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Fig. I. Schematic configuration of press forming and in-process assembly in a progressive die.
- Establishing new methods for fabrication and evaluation of die features smaller than 20 um,
- Keeping errors in dimension and allocation of the elements smaller than 1urn,
- Treatment of die surface for protection from wearing and degradation and for processes without lubricant,
- Removing burrs larger than several micron before assembly process,
- Miniaturization and digitization of press machine for reducing errors in vertical and horizontal positions during the processes.
Methodologies on fabrication of micro die in size of several ten micrometers and design of the press forming process with the assembly system will be discussed in chapter 3 and 4, respectively.
3. Fabrication of micro dies
3.1 Fabrication of micro die by combining machining and high energy beams
High-energy beams including short-wavelength laser beam and ion beam was introduced to the fabrication of micro die with feature of several micrometers. The short-wavelength Laser and ion beams, which are widely applied in semiconductor process, have ability of removing materials with the features of several micro- and nano-meters, respectively, while the conventional machining generally processes shape with features as small as sub-millimeter order. Here, we proposed a combination of the machining and high energy beam processes for fabrication of micro die in such manner to process profile of the die roughly by the machining and the smaller features by laser process and then, the shape with sub-micrometer or finish surface in nano-order by ion beam [3].
Fig. 2 shows photo of a micro die for punching micro parts with gear shape fabricated by using the combination. The micro gear has a diameter of pitch circle of200 urn and a module of20. After machining
the circle shape of punch, the micro gear with the feature of about 20 urn was fabricated by combination of short pulse laser and micro electrical discharge processes. Furthermore, the ion irradiation was applied to remove the traces on the sidewall and micro cracks on the leading edge. The detail conditions for the fabrication of the shape and finish of the surface were appeared in our previous works [3], [4]. It seemed that the surface roughness of punch was improved after the irradiation. For micro-die, improvement of surface roughness and mechanical properties is important for protection from wear and breakage due to increase in stress concentration
Fig.2.Surface treat for micro gear shaped punch by ion beam irradiation (800V,1.65mA,1 hr,incient angle 45 degree.)
3.2 Die surface coating using ion DLC film
The authors also coated a DLC film on the die for reduction of friction and protection from wear [4].In the case of metal forming in micro scale, the die bears larger stress on the shoulders or the leading edges than that in macro scale so that the DLC coating becomes easier to be damaged and the die becomes easier to be worn or broken. The authors evaluated the mechanical and adhesive properties of DLC coating due to the concentration of stress/strain by using a nano-indentation test, and tribological property depended on size by using a micro-bending test [4]. The results show that the DLC film coated with certain conditions could bear average stress as high as 10 GPa without delamination or breakage; the DLC film with larger hardness and elastic modulus show stronger wear-proof property but easier to delaminate; on the contrary, the DLC film with smaller hardness and elastic modulus show stronger adhesion strength with substrate but easier to wear. From the results of the wearing test, a DLC film with gradient properties by combining different coating conditions could be effective on both wear-proof and lower friction. In this study, a two-layered DLC film was coated on the substrate and the tribological property of the film was evaluated. Fig. 3 shows that the results for DLC film with gradient properties. It seemed that the significant damage was not observed after more than 50000 times for the die with radius of 600 urn. The results show that the DLC film with gradient properties is practical for real production.