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Procedia Engineering 184 (2017) 137 – 144
Advances in Material & Processing Technologies Conference
Burr Reduction of Micro-milled Microfluidic Channels Mould
Using a Tapered Tool
Kushendarsyah Saptajia*, Sathyan Subbiahb
a
Faculty of Manufacturing Engineering, University Malaysia Pahang, 26600 Pekan, Malaysia
b
Department of Mechanical Engineering, Indian Institute of Technology Madras, 600036 Chennai, India
Abstract
Moulds with micro-sizes features needed for many applications, such as for hot embossing, can be manufactured
using micro-milling process. However, the burrs formed in the micro-milling process are a challenge that needs to
be addressed. The burr sizes are comparable to the micro-milled feature sizes and the common types of burr seen
being the top/side and exit burrs. The use of a tapered geometry micro-milling tool is investigated in this paper that
enables reduction in both the top and exit burrs. The straight and tapered micro-milling tools of various angles are
used and the burrs formed are observed. Micro-milling experiments are conducted on an aluminium alloy by
producing common positive features seen in the mould for the production of polymer microfluidic devices. The
results show that the burr reduction can be attributed due to the increase of the taper angle. It is seen that the tapered
tool not only substantially reduces the top burrs, but also leaves behind inclined walls which further help in reducing
exit burrs formed during the subsequent finish face milling. Furthermore, embossing trials performed with the
micro-milled tapered geometry moulds show improved performance not only because burrs are reduced and also
because the taper eases mould release.
©©2017
2017TheTheAuthors.
Authors. Published
Published by Elsevier
by Elsevier Ltd. is an open access article under the CC BY-NC-ND license
Ltd. This
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of the Advances in Material & Processing Technologies
Peer review under responsibility of the organizing committee of the Advances in Materials & Processing Technologies Conference
Conference.
Keywords: burrs; micro-milling; tapered; mould; microfluidic devices
*
Corresponding author. Present address: 26600 Pekan, Pahang Darul Makmur,
Malaysia. Tel.: +609-424 5845; fax: +609-424 5888.
E-mail address: kushendarsyah@ump.edu.my.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer review under responsibility of the organizing committee of the Advances in Materials & Processing Technologies Conference
doi:10.1016/j.proeng.2017.04.078
138 Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144
1. Introduction
Burr formation is a common problem that occurs in micro-milling. In the micro-milling process, the burr size can
become comparable to the features being created. The two main types of burrs occurred in micro-milling are: (a) the
burr attached to the surface machined by the minor edge of the tool which is exit burr and (b) the burr attached to the
top surface of the workpiece which is called a top/side burr [1]. In general, there are two approaches reported in the
literature to overcome the burrs. One way is to remove the burr (deburring [2]) after the machining and the other is
to reduce the tendency for burr formation during the machining process.
Removing the burr on the micro-scale features produced by micro-milling is challenging; it must be performed
carefully to avoid damage to the features themselves. Incorrect selection of deburring techniques or parameters also
may introduce dimensional errors, damage, surface finish, and residual stresses. Many researchers have explored
and reported burr reduction through different machining strategies. Formation of burrs in ductile materials can also
be reduced using sharp single crystal diamond cutting tools with very small edge radii; diamond micro-milling tools
are commercially available in the market but are expensive and require good vibration control and precision on the
machine tool for effective usage.
It has been reported that by strengthening the edge of the machined wall using tapered milling tools can reduce
the burrs formation [3]. In this paper, the application of tapered shape geometry micro-milling tool in fabricating the
common micro-channel features exist in the microfluidic devices is studied. The micro-channel features are selected
to represent a typical feature seen on an embossing mould used in the hot embossing process in the production of
polymer microfluidic devices. Polymer microfluidic devices are micro analyzer tools used for integration of
monitoring and analysis of chemicals, such as sample collection, pretreatment, amplification and detection, all to be
performed in one easy to handle as if in a standard laboratory. The integration offered by the polymer microfluidic
devices is advantageous in that it can reduce the consumption of the chemical samples that may be rare and
expensive, provide faster and more accurate analysis, offer simplicity of use, provide higher sensitivity, and offer
lower cost compared to conventional devices [4]. The embossing mould for the production of polymer microfluidic
devices can be fabricated using micro-milling process. The micro-milling process can produce faster and cheaper
moulds when compared to lithography and MEMS based processes [5].
In addition, the effect of the subsequent finish face milling pass on the top burrs and surface quality of the
positive features produced by tapered tools is also studied. Lastly, hot embossing trials using moulds made of
tapered micro-milling tools are also performed to study the efficacy of the burr reduction technique. Therefore, this
work is needed to be conducted especially for ductile materials using carbide tools, to confirm that use of tapered
tool can solve the burr problems during fabrication of embossing mould using micro-milling.
2. Experimental Setup
The micro-channel feature is adopted as the experimental features to be produced by slot micro-milling followed by
a finish face milling pass. By increasing the edge angle between the wall of the slot and the top surface, the top burrs
are expected to decrease (Figure 1 (a1 and b1)). The formation of the higher edge angle also strengthens the edge,
which can lead to the reduction of exit burrs on the top surface of channel by the subsequent finish face milling pass
(Figure 1 (a2 and b2)).
The micro-milled micro-features designs in the experiments are shown in Figure 2. The designs were selected to
represent a typical feature seen in microfluidic devices. The proposed design for the straight wall mould consists of
a protruded straight wall with a rectangular cross-section (100 µm width and 100 µm depth) connected to a
cylindrical protrusion with a 1 mm diameter and 100 µm depth. Meanwhile the tapered mould consists of a
trapezoidal cross section straight protruded wall with a top width of 73 µm and depth of 100 µm. This is connected
to a conical frustum protrusion with the same depth.
Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144 139
Slot micro-milling
Straight Tapered
(a1) milling
(b1)
milling
tool tool
Top burr
Top burr
edge
Feed angle>900 Feed
edge direction
angle=900 direction
Finish face milling
Straight (a2) Straight (b2)
milling milling
tool tool
Side burr Side burr
edge Feed edge Feed
angle=900 direction angle>900 direction
Exit burr Exit burr
Figure 1. A tapered tool (b1 and b2) is expected to reduce top burrs formed and help in reducing the exit burrs formed in the subsequent finish
face milling compared to a non-tapered tool (a1 and a2).
The channels were manufactured by slot micro-milling process using a straight end-mill (0.8 mm diameter) and
various tapered end-mill (with a bottom diameter of 0.5 mm) of carbide milling tool (Figure 3). The wall angle is
varied by varying the taper angle of the micro-milling tool from 150 to 500. In order to observe the effect of the
additional finish face milling, the channel is micro-milled with excess depth (50 µm), and this was subsequently
removed using a straight 2.5 mm diameter milling tool. Two channel features were made using each tool in order to
observe the effects of taper angle and additional finish face milling pass on the burr formation and surface quality.
Subsequently, the hot embossing process is conducted to study the performance of the mould consisting of micro-
features produced by various taper angle and additional finish face milling.
100Pm 100Pm
100Pm
73Pm
Figure 2. Mold designs with straight wall (left) and tapered walls (right).
Experiments were conducted to produce the micro-channel features by micro-milling process using a Mazak
FJV-250 3-axis conventional CNC machine. The Al6061-T6 block workpiece was milled flat using 4 mm diameter
milling tool. Preliminary experiments are conducted to determine the optimum parameters for the micro-channel
milling and finish face milling process. Based on these results the micro-milling slot parameters for micro-channel
and finish face milling parameters to be used are listed in Table 1. Micro-milling of channels under dry conditions is
undertaken at a spindle speed of 10,000 rpm and a feed rate of 25 µm/rev. All experiments were conducted in dry
cutting condition. The micro-channels and burrs were observed using a scanning electron microscope (SEM) JEOL
5600L.
140 Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144
Figure 3. Micro-milling tools, from left to right: straight tool, 150, 300, 400 and 500 tapered tools.
Table 1. Experimental conditions.
Workpiece Al6061-T6
Milling tools Two flute end mills super micro grain carbide tool
0.8 mm diameter (straight)
0.5 mm bottom diameter tapered tool with 15 o, 30o, 40o and 50o taper angle
2.5 mm diameter (straight) for finish face milling
Slot micro-milling of micro-channels
Feed rate 250 mm/min
Axial depth per cut 25 µm
Spindle speed 10,000 rpm
Total depth of cut 100 µm (without finishing pass)
150 µm (with finishing pass)
Cutting condition Dry cutting
Finishing face milling
Feed rate 500 mm/min
Axial depth per cut 25 µm
Spindle speed 10,000 rpm
Total depth of cut 50 µm
Cutting condition Dry cutting
3. Results and Discussions
3.1. The influence of tapered channels design on a mould
The proposed tapered design when implemented in an embossing mould has some design implications such as in
the final microfluidic device, since the taper geometry changes the channel width and/or the cross sectional area. In
order to maintain the same flow rate of the fluids through the channels, it is expected that the channel cross-section
size produced by the tapered micro-milling tool should be in the same range as that in a non-tapered channel design.
However, by keeping the width at the top of the channel to be the same, the width of the bottom channel can now be
increased because of the taper tool angle (Figure 4). The width of the channel (L) with a value of 73 Pm is selected
for tapered tool in order to maintain slenderness of the channel by proportionately maintaining the ratio of the
channel height to the width. High aspect ratio can weaken the mould resulting in mould damage during embossing
mould and shorten its useful life. Microfluidic channels with angled walls are reported in the literature [6], where the
embossing moulds were made in Silicon by wet-etching. The etched plane preference causes sloped walls in the
Silicon channels. It is reported that such sloped walls result in better mould release and hence leads to better
embossed features; also the edges are now stronger resulting in less edge breakage. Table 2 shows the channel cross-
sectional areas and the differences for various taper angles compared with a non-tapered channel design.
Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144 141
Mold top surface
(channel bottom surface)
J
Edge angle
Mold base surface
(channel top surface)
Figure 4. Cross-section of the channel.
Table 2. Cross-section area of the channel.
Taper angle Edge angle Channel Cross-section Cross-section
L (μm)
J (o) area (μm2) Difference (%)
0 90 100 10,000 0
15 105 73 9,979 -0.21
30 120 73 13,074 30.74
40 130 73 15,691 56.91
50 140 73 19,218 92.18
3.2. Micro-channels Features
3.2.1. Effect of milling direction: Experiments conducted with slot milling in the up milling direction and down
milling direction showed severe differences in top/side burr formation. Top burrs are seen to be much more severe
when slot milling in the down milling direction. This was seen to be the case for both straight milling tool and the
tapered milling tool (Figure 5). However, the opposite was true for the side wall surface finish appearance. The side
wall finish was visibly better in SEM micrographs with down milling than with up milling; again this was true
regardless of whether the tool was straight or tapered. The side wall produced using up milling reveals regular
jagged like shape associated with the milling mark pattern [7]. Hence, in the subsequent parts, the discussions are
focused mainly for the results when using down milling process.
Figure 5. Effect of milling direction (150 tapered tool, before finish face milling pass) (a), (b) Down milling (c), (d) Up milling. Down milling is
seen to produce more severe top/side burrs.
142 Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144
3.2.2. Burr formation using straight milling tool: The burrs on the edges of the milled features when using a straight
micro end mill can be seen in Figure 6. The burrs are seen to be very severe with dimensions comparable to the wall
height (Figure 6(a) (b)). Burrs are seen to be severe both in the straight wall and in the cylindrical protrusions. After
undergoing the finish face milling pass, the top burrs generated earlier are replaced with exit burrs (Figure 6(c) (d)).
The edges also appear broken and the top burrs are seen to be fairly severe enough to interfere with the embossing
process.
Figure 6. Burrs in straight wall feature mold. (a), (b) Before finish face milling pass; top burrs generated from the slot milling are comparable to
the channel itself. (c), (d) After finish face milling pass; exit burrs generated from the finish face milling pass can be seen clearly.
3.2.3. Effect of taper angle: The effect of the taper angle in reducing the top burr formation during slot micro-
milling and exit burrs during the subsequent finish face milling pass can be observed in Figure 7 and Figure 8
respectively. As the tool taper angle is increased from 15 0 to 500, the top burrs formed both in the channel section
and in the cylindrical protrusion section of the moulds decreased substantially. From the micrographs of Figure 7 it
is not very evident that the top burrs have decreased in the channel sections because of the viewing angle and the
proximity of the burrs from both edges. The results are more easily evident in the conical frustum protrusion portion
of the mould. Higher the taper angle, lesser are the top burrs formed during the slot micro-milling process. In
addition, after the finish face milling pass, the exit burrs generated are minimized as the taper angle is increased
(Figure 8). Again, as the taper angle increases the burr condition at the edges is less. It may very well be possible
that a finish face milling pass may not be necessary if the process parameters is optimized further.
150 tapered tool 300 tapered tool 400 tapered tool 500 tapered tool
(edge angle 105 0) (edge angle 120 0) (edge angle 130 0) (edge angle 1400)
Figure 7. Effect of taper angle on top/side burr formation on the micro-features. The top row shows the channel section of the mould and bottom
row shows the conical frustum protrusion section of the mould. The reduction in top burr formation with increasing taper angle is evident.
Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144 143
150 tapered tool 300 tapered tool 400 tapered tool 500 tapered tool
(edge angle 1050) (edge angle 120 0) (edge angle 130 0) (edge angle 140 0)
Figure 8. Effect of taper angle on exit burr formation on the micro-features followed by finish face milling pass. The top row shows the channel
section of the mould and bottom row shows the conical frustum protrusion section of the mould. The reduction in exit burr formation with
increasing taper angle is evident.
The results obtained above with higher taper angle and minimal burrs are surprising given that the milling was
performed without any coolant. Hence, if there is flexibility in designing the microfluidic channels with tapered
walls, then it is possible to economically make these micro-channels for moulds such as for embossing using
conventional carbide cutting tools on conventional CNC machining centres. It is noted that the 50o taper channel has
resulted in an increase in cross sectional area by 92.18% (Table 2) while the burrs are the lowest when milling the
channel of this geometry (Figure 7). Increasing the width of the channel affects the final size of the complete
microfluidic device, especially in the case of a complex design with parallel multiple flow channels. This impact
needs to be considered when designing the microfluidic device embossing mould using tapered micro-milling tool
since there may be a limitation in the size of microfluidic devices required for certain applications.
3.3. Hot plate embossing
In order to observe the performance and quality of the different features produced with and without taper shape,
embossing trials were performed. The machined mould consists of micro-features was subsequently transferred onto
a polymer substrate such as PMMA (Polymethylmethacrylate) by hot embossing. PMMA is the most commonly
used polymer for moulding applications because of its biological compatibility, its optical properties and ease of
moulding [8]. It is noted that the hot plate embossing is sensitive to temperature, time, and pressure [9]. The
embossing process is performed using the hot plate embossing system Carver Manual Press 4386. The PMMA was
embossed with the following hot plate embossing parameters: 10 minutes time, base plate temperature 93 oC, top
plate temperature 115 oC and pressure 10 MPa. The embossing results for straight tool and 50 o taper angle tool are
shown in Figure 9. The embossed PMMA revealed similar geometry features with its embossing mould.
(a) (b)
Replication of
burrs
Figure 9. Embossed features on PMMA (a) Embossing mould produced using straight tool, (b) Embossing mould using 50o taper angle tool, with
down milling and without finish face milling pass.
144 Kushendarsyah Saptaji and Sathyan Subbiah / Procedia Engineering 184 (2017) 137 – 144
As seen in Figure 9, the embossed PMMA with straight wall exhibits replication of burrs especially at the edges
of the micro-features. In contrast, the micro-features of the embossed PMMA with 500 taper angle exhibits good
results with no sign of burrs. The tapered channel shape also improved the de-embossing process of hot plate
embossing. The taper angle provides easy mould release and avoids sticking between the embossing mould and
PMMA. Hence, the tapered mould design may have potential advantages in mass manufacturing the polymer
microfluidic devices.
4. Conclusions
The main conclusions of this work are:
x Down milling results in a smoother side wall surface however it produces larger top burrs compared to up
milling.
x The burr reduction observed as the taper angle of the micro-milling tools increases with a taper angle of 50o
angle shows no burr formation at the top side and has smooth surface on the side wall of the mould.
x The larger machined edge angle created by higher tapered tool angle can help in further reducing the exit burrs
formed in the following finishing face milling process.
x Moulds with burr-free micro-features for embossing moulds can be satisfactorily made using tapered micro-
milling tools on conventional machining centres. Hot embossing trials using the tapered mould design showed
good process performance especially during de-embossing process.
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