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.     

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.     

Performance and simulation of thermoplastic micro injection molding

 Originally developed for the replication of high aspect ratio LIGA structures, micro injection molding is presently on its way to become an established manufacturing process. Enhanced technological products like micro optical devices are entering the market. New developments like the different kinds of injection molding with several components open up opportunities for increasing economic efficiency as well as for new fields of applications. Software tools for the simulation of the thermal household of the molding tool and/or the moldfilling process itself can provide useful but not wholly sufficient assistance for the optimization of micro injection molding. Received: 10 August 2001/Accepted: 24 September 2001     

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 internally driven micro centrifugal force pumps based on synchronous micro motors

In this paper, the fabrication process and mounting of a micro centrifugal force pump based on a synchronous motor is presented. The pump unit as well as the electromagnetic drive unit is fully integrated and mainly realized by means of micro fabrication technologies including UV-depth lithography and electroplating. Furthermore, one specialty is the application of polymer magnets which allow a miniaturization of the toothed rotor down to 2 mm. The presented pump reveals a promising basic concept for other micro rotary pumps like spiral, impeller, or gear pumps.     

Ceramic micro parts produced by micro injection molding: latest developments

Powder injection molding is a preferred technology for the production of micro parts or microstructured parts. Derived from the well known thermoplastic injection molding technique it is suitable for a large-scale production of ceramic and metallic parts without final machining. To achieve good surface quality and control the part size and distortions is an important goal to allow mass production. This means that all process steps like part design adjusted for MIM/CIM-technology, appropriate choice of powder and binder components and injection molding simulation to design the sprue are required. Concerning the injection molding itself high quality mold inserts, high-precision injection molding with suitable molding machines like Battenfeld Microsystem50 or standard machine with special equipment like variotherm or evacuation of the molding tool and an adjusted debinding and sintering process have to be available. Results of producing micro parts by powder injection molding of ceramic feedstock will be presented.     

Manufacturing of isolated ceramic microstructures

The design of a micromold for gearwheels and components for a gearwheel micropump were adapted for the fabrication by micro powder injection molding (micro-PIM). Several variations were made to compensate the shrinkage during sintering and to meet the dimensions of the micro features. Mold inserts made by the LIGA- and the UV-LIGA-Process were used. The quality of the models was evaluated by SEM and tests of micro injection molding in PMMA. Subsequently, micro-PIM was carried out using different feedstocks. The green microparts were isolated from the substrate, debound and sintered. At sintered parts, the roughness of the surface was evaluated and the processes were optimized.     

Development of stacked conductive templates for electroforming of multi-level metallic micro components

Economic mass manufacturing of metallic micro components requires effective methods to produce the adequate partial conductive templates. Especially in case of stacked or multi-level micro components a considerable increase in economic efficiency could be achieved if originally made templates, e.g. by LIGA, would be replaced by much cheaper replicated ones. For this purpose a modified injection molding process to produce stacked and partially conductive polymeric templates had been developed. As suitable demonstrator a combination of a gear wheel with an adequate shaft was chosen. Electroforming trials using a standard nickel sulfamate electrolyte were carried out. Finally, the stepwise electroplating procedure filling the shaft cavity first subsequently the gear wheel cavity could be realized. The lateral dimensions were measured and the surfaces of the metal components were characterized by SEM.

     

Replication of micro components by different variants of injection molding

In microsystems technology, components made by micro injection molding are applied to an increasing extent. The variety of materials, low costs, the high number of sub-variants, and the relatively easy shaping are decisive factors. As further improvement, special variants like micro multi-component injection molding or micro powder injection molding are currently under development at Forschungszentrum Karlsruhe. The goals of these research activities are not only to increase economic efficiency but also to expand the range of materials to metals and ceramics.These R+D activities were mainly sponsored by the Deutsche Forschungsgemeinschaft (DFG) and the Federal Ministry of Education and Science (BMBF). We also wish to thank our industrial partners, namely STEAG microParts, Zumtobel Staff, and all colleagues at Forschungszentrum Karlsruhe for their always helpful assistance.     

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.     

A mechanism influencing micro injection molded weld lines of hybrid nano filled polypropylene

Nano fillers reinforced polymer composites have been widely applied in microsystem technologies. As one popular micro fabrication technology, micro injection molding is playing the key role in manufacturing micro scale parts made of nano thermoplastics composites. As a defect in micro injection molding parts hard to avoid, the weld line causes detrimental mechanical and physical properties, particularly in case of nano filled composites. In this study, the hybrid carbon nanofibers (CNFs)/TiO₂ nano particles (1:1) filled polypropylene (PP) was prepared by inner melts mixing process with various weight contents(10, 20, 30 and 35 wt%). Micro tensile samples with weld lines for all composites were formed by micro injection molding process at constant processing conditions. Mechanical properties of micro weld lines were tested by tensile test characterizing method. The results show that comparison with neat PP, the samples with weld lines made of hybrid nano PP composites are showing lower tensile strength and elongation percent, but higher E modulus. As increasing nano filler contents, the tensile strength and elongation of micro weld line samples are decreasing, while the E modulus is improved considerably.     

Simulation of the filling process in molding components with micro channels

In micro molding of components with micro features, the ability for the polymer melt to flow into the micro channels is a crucial factor for successful molding. In this case, the molded volume is about the same as the conventional molding. The penetration distance into the microstructure depends on the flow rate and the cooling rate of the micro features, which is function of the geometric dimensions. In this study, a simplified model was established to estimate the injection distance into the micro channels of a mold insert. The effect of the mold temperature, injection rate, and micro channel dimension on the filling distance was investigated based on the model. The filling distance increases dramatically with respect to the increase of the channel width. In molding of components with micro features as those analyzed in this study, decrease of the part thickness could enhance the filling in the micro features.The authors would like to thank for the financial support from National Science Council in Republic of China under the contract number of NSC 91-2212-E006-131.     

Influence of processing parameters on micro injection molded weld line mechanical properties of polypropylene (PP)

As a hot fabrication technology for micro scale parts, micro injection molding is receiving increasing market attention. Improving mechanical properties of micro parts should be an important issue in the micro injection molding process. The relation between weld line strength in micro injection molding parts and processing parameters is investigated. A visual mold with variotherm unit is designed and constructed, in which the micro tensile specimen with weld line are prepared. Polypropylene (PP) is used as the research material in this study, and six processing parameters were chosen as investigating factors, which were melt temperature, mold temperature, injection pressure, packing pressure, ejection temperature and injection speed. In order to achieve optimized processing parameters and their order of significance, Taguchi experiment method was applied in this presented study. The prediction formulation of the strength of micro weld line was built up by multiple regression analysis based on Chebyshev orthogonal polynomial. The results showed the influencing significance order of parameters from strong to week separately are mold temperature, melt temperature, injection speed, ejection temperature, packing pressure and injection pressure. And the tolerance of micro weld line prediction formulation was found to be lower than 21% through confirmation experiments.     

Molding technology for improvement on dimensional accuracy in micro metal injection molding

This study aims to develop processing techniques to improve dimensional accuracy of fine metal parts produced by metal injection molding. Micro dumbbell specimens were molded by the micro injection molding machine, which monitor the cavity pressure during injection molding. Among various injection molding conditions, the influences of injection volume on the cavity pressure, and dimension of both green compacts and sintered parts were investigated. For dimension control, the importance of monitoring cavity pressure at each shot was verified.     

Effect of micro tensile sample’s cross section shape on the strength of weld line in micro injection molding process

As a suitable mass and cost efficiency fabrication method, micro injection molding is doing a very good performance in micro plastic parts production. The mold design is an important part affecting micro parts properties. In this study, a micro injection mold with multi cavities of micro tensile bar is used. These micro cavities are fabricated by a micro milling process in different cross section shapes (semicircle R = 0.5 mm, equilateral triangle D = 0.3 mm, and trapezoid D = 0.336 mm t = 0.2 mm bottom angle = 95°). With an Arburg^® 320C injection molding machine, micro tensile test sample are prepared in different processing parameters so that a correlation between the cross section shapes with micro weld line strength in different conditions could be investigated by tensile test. Final results show that when the cross section shape is different, their corresponding weld line strength is also different. Equilateral triangle cross section is leading to strongest weld line, and then followed by trapezoid, semi-circle is the last. By analysis of these tensile test results, the quantitative factor a is defined as the ratio of perimeter to area of cross section shape, and higher a value is corresponding stronger weld line. After weld line strength comparison in different processing conditions, the results show that higher injection pressure induced to lower weld line strength whatever the cross section shape is. By higher mold and melt temperature, equilateral triangle cross section gives improved weld line strength. But mold and melt temperature affect weld line strength negative for other cross section shapes.     

Metal-ceramic-composite casting of complex micro components

Metal-ceramic-composite casting has a huge potential as a new manufacturing method for the production of complex-shaped micro sized parts or microsystems consisting of different metals and ceramics. The fundamental advantage of this method is the capability of multi-component part fabrication in one step avoiding first time consuming joining or assembling techniques; second the used material combinations can fulfill complex functionalities and enhanced mechanical properties. One of the most challenging factors in micro composite casting is a stable mechanical bonding between the used individual materials. But under consideration of the different physical properties like thermal expansion coefficient as well as of the wettability of the ceramic inserts and of the applied metal casting material it is possible to manufacture form and force fitting microsystems. Within the framework of this feasibility study complex metal-ceramic micro composites have been realized successfully using the lost-wax casting process. Casting experiments were performed at different muffle preheating temperatures with Al-bronze of the type CuAl10Ni5Fe4 as casting material. The ceramic parts, respectively inserts cast around by metal are micro gear wheels (2.5 mm diameter) consisting of ZrO₂ and Al₂O₃.     

A micro gearing system based on a ratchet mechanism and electrostatic actuation

This paper presents the design and fabrication of a silicon micro gearing system (MGS) that utilizes electrostatic comb-drive actuators to rotate a gear ring through a ratchet mechanism. The rotational comb-drive actuator is engaged with the gear ring through a spring system and ratchet teeth at one end, reciprocally rotates around an elastic point at the other end based on the electrostatic force. Rotational motion and torque from the driving gear ring are transmitted smoothly to driven gears through involute-shaped gear teeth. Smart design of anti-gap structures helps to overcome the unavoidable gap problem occurred in deep reactive ion etching (deep-RIE) process of silicon. The MGS has been fabricated and tested successfully by using SOI (silicon-on-insulator) wafer and one mask only. The angular velocity of the gear ring is proportional to the driving frequency up to 40 Hz.     

Micro molding for double-sided micro structuring of SU-8 resist

Advances in micro and nano fabrication technologies for MEMS require high-level measurement techniques with regard to sampling and sensitivity. For this purpose at the Institute of Microtechnology (IMT) highly sensitive piezoresistive 3D force sensors based on SU-8 polymer have been developed. In this paper we present an improved micro fabrication process for a double-sided micro structured design. The sensors are produced by multilayer processing techniques such as UV lithography and coating methods. The double-sided micro structured design demands a photoresist application method which simultaneously features a top side structuring and a casting from a mold. We use a new micro molding process to meet the demands. The micro fabrication technology is described, focusing on the development of the molding structure for shaping of the bottom side and a capable release process for the detachment of the molded structures. The fabrication process of the SU-8 mold layer is optimized to fabricate molding structures with heights from a few μm up to 350 μm. Therefore different SU-8 formulations, namely with classification numbers 5, 25, 50, and 100, have been used. The fundamental limitations for the mold design result from the lithography process, which defines the smallest lateral resolution, and from the characteristics of a molding process, e.g. the impossibility to realize an undercut. To allow for reliable release, the molding structures have to be coated with a sacrificial layer. Silicon nitride is deposited onto the substrate with accompanying monitoring of the deposition temperature during the PECVD process.     

Design and fabrication of a micro Alvarez lens array with a variable focal length

A 5 × 5 micro Alvarez lens array mold was fabricated using a 5-axis ultraprecision diamond machine and an Alvarez lens array was manufactured by injection molding process. Unlike conventional processes for asymmetrical element fabrication such as small tool grinding, this research demonstrates slow tool servo broaching process that allows the entire Alvarez lens array to be accurately machined on a metal mold in a single operation. To further reduce manufacturing cost, injection molding was used to fabricate the Alvarez lens arrays. The mold and molded lenses were both measured using an optical profiler. All measured profiles showed a good agreement with design and surface roughness also indicated an optical surface finish. The functionality of the molded polymeric lens arrays was achieved when the focal lengths were varied by laterally translating the molded Alvarez lens array pair. This research is a demonstration of the capability of fabricating complex optics using the same approach.     

A visual mold with variotherm system for weld line study in micro injection molding

In order to study the weld line developing process and its influence on mechanical properties in micro injection molding, a visual mold with variotherm system mold was designed and fabricated. In this mold, a visualization design and a rapid heating/cooling system were integrated, and specimens with different cross section shape and micro dimensions could be molded for weld line study. The building process for the visual and variotherm mold was presented and the experiments were executed. The specimens for weld line study of micro injection molding were produced applying different processing parameters. A problem of flash in molded specimens needs to be solved.