Anti-gravitational 3D printing of polycaprolactone-bonded Nd-Fe-B based on fused deposition modeling

Accepted Manuscript Anti-gravitational 3D printing of polycaprolactone-bonded Nd-Fe-B based on fused deposition modeling Jianlei Wang, Hongmei Xie, Lei Wang, T. Senthil, Rui Wang, Youdan Zheng, Lixin Wu PII:

S0925-8388(17)31412-3

DOI:

10.1016/j.jallcom.2017.04.210

Reference:

JALCOM 41614

To appear in:

Journal of Alloys and Compounds

Received Date: 24 January 2017 Revised Date:

18 April 2017

Accepted Date: 19 April 2017

Please cite this article as: J. Wang, H. Xie, L. Wang, T. Senthil, R. Wang, Y. Zheng, L. Wu, Antigravitational 3D printing of polycaprolactone-bonded Nd-Fe-B based on fused deposition modeling, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.04.210. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Anti-gravitational 3D printing of polycaprolactone-bonded Nd-Fe-B based on fused deposition modeling Lixin Wu1, 2

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Correspondence to: Lixin Wu (E-mail: [email protected])

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Jianlei Wang1, 3, Hongmei Xie1, Lei Wang1, T.Senthil1, Rui Wang3, Youdan Zheng3,

Key Laboratory of design and assembly of functional nanostructures, Fujian Institute

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of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China 2

Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on

the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China University of Chinese Academy of Sciences, Beijing 100049, China

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3

Abstract

Generally, fused deposition modeling (FDM) 3D printing proceeds in the dependence of gravity, which restrains its application scenario. In this study, polycaprolactone

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(PCL) bonded Nd-Fe-B material filament for FDM process was prepared and a novel approach based on FDM was proposed to achieve anti-gravitational printing process

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by means of designing a magnetic platform. The effects of Nd-Fe-B content, magnetic flux density of the platform and printing angle on mechanical, magnetic and thermal properties were investigated. Results indicate that the tensile strength of the fabricated part of 60 wt% Nd-Fe-B highly filled PCL approximates the neat sample in the presence of magnetic force. Also, when loading 60 wt% Nd-Fe-B, the presence of magnetic force in the FDM process exerts a positive influence, improving 23%, 29.8% and 24.1% in tensile strength, (BH)max and thermal conductivity, respectively. Keywords: Composite materials; Mechanical properties; Magnetic measurements; Fused deposition modeling

ACCEPTED MANUSCRIPT 1. Introduction With the latest advances, additive manufacturing (AM) is developing at an incredible pace in the speed and accuracy of printing models with complex geometries and low manufacturing cost, representing a new edge on prototyping process evolution, which

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has captured the world’s horizon nowadays[1-4]. Fused deposition modeling (FDM) invented and developed by Stratasys Inc. in the early 1990s is the trendiest technique among all AM technologies, showing high potentials for fabricating plastic parts with the capacity to compete with conventional processing techniques[5, 6]. The

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applications of FDM process are comprehensive, ranging from medical treatment[7, 8], mold design[9], engineering[10] to automotive[11], aeronautics[12] and

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semiconductor device area[13]. However, FDM printing process depends on the materials’ gravity nowadays, which restrains its application scenario, such as outerspace or a bumpy moving car. It is meaningful to explore novel approaches to achieve printing process independence of gravity, which further expands application domain of FDM 3D printing.

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As one of the most important functional materials in modern life, Nd-Fe-B has been widely used in many fields as vital components for various electromechanical applications[14, 15]. Polymer-bonded magnets are typically obtained by mixing

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magnetic powders with a binder, such as mixing a thermoplastic polymer in an extruder or mixer and then subjecting the granule extrudate to injection molding or compression molding, which has expanding applications because of their superior characteristics,

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mechanical

resistance

to

corrosion

and

facile

processing

conditions[16-18].

In this study, it is innovative to prepare Nd-Fe-B highly filled polymeric composites for FDM process to achieve anti-gravitational FDM process by means of designing a magnetic platform, combining bonded Nd-Fe-B with FDM 3D printing. Meanwhile, the effects of Nd-Fe-B content, magnetic flux density of the platform and printing angle on mechanical, magnetic and thermal properties were investigated.

2. Experimental details

ACCEPTED MANUSCRIPT 2.1 The preparation of materials In this study, the Nd-Fe-B powders provided by XND Co. were about 48 um in particle size. To satisfy the requirements of FDM process, choosing the polymer to bond Nd-Fe-B powders is important. In that case, we developed modified

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polycaprolactone (PCL) named Polymate based on CapaTM 6800 produced by Perstorp Co. to fit for FDM process[19]. It had low linear heat shrinkage rate, so the fabricated part had no warpage and accurate dimensional stability. The filament preparation contained three procedures. Firstly, Nd-Fe-B powders were treated with

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silane coupling agent γ-APTS to strengthen the interface bonding with Polymate and prevent oxidization during preparation. Secondly, Nd-Fe-B powders and Polymate

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were mixed for 10 min and then compounded in HAAKE twin-screw extruder to prepare a masterbatch. The content of Nd-Fe-B was 80 wt%. Thirdly, the masterbatch and Polymate were mixed and then extruded in a single-screw extruder to prepare filament with 1.75 mm in diameter. During the process, the barrel temperature ranged

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between 70 and 90 ℃.

2.2 The design of magnetic platform

As shown in Figure 1, to achieve printing platform with magnetism, the solenoid was

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employed[20]. Simultaneously, high-power step motor was applied to ensure printing stability and dimensional accuracy. Magnetic flux density was set at 0, 0.1, 0.2, 0.25 and 0.3 T by controlling the magnitude of the current, respectively. A thin iron sheet

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was placed on the solenoid to attach the fabricated part.

2.3 FDM printing process The specimens for tensile strength, dynamic mechanical analysis (DMA), magnetic hysteresis loop and thermal conduction were prepared by feeding the filament into a commercial desktop FDM unit with the modification of magnetic platform provided by HY3D Corporation. During printing, nozzle temperature was maintained at 95 ℃, while platform temperature was near room temperature. The layer height was set at 0.2 mm, and the infill density was set at 100%. The nozzle’s diameter of the FDM

ACCEPTED MANUSCRIPT unit was 0.5 mm, ensuring that no clogging happened during printing. The printing speed for the first layer and other layers were 30 and 50 mm/s, respectively. The printing angle of the nozzle was 0-180° with the interval of 45°. In the FDM process, the filament as feedstock was fed into the heating chamber by a stepping motor and

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extruded through the nozzle in a prescribed manner upon its melting temperature, then solidified and deposited on the platform on a layer by layer basis[6, 21]. There existed an assumption that each layer of the fabricated part was under the same magnetic force, taking into consideration that the thickness of specimens was less than 3 mm.

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The dimensions of specimens for ASTM D638 dog-bone, single-cantilever, magnetic hysteresis loop and thermal conduction were followed, respectively. The deposited

3. Results and Discussion

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direction and cross-section topography of specimens are illustrated in Figure 2.

3.1 The maximum Nd-Fe-B loading for FDM process

In order to acquire large magnetic force for the material, high Nd-Fe-B content is

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requisite. However, there exists an upper limit of Nd-Fe-B content for printable PCL/Nd-Fe-B filament for FDM printing. Experiments showed that the upper limit of Nd-Fe-B content for FDM process is between 70 and 75 wt%. As shown in Figure 3,

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the filament material is squeezed by the driving gear and driven wheel and then fed into the heating chamber, which means that the material needs deformability and toughness to a certain degree. Figure 4 shows the variation of notched impact strength

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of injection molded samples with Nd-Fe-B content. It can be seen that notched impact strength decreases dramatically as Nd-Fe-B content reaches 75 wt%, which coincides with the experimental phenomenon that the surface is coarse with granular sensation. It can be explained that the matrix is too little to form a continuous phase and thus to bond Nd-Fe-B particles effectively.

3.2 The effect of Nd-Fe-B content on mechanical properties The effect of Nd-Fe-B content was assessed, comparing printed samples with 0, 3, 10, 20, 40, 60 wt% in circumstances of magnetic flux density of 0 and 0.3 T. As shown in

ACCEPTED MANUSCRIPT Figure 5, the influence of Nd-Fe-B content is significant, presenting a peak in the graph. When introducing 3 wt% Nd-Fe-B powders, tensile strength increases, ascending from 20.2 Mpa to 22.3 Mpa. This is mainly due to 1) Nd-Fe-B powders are rigid, working similarly as reinforced materials, such as TiO2 [22] or CaCO3 [23]; 2)

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Nd-Fe-B powders regarded as nucleating agent accelerate crystallization process and increase the degree of crystallinity of the PCL matrix[24, 25]; 3) when adding slight Nd-Fe-B powders, cracks propagate along with the surface of particles, which lengthens the crack paths and thus consumes more energy[26, 27]. However, there

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exhibits a different trend when Nd-Fe-B content is high, declining by up to 27% as Nd-Fe-B content reaches 60 wt%, referring to the samples bearing no magnetic force.

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This can primarily be attributed to poor dispersion of powders. When introducing more than 10 wt% particles, the dispersion is difficult, and agglomeration is significant, which leads to stress concentration in the samples[28]. The phenomenon becomes remarkable as Nd-Fe-B content further rises. On the other hand, the matrix is too little to transfer stress effectively, resulting in lower tensile strength. Moreover,

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the die swell behavior out of the nozzle of pure PCL is severe during printing and the nozzle squeezes PCL matrix at the layer height of 0.2 mm and helps the entanglement of the molecular chain. While Nd-Fe-B particles decrease the die swell effect of PCL

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molecule and restrains the molecular chains’ movement, the deposited line of PCL/ Nd-Fe-B is inclined to maintain circular, which results in bigger voids and thus poorer adhesion between deposited lines[29]. Meanwhile, inter-particle voids accumulate. As

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a result, tensile strength decreases a lot. However, it presents a different trend as magnetic flux density reaches 0.3 T, showing a steady curve in the graph, which can be attributed to the significant effect of magnetic force. Compared with fabricated samples without magnetic force, the tensile strength of samples with 60 wt% Nd-Fe-B approaches 19Mpa, closing to the neat samples, which means that the positive effect on tensile strength is evident in samples with high Nd-Fe-B content. SEM of fracture interface of samples with different Nd-Fe-B content is exhibited in Figure 6. It can be observed that porosity, including the gaps between particles and matrix and the physical voids at layer/layer interfaces, increases significantly when adding more

ACCEPTED MANUSCRIPT Nd-Fe-B particles, especially samples with 60 wt% Nd-Fe-B. Figure 7 reveals the variation of storage modulus with Nd-Fe-B content. As Nd-Fe-B content increases, storage modulus increases, which can be attributed to the stiffness of Nd-Fe-B. However, polymer chains possess large free volume at high temperature,

3.3 The effect of printing conditions on mechanical properties

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resulting in the decrease in storage modulus.

The effects of magnetic flux density and printing angle were investigated by

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comparing samples with 60 wt% Nd-Fe-B on different printing conditions. As shown in Figure 8, tensile strength increases when magnetic flux density goes up in all

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circumstances. It can be attributed to that Nd-Fe-B particle under magnetic force moves to push the matrix to fill voids, which makes the interface between layers blurred and crack path longer[30, 31], as illustrated in Figure 9. Porosity is regarded as a crucial factor for mechanical properties. To understand the mechanism of porosity formation, a close observation of the FDM process is needed. The porosity in the

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sample consists of relatively large triangular voids which are similarly oriented. These voids are mainly gaps between deposited lines during printing. Although the molten material extruded through the nozzle is circular, then it is pressed down to the

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thickness of 0.2 mm and becomes elliptical. As the deposited line is still soft when being deposited, the bottom flattens under press while the top cools to form round edges before another layer is deposited on it, which illustrates the reason why the

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triangular gaps aligned in the direction of printing are only downwards[12]. Also, inner impetus enhances the entanglement of PCL molecular chain upon its melting temperature, improving the matrix strength[32]. However, as magnetic flux density is low, approximating 0 and 0.1 T, the printing process is not smooth and tensile samples are not reachable, with the printing angle of 180°. Figure 10 shows the results of the effect of printing angle on tensile strength. Obviously, it can be concluded that the augment in printing angle lowers tensile strength, especially upon 90°. It is easy to understand that the positive effect of materials’ gravity decreases as printing angle increases, which is adverse to the

ACCEPTED MANUSCRIPT entanglement of molecular chain and makes voids between deposited lines bigger. SEM of fracture interface of samples fabricated on the condition of 0°, 0.3 T, and 180°, 0.3 T are shown in Figure 11, which evidently demonstrates the positive effect of magnetic force introduced into FDM process and the adverse effect of enlarging

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printing angle. However, it is worth mentioning that FDM 3D printing process with anti-gravity can be achieved by magnetic force in the presence of magnetic platform, which suggests that it is possible to print plastic parts by FDM independence of gravity. Figure 12 shows the tensile samples printed under different circumstances of

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0°, 90° and 180° in the presence of 0.3 T. The printing quality of the samples are distinguishing, with the sample printed in 0° being entirely intact and the sample

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printed in 180° being porous on the surface. Results show that the porosity of the sample printed in 0° and 180° is 8.9% and 15.2%, respectively. There exist two categories of voids in samples, including the gaps between particles and matrix and the physical voids at layer/layer interface. The first type is mainly generated during the fabrication of feedstock filament in the extrusion process. While the second

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mainly results from the FDM process, which accounts for a larger proportion. The effects of magnetic flux density and printing angle on storage modulus are shown in Figure 13, comparing samples with 60 wt% Nd-Fe-B. The storage modulus is

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nearly 300 Mpa when printing with anti-gravity in room temperature, while it is about 600 Mpa when printing with zero-gravity. The former is less than half the latter, which verifies the very negative influence of the augment in printing angle on

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stiffness. Also, it can be found that the difference in storage modulus between 0 and 0.3 T is larger as printing angle increases, which further illustrates that the gravity in FDM process has a significant effect on the properties of the fabricated part.

3.4 The effect of printing conditions on magnetic properties The remanence Br, coercivity Hcj and maximum magnetic energy product (BH)max of Nd-Fe-B used in this study is 0.79 T, 788.71 kA/m, and 97.02 kJ/m3, respectively. Magnetic properties were evaluated by comparing samples with 60 wt% Nd-Fe-B that was the upper limit acquired by experiment. Figure 14 shows the magnetic hysteresis

ACCEPTED MANUSCRIPT loop of samples fabricated under different circumstances and the exact values of magnetic properties are listed in Table 1. It can be concluded that Br decreases evidently as printing angle increases and the magnetic platform with 0.3 T is beneficial to magnetic properties of samples. This can be attributed to that magnetic

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properties quite depend on the density of Nd-Fe-B particles[33, 34]. As discussed above, as printing angle increases with less gravity, the infiltration and encapsulation of Nd-Fe-B by PCL becomes uneven, which makes voids between deposited lines larger and thus decreases the density of Nd-Fe-B. However, the presence of magnetic

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force in the FDM process helps the compact integration of adjacent deposited lines and a better bond between PCL and Nd-Fe-B particles, decreasing the total porosity

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and making Nd-Fe-B much denser.

3.5 The effect of printing conditions on thermal properties

The thermal properties of fabricated parts by FDM were evaluated by steady-state heat flow method, comparing samples with 60 wt% Nd-Fe-B printed in different

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conditions. Results indicate that the thermal conductivity of the neat sample is 0.109 W/(m •K) while it rises to 0.183 W/(m •K) when introducing 60 wt% Nd-Fe-B, as shown in Figure 15. It can be attributed to that lattice vibration mainly results in

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thermal conduction in PCL, which is hindered by molecular entanglement and incomplete lattice[35]. However, it is electron conduction that is the principal mechanism in Nd-Fe-B when it comes to thermal conduction. The network of thermal

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conduction is constructed by introducing the high content of Nd-Fe-B particles, which leads to improvement in thermal conductivity. Because air is the poor conductor of heat, decreasing the porosity of fabricated part helps enhance the density of thermal conduction network[36]. This explains why the presence of magnetic force in FDM process can further improve 24.1% and 19.5% in thermal conductivity and thermal resistance of fabricated parts, respectively, comparing to those bearing no magnetic force.

3.6 Applications of the material

ACCEPTED MANUSCRIPT Bonded Nd-Fe-B has broad applications, such as automation equipment, instrument, cell phone, and so on. The combination of 3D printing and bonded Nd-Fe-B further broadens the scope of application, taking medical treatment as an example. There exist differences in the dimension and the degree of pathological change of patients in

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the clinical research, which makes inconvenience and complexity in the surgery. However, the appearance of 3D printing can solve the problem by fabricating customized and individual parts for patients to alleviate the agony and danger during the surgery, which facilitates the precision medicine. PCL/ Nd-Fe-B can be printed

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and used as magnetic anastomosis ring after magnetization to expand the feasibility of magnetic anastomosis technique in the complicated parts of the body. Figure 16

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shows the magnetic anastomosis ring made of PCL with 60 wt% Nd-Fe-B. As can be seen, the fabricated part by FDM has almost the same dimensions with the designed model, which indicates that the material has dimensional stability.

4. Conclusions

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In this study, Nd-Fe-B particles highly filled PCL filament material was prepared for FDM and a novel process based on FDM was proposed by designing a magnetic platform to achieve anti-gravitational printing process. Results indicate that the

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fabricated parts of PCL/ Nd-Fe-B are endowed magnetic and thermal properties without sacrificing mechanical properties. Also, when loading 60 wt% Nd-Fe-B, the presence of magnetic force in the FDM process exerts a positive influence, improving

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23%, 29.8% and 24.1% in tensile strength, (BH)max and thermal conductivity, respectively. This study provides a possible solution and plays an instructive role to achieve innovative FDM printing mode, which broadens the application scenario of FDM.

Acknowledgements This research was financially supported by the National Natural Science Foundation of China (Grant No.: U1205114 and 51403212) and Science Foundation of Fujian Province (Grant No.: 2014J01217 and 2015H0047).

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Author Contributions Jianlei Wang conceived and designed the experiments. Hongmei Xie and Youdan Zheng performed the experiments. T. Senthil was responsible for the SEM images.

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Lei Wang and Rui Wang helped to analyse the reasons to experimental results. Lixin Wu coordinated and supervised the project. All authors discussed the results and reviewed and commented the manuscript.

Table 1. The magnetic properties of Nd-Fe-B/PCL fabricated in different conditions Ms/ ( emu/g )

Br*/ T

Hcj/ ( kA/m )

( BH )max/ ( kJ/m3 )

0°, 0 T

42.68

0.536

771.9

17.31

0°, 0.3 T

53.37

0.671

680.1

22.48

180°, 0.3 T

38.95

0.489

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Printing Condition

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*B=H+4πM

767.0

Figure 1. The design of magnetic platform

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Figure 2. Types of specimens printed by FDM: A) magnetic hysteresis loop& thermal conduction; B) tensile;

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C) DMA and D) section topography of black rectangle plane in A), B) and C)

Figure 3. Schematic of filament material fed into heating chamber in FDM process

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Figure 4. The effect of Nd-Fe-B content on notched impact strength of injection molding specimens

Figure 5. The effect of Nd-Fe-B content on tensile strength

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Figure 6. SEM of fracture interface of samples with A) 0, B) 10, C) 20 and D) 60 wt% Nd-Fe-B

Figure 7. The effect of Nd-Fe-B content on storage modulus

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Figure 8. The effect of magnetic flux density on tensile strength

Figure 9. Schematic of Nd-Fe-B powders ( rufous balls ) in matrix ( gray area ) under magnetic force

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Figure 10. The effect of printing angle on tensile strength

Figure 11. SEM of fracture interface of samples printed under A) 0°, 0.3 T and B) 180°, 0.3 T

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Figure 12. The porosity of printed samples in different printing angles a) 0, b) 90 and c) 180° under 0.3 T

Figure 13. The effects of magnetic flux density and printing angle on storage modulus

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Figure 14. The effect of FDM printing conditions on magnetic properties

Figure 15. The effect of FDM printing conditions on thermal properties

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Figure 16. The designed (left) and printed (right) model of magnetic anastomosis ring

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Figure and table captions: Figure 1. The design of magnetic platform

Figure 2. Types of specimens printed by FDM: A) magnetic hysteresis loop& thermal conduction; B) tensile; C) DMA and D) section topography of black rectangle plane in A), B) and C)

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Figure 3. Schematic of filament material fed into heating chamber in FDM process Figure 4. The effect of Nd-Fe-B content on notched impact strength of injection molding specimens

Figure 5. The effect of Nd-Fe-B content on tensile strength

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Figure 6. SEM of fracture interface of samples with A) 0, B) 10, C) 20 and D) 60 wt%

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Figure 7. The effect of Nd-Fe-B content on storage modulus Figure 8. The effect of magnetic flux density on tensile strength Figure 9. Schematic of Nd-Fe-B powders ( rufous balls ) in matrix ( gray area ) under magnetic force

Figure 10. The effect of printing angle on tensile strength Figure 11. SEM of fracture interface of samples printed under A) 0°, 0.3 T and B) 180°, 0.3 T Figure 12. The porosity of printed samples in different printing angles a) 0, b) 90 and c) 180° under 0.3 T

ACCEPTED MANUSCRIPT Figure 13. The effects of magnetic flux density and printing angle on storage modulus Figure 14. The effect of FDM printing conditions on magnetic properties Figure 15. The effect of FDM printing conditions on thermal properties Figure 16. The designed (left) and printed (right) model of magnetic anastomosis ring

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Table 1. The magnetic properties of Nd-Fe-B/PCL fabricated in different conditions

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Highlights: 1. Polycaprolactone-bonded Nd-Fe-B material was prepared for FDM 3D printing. 2. Anti-gravitational FDM process was achieved by designing a magnetic platform. 3. The presence of magnetic force improves mechanical properties of printed

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sample. 4. The presence of magnetic force improves magnetic properties of printed sample.

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5. The presence of magnetic force improves thermal properties of printed sample.