**3.1 Binder formulation**

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

Powder Injection Molding of Metal and Ceramic Parts 71

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,

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

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

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 cheaper than chemical methods. However, chemical methods offer better control of the powder characteristics, such as shape

Mechanical methods are generally used to prepare powders from naturally occurring raw materials. Operations such as crushing, grinding and milling are classified as mechanical methods. Machines like jaw, gyratory and cone crushers are used to produce powders in the size range from 0.1 to 1mm. In order to achieve particles with less than 1 mm milling is generally used. Mills used today include high-compression roller mills, jet mills and ball

Chemical methods are generally used in the synthesis of powders of advanced ceramics from synthetic materials or from naturally occurring materials with a significant chemical refinement. The most common chemical methods are summarized in Table 1. It is important to mention that many chemical methods require a milling step to break down agglomerates and for determining the average particle size and particle size distribution (Rahaman, 2003).

acting as a backbone until being burned out during the sintering cycle.

the surfactants are composed of mostly short molecules (Tseng & Hsu, 1999).

**3.2 Powder manufacturing** 

**3.2.1 Ceramics** 

and size (Rahaman, 2003).

mills (Rahaman, 2003).

powders of various shapes and sizes.

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).

Fig. 5. Examples of polymers used in different binder formulations.

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

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).

Fig. 5. Examples of polymers used in different binder formulations.

during burnout, causing distortions and cracks (Tseng & Hsu, 1999).

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

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

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

product is reduced as its proportion increase after certain limits (Tseng & Hsu, 1999).

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).

#### **3.2 Powder manufacturing**

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.

#### **3.2.1 Ceramics**

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 cheaper than chemical methods. However, chemical methods offer better control of the powder characteristics, such as shape and size (Rahaman, 2003).

Mechanical methods are generally used to prepare powders from naturally occurring raw materials. Operations such as crushing, grinding and milling are classified as mechanical methods. Machines like jaw, gyratory and cone crushers are used to produce powders in the size range from 0.1 to 1mm. In order to achieve particles with less than 1 mm milling is generally used. Mills used today include high-compression roller mills, jet mills and ball mills (Rahaman, 2003).

Chemical methods are generally used in the synthesis of powders of advanced ceramics from synthetic materials or from naturally occurring materials with a significant chemical refinement. The most common chemical methods are summarized in Table 1. It is important to mention that many chemical methods require a milling step to break down agglomerates and for determining the average particle size and particle size distribution (Rahaman, 2003).

Powder Injection Molding of Metal and Ceramic Parts 73

2000). Water atomization leads to irregular particles with a good yield of particles below 45 m. Gas atomized powders leads to mostly spherical particles (Fig. 6) but the yield of

Fig. 6. SEM micrograph of gas atomized stainless steel powder (Hartwig *et al*, 1998).

are among the purest powders available, but they are pricier (Schrader *et al*, 2000).

Metal powder can also be electrolytically deposited under the right conditions. The deposited material may have to be broken up and ground to achieve the desired fineness, heated to be annealed, drive off hydrogen, sorted and blended. Electrodeposited powders

Other less common methods of manufacturing metallic powders include vapor condensation, chemical decomposition, ordinary machining, impacting of chips and scrap, and granulation by stirring vigorously during solidification (Schrader *et al*, 2000). The most common methods applied for obtaining powders of different metals can be found in Table 2.

**Production method Used for** 

Mechanical (comminution & atomization) Steels, titanium, intermetallics

With the growing demand for microparts (Fig.7) to be used in consumer electronics, automotive parts and in the biomedical field, there is also a requirement to use powders with smaller particles and perform what is called micro-PIM. It is generally accepted that the smallest feature of a part can only be ten times larger than the mean particle diameter, thus in order to obtain microparts with good edge definition and shape retention, submicron or nanoparticles need to be used. Also, submicron powders have an enhanced sintering activity, which is beneficial for attainment of high-density bodies at lower sintering

Table 2. Common production methods for metal injection molding powders

(Hartwig *et al*, 1998).

**3.2.3 Submicron powders** 

Chemical Tantalum, tungsten Electro-chemical Copper, iron, nickel Thermo-chemical Carbonyl-iron, nickel

particles below 45 mm is limited (Hartwig *et al*, 1998).


Table 1. Commonly used chemical methods for manufacturing of ceramic powders (Rahaman, 2003).

#### **3.2.2 Metals**

Most metal powders can be produced by comminution of the refined ore. Milling and grinding are methods to produce powders of any degree of fineness from friable or malleable metals (e.g. titanium or steel). However not all particle sizes are sinterable, in general particles have to be below 45 m. Currently, there is a tendency to use submicron and nanoparticles, since a better packing density can be achieved, their activation energy is higher and their sintering temperature is lower (Shearwood *et al*, 2005). Nevertheless, care must be taken when handling nano powders since they can be explosive or can easily oxidize; therefore most of the time powder metallurgy utilizes particles in the 1 to 45 m range (Hartwig *et al*, 1998). In particular, for metal injection molding the mean particle size ranges from 5 to 15 m (Krug *et al*, 2002). This type of size particles can generally be achieved in ball mills, rotary mills, planetary mills, jet mills, vibrating grinders, stampers and crushers (Schrader *et al*, 2000).

Nickel or iron can react with carbon monoxide to form metal carbonyls; carbonyl vapors undergo decomposition by instantaneous mixing with a large volume of hot inert gas followed by quenching the aerosol formed by diluting and cooling. Metal powders from carbonyls have high purity, small and uniform grain size, and particles that are dense and round (Neikov *et al*, 2009; Schrader *et al*, 2000). The most commonly used powders in today's MIM production are carbonyl powders. Simple mixtures of iron and nickel carbonyl powders are the basis of powder mixtures for metal injection molding; other metals included in these mixtures include heat treatable steels and stainless steels (Hartwig *et al*, 1998).

Metals or alloys that may be homogeneously melted can also be atomized in a stream or air or an inert gas (often argon). Some metals are melted first and later injected through an orifice into the stream to later be dropped in water, such process is called shotting. Other metals like iron and stainless steel may be fused in an electric arc and refractory metals in a plasma arc. For example, titanium droplets freeze to powder after being thrown from the end of a rapidly rotating bar heated by a plasma arc in a helium atmosphere (Schrader *et al*,

Inexpensive, simple equipment used

Small particles, chemical homogeneity, high purity, composition control

Non-aqueous liquid reaction Small particles, high purity Limited to non-oxides

sizes

Gas-liquid reactions Small particles, high purity Expensive, limited

inexpensive for oxides

Most metal powders can be produced by comminution of the refined ore. Milling and grinding are methods to produce powders of any degree of fineness from friable or malleable metals (e.g. titanium or steel). However not all particle sizes are sinterable, in general particles have to be below 45 m. Currently, there is a tendency to use submicron and nanoparticles, since a better packing density can be achieved, their activation energy is higher and their sintering temperature is lower (Shearwood *et al*, 2005). Nevertheless, care must be taken when handling nano powders since they can be explosive or can easily oxidize; therefore most of the time powder metallurgy utilizes particles in the 1 to 45 m range (Hartwig *et al*, 1998). In particular, for metal injection molding the mean particle size ranges from 5 to 15 m (Krug *et al*, 2002). This type of size particles can generally be achieved in ball mills, rotary mills, planetary mills, jet mills, vibrating grinders, stampers

Nickel or iron can react with carbon monoxide to form metal carbonyls; carbonyl vapors undergo decomposition by instantaneous mixing with a large volume of hot inert gas followed by quenching the aerosol formed by diluting and cooling. Metal powders from carbonyls have high purity, small and uniform grain size, and particles that are dense and round (Neikov *et al*, 2009; Schrader *et al*, 2000). The most commonly used powders in today's MIM production are carbonyl powders. Simple mixtures of iron and nickel carbonyl powders are the basis of powder mixtures for metal injection molding; other metals included in these mixtures include

Metals or alloys that may be homogeneously melted can also be atomized in a stream or air or an inert gas (often argon). Some metals are melted first and later injected through an orifice into the stream to later be dropped in water, such process is called shotting. Other metals like iron and stainless steel may be fused in an electric arc and refractory metals in a plasma arc. For example, titanium droplets freeze to powder after being thrown from the end of a rapidly rotating bar heated by a plasma arc in a helium atmosphere (Schrader *et al*,

Table 1. Commonly used chemical methods for manufacturing of ceramic powders

Gas-solid reactions Inexpensive for large particle

Reaction between gasses Small particles, high purity,

Solid-state reactions: - Decomposition - Reactions between solids

Liquid solutions: -Precipitation -Solvent vaporization -Gel routes

(Rahaman, 2003).

and crushers (Schrader *et al*, 2000).

heat treatable steels and stainless steels (Hartwig *et al*, 1998).

**3.2.2 Metals** 

**Method Advantages Disadvantages** 

Agglomerated powders, limited homogeneity for multicomponent powders

Agglomerated powders, poor for non-oxides, expensive

Low purity, expensive for fine powders

applicability

Agglomerated powders, expensive for non-oxides 2000). Water atomization leads to irregular particles with a good yield of particles below 45 m. Gas atomized powders leads to mostly spherical particles (Fig. 6) but the yield of particles below 45 mm is limited (Hartwig *et al*, 1998).

Fig. 6. SEM micrograph of gas atomized stainless steel powder (Hartwig *et al*, 1998).

Metal powder can also be electrolytically deposited under the right conditions. The deposited material may have to be broken up and ground to achieve the desired fineness, heated to be annealed, drive off hydrogen, sorted and blended. Electrodeposited powders are among the purest powders available, but they are pricier (Schrader *et al*, 2000).

Other less common methods of manufacturing metallic powders include vapor condensation, chemical decomposition, ordinary machining, impacting of chips and scrap, and granulation by stirring vigorously during solidification (Schrader *et al*, 2000). The most common methods applied for obtaining powders of different metals can be found in Table 2.


Table 2. Common production methods for metal injection molding powders (Hartwig *et al*, 1998).

### **3.2.3 Submicron powders**

With the growing demand for microparts (Fig.7) to be used in consumer electronics, automotive parts and in the biomedical field, there is also a requirement to use powders with smaller particles and perform what is called micro-PIM. It is generally accepted that the smallest feature of a part can only be ten times larger than the mean particle diameter, thus in order to obtain microparts with good edge definition and shape retention, submicron or nanoparticles need to be used. Also, submicron powders have an enhanced sintering activity, which is beneficial for attainment of high-density bodies at lower sintering

Powder Injection Molding of Metal and Ceramic Parts 75

conclusions. For example, regarding the flow characteristics, German (1990) highlighted the importance of low viscosity at the molding temperature, a characteristic stressed by Liu *et al* (2001) as even more important in the case of micro moldings. Ahn *et al* (2008) mentioned that a high drop in viscosity at the high shear rates (shear-thinning behavior) is also a desired property for cavity filling with less energy, especially for complicated geometries. However, stability of the mixture should be taken into account in order to avoid powder-

The binder should also be strong and rigid after cooling and present small molecules to fit between particles (German, 1990). No agreement is found regarding the viscositytemperature dependence, which is suggested to be the least as possible by German (1990) but high by Thomas-Vielma *et al* (2007). Nevertheless, Ahn *et al* (2009) shown that contrary to the viscosity-shear rate dependence, the viscosity-temperature dependence is more

High thermal conductivity, low thermal expansion coefficient, short chain length, no orientation, low contact angle and good adhesion with powder, capillary attraction of particles and be chemically passive with respect to the powder are also desired properties

Thomas-Vielma *et al* (2007) reported that defects as cracking (Fig. 8), slumping and sagging are partly related to the swelling of the binder component during solvent debinding, a problem that can be reduced by increasing the crystallinity of the polymer acting as a backbone. Besides this, the decomposition of such polymer should be non-corrosive, non-

(A) (B)

100 μm 100 μm

1

As can be observed in this section, the thermal and time-dependent properties of the different components of the binder play an important role in the quality of the final product. Furthermore, it also influences the choice of process parameters, as injection pressure and heating ramp, determining the cost and the productivity of the process. In this sense, understanding and improving the properties of this multi-component system or its single

Fig. 8. Appearance of (A) cracks and (B) transverse cracks after thermal debinding on

components have a great importance to the optimization of the PIM technology.

that should be taken into account when selecting binder components (German, 1990).

influenced by the powder selection than the selection of the binder itself.

toxic, and having low ash content (German, 1990).

alumina parts (Thomas-Vielma *et al* , 2007).

binder separation.

temperatures. Such powders are still under development since there are many issues that need to be resolved.

Fig. 7. Ceramic microparts produced by PIM (Pioter *et al*, 2001).

Submicron powders tend to be pyrophoric and therefore it is essential to avoid any oxidation during handling. One way to avoid oxidation is to handle submicron powders in a glove box under argon; inside this box powders are coated with a binder constituent, in order to avoid oxidation in the subsequent processing steps of feedstock compounding and injection molding (Zauner, 2006). Another issue that arises as the particle size decreases below 1 m is the greater tendency for particle-particle interaction, which results in agglomerates. If many agglomerates are present, the final sintered part could have a non-uniform distribution of particles and little benefit is achieved compared to coarse particles. Therefore, the use of submicron powders requires special handling and mixing procedures in order to minimize detrimental effects due to the presence of agglomerates (Rahaman, 2003).

### **3.3 Desired properties of feedstock**

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. As shown by Quinard *et al* (2009), using a twin-screw extruder yielded a better binder distribution. However, when using a z-blade mixer a better binder volume distribution around the powder particles was achieved, that would lead to an isotropic shrinkage after sintering.

Understanding which the desired properties are for a binder is a crucial step in the optimization of PIM regarding both productivity and quality of the final product. Only a few authors have been working in this direction and not all of them agree in their

temperatures. Such powders are still under development since there are many issues that

Submicron powders tend to be pyrophoric and therefore it is essential to avoid any oxidation during handling. One way to avoid oxidation is to handle submicron powders in a glove box under argon; inside this box powders are coated with a binder constituent, in order to avoid oxidation in the subsequent processing steps of feedstock compounding and injection molding (Zauner, 2006). Another issue that arises as the particle size decreases below 1 m is the greater tendency for particle-particle interaction, which results in agglomerates. If many agglomerates are present, the final sintered part could have a non-uniform distribution of particles and little benefit is achieved compared to coarse particles. Therefore, the use of submicron powders requires special handling and mixing procedures in order to minimize detrimental effects due

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. As shown by Quinard *et al* (2009), using a twin-screw extruder yielded a better binder distribution. However, when using a z-blade mixer a better binder volume distribution around the powder particles was

Understanding which the desired properties are for a binder is a crucial step in the optimization of PIM regarding both productivity and quality of the final product. Only a few authors have been working in this direction and not all of them agree in their

Fig. 7. Ceramic microparts produced by PIM (Pioter *et al*, 2001).

achieved, that would lead to an isotropic shrinkage after sintering.

to the presence of agglomerates (Rahaman, 2003).

**3.3 Desired properties of feedstock** 

need to be resolved.

conclusions. For example, regarding the flow characteristics, German (1990) highlighted the importance of low viscosity at the molding temperature, a characteristic stressed by Liu *et al* (2001) as even more important in the case of micro moldings. Ahn *et al* (2008) mentioned that a high drop in viscosity at the high shear rates (shear-thinning behavior) is also a desired property for cavity filling with less energy, especially for complicated geometries. However, stability of the mixture should be taken into account in order to avoid powderbinder separation.

The binder should also be strong and rigid after cooling and present small molecules to fit between particles (German, 1990). No agreement is found regarding the viscositytemperature dependence, which is suggested to be the least as possible by German (1990) but high by Thomas-Vielma *et al* (2007). Nevertheless, Ahn *et al* (2009) shown that contrary to the viscosity-shear rate dependence, the viscosity-temperature dependence is more influenced by the powder selection than the selection of the binder itself.

High thermal conductivity, low thermal expansion coefficient, short chain length, no orientation, low contact angle and good adhesion with powder, capillary attraction of particles and be chemically passive with respect to the powder are also desired properties that should be taken into account when selecting binder components (German, 1990).

Thomas-Vielma *et al* (2007) reported that defects as cracking (Fig. 8), slumping and sagging are partly related to the swelling of the binder component during solvent debinding, a problem that can be reduced by increasing the crystallinity of the polymer acting as a backbone. Besides this, the decomposition of such polymer should be non-corrosive, nontoxic, and having low ash content (German, 1990).

Fig. 8. Appearance of (A) cracks and (B) transverse cracks after thermal debinding on alumina parts (Thomas-Vielma *et al* , 2007).

As can be observed in this section, the thermal and time-dependent properties of the different components of the binder play an important role in the quality of the final product. Furthermore, it also influences the choice of process parameters, as injection pressure and heating ramp, determining the cost and the productivity of the process. In this sense, understanding and improving the properties of this multi-component system or its single components have a great importance to the optimization of the PIM technology.

1

Powder Injection Molding of Metal and Ceramic Parts 77

molecules can act as internal lubricants for the larger molecules facilitating their movement. However, when in the solid state the smaller molecules could fit between the larger ones, creating a closely packed structure that cannot be deformed as easily, translating into

Once good flowability of the binder has been obtained it is important to check the flowability of the feedstock material. The flowability of feedstock will not only depend on the viscosity of binder but also on the powder loading. The optimal powder loading refers to a concentration of powder for which a compound exhibits good flow properties, good dispersion and distribution, and flow stability in the shear rates applied during injection molding (102 - 105 s-1). Rheological data represents an extremely useful tool to evaluate optimum powder loading. The ratio of feedstock's viscosity to binder's viscosity known as the relative viscosity is an important parameter used to determine the maximum packing fraction for a particular powder-binder compound. As the powder concentration reaches the maximum level the flow of feedstock material is restrained and a sharp increase in relative

The maximum loading level in a feedstock material is dependent on the characteristics of the powders and how these particles are packed. The packing behavior of particulate materials depends largely on their particle size, shape and surface characteristics. Packing behavior has been explained by model systems with closely defined size and shape distributions. Even though real materials do not have a well-defined shape and size distribution, the principles derived from models appear to work reasonably well. Theoretical maximum packing has been studied for spherical, smooth, regular, mono-sized particles, which can readily move past another. With these particles a maximum ordered packing fraction of 0.74 has been established from geometrical principles. In order to increase the packing fraction of powders, small particles to fill in the pores in the packed structure obtained from larger particles are used and then use even smaller particles to fill in the remaining pores and so on. This approach is known as multimodal packing and much effort has been devoted to find the optimal particle size ratios that yield maximum packing fractions in the field of particulate composites (Rothon, 2003). German (1990) has already pointed out that an ideal PIM powder should combine large and small particles in a tailored size distribution. Hausnerová *et al* (1999) showed that feedstock material containing particles with a monomodal particle size distribution exhibit higher viscosity compared to feedstock with bimodal powder. The current explanation for this behavior is that the smaller particles fill the inter-particle voids created by large particles, thereby releasing previously

The second step in the PIM process is molding the feedstock into the desired shape. The most popular method is to use a reciprocating screw, horizontal, hydraulic or electric machine in which a screw stirs the feedstock inside the barrel while it is melting. After melting the feedstock the screw acts as a plunger to generate the pressure to fill the die (Stevenson, 2009). Conventional screw-type injection molding machines consist of a clamping unit, injection unit and control system. The clamping unit houses the mold which is generally comprised of two halves. When the clamping unit is closed, material can be injected into the mold, when the clamping unit is open the molded part can be removed.

improved mechanical properties.

viscosity is observed (Hausnerová, 2011).

immobilized molten binder (Hausnerová, 2011).

**4. Injection molding** 

#### **3.4 Possible feedstock optimization**

Rheological and time-dependant properties represent powerful tools for optimizing the PIM process. The binder's rheological properties (flowability) are important to determine the optimal binder formulation or selecting a proper additive. It has been suggested in the literature that the viscosity of the binder should be less than 0.1 Pa · s in order to provide the PIM feedstock with a viscosity below 1000 Pa · s (German, 1990). Having low viscosity allows for easy molding. However, it is generally known that low viscosity materials have also low mechanical properties when solidified. Having low mechanical properties transforms into a "green part" which is prone to deformation and breakage if not handle with care, especially right after molding when the binder is still soft. For this reason a compromise has to be made between the molten rheological properties and the solid mechanical properties of binder.

Bimodality in the molecular mass distribution (Fig. 9) of binders is still an issue that has not been extensively studied. Nevertheless, recent investigations have shown its potential to bring two main benefits to the process of PIM: lower the viscosity of the feedstock in the molten state and maintaining the mechanical strength of the green part, all of this without modifying the chemistry of the binder.

Fig. 9. Schematic representation of bimodal and monomodal molecular mass distribution in polymeric materials.

It has been observed that bimodality in the molecular mass distribution improves the shear relaxation modulus (Emri & von Bernstorff, 2006) and the shear creep compliance (Kubyshkina *et al*, 2011) of polyamide 6 in the solid state. Based on those results, Stringari *et al* (2011) studied the effect of bimodal molecular mass distribution in POM and it was shown that bimodality decreases viscosity without significantly affecting the mechanical properties (shear creep compliance) of the potential PIM binder. The mechanisms for such behavior could be explained as follows: The addition of small polymer chains through bimodality increases the flowability of the binder in the molten state, since such small

Rheological and time-dependant properties represent powerful tools for optimizing the PIM process. The binder's rheological properties (flowability) are important to determine the optimal binder formulation or selecting a proper additive. It has been suggested in the literature that the viscosity of the binder should be less than 0.1 Pa · s in order to provide the PIM feedstock with a viscosity below 1000 Pa · s (German, 1990). Having low viscosity allows for easy molding. However, it is generally known that low viscosity materials have also low mechanical properties when solidified. Having low mechanical properties transforms into a "green part" which is prone to deformation and breakage if not handle with care, especially right after molding when the binder is still soft. For this reason a compromise has to be made between the molten rheological properties and the solid mechanical properties of binder.

Bimodality in the molecular mass distribution (Fig. 9) of binders is still an issue that has not been extensively studied. Nevertheless, recent investigations have shown its potential to bring two main benefits to the process of PIM: lower the viscosity of the feedstock in the molten state and maintaining the mechanical strength of the green part, all of this without

Fig. 9. Schematic representation of bimodal and monomodal molecular mass distribution in

It has been observed that bimodality in the molecular mass distribution improves the shear relaxation modulus (Emri & von Bernstorff, 2006) and the shear creep compliance (Kubyshkina *et al*, 2011) of polyamide 6 in the solid state. Based on those results, Stringari *et al* (2011) studied the effect of bimodal molecular mass distribution in POM and it was shown that bimodality decreases viscosity without significantly affecting the mechanical properties (shear creep compliance) of the potential PIM binder. The mechanisms for such behavior could be explained as follows: The addition of small polymer chains through bimodality increases the flowability of the binder in the molten state, since such small

**3.4 Possible feedstock optimization** 

modifying the chemistry of the binder.

polymeric materials.

molecules can act as internal lubricants for the larger molecules facilitating their movement. However, when in the solid state the smaller molecules could fit between the larger ones, creating a closely packed structure that cannot be deformed as easily, translating into improved mechanical properties.

Once good flowability of the binder has been obtained it is important to check the flowability of the feedstock material. The flowability of feedstock will not only depend on the viscosity of binder but also on the powder loading. The optimal powder loading refers to a concentration of powder for which a compound exhibits good flow properties, good dispersion and distribution, and flow stability in the shear rates applied during injection molding (102 - 105 s-1). Rheological data represents an extremely useful tool to evaluate optimum powder loading. The ratio of feedstock's viscosity to binder's viscosity known as the relative viscosity is an important parameter used to determine the maximum packing fraction for a particular powder-binder compound. As the powder concentration reaches the maximum level the flow of feedstock material is restrained and a sharp increase in relative viscosity is observed (Hausnerová, 2011).

The maximum loading level in a feedstock material is dependent on the characteristics of the powders and how these particles are packed. The packing behavior of particulate materials depends largely on their particle size, shape and surface characteristics. Packing behavior has been explained by model systems with closely defined size and shape distributions. Even though real materials do not have a well-defined shape and size distribution, the principles derived from models appear to work reasonably well. Theoretical maximum packing has been studied for spherical, smooth, regular, mono-sized particles, which can readily move past another. With these particles a maximum ordered packing fraction of 0.74 has been established from geometrical principles. In order to increase the packing fraction of powders, small particles to fill in the pores in the packed structure obtained from larger particles are used and then use even smaller particles to fill in the remaining pores and so on. This approach is known as multimodal packing and much effort has been devoted to find the optimal particle size ratios that yield maximum packing fractions in the field of particulate composites (Rothon, 2003). German (1990) has already pointed out that an ideal PIM powder should combine large and small particles in a tailored size distribution. Hausnerová *et al* (1999) showed that feedstock material containing particles with a monomodal particle size distribution exhibit higher viscosity compared to feedstock with bimodal powder. The current explanation for this behavior is that the smaller particles fill the inter-particle voids created by large particles, thereby releasing previously immobilized molten binder (Hausnerová, 2011).
