During this thermal binder removal process, the binder in the parts softened and the risk of distortion was extremely high. Further, the time consuming binder removal process resulted in high processing cost.

Generally speaking, the binders used in the original metal injection moulding process were mixtures of a polymer like polyethylene or polypropylene, a synthetic or natural wax and stearic acid.

MIM 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.

During this thermal binder removal process, the binder in the parts softened and the risk of distortion was extremely high. Further, the time consuming binder removal process resulted in high processing cost.

Polyacetal binder systems

A significant progress towards a reliable metal injection molding 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. It can even be integrated in a continuous binder removal and sintering process as the inevitable slight vibrations imposed on the parts during transport through the furnace are not detrimental. Today a large portion of metal injection moulded parts are produced according to this patented process.

Solvent debinding

Other binder removal techniques have been developed, among which the most successful was solvent debinding. The binder composition includes a constituent that can be dissolved in a liquid at low temperature so that a network of interconnected porosity is formed in the part while immersed in the solvent.

Acetone 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.

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.

Supercritical debinding

Recently an innovative binder removal technique using supercritical carbon dioxide (CO2) has been reported. The supercritical phenomenon is exhibited by gases above a certain combination of temperature and pressure.

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 3 hours.

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