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MICROMACHINING OF MOLDS FOR MANUFACTURING OPTICAL DEVICES E. Kuljanic1, S. Sinesi2, M. Sortino1, G. Cattelan2, G. Totis1 1
Department of Electrical, Management and Mechanical Engineering, University of Udine, Italy 2 Centro Ricerche Plast-Optica, Amaro, Italy
KEYWORDS: Micromachining, Molding, Optical Devices ABSTRACT. There is an increasing interest in the development of new methodologies for massproduction of micro optics components. Among other methodologies, micro chip removal processes are promising in the realization of complex shape molds. In this paper, the most important issues regarding application of micro machining processes such as microturning, micromilling and microgrooving for manufacturing molds for optical devices are discussed. The differences between conventional and micro machining processes describing the main characteristics of tools, workpiece materials and machine tools for micro machining are also discussed. In addition, two case studies investigated at the Centro Ricerche PlastOptica, Amaro, Italy, in collaboration with the University of Udine, are presented. The first case study illustrates the realization of brass inserts for molding of aspheric laser optics. The second case study describes the production of micro prism master for manufacturing optical components.
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INTRODUCTION
In the last years, the development of telecommunications, photovoltaic technology, medical imaging-diagnostics, surveillance systems and new illumination systems requires the manufacturing of optic devices whose elements could have dimensions in the range from 10 μm to 100 μm (often referred to as micro-optics or MesoOptics). However, the precision and efficiency of manufacturing, testing and metrology techniques available for micro-optics production at present time are very limited and there is a need for new instrumentation and methodologies. There are three processes available for mass-production of MesoOptics: hot embossing, used to produce micro structured plastic films (holograms, wearable reflex, retro reflectors for street application, ….), compression molding and injection molding, both applied to produce single optical components (compact optics for CCD, micro-diffusers, Fresnel lenses, …). Obviously, the core component in all these processes is the mold. Production of the mold is a complex activity which involves manufacturing the master and post-processing, which usually requires several techniques, such as electroforming, replication and others. Compared to microelectronics, where devices have generally almost bi-dimensional shapes and can be efficiently produced by conventional lithography processes, the structures in micro-optics are three-dimensional with high aspect ratio [1]. In addition, both surface quality and form accuracy are critical in order to achieve a good optical behavior of the device. Therefore, the application of lithography processes (including gray scale lithography, LIGA techniques and electron beam lithography) for manufacturing masters are less economical.
Therefore, there is an increasing interest for micro-machining techniques such as micro-turning, micro-milling (including fly-cutting) and micro-grooving for this application [2][3]. The paper presents recent developments in micromachining processes with several examples of application.
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MICROMACHINING
There are several issues concerning the application of mechanical material removal principles to micro scale machining, as follows [1] [3]: • the precision of positioning system of the machine tool and tool geometry must be adequate; • the cutting forces should be low in order to reduce the stress, strain and heating on the tool and workpiece. Therefore very small feeds and depths of cut are necessary; • since the ratio between workpiece grain size and uncut chip thickness h is close to one, there is a strong fluctuation of the cutting forces because of the influence of grain orientation; • the temperatures of the tool and workpiece should be low: coolant is applied and environmental temperature is controlled; • since the feed and depth of cut are low, the cutting edge radius of the tool rn must be very small in order to avoid the minimum chip thickness effect (h/rn > 5-38%, [Filiz, 2007]) and rubbing of tool on the workpiece surface, see Figure 1; • the waviness and the roughness of the workpiece surface are required to be minimum, therefore, the waviness of the tool should be low; • deburring is problematic for micro components, therefore, the geometry of the tool and cutting parameters should be designed in order to minimize the burr formation.
FIGURE 1. a) Conventional machining and b) micro machining
A comparison of requirements for conventional machining and micro machining is given in Table 1.
TABLE 1. Comparison of requirements for conventional machining and micro machining Quantity
Conventional machining
Micro machining
Cutting edge radius
0.01 mm
