The development of two-component micro powder injection moulding and sinter joining

Two-component micro powder injection moulding experienced significant progress in the recent past. Starting as a manufacturing method for integrating two different plastics, the extension of two-component injection moulding (2C-IM) from pure plastics to more resistant materials like ceramics or metals (2C-PIM) provided sophisticated and challenging applications. With the transfer of 2C-PIM to micro systems, two-component micro powder injection moulding (2C-MicroPIM) was established. Up to a certain extend sinter joining is an alternative to 2C-PIM. It allows for component assemblies to be moulded as separate low-complexity parts which are then joined into complex assemblies. This procedure considerably reduces the time and cost required to manufacture the injection moulding tools. This article gives an overview of the development of 2C-MicroPIM??from two component plastic devices to the production of complex two-component micro assemblies made of two ceramic or metallic materials and the state-of-the-art of science and technology of sinter joining.     

New methacrylate-based feedstock systems for micro powder injection moulding

New methacrylate-based feedstocks have been developed according to fulfil the requirements of the micro powder injection moulding process chain, consisting of compounding, moulding and thermal post-processing for the realization of dense and warpage-free ceramic or metal parts. The different feedstock systems, consisting of polymethylmethacrylate (PMMA) as major binder component and microsized zirconia or stainless steel filler, were prepared by reactive compounding. The individual components—PMMA, solved in its monomer methylmethacrylate (MMA), additives and fillers—were mixed in a dissolver stirrer under ambient conditions and subsequently polymerized to a thermoplastic feedstock. Prior to injection moulding the mixtures’ flow properties were characterized by high pressure capillary rheometry as function of the composition, solid load and shear rate. After replication the obtained test specimen were debinded and sintered using optimized temperature programs. The achieved specimen densities reach 99% of the theoretical possible value for zirconia and 98.7% in case of stainless steel 17-4PH.     

Influence and limits of sintering temperatures on the movability of shaft-to-collar connections formed by two-component micro powder injection moulding

Two-component micro powder injection moulding (2C-MicroPIM) is a special variant of micro injection moulding. It enables the integration of two different functions in one micro assembly due to the combination of different materials. By 2C-MicroPIM it is possible to realise fixed and movable connections. For both applications, the two components have to be adapted well to each other. The adaptation has to cover the materials as well as the processes. In this context, especially the sintering process is of importance since the degree of shrinkage and physiochemical processes at the interface between both components determine either the formation of a fixed junction between the components or the formation of a gap. Both processes are temperature-dependent. Therefore, it can be assumed that by a certain temperature these processes can affect the interface in such a way that it may get in conflict with the formation of movable connections, i.e., there is a limiting temperature at which the components are not joined together. Above this temperature joining of the components occurs at least partially. For this study, shaft-to-collar connections were produced by 2C-MicroPIM. They consist of a gear wheel made of zirconia feedstock and an axle made of alumina feedstock. The 2C-MicroPIM process has influence on the characteristics of the micro parts, like functionality, density and mechanical properties. This work focuses on movability of the shaft-to-collar connection as the most important function. The sintering temperature required for movable connections constitutes important limits for the overall process. The aim of our presented work is to determine the limit temperature or temperature range at which movable connections cannot be formed due to the mentioned processes. Furthermore it was analysed if there is a smooth transition from movable connections to unmovable connections with increasing temperature or if this transition takes place within a small temperature range. Within a given temperature interval the portion of movable connections out of all sintered samples were counted for each peak temperature. The results showed that the number of movable connections decreases with increasing peak sintering temperature. Hence, production of movable connections has a limit with respect to sintering temperature. This temperature limit was found to be sharply defined at around 1,460?C1,470°C.     

Feedstock development for micro powder injection molding

Powder injection molding of microstructured parts with high aspect ratios requires feedstocks, which have a high mechanical stability for demolding. The binders of the feedstocks have to allow pressure free and complete debinding and sintering without deformation in the submillimeter range. For complete molding of especially small and complex detailed microstructures, powders with a small particle size have to be used. Additionally the microstructured mold inserts themselves must have an appropriate design, which allows complete filling of the cavity and an easy removal of the molded microstructures. By the development of new binder compositions, adapted micro mold inserts and optimized processing parameters it was possible to manufacture specimens for micromechanical investigations without substrate plates. Thus many machining and finishing worksteps, which have great influence on the mechanical properties of the microstructures, can be omitted. Received: 10 August 2001/Accepted: 24 September 2001     

Effects of material improvement and injection moulding tool design on the movability of sintered two-component micro parts

Two-component micro powder injection moulding (2C-MicroPIM) broadens the scope of possible microsystem applications since it facilitates the integration of two different materials and, hence, also two different functions in one micro assembly. As shown recently, not only two-component micro parts with fixed junctions can be produced, but, in principle, also parts with movable connections. This paper describes the difficulties associated with the realisation of shaft-to-collar connections with a movable junction. As an approach the gating concept of the injection moulding tool was changed. With the modified injection moulding tool, movable connections were obtained after sintering the micro parts to a maximum temperature of 1,450°C. Thus, the production of sintered movable connections does no longer require additional steps.     

New plastification concepts for micro injection moulding

Until now there are no suitable injection moulding machines available for the production of single micro parts, so injection moulders produce big, but precise sprues to achieve the necessary shot weight. Very often over 90 % of the polymer are wasted and this waste can be an important cost factor. Moreover, the big sprue increases cooling time and, along with that, cycle time. To open new dimensions for the minimum shot weight (<0.01 g), IKV, Aachen, Ferromatik Milacron Maschinenbau GmbH, Malterdingen, and Otto Männer Heißkanalsysteme GmbH, Bahlingen, with support of AGA Gas GmbH, Hamburg, have developed a new micro injection moulding machine. The very small amount of plastics needed is plasticised in an electrically heated cylinder and fed into the injection cylinder by a plunger. A second plunger with a diameter of just 2 mm injects the molten material into the cavity. It is driven by an electric motor and a precise linear drive. A central element for the operation of the machine is the nozzle. It is heated electrically and can be cooled down rapidly by injecting liquid CO₂ (TOOLVAC technology). In this way the gate is closed after the injection and the holding phase. The moulded part can be demoulded and a new injection moulding cycle starts. This processing technology reduces the sprue to about 15 to 20 mg. The ability to function of the thermal shut-off-nozzle could be proven theoretically by calculations with the IKV software CADMOULD/MEX. These calculations were verified by temperature measurements on a prototype machine. With the results of these investigations the design of nozzle and mould was optimized. After the injection unit and all the necessary drives and controlling devices have been integrated, the operation characteristics of the machine prototype are analysed. In the next step of the project, the focus is on improving the plasticising efficiency, because the prototype is equipped with a simply electrically heated plastification. For the thermal layout of the plasticising unit the recently developed 3D software SIGMASOFT was used. Moreover, different plasticising concepts are analysed, as there are a “Weißenberg”-extruder using the normal stress effect or plastification by ultrasonics. Especially the plastification by ultrasonics is a very promising idea for very small amounts of plastics and therefore analysed further. Ultrasonics are used successfully for wel- ding and riveting in plastics processing, as it is fast and neat bondings are produced. A test unit was built to prove the ability to operate as a plasticising device and to optimize the necessary components and processing parameters. The plastification results regarding homogenization and morphology of the molten mass have been evaluated by microscopy. The presentation will give detailed information about the trials and their results of the topics summed up above.     

Powder injection moulding of metallic and ceramic micro parts

Industrial use of micro components is determined by the availability of efficient manufacturing techniques. While micro injection moulding of plastic is common practice, metal and ceramic powder injection moulding (PIM) still is under development. High-pressure and low-pressure injection moulding methods complement each other ideally, covering the entire spectrum from prototype to large-scale production. With high-pressure PIM, micro gear wheels with diameters <300 μm can be fabricated using LIGA mould inserts. Densities between 97 and 99% of the theoretical values are achieved. Apart from oxide ceramics, metal materials like copper or powder metallurgical steels like 17-4PH or 316L are often applied. Multi-component injection moulding requires less mounting steps and, hence, offers decisive advantages for effective production of interesting material combinations like electrically conductive/insulating or hard/ductile. Studies relating to the fabrication of immobile as well as mobile shaft-wheel components were performed. Other activities focussed on in-mould labelling with foils containing ultra-fine particles to improve surface quality and detail accuracy. Low-pressure injection moulding allows for the manufacture of small series within 1–4 weeks at low cost. However, the process has features which are not compatible with high-pressure PIM. Although use of a low-viscous feedstock is associated with various benefits, low-pressure injection moulding has not met with acceptance in micro moulding.     

Mechanical structuring,surface treatment and tribological characterization of steel mould inserts for micro powder injection moulding

Manufacturing of ceramic and metallic micro components in micro powder injection moulding (μPIM) requires mould inserts offering high wear resistance and a sufficient demoulding behaviour. Within the frame of this research μPIM mould inserts made from low and high alloyed tool steel were structured by micro milling and finished by micro peening and ultrasonic wet peening. Influence of surface condition on wear and demoulding behaviour of the steels in μPIM with ceramic feedstock was characterized using a laboratory tribotester simulating powder injection moulding and a specially adapted static friction tester. Results indicate that performance of mould inserts in micro powder injection moulding depends not only on hardness, surface condition and homogeneity of the mould insert materials but also is strongly influenced by the characteristics of the feedstock, like composition of the binder or amount and hardness of the ceramic particles.     

Fabrication of micro gear by micro powder injection molding

Micro components made from polymers can be easily processed but they may not be suitable for all applications. One example is where good mechanical properties are required. Thus, the fabrication of micro components from non-polymeric materials such as metals and ceramics is essential. In this paper, the fabrication of 316L stainless steel micro gear by micro powder injection molding is reported. The specifications of the green micro gear were: 10 teeth, module of 0.08, outer diameter of 1 mm and a length of 1 mm. Injection molding was conducted on a conventional injection molding machine with a small screw diameter of 14 mm. The green micro gear was well replicated. The debound micro gear retained its shape and the teeth were well defined. After sintering, the shape was also retained but with some surface irregularities. The process differences between μPIM and PIM, such as the use of smaller particle size and higher mold temperature are also highlighted.     

Production of two-material micro-assemblies by two-component powder injection molding and sinter-joining

In the field of micro-technology the production of metallic and ceramic micro-components by powder injection molding (PIM) has become a more and more established fabrication method. But in order to fulfill the demand for more complex-shaped high-precision micro-components further development work has to be performed. This is especially true if more efficient production routes for multi-component-micro-assemblies consisting of different materials or sub-components are envisaged. To meet these challenges, investigations are performed to realize and to establish two primary shape micro-processes. These are two-component micro-injection molding (2C-MicroPIM) and sinter-joining. The realization of these technologies will lead to a markedly reduction of the efforts for handling, adjustment, and assembling of metallic and ceramic micro-assemblies. Furthermore, an increased integration level and functionality can be yielded. For an effective transfer of scientific results to industrial applications the whole process chain must be considered, from development and construction of the tooling as well as of the components to the quality assurance and determination of the properties of the assemblies after sintering. These primary shape processes shall enable the mutual processing of different materials within the fabrication process, so avoiding separate mounting or assembling steps. Additionally fixed and loose junctions between at least two components shall be realized. The progress in research and development will be demonstrated especially by the implementation of shaft-to-collar connections between micro-gearwheels and corresponding shafts. Regarding two-component micro-injection molding, the tool construction for shaft-to-collar connections will be presented as well as first experimental results on the properties of selected ceramic powders and feedstocks for the special requirements of the 2C-MicroPIM process. With the assembly step being performed outside the injection molding tool before sinter-joining different parts and geometries can be combined quite easily. The presented article gives an overview on the concept and on preliminary testing results for the fabrication of a shaft-to-collar-connection. Additionally, a solution for an automated assembly of a shaft and a toothed wheel outside the injection molding tool is presented.     

Compounding, micro injection moulding and characterisation of polycarbonate-nanosized alumina-composites for application in microoptics

The addition of nanosized ceramics to polymers allows for a tailoring of the thermomechanical and optical properties of the used thermoplastic matrix polymers. In this work, the impact of nanosized γ-alumina filler to polycarbonate APEC2097 on the compounding process and the resulting physical properties is reported. Polymer degradation during compounding affects a plasticising effect and a reduction of the Charpy impact strength. The refractive indices of the polycarbonate-alumina-composites increase with increasing solid load. Due to nanoparticle agglomeration the optical transmittance drops with growing alumina content.     

Polymethylmethacrylate/polyethyleneglycol-based partially water soluble binder system for micro ceramic injection moulding

In micro powder injection moulding polyethylene-wax binder systems have been widely established for many years enabling the fabrication of dense ceramic or metal micro structured parts. With respect to complete organic moiety removal a solvent debinding step prior to thermal decomposition using hexane as organic solvent has to be applied dissolving the wax prior to thermal decomposition of the polyethylene. The development of environmentally friendly binder systems must consider the substitution of any organic solvent or even the solvent pre-debinding. In this work a modified process chain, starting with a reactive feedstock mixture consisting of a thermally curable methylmethacrylate/polymethylmethacrylate resin, low molecular mass polyethyleneglycol and submicron-sized zirconia as ceramic filler, followed by feedstock polymerization at elevated temperature, pelletizing, injection moulding, debinding and sintering, will be presented. Prior to replication important feedstock properties like temperature and solid load dependent melt viscosity as well as the real solid load was measured guaranteeing a successful mould filling. Two different debinding strategies—with and without water-assisted predebinding—were pursued and the resulting sinter part densities as well as surface qualities were compared. Zirconia test specimen parts with a density around 99 % of the theoretical density could be obtained successfully.     

Micro injection molding of micro gear using nano-sized zirconia powder

Powder injection molding (PIM) is well established in macro molding. Scaling down to micro-PIM (μPIM) opens a new arena for cost effective production of micro components. One potential application is the manufacture of cutting tools with sharp edges for machining. However, scaling down to such a level produces challenges in raw material preparation, injection molding and thermal treatment. In this report, near dense (>98% of theoretical density) tensile bars and 3 mm micro gears were produced using 50 nm Yttria Tetragonal Zirconia Polycrystal (Y-TZP). The sintered gear teeth were well defined and visually defect free.     

Optimization of the reactive injection moulding process

The aim of the paper is to discuss the possibility of applying a genetic algorithm (GA) in conjunction with the finite-volume (FV) FLUENT package to the optimisation of the reactive injection moulding (RIM) technological process. Firstly, the fundamental flow, heat, mass, and rheokinetic relations are examined, and the possibility of numerically simulating this class of problems is examined, with a view to optimising the parameters governing the progress of the process. Then, the optimisation objective is formulated, taking into account both the total curing time of the RIM process and the set of control parameters, called the degree of cure, that is used as a measure of the effectiveness of the technological process. From among the various parameters characterising the process, the temperatures of the heat sources (electric heaters placed around the mould) are chosen as the design variables (in discretised form). A 2-D planar problem is solved to illustrate the effectiveness of the proposed method.     

Sensor-guided micro assembly of active micro systems by using a hot melt based joining technology

This article presents (the most) recent results of the subprojects B4 and B8 of the Collaborative Research Center 516—Design and Manufacturing of Active Micro Systems—which are concerned with the assembly of active micro systems. While subproject B4 investigates sensor guided assembly processes, subproject B8 develops suitable assembly techniques on the basis of non-viscous adhesive systems (hot melts). Process development focuses on the suitability for automation, process times and the applicability of batch processes. The article discusses certain hot melt application techniques that are suitable for batch production, a sensor-guided assembly system as well as different approaches for heat conduction in an automated assembly process for hot melt coated micro components.     

Numerical modeling of laser sintering of two-component powder mixtures

The process of direct sintering of metal powders induced by laser radiation is investigated. A two-component mix of powders consisting of low- and high-melting components is considered. The model is based on a self-consistent nonlinear equation of continuity for volume fractions of low- and high-melting components of the mixture and on the equation of energy transfer in the melt-powder mixture system. It includes the motion of hard particles due to shrinkage associated with the changed density of the powder mixture and with convective fluxes caused by the forces of surface stress and gravitation. The liquid flow is determined by the Darcy law of filtration. The effect of the finite width of the region of phase transition of the high-melting powder has been detected. The rate of melting zone expansion is shown to depend on both the parameters of laser radiation (beam power) and on the physical characteristics of the substance of particles, and to grow with the increase in penetrability or phase transition heat. A diagram of the evolution quality of the melting front is obtained, by which it is possible to predict the solution behavior depending on the parameters of the powder mixture (penetrability and melting heat).     

Determination of mechanical properties of slip cast, micro powder injection moulded and microcast high aspect ratio microspecimens

In a project of the German Research Council (DFG) Collaborative Research Centre (SFB) 499 Development, production and quality assurance of moulded micro-components made of metallic and ceramic materials [SFB03] three-point bending-specimens (dimension: 1.2 × 0.2 × 0.2 mm) made of slip cast [BAU02] or micro powder injection moulded ZrO₂ [RUP03] as well as tensile specimens (dimension of the gauge length: 0.78 × 0.26 × 0.13 mm) made of the micro-cast AuAgCu-alloy Stabilor G [BAU03] were investigated with respect to their behaviour under quasi-static loading.The support of the Deutsche Forschungsgemeinschaft is gratefully acknowledged. The authors appreciate the co-operation of all the project partners within the SFB 499.     

The application of a microcomputer to the control of an injection moulding machine

This paper reports a successful project that modified a locally made reciprocating injection moulding machine for microcomputer control. The microcomputer control system and its associated interfacing peripherals with the machine are described. The software including the control and interactive programs is written in assembly language for the sequence control of the machine and the closed-loop control of the injection speed.     

Micro assembly injection moulding for the generation of hybrid microstructures

Based on the micro injection moulding process, which has been investigated at the IKV for several years, a process called micro assembly injection moulding is developed and investigated. It combines the joining of hybrid elements with the generation of functional structures. Micro assembly injection moulding is a quite diverse process with many influencing parameters. For basic studies of this process, a special mould technology with maximum flexibility and precision is used. The mould concept includes a special system for the positioning of inlay parts and complex sensor equipment (pressure, temperature endoscopic devices) for controlling and analysing the process. The processing aspects which have been investigated are the production of movable microstructures by using incompatible polymers, the generation of fluidic hollow structures by lost core technology and the overmoulding of wires and optical fibres. In all cases, the choice of the different materials, the temperature layout of the process and the mechanical strain of inlay parts play a significant role. Furthermore, the precision in positioning of inlay parts and the demoulding of hybrid structures are of great importance.The investigations are part of the SFB (Sonderforschungsbereich) 440 Assembly of Hybrid Micro Systems and were financially supported by the DFG (Deutsche Forschungsgemeinschaft).This paper was presented at the Conference of Micro System Technologies 2001 in March 2001.