Robocasting - Direct Ink Writing

Gate Keeper Prof. Dr. Pedro Miranda University of Extremadura (UEx) Department of Mechanical, Energy and Materials Engineering. Industrial Engineering School. Avda. de Elvas s/n. 06006 Badajoz (SPAIN) T: + 34 924 289 300 (Ext. 86735) F: + 49 924 289 601 E-Mail: pmiranda@unex.es

Robocasting (RC) / Direct Ink Writing (DIW)

Robocasting (RC), also known as Direct Ink Writing (DIW) and, less frequently, as Direct-Write Assembly (DWA) or Microrobotic Deposition (µRD) is an additive manufacturing technology based on the direct extrusion of slurry-based inks. Robocasting was the term used by the original inventors of the technique and has found special echo in the ceramic AM community due to its single-word simplicity and resemblance to other slurry-based conventional ceramic processing methods such as slip-casting, tape-casting, etc. The technique was first developed at Sandia National Laboratories in 1996 as a method for Freeforming Objects with Low-Binder Slurry (U.S. Patent 6,027,326) [1-2]. In more detail, the technique consists of the robotic deposition of highly concentrated (~35-50 vol.%) colloidal suspensions of ceramic powders in water (typically), which are capable of supporting their own weight during assembly, thanks to their carefully tailored composition and viscoelastic properties. An ink with appropriate rheological properties for robocasting must be able to ow through the nozzle (i.e. possess relatively low viscosity under stress) and have excellent shape retention capacity (i.e. high elastic/storage modulus, and high yield stress) upon deposition. Because of this shear thinning rheological behavior the ink behaves as a pseudo-solid after deposition and shape retention does not rely, on the solidification or drying of the feedstock as in other techniques (e.g. fused deposition modelling).

Figure 1. Schematic illustration and in-situ images of the robocasting process within an oil bath

The preparation of colloidal inks for robocasting usually include two steps: (i) the preparation of a low viscosity, yet concentrated suspension using, typically, anionic polyelectrolytes as dispersing agents; (ii) the induction of a drastic rheological change to turn the suspension into a pasty-like system with tuned elastic properties through a pH or temperature change, or the addition of salts or cationic additives [3,4].

After the desired rheological behavior is achieved, the ink is housed in a syringe and extruded normally through a conical/cylindrical nozzle (inner diameter, d typically ≥ 100 μm) by the computer-controlled robotic deposition system (Fig. 1). The position of the nozzle is moved following the CAD model designed previously in the control software to build line-wise and layer by-layer the desired part. The ink flows through the nozzle at the volumetric flow rate required to maintain a constant linear deposition speed typically between 1-100 mm s−1. Lower speeds usually yield better shape tolerances at the expense of increased printing time. Line separation, s, and layer height, h, (Fig. 1) as well as raster pattern are also critical parameters in determining the microstructure and properties of the printed ceramic part [5]. When appropriately done, no interlayer defects are formed and interfaces between individual layers are indistinquishable in the microstructure. To prevent a non-uniform drying of the structure during assembly, the deposition process can be carried out within a paraffin oil reservoir (Fig. 1) or in humid environment, although direct printing in air is definitely possible and even advisable in some cases when printing bulk parts [5]. After assembly, the green sample are typically retrieved and dried at ambient conditions for 24 h and thermal treated at the appropriate conditions for debinding and sintering of the ceramic part.  

Lack of a powder bed can be counted as one of the advantages of this technique, as it allows one to build 3D structures using minimal amounts of material, minimizing waste and avoiding the need of implementing additional processes for excess powder re-use. This is especially useful when printing costly and rare materials as those under development in research activities. Nonetheless, the lack of a supporting powder bed implies that when printing complex parts with large overhangs or internal gaps by robocasting, a secondary support structure has to be built with a sacrificial material (Fig. 2). This is also the case for other extrusion-based AM techniques such as FFF/FDM. Graphite-based inks, among other options, have been proposed as suitable alternatives for this purpose, since the carbon can be readily eliminated by a thermal treatment at 800 ºC, well below typical sintering temperatures for ceramics [6]. Printing with such support structures requires the use of dual printing heads.

Figure 2. Robocasting complex structures with overhangs require co-printing a fugitive ink (with a dual-head) to generate a support structure that is eliminated during heat treatment.

Another advantage of the use of a ceramic high solid loading suspension for additive manufacturing is its potential to produce bulk samples with high initial green densities, typically exceeding 50 % of the theoretical density of the material. For these dense parts, it is also possible to increase the green density even further through the use cold isostatic pressing [7].

Beside the production of bulk ceramic parts, the high storage modulus of robocasting inks can span gaps in the underlying layers of many times the filament diameter in length. This makes it a especially useful technique for the fabrication of porous structures, which have been extensively used as catalyst supports, filters, acoustic metamaterials, photonic crystals, and scaffolds for tissue engineering and other biomedical applications [8-10]. Advanced load bearing composites can also be formed by infiltrating robocast porous structures with molten polymers, glasses or metals [11].

References

1.   Cesarano, J., III; Calvert, P. D. (Sandia National Laboratories). Freeforming Objects with Low-Binder Slurry. U.S. Patent 6,027,326, 1997.
2.   Smay, James E.; Cesarano, Joseph; Lewis, Jennifer A. (2002). "Colloidal Inks for Directed Assembly of 3-D Periodic Structures". Langmuir. 18 (14): 5429–5437.
3.   Miranda  P,  Saiz  E,  Gryn  K,  Tomsia  AP.  Sintering  and  robocasting  of  beta-tricalcium phosphate scaffolds for orthopaedic applications. Acta Biomater 2006, 2, 457–66.
4.   Eqtesadi S, Motealleh A, Miranda P, Lemos A, Rebelo A, Ferreira JMF. A simple recipe for direct writing complex 45S5 Bioglass-® 3D scaffolds. Mater Lett 2013, 93, 68–71.
5.   Feilden, E., Blanca, E. G.-T., Giuliani, F., Saiz, E., Vandeperre, L. J. Robocasting of structural ceramic parts with hydrogel inks. J. Eur. Ceram. Soc. 2016, 36, 2525–2533.
6.   Martínez-Vázquez, F.J,  Pajares A., Miranda P., A simple graphite-based support material for robocasting of ceramic parts. J. Eur. Ceram. Soc. In press.  https://doi.org/10.1016/j.jeurceramsoc.2017.10.016 

7.   Eqtesadi S, Motealleh A, Perera F.H., Miranda P, Pajares A., Wendelbo R, Guiberteau F., Ortiz A.L. Fabricating geometrically-complex B4C ceramic components by robocasting and pressureless spark plasma sintering. Scripta Materialia 2018,145, 14–18.
8.   Tubío C.R., Azuaje J., Escalante L., Coelho A., Guitián F., Sotelo E., Gil A., 3D printing of a heterogeneous copper-based catalyst, J. Catalysis 2016, 334, 110-115.
9.   Kruisová A., Seiner H., Sedlák P., Landa M., Román-Manso B., Miranzo P., Belmonte M., Metamaterial behavior of three-dimensional periodic architectures assembled by robocasting, Appl. Phys. Lett. 105 (2014) 211904.
10. Miranda P, Saiz E, Gryn K, Tomsia AP. Sintering and robocasting of beta-tricalcium phosphate scaffolds for orthopaedic applications. Acta Biomater 2006, 2, 457–466.
11. Martinez-Vazquez FJ, Perera FH, Miranda P, Pajares A, Guiberteau F. Improving the compressive strength of bioceramic robocast scaffolds by polymer infiltration. Acta Biomater 2010, 6, 4361–8.

Examples

Date: 13.12.2017

Made by: UEx
Material: Tricalcium Phosphate (TCP)
Description: Rhesus monkey mandibular scaffolds, CAD models and sintered samples

Date: 13.12.2017

Made by: UEx
Material: Boron carbide (B4C)
Description: As dried

Date: 13.12.2017

Made by: UEx
Material: Hydroxyapatite (HAp)
Description: Scaffolds for human mandibular defect reconstruction, as sintered