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Development of Build Strate-gies for Droplet-based Additive Manufacturing

MiMed | Andreas Schröffer (M.Sc.) | Development of Build Strategies for Droplet-based Additive Manufacturing 2

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

Outline

1. Institute of Micro Technology and Medical Device Technology (MiMed)

Institute ofMicro Technology and Medical Device TechnologyTechnical University of MunichDepartment of Mechanical Engineering85748 Garching

Department organisationProf. Dr. rer. nat. Tim C. Lüth16 scientific staff7 non-scientific staff

Quality managementCertified to ISO9001 and ISO 13485

Fields of research

Medical robotics Kinematics

Micro technologyAgeTech

pF


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

Uni

t cos

ts

Complexity/individualisation

Suitable for medium volume andhigh-volume production

(Zäh, 2006):

Increasing relevance of additive manufacturing

© ARBURG

Uni

t cos

ts

Unit volume

Small-volume batches: up to 20

Medium-volume batches: up to 1000

High-volume batches: over 1000

Individualised parts increasingly require:• Production of small-volume batches• Prototypes• Web configuration option


State-of-the-art

For current technologies the build material often needs to be specially developed for the relevant process

ARBURG is taking the approach of making commercial standard granulates used in injection mouldingsuitable for additive manufacturing as well

Restricted choice of materials and poorer component properties

Current development: semi-crystalline polymers



Semi-crystalline plastics

• Approx. 50 % of plastic production in Germany in 2015 (Consultic, 2015)

• Generally higher rigidity and temperature-resistance 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)


ARBURG freeformer

Operating principle of droplet generator (according to Hehl, 2010)

• 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

• Piezo frequency: up to 200 Hz

Standard granulate

Plasticisingunit

Nozzle actuator

Part carrier


• A process-specific NC program (GCode) must be created to control the machineindividually for each part

• The GCode can be generated automatically from the CAD data for a part


Workflow in use

Design Data slicing Production

STL GCode

Data slicing


Data slicing and build strategies

(according to Schwaiger, 2014)

GCodeG01 X1 Y2G01 X1 Y2

….

CreatingGCode

STL file Slicing Defining build strategyfor contours and filling

CreatingGCode



Influencing factors and optimisation objectives of build strategies

Material requirements

Build time

SurfaceStrength

Dimensional accuracy

…

• Discharge sequence• Material• Build strategy

• Material• Filling level• Filling direction

• Filling level

Build strategy

• Dry runs• Discharge sequence

• Edge contour• Overlap of filling

and contour


Material requirements and build time

Grid and honeycomb structures

Build strategies for support structures

Filling strategiesDischarge sequence

Reduction of dry runs

Minimising travel distance

(Schwaiger, 2014)


• Uneven cooling• Molecular orientation of polymer chains• Uneven crystallisation

(Domininghaus et al., 2012, Baur et al., 2013)

Special feature of additive manufacturing:Time-staggered formation (layer build-up).


Causes

Principle of origin of the “curling” effect (Gebhardt, 2016; Fahad and Hopkinson, 2016)

Material application

Shrinkage rate

Curling

Component layers

Tensile rod, PA6 Grilon F50, “curling effect”

Internal stresses result in component distortion (Held, 2009; Osswald, 2011)

• Reduced cooling rate due to build chamberand base plate heating More even cooling (Gebhardt, 2014)

• Optimised layer structure for reducingthermal gradients Less internal stress (Catchpole-Smith et al., 2017)

• Distribution of the heat input during the build process


Solutions

“Hexagonal”

"Line jump"21



Curling study

“Hexagonal”

“Line jump”

“Standard”

h1 h2d

2 “Curling factor”Reduction of distortion by approx. 50%

0 mm

1 mm

2 mm

3 mm

4 mm

5 mm

6 mm

Standard Hexagonal Liniensprung

Curling depending on build strategy


Shrinkage model

Model building Level of abstraction

Heat transitions• Droplet – Construction chamber• Droplet – Platform• Droplet – Droplet

Rate of shrinkage• Coefficient of shrinkage• Gradient of temperature• (Virtual) deformation

Tensions• E-Module• Resulting tensions

Time t

Time t+1

Time t+n



Shrinkage model

Free contraction of volumeEven in all directions

Limited contraction of volumeCaused 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 counteractthe shrinkage of parts


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 HanserVerlag, 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 2015 - Short Version)

Baur, E., Brinkmann, S., Osswald, T. A. & Schmachtenberg, E. (2013): Saechtling Kunststoff Taschenbuch. 30 Ed. Munich: Carl HanserVerlag Munich.

Hehl K. (2010): "Vorrichtung zur Herstellung eines dreidimensionalen Gegenstandes" (Device for producing a three-dimensional object). German Patent DE 102009030099B4.

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.

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