Small but Effective - Metal Injection Moulded Micro Gear Wheels

A Sponsored Editorial Feature by ARBURG

The miniaturisation of components is gaining in importance in many sectors such as medical technology, biotechnology and drive technology, with an increasing demand for so-called micro components. A research partnership involving a university and industrial companies in Germany has resulted in the successful production of micro gear wheels using metal injection moulding (MIM). ARBURG'S ALLROUNDER injection moulding technology has played its part in this partnership.

Micro components are not only becoming ever smaller they are also required to have several functions at the same time. For example, they must have sufficient load bearing strength and be able to transmit mechanical forces and movements independent of environmental influences. In this way, this type of part can take on an important role in micro system technology, such as for example mini submarines which have to travel through the human bloodstream in order to deliver the necessary medication directly to the seat of the disease. Such demanding applications often require measurements in nanometers.

The micro powder injection moulding process (micro PIM) has been significantly improved in recent years, with many of the technological advances having been developed jointly by universities and industry. A good example is the research partnership of the injection moulding mould specialist Herbert Kugele (Schömberg Langenbrand), the Institut für Werkzugmaschinen and Betriebstechnik (wbk) (Institute of Machine Tools and Production Science) at the University of Karlsruhe, which took on the task of automating the overall process, and the injection moulding machine manufacturer - ARBURG in Lossburg. This partnership has successfully worked together to vigorously demonstrate the potential of the micro PIM process for the production of high quality parts in medium to large quantities.

What is PIM?

The abbreviation PIM stands for powder injection moulding. In simple terms, ceramic or metal in powder form are made injection mouldable by adding a thermoplastic binder and by processing this into a moulding material (feedstock) by means of intensive mixing. Using a conventional injection moulding process this mixture is fed into one or more mould cavities under pressure and specific temperature, and the part is moulded. The part is then removed from the mould and is completed in two subsequent thermal process steps.

First, so-called debinding takes place in which the binder component is removed from the compact. Subsequently the metal or ceramic component is sintered to its final size and original material properties at high temperature. During this second sintering step the component shrinks in a reproducible and linear manner within a range of 18 to 25% of its original dimensions.

The Cooperative Partnership's Project

A micro gear wheel, which is manufactured by means of fully-automatic series production using the metal powder injection moulding process, is the result of cooperation between wbk, Kugele and ARBURG. These gear wheels can be used as components in micro-motors in medical technology or telecommunications sectors. The requirements of fully-automatic production made it necessary to automatically remove the sprue of the moulded part (which is much larger than the gear wheel itself) with a high degree of accuracy in the mould. The sprue material is 100% recyclable and can be reused in production.

The micro gear wheels, which are produced in a single cavity mould, are removed by means of a MULTILIFT H robotic system, which is equipped with a special gripping system using pneumatic action and pneumatic drives. The parts are positioned by the robotic system on ceramic trays. These are stacked in a stacking unit in a highly accurate manner and then are automatically transferred to the subsequent stations for debinding and sintering.

Moulding and Handling the Micro PIM Components

The micro parts are produced on an ALLROUNDER 320 C 500-100 injection moulding machine, which is equipped with a plasticising screw having a diameter of 15 millimetres. The screw with so-called PIM geometry is characterised by reduced compression and modified zone splitting.

The shot volume is 2.3 cm' and the injection flow is 10 cm'/ s. The switchover pressure is 900 bar and the holding pressure time is 0.1 s. Pressure decreases from 900 bar to 25 bar in only 0.5 s. Equipped with the 15 millimetres series screw, the machine reacts extremely sensitively to the smallest changes in the injection moulding process. This enabled filling analyses to be carried out by changing the switchover point parameters in steps of 0.05 cm' in order that optimum cavity filling could be determined. The result is a high-quality, absolutely perfect moulded part which can be manufactured in series production with a high degree of reproducibility.

The gripper system as well as the stacking and transport units for the entire automation process were developed or optimised by wbk. The gripper system can be positioned at the cavity with a high degree of accuracy. The demands on the robotic and gripper system with respect to positioning are less than ± 2 µm for the application described. The gripper is docked onto the moveable side of the mould via a centring system. This helps to position the gripper hand for parts removal with µm-accuracy and without realignment.

A vacuum gripper with pneumatically driven kinematics allows the components to be handled gently and ensures a gripper movement which is optimised for component set-down. The components are stacked by means of air¬assisted set-down. This is supported by the gripper kinematics. The components are placed 5 millimetres apart. The MULTILIFT H, which is used as a robot on the machine, has servo-electrically driven axes in order to be able to travel to each position with extreme accuracy.

High-Tech Mould Technology

High-tech mould technology has been developed to successfully produce MIM micro gear wheels. Despite the small component size of the gear wheel with an external diameter of only 1.4 millimetres, the sprue is separated directly in the mould using a three-platen mechanism. The first separation removes the sprue from the part by tearing, the second separation helps to demould the parts. The mould temperature required for optimum material preparation is 55°C, the material temperature is 155°C.

The finished moulded part is demoulded by means of a sleeve ejector. Centring cones aid accurate centring of both mould halves as well as the positioning of the gripper and robotic systems.

A few impressive figures concerning the micro gear wheel illustrate the degree of precision which had to be used when planning and designing the mould. The internal diameter of the mould is 525 µm, it has a thickness of 0.2 mm and the size of the root circle is 970 µm. The ten cog teeth are moulded as involutes.

The Material

A curable, rust-free 17-4 PH type stainless steel from Osprey was used to produce these micro parts. The gas atomised powder is very fine, meaning that the proportion of particle sizes below 4 µm is approx. 95%. The material was made into a compound directly at ARBURG with the Clariant binder Licomont and various additives. Trials have also been carried out with zirconium oxide powder, a ceramic material from Tosoh and the same binder system. These trials have shown that the micro gear wheel can also be produced from ceramics with the same good moulding and demoulding properties and the same level of quality

Partnership Receives Financial Support

The research consortium, which will also integrate the companies Cremer Thermoprozessanlagen (furnace construction), Inmatec (material preparation) and Förster MIM¬Technik (production) in the future, was formed on the initiative of the Institute of Machine Tools and Production Science (wbk) at the University of Karlsruhe. The consortium is supported by the Stiftung Industrieforschung (a German foundation for industrial research).

In order to demonstrate the further potential of the process described, additional approaches to the innovative miniaturisation of micro PIM parts having complex moulded part geometries are planned such bevel gears with maximum dimensions of less than one millimetre to the realisation of 3D physiognomies.
The Research Partners

Herbert Kugele: With ten employees the company manufactures injection moulds, prototype and pilot series as well as technical plastic parts up to a shot capacity of 120 grams. Areas of expertise include injection moulds for technical plastic parts in the field of strip conductor encapsulation in particular, as well as metal powder and two component injection moulds. Moulds and handling devices for micro MIM components are being developed in close cooperation with the wbk.

wbk: The Institute of Machine Tools and Production Science (wbk) is one of the four institutes in the Engineering faculty at the University of Karlsruhe (TH) which focus on 'production technology'. The institute's approximately 40 scientific assistants, under the leadership of Prof. Dr.-Ing. Jürgen Fleischer, Prof Dr.-Ing. Jürgen Schmidt and Prof. Dr.-Ing. Hartmut Weule, work in five research groups on tasks concerning the fields of drive technology and hydraulics, manufacturing technology, handling and assembly technology, computer science for production systems and quality management as well as operational planning and organisation. One of the institute's strategic focuses is the field of micro-production technology and micro-mechatronics. Within this area, machining-based production (turning, drilling, milling) of microcomponents as well as handling, automation and quality assurance with regards to injection moulding machines are of particular interest.

ARBURG : ARBURG, is one of the leading global manufacturers of injection moulding machines for plastics processing with clamping forces between 125 kN and 4000 kN. The product range is completed by robotic systems and other peripherals. In addition to the standard machine series, their range also includes special equipment variants for specialised sectors such as PIM. Production is carried out by approx. 1600 employees, exclusively at the parent factory in Lossburg (Germany). A further 300 employees work at the 21 subsidiaries and representative offices worldwide.

Further information:

ARBURG GmbH + Co KG
Postfach 1109
72286 Lossburg,
Germany
Tel.: +49 (0) 7446 33-0
Fax: +49 (0) 7446 33-3365
e-mail: pim@arburg.com
http://www.arburg.com

Herbert Kugele
Bruckenackerstrasse 18
75328 Schömberg
Langenbrand,
Germany
e-mail: kugele.werkzeugbau@cw-net.de

Institut für Werkzeugmaschinen and Betriebstechnik Universitat Karlsruhe:
Prof. Dr.-Ing. Jürgen Fleischer
Kaiserstr.l2
76128 Karlsruhe,
Germany
e-mail: fleischer@wbk.uka.de
http://www.wbk-ka.de