Microengineering of Metals and Ceramics: Special Replication Techniques, Automation, and Properties: 4 (Advanced Micro and Nanosystems) - Hardcover

Hardcover

ISBN 10: 3527314938 ISBN 13: 9783527314935

Verlag: Wiley-VCH Verlag GmbH, 2005

Inhaltsangabe

Microstructures, electronics, nanotechnology - these vast fields of research are growing together as the size gap narrows and many different materials are combined. Current research, engineering sucesses and newly commercialized products hint at the immense innovative potentials and future applications that open up once mankind controls shape and function from the atomic level right up to the visible world without any gaps. Continuing from the previous volume, authors from three major competence centres for microengineering here cover all aspects of specialized replication techniques and how to employ state-of-the-art technologies for testing and characterizing micro-scale components, and illustrate quality control aspects and strategies for automation of production procedures in view of future industrial production and commercialisation.

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Prof. Dr.-Ing. Detlef Lohe studied mechanical engineering at the Technical University of Karlsruhe, Germany and obtained his Ph.D. in 1980. After heading the microstructure and mechancal behaviour working group there, he was appointed in 1991 as professor for materials science at Paderborn University, Germany, where he received an award for outstanding teaching achievements in 1994. In the same year, he returned to the Institute for Materials Science and Enginering I at Karlsruhe Technical University as its Director. He is Speaker of the Collaborative Research Centre 499 "Design, production and quality assurance of molded microparts constructed of metals and ceramics" and has been a Senator of the Deutsche Forschungsgemeinschaft (DFG) since 2003. His research interests focus on metallic and ceramic materials properties and durability under different kinds of stress, component manufacture and behaviour, optimisation of heat treatment methods, and failure analysis. Prof. Dr.-Ing. Jurgen Haubetaelt studied Physics and Materials Sciences at the University of Erlangen, Germany. After his doctorate and a research stay at Stanford University he joined Degussa AG in 1977, starting in metals research. After having worked as technical director in Degussa-s subsidiary in New York City, he returned to Germany in 1985 and was first in charge of metals research, then managed the entire materials development und process technology of Degussa-s corporate division "Metals". In 1993 he joined Forschungszentrum Karlsruhe as head of the Institute of Materials Research III. In addition, he was appointed professor at Freiburg University as Chair for Micromaterials Process Technology at IMTEK in 1996. In 1998 he became member of the supervisory board of Norddeutsche Affinerie AG, Hamburg.

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The gateway to the micro and nano worlds: AMN provides cutting-edge reviews and detailed case studies by top authors from science and industry, covering technologies, devices and advanced systems. Together, these have an immense innovative application potential that opens up with control of shape and function from the atomic level right up to the visible world without any technological gaps.<br> <br> This and the preceeding volume cover all angles of micro-scale parts and components engineering from both metallic and ceramic materials, a very promising field which is a strong source of innovation and development for micro process technology, aerospace applications, sensors, actors, medical and dental as well as many other applications.<br> <br> In this volume, readers find casting and electroforming replication techniques, automation and quality control issues, and the characterization of the microengineered components.<br> <br> From the Contents:<br> Micro Casting<br> Micro Electroforming of Metals<br> Further Ceramic Replication Techniques<br> Automation PIM<br> Assembly<br> Quality Assurance<br> Metallic Materials<br> Ceramic Materials<br> Tribological Characterization of Mold Inserts and Materials for Micro Components<br> Development of a Simulation Tool for Wear in Microsystems<br> <br> Part I covers the introduction to this field and leads from the design and modeling aspects to tooling, molds, and micro injection molding as a powerful replication technology.

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Microengineering of Metals and Ceramics

Part II: Special Replication Techniques, Automation, and PropertiesBy Henry Baltes Oliver Brand Gary K. Fedder Christofer Hierold Jan G. Korvink Osamu Tabata Detlef Lhe Jrgen Hausselt

John Wiley & Sons

Copyright © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
All right reserved.
ISBN: 978-3-527-31493-5

Chapter One

13 Microcasting

G. Baumeister, J. Hausselt, S. Rath, R. Ruprecht, Institute for Materials Research III (IMF III), Forschungszentrum Karlsruhe, Germany

Abstract

Microcasting is a metal forming process based on the well-known lost-wax - lost-mold technology of investment casting. The further development of this technique for casting structures in the range of some tens of micrometers requires special patterns, investments and casting parameters. First, this chapter describes the general casting process, highlighting differences from conventional dental and jewelry casting. Additionally, the parameters of a typical microcasting process are given. Next, some alloys used for microcasting and their chemical compositions, melting and casting temperatures and phase transitions during the solidification process are described in detail. Thereafter, the two basic investments used for microcasting and the influence of the investment on the surface roughness of the cast parts are discussed. Finally, cast microparts are shown and their properties such as microstructure, dimensional accuracy, surface roughness, mechanical properties, smallest achievable structure size and highest obtainable flow length and aspect ratio are presented.

Keywords

investment casting; dental casting; gold base alloy; bronze; CoCrMo alloy

13.1 Introduction 358 13.2 Investment Casting 359 13.2.1 General Process 360 13.2.1.1 Process Description 360 13.2.1.2 Pattern Design 362 13.2.1.3 Melting 363 13.2.1.4 Casting 364 13.2.1.5 Solidification 365 13.2.2 Vacuum Pressure Casting 365 13.2.3 Centrifugal Casting 367 13.2.4 Example of a Typical Casting Process 368 13.3 Casting Alloys 369 13.3.1 Introduction 369 13.3.2 Gold Base Alloy as an Example of Precious Metals 369 13.3.3 Bronze as an Example of Typical Casting Alloys 371 13.3.4 CoCrMo Alloy as an Example of High Strength Materials 372 13.4 Investment Materials 373 13.4.1 Introduction 373 13.4.2 Phosphate Bonded Investments 375 13.4.3 Plaster Bonded Investments 377 13.4.4 Influence of the Investment on the Surface Roughness 378 13.4.4.1 Coating the Pattern 379 13.4.4.2 Infiltrating the Mold 379 13.4.4.3 Modifying the Investment 380 13.5 Cast Microparts and Their Properties 381 13.5.1 Examples of Cast Microparts 381 13.5.2 Microstructure/Grain Size 383 13.5.3 Dimensional Accuracy 385 13.5.4 Surface Roughness 386 13.5.5 Mechanical Properties 387 13.5.6 Achievable Structure Size, Flow Length and Aspect Ratio 388 13.6 Conclusions 390 13.7 References 391

13.1 Introduction

Microcasting is the manufacturing process of small structures in the micrometer range or of larger parts carrying microstructures by using a metal melt which is cast into a microstructured mold. Fields of application are, e.g., instruments for minimal invasive surgery, dental devices and instruments for biotechnology. Additionally, the manufacturing of miniaturized devices for mechanical engineering is a desired outcome.

At present, two different techniques for casting structures in the micrometer range are known: capillary action microcasting and microcasting based on investment casting. The first manufacturing method was developed by Bach et al. and Moehwald et al. They applied capillary action microcasting for form filling of structures in the range of some micrometers. Similar to die casting, this technique uses a permanent mold which can be opened in order to remove the cast structure. The cavities in the mold are shaped by high-precision grinding. For casting, two different principles to fill these cavities exist: the suction principle and the displacement principle. In the first case the melt is sucked into a specially coated mold by the capillary pressure. In the second case the casting alloy is melted inside the divisible mold and fills the microstructured cavities owing to the capillary force. Subsequently pressure is applied to the mold to displace the excess melt through the slit. Owing to absorption of the coating during solidification, the casting detaches from the mold's surface, but at the same time the alloy composition changes slightly compared with the original material. In capillary action microcasting the castable geometries are limited to structures which can be filled by application of capillary forces. Microcasting based on the investment casting technique, which will be discussed in the following, does not suffer from these limitations.

13.2 Investment Casting

Microcasting, also named microprecision casting, is generally identified with the investment casting process, a casting technology also known as the lost-wax, lost-mold technique. This forming process excels in near net shape manufacturing and is an established technology with great freedom in design. It offers the chance to produce very complicated formed parts in metal even with undercuts. Another advantage of the investment casting process over other shaping processes is the rapidity of the casting procedure itself and the low loss of material due to the possibility of recycling the runners and sprues. However, the process cannot be fully automated, so it is best suited for small and medium series and for parts with highly complex shape. This is the reason why investment casting has, in addition to technical application, a high relevance for jewelry and dental casting. For both applications, precise manufacturing is achieved, especially by using precious alloys. For jewelry and dental casting, the sizes of the produced parts are in the millimeter up to the centimeter range with structural details in the millimeter and submillimeter ranges. Further development and improvement of these techniques allowed the casting of microparts with structural details even in the micrometer range, which was confirmed by the replication of small-scale LIGA structures (see Section 13.2.1.1) with high accuracy. The new microtechnology, derived from the conventional production process, requires different pattern materials, other investments, special alloys and other casting parameters compared with the standard investment casting process. Additionally, microcast parts cannot be machined mechanically after manufacturing. Sand blasting, as applied in dental and jewelry casting, cannot be used to remove residue of the investment, nor can surfaces be polished to increase their quality. Sand blasting would reduce the sharpness of the edges and therefore influence the accuracy of the part, and polishing is not possible because of their small size. Therefore, precious alloys are particularly suitable for microcasting, because here the investment can be removed chemically from the cast metal part using hydrofluoric acid without influencing the cast part. Recent progress in the development of investments, however, opened the possibility of casting microstructures with bronze as a non-precious alloy. In the following, the typical microcasting process will be illustrated first. Later, details on the alloys and investments and variables specifically influencing this process are given.

13.2.1 General Process

The microcasting process, which is described in Section 13.2.1.1, has enormous potential in manufacturing microparts of high quality without the need for further processing, as opposed to the dental and jewelry casting technique. The patterns used in the microcasting process (see Section 13.2.1.2) guarantee a higher strength and are thus of advantage when assembling microstructures. Sections 13.2.1.3-13.2.1.5 give the basics on melting, casting and solidification.

13.2.1.1 Process Description

The microcasting process itself is based on the lost-wax, lost-mold technique. It is widely comparable to casting of dental protheses or jewelry. In contrast to the wax patterns used there, microtechnology mostly works with injection-molded plastic patterns which have much higher mechanical strength. The improved mechanical properties permit easier handling and assembling of the pattern during the manufacturing process.

The shaping of the microcavities in the mold insert, used for injection molding, can be achieved by several methods. We applied mainly the technique of micromilling, which is a further development and improvement of the standard milling process towards miniaturized manufacturing (see Chapter 4), but in some cases also microelectro discharge machining. More details on the latter process can be found in Chapter 7. Other ways for the production of microstructured mold inserts are the laser technique and the LIGA process. The LIGA process, which is described in detail in Chapter 8, includes a lithographic and a galvanic process and is beneficial for microreplication owing to the very good surface quality of the mold inserts and the high potential of generating minimum structures. However, in contrast to the milling process, which allows the production of free form faces and real 3D structures, the LIGA technique is limited to 2.5-dimensional structures because of their necessarily vertical walls.

The microcasting process requires a lost plastic pattern to be mounted on a gate and feeding system made of wax (Fig. 13-1). The assembly is then completely embedded in a ceramic slurry. This process differs from the technical investment casting process where normally a ceramic shell is built-up by repeatedly dipping the pattern in a ceramic slurry followed by stuccoing. After drying, the ceramic is sintered, resulting in a ceramic mold with high mechanical strength. Simultaneously, the plastic melts during the burning process and is pyrolyzed.

In order to fill the mold with the metallic melt, either the vacuum pressure casting or the centrifugal casting technique can be used. In the first case, the ceramic investment mold is evacuated, then the melt is poured into the mold, filling the cavity only due to gravitational forces. After that, pressure is applied to the melt. In the second case, the centrifugal force is used for form filling. Both techniques will be explained later in detail. After solidification, the investment is mechanically removed without destroying or influencing the cast surface. Depending on the casting alloy and the investment material, additional chemical cleaning processes may be sometimes necessary. Finally, the single parts are separated from the runner system. Unlike dental or jewelry casting, there is no further treatment such as grinding or polishing of the cast surface. This is due to the much smaller geometry and inapproachability of details on cast microparts and also to the necessity for high contour precision without any rounding of the edges.

Fig. 13-2 shows the most important replication steps for the example of a microturbine plate. On the left, the mold insert for injection molding of the pattern, the negative form, is shown. It is made by micromilling in brass. In the center is the injection-molded plastic turbine plate made of PMMA [poly(methyl methacrylate)]. This is the positive pattern required for microcasting. The plastic pattern is replicated by the investment and forms the negative mold. The third replication process - the real casting - yields the desired positive form, in this case made of a gold base alloy (right). It is worth mentioning that the replication is so precise that even scratches with depths of a few micrometers in the mold insert for injection molding are perfectly replicated on the cast part.

The investment casting procedure for manufacturing microparts is influenced by many different parameters. The most important ones are the casting alloy, the ceramic investment, the preheating temperature of the mold and the casting pressure. The molten casting alloys must exhibit a low viscosity in order to fill the small microstructures completely. Additionally, a minor tendency for oxidation is of high interest. For the ceramic investment, the most important factors are the ability for high-precision replication, an expansion behavior adjusted to the alloy used and a low surface roughness. The preheating temperature and the pressure influence the entire form filling process and, as a consequence, the achievable grain size and the resulting mechanical properties.

13.2.1.2 Pattern Design

For cost-effective casting, the assembly of single patterns in form of a so-called tree is necessary, whereas the design rules of good castability should be considered to allow homogeneous form filling of all mounted structures. In microcasting, single polymer patterns are normally fixed with wax. As an example, Fig. 13-3 a shows a pattern with 15 injection-molded polymer tensile test specimens fixed on a sprue system made of wax. In Fig. 13-3 b, the resulting cast part (gold base alloy) can be seen. Single microstructured patterns should be made at least with a small runner owing to the difficult handling of the small parts. Forming of complete plastic or wax assemblies is even better. Especially patterns which are injection-molded on a substrate plate proved to be advantageous because the substrate plate can be used as feeder. However, the melt flow in the plate is not easy to control. In industry, similar problems are solved by simulating the casting process. For microdimensions such specialized tools are not yet available.

Like patterns for macrocasting, patterns for microcasting should be constructed according to the well-known design rules for casting. In order to produce faultless patterns, different wall thicknesses and sharp edges should be avoided. Furthermore, the form filling process is of great importance. The cross-sectional thickness of the sprue system should increase in the direction of the sprue bottom, because solidification must begin in the microparts and end in the bottom of the tree. On the one hand, this design is beneficial for good form filling; on the other, it helps to avoid shrinkage holes in the casting. This design rule, however, does not normally cause any problems in microcasting, because the parts are generally distinctly smaller than the feeder and runner system. Nonetheless, the heat capacity of the mold should also be taken into account because the compact molds used for microcasting show a comparatively high heat capacity. This results in the inner part of the massive form being still hot while the surface cools rapidly after the mold has been taken out of the furnace. Therefore, thin-walled parts should be positioned in the outer and thick-walled parts in the inner area of the mold. The melt will then remain liquid in the thick-walled parts for the longest time so that they can work as feeder for the thinner parts. As mentioned before, an adequate sprue system is necessary in order to avoid shrinkage holes in the thick-walled parts. More detailed information on the sprue design is given in the literature.

A special aspect in microcasting is the flow behavior in very fine channels. Owing to the much higher surface to volume ratio in microchannels compared with macrostructures and the distinct influence of surface roughness, the occurrence of turbulent flow needs to be taken into account. Another aspect is the extremely high cooling rate and therefore extremely fast solidification in the small structures, which hinders form filling much more than in macrostructures. This aspect will be discussed in more detail in Section 13.5.6.

13.2.1.3 Melting

For casting in different atmospheres, various set-ups are available. Some casting machines work with vacuum, some with air and others with an inert gas atmosphere. Also the furnaces can vary. There is electrical resistant heating, heating by an open flame, induction heating and melting by an arc furnace.

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Details zur Ausgabe

• Verlag: Wiley-VCH Verlag GmbH
• Erscheinungsdatum: 2005
• ISBN 10: 3527314938
• ISBN 13: 9783527314935
• Einband: Tapa dura
• Sprache: Englisch
• Anzahl der Seiten: 306
• Herausgeber: Baltes Henry, Brand Oliver, Fedder Gary K., Hierold Christofer, Korvink Jan G., Tabata Osamu, Löhe Detlef, Hausselt Jürgen
• Kontakt zum Hersteller: Nicht verfügbar