Creation of Food Structures Through Binder Jetting

Chapter 9

Creation of Food Structures Through Binder Jetting Sonia Holland1, 2, Tim Foster1, Chris Tuck2 1 Division of Food Science, Sutton Bonington Campus, University of Nottingham, Loughborough, United Kingdom; 23D Printing and Additive Manufacturing Research Group, University Park, University of Nottingham, Nottingham, United Kingdom

Chapter Outline 9.1 Introduction to Binder Jetting 9.2 Powder Characteristics and Interlayer Cohesion 9.3 Powder Binding Mechanisms 9.4 Suggested Analysis 9.5 Formulating a Functional Printing Ink 9.5.1 Ink Jetting Theory 9.5.2 Viscosity Considerations for Ink Jetting 9.5.3 The Effects of Additives to Enhance Viscosity 9.5.4 Surface Tension and Density

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9.6 Controls and Considerations During and Postprinting 9.6.1 Printer Settings 9.6.1.1 Powder Deposition 9.6.1.2 Material Jetting 9.6.2 Combining Powder and Ink During Print 9.7 Postprocessing 9.8 Application and Outlook for Food Materials References

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9.1 INTRODUCTION TO BINDER JETTING In the 21st century, it is commonplace to have one or more inkjet printers within an office environment and in our own homes that are used frequently to record important information in the form of documents or capture moments through printing images. Inkjet printing technology has been around since the 1960s, though the first jetting concept is thought to have been William Thompson’s (Lord Kelvin) siphon recorder, patented in 1867 (Gibson et al., 2009; Thomson, 1870). This was an example of continuous ink jetting (CIJ) whereby ink is continually jetted from a nozzle and deflected toward or away from a chosen substrate, depending on the particulars of the process Fundamentals of 3D Food Printing and Applications. https://doi.org/10.1016/B978-0-12-814564-7.00009-2 Copyright © 2019 Elsevier Inc. All rights reserved.

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employed (Derby, 2015). Within the food industry, CIJ is widely used, particularly in barcoding or in the marking and coding of egg shells (Fellows, 2016), whereas colour labelling and packaging is more likely done with other ink application techniques such as lithography, flexography or gravure (Forrest, 2009). Drop on demand (DoD) inkjet printers can be used to produce images on substrates like icing sheets or edible papers for decorative purposes (Pallottino et al., 2016; Periard et al., 2007). As the name suggests, ink droplets are only produced as and when required through pulse actuation, unlike the constant nature of CIJ. This allows for more complex and highresolution designs to be created with the option of combining a number of nozzles containing inks of varying formulation, for example, with different dye colourants (Le, 1998). Binder jetting was predominantly developed at Massachusetts Institute of Technology (MIT) in 1993, stemming from the use of an inkjet printer to jet a binding ink onto ceramic powder for investment casting processes and was originally termed three-dimensional printing (3DP). A few years later, Z Corp licenced the technology from MIT and launched its first 3D printer and associated materials based on this concept (Wohlers and Gornet, 2014). This type of additive manufacturing (AM) technology is typically lower cost, faster and easier to incorporate multiple material components into, including the option of printing in 24-bit colour, compared with other AM techniques (Gibson et al., 2009). The schematic above (Fig. 9.1) depicts a typical binder jetting process. Once the file for printing has been received, the 2D ‘slices’ of the 3D image are sequentially printed from the inkjet nozzle onto the powdered material on the print bed. After one layer of ink is deposited on the powder below, the X-Y positioning system Inkjet print head Powder spreader Part

Binder droplets

Unused powder

Z

Build platform

FIGURE 9.1 A schematic of the binder jetting process. Reprinted with permission from Gibson et al., (2009).

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printing plate moves down by one ‘z’ unit as the feedstock chamber plate moves up to provide sufficient powder for the next layer. A doctor blade or counterrotating roller and powder leveller carry the raised powder from the feedstock across to fill the build platform with any excess being collected. The next layer of ink can then be printed and the process repeated until the 3D object or group of objects have been created. Powder hoppers on a separate level to the build platform are an alternative to having a feedstock chamber and could facilitate building with multiple materials; however, such systems are not commercially available and are in their research infancy. As with other powder-based processes, such as laser sintering, the presence of loose and unbound powder in the bed throughout printing acts naturally to support the growing 3D object, thus negating the requirement for extra support material as in other AM processes. This also means that structures with added complexity like overhangs are able to be produced. One thing to note when designing structures for binder jetting is that powder must have an exit point if the shape is to be hollow or have voids; for example, if the design is a spherical shell, there will be no exit point for the loose powder, so it will be enclosed within the shape. Further considerations for powder design, ink formulation and combining these in 3D applications will be discussed in subsequent sections of this chapter. Inherently, this process can be categorised as ‘multimaterial’, as it involves combining a powder and ink to form a single component. Thus, many powder and ink combinations capable of producing successful objects exist owing to knowledge in other powderebinder processes and adaptability of inkjet nozzles (within reason). A typical example of how a known process has been translated to use in binder jetting is with ceramics, which is now a relatively mature material feedstock for this type of AM. Yoo et al. (1993), the research group from MIT who pioneered work in this area (see Sachs et al., 1992), describe the adaptation of binder jet printing to produce ceramic parts from MgO powder and a latex binder to initially produce a ‘green part’ for isostatic pressing and sintering as postproduction steps. This work demonstrates the adaptability of binder jetting to different materials and can be a useful guide when thinking of other novel materials. Typically submicron-sized particles, as used in the MIT project, are notoriously difficult to spread due to higher interparticle friction and electrostatic forces, but deliver a higher green part packing density, accuracy and surface finish for this product. Therefore, discovering a way to improve the original binder jet process to gain these advantages has enabled further application in the ceramics area and the ability to translate this knowledge when working with other materials (Gibson et al., 2009; Yoo et al., 1993). In terms of food applications where a liquid binder is introduced to a powder phase, agglomeration in spray drying, and related processes, is an obvious example. Agglomeration involves the accumulation of particulate solids, gradually over time or as a result of processing, resulting in size enlargement of the product clusters. In long-life powdered products, natural

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caking over time is considered a negative effect on product quality, whereas in spray drying, controlled agglomeration, like in a fluidized bed, before packaging can improve qualities such as solubility, wetting, controlled release and flowability of the product (Bhandari et al., 2013; Cuq et al., 2013). Powdered food products tend to lend themselves well to agglomeration due to their stickiness and hygroscopicity. The addition of liquid to powder particles promotes adhesion forces between them. Within agglomeration techniques in the food industry, shear is typically added to promote particle collision, such as high-velocity air or other types of agitation. However, as each layer of the binder jetting process is static once laid down, it is important that packing characteristics of the powder ensure there are enough particles in contact with one another to promote adhesion without the application of an external force, i.e., the powder bed porosity should not be too high. This consideration and other mechanisms of particleeparticle adhesion will be discussed in the following sections along with powder and ink interactions. It is worth noting that agglomeration is not solely reliant on the application of a wet binder. For example, pressure compaction is used in the pharmaceutical industry for tabulation or in food factories to create concentrated stock cubes from bouillon powder. Other external influences such as temperature will further effect the physical or chemical bonding of particulates (Dhanalakshmi et al., 2011; Pietsch, 2003). Binder jetting allows for the inclusion of these in the form of pressure roller systems for spreading powder and also the ability to keep the bulk powder bed at a determined temperature, often just below the glass transition (Tg) of a powder component. The Tg is a second-order phase transition, with both a moisture and temperature dependence, during which an amorphous glassy solid becomes more ‘rubbery’. It occurs over a temperature range which can be determined through thermal analysis and, as will be discussed later, is an important parameter to take into consideration, as it can govern structuring and order within a material (Bhandari and Howes, 1999). Powder and ink formulation design will determine the microstructure of the printed object through the culmination of individual properties and interactions of each component, and macrostructure design is only limited to the imagination of the maker.

9.2 POWDER CHARACTERISTICS AND INTERLAYER COHESION Knowledge of powder material properties is paramount for success in binder jetting; it will allow for the design of an appropriate ink component and also selection of parameters during printing. When using powders in bed processes such as this, there are a number of general considerations: 1. Powder particles must flow freely past one another to allow easy spreading across the powder bed.

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There are a number of powder properties which could potentially cause failure in this respect. It is tempting in powder bed processes to use very fine particulates in the hope of reducing layer size for higher resolution and a better surface finish. However, electrostatic forces between particles increase with a decrease in particle size, causing more interparticle friction and hindering flowability of the powder. Further influencing the friction between particles is their shape (i.e., spherical or irregular) due to the effect on the angle of repose, where irregularly shaped particles can attain a steeper angle which is correlated to lower flowability, though overall powder particle size is more of an influence (Utela et al., 2008). The material coefficient of friction will determine the success of spreading the correct powder without disturbing the layer underneath, as well as ensuring the correct volume of powder is collected and evenly spread, which will differ as a result of the spreading mechanism employed, i.e., using either a counterrotating roller system or doctor blade with or without an integrated hopper system. There exists, too, a dependence on the surface that the powder is being dragged across, with particle size being independent of the resistance to slow if powder is being pulled along a stationary layer made of the same material. Therefore, it is worth noting that flowability and layer success may be different on the first pass over the actual print bed to when the layer underneath is the powder itself (Augenstein and Hogg, 1974; Bhandari et al., 2013; Gibson et al., 2009). Cohesion effects must also be considered at this stage; as well as being crucial for the creation of 3D structures, they can be very detrimental to powder flowability. The surface of glassy food powders can transition to the amorphous, rubbery state over time or through processes such as mechanical milling or spray drying if sufficient time is not allowed to form ordered, more thermodynamically stable, crystalline structures. Thus, identification of conditions exacerbating this for the particular feedstock is crucial, and steps should be taken to limit its occurrence during storage or printing, particularly if the aim is to collect and reuse excess unbound powder from a given printing run (Bhandari et al., 2013; Opali
nski et al., 2012). 2. The maximum particle size should not exceed the intended layer size. This will further help to ensure that layers placed down are smooth, thin and repeatable throughout the process. Equally, if the powder is prone to natural caking and agglomeration, the maximum agglomerate size should not exceed this thickness. Holland et al. (under review) also describe a phenomenon where ink interacts with powder to cause swelling and lifting of a given layer. If, after printing the ink layer, the powder plus ink layer thickness is greater than that which is programmed, then powder being spread across will cause misalignment and movement between layers. Thus, it is important

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that powders and inks are tested for interactive effects in 2D prior to use in a 3D scenario. 3. To enable efficient packing of the powder bed and sufficient ‘green density’ of the printed object, there should be a range of particle sizes within the powder. Green density refers to the relative particle packing density compared to the external volume of a printed object. The minimum required green density of any given part will depend on the powder material properties, binding mechanism (e.g., physical ‘gluing’ versus chemical bonding) and the effect of any postprocessing; for example, Yoo et al. (1993) state a green density of at least 60% is required when printing ceramics that are sintered via solid-state diffusion, whereas Bai et al. (2015) used metal powders in a binder jetting process and worked within a green density range of 44%e57%. The use of bimodal and multimodal powder formulations, whereby smaller particles fall in interstitial voids between coarse particles to provide more available necking sites between particles to improve green and sintered densities, is well documented in literature across a range of powder bed materials and processes (Bai et al., 2015; Brunello et al., 2016; Gibson et al., 2009; Olakanmi et al., 2015; Utela et al., 2008; Xia and Sanjayan, 2016). Apparent bed density and tapped density are simple measurements which can be used to determine the performance of a powder and as a screening tool in formulation development. It is also worth noting that as the surface area-to-volume ratio of a given powder particle increases, its reactivity increases. This introduces the risk of dust formation or risk of explosion as with other fine powders used in the food industry (and others), influencing handling and transportation aspects of the process (Bhandari et al., 2013; Gibson et al., 2009). Importantly for food powders and when thinking ahead to potential powder recycling during processing, higher powder surface area increases susceptibility to oxidation and related compound degradation reactions, depending on the chemical constituents of the powder. With this in mind; controls and monitoring steps should be put in place to ensure the safety and quality of end products, as in all food material processes.

9.3 POWDER BINDING MECHANISMS The powder agglomeration method will depend on the specific material properties of the powder and its interaction with the ink component. Generally, water-soluble materials are best suited to agglomeration. For hydrophobic or water-insoluble materials, the addition of small amounts of a better-suited material can provide sufficient interaction to bind; this should be taken into consideration during material scoping phases for binder jetting.

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Binding can occur physically, chemically or electrostatically. Fig. 9.2 depicts the various adhesion mechanisms that can occur to achieve agglomeration of particles. In nonfood applications, it is common to use heat or UV-curable resins such as acrylic or epoxy-based resins, but due to toxicity of those particular substances and related photoinitiators, they are unsuitable for use in edible products. In some instances, it is the ‘ink’ that provides the full binding effect where the binder cures or hardens around the powder particles to create particulate agglomerates. Of course, the alternative is that interactive effects between the ink and powder components initiate bonding and aggregation; thus, both components would have a part to play in the adhesion. Food powder particles have a tendency to exhibit caking upon addition of moisture; thus, it is likely that for the use of food materials in binder jetting, this is the most likely adhesion scenario. Added moisture will begin to wet the outside of each particle; the degree to which this occurs will depend on the saturation ratio of ink to powder. Ink addition (‘wetting’ stage) will have a number of effects to initiate agglomeration: l

l

l

Polarities and compatibility of both phases will determine the formation of chemical bonds. For food products, this would likely induce the formation of hydrogen bonds if a polar ink (water or ethanol based) is deposited onto a powder with predominantly polar components (the bulk of this would be carbohydrate monosaccharides or polysaccharides and proteins). Facilitation of further low-energy bonding such as van der Waals or ionic interactions and the occurrence of hydrophobic effects, potentially detrimental to adhesion, should be considered, depending on the composition of powder and ink materials. Particle surface viscosity will differ upon ink addition time and volume.

FIGURE 9.2 Agglomeration mechanisms between food particles. Reprinted with permission from Pietsch (1997).

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In the ‘nucleation’ phase of agglomeration, liquid bridges form between particles to initially sinter them to one another, stabilised by capillary forces; over time, soluble components may dissolve and enhance local viscosity to strengthen these sinter bridges. Descamps et al. (2013) state that below a viscosity of 108 Pa s, particles will start to exhibit ‘sticky’ behaviour. The powder bed during binder jetting is static; however, particles with lowviscosity surfaces may plastically deform to create a larger area of contact between them to aid adhesion and reduce porosity which is why a distribution of particle sizes and efficient bed packing is advantageous. In terms of interaction time for layers of a part, the lower layers of a 3D structure will have a longer time to interact due to the nature of the layer-by-layer process and are more likely to exhibit surface solubilisation and deform plastically to create a more viscous sinter bridge between particles. The top layer will experience the process for a shorter time and may not have the chance to absorb the binder, forming a liquid bridge instead which is stabilised primarily by capillary forces unless postprocess resting time or additional steps are factored in. ‘Growth’ interactions are crucial to maintaining cohesion in the z direction, i.e., any given layer interacts with that both above and below it, despite the lack of mixing to promote collision like fluidised beds would provide in conventional food agglomeration processes. Therefore, particle:binder affinity, particle:binder ratio and the thickness of each layer ultimately determine the growth of a 3D structure or of separated 2D slices. ‘Consolidation’, ‘rupture’ and ‘erosion’ effects present in other agglomeration processes are less important here, as there should be minimal perturbation of a given layer once the binder has been added. Subsequently irreversible interactions can be initiated, such as denaturation to covalently bond molecules or state transition may occur between amorphous, semicrystalline or crystalline supramolecular order. Such phenomena are dynamic and subject to not only the plasticization effect of the ink component, but also addition of thermal energy or pressure. This will act as a ‘stabilisation’ step, like in other food powder agglomeration processes. Knowledge of a material’s state diagram and basic chemistry is highly recommended prior to testing in a binder jet printer to be able to select likely ballpark parameters for a successful print, particularly for complex, multicomponent food powders. Fig. 9.3 gives an example of a state diagram of wheat flour, highlighting important areas such as Tg, melting and gelatinization that all need to be considered when using a complex material, particularly a mixed powder system for binder jetting. The state of a material will affect the mechanism and success of adhesion, particularly the ability of ink to penetrate a material if it is highly ordered (crystalline) versus one that is lacking in order (amorphous) and more heterogeneous. Changes to local surface viscosity and the ability for formation and sintering of liquid bridges will be affected by molecular ordering and mobility. Materials with favourable binding properties present in

Temperature (°C)

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220 200 180 ‘Theoretical Tm (starch) 160 flow state’ 140 120 100 Tr (proteins) 80 60 ‘Rubbery state’ 40 ‘Glassy Tgelat, Tg (starch) 20 state’ (starch) 0 Tg (proteins) –20 0 10 20 30 40 50 60 70 80 90 100 Water content (g per 100 g dry matter)

FIGURE 9.3 State diagram for the main constituents of wheat flour (starch and gluten proteins). Tm is the melting point, Tr is the minimum thermosetting temperature for proteins, Tg is the glass transition temperature and Tgelat is the starch gelatinisation; bold labels relate to the gluten proteins. Reproduced with permission from Cuq et al. (2003).

the powder formulation can complement those in the ink; a host of food-grade materials, not only low-molecular-weight carbohydrates, fall into this category. Equally, ‘bridging potential’ of a powder formulation can be negatively affected by the presence of hydrophobic constituents or ionic interactions caused by salts. Inclusion of materials with some water-binding capacity or an ability to form a highly viscous solution or gel upon hydration in the powder component would also be an advantage for creation of food structures by binder jetting. Further, material-specific information on Tg and agglomeration can be found in food science literature (Dhanalakshmi et al., 2011; Hartel et al., 2011; Bhandari et al., 2013; Cuq et al., 2013; Descamps et al., 2013; Roos and Drusch, 2016; Wang and Truong, 2016).

9.4 SUGGESTED ANALYSIS Given that there are many factors to consider in creating a suitable powder for binder jetting, there are a few key analysis techniques to consider undertaking on trial formations. Before any ink is deposited in binder jetting, the powder must first be spread; therefore, it is important to ensure the particles will perform well in this task. It is recommended that visual assessment of powder particles through light microscopy or scanning electron microscopy is undertaken. This will give an indication as to the particle shape and possible particle size distribution; however, it is crucial to conduct some other particle size analysis, such as light scattering, to fully assess the maximum size and distribution of particles. From these data formulation adjustments, further processing, such as milling or sieving, can be conducted if necessary.

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Particle size and shape are linked to the flowability of the powder (Schulze, 2006). Powder behaviour is important, as the powder must spread well over the base plate surface for the initial layer, but also over bound and unbound powder in subsequent layers. A number of methods exist, ranging from quick, crude visual assessments to more advanced setups such as powder flow rheometers. Jenike (1964) described the use of a shear cell to measure powder flow properties in order to design vessels for their transport and deposition which really began research in this area. A number of shear cells are available such as the direct shear cell, Schulze annular ring shear tester (ASTM International Standard D6773) and other devices serving as advances to Jenike’s technique as well as others which incorporate uniaxial compression techniques. Crucially in powder flow analysis, these techniques are undertaken to define the relationship between consolidation stress (s1) of a powder bulk to its unconfined yield stress (sc), yielding a dimensionless ‘flowability’ index value (ffc). ff c ¼

s1 sc

When ffc < 1, the material will not flow, whereas a ffc > 10 indicates a freeflowing material. An index value between 4 and 10 is deemed to be ‘good flow’; thus, this should be the minimum value range to aim for when designing a flowable powder for binder jetting, as any value below 4, which is ‘cohesive’, will be very detrimental to the process, particularly in terms of disturbing lower layers during spreading. In addition, a small angle of repose has been found to confer good flow characteristics of food materials in powder bed processes (Bhandari et al., 2013; Cagli et al., 2007; Descamps et al., 2013; Diaz et al., 2015; Ghosal et al., 2010; Jenike, 1964; Liu et al., 2017; Schulze, 2015). As discussed above, a variety of density measurements may be made to determine how the powder may pack in the powder bed after being spread. The bed porosity, i.e., ratio of the volume of voids to the total bed volume, will greatly determine the ability of particles to adhere to one another to create a cohesive structure, and the porosity of the final printed object. Sorption isotherms of powder formulations measured either through dynamic vapour sorption (as in Holland et al., 2017) or through equilibration over saturated salt solutions, measured by a humidity sensor (described in Descamps et al., 2013), will provide information of how water (or other solvent) molecules bind to the powder particles. In dynamic techniques, desorption can also be measured; thus, any sorption/desorption hysteresis is observed which may be insightful if solvent removal is important in the proposed binder jet process. Finally, thermal analysis through differential scanning calorimetry will provide experimental data to complement state diagram information obtained from literature. Printing parameters such as ink saturation (providing moisture plasticization of transitions) and required temperature settings can be adjusted accordingly to give desired states of order of the material.

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9.5 FORMULATING A FUNCTIONAL PRINTING INK 9.5.1 Ink Jetting Theory DoD inkjet is used in binder jetting; however, the actuation mechanism may vary. Thus, inkjet printheads will inevitably differ between binder jetting machines based on their manufacturer. Actuation mechanisms may be acoustic, thermal, electrostatic or piezo-driven for DoD systems, with the thermal and piezo methods being most commonly employed. In the thermal method, a current pulse is passed through a heating element (located adjacent to or as a ‘roof’ over the nozzle orifice) to locally superheat the ink and nucleate bubble formation. The bubble expands to eject a droplet of ink and then begins to collapse against the heater, triggering droplet break-off and refilling of the ink chamber. Piezo actuation involves deformation of a piezo ceramic, generating waves through the pressure chamber to eject ink droplets, and can be one of four modes: shear, bend, push or squeeze tube (Godoi et al., 2016; Le, 1998; Wijshoff, 2010). Despite variation in actuation mechanisms, the theory behind droplet ejection from the nozzle is universal and can be explained through PlateaueRayleigh instability of liquid jets (Plateau, 1873; Rayleigh, 1878). A volume of liquid is ejected from the nozzle as a jet which is subject to capillary thinning of the liquid bridge between the main droplet and nozzle until this ligament breaks to produce a single droplet, in an ideal scenario, or main droplet and associated satellites (Derby, 2010; McIlroy and Harlen, 2014). As a result, some key analyses can be performed on trial ink formulations before application to the printer to give an idea as to their anticipated success or ‘printability’. The printability parameter (Z) was initially proposed by Fromm (1984) but revised more recently by Reis and Derby (2000) using modelling techniques to better understand the ‘printable’ values of Z. pffiffiffiffiffiffiffiffiffi 1 Re grL Z¼ ¼ pffiffiffiffiffiffiffi ¼ Oh h We The Ohnesorge number (Oh) is a ratio of two dimensionless numbers, the Reynold’s number (Re) and Weber number (We), which are commonly used in fluid dynamics, Re particularly in the food industry when considering product flow through a pipe (Fellows, 2016). Re and We are influenced by velocity; using Oh directly allows for printability characterisation independent of fluid velocity, a parameter which can, of course, be altered in print settings (discussed later). In the liquid jet model, surface tension (g) forces act to destabilise the jet in order to minimise surface energy through breakup into spherical liquid droplets. This action is resisted by the ink’s viscosity (h). Achieving the correct balance between surface tension, viscous and inertial forces is key to developing a functional printing ink. Viscosity is the main driver of printability, determining the amount of thinning the liquid capillary

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undergoes prior to breakup. Surface tension should be sufficient so that the ink does not freely flow from the nozzle in a static state but only to allow breakup to single droplets after actuation. Density (r) and the nozzle diameter (L) of the formulation are also considered in this model. If Z is too low, it is likely that viscous forces are too large, preventing droplet ejection, whereas a high Z number is related to the formation of satellite droplets and potential inaccuracies in droplet positioning if these do not reform with the main droplet before coming into contact with the substrate (Derby, 2010; Hoath et al., 2009; Hoath, 2016; McIlroy and Harlen, 2014). It is worth noting that measurement of these parameters should be undertaken under printing shear rate and temperature conditions to give a Z calculation relevant to conditions the ink will be subjected to. A key figure in Derby (2010) has been converted to an interactive online tool by Abbott which can be used by formulators to estimate printability of trial inks. Measured ink parameters (h, g and r) are selected using the slide bars, along with the set nozzle diameter and projected print speed. The marker moves around the two graphs based on computed values for Z, Oh, Re and We to give an indication of droplet breakup during printing and thus the success of the formulation. It is also useful to hold parameters at given values and then change one at a time to observe the relative effects this would have on the estimated printability, e.g., viscosity fluctuation versus density. Fig. 9.4 is a screenshot of this online tool with values selected within the FujiFilm (2008) guidelines for a Dimatix printer; h ¼ 10e12 cP, g ¼ 32e42 m/Nm and relative r > 1.

Ohnesorge Number 1000

10.00

Atomization

Splashing Too Viscous

Wave

1.00

100

Oh Satellites

Z

Sine

0.10

10

Rayleigh

0.01 Low Energy 1

1 Viscosity cP 10 ρ Density g/cc Oh 0.352

10

Re

100 1000 Length μm 21

1

10 100 σ dyne/cm Re 32

1000

10000

Velocity m/s 6

1.2 Z=1/Oh 2.8

Re 15.1

We 28.3

FIGURE 9.4 An example of ink properties giving an ideal printing ink, visually represented by the online tool developed by Abbott (2017).

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Water is commonly used in fluid bed agglomeration to adhere amorphous, water-soluble particles to one another, whereas for crystalline particles, more viscous liquids tend to be employed, as water cannot readily migrate through this ordered matrix (Cuq et al., 2013). However, water is not an ideal material for printing, with physical properties being outside those reported as ‘printable’, particularly with regard to its low viscosity and high surface tension; h ¼ 1 cP, g ¼ 72.8 m/Nm and r ¼ 998.2 kg/m at 20 C (Crittenden et al., 2012). Despite this, water has actually been cited as a binding material in a number of food-specific binder jetting applications (Diaz et al., 2015; Von Hasseln et al., 2014). Aqueous-based inks are ideal for food printing due to the majority of food polymers being polar and water-soluble or exhibiting hygroscopicity and tendency to agglomerate in a powdered form when exposed to water. Therefore, such additives may be utilised to create printable, foodgrade inks based on water as a primary solvent.

9.5.2 Viscosity Considerations for Ink Jetting A material’s viscosity describes its resistance to flow when a shear stress is applied. This property of fluids in motion can be described simply by Eqs. 9.1 and 9.2 (below) in which s represents the shear stress as a consequence of a force (F) being applied over a given area (A); the ratio of s to the rate at which _ then gives the material’s shear viscosity (h). the shear is applied (g) F A s h¼ g_ s¼

(9.1) (9.2)

Newton’s law applies to ideally viscous materials, whereby viscosity is directly proportional to the shear stress, independent of the rate at which the shear is applied. Water is one example of a Newtonian fluid. Fluids are termed ‘non-Newtonian’ if the measured viscosity is dependent on the rate shear is applied. Typical non-Newtonian effects are viscosity decreasing with increasing shear rate (shear thinning) and viscosity increasing with increasing shear rate (shear thickening). Flow curves may be constructed by plotting s _ or a viscosity curve replacing s for h on the y-axis to visually depict versus g, a material’s flow behaviour. Fig. 9.5 shows an example of a viscosity curve, where (A) is the expected profile of a Newtonian fluid, (B) a shear-thinning sample and (C) a shear-thickening sample (Mezger, 2014). Food systems are typically composed of multiple materials and exhibit complex flow as a result of high-molecular-weight, multiphase or particulate components. Usually at rest, these components are randomly oriented (for suspended aggregates, anisotropic particles or emulsion droplets) or entangled (in the case of long-chain polymers). Initially, these systems can exhibit high

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FIGURE 9.5 A generalised curve showing the viscosity versus shear rate behaviour of (a) a Newtonian fluid; (b) shear thinning; (c) shear thickening with a dilatancy peak on a logarithmic scale.

resistance to flow when low shear is applied. However, as the shear rate increases, particles or molecules will align or orient in the direction of shear; aggregates are able to be broken up or liquid phase droplets deformed such that the resistance and, therefore, measured viscosity is lower at these high shear rates (shear thinning). The presence of such structures on different length scales can provide a myriad of other rheological effects such as the occurrence of a yield stress or temperature-dependent events that influence flow, such as gelation (Day and Golding, 2016; Dickinson, 2016). Though values used to calculate Z should be accurate under printing conditions (i.e., temperature and shear rate for the reported viscosity), these complex behaviours are not accounted for in the model and thus should only be considered a guide in these instances (Derby, 2010). Non-Newtonian effects can be advantageous depending on the application, particularly in the case of adding low concentrations of shear-thinning polymers which can inhibit the formation of satellite droplets (Derby, 2015; McIlroy et al., 2013). Shear-thinning materials also have the ability to reach a printable viscosity at the high elongational strain rates experienced in the nozzle, but then recover their higher, low shear viscosity after ejection and upon impact with the substrate. A simple viscosity measurement over a range of shear rates should be conducted at the intended ink printing temperature to obtain a flow curve. Generally, viscosity decreases with increasing temperature due to energy input and higher molecular mobility, so selection of an elevated printing temperature could be favourable to achieve a desired viscosity if the ink components are not degraded by such treatments. Rotational rheology in which a sample is contained between a stationary surface and a rotating surface, providing the shear, is a common analysis method for these substances. A number of different geometries exist for taking such measurements, and the best-suited one for analysis will depend on ink components and characteristics. Brookfield viscometers with spindle attachments conforming to International

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Standards Organisation (ISO) and Deutsches Institut fu¨r Normung e.V (DIN) standards may be used to measure low-viscosity liquids. Often, turbulent flow and secondary flow effects are observed when measuring low viscosity liquids, and the ‘Weissenberg effect’, where the liquid ‘climbs’ a rod during shear, can affect the validity of results in the case of a solution consisting of a polymer in water. Concentric cylinder geometries including standard cup and bob, Couette variants or double-gap geometries, which are designed specifically to measure low-viscosity liquids at high shear, can be useful alternatives. Cone and plate as well as parallel plate oscillatory tests using thin film techniques may also provide useful data, again, depending on the ink constituents, as secondary flow effects can result in the material flowing out of the gap at high shear (Davies and Stokes, 2008; Mezger, 2014; Zotek-Tryznowska, 2015).

9.5.3 The Effects of Additives to Enhance Viscosity Soluble sugars, starches, proteins and biopolymers may be used to achieve sufficient ink viscosity and can be selected based on their intended interaction with the powder component based on the binding mechanism. These types of additives affect the viscoelasticity and viscosity of the base solvent (Hoath et al., 2009); therefore, concentrations of these should be selected within a sensible range to confer the required viscosity for printing. Issues mainly arise when ink components are particulate or have a high molecular weight; many food colloids fit into one or other of these categories. Particles in a solution will increase the bulk viscosity by occupying space. In the context of food, this may include suspended granules, milled powders or flavoured flakes which can, in theory, range in size from the nanoscale up through to visible length scales (Dickinson, 2016; McIlroy and Harlen, 2014). It is recommended that the particle’s largest characteristic length is at least 5, if not 10, times lower than the selected test gap in a rheometer (Mezger, 2014) and, of course, should be lower than the intended nozzle diameter of the printhead. In conventional inkjet printing, particulates are often pigments and should not be larger than a few micrometres to avoid particle aggregation and nozzle blocking (Zotek-Tryznowska, 2015); any trapped particle around 15e20 mm can completely block nozzles, contribute to the formation of air bubbles and disrupt proper ink droplet formation (Wijshoff, 2010). Related to this, the challenge of maintaining a stable dispersion through viscosity modulation of the continuous phase could also result in unfavourable printing characteristics as a trade-off when trying to prevent particle aggregation. Droplet size is usually related to the nozzle diameter (Le, 1998; McIlroy et al., 2013). However, Hoath et al. (2009) found that as the concentration of a high-molecular-weight polymer increased, the total droplet volume ejected increased, as well as the proportion of this volume contributing to the trailing ligament rather than the main droplet. The modelled polymer in this work was monodisperse, linear polystyrene, conditions rarely found in the context of

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food materials. Xu et al. (2007) studied the effect of concentration (0.3%e 3.8%) and molecular weight (10.5e46.5 kDa) of cellulose esters in ink formulations. Cellulose esters exhibit weak rod properties, and chains have a random coil conformation in solution, a common characteristic amongst highmolecular-weight food-grade polymers. They show that as concentration remains at or below the coil overlap concentration, the polymer behaves as expected with respect to increasing viscoelasticity and length of the trailing ligament through polymer chain extension. Above this overlap, concentration extended polymers may entangle, with intramolecular H-bonds breaking and reforming as intermolecular H-bonds, greatly increasing the material relaxation time and its elasticity under shear and thus resisting the flow required to print. This was true across the molecular weight range tested. McIlroy et al. (2013) further explored jetting regimes corresponding to molecular weightdependent transitions in DoD printing using polystyrene as a model. It was identified that polymer coils stretch to extended chains as a result of high strain rates in the nozzle during printing as opposed to extensional flow within the extending ligament before break-off. At a critical strain rate, greater than the fracture strain rate (at which carbon-carbon bonds will break), central scission of the molecule may occur. In practical terms, the result of this flow-induced degradation is a reduction in the molecular weight of the polymer ink which could correlate to a loss of functionality. Wheeler et al. (2016) highlighted that polymer conformation also plays a vital role in the success or failure of ink jetting. For polymers of comparable molecular weight, those which were hyperbranched showed a much higher maximum printing concentration than their linear counterparts. This was related to the retention of a stable coil conformation and the longest chain length being smaller, thus resisting degradation during flow through the printhead when using these hyperbranched molecules. Therefore, if high-molecular-weight polymers are used in inks for a food binder jetting process, careful consideration of the balance between molecular weight and their concentration must be considered during the formulation stages; Holland et al. (2017) showed this through the inclusion of xanthan gum in a food-grade ink. Printability was shown to improve as xanthan gum molecular weight was reduced, and a higher effective concentration of xanthan gum was able to be included in the ink whilst maintaining other printable ink properties, which was advantageous for the proposed powder binding mechanism. Yoo and Kim (2013) also investigated the use of xanthan gum in an ink jetting scenario to monitor the impact of extensional properties on capillary breakup. Increases in the extensional viscosity due to the strain hardening of polymers affected ligament detachment from the nozzle; relaxation time of these types of polymers after extension during flow through the nozzle was also found to be an important factor. As a result, concentrations of xanthan gum used in their work were lower than those achieved by Holland et al. (2017) through molecular weight reduction.

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9.5.4 Surface Tension and Density Surface tension drives towards capillary thinning and breakup, whereas other forces act to resist this phenomenon (McKinley, 2005). In the case of water as a good solvent for food-grade materials, high surface tension is a key factor in inhibiting repeatable printing of water droplets, as described above. Some solvents possess lower surface tension than water so can be used as cosolvents to achieve more favourable printing characteristics without surfactants (Soleimani-Gorgani, 2016). Mallinson et al. (2016) describe viscosity and surface tension of aqueous mixtures of glycerol, ethyleneeglycol and 1-propanol at varying mole fractions for the context of inkjet printing. They show how even small additions of glycerol can reduce the surface tension of water, making it a favourable cosolvent for food inkjet printing. Glycerol also has a density greater than 1 (Segur, 1953), which is yet another desired characteristic for production of a repeatable printing ink. Ethanol is another popular cosolvent in edible aqueous ink formulations (Pallottino et al., 2016) and has the advantage of lowering surface tension. Selecting the correct volume fraction of ethanol is important in a formulation, as it has the potential to introduce a number of detrimental effects: its low boiling point can cause solvent evaporation and drying of soluble components at the nozzle, leading to blockages; it may be a less effective solvent for some ink components and cause precipitation of these out of solution. Ethanol and other low-molecular-weight alcohols are lower density than water which could also be detrimental to printing. Another way to control ink surface tension is through the addition of a surfactant. Many surface-active molecules are naturally present in or added to food products; low-molecular-weight molecules such as naturally occurring lecithin or synthesized sorbitan esters to high-molecular-weight components exhibiting surface activity such as proteins and polysaccharides (Kralova and Sjo¨blom, 2009). It has been discussed previously that there are many aspects to consider in terms of ink functionality when adding high-molecular-weight components to a formulation. The low-molecular-weight alternatives also have the advantage of being more mobile towards the interface which could be an important factor when considering timescales within printing processes. It is well documented in literature that temperature can affect the adsorption of surfactants at the interface, as well as their critical micelle concentration and thus the measured interfacial or surface tension (Kralova and Sjo¨blom, 2009; Mohajeri and Noudeh, 2012; Nin˜o and Patino, 1998); therefore, it is again important that surface tension values are measured close to the intended printing temperature. A number of simple surface tension measurement devices are available; the most commonly used techniques are the Du Nou¨y ring method, Wilhelmy plate method or drop shape analysis using a pendant drop approach (Berry et al., 2015). The addition of a surfactant may also affect the contact angle of ink droplets on the powder substrate with the ability to enhance wetting characteristics. A succinct description of contact angle, its

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measurement and relation to food powder wetting can be found in Section 14.4.3 of Gaiani et al. (2013). Ink density, recommended to be above 1, should be taken into consideration, particularly when formulating printing waveforms (discussed in the next section). The density will be affected by the solvent as well as dissolved or suspended constituents. It is likely that inks with suitable rheological and surface tension properties will not be too far off a printable density either; therefore, the two former properties should be fine-tuned preferentially. A final consideration with regard to ink formulation is that trapped air will disrupt droplet formation and ejection mechanisms (FujiFilm, 2008; Hoath et al., 2009).

9.6 CONTROLS AND CONSIDERATIONS DURING AND POSTPRINTING Measurable properties to aim for during powder and ink formulation are useful; however, a number of settings may be altered within a binder jetting machine and can allow leeway for the use of theoretically ‘nonideal’ materials.

9.6.1 Printer Settings 9.6.1.1 Powder Deposition Resolution achieved is, of course, directly related to the layer thickness, independent of the AM process being used (Chen and Zhao, 2016). If able to be varied in a given machine, changing the layer thickness is a process parameter which may be optimised to give a good surface finish and dimensional accuracy. Often, AM is limited in producing truly representative 3D objects based on computer-aided design (CAD) models due to the ‘stair-stepping’ effect; this would occur when 2D slices of the CAD image appear as stacked circles, for example, when considering a sphere. Though the nature of loose powder in binder jetting being able to support an object through printing is an advantage to achieve more complex geometries, the limitations of layer thickness and number of printed layers can oppose this. The smooth, curved edge may not be accurately represented when printing, rendering a printed object whose individual layers are visibly likened to a set of stairs. This is also apparent in other AM processes, particularly extrusion-based techniques (Sun et al., 2017). The powder feed rate, spreading and resulting layer density will largely be affected by the powder physical properties described earlier; the method of deposition will vary the exact parameters that can be changed within the printer. Optimisation of a given machine’s powder delivery should be undertaken to ensure the powder is deposited reliably in a smooth, even layer, and new layers do not drag or distort those underneath them. This will largely affect powder packing and print bed density, contributing to the necessary level of saturation to be set and, ultimately, the density and structural integrity of the printed object.

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9.6.1.2 Material Jetting Printed drops should be a uniform size and shape, thus maintaining a consistent drop volume. Ideally, each nozzle will eject this volume of ink when actuated, which necks and pinches off to allow a single, spherical droplet to fall in a straight line onto the substrate. The presence of longer ligaments is acceptable as long as they thin, pinch off and reform with the main droplet before hitting the substrate. The same is true for any satellite droplets which could be produced by capillary thinning action; satellites and ligaments only pose a problem if they cause the drop to deviate from flight or do not join the main droplet, potentially causing splashing. Sufficient time should be allowed for this merging to occur; thus the printhead is usually situated between 1 and 3 mm above the substrate at all times (Derby, 2015). Different actuation waveforms can be used to control the pulse delivered to each nozzle on the printhead in order to achieve optimal printing characteristics of drop volume, speed and jetting direction from each (Khalate et al., 2012). A waveform is composed of four segments, controlling the chamber filling, droplet ejection, break-off, then recovery and refilling of the chamber. The speed (slew rate), amplitude (voltage) and duration of bending a piezoceramic throughout one printing cycle are all changeable parameters (FujiFilm, 2008). Simple waveforms, such as trapezoidal, are often used successfully, with only small adjustments. Though a number of studies exist describing methods for designing waveforms from scratch based on material behaviour during jetting, manufacturers often include preset waveforms based on common solvents or known material properties for use in specific machines (FujiFilm, 2008; Hoath et al. 2011; Khalate et al., 2012; Kwon, 2009). As a general rule, the more viscous a material under printing conditions, the higher the voltage and steeper the slew rate required, whereas the opposite is true for low-viscosity inks. Hoath et al. (2009) attribute this to the nearlinear extension of a low-viscosity ink ligament until detachment compared with a more rapid deceleration and later detachment of a jet comprised of a higher-viscosity ink. The voltage pulse produces the necessary oscillations to eject ink from a given nozzle; if residual oscillations have not settled before the next pulse, intradrop variation will result (Khalate et al., 2012). Thus, the frequency will be affected if residual oscillations do not dampen within a reasonable time. Ideally, the frequency will be set as fast as possible for economic reasons and to maintain a reasonable print speed, but printing too quickly can be detrimental to the consistency of the print (Cie, 2015). Finally, cartridge ‘purges’ to clean nozzles should be incorporated into printing runs and can usually be set to occur between a given number of print passes. It is very important to include cleaning cycles midprint to clear any temporary blockages caused by dirt, particulates or solvent drying at the nozzle (Cie, 2015; FujiFilm, 2008; Wijshoff, 2010). As mentioned previously, the inclusion of high-molecular-weight polymers or particulates in food inks could be the cause of this issue, so observation during print tests should be undertaken to determine appropriate cleaning cycle intervals.

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9.6.2 Combining Powder and Ink During Print Commonly in inkjet printing, droplet arrays of a test ink are printed on a substrate at a large droplet spacing; assessments of droplet size, drop-to-drop uniformity and spreading behaviour or affinity for that substrate are undertaken. The droplet spacing is then reduced so that individual drops overlap to produce cohesive arrays, and further modification of this will determine the resolution achieved in 2D as desired. As the nozzle diameter is usually on a micron scale, the drop diameter is also within this range; the space between drops should be below an individual droplet diameter to ensure overlap. However, convention is to express drop spacing in ‘drops per inch’ (DPI) rather than as a distance in micrometres. For example, if printing from a 21-mm nozzle with droplet diameter similar to the nozzle size, one may wish to print with a drop spacing of 10 mm to ensure sufficient overlap; this is equivalent to 2540 DPI (Cie, 2015; Creamer, 2012; Wijshoff, 2010). Utela et al. (2008) highlights the importance of testing powders and inks outside of a 3D printer. They suggest observing the interaction between a pile of powder with ink deposited from a syringe to test the absorptive ability of the powder and subsequent cohesion, as well as selectively combining ink and powder in varying ratios to indicate required saturation and potential green strength of parts. Holland et al. (2017) screened food powder and ink formulations in a bespoke system comprised of a recessed plate with powder-containing wells of 100e400 mm thickness which could be used as a substrate in conventional inkjet printing to observe experimental powder and binder interactions in 2D. This enabled quick screening and adaptation of both powder and ink formulations as well as sensible defined parameters for future 3D application. When considering drop spacing in 3D, we tend to refer to the saturation level, which is defined as the ratio of binder to the void space available, calculated by taking into consideration the total print bed volume versus that taken up by solid particles which, of course, is dependent on the packing density (Chen and Zhao, 2016; Goole and Amighi, 2016). Saturation should be at a sufficient level to achieve binder liquid interaction with powder to facilitate liquid bridging between particles. The addition of a binding agent will cause a difference in the rheological behaviour of the particle at the surface compared with its dry core or native state (Cuq et al., 2013). Through achieving wetting on the surface, without excess binder addition causing further migration into the particle and swelling as a result, adjacent wetted particles will be able to bind to one another. If excess solubilising binder is added, the wetted particle may begin to dissolve slowly into single molecules within the liquid phase rather than forming a sticky outer layer to enable binding. Full dissolution of smaller particles may be desirable to ensure filling and solidifying of unwanted porous spaces; however, if all sized particles exhibit this behaviour, the print quality and resolution could be negatively affected.

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Related to the saturation is the timescale on which printing occurs. As with other food powder agglomeration techniques, the time, temperature and physicochemical reactivity of powders and inks for binder jetting will determine the final structure. Particles coated with an aqueous layer on a short timescale will have high surface stickiness compared to the particle centre, as described above. As time progresses, mass transfer of water occurs from the wetted surface to the particle core (either through capillarity in already porous particles or diffusion at rates governed by the crystalline to amorphous ratio in denser particles), and more soluble components may dissolve into the ink. This phenomenon promotes local plasticization, maybe even conversion of the whole particle to the rubbery state, and adhesive contact between neighbouring particles where ink has been deposited (Bhandari et al., 2013). It is obvious that during any print, the top layers of powder ‘see’ the ink for a shorter amount of time compared to the bottom layers; thus, there is a timescale gradient for interaction through the printed object. This could be critical and something to consider, depending on the materials used and any specific physicochemical interactions necessary to have the desired binding effect prior to postprinting treatments. In addition, some binder jetting setups will have options to retain a heated powder bed or to provide overhead heat for a given time to ensure sufficient binder evaporation or drying. With all of these heating steps, exact parameters will depend on what is required by the powder and binder materials to induce the desired crystallization, adhesion or evaporation. Again, the timescale and potential detrimental effects of heating on the native powder or object throughout printing should be considered. Generally, the addition of sufficient thermal energy increases strength; however, it has been shown that insufficient heating will cause brittleness and potentially failure to adhere properly, and excessive heat will contribute to shrinkage and warping phenomena due to thermal expansion and contraction (Chen and Zhao, 2016; Stansbury and Idacavage, 2016). From design to print, there are two major considerations which should also be measured here, accounting for known shrinkage effects and anisotropic effects resulting from print direction. Shrinkage may be due to thermal effects, as mentioned above, as a consequence of polymerization or due to the uneven packing of a powder bed. It is possible to model and account for shrinkage in design if the printing process itself is consistent and the material properties well known and defined (Chen and Zhao, 2016; Stansbury and Idacavage, 2016; Yoo et al., 1993). Shrinkage may occur to different degrees in the x, y and z directions, and reducing shrinkage in one direction may exacerbate the effects in another. This idea is linked to anisotropy in microstructure and structural strength that can occur based on the orientation of a build in the printer. Farzadi et al. (2014) demonstrate this by orienting the same porous scaffold design to print in the x, y and z directions and performing analysis on the resulting structures. They found printing orientation and layer thickness greatly affected the structural integrity and dimensional accuracy of printed

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designs, recommending that the most important dimension of the printed model should be oriented in the x direction, due to coinciding with the movement of the printhead.

9.7 POSTPROCESSING Inducing particle stickiness and adhesion forces through adequate saturation with an ink binding material is ideal so that each particle will be in contact with neighbours potentially on all sides, increasing strength. However, it does mean that particles that have not specifically had binder printed on them may stick to those at the end of a design, resulting in surface roughness and irregularity. Though depowdering via air blowing or vibration mechanisms can dislodge this weakly bound powder at the surface as a postprocessing step, surface roughness is a common phenomenon in powder-based AM processes in all industries and is difficult to avoid. Depending on the desired finish and product volume, a simple soft brush may be used to dust off excess without placing too much stress on the ‘green part’ (printed piece prior to postprocess sintering or firing) if fragility is an issue. In some instances, a wet depowdering mechanism can be employed with sonication or microwaving; however, the fluid used to complete this must not be a good solvent for the printed materials (Farzadi et al., 2014; Gibson et al., 2009; Utela et al., 2008; Wang and Zhao, 2017). For binder jetting of ceramics, the green part density and strength are significantly enhanced after a temperature-induced sintering process, much like in conventional ceramic casting (Derby, 2015; Yoo et al., 1993). In food materials, this could involve conversion of a predominantly amorphous or semicrystalline printed structure to a more thermodynamically and structurally stable crystalline form, demonstrated by Holland et al. (manuscript in preparation) for cellulosic powders. In nonfood applications, acrylate resins, waxes or varnishes are sometimes used to further boost printed part density. The material used to infiltrate remaining void space must have a melting point below that of the printed material, unless a coating step is also employed (Bai et al., 2015; Chen and Zhao, 2016; Utela et al., 2008). Von Hasseln et al. (2014) describe an example of a binder jet-printed structure with a composite powder (comprised of carbohydrate, milk protein, cocoa butter and flavouring) bound by application of water as an edible binder and infiltrated with melted cocoa butter after printing; this is the only example of an infiltration step in food binder jetting, to the authors’ knowledge.

9.8 APPLICATION AND OUTLOOK FOR FOOD MATERIALS Currently, the most widely demonstrated edible feedstock for binder jetting is sugar. 3D Systems have prepared a wide variety of full colour 3D printed sugar structures using the ChefJet printer with collaborators from the culinary world. Geometric complexity ranges from relatively simple cubes to lattice-type

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FIGURE 9.6 A printed sugar creation designed for serving with absinthe in place of the traditional sugar cube (3D Systems).

structures to organic, free-flowing shapes, including Gaudi-inspired chimney toppers for mixologists to serve with absinthe (seen below in Fig. 9.6). Originally based in the Netherlands but recently moved to Sydney, Australia, Julian Sing also uses binder jetting technology to create designs from sugar with the company 3dChef along with offering products created by other 3D methods. Designs range from cake toppers to customised corporate confections, again utilising colours added to the ink formulation to achieve more interesting designs, an example of which is shown below in Fig. 9.7. An interesting food material application has been showcased by Jason Mosbrucker (3Dible.co), printing structures from a number of food powders including cocoa and coffee, as seen below in Fig. 9.8. The materials are all food grade and enjoyable to eat.

FIGURE 9.7 An example of a printed piece by Julian Sing (3dChef) distributed at the 3D Food Printing Conference 2016 in Venlo, the Netherlands, alongside microcomputed tomography scans of the same piece to observe the internal structure (conducted by the authors at The Hounsfield Facility, School of Biosciences, University of Nottingham which received funding from the European Research Council (Futureroots Project), Biotechnology and Biosciences Research Council, Engineering and Science Research Council and the Wolfson Foundation).

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FIGURE 9.8 Left: Coffee cups printed out of ground coffee. Right: Cocoa and coffee printed structures. Kindly provided by Jason Mosbrucker, 3Dible.co.

As with all 3DP technology, many hobbyist printers trial different ink or powder formulations in their homes that often aren’t documented through press releases or on a scientific forum. Many open-source forums exist online where hobbyists share their creations. The RepRap project wiki page is one such forum (http://reprap.org/wiki/Main_Page). Though RepRap is an extrusion-based printer system, there are tried-and-tested powder and ink ‘recipes’ shared for binder jetting. Example inks include sake rice wine and other alcohol-containing formulations with or without colouring and other additives such as glycerol. Powder formulations are relatively limited, with mixtures of sugars to achieve certain particle size distributions, sugars with added meringue powder or salt and maltodextrin mixtures being the only edible examples posted (http://reprap.org/wiki/Powder_Printer_Recipes). The University of Washington also hosts an open-source website, aptly named Open3DP, to encourage sharing of thoughts and expertise of likeminded people. There are a multitude of blog post updates on process or material optimisation with their various 3DP setups. Their recipes generally feature on the RepRap wiki page also, where binder fluids are usually water and alcohol with optional glycerine, colour, flavour and surfactant components. Edible powders tested include sugar mixtures and a rice flour, sugar and maltodextrin mixture. A myriad of nonfood powders have also been trialled; one powder-tobinder ratio shown to be successful when printing with these formulations is 40:1 (https://depts.washington.edu/open3dp/category/formulas/). Southerland et al. (2011) from the University of West England set out to trial three AM processes for food products, including binder jetting. They used a 50:50 blend of caster sugar and icing sugar to give a suitable particle size distribution based on analysis of the original Z Corp powder used in the machine for production of casting moulds e a range between 37 and 149 mm with a minimum of 55% between 37 and 53 mm. At 35% binder saturation, teeth made of sugar were produced successfully, though it was noted that these structures were noticeably more brittle and had more surface roughness compared with a conventional Z Corp powder and binder structure. Though results are not explicitly mentioned in the publication, the researchers also recommend using starch or cornflour as the powder constituents.

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A few researchers have looked into using alternative powder materials in binder jetting techniques. Holland et al. (2017) describe the design and characterisation of cellulose-based powders as a feedstock for binder jetting. The mechanism of adhesion relies on a recrystallization event of the cellulose (amorphous through mechanical attrition) triggered through application of the aqueous ink and necessary thermal energy, to render the printed piece in a semicrystalline form but the surrounding powder unbound and still amorphous. Other stereochemically similar polysaccharides added in the powder or ink component modify this recrystallization temperature, provide additional adhesion and thus vary the resulting mechanical properties. The paper describes observing interactions in 2D; a manuscript is being prepared to show results in 3D from a bespoke, small-scale layering setup used with an inkjet printer. Unlike sugar, cellulose (a plant cell wall structural polysaccharide) cannot be digested by humans nor fermented in the gut, thus is classed as a dietary fibre and does not contribute to calorific intake. Thus, 3DP structures from cellulosic material open up options for reduced calorie structures to be used by both the food manufacturing industry and consumers. There are two main cited patents relating specifically to binder jetting of food materials: Von Hasseln et al. (2014) and Diaz et al. (2015). The former is related to the 3D Systems ChefJet technology covering the production of coloured and flavoured food structures with 25e75%wt maltodextrin and 25e75%wt confectioner’s sugar (similar to those pictured above) as well as a scenario where the product contains 1e25%wt seed crystals, i.e., cocoa butter polymorph V to produce structures similar to Fig. 9.9. They note that combining such powders with a binder containing water, alcohol, glycerine

FIGURE 9.9 A complex binder jet printed design containing chocolate elements. Source: 3D Systems.

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FIGURE 9.10 Top: TNO Powder Binder Printing (PBP) set up (right) and test squares (left). Bottom: Cookie formulation with texture modulation between layers achieved by altering printer settings. Source: TNO, The Netherlands.

and salt will give a harder, ‘candy’-type texture, whereas a binder of milk, alcohol and water would provide a smoother texture, akin to frosting. The latter patent is more detailed in the description of potential feedstock materials (incorporating protein and plant polysaccharides not used by Von Hasseln et al.) and recipes to create products. These range from pasta to cake to cookies and even protein bars. Through varying material composition and relative ratios (particularly of protein and maltodextrin), textures created can range from dry and hard to soft and rubbery. An example of a multitextured cookie produced by this technique can be seen in Fig. 9.10. By varying the printer settings, the top and bottom sections were designed to be more porous, with a dense section between them. After baking in a conventional oven. the material colour changed from white to yellowebrown, and the resulting cookie was multitexture with a soft top and bottom and hard middle (TNO, The Netherlands). Printed products can either be consumed directly after printing, though textural changes may occur on storage, baked in a conventional oven (as for the cookie embodiment), boiled in water immediately or dried for later use (when using pasta materials). The future for binder jetting of food materials is exciting. Through the research which has already been done in the area, it is clear that a variety of materials may be utilised to create structures. With researchers at universities and within companies looking beyond sugar as a feedstock and at the creation of composite powders, a lot of scope exists for the use of this technology, both by consumers at home or companies within the food industry. The technology

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is already creating headlines in the pharmaceutical world with the recent US Food and Drug Administration approval of a binder jet-printed drug ‘Spritam’ manufactured by Aprecia used to prevent epileptic seizures (FDA, 2015; Royal Society of Chemistry, 2015), so it is possible that we could see a parallel breakthrough with food products. Because the unbound powder acts as a support material, binder jetting has an advantage over other 3DP techniques relevant to foods in that it is easier to create more complex geometries than, for example, when using extrusion. A variety of end textures have already been demonstrated. Unlike other 3DP technologies used for foods, the achieved minimum feature size depends on three key aspects: powder particle size, layer thickness and ink nozzle diameter. By optimising materials in these areas, there is great potential not only to develop food macrostructures by binder jetting, but also design microstructures as well.

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