What Is Binder Material?
In metal injection molding, the binder plays a crucial role. Binders are typically composed of a mixture of various polymers, including a main phase and several additive phases (such as dispersants, stabilizers, and plasticizers). The main function of the binder is to increase the flowability of the powder during injection to facilitate molding and to provide a certain strength to the part after molding. As an intermediate component, the binder not only shapes the metal powder but also maintains its shape before sintering begins. Mixing the binder with the metal powder creates a feedstock, which is used in metal injection molding. The binder is removed after injection molding and before sintering begins.
And the final properties after sintering. The properties of the binder affect the distribution of metal particles, the injection molding process, the size of the injection part, and the final properties of the sintered part. Table 4.1 summarizes the characteristics of an ideal binder system for metal injection molding. The binder and metal particles should have a small contact angle, as a smaller contact angle allows the binder to better wet the powder surface, thus facilitating mixing and injection molding. The binder and metal particles should remain mutually inert; that is, the binder should not react with the metal particles, and the metal particles should not cause the binder to polymerize or degrade. The mixture of binder and powder, i.e., the feedstock, should meet various rheological requirements to successfully mold parts free of defects. The viscosity of the feedstock should be within a reasonable range to ensure smooth injection molding. Too low a feedstock viscosity will cause phase separation between the powder and binder during molding, while too high a feedstock viscosity will affect the mixing and injection molding process. In addition to requiring the feedstock viscosity to be within an ideal range during molding, it is also required that the feedstock viscosity increase significantly upon cooling, which helps the injection preform maintain a certain shape during cooling.
Table 4.1 Characteristics of Ideal Binder Systems for Metal Injection Molding
| Item | Ideal Characteristics |
|---|---|
| Interaction with Powder | Small contact angle, good bonding performance with powder, no chemical reaction with powder |
| Flow Characteristics | Low viscosity at molding temperature, small viscosity change during molding, rapid viscosity increase during cooling, high fluidity and fillability |
| Debinding Property | Stepwise decomposition, different decomposition temperatures for different components of binder, low residual carbon content after debinding, non-toxic and non-corrosive decomposition products |
| Manufacturing Processibility | Easy to obtain, low production cost, long shelf life, safe and environmentally friendly, no degradation due to cyclic heating, high strength and hardness, low thermal expansion coefficient, easy to dissolve in common solvents, high lubricity, no chain branching in molecules, no oriented distribution |
The binder should be rapidly removed during debinding without causing defects in the injection-molded part. Defects are most likely to form in the green body during the debinding stage because the likelihood of defects increases as the strength-providing binder is removed. The lack of open pores in the initial stage of thermal debinding leads to defects such as cracks and blistering in the injection-molded part. Stress generated by the failure of polymer degradation products inside the injection-molded part to escape can also cause defects. To avoid this, the binder is generally designed as a multi-component system that decomposes at different temperatures. In this case, the debinding process can be divided into two stages: In the first stage, the low-melting-point components of the binder system are removed, creating open pores in the green body. During this process, the remaining components of the binder system provide strength and maintain the shape of the injection-molded part. In the second stage, the other components of the binder system are gradually removed. This two-step debinding method allows for faster removal of the binder from the injection-molded part. The binder should also be completely decomposable without leaving carbon residue, and the decomposition products during thermal debinding should not be corrosive to the equipment.
Binders used in metal injection molding should be readily available, inexpensive, and have a long shelf life; gate and runner waste should be reusable during the injection molding process; the binder should have good recyclability and should not degrade during reheating; the binder should have high thermal conductivity and low coefficient of thermal expansion to prevent defects caused by thermal stress.
A single binder is unlikely to meet all feed characteristics; binder systems used in injection molding typically contain multiple components, each performing a specific function. A binder system usually contains a main component, with other components acting as additives to achieve the desired feed characteristics.