Binders and additives that used in metal injection molding

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In metal injection molding process, the addition and removal of binders are the most major characteristics. Compared with machining, casting as well as other powder metallurgy technologies, binder is the unique component of metal injection molding. It is only because organic binders are added in the forming process, the forming and mold filling of metal parts can become more fluent, thus it can provide freer shape and design space for parts production and designers.

Additives are used in assisting binders in the metal injection molding process, thus binders will play biggest effect. Based on the key function of binders in the whole process, the composition of binders and additives will determine the volume change and intensity change of forming blanks in the solidification and cooling process after demoulding as well as the release of residual stress in the blanks after removing binders, which will have direct influence on the shape preservation ability of forming blanks in the degreasing and sintering process as well as will even determine the comprehensive property and yield of final products.

binders and metal powdersRequirements on the ideal binders are as follows: it should have good wettability with metal powders, should have high mobility and good heat stability in the injection molding process, can rapidly resolve in the degreasing process or can be easy to volatilize and will not remain hazardous substance. Besides, it should have certain cohesive property and strength under room temperature so as to ensure that the blanks can keep certain shape, will not craze or bond with die wall after demoulding process. In addition, this kind of binder should be easy to remove. Non-toxic and non-pollution green material as well as materials that have reasonable price will be most appropriate.

Currently, the commonly used binder system can be divided into three kinds: thermoset system, thermoplasticity system and colloid base system. Among them, thermoplasticity system binder has been widely used and colloid base system binder also has achieved rapid development in recent years, while thermoset system binder is seldom used as it is limited by several specific materials.

When choosing binders, MIM manufacturing enterprises should not only consider the expansion coefficient of every component of binders under different temperature and compressibility under different pressure, they should also consider the viscosity and thermal composition condition of every component under different temperature.

Regarding binders that contain single component, as it has low viscosity, it can be easy to evenly mix with metal powders in the feeding preparation process as well as will also have good performance in the injection process, but it will be easy to generate excessive gases in the degreasing process, besides, part of binders will be left in the blanks and will be difficult to remove thoroughly. Moreover, due to the intense gas volatilization, it will cause collapse of degreasing blanks. However, regarding parts that have small size, they will seldom have this kind of defect. Therefore, single component binder could be a good choice for small size parts.

Multicomponent binder refers to the binder system that is composed by various components, in which the components will have different thermal properties as well as can volatilize and resolve under different temperature periods. Therefore, in the degreasing process, binders should be removed stepwise according to different level. The advantages of multicomponent binders are as follows: it can be targetedly combined and developed into appropriate binder system according to different kinds of metal powders, besides, as it has wide application range, it can be both used in small size and medium size parts. However, the advantages of multicomponent binders can also be transformed into disadvantages. This is because different kinds of components will be easy to separate with each other in the injection process or feeding preparation process. Therefore, by adding additives, it will prevent separation between different components as well as multicomponent binder will play larger effect.