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MIM Mixing
Dec 20, 2018

Mixing is the process of mixing metal powder with a binder to obtain a uniform feed. Since the nature of the feed determines the properties of the final injection molded product, the process step of mixing is very important. This involves various factors such as the manner and sequence of binder and powder addition, the mixing temperature, and the characteristics of the mixing device. This process step has been stuck at the level of empirical exploration. An important indicator for evaluating the quality of the mixing process is the uniformity and consistency of the obtained feed.

The mixing of the MIM feed is done under the combined effect of thermal and shear forces. The mixing temperature should not be too high, otherwise the binder may decompose or the viscosity of the binder may be separated due to too low viscosity. The shear force varies depending on the mixing method. MIM commonly used mixing devices include double screw extruder, Z-shaped impeller mixer, single screw extruder, ram extruder, double planetary mixer, double cam mixer, etc. Both are suitable for preparing a mixture having a viscosity in the range of 1-1000 Pa·s.

The mixing method generally involves first adding a high melting point component to melt, then cooling, adding a low melting point component, and then adding the metal powder in portions. This can prevent the gasification or decomposition of the low-melting component, and the addition of the metal powder in batches can prevent the torque from being rapidly increased due to too fast cooling, thereby reducing equipment loss.

For the feeding method of different particle size powders, the Japanese patent introduces: firstly add the thicker 15-40um water atomized powder into the binder, then add 5-15um powder, and finally add the powder degree ≤5um powder, thus obtained The final product shrinks very little. In order to uniformly apply a layer of binder around the powder, the metal powder may be directly added to the high melting point component, the low melting component may be added, and finally the air may be removed. For example, Anwar added the PMMA suspension directly to the stainless steel powder, then added the PEG aqueous solution, dried, and then removed the air while stirring. O'connor is mixed with solvent. The SA is mixed with the powder and then added to the tetrahydrofuran solvent. Then the polymer is added. After the tetrahydrofuran is removed from the heat, the powder is mixed to obtain a uniform feed.

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