Feature article: PIM International, Vol.3 No. 3 September 2009, pages 35-40, 3346 words

Author: Dr Georg Schlieper, Ingenieurbüro Gammatec, Germany

Ingenieurbüro Gammatec, Mermbacher Str. 28, D-42477 Radevormwald, Germany

Although MIM is a near-net shape process, the majority of components are not ready for shipping when they come out of the sintering furnace. A wide variety of secondary operations are applied before products are delivered to the end user with, in many instances, a number of processes having been performed. Dr. Georg Schlieper continues the ‘Back-to-Basics’ series with a review of the main secondary and finishing operations applied to MIM components.

The great design flexibility of injection moulding allows us to create almost any desired shape, however, surface finish and the dimensional accuracy of the final sintered product is influenced by a number of factors. Parts are usually relatively soft after sintering and it may be necessary to improve their strength by subsequent heat treatment. If the environment in which the parts are used might lead to corrosive attack, a protective coating is often applied. In this feature we will attempt to give an overview of the processes applied as secondary and finishing operations for MIM components, as well as drawing attention to the specific properties of MIM materials and the consequences of applying these finishing processes.

The surface roughness of a MIM part after sintering is influenced to some extent by the particle size of the starting powder. Sintered MIM parts usually have a dull surface appearance. Finer powders tend to improve the surface finish as compared to coarser powders and are used in many applications, but even the finest powders will not be able to produce a shiny surface, as is required on many products for convenience goods such as stainless steel watch cases or mobile phone parts (Fig. 1). A great variety of surface finishing processes are applied to improve the aesthetic appearance of MIM components or for functional reasons.

Further sections of this article include:

- Hot isostatic pressing
- Cold forming
- Fine machining
- Heat treatments
Quench and temper
Case hardening
Kolsterising
Nitriding
Induction hardening
Precipitation hardening
- Surface finishing
Barrel or vibrator finishing, polishing
Electro-polishing
Pickling
Burnishing
Anodising
- Coating techniques
Electroplating
Electroless nickel plating
CVD, PVD
- Joining techniques

Figures and Tables:

Fig. 1 Polished MIM 17-4 PH stainless steel part used in Motorola’s PEBL phones. These parts are coined, machined, polished and plated. Machining these parts, an alternative fabrication method, would have cost five times more than MIM (Courtesy Advanced Materials Technologies Pte Ltd, Singapore, & MPIF, USA)

Fig. 2 Hot isostatic press (HIP) schematic (Courtesy Bodycote)

Fig. 3 Pore structure of MIM-316L sintered (above) and HIPped (below) (Courtesy Bodycote)

Fig. 4 Pore structure at the surface of a MIM 316L stainless steel component (Courtesy CETEHOR, France, International PM Directory 13th Edition)

Fig. 5 A MIM Trigger Guard produced by Megamet Solid Metals Inc. Secondary operations on this part include reaming three holes, tapping two screw holes and deburring. The part is provided to the customer in the as sintered condition and the customer applies the black oxide surface finish (Courtesy MPIF)

Fig. 6 The microstructures of a PIM Fe - 2% Ni alloy with two post sintering heat treatments; slow cooling (left) giving a hardness of 79 HRB versus a quenched and tempered (right) giving a hardness of 45 HRC. Courtesy H Zhang. Reproduced with permission from the book ‘Injection Molding of Metals and Ceramics’, Randall M German and Animesh Bose

Fig. 7 Micrograph of kolsterised MIM 316L, surface layer thickness 0.42mm (Courtesy Bodycote)

Fig. 8 Quench and temper heat treated MIM rocker arm components for BMW (Courtesy Schunk Sintermetalltechnik GmbH, Germany)

Fig. 9 These injection nozzles are used in the stabilisation of cracked concrete structures. Manufactured from MIM-100Cr6, the parts are vacuum sintered to a density of 7.55 g/cm³, gas nitrocarburised with subsequent oxidation for a surface hardness of 830 HV. (Courtesy Listemann AG, Liechtenstein, International PM Directory 13th Edition)

Fig. 10 This MIM tensioning component has been polished for 4 hours in a Rostler polisher using ceramic stones (Courtesy Egide UK Ltd)

Fig. 11 An Alloy 42 component, gold plated to a thickness of 1.3 microns minimum, over electroless nickel (Courtesy Egide UK Ltd)

Fig. 12 Replaceable PIM coated cemented carbide drilling insert heads (Courtesy Seco, as published in “PIM of cemented carbides: a global industry with a bright future”, PIM International Vol.1 No.3, p17-24)

Table 1 General dimensional tolerances of MIM parts (Courtesy EPMA)

Table 2 Examples of the mechanical properties from heat treated MIM steels. Reproduced with permission from the book ‘Powder Injection Molding: Design & Applications’, Randall M German