- Email: [email protected]
Improvement and Evaluation of the Interlaminar Bonding Strength of FDM Parts by Atmospheric-Pressure Plasma
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 42 (2016) 754 – 759
18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)
Improvement and Evaluation of the Interlaminar Bonding Strength of FDM Parts by Atmospheric-Pressure Plasma Hiroyuki NARAHARAa*, Yota SHIRAHAMAa, Hiroshi KORESAWAa a
Kyushu Institute of Technology, Kawazu 680-4, Iizuka-shi, FUKUOKA 820-8502, JAPAN
* Corresponding author. Tel.: +81-948-29-7766 ; fax: +81-948-29-7751. E-mail address: [email protected]
Abstract Additive Manufacturing has the feature that the on-demand manufacture of the arbitrary shape can be performed. However, as one of weak points, interlaminar bonding strength is low. It is because the building principle is based on lamination. This research aims at improvement in the adhesive strength between laminations by performing hydrophilization treatment to the lamination plane of a FDM parts using atmospheric pressure plasma. Atmospheric-pressure plasma is characterized by generating glow-discharge condition under an atmospheric pressure. It can use for surface treatment etc. using active species (radical ion, an electron, etc.). For example, they are cleaning effects, such as soil removal of an organic substance, an effect to the roughened surface for forming unevenness in the surface, etc. Although a tensile test is usually performed on evaluation of adhesive strength, a lot of hours are needed for experimental preparation. The surface free energy of an object surface is effective as one of the evaluation indices for evaluating the adhesive strength of a lamination plane. However, in order to find an experimental condition in research initial stages, such as material search and manufacturing conditions, it can become effective in respect of an hour and cost to use surface free energy as an evaluation index. In this paper, the effect to the surface free energy by atmospheric pressure plasma is investigated first. And the influence on the rupture strength improvement by atmospheric pressure plasma is investigated. Next, it is searched for the effective irradiation conditions of atmospheric pressure plasma. The influence of the atmospheric pressure plasma to the building parts of FDM was experimented based on surface free energy. The rupture test was done and improvement in rupture stress was confirmed by plasma irradiation. The result that the irradiation range and irradiation distance of an atmospheric-pressure-plasma head had large influence on the increase in surface free energy was obtained. It was observed that surface free energy increases about 75% to an initial sample by irradiation of atmospheric pressure plasma.
© by Elsevier Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016 The The Authors. Authors. Published Published by (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: Additive manufacturing; Fused deposition modeling; Atmospheric pressure plasma; Surface free energy; Interlaminar bonding strength
1. Introduction Additive Manufacturing is the technology of realizing solid parts, defining three-dimensional shape as lamination of a thin sheet, and joining the material of plastics or metal little by little. Unlike removal-processing technology, such as machining, there is the feature which can be produced easily also in complicated shape. Fused Modeling Deposition (FDM) builds parts by lamination with a plastic material. Since the adhesive strength between this lamination is low, the strength of parts is lower rather than the parts by injection molding process.
If the adhesive strength between laminations of this FDM parts can be improved, expansion of the application range, for example, functional verification, use for a product, etc., will spread. In this research, the hydrophilization treatment by atmospheric pressure plasma is used for improvement in the adhesive strength between laminations. The experimental result of a rupture test is reported about the influence of the hydrophilization treatment by atmospheric pressure plasma. Change of the surface free energy of the FDM parts before and after an atmospheric pressure plasma treatment is also reported.
2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.314
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Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
Nomenclature ୟ ߛ ߛ ߛ ɀ ɀୢୗ ɀୢ ୮
ɀୗ
୮
ɀ
ɀୌ ୗ ɀୌ ȣ ߪ F A
Adhesion work Surface free energy of the surface A [J/m2] Surface free energy of the surface B [J/m2] Surface free energy of an A-B interface [J/m2] Surface tension of a liquid Dispersion force component of the Van der Waals force of a solid Dispersion force component of the Van der Waals force of a liquid Interface-interaction force by the intermolecular force based on the polarity a solid (polar force component) Interface-interaction force by the intermolecular force based on the polarity of a liquid (polar force component) Hydrogen-bond interaction force of a solid (hydrogen bonding component) Hydrogen-bond interaction force of a liquid (hydrogen bonding component) Angle of contact Rupture stress [MPa] Measuring load [N] Cross-sectional area [mm2]
surface treatment, and it is said that various application can be performed [1-4]. In this research, atmospheric pressure plasma is generated by making it discharge within the glass tube which helium gas passes by low frequency wave length and a high voltage power source (10 kV, 10 kHz) (Fig. 2, Fig. 3). He Gas
Electrodes
Plasma plume
Earth
Fig.2 Atomospheric pressure plasma
Fig.3 Plasma plume 1.3. Evaluation of the strength of adhesive bonding by surface free energy
1.1. Fused Deposition Modeling There is a Fused Deposition Modeling (FDM) in one of the Additive Manufacturing (AM). As shown in Fig. 1, it is the method of building three-dimensional shape by the thermoplastic resin dissolved from the nozzle head being sent out, and deposition and solidification being repeated. It has the feature which can perform on-demand manufacture of the arbitrary shape. However, since the principle is based on lamination, one of weak points is that interlaminar bonding strength is low. In this research, the effect to improvement in the adhesive strength of lamination by the atmospheric pressure plasma treatment on the surface of lamination of the FDM part during building is investigated. Material
rotor Heater
Fig.1 FDM building process 1.2. Atmospheric-pressure plasma Although the plasma can usually occur only under vacuum conditions, the feature of atmospheric pressure plasma is that it can generate under an atmospheric pressure. The active species (a radical, an ion, an electron, etc.) which arise by plasma have cleaning effects, such as a soil removal of an organic substance, the rough surface generation effects, such as uneven shape formation, and the activation effects, such as
Adhesion work is defined by energy needed to pull apart the two surfaces which have touched by the interface as shown in Fig. 4.
A
J AB
A
Wa
B
JA JB
B
Fig.4 Adhesion work It is expressed with the equation (1) called the Dupre equation [5]. ୟ ൌ ߛ ߛ െ ߛ
(1)
Here, ߛ is the surface free energy [J/m2] of the surface A, and ߛ is the surface free energy [J/m2] of the surface B, ߛ expresses the surface free energy [J/m2] of the A-B interface, respectively. Calculating this adhesion work can evaluate the strength of adhesive bonding of lamination. An extended Fowkes equation is one of the methods of calculating solid surface free energy [5]. It can be denoted by an equation (2) that the applied force which one side's surface gives to another surface is a geometric mean value of each surface free energy [6]. ୟ ൌ ɀ ሺͳ Ʌሻ ൌ ʹ൫ɀୢୗ ή ɀୢ ൯ ʹ൫ɀୌ ୗ
ή
ଵΤଶ
୮
୮ ଵΤଶ
ʹ൫ɀୗ ή ɀ ൯
ଵΤଶ ɀୌ ൯
(2)
Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
Here, ɀ is surface tension which is a liquid, ɀୗ is solid surface tension, ɀୢ is a dispersion force component of Van der ୮ Waals force, ɀ is the interface-interaction force (polar force component) by the intermolecular force based on polarity, ɀୌ expresses hydrogen-bond interaction force (hydrogen bonding component). As shown in Fig. 5, ș is an angle of contact. An angle of contact is an angle between the horizon and the tangent of a droplet boundary zone. Tangent angle
Liquid
Fig.5 Contact angle Three or more kinds of liquid reagents are used for measurement of surface free energy. It is used for these reagents that three component values (a dispersion force component, a polar force component, hydrogen bonding component) of surface free energy have become clear beforehand. Those reagents are dropped at a test piece and an angle of contact is measured. And each-component value of solid surface free energy is obtained by solving it with a leastsquares method to an equation (2). ଵ Τଶ ଵ Τଶ ଵ Τଶ Now, each component ሺ൫ɀୢୗ ൯ ǡ ൫ɀ୮ୗ ൯ ǡ ൫ɀୌୗ ൯ ሻ of solid surface free energy presupposes that it is unknown. And since each component of the surface free energy of reagents ଵ Τଶ ଵ Τଶ ୮ ଵ Τଶ ሺ൫ɀୢ ൯ ǡ ൫ɀ ൯ ǡ ൫ɀୌ ሻ is known, it replaces each with ൯ ሺ ͳ݅ݔǡ ʹ݅ݔǡ ͵݅ݔሻ. It will become a primary equation (3) if a solid unknown component is placed with a, b, and c, respectively.
ሺǡ ǡ ሻ ൌ ȭ߳ଶ ൌ ȭሼݕ െ ሺܽݔଵ ܾݔଶ ܿݔଷ ሻሽଶ
Here, it is investigated by atmospheric pressure plasma how the surface energy of resin changes. In this experiment, an atmospheric-pressure-plasma head is fixed above a PLA resin, and resin is irradiated with atmospheric pressure plasma only the specified hour. Then, surface free energy is calculated by measuring the angle of contact of droplet with multiple reagents. Five kinds of liquids were used for the reagent. Each element of the surface free energy of each reagent is shown in Table 1. An experimental condition is shown in Table 2. Table 1 Surface free energy of reagents
Solid
Ԗ୧ ൌ ݕ െ ሺܽ ݔ ڄଵ ܾ ݔ ڄଶ ܿ ݔ ڄଷ ሻ
2.2. Measurement of Influence in Surface Free Energy by Atmospheric Pressure Plasma
(3) (4)
A residual error is obtained to the measurement experiment of each angle of contact. It solves with a least-squares method so that S of the residual error sum (4) of all the experiments may become the minimum, and a, b, and c are obtained. 2. Effect of Atmospheric Pressure Plasma to Lamination Interfaces of Fused Deposition Modeling Parts 2.1. Objective First, the change of surface free energy by irradiation of atmospheric pressure plasma is investigated at Section 2.2. Next, the effect of the atmospheric pressure plasma treatment at the time of a part building is investigated, and the effect to rupture strength is observed at Section 2.4.
ࢽࢊ 30.1 37.4 29.1 46.8 44.4
Ethylene glycol Glycerin Water Diiodomethane 1-Bromonaphthalene
ࢽ 0 0.2 1.3 4 0.2
ࢽࡴ 17.6 25.8 42.4 0 0
ࢽࡸ 47.7 63.4 72.8 0 0
Table 2 Experimental conditions Glass tube inside diameter [mm] Glass tube outside diameter [mm] Plasma irradiation distance [mm] Helium gas flow rate [l/min] Helium gas pressure [MPa]
2 4 10 0.3 0.2
2.3. Experimental Result of Atmospheric Pressure Plasma Irradiation Change of surface free energy when a PLA resin is irradiated with atmospheric pressure plasma for 0 to 30 seconds is shown in Fig. 6. From Fig. 6, it was observed that the surface free energy on the surface of building parts was increasing by the atmospheric pressure plasma irradiation. Surface free energy increased to about 8% in the processing time for 5 seconds, and increased about 20% in 20 seconds. As shown in Fig. 6, the rate of the hydrogen-bond term in surface free energy increased greatly by plasma treatment. Surface free energy [mJ/݉^2]
756
80 70 60 50 40 30 20 10 0
Hydrogen
0
polar
dispersion
5 10 20 Irradiation time [s]
30
Fig.6 Surface free energy by atmospheric pressure plasma treatment 2.4. Rupture test In order to investigate the effect by an atmospheric pressure plasma treatment, the test piece for rupture tests is produced.
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FDM equipment (RepRap Mendel Plusa) was used for preparation of a test piece. The plasma irradiation head was attached to the FDM equipment. Resin lamination and plasma treatment are performed one after the other at the time of the building of a test piece. As shown in Fig. 7, a building head and a plasma head are placed on the distant position, and are being fixed to the moving mechanism. A top face is irradiated with a plasma plume whenever a lamination plane is built. Although two test pieces are built simultaneously, it irradiates with an atmospheric pressure plasma treatment only to one piece. That is, plasma treatment is performed only to 1 sample within the two samples. In this experiment, the helium gas flow rate at the time of a plasma generation is changed by using regulator with a flow meter.
2.5. Rupture test result The rupture stress of each test piece by a rupture test is shown in Fig. 9. As shown in Fig. 9, rupture stress became higher when irradiation speed was high. Furthermore, by performing an atmospheric pressure plasma treatment, fracture strength further improved. The rupture stress in the case of the atmospheric pressure plasma irradiation in 1000 mm/min and 1400 mm/min both increased from the case of not irradiating. 50
Rupture stress ı[MPa]
Atmospheric-pressure plasma head
In a rupture test, the maximum load at the time of a rupture and the area of a fracture surface are used. Image-analysis software is used for measurement of the cross-sectional area.
Head Motor Heater Test piece
40
without treatment plasma treatment
30 20 10 0 1000
Fig.7 Experimental setups of plasma The shape of the test piece of a rupture test is shown in Fig. 8. Experimental conditions are shown in Table 3. A test piece performs a rupture test using the force gauge attached to the electric stand, respectively.
Fig.8 Sample shape Table 3 Experimental conditions Glass tube inside diameter [mm] Glass tube outside diameter [mm] Plasma irradiation distance [mm] Helium gas flow rate [l/min] Helium gas pressure [MPa] Plasma irradiation speed [mm/min]
2 4 15 0.5 0.22 1000, 1400
Rupture stress ı [MPa] can be calculated from an equation (5). ߪ ൌ ܨȀܣ
(5)
Here, F is the load for measurement [N] and A is the crosssectional area [mm2]. In a rupture test, the maximum load at the time of a rupture and the cross-sectional area of a fracture surface are used. Image-analysis software is used for measurement of the cross-sectional area. F is the load for measurement [N] and A is the cross-sectional area [mm2] here.
1400
Building Speed[mm/min] Fig.9 Rupture stress by atmospheric pressure plasma treatment 2.6. Discussion From the experimental result of Section 2.3, it was observed that surface free energy increases by the atmospheric pressure plasma irradiation to a PLA resin. As a reason for this increase, the cleaning effect by plasma, generation of a hydroxyl group, etc. can be thought of. About a detailed mechanism, the further investigation is needed. When irradiation speed became high, the result to which rupture stress became high was obtained from the experimental result of Section 2.5. As this reason, if a scattering rate becomes high, a building hour will become short. On the other hand, if the building parts fabricated by heating resin are high-speed buildings, since cooling does not progress, the temperature of parts is still high. This elevated temperature is considered to have influenced adhesive strength. About this hypothesis, still more detailed investigation is needed. Anyway, the effect of the atmospheric pressure plasma irradiation was obtained to rupture strength. The surface free energy on the building conditions of lamination was not able to be measured without the ability to drop a reagent at a sample surface. It is necessary to investigate relevance with rupture strength in detail by considering a specimen configuration etc. Since the effect of atmospheric pressure plasma is influenced by irradiation conditions, it becomes very important from these results to find the effective irradiation conditions of atmospheric pressure plasma. In Chapter 3,
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Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
examination about the effective irradiation conditions of atmospheric pressure plasma is performed. 3. Investigation of Influence of Atmospheric Pressure Plasma Based on Surface Free Energy
Fig. 10 shows a test piece. The irradiation area of atmospheric pressure plasma is shown in Fig. 11. Measurement of surface free energy was performed in the position of area-a. PLA resin was used for the test piece and traveling speed was set to 660 [mm/min].
3.1. Objective
3.2. Search of Atmospheric-Pressure-Plasma-Irradiation Conditions by Orthogonal Array Experiment The orthogonal array experiment was conducted and investigation of the factor which has large influence on an atmospheric pressure plasma treatment, and improvement in irradiation conditions were investigated. In the experiment 1, a building conditions is changed and surface free energy is measured. Table 4 Experimental conditions 1 Level
1 2 3
Stage temperature [A]
40Υ 50Υ 60Υ
Sample size[B]
20x10mm 30x10mm 40x10mm
Irradiation area [C]
Measurement Interval time[D]
a ab abc
0h 24h 48h
Fig.10 Specimen
The experimental result is shown in Fig. 12. The influence of the surface free energy on each conditions was investigated. The result which the irradiation range of [C] affected most was obtained. When the stage temperature of [A] changed to 50 [ć] from 40 [ć], the effect of atmospheric pressure plasma decreased by about 30%. When the measurement interval of [D] changed to 48 [h] from 0 [h], the effect of atmospheric pressure plasma decreased to about 26%.
Table 5 Experimental No. based on orthogonal array No. No treatment 1 2 3 4 5 6 7 8 9
A 40Υ 40Υ 40Υ 40Υ 50Υ 50Υ 50Υ 60Υ 60Υ 60Υ
B 20x10mm 20x10mm 30x10mm 40x10mm 20x10mm 30x10mm 40x10mm 20x10mm 30x10mm 40x10mm
C a ab abc ab abc a abc a ab
D 0h 24h 48h 48h 0h 24h 24h 48h 0h
Fig.11 Irradiation area
3.3. Experimental Result 1
Surface free energy [mJ/m²]
Although a tensile test is usually performed on evaluation of adhesive strength, a lot of hours are needed for experimental preparation. The surface free energy of an object surface is effective as one of the indices for evaluating the adhesive strength of the surface of material. Especially, if surface free energy is used as an evaluation index in order to find an experimental condition, it can become effective in respect of an hour and cost in research initial stages, such as material search and manufacturing conditions. The objective of this experiment is to search for the effective irradiation conditions of atmospheric pressure plasma.
64
to maximize
59 54 49
to minimize
0 (untreated) 44 40 50 60 [A]
20 30 40 [B]
a ab abc [C]
0 24 48 [D]
Fig.12 Experimental results From these results, two combination conditions, set1 and set2, shown in Table 6 were set up for the next experiment. Table 6 Combination conditions Stage
irradiation
Measurement
temperature [A]
area [C]
Interval time[D]
Set1
40Υ
abc
0h
Set2
50Υ
a
48h
3.4. Experiment2 As shown in Table 4, the conditions of each factor are assigned to an orthogonal array, and it experiments in nine kinds of conditions combination (Table5). Then, it totals for every level of factor, and calculates an average value. As for these nine experiments, the same number of each level of factor is included. If the effect of each factor assumes the experiment model which is combined linearly and each levelof-factor average will be calculated, only the effect of a factor to investigate can be obtained.
In the experiment 2, the plasma generation conditions was given based on the result in the experiment 1. Surface free energy was measured on eight conditions shown in Table 7. A helium gas flow rate [X], atmospheric-pressure-plasmairradiation distance [Y], and combination 2 conditions in the experiment 1 were used.
Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
Table 7 Condition of Experiment2 No. 1 2 3 4
Flow rate of He gas [X] 0.5[L/min] 0.7[L/min] 0.5[L/min] 0.7[L/min]
Irradiation distance [Y] 10mm 10mm 5mm 5mm
Set1 Set1 Set1 Set1
5 6 7 8
0.5[L/min] 0.7[L/min] 0.5[L/min] 0.7[L/min]
10mm 10mm 5mm 5mm
Set2 Set2 Set2 Set2
Condition Set
4. Conclusion
3.5. Experimental Result 2 The experimental result in eight conditions shown in Table 7 is shown in Fig. 13. From Fig. 13, two conditions shown in Table 6 had the tendency for a result to be the same. On all the conditions, the conditions of Table 7-[Set1] was observed that the effect of atmospheric pressure plasma was higher than the conditions of Table 7-[Set2] about 5% on the average. It was observed that the effect of atmospheric pressure plasma increases notably by shortening the irradiation distance of [Y]. On the conditions of Table7-[No.3] , the effect of atmospheric pressure plasma became the maximum. By irradiating with atmospheric-pressure plasma, the result which surface free energy increases from no processing by about 75% was obtained. 90
hydrogen bond
80
polarity
dispersion force
70 Surface free energy [
composition are deeply related. Since surface free energy is evaluating only the ideal adhesive strength of materials, it does not guarantee 100% of the joining strength of real parts. Nevertheless, it seems that the evaluation by surface free energy is effective, and predominant. At AM research, it is because material development and development of a mechanism are needed and it can investigate the ideal condition of a system simple even in a development initial stage. Being used by development will be desirable, using together with a destructive test.
60 50 40 30 20 10
For improvement in the adhesive strength of the parts in a Fused Deposition Modeling, this research has proposed the surface treatment method by atmospheric pressure plasma. Moreover, examination was promoted by measuring the surface free energy of a FDM parts about the process of searching for the conditions which give an atmospheric pressure plasma treatment effectively. The influence of atmospheric-pressure-plasma-irradiation conditions was investigated, and the following results were obtained. 1. The improvement result of rupture strength was obtained by the rupture test. 2. The conditions of the irradiation range and irradiation distance at the time of an atmospheric pressure plasma irradiation gave the large effect to the increase in surface free energy. 3. Surface free energy increased about 75% from unprocessed condition by adjusting the irradiation conditions of atmospheric pressure plasma. 4. It was suggested that an atmospheric pressure plasma irradiation is an effective means to improvement in the joining strength of the parts of a Fused Deposition Modeling. Acknowledgements
0 1
2
3 4 5 Specimen nunber
6
7
8
Fig.13 Surface free energy of experiment2 3.6. Discussion It was searched for the factor and level which make surface free energy high by the orthogonal array experiment about the irradiation conditions parameter of atmospheric pressure plasma. It is necessary to perform part manufacture and a rupture test and to advance investigation further about the conditions that the effect of an atmospheric pressure plasma irradiation is high. 4. Discussion About the bond strength of material, not only the adhesive strength of materials but a defect, a crack, etc. of a plane of
A part of this research was done by grants-in-aid for scientific research (KIBAN (C) 24560137). References [1] Bárdos L, Baránková H. Cold atmospheric plasma: Sources, processes, and applications. Thin Solid Films. 2010;518(23):6705-13. [2] Chang JS. Physics and Chemistry of Atmospheric Plasmas. Journal of Plasma and Fusion Research. 2006;82(10):682-92. [3] Fukuda K, Kondo Y, Oishi K, Toda Y. The Development of Adhesion Technology Using Atmospheric Pressure Glow Plasma Processing. Konica Minolta technology report. 2004;1:35-8. [4] Tendero C, Tixier C, Tristant P, Desmaison J, Leprince P. Atmospheric pressure plasmas: A review. Spectrochimica Acta Part B: Atomic Spectroscopy. 2006;61(1):2-30. [5] Mitou M. Chap.5 The surface chemistry of adhesion. Advanced adhesive joint technology: NGT; 2000. [6] Nakajima A. Wetting property control of a solid surface: Uchida Rokakuho Publishing Co., Ltd.; 2007.
759
ScienceDirect Procedia CIRP 42 (2016) 754 – 759
18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)
Improvement and Evaluation of the Interlaminar Bonding Strength of FDM Parts by Atmospheric-Pressure Plasma Hiroyuki NARAHARAa*, Yota SHIRAHAMAa, Hiroshi KORESAWAa a
Kyushu Institute of Technology, Kawazu 680-4, Iizuka-shi, FUKUOKA 820-8502, JAPAN
* Corresponding author. Tel.: +81-948-29-7766 ; fax: +81-948-29-7751. E-mail address: [email protected]
Abstract Additive Manufacturing has the feature that the on-demand manufacture of the arbitrary shape can be performed. However, as one of weak points, interlaminar bonding strength is low. It is because the building principle is based on lamination. This research aims at improvement in the adhesive strength between laminations by performing hydrophilization treatment to the lamination plane of a FDM parts using atmospheric pressure plasma. Atmospheric-pressure plasma is characterized by generating glow-discharge condition under an atmospheric pressure. It can use for surface treatment etc. using active species (radical ion, an electron, etc.). For example, they are cleaning effects, such as soil removal of an organic substance, an effect to the roughened surface for forming unevenness in the surface, etc. Although a tensile test is usually performed on evaluation of adhesive strength, a lot of hours are needed for experimental preparation. The surface free energy of an object surface is effective as one of the evaluation indices for evaluating the adhesive strength of a lamination plane. However, in order to find an experimental condition in research initial stages, such as material search and manufacturing conditions, it can become effective in respect of an hour and cost to use surface free energy as an evaluation index. In this paper, the effect to the surface free energy by atmospheric pressure plasma is investigated first. And the influence on the rupture strength improvement by atmospheric pressure plasma is investigated. Next, it is searched for the effective irradiation conditions of atmospheric pressure plasma. The influence of the atmospheric pressure plasma to the building parts of FDM was experimented based on surface free energy. The rupture test was done and improvement in rupture stress was confirmed by plasma irradiation. The result that the irradiation range and irradiation distance of an atmospheric-pressure-plasma head had large influence on the increase in surface free energy was obtained. It was observed that surface free energy increases about 75% to an initial sample by irradiation of atmospheric pressure plasma.
© by Elsevier Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016 The The Authors. Authors. Published Published by (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: Additive manufacturing; Fused deposition modeling; Atmospheric pressure plasma; Surface free energy; Interlaminar bonding strength
1. Introduction Additive Manufacturing is the technology of realizing solid parts, defining three-dimensional shape as lamination of a thin sheet, and joining the material of plastics or metal little by little. Unlike removal-processing technology, such as machining, there is the feature which can be produced easily also in complicated shape. Fused Modeling Deposition (FDM) builds parts by lamination with a plastic material. Since the adhesive strength between this lamination is low, the strength of parts is lower rather than the parts by injection molding process.
If the adhesive strength between laminations of this FDM parts can be improved, expansion of the application range, for example, functional verification, use for a product, etc., will spread. In this research, the hydrophilization treatment by atmospheric pressure plasma is used for improvement in the adhesive strength between laminations. The experimental result of a rupture test is reported about the influence of the hydrophilization treatment by atmospheric pressure plasma. Change of the surface free energy of the FDM parts before and after an atmospheric pressure plasma treatment is also reported.
2212-8271 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.314
755
Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
Nomenclature ୟ ߛ ߛ ߛ ɀ ɀୢୗ ɀୢ ୮
ɀୗ
୮
ɀ
ɀୌ ୗ ɀୌ ȣ ߪ F A
Adhesion work Surface free energy of the surface A [J/m2] Surface free energy of the surface B [J/m2] Surface free energy of an A-B interface [J/m2] Surface tension of a liquid Dispersion force component of the Van der Waals force of a solid Dispersion force component of the Van der Waals force of a liquid Interface-interaction force by the intermolecular force based on the polarity a solid (polar force component) Interface-interaction force by the intermolecular force based on the polarity of a liquid (polar force component) Hydrogen-bond interaction force of a solid (hydrogen bonding component) Hydrogen-bond interaction force of a liquid (hydrogen bonding component) Angle of contact Rupture stress [MPa] Measuring load [N] Cross-sectional area [mm2]
surface treatment, and it is said that various application can be performed [1-4]. In this research, atmospheric pressure plasma is generated by making it discharge within the glass tube which helium gas passes by low frequency wave length and a high voltage power source (10 kV, 10 kHz) (Fig. 2, Fig. 3). He Gas
Electrodes
Plasma plume
Earth
Fig.2 Atomospheric pressure plasma
Fig.3 Plasma plume 1.3. Evaluation of the strength of adhesive bonding by surface free energy
1.1. Fused Deposition Modeling There is a Fused Deposition Modeling (FDM) in one of the Additive Manufacturing (AM). As shown in Fig. 1, it is the method of building three-dimensional shape by the thermoplastic resin dissolved from the nozzle head being sent out, and deposition and solidification being repeated. It has the feature which can perform on-demand manufacture of the arbitrary shape. However, since the principle is based on lamination, one of weak points is that interlaminar bonding strength is low. In this research, the effect to improvement in the adhesive strength of lamination by the atmospheric pressure plasma treatment on the surface of lamination of the FDM part during building is investigated. Material
rotor Heater
Fig.1 FDM building process 1.2. Atmospheric-pressure plasma Although the plasma can usually occur only under vacuum conditions, the feature of atmospheric pressure plasma is that it can generate under an atmospheric pressure. The active species (a radical, an ion, an electron, etc.) which arise by plasma have cleaning effects, such as a soil removal of an organic substance, the rough surface generation effects, such as uneven shape formation, and the activation effects, such as
Adhesion work is defined by energy needed to pull apart the two surfaces which have touched by the interface as shown in Fig. 4.
A
J AB
A
Wa
B
JA JB
B
Fig.4 Adhesion work It is expressed with the equation (1) called the Dupre equation [5]. ୟ ൌ ߛ ߛ െ ߛ
(1)
Here, ߛ is the surface free energy [J/m2] of the surface A, and ߛ is the surface free energy [J/m2] of the surface B, ߛ expresses the surface free energy [J/m2] of the A-B interface, respectively. Calculating this adhesion work can evaluate the strength of adhesive bonding of lamination. An extended Fowkes equation is one of the methods of calculating solid surface free energy [5]. It can be denoted by an equation (2) that the applied force which one side's surface gives to another surface is a geometric mean value of each surface free energy [6]. ୟ ൌ ɀ ሺͳ Ʌሻ ൌ ʹ൫ɀୢୗ ή ɀୢ ൯ ʹ൫ɀୌ ୗ
ή
ଵΤଶ
୮
୮ ଵΤଶ
ʹ൫ɀୗ ή ɀ ൯
ଵΤଶ ɀୌ ൯
(2)
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Here, ɀ is surface tension which is a liquid, ɀୗ is solid surface tension, ɀୢ is a dispersion force component of Van der ୮ Waals force, ɀ is the interface-interaction force (polar force component) by the intermolecular force based on polarity, ɀୌ expresses hydrogen-bond interaction force (hydrogen bonding component). As shown in Fig. 5, ș is an angle of contact. An angle of contact is an angle between the horizon and the tangent of a droplet boundary zone. Tangent angle
Liquid
Fig.5 Contact angle Three or more kinds of liquid reagents are used for measurement of surface free energy. It is used for these reagents that three component values (a dispersion force component, a polar force component, hydrogen bonding component) of surface free energy have become clear beforehand. Those reagents are dropped at a test piece and an angle of contact is measured. And each-component value of solid surface free energy is obtained by solving it with a leastsquares method to an equation (2). ଵ Τଶ ଵ Τଶ ଵ Τଶ Now, each component ሺ൫ɀୢୗ ൯ ǡ ൫ɀ୮ୗ ൯ ǡ ൫ɀୌୗ ൯ ሻ of solid surface free energy presupposes that it is unknown. And since each component of the surface free energy of reagents ଵ Τଶ ଵ Τଶ ୮ ଵ Τଶ ሺ൫ɀୢ ൯ ǡ ൫ɀ ൯ ǡ ൫ɀୌ ሻ is known, it replaces each with ൯ ሺ ͳ݅ݔǡ ʹ݅ݔǡ ͵݅ݔሻ. It will become a primary equation (3) if a solid unknown component is placed with a, b, and c, respectively.
ሺǡ ǡ ሻ ൌ ȭ߳ଶ ൌ ȭሼݕ െ ሺܽݔଵ ܾݔଶ ܿݔଷ ሻሽଶ
Here, it is investigated by atmospheric pressure plasma how the surface energy of resin changes. In this experiment, an atmospheric-pressure-plasma head is fixed above a PLA resin, and resin is irradiated with atmospheric pressure plasma only the specified hour. Then, surface free energy is calculated by measuring the angle of contact of droplet with multiple reagents. Five kinds of liquids were used for the reagent. Each element of the surface free energy of each reagent is shown in Table 1. An experimental condition is shown in Table 2. Table 1 Surface free energy of reagents
Solid
Ԗ୧ ൌ ݕ െ ሺܽ ݔ ڄଵ ܾ ݔ ڄଶ ܿ ݔ ڄଷ ሻ
2.2. Measurement of Influence in Surface Free Energy by Atmospheric Pressure Plasma
(3) (4)
A residual error is obtained to the measurement experiment of each angle of contact. It solves with a least-squares method so that S of the residual error sum (4) of all the experiments may become the minimum, and a, b, and c are obtained. 2. Effect of Atmospheric Pressure Plasma to Lamination Interfaces of Fused Deposition Modeling Parts 2.1. Objective First, the change of surface free energy by irradiation of atmospheric pressure plasma is investigated at Section 2.2. Next, the effect of the atmospheric pressure plasma treatment at the time of a part building is investigated, and the effect to rupture strength is observed at Section 2.4.
ࢽࢊ 30.1 37.4 29.1 46.8 44.4
Ethylene glycol Glycerin Water Diiodomethane 1-Bromonaphthalene
ࢽ 0 0.2 1.3 4 0.2
ࢽࡴ 17.6 25.8 42.4 0 0
ࢽࡸ 47.7 63.4 72.8 0 0
Table 2 Experimental conditions Glass tube inside diameter [mm] Glass tube outside diameter [mm] Plasma irradiation distance [mm] Helium gas flow rate [l/min] Helium gas pressure [MPa]
2 4 10 0.3 0.2
2.3. Experimental Result of Atmospheric Pressure Plasma Irradiation Change of surface free energy when a PLA resin is irradiated with atmospheric pressure plasma for 0 to 30 seconds is shown in Fig. 6. From Fig. 6, it was observed that the surface free energy on the surface of building parts was increasing by the atmospheric pressure plasma irradiation. Surface free energy increased to about 8% in the processing time for 5 seconds, and increased about 20% in 20 seconds. As shown in Fig. 6, the rate of the hydrogen-bond term in surface free energy increased greatly by plasma treatment. Surface free energy [mJ/݉^2]
756
80 70 60 50 40 30 20 10 0
Hydrogen
0
polar
dispersion
5 10 20 Irradiation time [s]
30
Fig.6 Surface free energy by atmospheric pressure plasma treatment 2.4. Rupture test In order to investigate the effect by an atmospheric pressure plasma treatment, the test piece for rupture tests is produced.
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FDM equipment (RepRap Mendel Plusa) was used for preparation of a test piece. The plasma irradiation head was attached to the FDM equipment. Resin lamination and plasma treatment are performed one after the other at the time of the building of a test piece. As shown in Fig. 7, a building head and a plasma head are placed on the distant position, and are being fixed to the moving mechanism. A top face is irradiated with a plasma plume whenever a lamination plane is built. Although two test pieces are built simultaneously, it irradiates with an atmospheric pressure plasma treatment only to one piece. That is, plasma treatment is performed only to 1 sample within the two samples. In this experiment, the helium gas flow rate at the time of a plasma generation is changed by using regulator with a flow meter.
2.5. Rupture test result The rupture stress of each test piece by a rupture test is shown in Fig. 9. As shown in Fig. 9, rupture stress became higher when irradiation speed was high. Furthermore, by performing an atmospheric pressure plasma treatment, fracture strength further improved. The rupture stress in the case of the atmospheric pressure plasma irradiation in 1000 mm/min and 1400 mm/min both increased from the case of not irradiating. 50
Rupture stress ı[MPa]
Atmospheric-pressure plasma head
In a rupture test, the maximum load at the time of a rupture and the area of a fracture surface are used. Image-analysis software is used for measurement of the cross-sectional area.
Head Motor Heater Test piece
40
without treatment plasma treatment
30 20 10 0 1000
Fig.7 Experimental setups of plasma The shape of the test piece of a rupture test is shown in Fig. 8. Experimental conditions are shown in Table 3. A test piece performs a rupture test using the force gauge attached to the electric stand, respectively.
Fig.8 Sample shape Table 3 Experimental conditions Glass tube inside diameter [mm] Glass tube outside diameter [mm] Plasma irradiation distance [mm] Helium gas flow rate [l/min] Helium gas pressure [MPa] Plasma irradiation speed [mm/min]
2 4 15 0.5 0.22 1000, 1400
Rupture stress ı [MPa] can be calculated from an equation (5). ߪ ൌ ܨȀܣ
(5)
Here, F is the load for measurement [N] and A is the crosssectional area [mm2]. In a rupture test, the maximum load at the time of a rupture and the cross-sectional area of a fracture surface are used. Image-analysis software is used for measurement of the cross-sectional area. F is the load for measurement [N] and A is the cross-sectional area [mm2] here.
1400
Building Speed[mm/min] Fig.9 Rupture stress by atmospheric pressure plasma treatment 2.6. Discussion From the experimental result of Section 2.3, it was observed that surface free energy increases by the atmospheric pressure plasma irradiation to a PLA resin. As a reason for this increase, the cleaning effect by plasma, generation of a hydroxyl group, etc. can be thought of. About a detailed mechanism, the further investigation is needed. When irradiation speed became high, the result to which rupture stress became high was obtained from the experimental result of Section 2.5. As this reason, if a scattering rate becomes high, a building hour will become short. On the other hand, if the building parts fabricated by heating resin are high-speed buildings, since cooling does not progress, the temperature of parts is still high. This elevated temperature is considered to have influenced adhesive strength. About this hypothesis, still more detailed investigation is needed. Anyway, the effect of the atmospheric pressure plasma irradiation was obtained to rupture strength. The surface free energy on the building conditions of lamination was not able to be measured without the ability to drop a reagent at a sample surface. It is necessary to investigate relevance with rupture strength in detail by considering a specimen configuration etc. Since the effect of atmospheric pressure plasma is influenced by irradiation conditions, it becomes very important from these results to find the effective irradiation conditions of atmospheric pressure plasma. In Chapter 3,
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Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
examination about the effective irradiation conditions of atmospheric pressure plasma is performed. 3. Investigation of Influence of Atmospheric Pressure Plasma Based on Surface Free Energy
Fig. 10 shows a test piece. The irradiation area of atmospheric pressure plasma is shown in Fig. 11. Measurement of surface free energy was performed in the position of area-a. PLA resin was used for the test piece and traveling speed was set to 660 [mm/min].
3.1. Objective
3.2. Search of Atmospheric-Pressure-Plasma-Irradiation Conditions by Orthogonal Array Experiment The orthogonal array experiment was conducted and investigation of the factor which has large influence on an atmospheric pressure plasma treatment, and improvement in irradiation conditions were investigated. In the experiment 1, a building conditions is changed and surface free energy is measured. Table 4 Experimental conditions 1 Level
1 2 3
Stage temperature [A]
40Υ 50Υ 60Υ
Sample size[B]
20x10mm 30x10mm 40x10mm
Irradiation area [C]
Measurement Interval time[D]
a ab abc
0h 24h 48h
Fig.10 Specimen
The experimental result is shown in Fig. 12. The influence of the surface free energy on each conditions was investigated. The result which the irradiation range of [C] affected most was obtained. When the stage temperature of [A] changed to 50 [ć] from 40 [ć], the effect of atmospheric pressure plasma decreased by about 30%. When the measurement interval of [D] changed to 48 [h] from 0 [h], the effect of atmospheric pressure plasma decreased to about 26%.
Table 5 Experimental No. based on orthogonal array No. No treatment 1 2 3 4 5 6 7 8 9
A 40Υ 40Υ 40Υ 40Υ 50Υ 50Υ 50Υ 60Υ 60Υ 60Υ
B 20x10mm 20x10mm 30x10mm 40x10mm 20x10mm 30x10mm 40x10mm 20x10mm 30x10mm 40x10mm
C a ab abc ab abc a abc a ab
D 0h 24h 48h 48h 0h 24h 24h 48h 0h
Fig.11 Irradiation area
3.3. Experimental Result 1
Surface free energy [mJ/m²]
Although a tensile test is usually performed on evaluation of adhesive strength, a lot of hours are needed for experimental preparation. The surface free energy of an object surface is effective as one of the indices for evaluating the adhesive strength of the surface of material. Especially, if surface free energy is used as an evaluation index in order to find an experimental condition, it can become effective in respect of an hour and cost in research initial stages, such as material search and manufacturing conditions. The objective of this experiment is to search for the effective irradiation conditions of atmospheric pressure plasma.
64
to maximize
59 54 49
to minimize
0 (untreated) 44 40 50 60 [A]
20 30 40 [B]
a ab abc [C]
0 24 48 [D]
Fig.12 Experimental results From these results, two combination conditions, set1 and set2, shown in Table 6 were set up for the next experiment. Table 6 Combination conditions Stage
irradiation
Measurement
temperature [A]
area [C]
Interval time[D]
Set1
40Υ
abc
0h
Set2
50Υ
a
48h
3.4. Experiment2 As shown in Table 4, the conditions of each factor are assigned to an orthogonal array, and it experiments in nine kinds of conditions combination (Table5). Then, it totals for every level of factor, and calculates an average value. As for these nine experiments, the same number of each level of factor is included. If the effect of each factor assumes the experiment model which is combined linearly and each levelof-factor average will be calculated, only the effect of a factor to investigate can be obtained.
In the experiment 2, the plasma generation conditions was given based on the result in the experiment 1. Surface free energy was measured on eight conditions shown in Table 7. A helium gas flow rate [X], atmospheric-pressure-plasmairradiation distance [Y], and combination 2 conditions in the experiment 1 were used.
Hiroyuki Narahara et al. / Procedia CIRP 42 (2016) 754 – 759
Table 7 Condition of Experiment2 No. 1 2 3 4
Flow rate of He gas [X] 0.5[L/min] 0.7[L/min] 0.5[L/min] 0.7[L/min]
Irradiation distance [Y] 10mm 10mm 5mm 5mm
Set1 Set1 Set1 Set1
5 6 7 8
0.5[L/min] 0.7[L/min] 0.5[L/min] 0.7[L/min]
10mm 10mm 5mm 5mm
Set2 Set2 Set2 Set2
Condition Set
4. Conclusion
3.5. Experimental Result 2 The experimental result in eight conditions shown in Table 7 is shown in Fig. 13. From Fig. 13, two conditions shown in Table 6 had the tendency for a result to be the same. On all the conditions, the conditions of Table 7-[Set1] was observed that the effect of atmospheric pressure plasma was higher than the conditions of Table 7-[Set2] about 5% on the average. It was observed that the effect of atmospheric pressure plasma increases notably by shortening the irradiation distance of [Y]. On the conditions of Table7-[No.3] , the effect of atmospheric pressure plasma became the maximum. By irradiating with atmospheric-pressure plasma, the result which surface free energy increases from no processing by about 75% was obtained. 90
hydrogen bond
80
polarity
dispersion force
70 Surface free energy [
composition are deeply related. Since surface free energy is evaluating only the ideal adhesive strength of materials, it does not guarantee 100% of the joining strength of real parts. Nevertheless, it seems that the evaluation by surface free energy is effective, and predominant. At AM research, it is because material development and development of a mechanism are needed and it can investigate the ideal condition of a system simple even in a development initial stage. Being used by development will be desirable, using together with a destructive test.
60 50 40 30 20 10
For improvement in the adhesive strength of the parts in a Fused Deposition Modeling, this research has proposed the surface treatment method by atmospheric pressure plasma. Moreover, examination was promoted by measuring the surface free energy of a FDM parts about the process of searching for the conditions which give an atmospheric pressure plasma treatment effectively. The influence of atmospheric-pressure-plasma-irradiation conditions was investigated, and the following results were obtained. 1. The improvement result of rupture strength was obtained by the rupture test. 2. The conditions of the irradiation range and irradiation distance at the time of an atmospheric pressure plasma irradiation gave the large effect to the increase in surface free energy. 3. Surface free energy increased about 75% from unprocessed condition by adjusting the irradiation conditions of atmospheric pressure plasma. 4. It was suggested that an atmospheric pressure plasma irradiation is an effective means to improvement in the joining strength of the parts of a Fused Deposition Modeling. Acknowledgements
0 1
2
3 4 5 Specimen nunber
6
7
8
Fig.13 Surface free energy of experiment2 3.6. Discussion It was searched for the factor and level which make surface free energy high by the orthogonal array experiment about the irradiation conditions parameter of atmospheric pressure plasma. It is necessary to perform part manufacture and a rupture test and to advance investigation further about the conditions that the effect of an atmospheric pressure plasma irradiation is high. 4. Discussion About the bond strength of material, not only the adhesive strength of materials but a defect, a crack, etc. of a plane of
A part of this research was done by grants-in-aid for scientific research (KIBAN (C) 24560137). References [1] Bárdos L, Baránková H. Cold atmospheric plasma: Sources, processes, and applications. Thin Solid Films. 2010;518(23):6705-13. [2] Chang JS. Physics and Chemistry of Atmospheric Plasmas. Journal of Plasma and Fusion Research. 2006;82(10):682-92. [3] Fukuda K, Kondo Y, Oishi K, Toda Y. The Development of Adhesion Technology Using Atmospheric Pressure Glow Plasma Processing. Konica Minolta technology report. 2004;1:35-8. [4] Tendero C, Tixier C, Tristant P, Desmaison J, Leprince P. Atmospheric pressure plasmas: A review. Spectrochimica Acta Part B: Atomic Spectroscopy. 2006;61(1):2-30. [5] Mitou M. Chap.5 The surface chemistry of adhesion. Advanced adhesive joint technology: NGT; 2000. [6] Nakajima A. Wetting property control of a solid surface: Uchida Rokakuho Publishing Co., Ltd.; 2007.
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