Bulk Metallic Glass Multiscale Tooling for Molding of Polymers with Micro to Nano Features: A Review
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INTRODUCTION
INJECTION molding is a long-established process for the manufacture of polymer components, of sizes in the range of 10 3 to 100 m and with surface features as fine as 10 4 m. However, there is a growing market for components for microengineering applications or for multiscale components in the 10 3 to 10 1 m size range with submicron surface features. The major applications are for MEMS sensors and microfluidic devices.[1] The global microfluidics market in 2009 was estimated at $2.1 b, growing at 13.5 pct p.a.[1] Polymer is and will remain the main substrate for microfluidic applications, and the market size for plastic microfluidics alone is forecast to be worth $2.5 b by 2016,[2] and clinical pointof-care diagnostics will account for 75 pct of this. For these reasons, this article is primarily concerned with enabling technology for production of the single use disposable microfluidic Lab-on-Chip (LoC) chemical and bio-chemical analysis devices needed for such diagnostic applications. As the devices will be used for rapid diagnosis on-site at, for example, a general medical practice, they will need to be mass-produced and of an inexpensive material: they will be of injectionmolded polymer. There is therefore a need to develop durable tooling in which to mold such polymer microbio-chips invested with the fine features available via lithography to silicon and glass. Other applications for molded components with surface features down to DAVID J. BROWNE, Senior Lecturer, DERMOT STRATTON, Graduate Student, MICHAEL D. GILCHRIST, Associate Professor and Head of School, and CORMAC J. BYRNE, Postdoctoral Researcher, are with School of Mechanical & Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland. Contact e-mail: [email protected] Manuscript submitted May 30, 2012. Article published online September 27, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
10 5 m in size include microlens arrays for LED illumination[3] and micro-gears.[4] In life science applications, molded LoC devices will be required with surface features of detail finer than the 10 5 m representative of a red blood cell—i.e., further down the ‘‘biological ruler.’’[5]
II.
TOOLING REQUIREMENTS
The current generation of microfluidic devices have channel dimensions in the range of tens to hundreds of micrometers. The requisite mold tool materials need to be amenable to being micro-machined to produce such patterns in negative, and must be resilient i.e., resistant to wear or other forms of surface or structural degradation, including those relating to thermal fatigue, over several thousands of molding cycles. However, tool steel—the workhorse of traditional injection mold tooling—is finding its limits in terms of minimum feature size as the number of microns in the scale becomes small.[6] This is due to the polycrystalline nature of tool steel, where even in the fine-grained alloys when the size of the surface feature gets down to the grain size, then the grain boundary and crystallographic misorientation effects start to inte
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