Binders and binder removal techniques
Essential in the forming of MIM parts, challenging to remove
The most difficult challenge in the early days of MIM technology was to find suitable binder compositions.
The binder is the most important part of the entire process and has to fulfill several tasks. The binder must:
- Be able to incorporate a high volume of fine metal or ceramic powders, typically 60% by volume
- Form a coherent mass that can be plastified and injection moulded at elevated temperature
- Allow removal of the main binder constituent in a reasonably short, environmentally friendly process
- Still provide enough strength after debinding by means of the ‘backbone binder’
- Be supplied in a regular granular form that can easily be fed into an injection moulding machine
- Have consistent, uniform properties from batch to batch
- Runners and green scrap should be easily recyclable
- Be cost effective
The development of MIM technology was to a great extent the development of binder compositions and the corresponding debinding technologies. The development can be traced from the late 1970’s when the potential of Raymond Wiech’s basic invention of the MIM process was recognised by the PM community, to the beginning of the 1990’s when the real industrialisation of the technology began.
Generally speaking, the binders developed in the original MIM process were mixtures of a polymer such as polyethylene or polypropylene, a synthetic or natural wax and stearic acid. Many of these types of binder systems are still successfully used today.
The feedstocks based on this type of binder were easy to mould, but the removal of the binder required very careful heating in a thermal process lasting 24 or more hours before a network of interconnected porosity was created through which the remaining binder could easily evaporate without destroying the part.
A significant progress towards a reliable MIM manufacturing process for volume production was made by the invention of a binder system based on polyoxymethylene (POM). This so-called polyacetal binder system provides good mouldability and excellent shape retention.
Binder removal is done in a gaseous acid environment, i.e. highly concentrated nitric or oxalic acid, at a temperature of approximately 120°C which is below the softening temperature of the binder. The acid acts as a catalyst in the decomposition of the polymer binder. Reaction products are burnt in a natural gas flame at temperatures above 600°C. The process yields parts with an interconnected porosity in approximately 3 hours.
Today a large portion of MIM parts are produced according to this patented process.
Acetone or heptane is sometimes used as the solvent although water soluble binder compositions are preferred since the handling of aqueous solvents is easier than that of organic solvents. The solvent which is contaminated with the binder after debinding is distilled and recycled. Fig. 8 shows several tanks for solvent debinding.
Although solvent debinding may take longer than catalytic binder removal, the investment and operating costs are lower so that the total processing costs are competitive.
Recently an innovative binder removal technique using supercritical carbon dioxide (CO2) has been reported. The critical point marks the temperature and pressure where the gas can no longer be brought to the liquid state of aggregation.
For carbon dioxide the critical temperature is 31°C and the critical pressure is 7.4 MPa. Under these conditions the density of CO2 is approximately 0.5 g/cm³, i.e. less than in the liquid state but much higher than in the gaseous form. Thus the supercritical state is somewhere between the liquid and the gaseous state. It is characterised by an extremely low viscosity which allows the molecules to penetrate into the fine pore channels that are created during debinding.
The processing time in debinding of MIM parts is claimed to be about three hours.