Computers and Electronics in Agriculture 162 (2019) 397–404 Contents lists available at ScienceDirect Computers and Electronics in Agriculture journal homepage: www.elsevier.com/locate/compag Original papers A comparative study between syringe-based and screw-based 3D food printers by computational simulation T Chao-Fan Guoa,b, Min Zhanga,d, , Bhesh Bhandaric ⁎ a State Key Laboratory of Food Science and Technology, Jiangnan University, 214122 Wuxi, Jiangsu, China International Joint Laboratory on Food Safety, Jiangnan University, 214122 Wuxi, Jiangsu, China c School of Agriculture and Food Sciences, The University of Queensland, Brisbane, QLD 4072, Australia d Jiangsu Province Key Laboratory of Advanced Food Manufacturing Equipment and Technology, Jiangnan University, China b ARTICLE INFO ABSTRACT Keywords: Computational simulation Screw-based 3D food printer Syringe-based 3D food printer Comparative study Extrusion-based three-dimensional (3D) printing is an emerging technology which has a high application potential in food manufacturing processes. Screw and syringe-based printers are two kinds of extrusion-based 3D printing methods that have been widely studied and reported. The objective of present work was to compare these two different kinds of extrusion-based food 3D printing methods, in fluid flow characteristics and printing profile by computational simulation model and printing experiment. Analysis of simulated model suggested that the screw-based 3D food printer had a complex fluid characteristic, and some backflows were found at the gap between walls and the screw flights in the extrusion tube. Whereas, the syringe-based 3D food printer showed more simple fluid characteristics, which could be easy to adjust. Moreover, the experimental 3D printing suggested that the screw-based 3D food printer were not suitable for extruding the inks with high viscosity. Results in present work provides information for suitable printing method selection, a theoretical base and technical guide for further 3D printing studies and new printer designing. 1. Introduction Additive layer manufacturing, which is also known as three-dimensional (3D) printing, is a kind of rapid prototyping technology, involving material and computer science, numerical control technology, precision transmission, and direct writing systems (Yang et al., 2018a). Since the very first 3D printer was reported in 1980s, 3D printing technologies have been researched in food manufacturing processes due to its great potential to create complex geometric structures for consumer demands with economic and environmental benefits (Kim et al., 2018). Recently, various printing methods have been investigated in food 3D printing manufacturing processes, such as hot melt/room temperature extrusion, selective laser sintering, hot air sintering, injection, and binder jetting printing. Among them, the extrusion-based 3D printing method is the most commonly used in food printing processes (Sun et al., 2018). Different to extrusion-cooking, the food extrusion 3D printing is a digitally controlled and robotic construction process for building up a complex 3D food object layer by layer (Huang et al., 2013). The food extrusion 3D printing starts with loading inks, extruding inks out of nozzle in a controlled manner, moving the ink stream according to the programmed path, and forming ⁎ a complex structure by depositing layer by layer. The extrusion-based printer consists of a digitally controlled multi-axis stage and one or more extrusion units. According to the extrusion mechanism, the extrusion unit can be classified into syringe-based extrusion, screw-based extrusion and air pressure driven extrusion (Sun et al., 2018). In the syringe-based extrusion unit, the plunger is driven by a programmed stepper motor, which provides a linear motion, pushing the food ink out of nozzle (Fig. 1a). The extrusion rate can be easily adjusted by varying the speed of motor. When the viscosity of food inks is too high, the motor requires more power to extrude it. The syringe-based 3D food printer has been widely applied into food printing processes and reported in many previous studies (Sun et al., 2018). On the other hand, the screw-based feeding mechanism has been successfully applied in food printing processes like nostoc sphaeroides biomass (An et al., 2019), mashed potatoes (Liu et al., 2018b), fish surimi (Wang et al., 2018) and dough (Yang et al., 2018b) 3D printing processes. In the extrusion tube, the screw driven by a programmed motor continuously brings the ink downwards from the cartridge and passes through the narrower tube nozzle (Fig. 1b). However, the screw-based extrusion 3D printing method has been reported not suitable for inks with high viscosity and Corresponding author at: School of Food Science and Technology, Jiangnan University, 214122 Wuxi, China. E-mail address: [email protected] (M. Zhang). https://doi.org/10.1016/j.compag.2019.04.032 Received 13 August 2018; Received in revised form 8 April 2019; Accepted 23 April 2019 Available online 28 April 2019 0168-1699/ © 2019 Elsevier B.V. All rights reserved. Computers and Electronics in Agriculture 162 (2019) 397–404 C.-F. Guo, et al. Fig. 1. Extrusion mechanisms: (a) syringe-based extrusion, (b) screw-based extrusion, and (c) air pressure driven extrusion. high mechanical strength which cannot achieve a proper extrusion for good printing precision (Liu et al., 2017). The mechanism of air pressure driven extrusion unit is similar to the syringe-based unit, where the materials in the food ink cartridge is pushed by the air pressure produced by a pneumatic pump (Fig. 1c). In food extrusion-cooking studies, the computational fluid dynamics (CFD) and finite element meshing (FEM) are efficient ways to simulate the fluid characteristics inside the extruder, used to understanding the inside underlying mechanisms. Dhanasekharan and Kokini (2003) using a computer simulation method to analyze a scale-up single screw extrusion of wheat dough, for its mixing and heat transfer. Emin and Schuchmann (2013) did numerical simulation of a twin screw extruder for analyzing the dispersive mixing efficiency of the plasticized starch. Singh Sushil and Muthukumarappan (2015) used the CFD and FEM to design and analyze the extrusion process of Jatropha seeds by a single screw extruder. As for extrusion-based 3D printing, the fluid characteristic and underlying mechanisms are similar to the extrusioncooking. Numerical models of 3D printing have also been established and widely studied. Woodfield et al. (2004) using the Hagen-Poiseuille law established the flow of biomaterials through the nozzle. Khalil and Sun (2007) studied the effect of the non-Newtonian fluid properties of the gel material on the printing and forming process of the scaffold, according to rheological theory. Li et al. (2009) established a numerical model for the relationship between the volumetric velocity of gel and the pore diameter and porosity of scaffold. Yang et al. (2019) used a commercial numerical simulation software, named Pollyflow, studying the fluid flow properties of lemon juice gel in the syringe for improving the 3D printing process. However, to our knowledge, limited study is found in the literature dealing with the simulation model of the screwbased extrusion printer. The comparison of the fluid characteristic and underlying mechanisms between screw-based and syringe-based extrusion 3D food printer have not yet been documented and this study is aimed to reveal this. In this work, CFD models were developed to investigate and compare the fluid characteristic of two 3D printing units. An experimental 3D printing study was also carried out for the comparison of two selected 3D food printers. software (V4.3a, COMSOL Multiphysics, Burlinton, USA) which is a commercially available software based on the FEM. In this work, the rotating machinery feature and the laminar flow features in the CFD Module were used to solve the fluid characteristic in the screw-based extrusion 3D printing equipment and the syringe-based extrusion 3D equipment, respectively. 2.1.1. Physical model The schematic diagram of the screw-based extrusion 3D printing unit (SHINNOVE-D1, Shiyin co. LTD, Hangzhou, China) and the syringe-based extrusion 3D printing unit (FSE 2, PORIMY co. LTD, Wuxi, China) used in present work for experimental study and computational study is shown in Fig. 2. Both types of printer consisted of a polar configuration multi-axis stage and an extrusion unit. In screw-based extrusion 3D printing equipment, food inks are added into the hopper or reservoir, which is designed to have a large opening at the top for material loading, and then an auger screw transports inks to the nozzle for continuous printing. On the other hand, in the syringe-based extrusion 3D printing equipment, food inks are stored in the syringe cartridge and are pushed out of the nozzle by a syringe plunger driven by a programmed stepper motor. To reduce the computation time, the physical models of both extrusion tube and syringe were considered for a length of 25 mm. 2.1.2. Basic assumptions governing equations and, initial and boundary conditions Mashed potatoes were used in this study as the ink for 3D printing. The temperature during printing was maintained constant at 26 ○C. The fluid was assumed an incompressible single-phase fluid with laminar flow interface. The density and viscosity of the ink were 9800 kg/m3 and 42,000 Pa s, respectively. Boundary conditions and initial values assumed for the extrusion process are listed below. – The screw speed was assumed as constant (n = 2 r/s) – The velocities of inlet were assumed as 0.04 m/s for syringe-based printing equipment, 0 m/s for screw-based printing equipment. The pressure of outlet of both these two printers was constant (P = 101,325 Pa) – The surface of screw was set as no slip. The wall of extrusion tube, syringe and nozzle were also set as no slip. – The initial values of velocities and pressures were 0 m/s and 101,325 Pa for both the syringe-based printing equipment and screw-based printing equipment. 2. Materials and methods 2.1. Computer simulation A computational study was used to understand and compare the flow field in the two selected extrusion-based 3D printing equipment. The CFD simulations were performed using COMSOL Multiphysics 398 Computers and Electronics in Agriculture 162 (2019) 397–404 C.-F. Guo, et al. Fig. 2. Schematic diagrams of (a) syringe-based 3D printing equipment, (b) screw-based 3D printing equipment and CAD models for (c) syringe and (d) extrusion tube. The governing equations for a time-dependent single-phase fluid is given by COMSOL as Eqs. (1) and (2). u + (u· ) u = t ·[ p I+ µ ( · u= 0 u+ ( u)T )] + F study need to run 92 s and 410,479 s respectively solving the result of laminar flow for syringe and extrusion tube. (1) 2.2. Printing experiments (2) 2.2.1. Sample preparation Potato flakes was purchased from Shanxi Sanlai Food Co. LTD (Shanxi, China), and was determined for the content of moisture (80.3 ± 0.6 g/kg) by AOAC 934.01, lipid (3.2 ± 0.2 g/kg) by AOAC 923.05, starch (684.6 ± 3.6 g/kg) by AOAC 979.10, and protein (103.2 ± 2.1 g/kg) by AOAC 920.87. 3 where ρ is the density (kg/m ), u is the velocity vector (m/s), p is pressure (Pa), F is the volume force vector (N/m3), T is the absolute temperature (K). Eq. (1) is the equation for continuity, and Eq. (2) is the momentum equation. These equations are applicable for the incompressible single-phase fluid which has a constant density, when the temperature variations in the flow are small. After meshing, the models consisted of 193,449 and 804,952 elements for syringe and extrusion tubes, respectively. The simulations were performed on a PC with Intel (R) Core (TM) i5-7300 CPU @ 2.60 GHz, 8.00 GB RAM with a Microsoft Windows 10, 64-bit operating system. Based on the above hardware conditions, the two models in this 2.2.2. 3D printing Mashed potato in this experiment was prepared by using the potato flakes with different ratios of water (potato flakes: water = 1:3, 1:4.5 and 1:6). Mixtures of potato flakes and water were stirred adequately, then covered with a thin layer of cling wrap to avoid the moisture loss. 399 Computers and Electronics in Agriculture 162 (2019) 397–404 C.-F. Guo, et al. Mixtures covered in containers were steamed at 100 ○C for 20 min, until starches in Mashed potato were fully gelatinized. After steamed, gelatinized mashed potato was cooled to 26 ○C to prepare for printing. Both of the screw-based and syringe-based printers were controlled by a computer installed with Repetier Host V2.0.5 and Slic3r software (Hot-World GmbH & Co. KG, Willich, Germany). The printed model was downloaded from internet as a stereolithography file (.stl), then loaded into the Repetier Host software and sliced by the Slic3r software into Gcode. The printing head was controlled in X, Y and Z axis by the G-code. A 1.0 mm diameter nozzle was used in this study for both screwbased and syringe-based printers. The nozzle height, extrusion rate, and nozzle movement speed for all printing experiments were all set at same values according to pre-tests. Thus, reducing the gap between walls and the screw flight in the extrusion tube can be an efficient way to improve the capacity of the screw-based 3D food printer for high viscosity inks. Another way to improve the extrusion performance is to reduce the viscosity of inks. Whereas, this measure may result in a reduction in the mechanical strength of inks, thereby reducing the precision of the extruded object and its supporting capacity (Liu et al., 2018b). The syringe-based 3D food printer is a better way to overcome the back flow issue in the printing process of high viscosity inks (Vaezi and Yang, 2015). Liu et al. (2017) also suggested that the syringe-based and air pressure-based extrusion units are more suitable for the materials with high viscosity and mechanical strength than screw-based extrusion unit. In another note, the syringe-based and air pressure-based 3D food printer have their natural drawbacks, for example the batch printing (Liu et al., 2017). Screw printing can operate continuously as it enables to feed the ink material through the inlet hopper. 2.2.3. Rheological properties measurements The rheological property of mashed potato was carried out by using a hybrid rheometer (Discovery HR-3, DHR, TA Instruments, USA) installed with a parallel plate with diameter of 20 mm and a gap of 1000 μm at 25 ○C. Mashed potato samples were loaded in the gap and were allowed to rest for 10 min before measurement to achieve a stable measurement results. The storage modulus (G′) and viscous modulus (G″) measurements were carried out by a dynamic oscillation frequency analysis with 0.1% strain and with linear viscoelastic range with frequency of 0.01–16 Hz. A sweep flow analysis was conducted at a shear rate range from 0.001 to 10 1/s for obtaining viscosity property. 3.2. Simulated shear rate distribution The rheological and viscosity properties of inks are critical for a precision and accuracy in printing (Godoi et al., 2016). Among these parameters, the viscosity directly affects the ink extrusion effect. A desirable low viscosity is crucial for providing a continuous extrusion (Liu et al., 2017). Whereas, some edible inks, such as mashed potato, fish surimi gel and lemon juice gel, are pseudoplastic or shear-thinning fluids (Liu et al., 2018b; Wang et al., 2018; Yang et al., 2018a). The shear rate distribution in extrusion tube/syringe is crucial for understanding the state of fluid, for example the viscosity, thus for better control the extrusion process. As shown in Fig. 4a, the shear rate of fluids in the syringe cartridge distributed homogenously with a low value. High shear rate values were found around the outlet of nozzle, rapidly increasing from top to bottom. These high shear rates could decrease the viscosity of shearthinning inks to a relatively low value. Thus, the ink could be easily ejected through the tiny nozzle outlet (Shastry et al., 2006). As for the screw-based 3D food printer, most of the shear rate values were homogenously distributed in the extrusion tube. Whereas, some high shear rate values were found at the gap between walls and the screw flight (Fig. 4b). This simulated result is similar with the previous simulation study on screw extrusion (Singh Sushil and Muthukumarappan, 2016). During extrusion, the screw spins rapidly to provide a push for ink. As a consequent, inks at the gap will suffer a high shear rate by the spinning flight. These high shear rates thin the inks at the gap resulting in an idle turning of the screw. 2.3. Data analysis Data analysis was carried out with the SPSS 20.0 software (IBM, Chicago, IL, USA). All experiments were performed in triplicates. 3. Results and discussion 3.1. Simulated velocity profile Velocity profile of syringe-based 3D food printer was simple topdown laminar flow (Fig. 3a). The velocity of fluid in the cylinder was relatively slow but uniformly distributed. Starting from the bottom part of the syringe, the flow line concentrated towards the center and its velocity gradually increased from the top to the bottom of the nozzle, with a maximum velocity at the outlet of nozzle. This simple, top-down flow field may cause an accumulation of dispersed phase in the multiphase ink at the junction of the cylinder and the nozzle. As shown in Fig. 3b, the velocity profile of screw-based 3D food printer was complex. High velocities were found around the screw, while fluids in nozzle showed low velocities. Most of the fluid in extrusion tube presented rotating around the screw with higher velocities. Note that, a vortex occurred at the top of nozzle, which can stir inks for a better uniformity. These velocity profiles appeared in extrusion tube and nozzle show a potential for the printing of non-homogenous (multiphases) inks, for example the mixture of solids and liquids. As shown in Fig. 3c, the longitudinal velocity field distribution in syringe-based 3D food printer was homogenous at the cylinder and increased from top to bottom of the nozzle. Moreover, the velocity of fluid in the nozzle gradually rose from the boundary to the center. This signified that the velocity of inks at the exit distributed non-uniformity, which may cause a jet expansion affecting the printing. On the contrast, the longitudinal velocity field distribution in screw-based 3D food printer was more complex than that in syringe-based printer (Fig. 3d). High longitudinal velocities were found under the screw thread, while some backflow occurred at the gap between walls and the screw flights. For powder-feeding printing, for example chocolate printing, once the molten materials flows back to the lower temperature region, it will solidify and affect the overall printing process (Tseng et al., 2018). 3.3. Simulated pressure distribution As shown in Fig. 5a, the pressure distributed homogenously in the cylinder of syringe-based 3D food printer, while decreased rapidly near the outlet of nozzle. The pressure gradient between the nozzle and the cylinder ensured that the ink can be extruded smoothly. Inks in cylinder suffered the highest pressure among the printer, which is directly applied through the plunger of syringe. For printing the inks with high viscosity, higher power needs to be applied to the motor of printer (Sun et al., 2018). For screw-based 3D food printer, the pressure distributed homogenously in the nozzle. However, some negative pressure values were found above the screw flight at the top of the extrusion tube Fig. 5b. This negative pressure forms a pressure gradient with atmospheric pressure, allowing the ink to be fed continuously from the hopper. However, the pressure of fluids in extrusion tube increased from top and was found highest at the bottom of the extrusion tube and nozzle. This simulated result indicated that a backflow may happen when the pressure at the nozzle point is too high which corresponds with the results of simulated velocity mentioned above. From the 400 Computers and Electronics in Agriculture 162 (2019) 397–404 C.-F. Guo, et al. Fig. 3. Simulated velocity (m/s) profile of (a) syringe-based 3D food printer, (b) screw-based 3D food printer and longitudinal velocity field of (c) syringe-based 3D food printer, (d) screw-based 3D food printer. pressure contour of the surface of the extrusion tube and the screw, the pressure experienced by screw was higher than walls of tube, while significantly lower than that of fluids in extrusion tube (Fig. 5b). Different to syringe-based extrusion, the screw suffers lower pressure than the plunger of syringe-based extrusion. Ignoring the problem of the backflowing issue of inks, the motor of a screw-based printer requires lesser power than that of syringe-based printer. previous studies (Liu et al., 2018a). This suggested that the mashed potato used in this study was pseudoplastic shear-thinning fluids (Yang et al., 2018a). Furthermore, the viscosity of mashed potato was decreased with the increasing of water content. Both of the storage modulus (G′) and loss modulus (G″) decreased with the increasing of the water content of mashed potato (Fig. 6b). This result indicated that the mechanical strength of mashed potato was highest at the lowest water content (potato flakes: water = 1:3), decreasing with the addition of water (Liu et al., 2018b). An experimental printing test was carried out by both screw and syringe-based 3D food printers. The printing profile is shown in Table 1. Both of the mashed potato formulated with 1:3 and 1:4.5 (potato flakes: water) were successfully extruded by the syringe-based printer, while 3.4. 3D printing experiment The rheological properties of mashed potato with different water content are shown in Fig. 6a. The viscosity of mashed potato was decreased with the increasing of shear rate which is similar with the 401 Computers and Electronics in Agriculture 162 (2019) 397–404 C.-F. Guo, et al. Fig. 4. Simulated shear rate (1/s) distribution of (a) syringe-based 3D food printer and (b) screw-based 3D food printer. the mashed potato with 1:6 (potato flakes: water) was found to be too thin to form a stable supporting structure. On the other side, the screwbased printer was found to had a difficulty with the printing with the high viscosity mashed potato (potato flakes: water = 1.3 and 1:4.5). During the extrusion process, the mashed potato with high viscosity adhered to the wall of hopper which was also difficult to be fed into the extrusion tube. Adhering of mashed potato was also found with the screw and walls in the extrusion tube. Shear-thinning occurred around the spinning screw flight, resulting an idle spinning of screw and slippage. Consequently, the screw in extrusion tube of printer couldn’t supply proper pressure to the mashed potato in nozzle, resulting in failure of extruding out. Mashed potato formulated with 1:6 (potato flakes: water) could be extruded by the screw-based printer successfully, the mechanical strength of printed object was too low to form a stable supporting structure. Fig. 5. Simulated pressure (Pa) distribution of (a) syringe-based 3D food printer and (b) screw-based 3D food printer. 402 Computers and Electronics in Agriculture 162 (2019) 397–404 C.-F. Guo, et al. Table 1 The printing profile of syringe-based printer and screw-based printer. Water content ((potato flakes: water)) Syringe-based printer Screw-based printer 1:3 Unable to print 1:4.5 Unable to print 1:6 Too thin to form a structure Fig. 6. Rheology properties of mashed potato with different water content (potato flakes: water). Too thin to form a structure 4. Conclusions Natural Science Foundation Program of China (No. 3187101297), China State Key Laboratory of Food Science and Technology Innovation Project (Contract No. SKLF-ZZA-201706), National First-class Discipline Program of Food Science and Technology (No. JUFSTR20180205), Jiangsu Province Key Laboratory Project of Advanced Food Manufacturing Equipment and Technology (No. FMZ201803), which enabled us to carry out this study. In current years, more and more researchers and companies have focused on improving the extrusion-based food printing process for achieving a new digital cooking device, promoting innovative designs and properties of foods in order to provide a healthy life of consumers. In addition, it is important to find a suitable extrusion method for proper inks before 3D printing based on the rheological properties of inks. To better understand the fluid characteristic and the inside underlying mechanisms, the mathematical model was built in this work. According to the simulation of velocity profile, shear rate and pressure distribution, the syringe-based 3D printing showed a simple fluid characteristic. The high shear rate and low pressure occurred around the outlet of nozzle which are beneficial for extruding the inks. On the other hand, the simulated fluid characteristic of screw-based 3D food printer was found to be complex, which may be positive for extruding the multiphase inks. However, in simulating study, high shear rate and backflows was found at the gap between walls and the screw flight. Moreover, in actual 3D printing experiment, the screw-based 3D food printer was found not suitable for printing the inks with high viscosity. Further studies can focus on the designing of new screw-based extruder for high viscosity inks, for example decreasing the gap between walls and screw flights, and its potential application in multiphases printing. References An, Y.-J., Guo, C.-F., Zhang, M., Zhong, Z.-P., 2019. 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