Sintered bonds are produced by mixing of metal powder and abrasive grits, molding, either hot pressing or cold pressing, and sintering (Fig. 3.14). In exceptional cases, the metallic powder is mixed with an organic binder (paraffin oil and/or wax granule). This binder helps to produce a green compact, which can be handled, and vaporizes in the sintering process [STOC86]. The binder also leaves pore space, which might be infiltrated with a soldering agent.
In the cold pressing method, the green compact is taken out and sintered in a melting oven under inert gas atmosphere [STOC86]. Sintering takes place with or without form [STOC86]. Iron or steel compacts are sintered at 1100 °C [YOUN66]. Bronze bondings are easy to process and press and they are sintered at temperatures between 500-700 °C gesintert.
In the hot pressing method, the mixture is compressed under additional heat directly in the graphite form. Three heating types, namely inductive heating, indirect and direct resistance heating, can be used in hot pressing [DRFR05].
The cold pressing procedure allows the application of fast, mechanical pressing methods as well as sintering at neutral or reduced atmosphere in melting ovens.
Hydrogen or other inert gases provide an oxygen-free surrounding, which is important for the protection of diamond grits at higher sintering temperatures. The hot pressing procedure is a one step operation and, therefore, a time-effective process with good quality control [STOC86].
Hot isostatic pressing is a competing method for traditional hot pressing [DRFR05]. Onishi et al. produced highly porous wheels from hot isostatic pressing (HIP) of 73 w% of cast-iron or iron powder, 25 w% of diamond grits and 2 w% of wax [ONIS97]. In this procedure, the material was pressed first, presintered to remove the wax, and HIP processed.
Porosity in sintered metal bonded tools is achieved by vaporizing fillers and adjusted pressing pressure. Porosity in bronze bonding is gained from adding carbon, e. g. up to 20 V% in diamond cup grinding wheels [BUTT68, p. 74].