Micro-milling is a common processing method for fabricating microfluidic chips or other micro products with high processing accuracy and low cost, suitable for mass production. The main concern of micro-milling is the surface roughness of the machined surface. However, the general study of the surface roughness of micro-milling can only find that only a small range of surface roughness can be obtained by changing the processing parameters. It is very difficult to obtain a specific roughness. In the process of micro-milling with end mills, due to the structural characteristics of the tool tip, the inclination angle of the tool will have a significant impact on the bottom surface of the machined channels. In this work, the influence of tool inclination on the surface roughness of machining was studied through the machining tests of inclined micro-milling on a poly(methyl methacrylate) (PMMA) surface, and it was proposed to realize the control of the machined surface roughness by inclined micro-milling. In addition, a theoretical model considering tool inclination was established to calculate the surface roughness of the machined bottom obtained by inclined micro-milling. The experimental results are consistent with the theoretical model results in the lower speed range. Afterwards, the polydimethylsiloxane (PDMS) is used to replicate the microchannel machined on the PMMA surface, and the microfluidic chips were prepared to control the fluid flow in the channel by adjusting the roughness of the bottom of the channel. Results show that the smoother channel will flow first under the same flow pressure. The study offers a new idea of surface roughness control, which can be applied to flow control in microfluidic chips.
Research Article
Effect of the inclined angle of micro-milling tool on the fabrication of the microfluidic channel
https://doi.org/10.21203/rs.3.rs-2373409/v1
This work is licensed under a CC BY 4.0 License
Journal Publication
published 30 Jan, 2023
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Micro-milling
roughness control
microfluidics
Microchannels are important part of microfluidic devices used in biology, chemistry, medicine and so on [1–3]. In general, because of the low price, commercial microfluidic devices used for bioanalysis are produced with biocompatible polymers employing molding processes such as injection molding [4, 5] and hot embossing [6]. However, the manufacturing cost of molds used in these processes is high and the process is complex. In most cases, especially in the prototype design stage, direct processing of polymer substrates may be more economical and suitable for preparation of microchannels. Laser ablation and micro-milling are typical direct processing processes [7–10], which can be used for rapid prototyping of microfluidic chips. Both methods can be used to process a variety of materials, including polymer materials, such as polymethylmethacrylate (PMMA)[11], polycarbonate (PC)[12], polyethylene terephthalate (PET)[13] and cyclic olefin copolymer (COC)[14], which are generally used in microfluidic chips. However, in the process of laser ablation, it is very common for debris to redeposit on the sidewall and floor. Therefore, due to the shape of the laser Gaussian beam, it is difficult to achieve high dimensional accuracy[15]. On the contrary, micro-milling can be used to manufacture three-dimensional structures with high dimensional precision. Due to the progress of tool manufacturing technology, micro-milling can be used to accurately manufacture microchannels and microstructure [16, 17]. There are more and more scholars using micro-milling technique to fabricate microchannel on polymer materials for the preparation of microfluidic chips.
Many machining parameters, such as tool geometry parameters, spindle speed, cutting depth and feed per tooth, have been proved to be closely related to machining performance. In the micro-milling process, some scholars have found that the tool inclination has an important influence on the quality of the machined surface [18]. Choosing the appropriate tool inclination can not only improve the machining quality and reduce the tool wear, but also control the machined surface roughness more conveniently by controlling the tool inclination in the micro-milling process. In addition, it has been found that the roughness of channel will affect the wettability of liquid in the channel [19], thus affecting the flow resistance, and then affecting the circulation of liquid. The flow condition of the reagent inside the microchannels have a large influence on the reaction speed and sequence in the chip, and the working efficiency and function of the microfluidic chips can be effectively improved by regulating the flow state of reagent. Using the inclined micro-milling process, the roughness of the bottom of the processed channel can be simply controlled, and then the reagent flow sequence in the microchannel can be regulated.
In this paper, the influence of inclined micro-milling on the roughness of groove bottom is studied by theoretical model and experimental tests. The effect of roughness on the wettability of the reagent is also studied. A microfluidic chip with microchannels of different roughness is prepared to control the flow sequence.
The simulation of structures formed on the groove bottom requires the establishment of micro-milling tool motion coordinate system as shown in Fig. 1(a). The coordinate system O’-X3Y3Z3, denoted {3}, is fixed on the workpiece and is known as the workpiece space coordinate system. The coordinate system O-X1Y1Z1, denoted {1}, in which the origin O is the axis end of the milling tool. X1-axis and Y1-axis parallel to X3-axis and Y3-axis respectively, which moves with spindle, are named as the spindle servo coordinate system. The coordinate system O-X0Y0Z0, denoted {0}, in which the Z0-axis coincides with the tool axis, is created by the coordinate system O-X0Y0Z0 rotating β degrees about the Y0-axis and is called the tool point coordinate system. β is the inclination angle of the micro-milling tool. The coordinate system O-X2Y2Z2, denoted {2}, in which the Z2-axis coincides with the Z2-axis, is produced by the rotation of the tool around the tool axis and is called the tool rotation coordinate system.
The structures on the bottom of the microchannel are mainly generated by the cutting motion of micro-milling tool. Figure 1(c) shows the cutting trajectory of the tool nose. The section of the tool nose can be regarded as composed of two straight lines and an arc. Therefore, the tool tip equation can be expressed as follows:
where α is the rake angle, β is the slant angle of the cutter and r is the radius of the tool nose. The transformation from coordinate system {0} to coordinate system {1} is
\(\mathop T\nolimits_{{01}} =\left[ {\begin{array}{*{20}{c}} {\cos \beta }&0&{ - \sin \beta }&0 \\ 0&1&0&0 \\ {\sin \beta }&0&{\cos \beta }&0 \\ 0&0&0&1 \end{array}} \right]\) (2)
The tool rotates about the spindle during milling, the transformation from coordinate system {2} to coordinate system {0} is
$$\mathop T\nolimits_{{20}} =\left[ {\begin{array}{*{20}{c}} {\cos \theta }&{ - \sin \theta }&0&0 \\ {\sin \theta }&{\cos \theta }&0&0 \\ 0&0&1&0 \\ 0&0&0&1 \end{array}} \right]$$
3
where θ is the rotation angle of the tool. The coordinates need to be transformed back to coordinate system {1}. The transformation from coordinate system {1} to coordinate system {2} is
$$\mathop T\nolimits_{{12}} =\left[ {\begin{array}{*{20}{c}} {\cos \beta }&0&{\sin \beta }&0 \\ 0&1&0&0 \\ { - \sin \beta }&0&{\cos \beta }&0 \\ 0&0&0&1 \end{array}} \right]$$
4
The cutter feeds along the X1-axis and the transformation from coordinate system {3} to coordinate system {1} is
$$\mathop T\nolimits_{{31}} =\left[ {\begin{array}{*{20}{c}} 1&0&0&{{x_0}+n{f_z}} \\ 0&1&0&{{y_0}} \\ 0&0&1&{{z_0}} \\ 0&0&0&1 \end{array}} \right]$$
5
where (x0,y0,z0) is the initial coordinate of coordinate system {1} with respect to frame coordinate system {3}.
Through the transformation, the coordinate of the point on the tool tip in the workpiece coordinate system can be obtained as
$${\left[ {{X_w},{Y_w},{Z_w},1} \right]^{\text{T}}}={T_{31}}{T_{12}}{T_{20}}{T_{01}}{\left[ {X,Y,Z,1} \right]^{\text{T}}}$$
6
For modeling of surface topography, the workpiece space is divided into XY grid nodes by meshing. The height coordinates of the workpiece at the discrete position swept by the tool tip are obtained and stored as the Z coordinates of the grid node through Eq. (6). Then feed and rotation motions of the micro-milling tool can be obtained by circulation, so as to construct the surface topography of the microchannel. The tool used in the model and subsequent experimental tests is a double edged flat-end micro-milling tool, as shown in Fig. 2. The rake angle α is 3° and the radius of the tool nose is about 6.026 µm measured by scanning electron microscope (SEM). Different simulated surface topographies can be obtained by changing the tool inclination angle and feed parameters, as shown in Fig. 3.
The model is obtained when the inclination angle of the tool is 5 °and the feed rate of each tooth is 5 µm/tooth as shown in Fig. 3. From Fig. 3, it can be found that the structure on the bottom of the microchannel obtained by the inclined micro-milling process is a periodic corrugated surface with a certain Radian. The corresponding contours can be extracted from the 3D topography, as shown in Fig. 3(c). And the corresponding surface roughness can be calculated based on the extracted contour data. This model can be used to predict the surface morphology and surface roughness obtained by inclined micro-milling for the follow-up experimental tests.
As shown in Fig. 4(a), a homebuilt small four-axis vertical micro-milling machine tool was used for performing all the micro-milling experimental tests. A ’high-speed electric spindle (NAKANISHI, NR-3060S, Japan) is employed for this micro-milling machine tool and the maximum rotation speed is 60,000 rpm. A Micro-Nano Optics high-precision rotating platform with 0.0005° resolution is used for Axis b rotating unit. High precision linear motion table is used in micro-milling feed motion(Micro-Nano Optics, WN200TA150H, China). The resolution is 1µm and the straightness is 3µm. In addition, the micro-milling machine is installed on the vibration isolation platform made of marble material to reduce the adverse effects of vibration.
The polymethyl methacrylate (PMMA) has good electrochemical properties, biocompatibility, high light transmittance, easy processing, low price, and is suitable for mass production. Thus, it is often used as a material or mold for microfluidic chips. In this study, the PMMA is selected as the workpiece. The micro-milling tool used for the experimental tests is a cemented carbide double-edged end mill (NS tools, AL2D, Japan) with a diameter of 1000 µm, as shown in Fig. 2. After the micro-milling process, a white light interferometer (Zygo, NewView 9000, America) was used to observe the topography of the microchannel bottom and the surface roughness of the microchannel bottom can be obtained.
4.1 Effects of micro-milling parameters on the surface roughness of the microchannels
In order to study the effects of the micro-milling parameters on the machining outcomes, the orthogonal tests are carried out by different combinations of the spindle speed, feed rate and cutting depth, while the tool inclination angle is set as 5 degrees. Detailed cutting parameters were given in Table 1.
| Tool inclination angle (°) | Spindle speed (r/min) | Feedrate (mm/min) | Depth of cut (mm) |
|---|---|---|---|
| 5 | 4000 | 20 | 0.2 |
| 6000 | 40 | 0.3 | |
| 8000 | 60 | 0.4 |
The typical surface morphology of the microchannel bottom is measured by the white light interferometer, as shown in Fig. 4. The surface roughness is measured at random positions for each cutting condition, and the measurement results are shown in Table 2. The morphology of the microchannel bottom shown in Fig. 4 (c) is a ripple surface with a certain period, which is mainly consistent with predicted surface shown in Fig. 3. However, there is difference between the machined and predicted surfaces, due to the influence of the runout of the micro-milling tool. The experimental results are analyzed using the simple and practical analysis of extreme variance (ANOVA) to determine the order of influence factors in the orthogonal tests, and to determine the optimal level of each factor in this test, and the results of ANOVA are shown in Table 3.
| Group | Spindle speed (r/min) | Feedrate (mm/min) | Depth of cut (mm) | Surface roughness Ra(µm) |
|---|---|---|---|---|
| 1 | 4000 | 20 | 0.2 | 0.103 |
| 2 | 4000 | 20 | 0.3 | 0.083 |
| 3 | 4000 | 20 | 0.4 | 0.081 |
| 4 | 4000 | 40 | 0.2 | 0.102 |
| 5 | 4000 | 40 | 0.3 | 0.095 |
| 6 | 4000 | 40 | 0.4 | 0.089 |
| 7 | 4000 | 60 | 0.2 | 0.188 |
| 8 | 4000 | 60 | 0.3 | 0.197 |
| 9 | 4000 | 60 | 0.4 | 0.199 |
| 10 | 6000 | 20 | 0.2 | 0.041 |
| 11 | 6000 | 20 | 0.3 | 0.044 |
| 12 | 6000 | 20 | 0.4 | 0.036 |
| 13 | 6000 | 40 | 0.2 | 0.047 |
| 14 | 6000 | 40 | 0.3 | 0.062 |
| 15 | 6000 | 40 | 0.4 | 0.079 |
| 16 | 6000 | 60 | 0.2 | 0.113 |
| 17 | 6000 | 60 | 0.3 | 0.095 |
| 18 | 6000 | 60 | 0.4 | 0.097 |
| 19 | 8000 | 20 | 0.2 | 0.078 |
| 20 | 8000 | 20 | 0.3 | 0.052 |
| 21 | 8000 | 20 | 0.4 | 0.054 |
| 22 | 8000 | 40 | 0.2 | 0.041 |
| 23 | 8000 | 40 | 0.3 | 0.060 |
| 24 | 8000 | 40 | 0.4 | 0.046 |
| 25 | 8000 | 60 | 0.2 | 0.077 |
| 26 | 8000 | 60 | 0.3 | 0.065 |
| 27 | 8000 | 60 | 0.4 | 0.045 |
| Spindle speed (r/min) | Feedrate (mm/min) | Depth of cut (mm) | |
|---|---|---|---|
| k1 | 0.126 | 0.064 | 0.088 |
| k2 | 0.068 | 0.069 | 0.084 |
| K3 | 0.058 | 0.120 | 0.081 |
| Extreme variance | 0.068 | 0.056 | 0.007 |
| Ranking of influencing factors | Spindle speed > Feedrate > Depth of cut | ||
| Optimal group | Spindle speed 6000r/min, Depth of cut 0.2mm, Feedrate 20mm/min | ||
According to the results of the extreme variance analysis, it can be found that the spindle speed has the greatest influence on the surface roughness among the cutting parameters, the feed is the second, and the cutting depth has the least influence. The average surface roughness value decreases with the increase of spindle speed, increases with the increase of feed, and decreases with the increase of cutting depth. According to the above experimental results, the spindle speed is the parameter that has the greatest influence on the surface roughness. In order to study the influence of spindle speed and tool inclination on the surface roughness of the bottom of the groove, and then to control the desired surface roughness by adjusting the machining parameters, four typical tool inclination angles and eight spindle speeds are selected in the subsequent micro-milling experimental tests, the feedrate and depth of cut are kept constant. The detailed cutting parameters are shown in Table 4.
| Feedrate (mm/min) | 40 | |||||||
|---|---|---|---|---|---|---|---|---|
| Depth of cut (mm) | 0.2 | |||||||
| Spindle inclination angle (°) | Spindle speed (r/min) | |||||||
| 5 | 4000 | 5000 | 6000 | 7000 | 8000 | 9000 | 10000 | 11000 |
| 10 | 4000 | 5000 | 6000 | 7000 | 8000 | 9000 | 10000 | 11000 |
| 15 | 4000 | 5000 | 6000 | 7000 | 8000 | 9000 | 10000 | 11000 |
| 20 | 4000 | 5000 | 6000 | 7000 | 8000 | 9000 | 10000 | 11000 |
From the above experimental tests, the surface roughness of the bottom of the microchannel is obtained when the spindle speed changes from 4000 r/min to 11000 r/min under different inclination angles. The prediction value of the surface roughness can be obtained by extracting the height value of the surface structure through the model established in the Section 2. The surface roughness prediction curve shown in Fig. 5 (a) is obtained by substituting the machining parameters used in the experimental tests shown in Table 4 into the surface prediction model. According to the prediction curve, under the ideal condition, the surface roughness obtained from the model decreases with the increase of the spindle speed, that is, with the increase of the feed per tooth, while the surface roughness increases with the increase of the tilt angle of the spindle, while the change of the surface roughness decreases with the increase of the feed per tooth. The results is obtained from the ideal surface prediction model without considering the influence of spindle runout, machine tool vibration and workpiece material. However, the actual machining situation will be different, and according to the machining condition, when the spindle speed exceeds 8000 r/min, the spindle vibration intensifies, the influence of vibration begins to increase, and the stability of micro-milling becomes worse, thus, the results obtained from the experimental tests at a high speed should have a large difference from the theoretical results. As shown in Fig. 5 (b), the variation of surface roughness with the spindle speed obtained from the experimental tests is given.
From Fig. 5(b), it can be found that the overall trend of surface roughness at the bottom of the microchannel with the spindle speed for four different tool inclination angles is decreasing first and then increasing, and the surface roughness basically reaches the minimum value at 7000–9000 r/min. Comparing with Fig. 5 (a), it can be seen that the change trend before 8000 r/min is basically in line with the results obtained by the prediction model, however, the surface roughness gradually increases after the speed is greater than 8000r/min, which is consistent with the previous discussion. The possible reason is with the increase of the spindle speed, the spindle runout and the overall vibration of the machine intensifies, and the cutting becomes more and more unstable. Moreover, there is no cutting fluid and coolant during the micro-milling process, as the spindle speed increases, the friction and heat between the tool and the workpiece increases, which also leads to a certain impact on the morphology of the machined surface. At the same time, when the spindle speed is too high, the feed per tooth is too small may lead to the cutting layer thickness less than the minimum cutting thickness of the sample material, the PMMA is extruded and scraped but not removed, with the movement of the cutting tool and the material accumulation. When the material accumulation exceeds the minimum cutting thickness, the workpiece material could be removed by the cutting tool. In addition, this instable material removal state will also lead to poor quality of the bottom of the microchannel and the increase of surface roughness.
The variation of the surface roughness with the tool inclination is consistent with the results obtained by the prediction model that the surface roughness increases with the increase of the inclination angle. The difference between the prediction values and that experimental results for the four different inclination angles at low speed are relatively small, and the deviation of the prediction model at 6000 r/min is from 1–12.5%, which shows that the effect of spindle runout and vibration at lower speed is not significant. However, as the spindle speed increases, especially when the speed exceeds 8000 r/min, the effect of tool runout and vibration increases sharply, due to the insufficient rigidity of the machine tool system. Furthermore, as the spindle speed continues to increase, due to the lack of cutting fluid lubrication and coolant, and the melting point of PMMA is relatively low, only 130–140 ℃, the cutting heat will also become one of the important factors affecting the quality of the machined surface, the tool nose cutting the workpiece surface at the extreme speed, and the rapid rise in local temperature may cause the bottom structure of the groove to partially melt, which will also affect the value of surface roughness. When the feed per tooth is too small, the plowing material removal state is also a possible factor of the quality of the machined surface. Based on the experimental results, it is found that the desired surface of the microchannel bottom with different bottom surface roughness can be obtained within a certain range of the cutting parameters.
4.2 Modulation of roughness and wettability of the machined surface
When preparing microfluidic channels, the surface characteristics of microfluidic channels, such as wettability and chemical properties [20], should be taken into account. Wettability is usually evaluated by the contact angle of droplets on its surface [21]. When the contact angle is less than 90°, it is hydrophilic and more than 90 °is hydrophobic. The contact angle on the same material will be affected by the surface characteristics [22–24]. And surface wettability can be influenced by surface roughness, which will affect the flow of liquid in the microfluidic channel [25, 26]. Therefore, the liquid flow state in the microfluidic channel can be controlled by the surface roughness. Based the prediction model and experimental tests, surfaces with different bottom roughness are processed and the contact angles of water droplets are measured using a contact angle measuring instrument (KRUSS DSA100S). The machining parameters are shown in Table 5. The experimental tests measured the contact angle change of another material commonly used for microfluidics, polydimethylsiloxane (PDMS), in addition to the original workpiece material, PMMA, which was used as a mold to replicate the machined surface for measurement. Since PDMS microfluidic chips require oxygen plasma treatment to achieve bonding, the treated PDMS was also tested. The measurement results of the contact angle are shown in Fig. 6 (a).
| Group | Feedrate (mm/min) | Spindle speed (r/min) | Spindle inclination angle (°) |
|---|---|---|---|
| 1 | 40 | 4000 | 5 |
| 2 | 40 | 5000 | |
| 3 | 40 | 6000 | |
| 4 | 40 | 7000 | |
| 5 | 40 | 8000 | |
| 6 | 40 | 4000 | 20 |
| 7 | 60 | 4000 | |
| 8 | 80 | 4000 | |
| 9 | 100 | 4000 | |
| 10 | 140 | 4000 | |
| 11 | 180 | 4000 |
According to the experimental results, PDMS changed from hydrophobic to hydrophilic after oxygen plasma treatment, and the variation of water contact angle on different roughness surfaces on PDMS before and after the treatment showed an overall increase with the increase of roughness. The effect of different roughness surfaces, including PMMA, PDMS and PDMS after oxygen plasma treatment, on water contact angle is the same, and the contact angle mainly increases with the increase of surface roughness. For the explanation of contact angle on the rough surfaces, Wenzel model and Cassie-Baxter model are the most representative wetting models. According to the Wenzel model, an increase in surface roughness leads to an increase in the contact angle of hydrophobic surfaces and a decrease in the contact angle of hydrophilic surfaces. On the contrary, according to the Cassie-Baxter model, an increase in air pocket caused by the increase in surface roughness below the droplet leads to an increase in contact angle, regardless of whether the surface is hydrophobic or hydrophilic. According to the measured results, the variation of the contact angle is consistent with the prediction of the Cassie-Baxter model, where air pockets may form within the regularly undulating structure of the bottom surface due to the bottom structure formed by the inclined micro-milling process. In addition, according to the equations of the model, as the roughness increases, the depth and period of the bottom structure increases, leading to a decrease in the contact area fraction between the liquid and the solid, which results in the contact angle increases. With the increase in contact angle, the flow of liquid in the microchannel will also be affected. From the general rule of capillary phenomenon is known, when the contact angle is larger, it is more difficult for the liquid to enter the capillary. Thus, it can be presumed that the liquid flow in the microchannel will also be impeded with the increase in contact angle. From the above experimental tests, it can be found that the wettability of these three surfaces can be changed by different surface roughness, thus, the microfluidic chip with different bottom roughness of the microchannel can be processed to realize the flow control of the microfluidic channel.
4.3 Preparation Of Microfluidic Chips
The above method of inclined micro-milling to control the roughness at the bottom of the groove can be applied to prepare microfluidic channels to achieve control of fluid flow in microfluidic channels. Wu et al. reported that the flow friction in silicon microchannels increases with increasing the surface roughness. The lower the value of surface roughness, the lower the flow friction. Based on this, a better control of the channel bottom roughness by inclined micro-milling, a larger flow pressure is required to pass through the microchannel with a larger flow friction. Therefore, the flow path can be controlled by changing the surface roughness at the bottom of the mirochannel. Three channels with different bottom roughness were processed to achieve selective fluid flow. The bottom surface roughness of the smooth channel was 0.0384 µm, the bottom surface roughness of the rougher channel was 0.156 µm, and the bottom surface roughness of the roughest channel was 0.351 µm, and fluid flow experiments were conducted to control the pressure by changing the flow rate. The experimental results are shown in Fig. 7.
When the liquid speed of 5 µL/min, only the middle smooth channel liquid circulation When the flow rate increases to 20 µL/min, the upper rougher channel liquid circulation. While, when the flow rate of more than 50 µL/min, the three channels are circulating. The experimental results show that the order of circulation of the three channels and through to the bottom surface roughness, the smoother the bottom, the smaller the water contact angle, the easier it is for red ink to go through the channel and become the lead in circulation. The rougher the surface, the greater the through-liquid pressure required for circulation. According to the experimental results, microchannels machined by inclined micro-milling can be used to prepare the microfluidic chips with controllable circulation order of the microchannels.
In this paper, the effects of tool inclination and cutting amount on the surface roughness of the bottom of a tilted micro-milling groove were investigated. A structural model of the bottom of the groove was established and roughness prediction was performed. The relationship between surface roughness and droplet contact angle is investigated, and a microfluidic chip with controlled flow is prepared based on the experimental results. The conclusions were drawn as follows:
-
The structure model of the bottom of the groove for tilting micro milling can reflect the actual surface shape and the predicted roughness within a certain range, and can be used to guide the actual machining with a small error at a low spindle speed. When the milling speed is high, the error between actual machining and prediction becomes larger and larger due to the influence of tool runout and machine tool vibration. It can be seen that the prediction accuracy is limited by the accuracy of the machine tool.
-
Among the machining parameters the spindle speed has the greatest influence on the surface roughness of the bottom of the groove. The roughness decreases and then increases with the increase of the spindle speed and increases with the increase of the tool inclination angle. The inclination angle of the tool can significantly affect the machined surface roughness. Therefore, according to the model and experimental results, the surface roughness can be easily controlled by controlling the inclination angle of the tool to obtain a specific surface roughness.
-
According to the study of droplet contact angle on the surface with different roughness and the liquid flow experiment of microfluidic chip, different bottom surface roughness affects the wettability of liquid droplets, which in turn affects the flow of liquid in the microchannel. In the smooth fluid channel at the bottom, the liquid will flow first, and the rougher channel will flow by increasing the pump pressure, so that the flow can be controlled by directly machining the microfluidic channel with different bottom roughness.
Authors’ Contributions:
Yanquan Geng: Methodology, Validation, Investigation, Visualization, Writing-Original Draft; Suyu Zhang: Investigation, Writing-Original Draft and Editing; Jiqiang Wang: Conceptualization, Methodology; Guijian Xiao: Writing–Reviewing and Editing; Chen Li: Supervision, Visualization, Writing–Reviewing and Editing; Yongda Yan: Supervision, Visualization, Writing–Reviewing and Editing
Funding: The authors gratefully acknowledge the financial supports of the National Natural Science Foundation of China (52222512, 52005134), Self-Planned Task (No. SKLRS202214B) of State Key Laboratory of Robotics and System (HIT), and the Fundamental Research Funds for the Central Universities (Grant No. HIT.BRET.2022008 and FRFCU5710050521).
Ethical Approval: Not applicable.
Consent to Participate: Not applicable.
Consent to Publish: We would like to submit the manuscript entitled “Effect of the inclined angle of micro-milling tool on the fabrication of the microfluidic channel”, for your consideration for publication in International Journal of Advanced Manufacturing Technology. No conflict of interest exits in the submission of this manuscript, and the manuscript is approved by all authors for publication. On behalf of the co-authors we declare that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
Competing Interests: The authors declare that they have no conflict of interest.
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