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Improvement in Surface Quality of Microchannel Structures Fabricated by Revolving Tip-Based Machining

  • Bo Xue
  • Yanquan Geng
  • Dong Wang
  • Yazhou Sun
  • Yongda YanEmail author
Original Articles
  • 95 Downloads

Abstract

In the present paper, the process parameters of revolving tip-based machining were optimized for the fabrication of microchannel structures. It was found that in comparison with the micromilling process, the main factor affecting the surface quality of revolving tip-based machining originated from residual materials produced in each revolution. Three process parameters, including cutting depth, feeding rate, and tool path strategy, were studied experimentally to optimize the surface quality of the machined aluminum alloy and polymethylmethacrylate (PMMA). It was noticed that at smaller cutting depths (< 3 µm) and feeding rates (< 20 µm/s) with fixed revolving parameters (50 Hz frequency and 6 µm radius), microchannels with better bottom surfaces were formed. Two different types of tool path strategies were designed and compared to obtain the best surface quality (Sa) of aluminum alloy (21 nm) and PMMA (19 nm).

Keywords

Revolving tip-based machining Microchannel structures Process parameter optimization Surface roughness 

1 Introduction

Microfluidic chip is the main building block of micrototal analysis system (µTAS), which performs the traditional analysis of biology and chemistry on a small chip with dimensions of few centimeters or less (called as lab on a chip) [1]; therefore, the incorporation of microfabrication techniques is essential during the manufacture of microfluidic chips [2]. Micromechanical machining has attracted significant scientific attention due to its ability to fabricate a wide range of materials with three-dimensional features [3]. Micromilling is proved to be the best way to fabricate complex-shaped microfluidic chips at a low cost [4]; however, with the increasing integration of microfluidic chips, micromilling technique has started to manifest its limitations for machining smaller channels with widths of several micrometers. Hence, some new methods based on the milling process have been developed in recent years [5, 6, 7].

Gozen et al. [5] presented a tip-based nanomilling method: An atomic force microscopy (AFM) tip was directly actuated by a piezoelectric actuator in order to machine microchannels with widths of several micrometers. Similarly, Park et al. [6] used a three-axis piezoelectric stage to actuate the workpiece and, simultaneously, made scratches on the workpiece surface using an AFM tip. Heamawtanachai et al. [7] developed a cutting module using a piezoelectric tube scanner to drive a microconical tip and, consequently, mounted the module on a computerized numerical control (CNC) machine tool to fabricate microchannels with widths of hundreds of micrometers. In order to fabricate microchannels with smooth sides, our group [8, 9] developed a revolving tip-based machining method to revolve a three-sided pyramidal tip horizontally by a multi-axis piezo-stage.

Surface quality of microchannel sidewalls is an important factor for a microfluidic chip because it affects the optical transparency of the chip [10], as well as undermines the viscosity of fluid inside it [11]. Huang et al. [12] proposed to heat the polymethylmethacrylate (PMMA) substrate in a water pool during laser drilling and noticed an improvement in surface roughness of chamber sidewalls. Chen et al. [13] experimentally investigated the effects of micromilling parameters on surface roughness of PMMA substrate. Yousuff et al. [14] optimized the milling parameters to obtain the best surface roughness of aluminum alloy. Ogilvie et al. [10] developed a solvent-vapor post-processing method to effectively reduce the surface roughness of polymer chips machined by micromilling.

When a novel fabricating method is developed, it is very helpful to give other researchers the reference about level of surface roughness that can be obtained by this method. For example, Lin et al. [15] proposed to fabricate the metal mold for microfluidic chip by using the 3D printing technique and paid more attention to analyze the surface quality obtained by this method. Therefore, in the current paper, for improving revolving tip-based machining, the process parameters of revolving tip-based machining were optimized to evaluate the surface roughness of aluminum alloy and PMMA. Based on the characteristics of revolving tip-based machining, the influence factors on surface quality were analyzed and used to guide the design of following process experiments.

2 Experimental Details and Methods

The proposed revolving tip-based machining system is displayed in Fig. 1. Three precision positioning stages (X–Y–Z) were used to locate the tip (Z) and the workpiece (X–Y). The core part of the system was formed by a three-axis piezo-stage and a three-sided pyramidal tip (enlarged view of Fig. 1). The piezo-stage caused the necessary trajectory movements in the space and controlled the machining depth. The tip was a commercial three-sided pyramidal nanoindenter, which had three sharp edges with radii of < 40 nm [8, 9]. The workpiece was machined under ambient conditions without any cutting fluid and compressed air.
Fig. 1

Revolving tip-based machining system

Figure 2 illustrates the flowchart of control signals and acquired signals of the machining system. The digital signals of trajectory movements were converted into analog signals by a data acquisition card and then amplified by a controller to drive the piezo-stage. During the movement of piezo-stage, strain gauges measured the actual displacement of each axis and generated the corresponding feedback signal. The force transducer with six components of force/torque was used to sense the three-dimensional forces and send their signals to the computer. The positioning stages were controlled by a motion control card, which was inserted into the motherboard of the computer.
Fig. 2

Flowchart of control signals during the machining process

Owing to the independence of each axis of the piezo-translation stage, the movement of the tip was achieved by the combination of the motions of any two axes. Figure 3a shows the horizontally revolving condition designed for the machining of microchannel structures. It is observable that z-axis was only used to control the machining depth, whereas x- and y-axes yielded the outputs of phase sine harmonic vibrations. Therefore, by changing the amplitudes of x- and y-axes (A1 and A2, respectively) separately, different trajectories of various shapes could be obtained. In our study, circular trajectory was only employed to machine the desired microchannel (A1 = A2).
Fig. 3

Comparison between the conventional milling process and revolving tip-based machining process

During conventional milling process, actuated by a high-speed rotating spindle, the cutting edge(s) of the mill can obtain a certain cutting speed, which depends on the product of rotational speed and mill radius (Fig. 3b). However, in order to machine smaller structures, such as microchannels, the radius of the mill should be reduced, and it causes new problems, such as severe tool damage and high equipment cost. In revolving tip-based machining, the mill is replaced with a tiny tip, and its rotating motion is converted into revolving motion. Although revolving tip-based machining is similar to single cutting-edge milling process (Fig. 3c), some pronounced differences exist between them: (1) The tip has sharpened cutting edges and possesses higher strength, (2) the widths of the machined channels are decided by the adjustable trajectory dimension, and (3) the sidewalls of the channels machined by revolving tip-based machining are tilt rather than vertical.

3 Influences of Revolving Tip-Based Machining Parameters on Surface Roughness

3.1 Cutting Depth

In our experiment, microchannels with flat bottoms and tilted sidewalls were machined by a two-dimensional horizontal revolving trajectory. It is well known that machining trajectories would be different along different directions, thus resulting in different topographies of microchannels [8]. The selected machining trajectory for the present work is illustrated in Fig. 4a, which was due to its ability to not only machine the channel with both two smooth sides but also machine the channel with the symmetry shape.
Fig. 4

Microchannels machined under different cutting depths: a machining trajectory, b 2 μm, c 3 μm, d 4 μm, e 5 μm, f 6 μm

Cutting depth is an important parameter for revolving machining. Due to the tapered shape of the tip, larger triangles were obtained as shown in Fig. 4a for deeper cutting depths, thus resulting in different cutting lengths for different cutting edges and, consequently, weakening the effects of revolving trajectory. Figure 4d–f displays the SEM images of microchannels machined under different cutting depths. With the increasing cutting depths, the shape of the revolving trajectory changed significantly; hence, under larger cutting depths, the piezo-stage could not provide enough stiffness against cutting resistance. Moreover, it is noticeable that two sides of the channels in Fig. 4b–d exhibit different topographies, whereas in Fig. 4e–f, two sides are nearly the same, it signifies that when the cutting depth reaches up to a certain amount, the scratching effect of tip producing the side burrs difficult to be removed overwhelms the revolving machining process.

3.2 Feeding Rate

The cross-sectional area along the vertical direction of the tip was not consistent. Moreover, the uncut chip thickness of revolving tip-based machining increased to the maximum and then decreased to zero. It is evident from Fig. 5a, b that at height h2, the triangle was large enough to cover the whole uncut chip thickness, whereas at height h1 the triangle was not large enough, and thus a part of materials remained unmachined. Figure 5c presents the schematic diagram of unmachined materials formed near the tip apex. It is already reported that in revolving tip-based machining, residual materials undermine the bottom surface quality of microchannels machined from aluminum alloy [9]. The amount of residual material depends on the value of uncut chip thickness, (which is calculated from feeding rate and revolving frequency), thus for smaller uncut chip thicknesses, less residual materials would be left. However, if the revolving frequency is fixed, a smaller feeding rate causes lower machining efficiency. Therefore, it is significantly worthy improving the efficiency as much as possible while ensuring the machined surface quality.
Fig. 5

Formation of residual materials during tip-based machining

3.3 Workpiece Material

In the current study, bulk materials of aluminum alloy and PMMA were chosen. Aluminum alloy (2024) can be used as microfluidic chip mold due to its excellent ductility and high strength [14, 16]. PMMA is a very popular material for microfluidic chips due to its transparency and superb insulation and corrosion resistance properties [12, 13]. However, as a polymer material, except for high elasticity and viscoelasticity and other temperature-dependent properties, PMMA manifests some brittleness, which increases the difficulty of fabrication [17].

Figure 6a, b displays the channels machined by four different types of trajectories on aluminum alloy and PMMA surfaces, respectively. The channels machined on two surfaces by the same trajectory were marked with the same number. It can be found that more burrs were formed in aluminum alloy channel, whereas no pronounced plastic burr was produced in PMMA (after ultrasonic washing); however, most of the PMMA channels became cracked at the sides. Furthermore, the topographies of bottom surfaces also manifested a big difference: periodic tool masks appeared in aluminum alloy, whereas no regular topography was found in PMMA.
Fig. 6

Microchannels machined on aluminum alloy and PMMA surfaces

3.4 Tool Path Strategy

Two different process routes for the fabrication of microchannels are presented in Fig. 7a, b. It is discernible from the SEM images in Fig. 4 that left sides of the channels were relatively smooth, whereas more burrs were formed on right sides. It is posited that right burrs could be removed by an additional lateral feed while revolving the tip reversely [18]. Therefore, in order to machine a surface with two smooth sides, the revolving tip should be moved to the right, and after finishing the surface, the deburring process should be performed for the right side. In Mode 1 (Fig. 7a), after finishing one channel (from positions 1 to 2), the revolving tip moves laterally with an amount of lateral feed (LF) and then machines the next neighboring channel (from 3 to 4), and thus a plane surface is produced by repeating this process. It can be found that the neighboring channels are machined by different machining trajectories: face-forward revolving and edge-forward revolving. In Mode 2 (Fig. 7b), the revolving tip backtracks first without any lateral feed after going forward and then moves laterally to machine the next channel. In this case, each channel is machined by the same way, and the returned feed might have the trimming effect on bottom surface quality.
Fig. 7

Two different tool path strategies: a Mode 1 and b Mode 2

Due to the characteristics of changing cutting angles and multiple cutting edges participating in one machining period of revolving tip-based machining, lateral feed plays a major role during the machining of plane surfaces (Fig. 8). When the size of lateral feed is set as condition 1, single cutting edge first cuts the workpiece with a large negative rake angle and then other cutting edges join the machining process. If the condition 2/3/4 happens, two cutting edges together cut the workpiece: One has an extremely negative rake angle and the other has a positive one. However, in conditions 5 and 6, channels are machined by a single cutting edge with a positive rake angle. Therefore, as discussed above, the different machining processes caused by the lateral feed would inevitably affect the bottom surface quality.
Fig. 8

Effects of lateral feed on surface quality in revolving tip-based machining

4 Results and Discussion

4.1 Surface Quality of Aluminum Alloy

Single-channel microstructure was first machined under 50 Hz revolving frequency and 6 µm revolving radius, and then the bottom surface was scanned in AFM tapping mode to calculate the 3D surface texture parameters, including arithmetic mean height deviation of the surface (Sa) and root-mean-square height deviation of the surface (Sq) [19]. AFM is often employed to characterize the surface topography due to its advantages of high resolution and 3D imaging; however, tip radius is convolved into scanned results during this method, thus feedback control signals, sampling interval, and tip radius become the main sources of uncertainty for AFM surface characterization. It was found that due to the averaging effect of calculation of Sa and Sq, tip radius had little effects on surface roughness [20]. In addition, AFM feedback parameters were adjusted to obtain the optimal tip tracing state with a low tapping force [21]. Hence, in this study, sampling interval was considered as the main influencing factor for surface roughness. The sampling interval was chosen in the range of 0.2–0.3 µm, which should be acceptable for a feature period larger than 2 µm [22].

Owing to the ductile property of aluminum alloy, residual materials significantly affected the surface qualities of the fabricated microchannels; hence, feeding rate and cutting depth were optimized as the main process parameters, and the corresponding results are plotted in Fig. 9a. It can be noticed that when cutting depth was less than 4 µm, the values of Sa increased sharply with the increasing feeding rates and then varied gently after reaching a certain amount of feeding rate. In the range of 20–80 µm/s, larger feeding rates resulted in more unmachined materials in one revolution, and consequently, increased the value of Sa. However, after 80 µm/s, with the increasing feeding rates, machining trajectory was converted into a spiral line, and the volume of residual materials in each revolution became constant. It was also found that smaller cutting depths caused a better surface quality, and the best result was obtained under the optimal condition of 20 µm/s feeding rate with a cutting depth of 1.6 µm. However, for a larger cutting depth (4 µm), machining process behaved like the direct scratching process, and thus feeding rate manifested little effects on surface roughness.
Fig. 9

Influences of a feeding rate and cutting depth, b different lateral feeds on surface roughness

Plane surfaces were machined in both Modes 1 and 2 using the optimized parameters (20 µm/s feeding rate and 1.6 µm cutting depth). It is evident from Fig. 9b that larger lateral feeds caused a worse surface quality in both modes; however, Mode 2 yielded a better surface quality (Sa = 21 nm and Sq = 34 nm). In addition, a sharp increase in surface roughness curves in Mode 1 was observed after 6 µm lateral feed (Fig. 8); thus, it signifies that the machining of the next channel would be started by the middle part of the machining trajectory in Fig. 8 (2/3/4), which is a two-edge cutting process with the largest uncut chip thickness. However, the sharp increase in surface roughness did not occur in Mode 2, and it indicated that returned feed effectively removed residual materials from the bottom surface of each channel. When the lateral feed was less than 6 µm (Fig. 8 (5/6)), smaller uncut chip thickness was required to begin the machining of the next channel; hence, the volume of residual materials was reduced.

Figure 10 displays the topographies and the cross sections of three surfaces machined under different lateral feeds in Mode 2. Under the same height scale of AFM images, it can be noticed that with the increasing lateral feeds, ridges started to form and undermined the surface quality, whereas other parts of the machined surfaces remained flat and had nearly the same color distribution meaning the same height range. It is detectable that ridges originated from the residual materials, which were produced in the middle of the channel bottom during the first feed and then pushed to the side by the tip during the returned feed.
Fig. 10

Surface topographies for aluminum alloy material for different lateral feeds: a 4 µm: Sa = 24 nm, Sq = 31 nm. b 8 µm: Sa = 52 nm, Sq = 77 nm. c 12 µm: Sa = 155 nm, Sq= 234 nm

4.2 Surface Quality of PMMA

As the bottom surface of PMMA manifested an irregular topography, residual materials were not the main influencing factor for surface quality; hence, the factor analysis of orthogonal experiment for channel machining experiment was performed in the case of PMMA in order to study the influences of each process parameter, as well as to obtain the optimal parameter combination. Three factors including revolving frequency, feeding rate, and cutting depth were selected with four levels for our analysis (Table 1).
Table 1

Four levels of three factors for channel machining experiment

Level

Revolving frequency (Hz)

Feeding rate (μm/s)

Cutting depth (μm)

A

B

C

1

50

5

1.6

2

75

10

2.4

3

100

15

3.2

4

125

20

4

Table 2 depicts the results of 16 channels machined under different process parameters. The mean values of Sa and Sq of each factor under each level were calculated and plotted (Fig. 11). It can be noticed that the increasing values of these three factors degraded the surface quality of microchannels. Revolving frequency played the biggest role in surface quality due to the decline in piezo-stage stiffness. Although higher revolving frequencies yielded improved machining efficiencies, machining trajectory became uncertain due to weaker stiffness. In the range of 50–75 Hz, the increase in surface roughness was much slower than that in higher ranges. Furthermore, similar with the condition occurring in aluminum alloy, no sharp increase in surface roughness was observed after a certain value of cutting depth. Moreover, smaller feeding rates manifested little effects on surface quality; hence, it is recommended to choose larger feeding rates in order to improve machining efficiency. Therefore, the machining of PMMA microchannels was carried out under the optimal condition of 50 Hz revolving frequency, 1.6 µm cutting depth, and 20 µm/s feeding rate.
Table 2

Experimental results of various levels for revolving frequency, feeding rate, and cutting depth

No.

A

(Hz)

B

(μm/s)

C

(μm)

S a

(nm)

S q

(nm)

1

50 (1)

5 (1)

1.6 (1)

23

33

2

50

10 (2)

2.4 (2)

32

44

3

50

15 (3)

3.2 (3)

41

56

4

50

20 (4)

4 (4)

48

63

5

75 (2)

5

2.4

38

59

6

75

10

1.6

24

35

7

75

15

4

53

76

8

75

20

3.2

50

71

9

100 (3)

5

3.2

60

102

10

100

10

4

60

101

11

100

15

1.6

52

68

12

100

20

2.4

53

69

13

125 (4)

5

4

108

144

14

125

10

3.2

117

157

15

125

15

2.4

107

150

16

125

20

1

90

125

Fig. 11

Influences of revolving frequency, feeding rate, and cutting depth on surface roughness

With the optimal process condition, the plane surface machining experiment for PMMA was conducted at different lateral feeds. Similar to aluminum alloy, larger lateral feeds resulted in worse surface qualities in both modes. The best result (Sa = 19 nm and Sq = 24 nm) was obtained in Mode 2 for a lateral feed of 2 μm. However, the surface topographies of PMMA differed from those of aluminum alloy. It is noticeable from Fig. 12a, b that with the increasing lateral feeds in Mode 2, no ridges were formed on the PMMA surface; however, sine-like wave structures whose period was related to the feed improved the surface roughness of PMMA. Moreover, residual materials on PMMA surface were mostly removed and thus did not move with the revolving tip during the returned feed. Furthermore, without the returned feed in Mode 1, the surface quality of PMMA microchannels became worse; however, the profile along the lateral direction did not present any periodic feature (Fig. 12b, c).
Fig. 12

Surface topographies of PMMA for different process parameters

5 Conclusions

Revolving tip-based machining is a viable process for the fabrication of microchannel structures with dimensions of several micrometers to hundreds of micrometers. In the afore-discussed paper, the process parameters of revolving tip-based machining were optimized to evaluate the surface roughness of aluminum alloy and PMMA. The obtained results are depicted below:
  1. 1.

    Although the properties of these two materials are different, the influences of process parameters were similar. With a fixed revolving radius, large cutting depths weakened the effects of revolving trajectory and caused a decline in surface quality. Larger feeding rates produced more residual materials in each revolution and undermined the surface quality.

     
  2. 2.

    During the machining of a single channel with a returned feed, significant improvements in bottom surface quality were noticed. Moreover, during the machining of a plane surface, smaller lateral feeds between two neighboring channels yielded better surface quality.

     
  3. 3.

    The optimal surface roughness (Sa = 21 nm and Sq = 34 nm) for aluminum alloy was obtained under a feeding rate of 20 µm/s, a cutting depth of 1.6 µm, and a lateral feed of 2 μm. With the same process parameters, PMMA manifested a better surface quality (Sa = 19 nm and Sq = 24 nm).

     

Notes

Acknowledgements

The authors gratefully acknowledge the financial supports of the Foundation for the National Natural Science Foundation of China (51475108), Innovative Research Groups of the National Natural Science Foundation of China (51521003), Self-Planned Task (SKLRS201606B) of State Key Laboratory of Robotics and System (HIT), and the National Program for Support of Top-notch Young Professors.

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Copyright information

© International Society for Nanomanufacturing and Tianjin University and Springer Nature 2018

Authors and Affiliations

  1. 1.Key Laboratory of Micro-Systems and Micro-Structures Manufacturing of Ministry of EducationHarbin Institute of TechnologyHarbinPeople’s Republic of China
  2. 2.Center for Precision EngineeringHarbin Institute of TechnologyHarbinPeople’s Republic of China
  3. 3.Harbin Institute of TechnologyHarbinPeople’s Republic of China

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