How to choose adhesives for MIM process
In MIM process, the function of the binder is to fill the pores between the powders to reduce the friction between the metal powders, so that the projectile can be injected with fluidity. However, after the embryo body is formed, its task has been completed and it must be removed.
Because in MIM formula, the proportion of metal powder is quite low, body in the process of removing adhesives could collapse and deformation or fracture, so skim must be slow and gradual manner, and that is why most of the adhesives have two to five kinds of ingredients, the function of each component is different, degreasing methods are also different, The first component is backbone binder or backbone binder, which functions as bonding powder. These main adhesives are polymer plastics, such as polyethylene (PE), poly propylene (PP), etc. The other major component is filler, whose main function is to fill the pores between powders and make the propellant have a low viscosity. Paraffin wax (PARAFFin wax,PW,C20H42 ~ C 40H82) is usually the main component of the binder. Also commonly used are coupling agents, such as stearic acid (SA), whose functional groups form bonds with the powder surface to enhance the binding force between the binder and powder and improve the strength of the embryonic body. In addition, small amounts of plasticizer are sometimes added to reduce the viscosity of the injection material. These small amounts of plasticizer can also reduce the adhesion between the material and the mold, making it easier to release the mold.
Mixing these different binders creates a material with multiple melting points, Differential scanning Calorim-ETRY curve (DSC curve) of the binder containing polypropylene, paraffin and stearic acid shows that the mixed binder system still has the melting point of each single component, but the temperature is slightly lower than that of the single component. This indicates that a small amount of intersolubility occurs between the components of the binder, but each component retains roughly its original properties. The tremogravimetric curve (TGA Curve) of the binder during thermal decomposition in nitrogen indicates that the decomposition of the binder is segmented, and each single component is decomposed within its specific temperature range. In contrast, if the binder is a single component, it is easy to produce foaming, cracking, collapse and other defects due to rapid mass decomposition in a narrow temperature range, so most of the industry uses multi-component binder.