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4D printing using anisotropic thermal deformation of 3D-printed thermoplastic parts
Materials and Design 188 (2020) 108485
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4D printing using anisotropic thermal deformation of 3D-printed thermoplastic parts Bona Goo a, Chae-Hui Hong a,b, Keun Park a,⁎ a b
Department of Mechanical System Design Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea Smart Information Technology Team, Samsung Display Co. Ltd., Asan 31454, Republic of Korea
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• An efficient 4D printing method was developed using a single thermoplastic polymer without a shape memory function. • To enable shape transformation, the printing paths were programmed to impose bidirectional anisotropy intentionally. • The difference in thermal deformation of transversely and longitudinally printed layers were caused bending deformation. • The selective lamination of isotropic and anisotropic regions enabled a localized bending for a self-assembly function.
a r t i c l e
i n f o
Article history: Received 18 November 2019 Received in revised form 7 January 2020 Accepted 8 January 2020 Available online 10 January 2020 Keywords: Additive manufacturing 4D printing Material extrusion Anisotropic thermal deformation Bidirectional printing
a b s t r a c t Four-dimensional (4D) printing is an advanced application of 3D printing with the additional shapes changes over time. For appropriate shape changes, 4D printing has been developed using shape memory materials or multi-material structures with different deformation behaviors in response to an external stimulus. In this study, an efficient 4D printing method was developed using a material extrusion (ME) type 3D printer and a single thermoplastic polymer without a shape memory function. To impose different deformation behaviors in a single thermoplastic material, the printing paths were programmed intentionally, and the resulting anisotropy was used to generate a unique thermal deformation in response to a thermal stimulus. The anisotropic thermal deformations of the longitudinally and transversely printed parts were investigated by analyzing the directional size changes of homogeneously laminated bars. This anisotropy was then used to enable 4D printing by applying heterogeneous lamination in which the transverse and longitudinal printing paths were used consecutively. This heterogeneous lamination showed bending deformation after heat treatment, and the effect of lamination strategy on the order of bending was investigated experimentally. The proposed 4D printing method was further applied to induce localized bending deformation by laminating isotropic and anisotropic regions selectively, which enabled a self-assembly function. © 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (K. Park).
Additive manufacturing (AM), also known as three-dimensional (3D) printing, has extended its application area from conventional
https://doi.org/10.1016/j.matdes.2020.108485 0264-1275/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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rapid prototyping (RP) to the direct fabrication of functional parts. A unique extension of AM is 4D printing, which adds changes of shapes or properties over time to conventional 3D printing [1]. Fourdimensional (4D) printing enables the manufacture of dynamic structures with adjustable shape transformations and has various applications, such as self-assembly, self-repair and self-adaptability [2]. Fundamental elements of 4D printing can be categorized as 3D printers, external stimuli, stimulus-responsive materials, interaction mechanisms and mathematical modeling [3]. Various studies have been performed to achieve shape transformations of 3D-printed parts using various combinations of stimuli and stimulus-responsive materials [4,5]. The most popular 4D printing method is to use smart materials that respond to external stimuli by changing their shapes and physical properties [6]. The most widely used smart material is shape memory polymers (SMPs), which can recover their original shapes by triggering certain environmental changes, such as thermal or solvent stimuli [7–10]. Thermal response SMPs make transformations by a thermal stimulus from a certain temporary shape to its inherent permanent shape. The thermal SMPs have generally been used as a composite of SMP fibers in an elastomeric matrix to realize 4D printing [11–14]. Water response SMPs, such as hydrogels, undergo size change in response to water [15], and have been used as a smart material composite to achieve 4D printing effects [16–19]. While these applications use smart materials to change the shapes of 3D-printed parts, 4D printing has also been attempted in the case of printing multi-materials that have different swelling or deformation properties [20–23]. This multi-material structure, however, requires 3D printers to support multi-material printing, which limits the use of 4D printing to particular materials and equipment. This study developed a 4D printing method using a material extrusion (ME) type 3D printer, also known as a fused filament fabrication (FFF) type printer. In ME type 3D printing, a thermoplastic filament is extruded through a narrow nozzle, and it thus possesses anisotropic mechanical properties due to the directional pre-strain and residual stress [24]. This directional pre-strain and residual strain was used in 4D printing by generating thermal shrinkage of 2D patterned lattice materials [25], and by enabling 3D bending deformation of 2D printed composite sheets [26]. The ME printing was also used to investigate the shape transformation characteristics by printing thermos-responsive SMP filaments [27,28]. Recently, the ME printing of SMP filaments was used in 4D printing by controlling the printing path in a unidirectional manner, in order to enhance anisotropy and the resulting shape transformation [29,30]. In this study, we de developed an efficient 4D printing method which uses an ME-type 3D printer and a single thermoplastic filament without a shape memory effect. To intensify shape transformation of the ME-printed parts, the printing paths were programmed in a bidirectional manner. That is, a number of transversely printed and longitudinally printed layers were laminated in sequence. We investigated the thermal deformations of the longitudinally and quantitatively analyzed transversely printed specimens, and the resulting shape changes and directional thermal strain. Printing paths were then programmed by laminating the transverse and longitudinal layers consecutively, which resulted in bending deformation when the printed sample was heated above its softening temperature. The deformation mechanism of the proposed method was investigated theoretically and numerically, and the effect of the longitudinalto-transvers ratio on the amount of bending deformation was analyzed and compared with experiments. This anisotropic thermal deformation was then used to transform various 3D printed parts by changing their printing paths and heat treatment conditions. The proposed 4D printing method was further applied to perform a self-assembly function by selectively printing anisotropic regions between isotropic regions and by localized bending deformation in the anisotropic region.
2. Materials and methods 2.1. Materials As a printing material, we used acrylonitrile butadiene styrene (ABS) filaments that were 1.75 mm in diameter (Shenzhen ESUN Industrial Co. Ltd., China). The density, thermal expansion coefficient, and glass transition temperature of the ABS filament were 1.04 g/cm3, 8.82 × 10−5/°C, and 107.8 °C, respectively. Additively manufactured parts using this filament are known to have orthotropic mechanical properties according to the printing direction; the elastic moduli for the X-, Y-, and Z-directions were 2.18, 2.50, 2.26 MPa, respectively [31]. 2.2. Additive manufacturing with homogeneous lamination AM was performed using an ME type 3D printer (Cubicon Single, Cubicon Inc., Korea). This printer has a printing chamber in which the chamber temperature is controlled in a convective manner. In the additive manufacturing step, the temperatures of the nozzle, bed, and chamber were set to 240, 115 and 60 °C, respectively. The layer thickness was set to 0.2 mm, and the printing speed was set to 80 mm/s. In ME type 3D printing, it is generally recommended to use 45° and − 45° raster angles alternatively to minimize in-plane anisotropy [32]. In this study, however, the raster angle was programmed intentionally to impose in-plane anisotropy. Fig. 1 illustrates two types of printing paths for a rectangular bar specimen (60 × 6 × 1.6 mm): a longitudinal printing path with a 0° raster angle and a transverse printing path with a 90° raster angle. In both cases, eight layers were printed and laminated with a consistent printing path, which is called as a homogeneous lamination. In homogeneous lamination, the two printed bars are expected to have different material orientations that may result in anisotropic thermal deformation by an external thermal stimulus. 2.3. Additive manufacturing with heterogeneous lamination The anisotropic thermal deformation was then used to realize 4D printing by mixing the transverse and longitudinal printing paths, socalled heterogeneous lamination. For heterogeneous lamination, a rectangular bar (60 × 6 × 1.6 mm) was printed by laminating a number of transversely printed layers (nt) and a number of longitudinally printed layers (nl) consecutively. The layer thickness was also set to 0.2 mm, and a total of eight layers were laminated to build a 1.6 mm thickness. Thus, we expected the printed bar with the bidirectional anisotropy to have different thermal deformations across the border between the transverse and longitudinal regions, and thereby to show bending deformation when a thermal stimulus was applied. 2.4. Heat treatment As a thermal stimulus, heat treatment was performed using an electric furnace with forced convection (DHG-9070A, NeuronFit Co. Ltd., Korea). The furnace was preheated to 150 °C before the heat treatment. The printed samples were placed on an AA6061 plate (220 × 180 × 5 mm),
Fig. 1. Configuration of longitudinal and transverse printing paths for rectangular bar specimens.
B. Goo et al. / Materials and Design 188 (2020) 108485
and put into in the preheated chamber for heat treatment. Heat treatment was performed for 15 min while maintaining the chamber temperature at 150 °C. During the heat treatment, shape transformations of printed samples were observed through a glass window of the furnace. The heated samples were then removed out of the furnace, and were cooled at a room temperature for 1 h. 2.5. Characterization To measure the glass transition temperature of the ABS filaments, a differential scanning calorimetric (DSC) test was performed. A DSC thermal analyzer (DSC25, TA Instruments, USA) was used at a rate of 10 °C/min from 20 to 300 °C in a nitrogen atmosphere. To observe sectional images of the printed parts, a digital optical microscope (Mi-9000, Jason Electro-Tech, Korea) was used. The amount of bending deflection after heat treatment was measured using a high resolution 3D scanner (Rexcan DS2 5.0, Solutionix Co., Korea). To measure directional thermal expansion or shrinkage, a digital Vernier caliper with the resolution of 0.01 mm (H500-20, Mitutoyo Co., Japan) was used. The size changes of the rectangular specimens were measured for the specimens before and after heat treatment. Five specimens were measured for each printing direction, and directional strain components (εi) were calculated based on the measurement results. The effective thermal expansion coefficient (αi) was then calculated by considering these directional strain components as the thermal strain, as given in Eq. (1). 0 ε 1 Li −Li ai ¼ i ¼ ð1Þ Li ΔT ΔT where the subscript i denotes the Cartesian direction (x, y, and z), and Li and Li′ are the corresponding directional dimensions before and after heat treatment with a temperature change of ΔT, respectively.
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3. Results and discussion 3.1. Thermal deformation of homogeneously laminated specimens 3.1.1. Directional deformation and thermal strain As a homogeneous lamination, rectangular bar samples were printed using three printing paths: standard (45° and −45°), longitudinal (0°) and transverse (90°) paths. Heat treatments were performed for 15 min at 150 °C temperature, as explained in Section 2.4. Fig. 2a compares the shape changes of the bar specimens after the heat treatment. The standard specimen showed isotropic shrinkage such that both the length and width of the specimen were reduced after heat treatment. On the other hand, the longitudinally printed specimen underwent anisotropic deformation with a decrease in length and an increase in width. The transversely printed specimen showed the opposite trend with an increase in length and a decrease in width. These contrary results can be explained by the difference in the residual stresses according to the printing direction. In the ME type 3D printing, the polymer material undergoes two-step extrusion processes. The first extrusion is the filament manufacturing process in which the molten polymer is extruded through an extrusion die. The second extrusion is the printing process in which a thick filament 1.75 mm in diameter is extruded through a thin nozzle with a 0.4 mm diameter. Due to these extrusion processes, the printed part becomes highly oriented along its longitudinal direction with tensile residual stress in its length and compressive residual stress in its cross section. Therefore, the relaxation of these residual stresses results in an anisotropic size change, a decrease along the printing direction and an increase along the transverse direction, as shown in Fig. 2a. Table 1 compares the detailed size changes for the longitudinally and transversely printed specimens. It can be seen that the calculated thermal expansion coefficients have negative values along the printing
Fig. 2. Thermal deformation of the rectangular bars. (a) Photographs of standard, longitudinal and transverse samples before and after heat treatment. (b) Comparison of the vertical deflection along the longitudinal centerline. (c) 3D plot of the vertical deflection (Δz) of the standard sample.
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direction (i.e. the x-direction for the longitudinal printing and the ydirection for the transverse printing), and positive values along the lateral direction (i.e. the y-direction for the longitudinal printing and the xdirection for the transverse printing). On the other hand, the thermal expansion coefficients along the thickness direction have positive values along the both printing cases, of which magnitudes (2.22 and 2.26 × 10−3/°C) were much greater than those of the other directions. 3.1.2. Deflection along the vertical direction Fig. 2b compares the vertical deflections of three bars after heat treatment. Here, the center point of each top face was selected as the origin point, and the relative z-position was plotted along the centerline of the top face. The longitudinally-printed bar has a positive deflection, with the maximum deflection of 0.41 mm. This can be explained by the sequential lamination and solidification. That is, a newly printed layer is solidified and shrunk on the previously printed layer that was already solidified and shrunk. Accordingly, the lower layer undergoes compressive residual stress whereas the upper layer undergoes tensile residual stress at every layer interface. As the residual stresses are relaxed by appropriate heat treatment, the resulting thermal deformation occurs in the opposite direction: tensile deformation in the lower layers and compressive deformation in the upper layer. Consequently, the relaxation of these residual stresses causes a bending deflection to the upward direction. By contrast, the transversely-printed bar has a negative deflection with the maximum deflection of 0.20 mm. This opposite trend can also be explained by the opposite thermal strain of the transverse printing. That is, the printed bar had a tensile strain along the x-direction and a compressive strain along the y-direction, as shown in Table 1. Accordingly, the residual stress along the x-direction becomes tensile for the lower layer and compressive for the upper layer. This residual stress distribution causes the compressive deformation in the lower layers and tensile deformation in the upper layer by the stress relaxation due to heat treatment. As a consequence, the resulting bending deflection occurs in the downward direction. Compared with these two results, the standard sample shows more complicated trend in which positive and negative deflections were mixed. This can be explained by the alternative raster angles of the standard printing path (45° and −45°). As a result, the printed bar was slightly twisted after heat treatment as shown in Fig. 2c. However, the maximum deflection was as small as 0.20 mm, which was even smaller than the thermal deformation along the z-direction (0.44–0.46 mm). Moreover, the maximum deflections of the homogeneously laminated bars were also insignificant: 0.41 mm for the longitudinal case and 0.26 mm for the transverse case. These small deflections cannot be regarded as a significant shape transformation considering the dimensions of the bar sample (60 × 6 × 1.6 mm). 3.1.3. Investigation of sectional deformation Fig. 3a and b show sectional photographs of a longitudinally printed specimen before and after heat treatment, respectively. It can be seen that a number of voids exist among adjacent filaments. These voids show a uniform distribution before heat treatment (Fig. 3a.) while their distribution becomes non-uniform after heat treatment (Fig. 3b), in which the voids near the bottom face are significantly decreased. Table 1 Directional thermal deformations and the resulting thermal expansion coefficients of homogeneously-printed bars according to printing paths. Type
Direction
Li (mm)
Li' (mm)
εi
αi (×10−3/°C)
Longitudinal printing
Length (x) Width (y) Height (z) Length (x) Width (y) Height (z)
59.89 6.41 1.58 60.01 6.15 1.57
48.18 7.19 2.02 61.25 5.83 2.03
−0.196 0.121 0.284 0.021 −0.052 0.290
−1.53 0.95 2.22 0.16 −0.40 2.26
Transverse printing
To discuss this non-uniform void density, cross-sectional filament sizes were compared according to their positions. Fig. 3c shows enlarged photographs of the cross section before and after heat treatment. A single filament can be seen to have an elliptical shape because an extruded filament through a nozzle with a 0.4 mm diameter was laminated with an interval of 0.2 mm. Indeed, the printed filaments in a layer were squeezed by the next layer, which created compressive residual stress along the thickness direction. Thus, the heat treatment increased the thickness due to the relaxation of this compressive residual stress, as shown in Fig. 3c. To investigate the thickness increase quantitatively, the minor diameters of the printed filaments (bi in Fig. 3c) were measured for the i-th layer in a column. The measurements were performed for three different columns for the printed specimen, before and after heat treatment. The measured minor diameters for each layer are compared in Fig. 3d. It can be seen that the minor diameters before heat treatment were near the given layer thickness (0.20 mm) from the second layer to the seventh layer. On the other hand, the minor diameter of the first layer (0.161 ± 0.011 mm) was smaller than the given layer thickness. This result can be explained by an ME printing function that adjusts the gap between the nozzle tip and the printing bed automatically. At the beginning of the printing, the printing bed position was adjusted automatically for proper leveling wherein the gap was set to a value smaller than the given layer thickness to increase the adhesion of the printed filament to the bed. On the contrary, the last layer showed a minor diameter (0.239 ± 0.012 mm) that was larger than the given layer thickness. This result can be explained by the fact that the last layer did not have any constraint in its elastic recovery along the thickness direction while the intermediate layers were constrained by the next layer deposition. As shown in Fig. 3d, the minor diameters increased after the heat treatment. The resulting minor diameters from the second to the seventh layer were approximately 0.245 mm, which corresponded to 21.2% thermal expansion. The first layer showed the largest thermal expansion (37.1%) since this layer was printed under the largest compressive residual stress. On the other hand, the last layer showed a relatively large thermal expansion (25.8%), which can also be explained by its free elastic recovery condition. As a result, the overall thermal expansion was 23.7% along the thickness direction. 3.2. Bending of heterogeneously laminated specimens 3.2.1. Theoretical analysis Thermal deformation of a heterogeneously laminated beam with different thermal expansions can be predicted by an analytic model of a bilayer beam. Fig. 4a illustrates a plane strain pure bending model that is composed of two materials with different thermal expansion coefficients (αl and αt). The radius of curvature (ρ) of the bilayer beam can be expressed in the following equation [33]: 1 ¼ ρ
6ð1 þ mÞ2 ðat −al ÞΔT mEl Et m2 þ ðt t þ t l Þ 3ð1 þ mÞ2 þ 1 þ Et mEl
ð2Þ
where E is the elastic modulus, t is the thickness, and ΔT is the temperature difference. Here, subscripts l and t denote the longitudinally and transversely printed layers, respectively. m is the thickness ratio between the longitudinal and transverse layers, which is defined as the following form: m¼
t l nl ¼ t t nt
ð3Þ
Considering that the designed rectangular bar (60 × 6 × 1.6 mm) was composed of eight layers, the number of transverse layers changed from one to seven, and the resulting radii of curvatures were calculated from Eq. (2). The calculated radii of curvature were plotted in Fig. 4b.
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Fig. 3. Sectional investigation of a longitudinally printed specimen with an eight-layer deposition. (a) Cross-sectional photograph before heat treatment. (b) Cross-sectional photograph after heat treatment. (c) Comparison of the laminated filament dimensions before (left) and after (right) heat treatment. (d) Comparison of the minor diameters of the printed filaments for each layer.
These results indicate that the number of transverse layers (nt) should be maintained at between three and five in order to increase the order of bending sufficiently (i.e., the smaller radius of curvature).
3.2.2. Experimental observation A rectangular bar was then printed by laminating three transversely printed layers and five longitudinally printed layers consecutively (i.e., nt = 3). This heterogeneously printed specimen was then heated in the electric furnace, according to the heat treatment condition described in Section 2.4. Fig. 5 shows shape changes with an increase in the heating time. One end of the bar was observed to begin to bend after 7 min of heating. This bending indicates that the specimen was heated enough to initiate thermal deformation by overcoming its own weight. That is, the length increase of the lower region (transversely printed) and the length decrease of the upper region (longitudinally printed) caused a bending deformation. This bending deformation continued with an increase in the heating time, and the rectangular bar specimen was rolled into an elliptical shape, as shown in Fig. 5a (please refer to the supplementary Movie 1). To compare the shape transformation of the heterogeneous lamination with that of the homogeneous lamination, the bending height was measured for the sample after nine-minute heat treatment. The bending height was measured to be 16.1 mm based on the image of Fig. 5a. This value is considerably larger than those of the homogeneous lamination cases, 0.41 mm for the longitudinally printed sample and 0.26 mm for the transversely printed sample. Therefore, the proposed heterogeneous lamination provides a significant shape transformation in comparison with the homogeneous lamination, and hence can be applied to 4D printing of a plain thermoplastic material without a shape memory function.
The bending behavior was further investigated by changing the ratios of the transverse layers to the longitudinal layers; the number of transverse layers (nt) was increased to four and five, and the number of longitudinal layers (nl) was decreased to four and three, accordingly. Therefore, the rectangular bars were printed with three different transverse-to-longitudinal ratios (3,5, 4:4, and 5:3), and heat treatments were performed for 15 min. The resulting shape changes are compared in Fig. 5b, which shows that the order of bending increased as the number of transversely-printed layers increased. This trend is different from the result of theoretical calculation (Fig. 4b), in which the bending deformation was the largest in the 4:4 lamination case (nt = 4).
3.2.3. Numerical simulation Numerical simulation was performed to investigate the deformation behavior of the heterogeneously laminated bar. Thermal finite element (FE) analysis was conducted to predict temperature change of the bar during the heat treatment. ANSYS Workbench® was used to conduct transient thermal analysis for the given problem. The ABS bar specimen and AA6061 plate were included in the analysis model, and their thermal properties are given in Table 2. The initial temperatures of the ABS bar and AA6061 plate were set to 25 °C, and thermal convection conditions were imposed for all outer faces with a 5.0 W/m2-K film coefficient and 150 °C temperature. Fig. 6a shows temperature change of the ABS bar, at the center and end positions of the top face. It can be seen that the temperatures at both positions rise higher than 140 °C after the 7 min heating and increases slowly with an increase in the heating time. Considering that the bending deformation was initiated after 7 min of heating, as shown in Fig. 5a, the proposed heterogeneous lamination requires a higher
Fig. 4. Theoretical investigation of heterogeneous bending deformation. (a) Plane-strain pure bending model for a bilayer beam. (b) Radii of curvature with a change in the number of transverse layers (nt).
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Fig. 5. 4D printing of a heterogeneously printed rectangular bar. (a) Shape changes of the heterogeneous bar with an increase in the heating time (nt = 3, nl = 5). (b) Deformed shapes of the heterogeneous bar after the 15 min heat treatment for different transverse-to-longitudinal ratios.
temperature than 140 °C to initiate a shape transformation by overcoming its own weight. Structural FE analyses were then conducted to investigate the thermal deformation behavior of the heterogeneously printed bar. In particular, deformation behaviors of three different transverse-tolongitudinal ratios (3:5, 4:4, and 5:3) were compared by the FE analysis. The FE model was generated using a 20-node quadratic hexahedral mesh, and the mesh size was set to 0.4 mm to consider the layer-wise material properties. To consider the thermal softening effect of ABS material, the elastic modulus was regarded as a function of temperature [34], as shown in Fig. 6b. Fig. 6c to e show the deformed shapes and distributions of vertical displacements for the analysis models with different transverse-tolongitudinal ratios. It can be seen that the amount of bending increased as the number of transversely-printed layers (nt) increased, which show trends similar to the results of the experiments shown in Fig. 5b. These results can explain the difference between the experimental results (Fig. 5b) and the theoretical solution (Fig. 4b) in which the 4:4 lamination case (nt = 4) had the largest bending. This indicates that the analytical model in Eq. (2) was based on the assumption of a 2D plane strain model, and thus it has a limitation in describing 3D deformation behaviors accurately. The consideration of thermal nonlinear effects in the FE analysis also provided more accurate estimation of the deformation characteristics than the theoretical solution with an assumption of the constant elastic modulus.
3.3. Applications to 2D-to-3D shape transformation 3.3.1. Shape transformation of a 2D cross shape The proposed heterogeneous lamination enabled 4D printing as 1Dto-2D shape transformations. This 4D printing method was extended to 2D-to-3D shape transformations by programming directional printing paths for a two-dimensional cross shape, as shown in Fig. 7a. In this case, the longitudinal and transverse printing paths were generated in the base region, as indicated by the shaded region in Fig. 7a, and the Table 2 Material properties for thermal FE analyses. Material Thermal conductivity (W/m-K) Specific heat (J/kg-K) Density (kg/m3)
ABS 0.1 1470 1076
AA-6061 180 896 2700
generated paths were copied by considering rotational repeats. Based on these printing paths, three transversely printed layers and five longitudinally printed layers were consecutively laminated, as shown in Fig. 7b. All the printing conditions and heat treatment conditions were set to be the same as those of the 1D-to-2D shape transformation case. Fig. 7c shows the shape changes of the cross-shape specimen during the heat treatment. It was observed that four ends of the cross began to rise after 7 min of heating and bent to gather at its center as the heating proceeded (please refer to the supplementary Movie 2). This shape transformation can be applied to a permanent gripper by considering its irreversible thermal deformation. 3.3.2. Shape transformation of a 2D star shape The heterogeneous lamination for 2D shapes was further extended to have more complicated printing paths. Fig. 8a shows the longitudinal and transverse printing paths for the base region of a star shape. Three transversely printed layers and five longitudinally printed layers were consecutively laminated based on these printing paths, as shown in Fig. 8b. Fig. 8c shows the sequential shape changes with an increase in the heating time, in which the 2D star specimen curled as the heating proceeded. This shape transformation mimics a curling motion of a starfish (please refer to the supplementary Movie 3). 3.4. Localized bending for self-assembly The proposed heterogeneous lamination was selectively performed to induce localized bending. Fig. 9a shows the configuration of a composite bar (54 × 6 × 1.6 mm) that contains isotopic and anisotropic regions. The isotropic regions were built by laminating eight layers with 45° and −45° raster angles alternatively. Here, the length for the isotropic (li) and anisotropic (la) regions was set to 8 and 15 mm, respectively. The thickness of each layer was set to 0.2 mm. The anisotropic regions were built by laminating four transverse layers and four longitudinal layers consecutively. All the printing and heat treatment conditions were set to be the same as the previous cases. Fig. 9b shows the shape changes of the composite bar during the heat treatment at the 150 °C temperature condition. It can be seen that bending deformation proceeded at the two anisotropic regions while three isotropic regions remained as straight shapes, and the resulting bending angle became N90° after 15 min of heat treatment. This indicates that the bending deformation can be controlled locally by selectively laminating isotropic and anisotropic regions.
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Fig. 6. Results of numerical simulation. (a) Temperature changes of the rectangular bar during heat treatment. (b) Elastic modulus change according to temperature change. Distribution of the vertical displacements with deformed shapes for different transverse-to-longitudinal ratios (unit: mm): (c) nt: nl = 3: 5. (d) nt: nl = 4: 4. (e) nt: nl = 5: 3.
This localized bending was then applied to perform a self-assembly function. The composite bar was placed reversely on the top surface of two half metal blocks, as illustrated in Fig. 9c. Heat treatment was also performed for 15 min at 150 °C temperature to obtain a large bending angle (N 90°) enough to fasten two metal blocks stably. Fig. 9d shows the resulting assembly structure in which two metal blocks were fastened by the locally-bent composite bar (please refer to the supplementary Movie 4). These blocks were not separated by their weights or an external excitation, which indicates that they were assembled by compressive force due to the bending of the composite bar (please refer to the supplementary Movie 5). 4. Conclusion In this study, an efficient 4D printing method was developed using the anisotropic thermal deformation of 3D printed parts. An ME type
3D printer and a single thermoplastic filament (ABS) were used. The printing paths were programmed to generate thermal anisotropy in a bidirectional manner by combining the longitudinal and transverse printing paths effectively. This intentional anisotropy that contained residual stress caused unique thermal deformation after heat treatment, with the dimensions contracting in the printing direction and expanding in the lateral and lamination directions. The relevant thermal deformation characteristics were investigated by analyzing the directional size changes of the homogeneously laminated specimens. This directional thermal deformation was then used to enable 4D printing by applying heterogeneous lamination in which transverse and longitudinal printing paths were used consecutively. The heterogeneously laminated bars showed bending deformation after heat treatment, which corresponded to 1D-to-2D shape transformations. Numerical simulation was conducted to predict the deformation behaviors of the heterogeneous bars with different transverse-to-longitudinal
Fig. 7. 4D printing of a 2D cross-shape specimen. (a) Printing path designs for the cross shape. (b) 3D view of the heterogeneous lamination. (c) Shape changes of the cross-shape specimen with an increase in the heating time.
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Fig. 8. 4D printing of a 2D star-shaped specimen. (a) Printing path designs for the star shape. (b) 3D view of the heterogeneous lamination. (c) Shape changes of the star-shaped specimen with an increase in the heating time.
ratios, which enabled to control the order of bending by changing the transverse-to-longitudinal ratio. The heterogeneous lamination was further extended to print 2D cross and star shapes, which enabled 2Dto-3D shape transformations. Moreover, a selective printing of the anisotropic region between isotropic regions induced localized bending deformation, which could be used to perform self-assembly function. Considering that the proposed method uses a personal ME printer and a single thermoplastic polymer (ABS) without a shape memory function,
this method has advantages in its usefulness and adaptability over other 4D printing methods that use shape memory materials or multiple materials. The proposed method is also different from conventional 4D printing in terms of its reversibility. That is, the printed parts undergo permanent thermal deformation after appropriate heat treatment while most shape transformations in 4D printing are temporary and reversible. These unique characteristics can be utilized in various applications that require permanent deformation after thermal stimulation.
Fig. 9. Application of 4D printing to self-assembly. (a) Printing path designs for the composite bar. (b) Local bending behavior of the composite bar during the heat treatment. (c) Configuration for self-assembly. (d) Assembly result by the localized bending deformation.
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2020.108485. CRediT authorship contribution statement Bona Goo: Methodology, Investigation, Formal analysis. Chae-Hui Hong: Formal analysis. Keun Park: Conceptualization, Supervision, Writing - original draft. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT, Republic of Korea (Grant number: 2019R1A2C1002799). References [1] A. Mitchell, U. Lafont, M. Hołyńska, C. Semprimoschnig, Additive manufacturing-a review of 4D printing and future applications, Addit. Manuf. 24 (2018) 606–626. [2] S. Tibbits, 4D printing: multi-material shape change, Archit. Des. 84 (1) (2014) 116–121. [3] F. Momeni, S.M.M. Hassani, N.X. Liu, J. Ni, A review of 4D printing, Mater. Des. 122 (2017) 42–79. [4] S. Miao, N. Castro, M. Nowicki, L. Xia, H. Cui, X. Zhou, W. Zhou, S. Lee, K. Sakar, G. Vozzi, Y. Tabata, J. Fisher, L.G. Zhang, 4D printing of polymeric materials for tissue and organ regeneration, Mater. Today 20 (10) (2017) 577–591. [5] X. Kuang, D.J. Roach, J. Wu, C.M. Hamel, Z. Ding, T. Wang, M.L. Dunn, H.J. Qi, Advances in 4D printing: materials and applications, Adv. Func. Mater. 29 (2) (2019), 1805290. [6] D.-G. Shin, T.-H. Kim, D.-E. Kim, Review of 4D printing materials and their properties, Int. J. Precis. Eng. Manuf. Green Tech. 4 (3) (2017) 349–357. [7] S. Li, W.M. Huang, Mechanisms of the multi-shape memory effect and temperature memory effect in shape memory polymers, Soft Matter 6 (18) (2010) 4403–4406. [8] W.G. Yang, H. Lu, W.M. Huang, H.J. Qi, X.L. Wu, K.Y. Sun, Advanced shape memory technology to reshape product design, manufacturing and recycling, Polymers 6 (8) (2014) 2287–2308. [9] H. Wu, P. Chen, C. Yan, C. Cai, Y. Shi, Y. Four-dimensional printing of a novel acrylatebased shape memory polymer using digital light processing, Mater. Des. 171 (2019) 107704. [10] F.A. Chávez, J.G. Siqueiros, I.A. Carrete, I.L. Delgado, G.W. Ritter, D.A. Roberson, Characterisation of phases and deformation temperature for additively manufactured shape memory polymer components fabricated from rubberised acrylonitrile butadiene styrene, Virtual Phys. Prototyp. 14 (2) (2019) 188–202. [11] Q. Ge, H.J. Qi, M.L. Dunn, Active materials by four-dimension printing, Appl. Phys. Lett. 103 (13) (2013), 131901.
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Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
4D printing using anisotropic thermal deformation of 3D-printed thermoplastic parts Bona Goo a, Chae-Hui Hong a,b, Keun Park a,⁎ a b
Department of Mechanical System Design Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea Smart Information Technology Team, Samsung Display Co. Ltd., Asan 31454, Republic of Korea
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• An efficient 4D printing method was developed using a single thermoplastic polymer without a shape memory function. • To enable shape transformation, the printing paths were programmed to impose bidirectional anisotropy intentionally. • The difference in thermal deformation of transversely and longitudinally printed layers were caused bending deformation. • The selective lamination of isotropic and anisotropic regions enabled a localized bending for a self-assembly function.
a r t i c l e
i n f o
Article history: Received 18 November 2019 Received in revised form 7 January 2020 Accepted 8 January 2020 Available online 10 January 2020 Keywords: Additive manufacturing 4D printing Material extrusion Anisotropic thermal deformation Bidirectional printing
a b s t r a c t Four-dimensional (4D) printing is an advanced application of 3D printing with the additional shapes changes over time. For appropriate shape changes, 4D printing has been developed using shape memory materials or multi-material structures with different deformation behaviors in response to an external stimulus. In this study, an efficient 4D printing method was developed using a material extrusion (ME) type 3D printer and a single thermoplastic polymer without a shape memory function. To impose different deformation behaviors in a single thermoplastic material, the printing paths were programmed intentionally, and the resulting anisotropy was used to generate a unique thermal deformation in response to a thermal stimulus. The anisotropic thermal deformations of the longitudinally and transversely printed parts were investigated by analyzing the directional size changes of homogeneously laminated bars. This anisotropy was then used to enable 4D printing by applying heterogeneous lamination in which the transverse and longitudinal printing paths were used consecutively. This heterogeneous lamination showed bending deformation after heat treatment, and the effect of lamination strategy on the order of bending was investigated experimentally. The proposed 4D printing method was further applied to induce localized bending deformation by laminating isotropic and anisotropic regions selectively, which enabled a self-assembly function. © 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (K. Park).
Additive manufacturing (AM), also known as three-dimensional (3D) printing, has extended its application area from conventional
https://doi.org/10.1016/j.matdes.2020.108485 0264-1275/© 2020 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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rapid prototyping (RP) to the direct fabrication of functional parts. A unique extension of AM is 4D printing, which adds changes of shapes or properties over time to conventional 3D printing [1]. Fourdimensional (4D) printing enables the manufacture of dynamic structures with adjustable shape transformations and has various applications, such as self-assembly, self-repair and self-adaptability [2]. Fundamental elements of 4D printing can be categorized as 3D printers, external stimuli, stimulus-responsive materials, interaction mechanisms and mathematical modeling [3]. Various studies have been performed to achieve shape transformations of 3D-printed parts using various combinations of stimuli and stimulus-responsive materials [4,5]. The most popular 4D printing method is to use smart materials that respond to external stimuli by changing their shapes and physical properties [6]. The most widely used smart material is shape memory polymers (SMPs), which can recover their original shapes by triggering certain environmental changes, such as thermal or solvent stimuli [7–10]. Thermal response SMPs make transformations by a thermal stimulus from a certain temporary shape to its inherent permanent shape. The thermal SMPs have generally been used as a composite of SMP fibers in an elastomeric matrix to realize 4D printing [11–14]. Water response SMPs, such as hydrogels, undergo size change in response to water [15], and have been used as a smart material composite to achieve 4D printing effects [16–19]. While these applications use smart materials to change the shapes of 3D-printed parts, 4D printing has also been attempted in the case of printing multi-materials that have different swelling or deformation properties [20–23]. This multi-material structure, however, requires 3D printers to support multi-material printing, which limits the use of 4D printing to particular materials and equipment. This study developed a 4D printing method using a material extrusion (ME) type 3D printer, also known as a fused filament fabrication (FFF) type printer. In ME type 3D printing, a thermoplastic filament is extruded through a narrow nozzle, and it thus possesses anisotropic mechanical properties due to the directional pre-strain and residual stress [24]. This directional pre-strain and residual strain was used in 4D printing by generating thermal shrinkage of 2D patterned lattice materials [25], and by enabling 3D bending deformation of 2D printed composite sheets [26]. The ME printing was also used to investigate the shape transformation characteristics by printing thermos-responsive SMP filaments [27,28]. Recently, the ME printing of SMP filaments was used in 4D printing by controlling the printing path in a unidirectional manner, in order to enhance anisotropy and the resulting shape transformation [29,30]. In this study, we de developed an efficient 4D printing method which uses an ME-type 3D printer and a single thermoplastic filament without a shape memory effect. To intensify shape transformation of the ME-printed parts, the printing paths were programmed in a bidirectional manner. That is, a number of transversely printed and longitudinally printed layers were laminated in sequence. We investigated the thermal deformations of the longitudinally and quantitatively analyzed transversely printed specimens, and the resulting shape changes and directional thermal strain. Printing paths were then programmed by laminating the transverse and longitudinal layers consecutively, which resulted in bending deformation when the printed sample was heated above its softening temperature. The deformation mechanism of the proposed method was investigated theoretically and numerically, and the effect of the longitudinalto-transvers ratio on the amount of bending deformation was analyzed and compared with experiments. This anisotropic thermal deformation was then used to transform various 3D printed parts by changing their printing paths and heat treatment conditions. The proposed 4D printing method was further applied to perform a self-assembly function by selectively printing anisotropic regions between isotropic regions and by localized bending deformation in the anisotropic region.
2. Materials and methods 2.1. Materials As a printing material, we used acrylonitrile butadiene styrene (ABS) filaments that were 1.75 mm in diameter (Shenzhen ESUN Industrial Co. Ltd., China). The density, thermal expansion coefficient, and glass transition temperature of the ABS filament were 1.04 g/cm3, 8.82 × 10−5/°C, and 107.8 °C, respectively. Additively manufactured parts using this filament are known to have orthotropic mechanical properties according to the printing direction; the elastic moduli for the X-, Y-, and Z-directions were 2.18, 2.50, 2.26 MPa, respectively [31]. 2.2. Additive manufacturing with homogeneous lamination AM was performed using an ME type 3D printer (Cubicon Single, Cubicon Inc., Korea). This printer has a printing chamber in which the chamber temperature is controlled in a convective manner. In the additive manufacturing step, the temperatures of the nozzle, bed, and chamber were set to 240, 115 and 60 °C, respectively. The layer thickness was set to 0.2 mm, and the printing speed was set to 80 mm/s. In ME type 3D printing, it is generally recommended to use 45° and − 45° raster angles alternatively to minimize in-plane anisotropy [32]. In this study, however, the raster angle was programmed intentionally to impose in-plane anisotropy. Fig. 1 illustrates two types of printing paths for a rectangular bar specimen (60 × 6 × 1.6 mm): a longitudinal printing path with a 0° raster angle and a transverse printing path with a 90° raster angle. In both cases, eight layers were printed and laminated with a consistent printing path, which is called as a homogeneous lamination. In homogeneous lamination, the two printed bars are expected to have different material orientations that may result in anisotropic thermal deformation by an external thermal stimulus. 2.3. Additive manufacturing with heterogeneous lamination The anisotropic thermal deformation was then used to realize 4D printing by mixing the transverse and longitudinal printing paths, socalled heterogeneous lamination. For heterogeneous lamination, a rectangular bar (60 × 6 × 1.6 mm) was printed by laminating a number of transversely printed layers (nt) and a number of longitudinally printed layers (nl) consecutively. The layer thickness was also set to 0.2 mm, and a total of eight layers were laminated to build a 1.6 mm thickness. Thus, we expected the printed bar with the bidirectional anisotropy to have different thermal deformations across the border between the transverse and longitudinal regions, and thereby to show bending deformation when a thermal stimulus was applied. 2.4. Heat treatment As a thermal stimulus, heat treatment was performed using an electric furnace with forced convection (DHG-9070A, NeuronFit Co. Ltd., Korea). The furnace was preheated to 150 °C before the heat treatment. The printed samples were placed on an AA6061 plate (220 × 180 × 5 mm),
Fig. 1. Configuration of longitudinal and transverse printing paths for rectangular bar specimens.
B. Goo et al. / Materials and Design 188 (2020) 108485
and put into in the preheated chamber for heat treatment. Heat treatment was performed for 15 min while maintaining the chamber temperature at 150 °C. During the heat treatment, shape transformations of printed samples were observed through a glass window of the furnace. The heated samples were then removed out of the furnace, and were cooled at a room temperature for 1 h. 2.5. Characterization To measure the glass transition temperature of the ABS filaments, a differential scanning calorimetric (DSC) test was performed. A DSC thermal analyzer (DSC25, TA Instruments, USA) was used at a rate of 10 °C/min from 20 to 300 °C in a nitrogen atmosphere. To observe sectional images of the printed parts, a digital optical microscope (Mi-9000, Jason Electro-Tech, Korea) was used. The amount of bending deflection after heat treatment was measured using a high resolution 3D scanner (Rexcan DS2 5.0, Solutionix Co., Korea). To measure directional thermal expansion or shrinkage, a digital Vernier caliper with the resolution of 0.01 mm (H500-20, Mitutoyo Co., Japan) was used. The size changes of the rectangular specimens were measured for the specimens before and after heat treatment. Five specimens were measured for each printing direction, and directional strain components (εi) were calculated based on the measurement results. The effective thermal expansion coefficient (αi) was then calculated by considering these directional strain components as the thermal strain, as given in Eq. (1). 0 ε 1 Li −Li ai ¼ i ¼ ð1Þ Li ΔT ΔT where the subscript i denotes the Cartesian direction (x, y, and z), and Li and Li′ are the corresponding directional dimensions before and after heat treatment with a temperature change of ΔT, respectively.
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3. Results and discussion 3.1. Thermal deformation of homogeneously laminated specimens 3.1.1. Directional deformation and thermal strain As a homogeneous lamination, rectangular bar samples were printed using three printing paths: standard (45° and −45°), longitudinal (0°) and transverse (90°) paths. Heat treatments were performed for 15 min at 150 °C temperature, as explained in Section 2.4. Fig. 2a compares the shape changes of the bar specimens after the heat treatment. The standard specimen showed isotropic shrinkage such that both the length and width of the specimen were reduced after heat treatment. On the other hand, the longitudinally printed specimen underwent anisotropic deformation with a decrease in length and an increase in width. The transversely printed specimen showed the opposite trend with an increase in length and a decrease in width. These contrary results can be explained by the difference in the residual stresses according to the printing direction. In the ME type 3D printing, the polymer material undergoes two-step extrusion processes. The first extrusion is the filament manufacturing process in which the molten polymer is extruded through an extrusion die. The second extrusion is the printing process in which a thick filament 1.75 mm in diameter is extruded through a thin nozzle with a 0.4 mm diameter. Due to these extrusion processes, the printed part becomes highly oriented along its longitudinal direction with tensile residual stress in its length and compressive residual stress in its cross section. Therefore, the relaxation of these residual stresses results in an anisotropic size change, a decrease along the printing direction and an increase along the transverse direction, as shown in Fig. 2a. Table 1 compares the detailed size changes for the longitudinally and transversely printed specimens. It can be seen that the calculated thermal expansion coefficients have negative values along the printing
Fig. 2. Thermal deformation of the rectangular bars. (a) Photographs of standard, longitudinal and transverse samples before and after heat treatment. (b) Comparison of the vertical deflection along the longitudinal centerline. (c) 3D plot of the vertical deflection (Δz) of the standard sample.
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direction (i.e. the x-direction for the longitudinal printing and the ydirection for the transverse printing), and positive values along the lateral direction (i.e. the y-direction for the longitudinal printing and the xdirection for the transverse printing). On the other hand, the thermal expansion coefficients along the thickness direction have positive values along the both printing cases, of which magnitudes (2.22 and 2.26 × 10−3/°C) were much greater than those of the other directions. 3.1.2. Deflection along the vertical direction Fig. 2b compares the vertical deflections of three bars after heat treatment. Here, the center point of each top face was selected as the origin point, and the relative z-position was plotted along the centerline of the top face. The longitudinally-printed bar has a positive deflection, with the maximum deflection of 0.41 mm. This can be explained by the sequential lamination and solidification. That is, a newly printed layer is solidified and shrunk on the previously printed layer that was already solidified and shrunk. Accordingly, the lower layer undergoes compressive residual stress whereas the upper layer undergoes tensile residual stress at every layer interface. As the residual stresses are relaxed by appropriate heat treatment, the resulting thermal deformation occurs in the opposite direction: tensile deformation in the lower layers and compressive deformation in the upper layer. Consequently, the relaxation of these residual stresses causes a bending deflection to the upward direction. By contrast, the transversely-printed bar has a negative deflection with the maximum deflection of 0.20 mm. This opposite trend can also be explained by the opposite thermal strain of the transverse printing. That is, the printed bar had a tensile strain along the x-direction and a compressive strain along the y-direction, as shown in Table 1. Accordingly, the residual stress along the x-direction becomes tensile for the lower layer and compressive for the upper layer. This residual stress distribution causes the compressive deformation in the lower layers and tensile deformation in the upper layer by the stress relaxation due to heat treatment. As a consequence, the resulting bending deflection occurs in the downward direction. Compared with these two results, the standard sample shows more complicated trend in which positive and negative deflections were mixed. This can be explained by the alternative raster angles of the standard printing path (45° and −45°). As a result, the printed bar was slightly twisted after heat treatment as shown in Fig. 2c. However, the maximum deflection was as small as 0.20 mm, which was even smaller than the thermal deformation along the z-direction (0.44–0.46 mm). Moreover, the maximum deflections of the homogeneously laminated bars were also insignificant: 0.41 mm for the longitudinal case and 0.26 mm for the transverse case. These small deflections cannot be regarded as a significant shape transformation considering the dimensions of the bar sample (60 × 6 × 1.6 mm). 3.1.3. Investigation of sectional deformation Fig. 3a and b show sectional photographs of a longitudinally printed specimen before and after heat treatment, respectively. It can be seen that a number of voids exist among adjacent filaments. These voids show a uniform distribution before heat treatment (Fig. 3a.) while their distribution becomes non-uniform after heat treatment (Fig. 3b), in which the voids near the bottom face are significantly decreased. Table 1 Directional thermal deformations and the resulting thermal expansion coefficients of homogeneously-printed bars according to printing paths. Type
Direction
Li (mm)
Li' (mm)
εi
αi (×10−3/°C)
Longitudinal printing
Length (x) Width (y) Height (z) Length (x) Width (y) Height (z)
59.89 6.41 1.58 60.01 6.15 1.57
48.18 7.19 2.02 61.25 5.83 2.03
−0.196 0.121 0.284 0.021 −0.052 0.290
−1.53 0.95 2.22 0.16 −0.40 2.26
Transverse printing
To discuss this non-uniform void density, cross-sectional filament sizes were compared according to their positions. Fig. 3c shows enlarged photographs of the cross section before and after heat treatment. A single filament can be seen to have an elliptical shape because an extruded filament through a nozzle with a 0.4 mm diameter was laminated with an interval of 0.2 mm. Indeed, the printed filaments in a layer were squeezed by the next layer, which created compressive residual stress along the thickness direction. Thus, the heat treatment increased the thickness due to the relaxation of this compressive residual stress, as shown in Fig. 3c. To investigate the thickness increase quantitatively, the minor diameters of the printed filaments (bi in Fig. 3c) were measured for the i-th layer in a column. The measurements were performed for three different columns for the printed specimen, before and after heat treatment. The measured minor diameters for each layer are compared in Fig. 3d. It can be seen that the minor diameters before heat treatment were near the given layer thickness (0.20 mm) from the second layer to the seventh layer. On the other hand, the minor diameter of the first layer (0.161 ± 0.011 mm) was smaller than the given layer thickness. This result can be explained by an ME printing function that adjusts the gap between the nozzle tip and the printing bed automatically. At the beginning of the printing, the printing bed position was adjusted automatically for proper leveling wherein the gap was set to a value smaller than the given layer thickness to increase the adhesion of the printed filament to the bed. On the contrary, the last layer showed a minor diameter (0.239 ± 0.012 mm) that was larger than the given layer thickness. This result can be explained by the fact that the last layer did not have any constraint in its elastic recovery along the thickness direction while the intermediate layers were constrained by the next layer deposition. As shown in Fig. 3d, the minor diameters increased after the heat treatment. The resulting minor diameters from the second to the seventh layer were approximately 0.245 mm, which corresponded to 21.2% thermal expansion. The first layer showed the largest thermal expansion (37.1%) since this layer was printed under the largest compressive residual stress. On the other hand, the last layer showed a relatively large thermal expansion (25.8%), which can also be explained by its free elastic recovery condition. As a result, the overall thermal expansion was 23.7% along the thickness direction. 3.2. Bending of heterogeneously laminated specimens 3.2.1. Theoretical analysis Thermal deformation of a heterogeneously laminated beam with different thermal expansions can be predicted by an analytic model of a bilayer beam. Fig. 4a illustrates a plane strain pure bending model that is composed of two materials with different thermal expansion coefficients (αl and αt). The radius of curvature (ρ) of the bilayer beam can be expressed in the following equation [33]: 1 ¼ ρ
6ð1 þ mÞ2 ðat −al ÞΔT mEl Et m2 þ ðt t þ t l Þ 3ð1 þ mÞ2 þ 1 þ Et mEl
ð2Þ
where E is the elastic modulus, t is the thickness, and ΔT is the temperature difference. Here, subscripts l and t denote the longitudinally and transversely printed layers, respectively. m is the thickness ratio between the longitudinal and transverse layers, which is defined as the following form: m¼
t l nl ¼ t t nt
ð3Þ
Considering that the designed rectangular bar (60 × 6 × 1.6 mm) was composed of eight layers, the number of transverse layers changed from one to seven, and the resulting radii of curvatures were calculated from Eq. (2). The calculated radii of curvature were plotted in Fig. 4b.
B. Goo et al. / Materials and Design 188 (2020) 108485
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Fig. 3. Sectional investigation of a longitudinally printed specimen with an eight-layer deposition. (a) Cross-sectional photograph before heat treatment. (b) Cross-sectional photograph after heat treatment. (c) Comparison of the laminated filament dimensions before (left) and after (right) heat treatment. (d) Comparison of the minor diameters of the printed filaments for each layer.
These results indicate that the number of transverse layers (nt) should be maintained at between three and five in order to increase the order of bending sufficiently (i.e., the smaller radius of curvature).
3.2.2. Experimental observation A rectangular bar was then printed by laminating three transversely printed layers and five longitudinally printed layers consecutively (i.e., nt = 3). This heterogeneously printed specimen was then heated in the electric furnace, according to the heat treatment condition described in Section 2.4. Fig. 5 shows shape changes with an increase in the heating time. One end of the bar was observed to begin to bend after 7 min of heating. This bending indicates that the specimen was heated enough to initiate thermal deformation by overcoming its own weight. That is, the length increase of the lower region (transversely printed) and the length decrease of the upper region (longitudinally printed) caused a bending deformation. This bending deformation continued with an increase in the heating time, and the rectangular bar specimen was rolled into an elliptical shape, as shown in Fig. 5a (please refer to the supplementary Movie 1). To compare the shape transformation of the heterogeneous lamination with that of the homogeneous lamination, the bending height was measured for the sample after nine-minute heat treatment. The bending height was measured to be 16.1 mm based on the image of Fig. 5a. This value is considerably larger than those of the homogeneous lamination cases, 0.41 mm for the longitudinally printed sample and 0.26 mm for the transversely printed sample. Therefore, the proposed heterogeneous lamination provides a significant shape transformation in comparison with the homogeneous lamination, and hence can be applied to 4D printing of a plain thermoplastic material without a shape memory function.
The bending behavior was further investigated by changing the ratios of the transverse layers to the longitudinal layers; the number of transverse layers (nt) was increased to four and five, and the number of longitudinal layers (nl) was decreased to four and three, accordingly. Therefore, the rectangular bars were printed with three different transverse-to-longitudinal ratios (3,5, 4:4, and 5:3), and heat treatments were performed for 15 min. The resulting shape changes are compared in Fig. 5b, which shows that the order of bending increased as the number of transversely-printed layers increased. This trend is different from the result of theoretical calculation (Fig. 4b), in which the bending deformation was the largest in the 4:4 lamination case (nt = 4).
3.2.3. Numerical simulation Numerical simulation was performed to investigate the deformation behavior of the heterogeneously laminated bar. Thermal finite element (FE) analysis was conducted to predict temperature change of the bar during the heat treatment. ANSYS Workbench® was used to conduct transient thermal analysis for the given problem. The ABS bar specimen and AA6061 plate were included in the analysis model, and their thermal properties are given in Table 2. The initial temperatures of the ABS bar and AA6061 plate were set to 25 °C, and thermal convection conditions were imposed for all outer faces with a 5.0 W/m2-K film coefficient and 150 °C temperature. Fig. 6a shows temperature change of the ABS bar, at the center and end positions of the top face. It can be seen that the temperatures at both positions rise higher than 140 °C after the 7 min heating and increases slowly with an increase in the heating time. Considering that the bending deformation was initiated after 7 min of heating, as shown in Fig. 5a, the proposed heterogeneous lamination requires a higher
Fig. 4. Theoretical investigation of heterogeneous bending deformation. (a) Plane-strain pure bending model for a bilayer beam. (b) Radii of curvature with a change in the number of transverse layers (nt).
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Fig. 5. 4D printing of a heterogeneously printed rectangular bar. (a) Shape changes of the heterogeneous bar with an increase in the heating time (nt = 3, nl = 5). (b) Deformed shapes of the heterogeneous bar after the 15 min heat treatment for different transverse-to-longitudinal ratios.
temperature than 140 °C to initiate a shape transformation by overcoming its own weight. Structural FE analyses were then conducted to investigate the thermal deformation behavior of the heterogeneously printed bar. In particular, deformation behaviors of three different transverse-tolongitudinal ratios (3:5, 4:4, and 5:3) were compared by the FE analysis. The FE model was generated using a 20-node quadratic hexahedral mesh, and the mesh size was set to 0.4 mm to consider the layer-wise material properties. To consider the thermal softening effect of ABS material, the elastic modulus was regarded as a function of temperature [34], as shown in Fig. 6b. Fig. 6c to e show the deformed shapes and distributions of vertical displacements for the analysis models with different transverse-tolongitudinal ratios. It can be seen that the amount of bending increased as the number of transversely-printed layers (nt) increased, which show trends similar to the results of the experiments shown in Fig. 5b. These results can explain the difference between the experimental results (Fig. 5b) and the theoretical solution (Fig. 4b) in which the 4:4 lamination case (nt = 4) had the largest bending. This indicates that the analytical model in Eq. (2) was based on the assumption of a 2D plane strain model, and thus it has a limitation in describing 3D deformation behaviors accurately. The consideration of thermal nonlinear effects in the FE analysis also provided more accurate estimation of the deformation characteristics than the theoretical solution with an assumption of the constant elastic modulus.
3.3. Applications to 2D-to-3D shape transformation 3.3.1. Shape transformation of a 2D cross shape The proposed heterogeneous lamination enabled 4D printing as 1Dto-2D shape transformations. This 4D printing method was extended to 2D-to-3D shape transformations by programming directional printing paths for a two-dimensional cross shape, as shown in Fig. 7a. In this case, the longitudinal and transverse printing paths were generated in the base region, as indicated by the shaded region in Fig. 7a, and the Table 2 Material properties for thermal FE analyses. Material Thermal conductivity (W/m-K) Specific heat (J/kg-K) Density (kg/m3)
ABS 0.1 1470 1076
AA-6061 180 896 2700
generated paths were copied by considering rotational repeats. Based on these printing paths, three transversely printed layers and five longitudinally printed layers were consecutively laminated, as shown in Fig. 7b. All the printing conditions and heat treatment conditions were set to be the same as those of the 1D-to-2D shape transformation case. Fig. 7c shows the shape changes of the cross-shape specimen during the heat treatment. It was observed that four ends of the cross began to rise after 7 min of heating and bent to gather at its center as the heating proceeded (please refer to the supplementary Movie 2). This shape transformation can be applied to a permanent gripper by considering its irreversible thermal deformation. 3.3.2. Shape transformation of a 2D star shape The heterogeneous lamination for 2D shapes was further extended to have more complicated printing paths. Fig. 8a shows the longitudinal and transverse printing paths for the base region of a star shape. Three transversely printed layers and five longitudinally printed layers were consecutively laminated based on these printing paths, as shown in Fig. 8b. Fig. 8c shows the sequential shape changes with an increase in the heating time, in which the 2D star specimen curled as the heating proceeded. This shape transformation mimics a curling motion of a starfish (please refer to the supplementary Movie 3). 3.4. Localized bending for self-assembly The proposed heterogeneous lamination was selectively performed to induce localized bending. Fig. 9a shows the configuration of a composite bar (54 × 6 × 1.6 mm) that contains isotopic and anisotropic regions. The isotropic regions were built by laminating eight layers with 45° and −45° raster angles alternatively. Here, the length for the isotropic (li) and anisotropic (la) regions was set to 8 and 15 mm, respectively. The thickness of each layer was set to 0.2 mm. The anisotropic regions were built by laminating four transverse layers and four longitudinal layers consecutively. All the printing and heat treatment conditions were set to be the same as the previous cases. Fig. 9b shows the shape changes of the composite bar during the heat treatment at the 150 °C temperature condition. It can be seen that bending deformation proceeded at the two anisotropic regions while three isotropic regions remained as straight shapes, and the resulting bending angle became N90° after 15 min of heat treatment. This indicates that the bending deformation can be controlled locally by selectively laminating isotropic and anisotropic regions.
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Fig. 6. Results of numerical simulation. (a) Temperature changes of the rectangular bar during heat treatment. (b) Elastic modulus change according to temperature change. Distribution of the vertical displacements with deformed shapes for different transverse-to-longitudinal ratios (unit: mm): (c) nt: nl = 3: 5. (d) nt: nl = 4: 4. (e) nt: nl = 5: 3.
This localized bending was then applied to perform a self-assembly function. The composite bar was placed reversely on the top surface of two half metal blocks, as illustrated in Fig. 9c. Heat treatment was also performed for 15 min at 150 °C temperature to obtain a large bending angle (N 90°) enough to fasten two metal blocks stably. Fig. 9d shows the resulting assembly structure in which two metal blocks were fastened by the locally-bent composite bar (please refer to the supplementary Movie 4). These blocks were not separated by their weights or an external excitation, which indicates that they were assembled by compressive force due to the bending of the composite bar (please refer to the supplementary Movie 5). 4. Conclusion In this study, an efficient 4D printing method was developed using the anisotropic thermal deformation of 3D printed parts. An ME type
3D printer and a single thermoplastic filament (ABS) were used. The printing paths were programmed to generate thermal anisotropy in a bidirectional manner by combining the longitudinal and transverse printing paths effectively. This intentional anisotropy that contained residual stress caused unique thermal deformation after heat treatment, with the dimensions contracting in the printing direction and expanding in the lateral and lamination directions. The relevant thermal deformation characteristics were investigated by analyzing the directional size changes of the homogeneously laminated specimens. This directional thermal deformation was then used to enable 4D printing by applying heterogeneous lamination in which transverse and longitudinal printing paths were used consecutively. The heterogeneously laminated bars showed bending deformation after heat treatment, which corresponded to 1D-to-2D shape transformations. Numerical simulation was conducted to predict the deformation behaviors of the heterogeneous bars with different transverse-to-longitudinal
Fig. 7. 4D printing of a 2D cross-shape specimen. (a) Printing path designs for the cross shape. (b) 3D view of the heterogeneous lamination. (c) Shape changes of the cross-shape specimen with an increase in the heating time.
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Fig. 8. 4D printing of a 2D star-shaped specimen. (a) Printing path designs for the star shape. (b) 3D view of the heterogeneous lamination. (c) Shape changes of the star-shaped specimen with an increase in the heating time.
ratios, which enabled to control the order of bending by changing the transverse-to-longitudinal ratio. The heterogeneous lamination was further extended to print 2D cross and star shapes, which enabled 2Dto-3D shape transformations. Moreover, a selective printing of the anisotropic region between isotropic regions induced localized bending deformation, which could be used to perform self-assembly function. Considering that the proposed method uses a personal ME printer and a single thermoplastic polymer (ABS) without a shape memory function,
this method has advantages in its usefulness and adaptability over other 4D printing methods that use shape memory materials or multiple materials. The proposed method is also different from conventional 4D printing in terms of its reversibility. That is, the printed parts undergo permanent thermal deformation after appropriate heat treatment while most shape transformations in 4D printing are temporary and reversible. These unique characteristics can be utilized in various applications that require permanent deformation after thermal stimulation.
Fig. 9. Application of 4D printing to self-assembly. (a) Printing path designs for the composite bar. (b) Local bending behavior of the composite bar during the heat treatment. (c) Configuration for self-assembly. (d) Assembly result by the localized bending deformation.
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Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2020.108485. CRediT authorship contribution statement Bona Goo: Methodology, Investigation, Formal analysis. Chae-Hui Hong: Formal analysis. Keun Park: Conceptualization, Supervision, Writing - original draft. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgement This research was financially supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT, Republic of Korea (Grant number: 2019R1A2C1002799). References [1] A. Mitchell, U. Lafont, M. Hołyńska, C. Semprimoschnig, Additive manufacturing-a review of 4D printing and future applications, Addit. Manuf. 24 (2018) 606–626. [2] S. Tibbits, 4D printing: multi-material shape change, Archit. Des. 84 (1) (2014) 116–121. [3] F. Momeni, S.M.M. Hassani, N.X. Liu, J. Ni, A review of 4D printing, Mater. Des. 122 (2017) 42–79. [4] S. Miao, N. Castro, M. Nowicki, L. Xia, H. Cui, X. Zhou, W. Zhou, S. Lee, K. Sakar, G. Vozzi, Y. Tabata, J. Fisher, L.G. Zhang, 4D printing of polymeric materials for tissue and organ regeneration, Mater. Today 20 (10) (2017) 577–591. [5] X. Kuang, D.J. Roach, J. Wu, C.M. Hamel, Z. Ding, T. Wang, M.L. Dunn, H.J. Qi, Advances in 4D printing: materials and applications, Adv. Func. Mater. 29 (2) (2019), 1805290. [6] D.-G. Shin, T.-H. Kim, D.-E. Kim, Review of 4D printing materials and their properties, Int. J. Precis. Eng. Manuf. Green Tech. 4 (3) (2017) 349–357. [7] S. Li, W.M. Huang, Mechanisms of the multi-shape memory effect and temperature memory effect in shape memory polymers, Soft Matter 6 (18) (2010) 4403–4406. [8] W.G. Yang, H. Lu, W.M. Huang, H.J. Qi, X.L. Wu, K.Y. Sun, Advanced shape memory technology to reshape product design, manufacturing and recycling, Polymers 6 (8) (2014) 2287–2308. [9] H. Wu, P. Chen, C. Yan, C. Cai, Y. Shi, Y. Four-dimensional printing of a novel acrylatebased shape memory polymer using digital light processing, Mater. Des. 171 (2019) 107704. [10] F.A. Chávez, J.G. Siqueiros, I.A. Carrete, I.L. Delgado, G.W. Ritter, D.A. Roberson, Characterisation of phases and deformation temperature for additively manufactured shape memory polymer components fabricated from rubberised acrylonitrile butadiene styrene, Virtual Phys. Prototyp. 14 (2) (2019) 188–202. [11] Q. Ge, H.J. Qi, M.L. Dunn, Active materials by four-dimension printing, Appl. Phys. Lett. 103 (13) (2013), 131901.
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