Printing direction dependent microstructures in direct ink writing
Graphical abstract
Introduction
Direct ink writing (DIW) is an additive manufacturing technique that can produce complex geometries across a broad range of compositions and microstructures, including ceramics [1], [2], hydrogel cell scaffolds [3], [4], [5], conductive alloys [6], [7], and polymer-ceramic composites [8], [9]. Especially for two-phase or composite materials, spatial gradients in structural and functional properties can be integrated with geometric features via techniques which control microstructures “on-the-fly” during printing such as acoustophoresis [10], [11], [12], [13], [14], magnetic fields [15], and multimaterial mixing [16], [17], [18]. Advancing microstructural control requires a fundamental understanding of processing-structure relationships in DIW of multi-phase materials. One of the most critical processing-structure relationships is between printing direction and the resultant filament microstructure.
One of the central challenges of controlling microstructure during DIW is the presence of complex flow fields that connect the particle distribution in the nozzle to the distribution in the deposited line. Here, acoustic focusing is used to establish a narrow, anisotropic particle distribution in the nozzle (Fig. 1C). Critically, flows transverse to the printing direction on the substrate may disrupt this “target” microstructure and influence the final distribution of particles in the printed filament. These flows can cause narrow particle distributions established inside the nozzle to shift within the print path or widen as the filament exits the nozzle (Fig. 1A and B). Thus, if these flows depend on print direction, they could cause variations in properties throughout a printed part. For example, when using DIW with acoustophoresis to write carbon fibers in an insulating polymer matrix [12], direction dependent flows could cause variations in conductivity throughout the part. Alternatively, direction dependence could cause different sides of a printed SiC fiber-epoxy polygonal prism to have different stiffnesses [10]. In this work, we measure transverse flow velocities on the substrate experimentally, using inks with suspended particles and in situ optical imaging to track fluid movement. By writing single-layer, three-pass polygons, we probe various boundary conditions and print directions.
Several effects lead to print direction dependent microstructures. In this study, square nozzles are used because they enable effective acoustic control over microstructures in the nozzle [10], [11], [12]. While conceptually such anisotropic nozzles can be rotated to preserve alignment with the print path, doing so is a complicated controls problem. By investigating the mechanisms through which this misalignment influences filament microstructure, we highlight more elegant strategies for achieving microstructural control in DIW. A yield stress support material, implemented in a layer-by-layer or bath geometry, introduces a second source of direction dependent microstructures (Fig. 1D). Additionally, calibration of the stage introduces direction dependence. Understanding all of these sources is critical for controlling the microstructure of printed parts.
To diagnose sources of print direction dependent flows and particle distributions, we postulate several physical mechanisms and propose corresponding idealized analytical models of the flow patterns as a function of printing direction. To illustrate how to use the models and identify strategies for controlling the mechanisms, we fit these models to experimental observations. This identifies dominant mechanisms within our printing system and more general guidelines for selecting printing parameters which mitigate or accentuate direction dependence. Briefly, the mechanisms considered here are: (1) misalignment of the acoustic focusing direction and print direction, which alters the orientation of particle packing, (2) flows in a zone around the nozzle that disturb flows parallel to the printing direction (i.e. the disturbed zone), (3) reshaping of the fluid column exiting the nozzle as it is driven to lie flat on the substrate with a symmetrical fluid surface, (4) overlaps between the written line and existing features, and (5) motor errors which misalign designed and written paths. The models that describe these effects assume idealized flows and assume that changes in particle distributions result directly from transverse flows within the filament.
This work is part of a group of three papers which consider different aspects of the same data set. Ref. [19] considers printing direction-independent effects which manifest in straight lines. This paper considers direction-dependent effects in straight lines. Ref. [20] considers changes in microstructure at printed corners.
Section snippets
Hypotheses
We propose six effects which contribute to printing direction dependent microstructures (Table 1). For each effect, we formulate a dependence of either particle distribution position and transverse flow velocity or particle distribution width on the printing direction ϕ. A linear superposition of these models is used to fit experimental data and thus diagnose sources of print direction dependent microstructures and flows. Microstructure anisotropy involves a rotationally asymmetric distribution
Experimental approach
This paper draws from the same data set that is used in Refs. [19], [20], but this work focuses specifically on direction-dependent behaviors. Data and code can be found at Ref. [22].
Inks consisted of diurethane dimethacrylate (UDMA) (Sigma Aldrich, mixture of isomers with topanol inhibitor), triethylene glycol dimethacrylate (TEGDMA) (Sigma Aldrich, with MEHQ inhibitor), fumed silica (Evonik Aerosil R106), camphorquinone (CQ) (Sigma Aldrich), and 2-(Dimethylaminoethylmethacrylate) (DMAEMA)
Particle distribution width
Digital image analysis of in situ videos of the printed line just behind the nozzle indicates that the width of the distribution of particles in the printed line varies as a function of the print direction. Focusing anisotropy predicts that the distribution width should vary as a function of print direction because the distribution of particles in the nozzle is rectangular, and the width of the distribution in the line will be the projection of the rectangle onto the cross-section of the print
Discussion
The experimental particle distribution width in layer-by-layer support has a stronger dependence on the printing direction ϕ than the distribution in bath support. Comparing the experimental distribution width to the theoretical width, the shape of the printing direction-width profile indicates that the distribution of particles in the nozzle is rectangular rather than square, with a large aspect ratio. Bath support has a smaller dependence on printing direction than layer-by-layer support
Conclusions
Herein we proposed an analytical model that describes printing direction-dependent effects on the flow field near the nozzle and the microstructure of the printed line. Using acoustophoresis, we established a narrow distribution of particles in the nozzle which changes upon exiting the nozzle. By tracking fluid flows transverse to the printing direction and measuring the width and peak position of the particle distribution in the printed line, we determined that the proposed model predicts many
Authors’ contribution
Leanne Friedrich: conceptualization, methodology, software, formal analysis, investigation, data curation, writing – original draft, visualization. Matthew Begley: conceptualization, writing – review & editing, supervision.
Conflict of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Institute for Collaborative Biotechnologies through contract no. W911NF-09-D-0001 from the U.S. Army Research Office and a UCSB Chancellor's Fellowship. The work used the Microfluidics Laboratory at the California Nanosystems Institute and the Polymer Characterization Facility supported by the MRSEC Program of the National Science Foundation under award NSF DMR 1121053 at UCSB.
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