**2. Overview of process**

The PIM process presents countless variations which are used in the industry today. Invariably, it consists of four steps (Gonçalves, 2001; Tay *et al*, 2009):


In Fig. 1 a flow chart illustrating the main stages of PIM is presented.

Fig. 1. Flow chart illustrating the main stages of PIM.

this, several contract electronic assembly firms that used to purchase MIM components from custom molders have now elected to make from MIM a captive operation (German, 2008). This could lead to rapid growth for those operations since the relationship between design

Stainless steel continues to dominate MIM applications, accounting for around half of the global production, reflecting the capture of components that would have otherwise gone to investment casting. Nevertheless, other materials as copper, nickel alloys, bronze and more recently tungsten alloys and titanium also represent important markets (German, 2008). On the other hand, the CIM market is mainly dominated by products made out of alumina,

Typical components produced by PIM, either MIM or CIM, range from 0.002 to over 100 g (German, 2008). Furthermore, in both segments it is clear the progressive move towards smaller products, demanding improved technologies regarding machinery, process, and

The PIM process presents countless variations which are used in the industry today.

Binder

Binder

**Solvent Catalyst Heat**

Injection molding of the mixture (feedstock)

zirconia, and silicon or aluminium nitride powders (Ruprecht et al, 2002).

materials, hence leaving a still open field for applied research.

Invariably, it consists of four steps (Gonçalves, 2001; Tay *et al*, 2009):

In Fig. 1 a flow chart illustrating the main stages of PIM is presented.

**stock**

Debinding of green part Sintering of brown part

and production can be improved.

**2. Overview of process** 

 Feedstock preparation; Injection molding; Debinding; Sintering.

**TEMP**

Fig. 1. Flow chart illustrating the main stages of PIM.

**Binder Powder Feed-**

Mixing of multi-component binder and metal (or ceramic) powder

The starting material for PIM, usually termed feedstock, is a homogeneous pelletized mixture of metal or ceramic powder and an organic multi-component binder. The binder and the powder are combined in a variety of compounding equipment, such as extruders and mixers. The mixture is then pelletized to an appropriate shape for feeding into the molding machine. The binder is simply a carrier medium for the powder and once a part is molded, the binder is removed in a subsequent step.

The injection molding process is mainly identical to conventional plastic injection molding. Nevertheless, some machine hardware changes are usually required to process a specific feedstock based on their compressibility and viscosity. Control of the molding process is vital for maintaining tight tolerances in subsequent steps. Most design advantages of PIM technology are captured during molding by relying on the flexibility of incorporating complexities in the tool. A molded part is called a "green part" and is oversized to allow shrinkage during sintering (Tandon, 2008).

Before sintering, it is necessary to remove the binder from the molded part. The debinding is the most expensive and time-consuming stage in the PIM technology. This removal process should be based on a progressive opening of the surface channels to facilitate the removal of vapors inside the nucleus (Gonçalves, 2001). Three main methods can be applied depending on the composition of the binder: thermal, solvent, and catalytic. In thermal debinding, the binder is removed by degradation, evaporation, or liquid extraction, at temperatures ranging from 60 to 600 °C. The relatively long time associated with thermal debinding is greatly reduced using an organic solvent or in some cases even water to dissolve the soluble components of the binder, in the so called solvent debinding (Tandon, 2008). The catalytic debinding, in turn, focus in a solid-to-vapor catalytic degradation, as it is the case of exposing acetal-polyolefin-based feedstocks to acid vapors, resulting in much faster binder removal and superior handling strength when compared to thermal or solvent debinding (Krueger, 1996; Mathew & Mastromatteo, 2003). Nevertheless, it is worth to point out that in all debinding methods, a skeleton of backbone binder often remains to impart adequate strength and shape retention up to the onset of sintering. This remaining backbone is thermally removed between 200 °C and 600 °C in a pre-sintering step (Tandon, 2008).

Sintering is the last stage of the process, providing the inter-particle bonding that generates the attractive properties from otherwise loose powder mass. Depending on the material, debound parts or "brown parts" are sintered at temperatures ranging from 1200 to 1600 °C. It is essentially a removal of pores, accompanied by growth and strong adhesion among the adjacent particles, causing the retraction of the product whose dimensions usually reduce between 14 and 20% (Gonçalves, 2001; Krug, 2002; Tandon, 2008). Therefore, green parts are oversized to compensate for the sintering shrinkage. The fine particle size used in the PIM process results in high sintered density ranging from 95% to 99.5% of theoretical, thus providing superior mechanical and corrosion properties as compared to press and sinter technology.

The following sections of this chapter have the goal to describe in more detail the different steps of the PIM and the desired characteristics of raw materials to be used during this process.

Powder Injection Molding of Metal and Ceramic Parts 69

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

 Fig. 3. Z-blade mixers for batch production of feedstock materials (courtesy of Winkworth

 Fig. 4. Shear rolls for continuous production of feedstock materials (courtesy of Bellaform

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

Binder vehicles used for PIM are usually designed as multi-component systems. One of the main components is termed backbone, which is a thermoplastic polymer that supports and maintains the shape of the molded part until the last stages of debinding (Thomas-Vielma *et al*, 2008). As examples of currently used backbones, it is possible to mention ethylene vinyl

feedstock preparation (Hausnerová, 2011).

Mixer Co., UK, www.mixer.co.uk).

GmbH, Germany, www.bellaform.com).

**3.1 Binder formulation** 

processes, and their desirable properties are indicated below.
