Debinding in Metal Injection Molding

In most MIM feedback, the binder materials consist of polyethylene or polypropylene, wax. As these materials have different melting and vaporization temperature. Therefore, we need to remove these materials in 2 separate stages, and avoid any deformation and defect in the debinding process. Because these kinds of binder removal depends on the part thickness, ZCMIM set upper limits on section thickness to 50 mm.

Normal MIM binder system composition:

As we can notice that paraffin wax, has lower melting temperature than polymer materials like polymethyl methacrylate(PMMA) and Polypropylene(PP). In order to increase binder removal efficiency and avoid any deformation and defect of green parts. Separate debinding normally remove wax at first, and then polymer next.

As wax removal is the first step. In this step, we always apply 2 different methods to remove paraffin wax or polythylene glycol at low temperature.

Thermal wax debinding is the most common method. Green parts will be set in debinding furnaces on chambers. Then increase furnaces temperature above the wax melting temperature, and pitch the chamber on a 3° tilt. In order to collect molten wax from the chamber wall and bottom into the wax reservoir pot. Meanwhile, gas flow will sweep away a large volume of wax vapor, the gas flow rate should be up to 25 time greater than hot zone volume per hour for effective wax removal. Therefore, The retort gas flows over trays to sweep away wax off-gases and provide constant pressure in resort during debinding. This wax debining method is successful for stainless steel MIM processing, it has advantage of lower initial investment, lower operation and maintenance cost.

In this debinding process, solvent extracts out paraffin wax or other soluble binder materials. We normally apply acetone or heptane as solvent for this binder composition. There are 2 kind of solvent wax removal methods. In the first method, green parts will be immersed in 60℃ temperature solvent with continuous stirring for hours, then dried in ovens to remove remaining solvent and cooled down. In the second method, we place green parts in a solvent bath of vaporized heptane or acetone at the temperature of 60℃ for hours.

Temperature of solvent is the key factor to affect wax removal efficiency. therefore, the solvent debinding temperature affect the diffusion and solubility of wax directly. 60℃ is the optimum temperature close to paraffin wax melting temperature. In conclusion, this temperature can encourage the highest percent of wax loss and avoid any deformation and defect on green compact. Otherwise, too high extracting temperature will affect debound compacts quality, give rise to swelling and crack generation on debound parts surface.

Solvent debinding still take longer time than catalytic binder removal. However, it has advantage with lower investment and operation cost in total process.

Polymer materials in binder system have higher melting and degradation temperature than paraffin wax. For instance, polymethyl methacrylate (PMMA).  So in the polymer removal process, thermal debinding at optimum degradation temperature is the most effective method. At this temperature, ploymer materials will be broken down into CO, CO2 and low molecular weight hydrocarbon gases. Furthermore, this polymer debinding is in a vacuum furnace, in order to avoid oxide formation on the surface of metal powder.

After polymer debinding, there are still some porous polymer ligament in brown parts. It will bind and hold metal particles together to provide sufficient supporting strength for brown parts. These component will be removed in the final sintering process.