Microsyst Technol (2006) 12: 702–706 DOI 10.1007/s00542-006-0100-8

T E C H N I C A L P A PE R

J. Fleischer Æ A.-M. Dieckmann

Automation of the powder injection molding process

Received: 28 June 2005 / Accepted: 7 November 2005 / Published online: 28 April 2006
Springer-Verlag 2006

Abstract 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.

1 Introduction Miniaturization is one of the most important technological trends, which will gain further importance in the future. It offers the decisive advantage of a higher functional integration on smaller room and thus the possibility to develop new product concepts. This opens a wide range of new application fields associated with handy, portable products [1]. Required quantities of miniaturized highly integrated components and products vary—from mass production for e.g. the automotive sector or medical applications to middle and small series in e.g. mechanical engineering. To ensure an economic production of micro parts in high series automation of the process has to be developed and integrated to an overall process-chain.

This work is based upon the Collaborative Research Centre SFB 499 funded by the German Research Foundation (DFG) J. Fleischer (&) Æ A.-M. Dieckmann wbk Institute of Production Science, Universita¨t Karlsruhe, Karlsruhe, Germany E-mail: fl[email protected] Tel.: +49-721-6084009 Fax: +49-721-699153

One of the production technologies to meet the increasing industrial requirements of an economical production of high quality parts in small to large series is the powder injection molding (PIM) process. It offers the production of three-dimensional micro parts as well as micro-structured parts in a wide variety of metallic and ceramic material. Due to the strength of the wearresistant materials highly precise metallic or ceramic parts for the transmission of forces and torques can be produced. As a result, the PIM technology enables the economic production of micro-structured parts and systems in various branches of the industry like biotechnology, telecommunication, automotive, etc. with a throughput of more than 1 Mio. parts per year. However, in order to utilize this potential and to transfer this technology into the industry, the complete automation of the micro-PIM process chain from the separation of the micro-PIM parts from the sprue through the sintering to the stacking is inevitable [2].

2 Microproduction technology for micro-mechanical parts The approach on the automation of the PIM-process is divided into sprue separation and demolding of micro parts, handling and automation of fragile green compact parts produced by micro-PIM and the integration of the debinding and sintering process in the complete processchain developed at the wbk Institute of Production Science, Universita¨t Karlsruhe.

2.1 PIMprocess For the processing of metal powder or ceramic powder (diameter of powder d50<4 lm), it is first necessary to homogenize and granulate the material with a binder in a mixing machine. This compound, which is capable of

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being molded, is then injected into the cavity giving it the form of the desired toothed wheel. After injection the parts (green compact) have to be separated from the sprues, demolded and transferred to a furnace. In the subsequent debinding process the binder is first removed from the molded part (green compact) resulting in the ‘‘brown compact’’ which then is sintered. Within these process steps the reproducible and material-specific shrinkage of the part is typically 18–30% [3, 4]. 2.2 Mold-internal separation of sprue from parts There are different possibilities of separation: within the mold via three-plate-mold or hot-runner-systems as well as inside the mold via grippers or outside the mold via manufacturing technologies (ablation, milling or grinding). The decision for the three-plate mold was based on experiments and studies with experts. By use of a two-stage opening movement of the mold, sprue and parts are separated from each other and the gripping system can handle the parts from the ejection side of the mold. Due to the challenging demand regarding the required precision of the tooling this technology is well known in the field of macro molding but does seldom find application within micro molding [5]. The separation process has been realized using the example of a toothed wheel with an outer diameter of 1,400 lm, a thickness of 230 lm (dimensions of the green compact, shrinking to 1,200 and 200 lm during sintering) and a module of 0.1 (involutes). The results show that the realized injection area has reached surface qualities meeting the design specifications and show non-destructive feedstock characteristics with a centered gap of 0.04 mm2 [6]. The principal set-up of the threepart mold with a cavity of a toothed wheel within the millimeter-range is shown in Fig. 1 (for better illustration only a one-cavity mold is shown).

Fig. 1 Mold-internal separation of parts and sprue (three-part mold), source: wbk, Kugele

2.3 Demolding When the cavities are opened the part has to be demolded from the mold. This process can either be done with mechanical ejector pins, application of pneumatic pressure within the mold or by the handling system. Due to the shrinkage (about 2% of the nominal value) of the part during the cooling time, the part may stick on mold cores for example. Research work at the wbk is aimed at the evaluation of the demolding forces imposed upon the produced parts. This analysis is being carried out with respect to boundary conditions e.g. necessary surface qualities, tribological characteristics, demolding slants of the mold in order to determine the position and size of ejector pins in the mold as a basis for technical designers and moldmakers. For these objectives to be achieved, correlations have to be set up between variable structures, sizes and alternative technical solutions (e.g. pneumatically) for demolding parts and structures by means of geometrical sets. These sets include structures like circles, triangles and hollow cylinders to model real parts e.g. toothed wheels (see Fig. 2). The dimensions are below 300 lm with part lengths from 400 to 1,000 lm and depths from 400 to 1,000 lm. Subsequent to this process all parts are measured to detect damages. The upper points in Fig. 2 show the measured demolding forces necessary for three different parts all of them having the same length. The distinction between the structures is not only their form but also the resulting surface causing cohesive forces inflicted upon it being the reason for higher demolding forces. The lower set of points in Fig. 2 shows the necessary demolding forces with demolding spray in use. The experiments show small demolding forces below 1 N including an extensive reduction of the forces by use of demolding spray and coating on the cavity for better surface qualities [7] but without further analysis of the effects of sprays to debinding and sintering processes. Within the automated process chain, demolding has

704 Fig. 2 Demolding forces at micro-PIM test structures

been realized by mechanical ejector pins having an inner diameter of 525 lm, surrounding the inner core of the toothed wheel (see Fig. 1). 2.4 Handling After demolding the parts from the cavities the handling system has to transfer the parts to the stack unit. There are two critical process steps: • Accurate positioning of the handling system to the cavities within a range of 1–3 lm in six degrees of freedom in production area (contamination, temperature, etc.) • Gripping of the fragile micro-PIM parts. A molded feedstock has a tensile strength of about 4 N/mm2; this restricts the gripper forces acting on the micro parts to the milliNewton range [8].

Due to the sensitivity of green and brown compacts to vibrations, there are high machine requirements for conveyance systems. To meet the requirements a paternoster and linear modules with a high gear reduction and a vibration absorber were installed in the system. A prototype of the process chain between the injection molding machine and the furnace is shown in Fig. 4. Stacking is done by means of a mechanical stage with a repeating accuracy of 50 lm—the movement in the third dimension has been realized by the handling system. A linear module transfers the sinter trays to a paternoster for flexible storing. The sintertrays are clustered with the help of a production planning and control system and transferred to the furnace conveyer belt. Subsequent to the debinding and sintering process the sinter trays are extracted by means of a linear module with a hexapod mounted below. Therefore the linear

The pneumatical gripper (see Fig. 3), based on the Venturi principle, consists of a housing with modular pestles for part variation. The movement of the pestle, essential for gripping the part at the cavity and releasing the part at the sintertray is realized by use of air-bearings with slip-stick-free movement within the millimeter-range.

2.5 Automated material flow To ensure a reliable and complete demolding of the part an integrated quality assurance is necessary. Due to the process restrictions e.g. cycle time a 100% visual inspection of the part is done after demolding and handling of the part to the sintertray. Damaged or missing structures of below 1 lm can therefore be detected. Combined with the quality assurance the parts are magazined on a sinter tray.

Fig. 3 Schematic view of a pneumatical gripper with air-bearings

705 Fig. 4 Schematic view of the process chain for micro-PIM parts between molding machine and furnace

module transfers the sinter trays and the hexapod with a repeating accuracy below 1 lm handling the parts by use of a mechanical gripper developed by the Institute of Machine Tools and Production Technology in Braunschweig [5]. Due to the integrated vision system (see Fig. 5) the parts can be localized, centered and handled by the mechanical gripper for stacking on 0.2 mm pins with total system accuracy below 2 lm in 6 degrees of freedom. 2.6 Debinding and sintering To provide an automated production of micro-PIM parts the thermal processes of debinding and sintering have to be integrated. Generally continuous and discontinuous furnaces are used for producing macro parts. In contrast to discontinuous (batch) furnaces distincFig. 5 Hexapod kinematics for stacking of micro-PIM parts and mechanical gripper, source: wbk, iwf

tively tuned for the use in metal injection molding (MIM), which are mostly used in laboratory environments, the most economic solution for large lot sizes is the usage of a continuous furnace. The furnace (see Fig. 6) consists of a catalytical as well as thermal debinding area (max. temperature 600C), a sintering zone (max 1,450C), a cooling area as well as a loading and unloading area. The total dimensions of the furnace are shown above. The maximum capacity of the furnace (atmosphere: nitrogen and hydrogen at variable mixing ratios) is about 9 Mio. parts/year with total cycle times below 9 h.

3 Conclusion The demand for complex three-dimensional micromechanical parts in wear-resistant material in different

706 Fig. 6 Rough draft of the continuous furnace for microPIM

branches like medical, biological or even telecommunication areas can be supplied by the PIM process [9]. To obtain a breakthrough of this technology, an economic and high-quality production of medium to large series has to be set up. This can only be achieved with an automated process and with a special focus on critical sub-processes, e.g. separation of parts and sprue, demolding and handling. To validate the concepts of an automated micro-PIM production a process chain for micro-PIM parts—here toothed wheels with a diameter of 1,400 lm—has been set up. Innovative separation solutions for micro-PIM parts with a three-part mold as well as demolding of microstructures, quality assurance, handling and transportation have been integrated in this automated process chain. To counter problems arising from producing smaller parts, dependencies between structure sizes, material and specific technology criteria, e.g. gripping principles or force control systems have to be developed as a basis for the respective sub-processes [10].

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