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.     

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.

     

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.     

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.     

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.     

Scale effect on filling stage in micro-injection molding for thin slit cavities

Micro-injection molding is the main method molding complex micro plastic parts accurately at one process. It is more complex than traditional injection molding because of the micro-scale effect. The polymeric flow in micro channels differs from those in macro ones significantly, and the molding theories of traditional injection molding can not be used in micro-injection molding. In this study, the effects of micro-scale, such as micro-viscosity and wall slip, were considered based on the flow characteristics of micro-injection molding, and the mathematical model and the numerical model were built. The simulation of filling stage in micro-injection molding was implemented by hybrid finite element/finite difference/control volume method accordingly. The influence of micro-viscosity and wall slip on filling stage was investigated by numerical analysis. The results indicated that the micro-scale effects have important influence on the filling stage of micro-injection molding, and the micro-scale effects become more and more significant as the gap thickness drops.     

Replication processes for metal and ceramic micro parts

To cover the demand for effective manufacturing of metal and ceramic micro components two process technologies are described. The first one can be regarded as a special variant of micro powder injection molding (MicroPIM): inmold-labeling using powder filled feedstocks. Its basic procedures are the backfitting of powder filled foils by an injected PIM-Feedstock and the subsequent co-debinding and co-sintering steps. For example, two-material ceramic parts with microstructured surfaces could be produced and compacted with mostly tight interfaces. The second process conduct combines two-component and insert injection molding with an electroforming process. Since all process steps involved are based on technologies suitable for series production, low product costs per unit will be realistic. Surface qualities and dimensional accuracies are comparable or even better than achieved if applying alternative processes like MicroPIM.     

Galvanic deposition in partially conductive plastic molds for manufacture of finely detailed microcomponents

Combining multicomponent injection molding and electroforming processes, a method for manufacturing medium and large quantities of high-quality metallic parts had been developed. Since the process steps involved are based on two technologies suitable for series production, low product costs per unit will be realistic. The high quality of the metallic microparts was verified, among other things, by means of demonstrator surface and dimensional measurements. Compared with competing methods (MicroPIM), significantly better or at least equal values were obtained. In addition, multi-material micro components containing e.g., ceramic inserts were manufactured successfully. There are certain potentials for optimizing the dimensional accuracy or scatter of the finished galvanic parts. Besides, 2C film injection molding, which allows replication of very fine exposed structures, has been examined successfully and will be developed further.     

How mold inserts influence the replication of metallic microparts produced by electroplating into two-component templates

Multi-component injection molding combined with electroplating, the so-called MSG process, represents a promising process chain to replicate metallic microstructures. MSG is the German acronym of ‘Mehrkomponenten-Spritzgießen und Galvanoformung’, in English ‘Multi-component Injection Molding and Electroplating’. The process is based on the highly accurate reproduction of surface details through injection molding to build a microstructure into a two-component template and electrodeposition of e.g. nickel into this cavity. This electroplated micropart is the replication of the former structure. To study the influence of the mold insert on the accuracy of the molded part and the produced microparts, a test specimen was fabricated and analyzed using a milled mold insert. On the mold insert, three microcoil parts, the components of a microgripper, were micromilled. The effect of the individual process steps on the surface quality and dimensional changes of the final microcoil will be presented. It will also be shown how the quality of the injection molding insert influences the dimensional accuracy of the produced microparts. Finally, potential process improvements will be outlined.     

Quasi-static and cyclic testing of specimens with high aspect ratios produced by micro-casting and micro-powder-injection-moulding

 The presented investigations of material properties of micro-components have been carried out in one project of the DFG Collaborative Research Centre (SFB) 499 “Development, production and quality assurance of moulded micro-components made out of metallic and ceramic materials” [SFB01]. The aim of this SFB is to develop production methods for micro-components in large-, middle and short-batches, where the applied metallic and ceramic materials can sustain high stresses and are wear-resistant, e.g. in relation to plastics. A micro-turbine and a sun-and-planet gearing with three stages (outside diameter: 1.9 mm; already realised in plastic [THü99]) are planned as a demonstration device. However, for the design of such micro-components, no reliable data for material states in small dimensions are available. Therefore, bending-specimens made out of micro-cast Stabilor G and out of micro-powder-injection-moulded ZrO₂ were investigated with respect to their behaviour under quasi-static as well as under cyclic loading. Received: 10 August 2001/Accepted: 24 September 2001     

Large-scale hot embossing

The hot embossing process is a flexible molding technique to produce delicate microstructures with high aspect ratios on thin layers. Large-scale hot embossing is one effective way to meet future requirements and produce high-quality microstructures at low costs. For this, however, principal changes of the molding process and molding tool design will be required. In the present paper, constructive solutions for large-scale hot embossing shall be described. Based on a simulation of the hot embossing process, solutions shall be presented that are aimed at reducing shrinkage of the molded parts and demolding forces and, hence, at avoiding damages of microstructures during demolding.     

Automated array assembly: a high throughput, low cost assembly process for LIGA-fabricated micro-components

 The development of a viable LIGA-based micro-manufacturing capability requires the assembly of discrete micro-components into more complex devices. The size and tolerances of micro-components preclude the direct use of traditional macroscopic assembly process to micro-device manufacturing. Instead specific micro-assembly technologies must be developed and validated for producing complex micro-systems. Automated array assembly, a high throughput, low cost approach to micro-assembly, uses specialized fixtures allowing an array of micro-components to be picked-up and inserted into a corresponding array of devices using automated and highly accurate positioning stages. Assembly rates are measured here for insertion assembly experiments using automated array assembly and these rates compared with manual assembly rates. These results show that the automated array approach can assemble micro-components with rates at least an order of magnitude greater than manual assembly rates. Received: 25 August 1997/Accepted: 3 September 1997     

Fabrication of high-aspect-ratio ceramic microstructures byinjection molding with the altered lost mold technique

A fabrication process for complex ceramic microstructures was proposed that combines a lost mold technique and ceramic injection molding. Two key points in this process were studied. First, the solubility of several engineering plastics in various organic solvents was tested to find appropriate combinations of mold material and solvent for dissolving molds. Secondly, the binder extraction rate and the strength of a green body during debinding were investigated. Experimental results indicate that acrylonitrile-butadiene styrene and acetone are the best combinations selected for this lost mold technique. We also propose that using gasoline as the debinding solvent and performing the debinding at room temperature will give a good time-saving effect and avoid toppling the microstructure if paraffin wax, stearic acid, and polyethylene were selected to compound the binder system. This process has been successfully applied to fabricate several ceramic microstructures, such as an integrated punch     

Microfabrication of ceramic microreactors

 Ceramic microreactors can be used for applications that cannot be covered by metal or polymer systems, because special material properties, such as high thermal and chemical resistance are required. However, application of ceramic microcomponents often fails due to the time-consuming and costly manufacturing of components with patterning details in the micrometer range. A promising solution to this problem is a rapid prototyping process chain. It offers a fast and precise fabrication of ceramic components down to the micrometer range by combining stereolithography and low-pressure ceramic injection molding. Its fast and flexible tooling allows rapid product development and manufacturing of ceramic components as functional models or in small series. For use in chemical microreaction technology, a modular ceramic microreactor with inner dimensions in the sub-millimeter range has now been developed by means of this process chain.     

Automation of the powder injection molding process

Powder injection molding (PIM) offers a high potential for fabrication of micro-mechanical parts manufactured in metal or ceramic material providing a large variety of properties. To ensure an economical micro-PIM production in large lot sizes and high quality automation of the process beginning with demolding, handling, debinding and ending with sintering is a necessity. Within the field of automation research focus is to optimize critical processes like sprue separation, demolding and handling as well as the set-up of an autonomous and automated process-chain which are presented in this article.This work is based upon the Collaborative Research Centre SFB 499 funded by the German Research Foundation (DFG)     

Injection molding of polymeric LIGA HARMs

The primary goal of an ongoing research effort at LSU is to develop the three-step LIGA process to inexpensively manufacture high aspect ratio microstructures (HARMs). The first two steps of the process (lithography and electroplating) produce a metallic mold insert that can be used as a template for molding microstructures. The final step of LIGA is molding. This paper focuses on injection molding of thermoplastics to produce surfaces covered with HARMs. The resulting microstructures are hundreds of micrometers in height, tens of micrometers in width, and separated by gaps on the order of tens of micrometers. Injection molding experiments using high density polyethylene were performed using a commercially available injection molding machine. Experimental variables included injection speed, the tool temperature, and air pressure in the mold cavity. Elevating the tool temperature above the melting point ensured that the polymer completely filled the mold, producing microstructures with the desired geometry. As the temperature of the mold was reduced, higher injection speeds did not necessarily ensure filling of the mold cavity. The cycle time is shorter than the values previously reported in the literature [Madou (1996)]. Received: 30 March 1999 / Accepted: 12 April 1999     

Fabrication method of two-level polymeric microstructure with the help of deep X-ray lithography

Two- or multi-level microstructures are getting more important in several applications such as multi-component micro optical elements and various microfluidic systems. In the present study, a simple and efficient method is newly proposed for a fabrication of the two-level polymeric microstructures. Making a mother two-level microstructure consists of two processes: (1) the hot embossing process for a fabrication of microstructures on a PMMA substrate, and (2) the deep X-ray lithography using the hot embossed substrate for a high aspect ratio microstructure fabrication, resulting in a high aspect ratio microstructure containing smaller microstructures on its surface. Making use of so fabricated two-level microstructures as a mother structure, one could achieve a mass replication of the same microstructures via injection molding process with a metallic mold insert obtained by a nickel electroforming onto the mother microstructure. In order to demonstrate the proposed method, a polymeric high aspect ratio microstructure having smaller square microstructures on its top surface was fabricated. The fabricated two-level microstructure shows fine vertical sidewalls, which is a characteristic feature of the deep X-ray lithography. In addition, a metallic mold insert for a mass replication was fabricated by a nickel electroforming process.     

Fabrication of ceramic microcomponents using deep X-ray lithography

In the last years the fabrication of micro components made from ceramic materials became more and more evident with respect to the pronounced chemical stability and the outstanding thermomechanical properties in comparison to plastics and metals. The aim of this work is the lithographic generation of ceramic microstructures avoiding an intermediate molding step using SU8 as pronounced sensitive resist matrix filled with fine ceramic powder in the submicron range. Focus of the research was to investigate the composite formation, patterning by x-ray lithography, developing, debinding and sintering to form stable ceramic parts. The addition of fine ceramic particles to low viscous liquids like SU8-10 leads to an increase of the viscosity. For a successful debinding and sintering a volume content of at least 40% ceramic is required resulting in a change of the viscosity from around 2 Pas up to a value of 1000 Pas at 25 °C and low shear rates. A modified casting procedure was developed for the formation of uniform resist films with a thickness around 300 m. Optimized exposure and development parameters allow the fabrication of good quality resist structures that can be further transformed into ceramic structures by sintering. Details of the work and results will be presented and discussed in this paper.At this point the authors would like to thank all people who supported this work.     

Micro-injection molding using a polymer coated mold

We applied an insulating polymer layer to the surface of mold cavity to injection-mold large plastic parts with a microsized thickness. Polyimide (PI) was coated on the mold wall using spray coating method. A thin light guide plate (LGP) was designed and fabricated via micro-injection molding. The polymeric coating layer could enhance the fluidity of polymer melt in the cavity during filling stage by minimizing the formation of the skin layer during injection molding. The surface roughness and pattern transfer rate of the LGPs were analyzed experimentally. In addition, numerical simulation was carried out to understand the insulation effect of the layer in the injection molding.

     

LIGA-Technology for the fabrication of positioned planar structures

 A fabrication process is presented for polymer inserts in micro-optical benches, which combine the mechanical precision of the LIGA-process with the wide variety of optical functions offered by diffractive optical elements. For this purpose metal masters with continuous relief elements and LIGA-structures were used in a combined molding tool and precision micro-optical elements were replicated by injection molding. Received: 25 August 1997/Accepted: 22 September 1997