Binder can be regarded as the key element from feeding preparation process to the degreasing and sintering of forming blanks in the whole injection molding process. It mainly has two effects in the whole process, which are as follows: a. it can improve metal powder mobility, thus metal injection feeding will have high mobility; b. it can maintain the shape of forming blanks. However, as it also will influence the performance of final products, it should be removed clearly after injection molding.
As rich experience and scientific methods are both needed in adding and removing binders, research on binder system has drawn many attentions by the international industry. Therefore, the adding and removing technology of binders are both very important. It can be seen that binder is the core technology of metal powder metallurgy MIM technology.
Currently, in the research and development of binders, binder function can be designed according to different degreasing technologies. As a result, several major binder systems have been formed. Among them, the most important system will be thermoplastic binder system. Below we will learn about the knowledge of this system.
Thermoplastic binder system mainly includes paraffin base, oil base and thermoplastics polymer base binders. Among them, paraffin base binder system was used earliest and still has strong competitive force until now, which will have obvious advantages in producing parts that have the wall thickness of less than 3mm. Paraffin will have low viscosity under high temperature, will have good compatibility with plastics as well as will have high powder loading capacity, but it will largely shrink in the cooling process as well as will generate high internal stress and will degrease slowly.
Oil base binder mainly follows the principle that oil will appear to be liquid state or semi-solid state under room temperature. Compared with paraffin base binder, it will have lower internal stress as well as can degrease quickly by using solvent degreasing methods. However, the difficulty of using oil base binder may lie in that it has to maintain certain green part strength when the oil content has increased so as to prevent two-phase separation, besides, it has to solve swelling and stress cracking problem in the rapid solvent degreasing process.
Generally speaking, as a large amount of polymers will be used in thermoplastic polymer base binders, green parts will have high strength, however, the excessive polymers will also lead to slow degreasing and low powder loading capacity. Binders that produced by BASF SE in 1990’s have been successfully used in the large-scale industrial production. This company has successfully solved the disadvantages of binder system by using unique methods. This kind of binder is composed by more than 90 percent of modified aldehyde resin and a small amount of additives, thus it can maintain shape under high temperature and reduce viscosity, besides, it not only has high powder loading capacity, but also has same magnitude order of feeding viscosity with paraffin base, which can be used in a wide range of powders.
Except for thermoplastic binder system, there are also other main kinds of binder systems including gel system, thermoset system and water soluble system. Among them, parts that produced by using gel system will have the wall thickness of 20mm and the weight can reach 800g, but the blanks will have low strength as well as is also difficult to demold; as the waste of thermoset system cannot be repeatedly used, it cannot realize large-scale application; due to the immature technology of water soluble system, it has always stayed in laboratory stage.
In summary, metal powder metallurgy has numerous binder systems. Each system both has advantages and disadvantages. Besides, binder characteristics will have large influence on the final performance of metal products. Therefore, in order to produce MIM products that have higher quality and better performance, it will become one of the important research contents of MIM technology in the future by developing new type and high-efficiency binder system.
