**3. Feedstock preparation**

The first step in the powder injection process is the preparation of feedstock materials. The powder and binder are hot mixed above the softening point of the binder constituents to provide a uniform coating on the powder surface (Fig. 2A). The feedstock is prepared by compounding polymeric binders with fine metallic or ceramic powders. Commercially available feedstock material is generally supplied in the shape of pellets (Fig. 2B), so that is easy to handle before and during the injection molding step.

Fig. 2. (A) Micrograph of feedstock material showing metallic particles surrounded by polymeric binder, (B) Pellets of commercially available feedstock material Catamold® by BASF.

The powder content usually ranges from 50 to 65% in volume, although there are claims of optimized commercial formulations in which even more than 80% is used. If the powder content is found to be lower than 50 vol.%, the sintering ability of the feedstock and the final density of the part are significantly lowered. From another standpoint, it is also important to keep the viscosity of the feedstock as low as possible in order to facilitate the injection molding process, reason for why a powder content higher than 65 vol.% should be handled with care (Merz *et al*, 2002).

One of the most important properties of the feedstock is certainly its homogeneity. A homogeneous distribution of powder particles and binder in feedstock is important as it helps to minimize segregation during the injection molding stage and later on to obtain isotropic shrinkage after debinding and sintering (Quinard *et al*, 2009). Avoiding segregation of feedstock components is necessary to prevent visual defects, excessive porosity, warpage and cracks in the sintered part (Thornagel, 2010).

The technique used for mixing binder and powder can influence the homogeneity of feedstock materials. Feedstock materials can be either produced in a batch process or continuously. Four different types of machines are generally used: high-shear mixers, roll mills, screw extruders and shear rolls. The first two are examples of batch operations while the last two are continuous. Which approach to take depends on the details of the

The first step in the powder injection process is the preparation of feedstock materials. The powder and binder are hot mixed above the softening point of the binder constituents to provide a uniform coating on the powder surface (Fig. 2A). The feedstock is prepared by compounding polymeric binders with fine metallic or ceramic powders. Commercially available feedstock material is generally supplied in the shape of pellets (Fig. 2B), so that is

(A) (B)

The powder content usually ranges from 50 to 65% in volume, although there are claims of optimized commercial formulations in which even more than 80% is used. If the powder content is found to be lower than 50 vol.%, the sintering ability of the feedstock and the final density of the part are significantly lowered. From another standpoint, it is also important to keep the viscosity of the feedstock as low as possible in order to facilitate the injection molding process, reason for why a powder content higher than 65 vol.% should be handled

One of the most important properties of the feedstock is certainly its homogeneity. A homogeneous distribution of powder particles and binder in feedstock is important as it helps to minimize segregation during the injection molding stage and later on to obtain isotropic shrinkage after debinding and sintering (Quinard *et al*, 2009). Avoiding segregation of feedstock components is necessary to prevent visual defects, excessive porosity, warpage

The technique used for mixing binder and powder can influence the homogeneity of feedstock materials. Feedstock materials can be either produced in a batch process or continuously. Four different types of machines are generally used: high-shear mixers, roll mills, screw extruders and shear rolls. The first two are examples of batch operations while the last two are continuous. Which approach to take depends on the details of the

Fig. 2. (A) Micrograph of feedstock material showing metallic particles surrounded by polymeric binder, (B) Pellets of commercially available feedstock material Catamold® by

**3. Feedstock preparation** 

BASF.

with care (Merz *et al*, 2002).

and cracks in the sintered part (Thornagel, 2010).

easy to handle before and during the injection molding step.

application and the materials to be used to prepare the feedstock (Clemens, 2009). When using fine particles, which have a tendency to agglomerate batch mixing in planetary or zblade mixers (Fig. 3) is preferred, even though the process can take a couple of hours. In high volume productions, twin-screw extruders or shear rolls (Fig. 4) are employed for feedstock preparation (Hausnerová, 2011).

Fig. 3. Z-blade mixers for batch production of feedstock materials (courtesy of Winkworth Mixer Co., UK, www.mixer.co.uk).

Fig. 4. Shear rolls for continuous production of feedstock materials (courtesy of Bellaform GmbH, Germany, www.bellaform.com).

The following sections have the purpose to further describe the two main components of the feedstock material: binder and powder. The binder formulation, powder synthesis processes, and their desirable properties are indicated below.
