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As with macroscopic PIM, the removal of the binder is a critical step in the MicroPIM process, both from the technical and the economic points of view In general, it can be performed by thermal melting or degradation, solvent extraction or catalytic decomposition of the binder components, or by a combination of these methods.
In the case of so-called microstructured parts (i.e., devices of relatively large overall dimensions but with microscale features on the surface), the debinding times, which depend on the wall thickness of the part, have the potential to limit the economic viability of the whole process. For microparts with wall thicknesses significantly below 1mm, however, debinding times play a minor role. Here the interaction of the material with the sintering tray and the shape conservation of filigree details are more important than shortening the debinding cycle. Furthermore, one run in a typical debinding furnace usually produces a large number of parts, meaning that operational capacity is very high even if debinding times are quite long.
Again, similarly to macroscopic PIM, the sintering conditions depend mainly on the type of metal processed and parameters differ little from the usual ones. Owing to the high specific surface of the fine powders for MIM, holding times, as well as peak sintering temperatures, can be reduced for microparts. On the other hand, to avoid exorbitant grain growth, the heating and cooling procedures usually have to be accelerated. Heating and cooling rates, however, can only be increased to values which will not cause distortion effects.
Batch furnaces running under reducing atmospheres (H2 or N2/H2) or vacuum are quite common. During the sintering process the parts experience a linear shrinkage of up to 23% due to the comparably lower powder loadings. Values for densities reached are in the range 95%-99% of theoretical density.
Metrology and quality assurance are crucial matters in microtechnology. When measuring the outer dimensions of green bodies and sintered parts one can rely on the measurement systems developed for application in microelectronic or microelectromechanical systems/microopto-electro-mechanical systems (MEMS/MOEMS) fabrication. For example, test stands based on coordinate measuring machine (CMM) units, or even white light interferometry and atomic force microscopy (AFM) are in use for geometrical inspection. These test systems are quite expensive. If an automated system is built up, it can represent a useful quality inspection system. Reliable measurement and quality inspection is an important matter for microtechnology. As a result, a lot of research and development approaches are working in this field and no additional efforts are necessary for MicroPIM.
Much more complicated is the "view into the body" to determine microcracks, cavities, and areas of powder/binder segregation . This internal inspection has to be carried out rapidly, preferably online, to avoid excessive failure production. Therefore, the classical way of cutting and grinding, and the subsequent optical investigation of the cross section, is not the most effective method of testing. Alternative methods, such as ultrasonic inspection and/or thermographical testing, show much more promise in terms of performance potential. As they are already under advanced development, and are already in use for macroscopic PIM, they are not described in detail here.
Finally, the two- and three-dimensional inspection methods based on X-ray irradiation should be mentioned. MicroMIM parts profit by their small thicknesses, meaning that they can be irradiated without thorough energy dissipation and beam widening.. Using monochromatic synchrotron radiation a three-dimensional profile of the powder distribution over a whole MicroMIM sample can be generated and the determination of powder/binder segregation phenomena becomes possible. Nevertheless, such investigations are costly and time consuming, meaning that faster and less complex variants have to be derived for industrial applications. It is essential that such developments continue as the down-scaling of functional test procedures has the potential to optimize the production of microparts and microsystems.
As in macroscopic PIM, handling, automation, and the interfaces of the relevant production facilities play an important role in MicroMIM. Existing or soon to be developed tools can be used for MicroMIM on the condition that they are adapted to the desired small dimensions. This is a challenge for the whole micro fabrication world and considerable research and development efforts are underway from which MicroPIM will also benefit.
In the case of singular microparts, the precise positioning of gripper to part is essential as tolerances are 1μm or less. This positioning of gripper to part has to be considered thoroughly during process planning. This is also true for automated quality assurance. In the case of MicroPIM, the relatively low green strength, which might cause problems if mechanical grippers are used, has to be considered. Similarly, the higher weights due to the powder loading can be a disadvantage in the case of vacuum grippers. On the other hand, metal-filled components reveal some advantages for handling. For example, unlike plastic ones, they are not charged electrostatically. Thus, MicroMIM has an advantage over polymer microinjection molding as regards the easier gripping or moving of parts.
Microcomponent PIM is an attractive method both from the technical and economic points of view. It is not surprising, therefore, that major research efforts are not limited to a closely restricted circle of scientists. In recent years, there has been an increase in MicroPIM research in East Asia. An example for industrial application is given by Fig. 15.5.
Fig. 15.5 Gear wheels for linear motor guides to be applied in industrial machines. Diameter 2.95mm, module 0.3mm. Material 17-4PH, weight 0.03 g. With this module and diameter, it was not possible to mass produce such parts by machining.
Among the research activities on MicroPIM in North America during recent years were analyses of ultrasonic PIM feedstock flow behavior . Economic studies and MicroCIM experiments, with an emphasis on the molding of wireeroded mold inserts using commercial aluminum oxide feedstocks (Catamold AOF), have also been carried out. The pronounced powder-binder segregation that occurred during these experiments may be assumed to have been due to the neglect of variothermal process control. Comprehensive studies on MicroPIM, for example on the production of medical components, have also been performed at Los Alamos National Laboratory.
In Europe, intensive research on MicroPIM has been carried out at Fraunhofer Institut IFAM in Bremen, Germany. The researchers developed microstructured metal components for heat sinks. WCu and MoCu had been chosen due to their high thermal conductivity and low coefficient ofthermal expansion. Further research and development approaches deal with the processing of biocompatible materials for implant applications. For example, a special 316L powdermixture has been created bythe addition of ultra-fine and nanoiron fractions. This mixture enabled the replication of microstructured surfaces with hemispheres of 5μm diameter.
FOTEC Research and Development GmbH in Wiener Neustadt, Austria, had developed an advanced X-Cooler to be used for central processing unit (CPU) temperization. The design was characterized by thin Cu tubes located on a carrier plate, all manufactured by MIM using pure Cu feedstocks. The wall thickness of the Cu tubes was 0.3mm at a height of 30mm, thus, a record holding calculative aspect ratio of 100 had been achieved. More material related, and not exclusively focused on MicroMIM, are investigations for processing of hard magnetic powders. It should be noted here that PIM of filigree precision parts or microparts is not confined to research but has already been adopted by the industry.
The joining and assembly of singular microcomponents into complex products is carried outthrough extremelyintricate and quite expensive processesthat risk causing damageto the often filigree structures of the individual parts. Problems of this kind can be reduced considerably by combining single-component molding with a joining step, as is done in 2C-MicroPIM. It is a particular challenge of 2C-PIM that, in addition to joining two different materials during the molding process, a defined bond between these materials must be maintained during debindering and sintering. To ensure optimum procedures, the debindering and sintering behaviors of the feedstocks and powders must be coordinated and adapted precisely regarding quantities, morphologies, and thermal and chemical behavior.
With the intention of adapting PIM techniques to the requirements of microdimensions, a joint research project was carried out in Germany some years ago: "2KPIM" was aimed at developing multicomponent PIM for the manufacture of microparts or microcomponents from two types of metal or ceramics and with corresponding different local properties and functions. Both partners, the Karlsruhe Institute of Technology and Fraunhofer Institut IFAM, hadto copewith the task of developing suitable materials and process technologies, placing emphasis on the identification of appropriate material combinations and feedstocks for microapplication, and the development of an injection molding, debindering and sintering method suited for multicomponent parts. Within 2K-MIM, the composites 17-4PH/316L (Fig. 15.6) and iron/316L, two soft magnetic/nonmagnetic composites, were developed successfully and used to manufacture two demonstrator specimens. The results obtained for iron/316L showed that different materials, with strongly differing sintering characteristics, can be tailored for joint sintering.
Fig. 15.6 2C-MicroPIM demonstrator consisting of nonmagnetic 316L and magnetized 17-4PH steel.
2C-PIM allows the manufacture of both fixed and movable structures if use is made of the powders' different sintering behaviors. The necessary preconditions can be provided by adapting the powder contents and selecting either sinter-active powders or powders that are inert to sintering. In addition, care must be taken to choose temperature programs that are adapted to the requirements of the respective materials and to use sintering underlays that are suited, for example, to free sintering of movable structures.
While fixed structures require equal sintering shrinkages and temperatures to avoid residual stresses or even self-destruction during compaction, movable structures require separation of the partial volumes during sintering as well as different powder contents and sintering temperatures. This ensures that shrinkage of one component sets in prior to that of the other component. Macroscopic movable 2C-PIM components have been developed for some time.
Typical two-component injection molding that blends at least two different melt flows in a cavity is not the only method available for PIM manufacturing of multicomponent parts. Equally promising results can be achieved combining in-mold labeling with PIM processes (IML-MicroPIM). PIM feedstock is injection-molded around a green film placed in an injection molding tool and filled with metal or ceramic particles. Both partial volumes are debindered and sintered at the same time to obtain a material composite. The material and process conditions are similar to those selected for 2C-MicroPIM of fixed structures.
While macroscopic methods of that kind have been developed for sometime,the novel microscopic applications offer further possibilities. Extremely fine (nano-) particles can be added to the film feedstocks without the workability being affected by the increase in viscosity. This means that functional particles or nanoparticles can be applied to surfaces of metal or ceramic components while the components' three-dimensional bodies themselves are manufactured at lesser costs by means of common PIM.
Micro in-mold labeling using PIM feedstocks has been developed, for example, within the framework of the EU Multilayer project (FP7-NMP4-2007-214122). Early interesting results were obtained with the successful injection molding of zirconium oxide-filled ceramic films on green parts of another zirconium oxide type. The solid, mostly pore-free structures of the material composites were maintained during debindering and sintering. The microstructures, imprinted in the outer surface of the film through the injection pressure, remained intact after debindering and sintering. The micro in-mold labeling tests described earlier were performed using mainly ceramic and metal/ceramic materials. Further research and development activities showed that the same processes can be used on purely metallic films or feedstocks.
Composites can also be produced by sinter joining; that is, by joining green parts after cooling and demolding. Basically, two or several green parts are assembled, debindered and, finally, sintered into a fixed structure. Since green parts can be assembled in various different ways (for example automatically by use of modular computer-assisted robots), this method is of considerable geometric flexibility. In addition, sinter joining may serve to manufacture components with undercuts or hollow parts with inside inserts.
One disadvantage of sinter joining is that the sintering of green parts, especially those of different powders or materials, may not be easy once they have been cooled and demolded. It is advisable, therefore, to provide subcomponents with joining elements that ensure positive locking. If necessary, the green parts that have been assembled may be weighted during sintering to obtain tight joints. Another drawback of sinter joining as opposed to multicomponent injection molding is that the adjustment and assembly of the parts is more intricate.
As the challenges and solutions related to a meaningful simulation of the PIM process are thoroughly described elsewhere in this chapter, at this point only the microspecific considerations shall be discussed. In general, designing a MicroPIM part follows the same basic rules as for macroscopic PIM. It should be mentioned here that, since it is very difficult to modify or alter the microstructured molding tools, computer programs would be extremely useful for avoiding errors at the design stage. Further remarkable differences between the macro- and microworlds are the increasing heat losses due to the larger surface-to-volume ratio and the (usually detrimental) influence of relatively high shear rates. Additionally, the limitations given by micromold insert production and powder size have to be considered.
Although widely in use, commercial simulation programs have weaknesses that limit their reliability for predicting typical MicroPIM effects. Owing to the singlephase material models, the phenomena of segregation cannot be simulated unless additional features are provided. Moreover, certain properties of the metal particles, for example higher inertia due to higher density, cannot be assessed sufficiently and the typical effects of PIM, such as the disproportionate formation of strands and folds, wall friction, and yield points, are often quite difficult to define correctly.
Since shear rates are often above average and changes in cross-sections of the flow channels can be quite abrupt, microinjection molding applications are more affected by the weaknesses described earlier than macroscopic PIM. Moreover, the higher surface/volume ratios of microcomponents must be assumed to aggravate all surfacedependent effects. With this in view, the principles described later are particularly relevant for MicroMIM. It must also be mentioned that commercial software tools do not consider any of the special microinjection molding methods such as variothermal process control or tool evacuation. Modified or even new simulation tools, adapted to the needs of microtechnology, must be developed in the future.
Tests had been performed at Karlsruhe Institute of Technology to find out whether it makes sense to use the popular commercial programs to simulate microinjection molding processes. Components with certain test geometries were defined and a suitable injection molding tool was manufactured to obtain and compare the calculated values and experimental process data for an exemplary part. The first experiments using data not specific to the feedstocks revealed considerable discrepancies between the quantitative simulation data and the measured values. The pressure that is actually required for complete molding, for example, was found to be above the calculated pressure by up to 100%. Results turned out to be much better and to deviate by <10% when measuring and using specific feedstock data. It was also found that fluid mechanical processes, such as mold filling, splitting and convergence of melt flows, cannot always be calculated qualitatively in advance. With this in view, profound knowledge and experience are required in order to interpret simulated calculations. Although the process of injection molding is not described exhaustively, the specifically determined material data and experienced interpretation of the results obtained already allow for a limited utilization of simulation tools within MicroPIM development projects.
Thorough and reliable simulations of MicroPIM processes are unlikely to be achieved without multiphase material models. Several research facilities have developed approaches that may turn out to be attractive. A quite promising approach for simulating MicroMIM has been evaluated at IMTEK, University of Freiburg. The scientists generated a new material model based on the smoothed particle hydrodynamics (SPH) method. To enhance this model for sufficient MicroPIM prediction they incorporated two new features, an inherent yield stress and shearinduced powder segregation. The first has been performed by means of a bi-viscosity approach. For the simulation of shearinduced powder segregation during injection, three important effects have to be considered: the migration of powder particles toward areas of lower concentration, which is caused by different collision frequencies; the migration of powder particles toward areas of lower shear rates; and the migration of powder particles toward areas of lower viscosities caused by viscosity inhomogeneities. After incorporating these approaches into the SPH model, a correct prediction of powder distribution in MicroMIM parts was achieved as proven by computer tomography (CT) measurements using synchrotron radiation.
Development of entirely new calculation routines, however, is costly, so commercial software suppliers usually tend to modify their existing tools. One rather new approach is to implement specific equation terms describing the collision- and viscosity-induced flow behavior of the powder particles in the organic fluid. By this improvement, a significantly better estimation of the risks of shear-induced powder-binder segregation could be achieved so up to a certain extend an optimized filling procedure becomes feasible. As further benefit, by following this so-called autonomous optimization approach, it became possible to optimize part design and parameters by a reduced number of simulation runs.
In summary, MicroPIM is a key technology that is suited to the manufacture of medium-sized to large quantities of heavy-duty microcomponents of differently complex geometries by use of metals, metal alloys, hard metals or ceramics. It is primarily due to the higher availability of finest ceramic powders that high aspect ratio microcomponents are predominantly made from ceramics. Metal powders made up of micrometer- or submicrometer-sized particles, however, can also be used for manufacturing precision mechanical components or microparts that require little or no reworking. Industrial applications are already available on the market.
Probably the most important demand for research and development is the comprehensive investigation of the material-parameter-properties interdependencies. This covers many singular effects including powder-binder segregation, jetting and wrinkling, grain growth and size distribution, and distortion during sintering.
Directly related to the exploration of how materials and parameters effect green and sintered parts performance is the improvement of analytical methods including the detection of defects such as microcuts and cavities. Equally as important is the demand for the monitoring of powder distribution in the green bodies to detect areas in danger of distortion during thermal treatment as early as possible.
Much as in the case of macroscopic PIM, improvements in dimensional accuracy and surface quality are necessary optimizations in the case of MicroPIM. Mostly, one tries to achieve enhancements by reducing the size of particles through use of particularly fine (for instance screened) powder fractions. Since the material costs are low enough that they do not significantly raise production costs, MicroPIM can be assumed to have advantages over the macroscopic methods. Dimensional reproducibility through minimization of the tolerances between the sintered part and nominal dimensions is known to play a significant role in industrial series production. In this regard, much research goes into optimizing feedstocks, placing emphasis on studies of suitable dispersants for coupling powder particles and organic binders. Investigations on the wear behavior and long-term stability of tools and mold inserts are scarce and have to be expanded. Among the future trends observed at present for MicroPIM is the enlargement of the range of suitable materials for the development of future fields of application. Tungsten or titanium are typical examples. These trends are driven by the lack of adequate alternative procedures and by material savings compared to the ablating methods. Now as before, steel, mainly 316L and 17-4PH, seems to be the material mostly used for micromechanical applications. At the same time, the use of copper or copper-diamond is expected to increase owing to the material's high electrical and thermal conductivity. The latter, of course, might result in unintended short shots.
Other important trends are the multicomponent manufacturing methods including 2C-MicroPIM, lost-core technology, micro in-mold labeling or sinter joining of metal or ceramic microparts. Further intensive research and development is expected due to these methods' particular attractiveness to microsystems technology. 2C-MicroPIM has, with the use of polymers, achieved a high technical standard which has already led to industrial applications and can therefore rely on efficient equipment and machinery in the future.
Both macroscopic and microscopic productions increasingly demand effective and realistic simulation tools enabling the modeling and calculation of multicomponent material systems for PIM and allowing the prediction of segregations and sintering deformations. Moreover, the relevant material models should admit unconventional flow profiles and consider typical PIM properties such as extreme strand formation. It is opportune that such software routines are already being developed to support the commercial programs and that novel research approaches are being taken to develop multiphase material models. Finally, it should be mentioned that holistic design rules, material data bases, standardized peripheric equipment, and the setup of PIM-specific standards are yet to be developed and it is hoped that these areas will open up opportunities for future research and investigation.
Contact: Cindy Wang
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Tel: 0512-55128901
Email: [email protected]
Add: No.6 Huxiang Road, Kunshan development Zone, JiangsuShanghai Branch: No. 398 Guiyang Rd, Yangpu District, Shanghai, China