Micro Rubber Milling

8/10/2019 Micro Rubber Milling

http://slidepdf.com/reader/full/micro-rubber-milling 1/10

Journal of Advanced Mechanical Design,Systems, andManufacturing

Vol. 2, No. 4, 2008

560

Micromachining of Soft Polymer Material

applying Cryogenic Cooling*

Yasuhiro KAKINUMA**, Nobuhito YASUDA** and Tojiro AOYAMA** **Dept. of System Design Engineering, Faculty of Science and Technology, Keio University,

3-14-1 Hiyoshi, Kouhoku-ku, Yokohama, 223-8522, Japan

E-mail: [email protected]

Abstract

Polydimethylsiloxane (PDMS) is one of the important materials for microfluidic

chips. The pattern of micro channels on the PDMS plate is usually fabricated

through the photolithography and micro molding process. However, the

photolithographic method requires multi chemical and mechanical processes and

resultant long process time. The micro milling process is a feasible method forrapid fabrication of various patterns of micro channels .However, micromachining

has not yet been applied to soft polymer materials. It is difficult to machine elastic

materials such as PDMS because of their low toughness. In order to machine a

micro grooves on soft polymer materials, the cutting process applying cryogenic

cooling is proposed because the elastic properties of soft polymer materials

remarkably change from rubbery state to glassy state below the glass transition

temperature. In this study, the freezing milling method using liquid nitrogen is

applied to the micro grooving of PDMS. The result of a cutting test shows that

micro grooves can be shaped easily and machined accurately in PDMS by the

proposed method.

Key words: Micromachining, PDMS, Cryogenic Cooling, µ-TAS, Microfluidic

Chip

1. Introduction

Recently, the study of µ-TAS (micro total analysis system) has developed rapidly and its

achievements have become the center of attention in the fields of micro fluidics and

biochemistry. The µ-TAS system, formed on one microfluidic chip [1, 2], consists of a

capillary that is a flow channel of liquid containing a sample and reagent, a reaction space

and a detection space. This system is expected to save micro samples, speed up the analysis,

automate the measurement, render the device portable, and reduce analysis costs.

The microfluidic chip used for µ-TAS is manufactured mainly from glass plate and the soft

polymer resin PDMS (polydimethylsiloxane) [3]. PDMS has been applied to crucial

components of micro devices in state-of-the-art research in bioengineering or micro

fluidics. The pattern of micro channels is usually fabricated through the micro molding

process, in which the micro molds are made by photolithographic methods. The

photolithographic method consists of the masking process, photoresist exposure and etching

processes, shown in Fig. 1. Although highly accurate processing is possible using

photolithography, the processing time lasts several days because of the multi process and its

need for a great amount of energy and hazardous chemicals is detrimental to the

environment. Other approaches, therefore, are required to manufacture a microfluidic chip

with high productivity and easier processing. The micro machining process, which is a

direct patterning method and is environmental friendly, is one of the potential approaches

for manufacturing microfluidic chips. Micromachining can provide a short turn around time*Received 21 Mar., 2008 (No. 08-0205)

[DOI: 10.1299/jamdsm.2.560]




561

Vol. 2, No. 4, 2008

Si platePatterning photoresist

(Si Mold)

Thermosetting PDMS

Micro groove formed

Si

Mask

PDMSPhotoresist coating

Development

Figure 1: Multi process of photolithography

for manufacturing and produce three-dimensional channel shapes. The purpose of this study

has been to develop the micro milling technology of soft polymer materials such as PDMS

in order to manufacture microfluidic chips. It is difficult to machine the PDMS at room

temperature because of the low hardness of PDMS. On the contrary, below the glass

transition temperature its hardness increases suddenly and, in this state, it is expected that

PDMS can be precisely machined. Therefore, in this study, to realize PDMS machining,cryogenic cooling with liquid nitrogen was applied and the performance of the cryogenic

milling process for PDMS was experimentally evaluated.

2. Property of polymer materials

Polymer materials have a glass transition temperature, which depends on the main chain

structure of the polymer. At the glass transition temperature, the polymer changes from a

rubbery state to glassy state. Table 1 shows the glass transition temperatures for several

polymer materials. The glass transition temperature of PDMS is lower than that of any other

polymer material. Figure 2 shows the molecular architecture of PDMS and the PMMA

(polymethylmethacrylate). In PDMS, the main chain is Si-O bonding. The Si-O bonding

makes molecular motion easy because the atomic radius of silicon is larger than that of

carbon. Therefore, the glass transition temperature of PDMS becomes very low.This study proposes a new machining technology for elastic polymer material.

Specifically, this study proposes the micromachining of PDMS after it is cooled below the

glass transition temperature with liquid nitrogen. The temperature of liquid nitrogen is

lower than –196˚C. Since PDMS changes its state from rubbery to glassy when cryogenic

cooling is applied, it is expected that its machinability is improved.

Table 1: Glass-transition temperature of polymers

Glass-transitiontemperature centigrade

Polydimethylsiloxane -123

Polyethylene -30Polypropylene -10

Polymethylmethacrylate 105

Polycarbonate 150

Nylon 66 60

Glass-transitiontemperature centigrade

Polydimethylsiloxane -123

Polyethylene -30Polypropylene -10

Polymethylmethacrylate 105

Polycarbonate 150

Nylon 66 60

Si O

CH3

CH3

CH2 C

C=O

CH3

O

CH3

(a) PMMA (b) PDMS

Si O

CH3

CH3

CH2 C

C=O

CH3

O

CH3

Si O

CH3

CH3

Si O

CH3

CH3

CH2 C

C=O

CH3

O

CH3

CH2 C

C=O

CH3

O

CH3

(a) PMMA (b) PDMS

Figure 2: Molecular structure of PMMA and PDMS




562

Vol. 2, No. 4, 2008

0

10

20

30

40

50

60

70

80

90

100

温液窒素アクリル

H a r d n e s s ( J I S K 6 2 5 3 T y p e A )

PDMS atroom temperature

PDMS atultralow temperature

PMMA atroom temperature

0

10

20

30

40

50

60

70

80

90

100

温液窒素アクリル

H a r d n e s s ( J I S K 6 2 5 3 T y p e A )

PDMS atroom temperature

PDMS atultralow temperature

PMMA atroom temperature

Figure 3: Hardness of the PDMS at room temperature and at ultralow temperature

Table

Dynamometer

Bakelite flame

Water circulationsystem

End millPDMS

Liquid nitrogenchamber

Liquid nitrogen

6 0 m m

110mm

Workpiece holder

: Temperature-measured point

Figure 4: Structure of cryogenic jig

PDMS

Water

Cryogenic cooling fixture

Micro square end mill

PDMS

Water

Cryogenic cooling fixture


Figure 5: Appearance of cryogenic machining system

3. Applying cryogenic cooling to the cutting process

3.1 Design of a special jig for cryogenic cooling

The change of hardness is measured by a durometer when the PDMS is cooling down with

liquid nitrogen. Figure 3 shows the hardness of PDMS at room temperature and at ultralow

temperature (below the glass transition temperature) and PMMA at room temperature. It is

clear that the hardness of PDMS at ultralow temperature is much higher than that at roomtemperature and the property of PDMS under cryogenic cooling changes into an acrylic. In

other words, PDMS changes from elastic to hard with cryogenic cooling.

In order to apply cryogenic cooling to the cutting process, a special jig with a liquid

nitrogen chamber was designed. The structure of the jig is shown in Fig. 4. The reservoir of

liquid nitrogen and the workpiece holder stage placed at the center of the reservoir are made

of aluminum because of its high heat conductivity. The PDMS plate is set on the stage and

fixed by an aluminum clamper. On the contrary, the outer flame made of bakelite has the

important role of insulating the heat transfer and suppressing the change of liquid nitrogen

to gas. The cutting force is measured by a dynamometer positioned under the special jig. To

keep the temperature at the bottom face of the jig constant and to prevent generation of the

heat drift of the dynamometer, a water circulation system is incorporated between the jig

and dynamometer. A photo of the special jig is shown in Fig. 5.




563

Vol. 2, No. 4, 2008

-200

-150

-100

-50

0

50

0 500 1000 1500 2000 2500 3000

Time s

T e m p e r a t u r e

Adding the liquid nitrogen No liquid nitrogen

Under the water circulation system

Aluminum

Machining space

PDMS surface

-200

-150

-100

-50

0

50

0 500 1000 1500 2000 2500 3000

Time s

T e m p e r a t u r e

Adding the liquid nitrogen No liquid nitrogen

Under the water circulation system

Aluminum

Machining space

PDMS surface

Figure 6: Temperature property of the cryogenic jig

3.2 The heat characteristic of the special jig

To evaluate the heat characteristic of the jig before cryogenic cutting test, the temperatures

at four points of the jig were measured using a T-type thermocouple. The four measurement

points are on the aluminum stage, in the machining space, under the water circulation

system and on the surface of PDMS. In this evaluation, the end mill is away from the jig.

The results of the measurements are shown in Fig. 6. Whenever the liquid nitrogen

evaporates spontaneously and its amount diminishes, new liquid nitrogen is infused into thereservoir by manual operation. As the result of this operation, it takes approximately 1000

seconds for the temperature to reach steady state. After the steady state, the temperature of

PDMS is kept below the glass transition temperature, although it changes repeatedly

between -170˚C and -196˚C. It is estimated that this temperature change is caused by the

quantity of liquid nitrogen in the reservoir. The temperature of PDMS dipped in liquid

nitrogen is nearly -196˚C. On the other hand, when the amount of liquid nitrogen decreases

and PDMS also comes out from the liquid nitrogen, its temperature rises to approximately

-170˚C.

These results indicate that the developed jig is capable of cooling down PDMS below the

glass transition temperature and can be used as the cooling system for cryogenic machining

of PDMS.

4. Effect of applying cryogenic cooling in the cutting process

4.1 Experimental setup and procedure

The prepared sample of PDMS is produced by curing the mixed two components at room

temperature under a vacuum of 10kPa. The size of the PDMS sample is 20mm × 30mm ×

5mm (Width × Length × Thickness).

Using the developed jig, a cryogenic micro milling test of PDMS was carried out. The

cutting condition is summarized in Table 2 [4,5]. A three-axis vertical machining center

(NVD1500, Mori Seiki) was used. The non-coated micro square end mill with helical angleof 30˚ (RSE230, NS Tool), shown in Fig. 7, was used for the grooving operation. This end

mill is made of tungsten carbide. The diameter of the milling tool with two cutting edges is

0.5 mm. To investigate the difference between the cutting performance at room temperature

and at ultralow temperature, the cutting force in each condition was measured. Also, the

machined surface and shape were evaluated at room temperature using scanning electron

microscope (SEM) (VE7800, KEYENCE) and the 3D optical profiler (New View 6200,

Zygo).The SEM used in this study can observe the insulating material without Au or Pt

coating, applying low acceleration voltage. The optical profiler, based on the scanning

white-light technology, can measure the surface roughness with resolution of 0.1nm in

vertical direction. The measurement area of this profiler is 500µm × 700µm.




564

Vol. 2, No. 4, 2008

Table 2: Cutting condition of PDMS at room temperature and at ultralow temperature

Rotational speed

Feed rate

Feed per tooth

Tool

Diameter of tool

WorkpieceMachine tool

5000min-1

10.0mm/min

1.0µ

m


0.5mm

PDMS3-axis vertical type

(b) Bottom of edge(a) Side view

100µm 66.6µm

(b) Bottom of edge(a) Side view

100µm100µm 66.6µm66.6µm

Figure 7: Micro square end mill ( 0.5)

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 0.01 0.02 0.03 0.04 0.05

Time s

C u t t i n g f o r c e i n f e e d d i r e c t i o n N At room temperature

At ultralow temperature1 revolution

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0 0.01 0.02 0.03 0.04 0.05

Time s

C u t t i n g f o r c e i n f e e d d i r e c t i o n N At room temperature

At ultralow temperature1 revolution

Figure 8: Cutting force at room temperature and ultralow temperature

4.2 Result of cutting test

4.2.1 Cutting force

Figure 8 shows the behavior of the cutting forces in the feed direction both at room

temperature and at ultralow temperature. At room temperature, the cutting force is not

periodic and the absolute value is so low that the milling process could not be performed.

On the contrary, the cutting force in the cryogenic condition is larger than that at room

temperature and changes periodically in synchronization with the rotation of the milling

tool. This result confirms that the machining process can work on PDMS under the

cryogenic condition.

4.2.2 Evaluation of the machined surface

Figure 9 shows the shape of the machined groove observed by SEM as long as the PDMS

warm to room temperature after cryogenic cutting test. Although the PDMS cannot be cut at

room temperature, it can be machined in the cryogenic condition and the micro groove is

precisely formed. In order to investigate the machining accuracy, the shape of the groove

was evaluated using image measurement software. The actual width of the machined groove

is approximately 560 µm and the depth is approximately 200 µm. The desired shape is

500 µm wide and 200 µm deep. Both tool runout and the difference of shrinkage between

end mill and PDMS under ultra low temperature causes the machining error. The tool

runout was nearly 20µm in this test, measured statically by dial gauge with resolution of

1µm. Therefore, it is necessary to suppress the tool runout and consider the difference of

shrinkage between tool and workpiece for accurate machining.




565

Vol. 2, No. 4, 2008

Top view

100µm100µm100µm

100µm100µm100µm 66.6µm66.6µm66.6µm

100µm100µm100µm

Top view

Side view

(a) At room temperature

Side view

(b) At ultralow temperature

Figure 9: Shape of machined groove (PDMS)

Table 3: Cutting condition for evaluation of machining characteristics

at room temperature and at ultralow temperature

Rotational speed

Feed rate

Feed per tooth

Depth of cut

Tool

Diameter of tool

Workpiece

Machine tool

5000min-1

1.0,10.0,50.0,100.0mm/min

0.1, 1.0, 5.0, 10.0µm

100µm

1000,5000,10000,20000min-1

10.0mm/min

5.0, 1.0, 0.5, 0.25µm

100µm


0.5mm

PDMS

3-axis vertical machining center

5000min-1

10.0mm/min

1.0µm

10,50,100,200µm

Feed rate change Rotational speed change depth change

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 0.01 0.02 0.03 0.04

Time s

C u t t i n g f o r c e i n f e e d d i r e c t i o n N

100.0mm/min

1.0 50.010.0

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

0 0.01 0.02 0.03 0.04

Time s

C u t t i n g f o r c e i n f e e d d i r e c t i o n N

100.0mm/min

1.0 50.010.0

Figure 10: Behavior of cutting force at various feed rates

5. Machining characteristics

5.1 Influence of the feed rate on the machined surface

The milling test was carried out at feed rates of 1.0, 10.0, 50.0 and 100.0 mm/min. The

cutting conditions for evaluation of the machining characteristics are shown in Table 3.

Cutting tests to investigate the influence of the feed rate were performed at a rotational

speed of 5000 min-1

for a depth of cut of 100 µm. Figure 10 shows the cutting force in feed

direction at each feed rate. The faster the feed rate is, the larger the cutting force. Figure 11

shows the machined surface at each feed rate.




566

Vol. 2, No. 4, 2008

(a) Feed rate, 1.0mm/min

55.5 m55.5 m55.5 m

55.5 m55.5 m55.5 m 55.5 m55.5 m55.5 m

55.5

m55.5

m55.5

m

(b) Feed rate, 10.0mm/min

(c) Feed rate, 50.0mm/min (d) Feed rate, 100.0mm/min

Figure 11: Influence of feed rate on the machined surface

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 51.0 10.0 50.0 100.0Feed rate mm/min

S u r f a c e r o u g h n e s s R a

m

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 51.0 10.0 50.0 100.0Feed rate mm/min


m

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 51.0 10.0 50.0 100.0Feed rate mm/min


m

Figure 12: Relation between surface roughness and feed rate

While the cutting mark is confirmed on the bottom surface at the 10.0, 50.0 and 100.0

mm/min feed rates, no cutting mark is observed at the 1.0 mm/min feed rate. The cutting

mark becomes more distinct as the feed rate increases. The bottom surface roughness was

measured using the surface profiler (New View 6200, Zygo). The relation between the

surface roughness and the feed rate is shown in Fig. 12. It is clear that the surface roughnessdecreases with the feed rate.

5.2 Effect of the rotational speed on the machined surface

The milling test was also carried out at various rotational speeds of 1000, 5000, 10000 and

20000 min-1

. The feed rate and depth of cut were set to 10 mm/min and 100 µm,

respectively. Figure 13 shows the machined surface at each rotational speed. As the

rotational speed increases, the cutting marks gradually disappear. The relation between the

surface roughness and the rotational speed is shown in Fig. 14. It is clear that a high

rotational speed is appropriate in PDMS cutting.




567

Vol. 2, No. 4, 2008

(a) Rotational speed,1000min-1

55.5 m55.5 m

55.5 m55.5 m 55.5 m55.5 m

55.5 m55.5 m

(b) Rotational speed, 5000min-1

(c) Rotational speed, 10000min-1 (d) Rotational speed, 20000min-1

Figure 13: Influence of rotational speed on the machined surface

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 51000 5000 10000 20000

S u r f a c e r o u g h n e s s

R a

m

Rotational speed min-1

0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 51000 5000 10000 20000


R a

m


0.000

0.050

0.100

0.150

0.200

0.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 51000 5000 10000 20000


R a

m


Figure 14: Relation between surface roughness and rotational speed

5.3 Relation between the cutting depth and form of machined grooveFigure 15 shows the cutting forces in the feed direction at various depths of cut of 10, 50,

100 and 200 µm. It is clear that the deeper the cutting depth is, the larger the cutting force.

Figure 16 shows the micrograph of the cross-sectional shape and Fig.17 shows the bottom

surface roughness of the machined groove, respectively. The shapes of all the machined

grooves are precisely formed. However, for cut depths of 100 and 200 µm, the surfaces

become rough. Image measurement software (KEYENCE) was used on the micrograph

shown in Fig. 16 to measure the cutting depth and width to investigate the machining

accuracy. The results of the machined depth and width are shown in Table 4. It is clear that

the machined depth is almost the same as the desired depth, although a noticeable error of

approximately 10% is confirmed in the machined width.




568

Vol. 2, No. 4, 2008

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0 0.01 0.02 0.03 0.04

Time s

C u t t i n g f o r c e i n f e e d d i r e c t i o n N 200µm

10100 50

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

0 0.01 0.02 0.03 0.04

Time s

C u t t i n g f o r c e i n f e e d d i r e c t i o n N 200µm

10100 50

Figure 15: Behavior of cutting force at various depth of cuts

Table 4: Depth and width of the machined groove

Set depth m Machined depth m Machined width m

10 12.8 560.3

50 47.4 564.1

100 103.8 556.4

200 201.3 559.0

(a) 10 m depth

66.6 m66.6 m

66.6 m66.6 m 66.6 m66.6 m

66.6 m66.6 m

(b) 50 m depth

(c) 100 m depth (d) 200 m depth

Figure 16: Cross-sectional view of the machined groove




569

Vol. 2, No. 4, 2008

0.000

0.050

0.100

0.150

0.2000.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 510 50 100 200

Depth of cut µm


m

0.000

0.050

0.100

0.150

0.2000.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 510 50 100 200

Depth of cut µm


m

0.000

0.050

0.100

0.150

0.2000.250

0.300

0.350

0.400

0.450

0.500

0 1 2 3 4 510 50 100 200

Depth of cut µm


m

Figure 17: Relation between surface roughness and depth of cut

6. Conclusion

Cryogenic micro milling of PDMS is proposed for the fabrication of microfluidic chips. A

special jig with a liquid nitrogen chamber was developed as the cooling system for

cryogenic machining. The validity of the proposed milling technology was evaluated by

several cutting tests. The results are summarized as follows.

1) With cryogenic cooling, PDMS can be machined and a micro groove can be shaped

precisely. The machining process works on PDMS at temperatures below the glass

transition temperature.

2) Under the cutting condition of a lower feed rate, cutting marks disappear and the

machined surface becomes fine. Especially at a feed rate of 1.0 mm/min, the machined

surface roughness achieves Ra=80 nm.

3) When the rotational speed is higher and the depth of cut is smaller, the machined surface

is also improved.

Since PDMS has the lowest glass transition temperature of polymer materials, it is

suggested that the cryogenic cutting process with liquid nitrogen will also be useful for

micromachining of other elastic polymer materials.

Acknowledgement

The machining center used in the research has been provided by Machine Tool

Technologies Research Foundation (MTTRF) and a part of this research is supported by

Machine Tool Engineering Foundation. Authors would like to express our appreciation for

these supports.

References

(1) Gracias A., Xu B. and Castracane J., Fabrication of Three-Dimensional

Micro-Channels in SU8, Proceedings of 9th International Conference of Miniaturized

Systems for Chemistry and Life Sciences, (2005) pp. 663-665.

(2) Bundgaard F. et al., Rapid Prototyping Methods for All-COC/Topas Waveguides and

Microfluidic Systems, Proceedings of 9th International Conference of Miniaturized

Systems for Chemistry and Life Sciences, (2005) pp. 1200-1202.

(3) Yokota K., An Introduction to Polymer materials, KAGAKU-DOJIN PUBLISHING

CO. 1999, pp. 95-101, 143-161.

(4) Hira S., Micro-cutting of Polytetrafluoroetylene (PTFE) for Application of

Micro-Fluidic Device, Key Eng Mater, Vol.329, (2006), pp. 577-582.

(5) Matsumura T., Machining System for Micro Fabrication on Glass, Proceedings of 2004

JUSFA, (2004), pp. 1-6.

Micro Rubber Milling

Documents

Transcript of Micro Rubber Milling