The Development of Alternative Molds for Micromolding
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The Development of Alternative Molds for Micromolding Terry Garino1, Joseph Cesarano1 and, Alfredo Morales2 1 Ceramic Materials Department, Sandia National Laboratories, Albuquerque, NM 87185-1411 U.S.A.; 2Microsystems Processing Department, Sandia National Laboratories, Livermore, CA 94550 U.S.A. ABSTRACT Micromolding is a recently developed technique for fabricating parts in the millimeter size range with minimum features and dimensional tolerances less than 10 µm. This degree of precision is achieved by fabricating a master using the LiGA process. The molds made from the master are then filled with powder of the desired material to form parts that are then released from the mold and sintered to high density. Currently, molds are made of either poly(methyl)methacrylate (PMMA) or poly(dimethylsiloxane) (silicone rubber, PDMS). The PMMA molds must be dissolved to release the parts using a solvent that limits the choice of binder used to hold together the powder in the part. The PDMS molds are soft and therefore not suitable for lapping the parts prior to removal from the mold. Recently, two new types of molds materials have been investigated that can be removed utilizing a phase change, either melting or sublimation. The development of these new mold materials, which allow a wider choice of binders, will be described.
INTRODUCTION Micromolding is being studied as a technique to fabricate components with in-plane dimensions in the sub-millimeter to several millimeter size range up to several hundred microns thick, a size range between that of silicon micromachining and conventional miniature machining.1-5 Micromolding is a variety of the LiGA process where micromolds produced using electroplated LiGA masters are filled with a powder mixture or a pre-ceramic polymer. Thus micromolding has all the advantages of LiGA: high dimensional precision, high aspect ratio, vertical sidewalls and parallel production of numerous parts. In the micromolding process, a mixture of a nanopowder and a binder, or a pre-ceramic polymer, is first applied to the top surface of the mold, filling the cavities and forming a layer along the top surface of the mold. Then after drying or curing, the mixture is lapped to the top of the mold, if possible, and then released from the mold. The parts are then heated, first to thermally pyrolyzed the binder or the volatiles in the pre-ceramic polymer and then to a higher temperature to sinter the material to achieve high density. There are several reasons why micromolding may be preferable to electroplating, the most common process used in LiGA for producing metal parts. First, electroplating is limited to certain metals and alloys whereas micromolding can be used with any material available as a fine, sinterable powder. This includes ceramics as well as metals such as stainless steel. Even for metals that can be electroplated, micromolding may be preferable for alloys where compositional control during plating is difficult and for thicker components where plating times can be excessive. Also
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