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

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... 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. 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. 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 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 z- blade 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). 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. 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 acetate (EVA), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyethylene glycol (PEG), polymethyl methacrylate (PMMA) among others (Ahn et al , 2009; Chuankrerkkul et al , 2007; Krug et al , 2002; Thomas-Vielma et al 2008; Yang et al , 2002) (Fig. 5). The second component, usually in a proportion similar to the backbone is commonly a wax, as paraffin or carnauba wax, or in some cases even agarose, that improves the material flowability (Ahn et al , 2009). Besides improving flowability, such component should be easily removed in early stages of debinding, in general via solvent methods, leaving open pores that will allow the gaseous products of the remaining polymer to diffuse out of the structure (Thomas-Vielma et al , 2008). Even though this low-melting temperature component has an important role in the process, it is worth to mention that the mechanical integrity of the final product is reduced as its proportion increase after certain limits (Tseng & Hsu, 1999). The importance of each of these two main binder components can be better understood with a further description of the debinding mechanism. It is worth to remember that at the beginning of debinding no pores or free space are shown in the molded part, hence the backbone component has a crucial role retaining the shape of the part, and avoiding cracks while the low-melting component leaves this molded structure (Thomas-Vielma et al , 2008). In the last stages of debinding, it is due to the open porous created by this second component that the backbone can diffuse out without damaging the structure of the product. If not by these pores, an excessive pressure would easily build up within moldings from the degradation species during burnout, causing distortions and cracks (Tseng & Hsu, 1999). However, the emergence of a POM-based binder system for PIM has made it possible to remove the polymer vehicle from up to 35 mm thick sections without the use of any wax or low molecular weight component (Krug et al , 2000). As previously described, POM (Fig.5) decomposes predominantly to formaldehyde in the presence of an acid vapor (as oxalic or nitric acid) well below its softening point, that is, in the solid state, avoiding the cracks and bloating that can be caused by the boiling of the binder (Krug et al , 2001). It is also important to mention that the polymer is not penetrated by the gaseous acid and the decomposition proceeds exclusively at the gas-binder interface with a nearly planar debinding front moving through the compact. In this sense, gas exchange is limited to the already porous shell and the buildup of an internal pressure is avoided. Nevertheless, POM-based binder systems often contains up to 30% of polyethylene which does not react with acid vapors, acting as a backbone until being burned out during the sintering cycle. Finally, additives as surfactants can compose the binder, being stearic acid the most common example of them. These surface-active dispersants normally present a low melting temperature and affinity to preferentially adsorb onto powder surfaces, forming a densely thin outer layer on a particle surface which leads to a more homogeneous packing structure (Chan & Lin, 1995). However, bubbles and cracks were reported to occur as the amount of the surfactants increases, presumably owning to the reduced vaporization temperature since the surfactants are composed of mostly short molecules (Tseng & Hsu, 1999). It is from the powder material that the final product will be constituted, and its selection often involves the combination of a tailored particle size distribution to maximize packing densities. Powders for ceramics and metals can be obtained from a variety of methods; the following section will describe some of the methods used for obtaining ceramic and metallic powders of various shapes and sizes. The methods used for synthesis of ceramic powders range from mechanical methods that involve grinding or milling (commination) for size reduction of a coarse, granular material to chemical processes involving chemical reactions under carefully controlled conditions. Generally speaking, mechanical methods are considerably ...