What are the differences among 【Powder metal】, 〖Sintering〗, 『powder metallurgy』 and Metal injection molding ?
People always get confused by these various terms! No worries, let me introduce them to you.
Actually, Powder metal or PM is the short form of powder metallurgy, and some people prefer to use the word of Sintering which is the step contained in powder metal process to express the term. Metal injection molding is a more advanced process that develops from powder metallurgy.
The main differences between Powder metal and Metal injection molding are:
- Powder metal can only make the part whose structure is simply Up-down way or Left-right way, such as gear and flange; metal injection molding can deal with any complex structure.
- Normally, the weight of a single powder metal part is upon 30 grams; while the weight of a single metal injection molding part is usually below 60 grams.
- The main steps of Powder metal process are die pressing and sintering; while the main steps of Metal injection molding are to inject feedstock into the mold and sintering.
Check the first picture above, and the steps of Metal injection molding are as following:
- Prepare the feedstock which is formed by mixing and granulating metal powders with polymer binders.
- To inject the feedstock into a well machined, net shaped and precise mold, the mold cavities are designed approximately 20% larger to account for the part shrinkage during sintering.
- To eject the part when it cools and solidifies into a particular shape, we call it “green part” here.
- Debinding – This step removes the polymer binder from the metal. In some cases, solvent debinding is first performed in which the “green” part is placed in a water or chemical bath to dissolve most of the binder. After (on in place of) this step, thermal debinding or pre-sintering is performed. The “green” part is heated in a low temperature oven, allowing the polymer binder to be removed via evaporation. we call it “brown part” here, and it still contain approximately 40% empty space by volume
- The final step is to sinter the “brown” part in a high temperature furnace (up to 2500°F) in order to reduce the empty space to approximately 1-5%, resulting in a high-density (95-99%) metal part. The furnace uses an atmosphere of inert gases and attains temperatures close to 85% of the metal’s melting point. This process removes pores from the material, causing the part to shrink to 75-85% of its molded size. However, this shrinkage occurs uniformly and can be accurately predicted. The resulting part retains the original molded shape with high tolerances, but is now of much greater density.
Check the second and the third pictures above, we can see that after the sintering process, no secondary operations will be required to improve tolerance or surface finish.