No abstract

In this work the influence of the casting parameters on the microstructure and the mechanical properties of extremely small parts produced by microcasting was investigated. Different specimens were cast using an aluminum bronze with a composition defined by DIN EN 17 665. It was found that the yield stress and the ultimate tensile strength of micro tensile test specimens increase when decreasing the mold temperature from 1,000 to 700°C. This correlates with the increasing fineness of the microstructure. Additionally, the investigation showed that the mechanical strength of micro-specimens is higher than the one of macrospecimens cast at the same mold temperature due to a finer microstructure which hinders dislocation movement.

Microcasting is a competitive process for the production of metallic microparts. The remarkable suitability of this technique especially for extreme aspect ratio microsamples made of metal alloys will be shown. Based on the well known process of investment casting it is possible to produce microstructures by a gold basis alloy called Stabilor G Ò with an aspect ratio of up to 90. Basic investigations on the ability of form ®lling and¯ow length show that microcasting can be improved by a increasing of the molding pressure to about 25 bar. Basic results will be presented as well as a testing structure for estimating the quality of the casting process in regard of its suitability for microcasting of high aspect ratio parts. Introduction and state of the artThe production of components in the micro range is one of the leading technologies for the present century and is expected to have a high market potential [1]. The trend of miniaturization requires extremely small components for instance in minimal invasive surgery, biotechnology or information technology. They can be manufactured by various microsystem technologies as LIGA technique [2], silicon bulk and surface machining [3], laser ablation [4] and injection molding [5]. The process technologies powder injection molding [6] and microcasting [7] are under development.Microcasting is a suitable process for the shaping of microparts made of metal alloys with high mechanical strength and high ratio of¯ow length to wall thickness (aspect ratio). The process is an improvement of investment casting concerning the manufacturing of microstructures. This includes structures smaller than 1 mm or bigger ones but with extremely ®ne structures in the micrometer range. Like the investment casting process, microcasting is a lost-wax lost-mold process, which is known to obtain the best results concerning precision and surface quality [8]. The microcasting process consists of several steps shown in Fig. 1. First a plastic master is embedded in a ceramic slip. After drying the ceramic mold is sintered and during this process the master is lost by melting and burning. The preheated ceramic mold is then ®lled with a metal melt by centrifugal or pressure casting. Recent investigations on microcasting showed the ability to manufacture small components with complex geometry, lateral dimensions in the range of more than 50 lm and aspect ratios of about 8±9 [9]. Further improvements of the process can be achieved either by increasing the mold temperature which is detrimental for the surface quality [10] or better by increasing the molding pressure. Therefore basic investigations on the relationship between¯ow length and ®lling pressure were performed. These results are presented in this paper. Calculations concerning filling pressureIdealized calculations of the necessary ®lling pressure for the melt entering a cylindrical sample were done neglecting the increase in pressure losses along the¯ow length as well as the change of viscosity with decreasing temperature during the ®lling. Based...

Microcasting with a gold base alloy and an Al-bronze as a replication technique for LIGA structures is shown. A LIGA mold insert for injection molding is used to produce PMMA patterns as lost wax models in the microcasting process. Although there are several steps of replication the accuracy of the process is high and allows the fast manufacturing of middle size batches of microstructured parts. The mechanical properties can be varied within a wide range by choosing different casting alloys or cooling rates. Additionally the influence of the investment on the surface roughness is presented as well as results of flowlength tests showing maximal achievable aspect ratios with the gold base alloy and the Al-bronze.

The production of micro parts made of alloys like Al-bronze by casting techniques requires specially designed investments with sufficient mechanical and thermal stability. For damage free deflasking, good solubility of at least one component of the burned mold is required. Furthermore, especially when the micro parts are meant to mechanically interact with each other in a micro system, the surface roughness of the micro parts has to be as low as possible. With phosphate-based investments damages caused by mechanical removal of the investment cannot be avoided. Therefore, other systems that allow chemical removal of the investment have to be developed. An investment based on hard plaster loaded with fine-grained quartz powder and good chemical removability was investigated. The results showing the high performance of the optimized system will be presented.