Study on precision polishing of micro structured mold －Development of micro polishing system using piezoelectric actuator

incorporated with mechanical transformer－

Sze Keat Chee1, Hirofumi Suzuki2, Mutsumi Okada2, Kazufumi Yoshida3, Weimin Lin4, Takeshi Yano1, Toshiro Higuchi5

1Mechano Transformer Corporation, 2Chubu University, 3Nagatsu Precision Mold Co., Ltd., 4Akita Prefectural University, 5The University of Tokyo

Demands of precision molds with complicated microstructures for digital devices, µ-TAS and etc. are increasing

[1]. The structured molds must be polished after grinding or cutting in order to improve the surface roughness. In this report, a two dimensional low frequency vibration (2DLFV) polishing actuator using PZT is proposed and developed. The 2DLFV consists of 4 MechaTrans, a multiplayer stacked piezoelectric actuator (PZT) incorporated with mechanical transformer, and a center piece. Mechanical transformer is a kind of flexure hinges structured that magnifies the stroke of the multiplayer stacked piezoelectric actuator around 7 times to 120 µm. With this actuation, the tool vibrates in a circle trajectory to achieve polishing effect [2]. In the polishing experiments, tungsten carbide molds (work pieces) are polished with diamond slurry. The surface roughness of the work pieces and polishing efficiency are evaluated. From the experimental results, we found low frequency vibration especially 2DLFV is a useful micro polishing system for micro structured mold.

1. Introduction Recently, demands of precision molds with

complicated microstructures for digital devices, µ-TAS and etc. are increasing [1]. The structured molds need polishing process after grinding or cutting in order to improve the surface roughness. The precision polishing technology of structured molds is becoming more and more important due to the insufficient of the skilled worker in realizing the complicated micro-structured mold. In this report, a two dimensional low frequency vibration (2DLFV) polishing actuator is proposed and shown to be effective for precise structured surface polishing. 2. Configuration and fabrication of 2DLFV

The concept of 2DLFV is moving the tool in 2 dimensional directions such as circle with low frequency vibration to polish the work through abrasive material such as diamond slurry. Figure 1 shows the idea of polishing process.

The 2DLFV can be realized with many kinds of configuration of actuators. As an example, 2DLFV introduced here consists of 4 MechaTrans and centerpiece as shown in Fig.2. The polishing tool is fixed at the centerpiece. These MechaTrans are driven with a particular frequency of sine wave with 90 degree phase different to each other. This cause the polisher tool

vibrates in a circle trajectory. The trajectory can achieve polishing effect at the particular frequency [2].

Fig.3 shows a MechaTrans which its actuation is diagonal direction to the actuation direction of the PZT. The stroke of the PZT is magnified 7 times to 120 µm. Fig. 4 shows the fabricated 2DLFV. The 2DLFV actuates about 120 µm at around 900 Hz with applied voltage 15 V. The stroke measurement result is shown in Fig. 5. 3. Basic polishing characteristics

In order to study basic polishing performances, the Fig. 1 Concept of 2DLFV polishing Fig. 2 Configuration of 2DLFV

Fig. 3 Actuation of MechaTrans

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Fig. 4 Fabrication of 2DLFV

Fig. 5 Actuation of 2DLFV

Table 1 Major polishing conditions Work Binderless tungsten carbide Polisher head Radius Hardness

Polyurethane 0.5 mm IRHD 90 (International Rubber Hardness Degree)

Polishing load 5 mN Vibrating mode

Frequency Amplitude

circle 900 Hz 120 µm

Abrasive Grain size Density

Diamond slurry 0.5 µm 1 wt%

Time 1, 3, 5, 7, 10 minutes tungsten carbide workpiece was polished at a fixed position for 1, 3, 5, 7, 10 minutes under a polishing load of 5 mN. Polish conditions are shown in Table 1.

Fig. 6 shows a polished profile of a fixed point polishing results. The removal shapes are confirmed in convex in both X and Y direction, which is important in micro structured mold polishing. The removal depth of the polished point was measured and the relationship between the removal depth and the polished time was shown in Fig. 7. The removal depth is proportional to the polishing time. The maximum removal depth was 1.8 µm when the polishing time was 10 minutes. 4. Plano raster polishing experiments

Plano raster polishing experiments were conducted. The major polishing conditions were same as Table 1 and the raster tool path is shown as Fig. 8. The total polishing

time was 15 minutes. The polished surfaces were measured with ZYGO (NewView6200). The removal depth was 0.3 µm while the surface flatness was 12 nm PV as shown in Fig. 9. 5. Summary 2DLFV as a new polishing method is proposed and

proved to be efficient for precision polishing of micro structured mold. 6. Acknowledgement

This work is funded by Grant-in Aid for promoting the development of Strategic and Basic Technology for Advanced Assisting Project of small and medium enterprises (SMEs) (2009-2011), “Development of ultra-precision machining and micro fabrication technology for precise structured mold and high hardness die”. References [1] Brinksmeier, E. Riemer, O., Gessenharter, A.,Autschbach, L.,

2004, Polishing of Structured Molds, Annals of the CIRP, 53/1, 247-250.

[2] H. Suzuki, S. Hamada, T. Okino, M. Kondo, Y. Yamagata, T. Higuchi, 2010, Ultraprecision finishing of micro-aspheric surface by ultrasonic two-axis vibration assisted polishing, CIRP Annals-Manufacturing Technology, 59(2010), 347-350.

Fig. 6 Polished profiles at a fixed point

Fig. 7 Removal depth Vs polishing time

Fig. 8 Raster tool

path

12 nm P-V

0.3 µm

Fig. 9 Polished surface profiles measured

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