Focus: IMMCAutomated production of micro-MIM gear validated by a consortium

Working together, a research institute, a toolmaker specializing in micromolds, and a machine supplier have shown the way to fully automated production of a microgear wheel by metal powder injection. And it works with ceramic, too.

There are surely differences between molding powdered metal and molding plastic, but in one important sense they are exactly the same. If you can automate the process for an extended run, your chances of making a profit increase dramatically. A group of organizations recently proved that could be done, even when the PIM (powder injection molded) part is a gear wheel with a finished diameter of 1200 µm and a thickness of only 200 µm.

The all-German team for this project consisted of The Institute for Machine Tools & Production Science (wbk) of the University of Karlsruhe; Kugele, a moldmaker specializing in tools for micro parts (less than 120g); and machine builder Arburg. Each of them faced significant challenges in making this project work, each of which related to the need for ultraprecision.

Machine, Mold, Materials

The production machine is a 50-ton Arburg Allrounder 320 C with a 15-mm PIM-geometry plasticating screw featuring reduced compression and modified zone splitting. For this gear, the shot volume is 1.1 cu cm and the injection flow rate is 10 cu cm/sec. The switchover pressure is 900 bar (13,053 psi); holding pressure time is .5 second; and the pressure decreases from 900 bar to 25 bar (363 psi) in .5 second. Arburg says that the 15-mm screw is very sensitive to the smallest process changes, which permitted filling analyses to be done with switchover point parameter steps of .005 cu cm to determine optimum cavity filling.

The mold from Kugele uses a three-plate mechanism to handle sprue separation, a critical step in making this process fully automatic. The first opening removes the sprue from the part while the second opening begins the demolding process, which is completed by a sleeve ejector. The centering cones used for accurate positioning of the mold halves also support precise positioning of the gripper and robotic systems. The optimum temperature for the mold is 55ºC (131°F); for the material it is 155ºC (311°F). Scrap material is 100% reusable.

The gripper system for the robot and the stacking and transport technology for the automation process were developed and optimized by wbk. The positioning tolerance for both the gripper system and the robot are less than ±2 µm for this application.

The robot is a horizontally operating Arburg Multilift H, which uses servodriven movement axes for positioning accuracy. A centering system docks the gripper on the movable mold half, positioning the gripper hand for accurate part removal without realignment. A pneumatically actuated vacuum gripper ensures gentle handling of the green parts, including air-assisted set-down and stacking. Parts are positioned on ceramic trays that are stacked and then automatically transferred to debinding and sintering stations.

A curable, rust-free 17-4PH steel from Osprey is used to mold the gear wheels in this program. It is supplied as a gas-atomized powder with 95% of the grains smaller than 4 µm. The powdered steel was compounded in Arburg’s PIM lab using Clariant’s Licomont binder and unspecified additives. Subsequent trials done using a powdered zirconium oxide supplied by Tosoh and the same binder system showed that the microgear could be produced in ceramic with the same good molding and demolding parameters, and the same level of finished part quality.

This cooperative project, which was initiated by wbk, will also be joined by furnace maker Cremer, materials supplier Inmatec, and micromolder Förster MIM-Technik. Additional approaches to miniaturizing PIM parts outside of automation are being considered—for example, how to produce more complex part geometries. Among the objectives are bevel wheels with submillimeter maximum dimensions and 3-D structures; the group is also focused on how to achieve free-form faces.