Feature article: PIM International, Vol.3 No. 1 March 2009, pages 21-27, 3251 words

Author: Dr. Georg Schlieper, Ingenieurbüro Gammatec, Germany

Ingenieurbüro Gammatec, Alleestr. 101, D-42853 Remscheid, Germany

Sintering is the binding together of metal or alloy powder particles when heated to high temperatures. As will become clear in this latest feature article in our ‘back-to-basics’ series by Dr. Georg Schlieper, it is also the most complicated aspect of MIM, requiring a detailed understanding of sintering practice and the furnaces used.

Introduction

A sintering furnace has to provide sufficient heat to sinter parts to full or near full density. At first glance this is a relatively simple task. If one looks at the sintering process in detail, however, a sintering furnace turns out to be a very complex system. Temperature is, however, not the only key factor. Atmosphere, pressure and the chemical interactions between the atmosphere and other materials in the furnace such as residual binders, carrier trays, walls, linings and heating elements, all play important roles. The speed of chemical reactions at elevated temperatures is of course much faster than those at ambient temperatures. A clear understanding of these interactions is therefore necessary for the production of high quality MIM products.

A ‘green’ MIM part contains approximately 40-50% by volume, or 6-8% by mass, organic binder. At least two thirds of the binder is removed in the debinding step (through either catalytic, thermal, solvent or supercritical debinding) prior to sintering so that the ‘brown’ MIM compacts that enter the sintering furnace have an open pore network. At this stage the compacts still contain the so-called ‘backbone’ binder, a polymer that holds the powder particles together and guarantees the stability and shape retention of the parts.

In some cases the debinding step is integrated in the sintering furnace, however we will not cover this stage of the process in detail here. This report will focus on sintering practice and the furnace types currently used in the MIM industry......

Further sections of this article include:

- The initial stage of sintering
- Interaction with the furnace atmosphere
- The later stages of sintering
- Sintering furnace types for MIM
- A comparison of continuous vs. batch furnaces
- Drawbacks
- Conclusions

Figures and Tables:

Fig. 1 Diffusion during sintering: 1-volume, 2-grain boundary, 3-surface diffusion (Courtesy EPMA)
Fig. 2 The three stages of solid state sintering (Courtesy EPMA)
Fig. 3 Proportion of open and closed porosity in sintered materials (Courtesy EPMA)
Fig. 4 Centorr’s MIM-Vac MTM modular style MIM debind and sinter furnace. This 3570 series model can operate up to 1650°C
Fig. 5 An industrial vacuum furnace for MIM from Fours BMI Industriels. This B53TM model offers a vacuum up to 5x10-6 mbar and is designed to operate at temperatures of up to 1450°C
Fig. 6 Integrated debinding and sintering pusher furnace (Courtesy Elino GmbH)
Fig. 7 Basic design of a pusher furnace (Courtesy EPMA)
Fig. 8 Principle of a walking beam transport mechanism (Courtesy Cremer GmbH)
Fig. 9 Pusher furnace for MIM production at Polymer Technologies Inc., (PTI), USA, as published in PIM International, Vol 1 No 3.

Table 1 Manufacturers of sintering furnaces for MIM
Table 2 Comparison of batch and continuous furnaces