Fine lattice structures fabricated by extrusion freeforming: Process variables

Journal of Materials Processing Technology 209 (2009) 4654–4661

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Fine lattice structures fabricated by extrusion freeforming: Process variables Xuesong Lu a , Yoonjae Lee b , Shoufeng Yang c , Yang Hao b , Julian R.G. Evans a,∗ , Clive G. Parini b a

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK Department of Electronic Engineering, Queen Mary, University of London, Mile End Road, London E1 4NS, UK c Department of Materials, Queen Mary, University of London, Mile End Road, London E1 4NS, UK b

a r t i c l e

i n f o

Article history: Received 10 May 2008 Received in revised form 18 November 2008 Accepted 21 November 2008 Keywords: Extrusion freeforming Paste Lattice Defects

a b s t r a c t This paper describes the main factors affecting the rapid prototyping of fine lattices by extrusion freeforming of powder, notably equipment accuracy, paste preparation, extrusion and post-processing and their effects on filament deposition and the defects that might be caused. Effective methods were devised in order to reduce the incidence of these defects. The results provide guidance for fabrication of very fine lattices from powder extrusion (comprising <100 ␮m diameter filaments) and improvement of sample quality. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Extrusion freeforming, which involves the direct assembly of filamentary lattices by downloading a computer design file, can be used for fabrication of hard tissue scaffolds (Gomes de Sousa and Evans, 2003) and electromagnetic (microwave) crystals (Lu et al., 2008). Extruded filaments of a paste consisting of polymer, fine particles (e.g. ceramic and metal) and solvent are assembled to construct 2D or 3D structures. The overall process includes paste preparation, deposition of fine filaments by an extruder and postprocessing, consisting of solidification, debinding and sintering. Compared to fused deposition (Bellini, 2002) and gel extrusion (Smay et al., 2002a), paste extrusion using a solvent is a relatively simple technique without heating, cooling or polymerization processes to contend with and can be easily controlled (Yang et al., 2006). In similar work, pastes using a low-volatility solvent needing special drying conditions (typically 110 ◦ C under 6 kPa vacuum) (Morissette et al., 2001) were prepared from ceramic powder, low-volatility solvent (␣-terpineol)), binder (ethyl cellulose with 45.9% ethoxy content) and deflocculants. Before use, ageing for up to 4 Ms was needed if prepared from as-received powders to attain steady state viscosity; even pastes prepared from dispersant-coated powders needed 90 ks of aging (Morissette et al., 2001). In our work, a high volatility solvent was used so that the yield stress of the paste increased quickly as the solvent evapo-

∗ Corresponding author. Tel.: +44 20 7679 4689. E-mail address: [email protected] (J.R.G. Evans). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.11.039

rated. This brings several benefits. During deposition, the filament shape is easily retained because the yield stress increases with solvent evaporation and solidification is quite quick in ambient atmospheres. Different products require distinct qualities such as dimensional accuracy and freedom from defects. For example, most solid freeforming processes produce surfaces with ripples, steps or striations, but in applications involving open structures such as scaffolds in tissue engineering or in bone-substitute and ceramic preforms for metal matrix composites (Grida and Evans, 2003), these are absent. Electromagnetic crystals require high dimensional accuracy and absence of large defects. If the dimensional deviations are more than 5%, the frequency bandgap varies (Martínez et al., 2007). Defects influence the dielectric properties of ceramics and the dielectric constant varies with the relative amount and arrangement of pores, such as size, shape and orientation (Ota et al., 1992). Experiments and modelling show that varying the amount of dielectric in the unit cell changes the bandgap frequencies (Martínez et al., 2007). Microscopic defects such as flaws and voids act as stress concentration sites and are detrimental to strength (Song and Nur, 2004). At present, highly accurate dimensional lattices with intricate hierarchical structures still provide a challenge for extrusion freeforming from powders (Yang et al., 2006). Electromagnetic crystals operating at millimeterwave frequencies require ceramic filament diameters and cell dimensional resolutions from 0.1 mm to 1 mm, but terahertz electromagnetic crystals require filament diameter and dimensional resolution below 0.1 mm owing to the low dielectric constants of available materials. Extrusion of very fine lattices requires an accurate extruder, precise control of the XY table

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Fig. 2. Levelling error in the building platform.

for extrusion was prepared by solvent evaporation and stirring to leave approximately 12% solvent. Lattice structures were fabricated by the extruder at ambient temperature using the procedure previously described (Lu et al., 2008). After the extruded sample was dried at the ambient temperature, it was heated to 400 ◦ C for 1 h at a heating rate of 2 ◦ C/min from room temperature and then sintered at the required temperature. The sintered samples were observed by SEM (JEOL JSM 6300F field emission microscope).

Fig. 1. (a) The experimental setup and (b) schematic diagram of the extrusion axis and XY table.

velocity and extrusion ram velocity as well as paste preparation methods that eliminate agglomerates and debris. The investigation of factors that affect the fabrication of lattices by extrusion of powder was undertaken to establish these conditions. 2. Experimental details The experimental setup is shown in Fig. 1(a) and the schematic arrangement is described in Fig. 1(b). The extrusion freeforming platform (Lu et al., 2008) has four axes: X, Y, Z and extrusion. The XY table (MX80L Miniature Stage, Parker Hannifin Automation, Dorset, UK) is capable of high acceleration (39.2 ms−2 ) and speed (0.1 ms−1 ). The stainless steel syringe with internal diameter of 9 mm is mounted on the Z-axis and the sample substrate is placed on the XY table. The extrusion pressure was measured by a load cell (Flintec, Redditch, UK), which was mounted on the extrusion axis. The filament pattern in each layer is defined by the trajectory of the die in relation to the XY table movement and the Z-axis moves in steps of one layer thickness to produce a 3D assembly. Al2 O3 of high purity (99.992%, d50 = 0.48 ␮m, dielectric constant εr = 9.6 at 100 GHz, ex Condea Vista, Tucson Arizona), La(Mg0.5 , Ti0.5 )O3 (LMT) (mean particle size: 1.9 ␮m), (Zr0.8 , Sn0.2 )TiO4 (ZST) (mean particle size: 1.8 ␮m) and silica (mean particle size: 2 ␮m, PI-KEM Co., UK) were used as the dielectric materials. A mixture of poly (vinyl butyral) (PVB) and poly (ethylene glycol) (PEG, molecular weight (MW): 600) was used as the binder. PVB and PEG were first fully dissolved in the solvent, propan-2-ol. Then, the dielectric powder was added to the solution and dispersed by an ultrasonic probe (U200S, IKA Labortechnik). The volume ratio of the alumina powder to the dry polymer mixture was 3:2. After mixing, a suitable paste

3. Results and discussion 3.1. Equipment: precision and control parameters Normally, the gap between nozzle tip and substrate is maintained at 2–3 times the filament diameter during construction and the accuracy of the vertical position of the syringe and nozzle together with the level accuracy of the XY table strongly influence fabrication. If the substrate is tilted, the nozzle tip could touch the existing build or even, during construction of the first layer, the substrate. So the angles defining the level determine how large a sample can be fabricated as shown in Fig. 2. For example, if a lattice composed of filaments with 0.1 mm diameter is required to be 38 mm × 38 mm, the level error must be below 0.15◦ and for 50 ␮m diameter below 0.075◦ . Fig. 3 shows how the permissible

Fig. 3. Relationship between fabrication dimensions and maximum level accuracy.

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error decreases as the sample size increases. Using a structure prepared by milling aluminium supports and testing with slip gauges, the error for the substrate of the extruder was 0.057◦ , which satisfies the requirements for fabrication of 38 mm × 38 mm samples with the filament diameter of 50 ␮m. The vertical error associated with the stepper drive was 0.2 ␮m for a vertical displacement of 0.1 mm. An important factor in fabricating fine filament lattices of large horizontal dimensions is control of XY table path velocity in relation to extrusion ram velocity. Once steady state is reached, the XY table path velocity should be equal to the paste extrudate velocity which is regulated by the extrusion ram velocity. If the latter exceeds the XY table velocity, the filament path is not straight and “snaking” develops. If lower, the filament can be stretched and broken during fabrication. For extrusion of very fine filaments, the XY table path velocity can be a thousand times higher than the extrusion ram velocity depending on the ratio of syringe barrel diameter to die diameter. The relationship between extrudate velocity and extrusion ram velocity follows Eq. (1) under steady state extrusion conditions: Vpaste = Vram

D

barrel

Dext

2

(1)

where, Vram is extrusion ram velocity, Vpaste paste extrudate velocity, Dbarrel syringe barrel diameter and Dext the extrudate diameter. This supposes no die swell, ram leakage or fibre draw. Thus, when using a die with 100 ␮m diameter and barrel with 9 mm diameter, the XY table path velocity must be 8100 times that of the ram. If the XY table path velocity is 5 mm/s, the extrusion ram velocity should be 600 nm/s. When regulating these two veloci-

ties, a small change in ram velocity demands a large change in XY table path velocity. If the ram velocity increases by 0.001 mm/s, the XY table path velocity should increase by 8.1 mm/s. It may take a long time to reach steady state after adjusting these velocities. So the selection of extrusion barrel diameter not only depends on the required paste content determined by the overall build size but also on operational matching of XY table and extrusion ram. At present, the maximum Vtable /Vram value used for silica paste extrusion was 1500. So if filament with 100 ␮m diameter is to be extruded, the barrel diameter should be 4 mm for this extruder. Thus, it can be seen that the barrel diameter should be selected in relation to the die diameter. Another reason for doing so is that a large ratio of barrel to die diameter could induce high extrusion loads. The nozzle path also influences the build quality (Qiu et al., 2001; Qiu and Langrana, 2002) because it determines the movement of the XY table and changes in direction require acceleration and deceleration along the X- or Y-axes. Even if the extrusion is in steady state and the paste flow is constant, acceleration or deceleration associated with direction changes can produce under-fill and overfill. So adjustment of extrusion ram velocity is needed to match the nozzle path. At the 180◦ turns during fabrication of woodpile structures, overfill is dominant as the XY table path velocity is first decreased from the operational velocity to zero and then increased to the operational velocity in a new direction. Fig. 4(a) shows overfill at a corner path; the filament diameter has increased locally. Overfill and under-fill can also occur when the extrudate velocity is more than or less than the XY table path velocity respectively. Fig. 4(b) shows overfill when the extrudate velocity is more than the XY table velocity and results in snaking. Fig. 4(c) shows under-fill

Fig. 4. Photos of (a) overfill caused by path change, (b) overfill caused by higher paste extrudate velocity than XY table path velocity and (c) under-fill caused by lower paste extrudate velocity than XY table path velocity.

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Fig. 5. Defects generated in paste preparation: (a) defects in syringe barrel, (b) particle agglomerates and (c) air bubble in the filament.

when the extrudate velocity is less than the XY table path velocity and results in filament thinning. 3.2. Paste preparation PVB was selected partly because it is a strong adhesive and partly because it enhances flow properties and dispersion stability of the powder by preferential adsorption on the particle surface in the liquid medium, providing a steric barrier which induces particle repulsion (Tseng and Lin, 2002). The most common plasticizer used with PVB binder systems is poly (ethylene glycol). Lim et al. (2003) emphasized the importance of molecular weight (MW) of PEG: a lower MW of PEG causes a higher degree of plasticization and hence decreases viscosity of high solids loading suspensions but low MW PEG can give rise to lower mechanical strength. Another potential binder system is polyvinyl alcohol (PVA) + PEG. Su and Button (2009) compared the rheology, microstructure and formability of both PVA/water and PVB/organic solvent (cyclohexanone) and concluded that the PVB system is more suitable for extrusion because it is more stable and without significant migration of polymer and solvent. Other potential vehicles include ethyl cellulose (EC). Morissette et al. (2001) used EC and ␣-terpineol. Colloidal gels which undergo a fluid-to-gel transition are effective for fine filaments (Lewis et al., 2006). The PVB system used here is compatible with a wide range of ceramic powders. For fabrication of fine filaments, the paste preparation must be carefully controlled. In stage 1, a high solids content suspension

is achieved by mixing powder, PVB, PEG and solvent. In stage 2, the solvent is evaporated until a suitable paste is obtained. The paste should be without particle agglomerates, entrained debris or air bubbles. The agglomerates and air bubbles shown in Fig. 5(a) are defect initiators which make the paste display non-isotropic strength producing filament defects during extrusion (Pugh and Low, 1964; Russell et al., 2006). For extruding sub-100 ␮m filaments, the particle size should be less than 2 ␮m but fine particles tend to agglomerate. Fig. 5(b) shows examples in the filament. Generally, agglomerates exist in the original powder and in poorly mixed pastes such as those mixed by hand or by a low shear mixer. Choice of mixing method influences paste characteristics (Yang and Jennings, 1995). There are two basic mixing operations: dispersive (intensive) and distributive (extensive). The former disperses agglomerates, ideally to primary particles, and extensive mixing or blending reduces the extent of compositional inhomogeneity (Yang and Jennings, 1995). In stage 1, an ultrasonic probe was used for intensive mixing, providing high frequency vibration to disperse the particles in a low viscosity medium primarily by cavitation effects. Either the container should be small in order to treat the whole volume or the liquid should be pumped past the sonotrode nozzle. Enhanced cooling or a limited duty cycle is needed to prevent heating. In stage 2, the paste was stirred as solvent evaporated to obtain the paste viscosity suitable for extrusion. After stirring, the paste was aged before extrusion. Aging phenomena in pastes prepared from as-received powders can involve adsorption of surface active agents (Morissette et al.,

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Fig. 6. Defects associated with filament deformation: (a) spanning deformation and (b) loss of deposition height.

2001; Liu et al., 1997) but in this case redistribution of solvent and homogenising the composition is the main aim and aging times were at least 5.4 ks. Air bubbles in the filament as shown in Fig. 5(c) were inadvertently introduced during stirring while evaporating excess solvent. For de-airing, Leong and Chung, 2004 placed the paste in a vacuum chamber after mixing ethyl cellulose, PEG, butyl ether and powder. Huang et al. (2007) used vacuum de-airing after removing excess water by evaporation at 70 ◦ C. Vacuum treatment during mixing may enhance the integrity of the paste provided solvent loss is limited. Solvent content has a commanding influence on viscosity, yield stress and shrinkage. On the one hand, high solvent content prevents high extrusion pressure developing that can exceed the loading capacity of the press; on the other hand, low solvent produces a rigid paste that retains the extruded filament shape defined

by the die geometry and prevents sagging of the filament between supports in the layer below. Nevertheless, the yield stress must allow the extruded filament to bend through 90◦ to change the flow direction because Z is the extrusion axis but the filament movement path is in the XY plane. Characteristic defects in extruded filaments are shown in Fig. 5(c) and Fig. 6. Fig. 5(c) shows an alumina paste filament unable to retain its circular cross-section when extruded through a nozzle with diameter 500 ␮m. Fig. 6(a) shows the collapse of a filament when spanning two filaments in the previous layer. Smay et al. (2002b) estimated this deformation from the filament modulus. Fig. 6(b) shows cylindrical structures of different heights (nozzle diameter: 0.5 mm) which demonstrate slumping. Fig. 7 shows the designed height and actual height against the number of layers. This limits the height to which a lattice can be built without introducing pauses between layers to allow drying. The reason for these defects is that the paste was not rigid enough to prevent deformation. 3.3. Extrusion process

Fig. 7. Relationship between theoretical height or actual height and layer number.

During steady state extrusion freeforming, the XY table velocity should be equivalent to the extrudate velocity and the relationship between XY table path velocity and ram extrusion velocity should approach Eq. (1). The XY table path velocity is sometimes far from the ideal value as shown in Fig. 8. For example, when the silica paste was extruded from a 9-mm barrel with 100 ␮m die, the observed Vtable /Vram was 1500 which deviates greatly from the ideal value of 8100. The reason is that there is a leakage path through the gap between the ram and barrel wall and in this case, the leakage rate reached 13 times that of extrudate causing the real extrudate velocity to deviate significantly from the expected value. When a PTFE seal was introduced to limit back-flow this ratio was reduced to 4.4. Surface defects such as ‘shark skin’, ‘dragon teeth’ and ‘strain rate chaotic deformation’ can be found in thermoplastic and metal extrusion (Arda and Mackley, 2005; Kulikov and Hornung, 2001)

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in the entrance region does not completely recover and flow in the die is mainly elongational in a short die (Liang, 2004). Gifford (2003) indicated that die swell can be compensated by die profile design. In these experiments, we used relatively low extrusion ram velocity (below 0.02 mm/s) and low XY table velocity (below 6.5 mm/s) to restrict die swell. 3.4. Drying, debinding and sintering

Fig. 8. Differences in the relationships between Dbarrel /Ddie and Vtable /Vram in ideal and real conditions caused by back-flow and die swell.

but did not appear in filaments deposited by solvent-based extrusion freeforming, but wave distortion as shown in Fig. 9(a) could be generated. There are two reasons for this. First, the filament was not always extruded out of the die exit vertically in the Z-axis. If the XY table moves slightly faster than paste extrusion, the filament is drawn from the die at an angle and the stresses around the filament are not uniform in all directions. Furthermore, the filament needs to bend 90◦ when it deposits to the substrate. Bending causes different stresses in the two sides of the filament. Thus, wavy distortion on one side is produced. Die swell as shown in Fig. 9(b) makes the final diameter of filaments differ from the die diameter and makes final sample dimensions difficult to control. The die swell equation for the PTT (Phan-Thien-Tanner) family of models (Tanner, 1970) is



2 D = 1 + w2 Ddie 2G0

1/6 (2)

where D is the diameter of the extrudate;  w , the wall shear stress and G0 = 0 /, where  is the relaxation time and 0 , the zero-shear viscosity. Thus, die swell is mainly attributed to the wall shear stress (Liang, 2004) and is affected by extrusion ram velocity and the die configuration. Low extrusion ram velocity limits die swell. A die with a short land length may cause a large amount of swell (Masood and Song, 2004). This is because elastic extensional strain produced

After building, the extruded lattices are dried, debinded to remove organic vehicle and sintered. The defects that may develop at these stages include warping, void formation and cracking. Warping often happens in conventional FDM because of the need to cool the deposited thermoplastic filament (e.g. ABS P400) rapidly (typically in ∼1 s) from the melt temperature (270 ◦ C) to the glasstransition temperature (94 ◦ C). The deposited thermoplastic fibre adheres and contracts and the resulting stresses within one layer can induce side-by-side or layer-by-layer warping (Wang et al., 2007). The volume change in solvent-based pastes arises from solvent loss and this is a slower process. If the first layer adheres to the substrate, warping does not occur. If it does not, the filament shrinkage also causes warping as shown in Fig. 10(a) where the substrate was filter paper. Generally, compared to melt solidification, change of state induced by solvent loss avoids the abrupt transition and warping is not severe if the first layer is attached to the substrate. In some cases, cracking of the filament was observed during drying. Adhesion to the substrate prevents shrinkage and the resulting tensile stress, if it exceeds the filament yield stress, results in tensile cracks. Fig. 10(b) shows this defect in a paste consisting of 63 vol.% alumina based on the dried ceramic–polymer system. The sensitivity of cracking to polymer–ceramic ratio is partly because the elastic modulus of the binary composite increases steeply as ceramic powder loading increases and partly because in the early drying stage the more crowded system inhibits particle rotation needed to achieve higher packing and hence higher prefired strength. For the solvent-based system, cracks inside the filament as shown in Fig. 10(c) can be produced in the debinding stage as well because an internal core–shell structure might be formed. The cracking can be explained by the consequence of non-uniform volume shrinkage on the removal of polymer. During the debinding process, as loss of volatiles occurs from the filament surface, the effective volume fraction of particles at the surface rises. A rigid exoskeleton can be formed in the outer layer at an early stage but in the core, the polymer is still present and shrinkage has yet to occur. Thus, stresses produced by the core shrinking from the rigid outer layer could be generated at this stage and cause circumferential cut-like rupture (Wright et al., 1990).

Fig. 9. Defects generated in extrusion: (a) wavy distortion and (b) die swell.

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Fig. 10. Defects generated in post-processing: (a) warping, (b) crack in filament cross-section and (c) core–shell defects.

4. Conclusions Extrusion freeforming is a new and promising solid freeforming method. It can be used, inter alia, in fabrication of hard tissue scaffolds and photonic crystals. The factors influencing the quality of extrusion freeformed open structures are described in terms of sample quality and limitations on filament diameter. (1) Equipment accuracy, particularly the levelling error, match of XY table path velocity with extrusion ram velocity and match of nozzle path and extrusion ram velocity are decisive settings for fine powder-based filament work. The levelling error determines how large a sample can be fabricated. Because the XY table velocity varies with the square of the ratio of ram to die diameter, it can reach more than a thousand times the extrusion ram velocity and some control difficulties are introduced to make the XY table and extrusion work in harmony. So the ratio of ram to die diameter should be selected to adjust these two velocities. XY table velocity should match the extrusion ram velocity and barrel diameter should be compatible with the die diameter. Overfill and under-fill of filament deposition can happen in acceleration and deceleration paths where extrusion is not in steady state. (2) The defects which are often produced in the paste preparation stage are air bubbles and particle agglomerates. Paste was prepared using an ultrasonic probe for dispersion of powder, drying to increase viscosity and limited vacuum de-airing. The solvent content of paste is the most important parameter which determines the filament’s ability to span and retain planned height. However, the paste rigidity should allow deliverable extrusion pressures and confer the ability of the filament to bend 90◦ .

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