2015 (Consultic, 2015) Generally higher rigidity and temperatureresistance than amorphous polymers Formation of crystalline structures Increased volume shrinkage, because crystalline areas have a higher density Subsequent shrinkage due to subsequent crystallisation (depending on time and temperature) (Baur et al., 2013 ) Arrangement of polymer chains of amorphous (left) and semi-crystalline (right) plastics with crystal structures shown in red (according to Baur et al., 2007; Osswald, 2011) 4. Droplet-based additive manufacturing ARBURG freeformer Standard granulate Melting of standard granulates in a plasticising unit Singulation of plastic droplets by means of a nozzle actuator Discharge of discrete droplets onto the moving part carrier Build chamber temperature: up to 120 C Nozzle actuator Plasticising unit Piezo frequency: up to 200 Hz Part carrier Operating principle of droplet generator (according to Hehl, 2010) Droplet-based Additive Manufacturing 4

5 4. Droplet-based additive manufacturing Workflow in use STL GCode Design Data slicing Production Data slicing A process-specific NC program (GCode) must be created to control the machine individually for each part The GCode can be generated automatically from the CAD data for a part 4. Droplet-based additive manufacturing Data slicing and build strategies GCode G01 X1 Y2 G01 X1 Y2. STL file Slicing Defining build strategy for contours and filling Creating GCode Creating GCode (according to Schwaiger, 2014) Droplet-based Additive Manufacturing 5

6 5. Process optimisations Influencing factors and optimisation objectives of build strategies Build time Dry runs Discharge sequence Build strategy Material Filling requirements level Surface Edge contour Overlap of filling and contour Strength Material Filling level Filling direction Dimensional Discharge accuracy sequence Material Build strategy 5. Process optimisations Material requirements and build time Discharge sequence Filling strategies Reduction of dry runs Minimising travel distance (Schwaiger, 2014) Grid and honeycomb structures Build strategies for support structures Droplet-based Additive Manufacturing 6

7 6. Shrinkage and distortion Causes Uneven cooling Molecular orientation of polymer chains Uneven crystallisation (Domininghaus et al., 2012, Baur et al., 2013) Shrinkage rate Material application Component layers Special feature of additive manufacturing: Time-staggered formation (layer build-up). Internal stresses result in component distortion (Held, 2009; Osswald, 2011) Curling Principle of origin of the curling effect (Gebhardt, 2016; Fahad and Hopkinson, 2016) Tensile rod, PA6 Grilon F50, curling effect 6. Shrinkage and distortion Solutions Reduced cooling rate due to build chamber and base plate heating More even cooling (Gebhardt, 2014) Optimised layer structure for reducing thermal gradients Less internal stress (Catchpole-Smith et al., 2017) Distribution of the heat input during the build process 1 2 Hexagonal "Line jump" Droplet-based Additive Manufacturing 7

8 6. Shrinkage and distortion Curling study Standard Curling depending on build strategy 6 mm 5 mm Hexagonal 4 mm 3 mm Line jump 2 mm 1 mm 0 mm Standard Hexagonal Liniensprung h 1 h 2 d Reduction of distortion by approx. 50% 2 Curling factor 6. Shrinkage and distortion Shrinkage model Model building Time t Level of abstraction Heat transitions Droplet Construction chamber Droplet Platform Droplet Droplet Time t+1 Rate of shrinkage Coefficient of shrinkage Gradient of temperature (Virtual) deformation Time t+n Tensions E-Module Resulting tensions Droplet-based Additive Manufacturing 8

9 6. Shrinkage and distortion Shrinkage model Free contraction of volume Even in all directions Limited contraction of volume Caused by connection to platform / neighbouring droplet Cable tensions 7. Summary Droplet-based additive manufacturing offers a cost-effective alternative to injection moulding, especially for the production of prototypes Data slicing and build strategies have an enormous influence on the build process and achievable part properties Optimisation and application options with regard to dimensional accuracy, surface quality, strength and production costs Using a shrinkage model it shall be possible to calculate and counteract the shrinkage of parts Droplet-based Additive Manufacturing 9

10 Thank you very much for your attention References: Johannaber F.,Michaeli W. (2001): Handbuch Spritzgießen (Handbook of Injection Moulding), 1st Edition, Carl Hanser Verlag, Munich. Zäh M. F. (2006): Wirschaftliche Fertigung mit Rapid Technologien (Cost-effective Manufacturing with Rapid Technologies). Carl Hanser Verlag, Munich Consultic (2015): Studie zu Produktion, Verarbeitung und Verwertung von Kunststoffen in Deutschland 2015 Kurzfassung (Study on the Production, Processing and Recycling of Plastics in Germany Short Version) Baur, E., Brinkmann, S., Osswald, T. A. & Schmachtenberg, E. (2013): Saechtling Kunststoff Taschenbuch. 30 Ed. Munich: Carl Hanser Verlag Munich. Hehl K. (2010): "Vorrichtung zur Herstellung eines dreidimensionalen Gegenstandes" (Device for producing a three-dimensional object). German Patent DE B4. Schwaiger J. (2014): "GCode Generierung für einen neuen 3D-Druckprozess auf Tropfenbasis. (GCode Generation for a new 3D Printing Process based on Droplets)." Dissertation. Technical University of Munich Prša J., J. Schwaiger J., Irlinger F., T. C. Lüth (2013): Dense 3D- packing algorithm for filling the offset contours of a new printing process based on 3D plastic droplet generation. Proceeding of the IEEE International Conference on Robotics and Biomimetics (ROBIO) Prša J., Irlinger F., T. C. Lüth (2014): Algorithm for Detecting and solving the problem of under-filled pointed ends based on 3D printing droplet generation. Proceedings of the ASME 2014 International Mechanical Engineering Congress & Exposition Domininghaus, H., Elsner, P., Eyerer, P., Hirth, T. (2012): Kunststoffe. 8. Ed. s.l.:springer Verlag Held, M.; Pfligersdorffer, C. (2009): Correcting warpage of laser-sintered parts by means of a surface-based inverse deformation algorithm. Osswald, T. A. (2011): Understanding polymer processing. Processes and governing equations. Munich: Hanser. Gebhardt, A., Hötter, J.-S. (2016): Additive Manufacturing. 3D printing for prototyping and manufacturing. Munich, Cincinnati, OH: Carl Hanser Fachbuchverlag Catchpole-Smith, S., Aboulkhair, N., Parry, L., Tuck, C., Ashcroft, I. A., Clare, A. (2017): Fractal scan strategies for selective laser melting of 'unweldable nickel' superalloys. In: Additive Manufacturing. Droplet-based Additive Manufacturing 10