**5. Debinding**

78 Some Critical Issues for Injection Molding

The injection unit consists of a screw, a heating system and a nozzle. The screw transports the material inside a barrel, compresses it and removes any bubbles. The heating system brings the material to an appropriate temperature for easy flow. The nozzle is the conduct through which the heated feedstock is injected into the mold under pressure. The control system of a modern injection molding machine includes hardware and software where the processing conditions are set and saved to ensure the reproducibility of previously

Molding of the feedstock is comparable to the injection molding of plain thermoplastics and

c. The molten material is injected under high pressure (60 MPa or even more) into the mold cavity which is mounted in the clamping unit. The feedstock must have low

d. The mold remains closed while cooling channels in the die extract heat from the molten

e. After solidification of the binder, the nozzle of the injection die is pulled away from the mold by moving the injection unit. The clamping unit opens and the molded part is

f. The green part is removed from the mold. Due to the fragile nature of most green parts, the removal process is done by hand or by a robotic system in order to prevent shocks

The shaping equipment used in PIM is the same as the one used for plastic injection molding. Due to the size of molded parts, injection molding machines used for PIM are in the lower range, with clamp force typically less than 100 tons, 18 to 25 mm screws and shot size of less than 30 cm3 (Stevenson, 2009). However, the main important difference when dealing with injection molding of any powdered part is that many of the components of the molding machine are subject to a more intense wear, particularly screws, non-return valves,

Injection molding machines for processing of powdery materials are optimized with wearresistant components, through special hardening processes or utilization of alloys. For example, when dealing with stainless steel feedstock materials, hardening with carbon nitride is recommended by feedstock manufacturers. And when working with ceramic and hard metals, boride cladding or carbide hard facing are recommended. Since harder screws are more brittle, lower torque limits during startup are used to prevent screw breakage in PIM. The solid feedstock pellets cause the most abrasive wear in the feed section of the screw, thus the feedstock should be melted as early as possible in the injection cycle

Screw geometry of PIM machine is adopted to lower the compression rate and extend the compression zone as compared to screws used for thermoplastics (Hausnerová, 2011). Compression ratios used in PIM tend to be in the lower range. Ratios between 1.2 and 1.8 are considered acceptable and a ratio of 1.6 is considered to be optimal for the removal of air between granules. It is also important to mention that when calculating the barrel capacity of the injection unit, the barrel rating must be scaled up to take into account the higher

a. The pelletized feedstock is placed in the hopper of the injection molding machine.

enough viscosity that it can flow into the die cavity under pressure.

feedstock and solidify the polymer to preserve the molded shape.

or impacts which could deform or even break the molded part.

employed production cycles (Arburg, 2009).

it has the following stages (Arburg, 2009; German & Bose, 1997):

b. The binder in the feedstock is melted by the heating system.

ejected by the ejector system of the machine.

cylinders and molds (Rosato & Rosato, 1995).

density of the PIM feedstock (Stevenson, 2009).

(Stevenson, 2009).

Before sintering, the organic binder must be removed without disrupting the molded powder particles; this process is commonly referred as debinding. Organic polymers have to be removed completely from the "green part", since carbon residues can influence the sintering process and affect the quality of the final product negatively. Moreover, binder removal is one of the most critical steps in the PIM process since defects can be produced by inadequate debinding, like bloating, blistering, surface cracking and large internal voids. It has been shown that the rate of binder removal plays a main role in the defect production due to structural changes in capillaries inside the green part (Oliveira *et al*, 2005).

The most commonly used debinding techniques include: thermal, solvent and catalytic. However there also exist some experimental techniques such as plasma debiding (dos Santos *et al*, 2004). The following sections have the aim to provide the reader with a description of the different debinding techniques showing their benefits and limitations.

#### **5.1 Thermal debinding**

Thermal debinding utilizes the mechanisms of thermal degradation of organic binders, which is based on the successive dissociation of polymers to produce light molecules that are later evaporated out of the surface of the molded part. Since the thermal degradation process is different for different polymers then thermal debinding time is greatly influenced by the type of polymer used. The binders developed in the original PIM process were a mixture of polyethylene or polypropylene, a synthetic or natural wax and stearic acid. Feedstock materials based on such binders can be removed thermally. However, it has been shown in the literature that POM and polybutyl-methacrylate (PBMA) have a much faster degradation than other polymers such as polypropylene (PP) and ethylene-vinyl-acetate (EVA) (Kankawa, 1997). It should also be noted that the choice of atmosphere under which thermal debinding is performed influences the rate of binder removal and some characteristics of the final piece such as density, carbon or oxygen content (Quinard *et al*, 2009).

In general, it can be said that thermal debinding is an inefficient process that can result in a poor etching of the piece surface if not properly controlled. Additionally, increasing the temperature too fast may produce an excessive increase of vapor pressure in the core of the molded piece leading to defects. Consequently, in order to reduce the risk of cracks or shape deformation, low heating rates are generally used, resulting in a long debinding time, ranging from 10 to 60 h (dos Santos *et al*, 2004). However, thermal debinding is still selected due to its simplicity, safety and respect for the environment as compared to solvent and catalytic binder removal (Quinard *et al*, 2009).

Powder Injection Molding of Metal and Ceramic Parts 81

evolution. The temperature effect is related to an increase in interaction between soluble binders and solvents as temperature increases, in other words temperature changes the

There is a tendency to try to use binders that are soluble in water, since handling the aqueous solvent is much easier than handling organic solvents. Good examples of watersoluble binders include polyethylene glycols, polyethylene oxide, polyvinyl alcohols, starches and polyacrylamide. All of these polymers have monomers containing oxygen and nitrogen that are hydrophilic. The time to debind is a function of the powder material, particle size, part geometry, water temperature, water circulation and water volume relative to mass of the "green part". All of these variables are interrelated with each other and must be optimized for a particular piece. After debinding, parts are usually dried in a forced air furnace at 65 to 75 C for at least three hours. The drying step can be included as part of the sintering program. Water can be regenerated after debinding by distillation and therefore a

Another type of solvent debinding is the use of a supercritical process. When applying supercritical extraction, the operative pressures of solvents such as carbon dioxide or propane are kept around 10 MPa and temperatures are less than 100 C. The use of low temperatures results in slow diffusion of the binder to the surface of the green part, resulting in long processing times. Super critical debinding is not widely employed in commercial operations due to the long processing times and elevated cost of the necessary equipment, which require high precision control in temperature and pressure

Catalytic debinding can be used for binders that decompose into smaller molecules in the presence of a catalyst when exposed to the appropriate temperature. The most common example is a binder based on POM sometimes also referred as polyacetal. A commercial example of a feedstock with a POM based binder is Catamold® produced by BASF. By using a catalyst, the polymer at the surface of the "green part" is cracked into monomers and evaporates. As the monomers evaporate, pores are created that expose the polymer beneath the surface and the depolymerization process continues deeper into the molded part. Thus the debinding occurs from the outside inwards. Shorter debinding times are achieved with the use

Fig. 11. Solvent vapor debing process (adapted from German & Bose, 1997).

solubility and diffusion coefficient of the binder (Oliveira *et al*, 2005).

closed process water circuit can be used (Auzene & Roberjot, 2011).

(dos Santos *et al*, 2004).

**5.3 Catalytic debinding** 

In order to increase the efficiency of the thermal debinding process a vacuum pump is used to continuously pull binder vapor away from the heated parts. Initially, the heating rate is typically 0.5 C/min or less until a temperature of 100 C is reached; this temperature is hold for approximately 4 h and later it is increased to 400 C using a heating rate close to 1 C/min, this temperature is maintained for 2 h. During the entire process the pieces are constantly exposed to a gas flow, in order to remove the binder vapors, which are condensed and collected in a trap. Finally when all the binder is removed the temperature is increased and the sintering process can begin. This is one of the main advantages of vacuum debinding. However, the debinding process is still around 10 hours long, since the main mechanisms of debinding is still thermal degradation (dos Santos *et al*, 2004).

Another variant of the thermal debinding process includes the use of a wicking material (porous substrate) which is in contact with the "green part" or compact. In industrial practice the compact is buried in the wicking material and therefore the binder is removed from the compact in all directions (Somasundram *et al*, 2008). A simplified model is shown in Fig. 10. The porous substrate provides a medium for capillary flow as the binder viscosity decreases due to the increase of temperature. Wicking thermal debinding is performed at temperatures where the binder melts, therefore the binder can flow out of the component into the pores of the contacting substrate. Wicking involves liquid extraction; while other thermal process requires the binder to be in gaseous state and thus the temperatures are generally lower but the process is slower due to the slower transport of liquid compared to a gas (German & Bose, 1997).

Fig. 10. Schematic cross-section of compact (green part) surrounded by wicking powder during thermal debinding (adapted from Somasundram *et al*, 2008).

#### **5.2 Solvent debinding**

Solvent debinding is done by immersing the molded part in a gaseous (Fig. 11) or liquid solvent such as ethanol, hexane, heptanes and acetone, at low temperature, typically 50 to 60 C (Torralba *et al,* 2011*)*. The solvent removes at least one of the binder components and produces an open porosity. The next step in solvent debinding is binder burnout to remove the backbone of the binder that provides adequate shape retention up to the onset of sintering (dos Santos *et al*, 2004; Aggarwal *et al*, 2007). Binder backbone removal is generally done thermally between 200 and 600 °C in a pre-sintering step (Tandon, 2008).

The effectiveness of solvent debinding is strongly related to the geometry of the "green part" in particular to the surface to volume ratio, since the solvent needs to penetrate the part. Other factors that influence solvent debinding include temperature and porosity

In order to increase the efficiency of the thermal debinding process a vacuum pump is used to continuously pull binder vapor away from the heated parts. Initially, the heating rate is typically 0.5 C/min or less until a temperature of 100 C is reached; this temperature is hold for approximately 4 h and later it is increased to 400 C using a heating rate close to 1 C/min, this temperature is maintained for 2 h. During the entire process the pieces are constantly exposed to a gas flow, in order to remove the binder vapors, which are condensed and collected in a trap. Finally when all the binder is removed the temperature is increased and the sintering process can begin. This is one of the main advantages of vacuum debinding. However, the debinding process is still around 10 hours long, since the main

Another variant of the thermal debinding process includes the use of a wicking material (porous substrate) which is in contact with the "green part" or compact. In industrial practice the compact is buried in the wicking material and therefore the binder is removed from the compact in all directions (Somasundram *et al*, 2008). A simplified model is shown in Fig. 10. The porous substrate provides a medium for capillary flow as the binder viscosity decreases due to the increase of temperature. Wicking thermal debinding is performed at temperatures where the binder melts, therefore the binder can flow out of the component into the pores of the contacting substrate. Wicking involves liquid extraction; while other thermal process requires the binder to be in gaseous state and thus the temperatures are generally lower but the process is slower due to the slower transport of liquid compared to a

Fig. 10. Schematic cross-section of compact (green part) surrounded by wicking powder

Solvent debinding is done by immersing the molded part in a gaseous (Fig. 11) or liquid solvent such as ethanol, hexane, heptanes and acetone, at low temperature, typically 50 to 60 C (Torralba *et al,* 2011*)*. The solvent removes at least one of the binder components and produces an open porosity. The next step in solvent debinding is binder burnout to remove the backbone of the binder that provides adequate shape retention up to the onset of sintering (dos Santos *et al*, 2004; Aggarwal *et al*, 2007). Binder backbone removal is generally

The effectiveness of solvent debinding is strongly related to the geometry of the "green part" in particular to the surface to volume ratio, since the solvent needs to penetrate the part. Other factors that influence solvent debinding include temperature and porosity

done thermally between 200 and 600 °C in a pre-sintering step (Tandon, 2008).

during thermal debinding (adapted from Somasundram *et al*, 2008).

mechanisms of debinding is still thermal degradation (dos Santos *et al*, 2004).

gas (German & Bose, 1997).

**5.2 Solvent debinding** 

Fig. 11. Solvent vapor debing process (adapted from German & Bose, 1997).

evolution. The temperature effect is related to an increase in interaction between soluble binders and solvents as temperature increases, in other words temperature changes the solubility and diffusion coefficient of the binder (Oliveira *et al*, 2005).

There is a tendency to try to use binders that are soluble in water, since handling the aqueous solvent is much easier than handling organic solvents. Good examples of watersoluble binders include polyethylene glycols, polyethylene oxide, polyvinyl alcohols, starches and polyacrylamide. All of these polymers have monomers containing oxygen and nitrogen that are hydrophilic. The time to debind is a function of the powder material, particle size, part geometry, water temperature, water circulation and water volume relative to mass of the "green part". All of these variables are interrelated with each other and must be optimized for a particular piece. After debinding, parts are usually dried in a forced air furnace at 65 to 75 C for at least three hours. The drying step can be included as part of the sintering program. Water can be regenerated after debinding by distillation and therefore a closed process water circuit can be used (Auzene & Roberjot, 2011).

Another type of solvent debinding is the use of a supercritical process. When applying supercritical extraction, the operative pressures of solvents such as carbon dioxide or propane are kept around 10 MPa and temperatures are less than 100 C. The use of low temperatures results in slow diffusion of the binder to the surface of the green part, resulting in long processing times. Super critical debinding is not widely employed in commercial operations due to the long processing times and elevated cost of the necessary equipment, which require high precision control in temperature and pressure (dos Santos *et al*, 2004).

#### **5.3 Catalytic debinding**

Catalytic debinding can be used for binders that decompose into smaller molecules in the presence of a catalyst when exposed to the appropriate temperature. The most common example is a binder based on POM sometimes also referred as polyacetal. A commercial example of a feedstock with a POM based binder is Catamold® produced by BASF. By using a catalyst, the polymer at the surface of the "green part" is cracked into monomers and evaporates. As the monomers evaporate, pores are created that expose the polymer beneath the surface and the depolymerization process continues deeper into the molded part. Thus the debinding occurs from the outside inwards. Shorter debinding times are achieved with the use

Powder Injection Molding of Metal and Ceramic Parts 83

The major differences, advantages and disadvantages between the three types of debinding

One-step process, no need to handle product between debinding and sintering (unless wicking is used). Low cost

installation. Applicable to a wide range of binders.

Component remains rigid

Rapid process (4 to 6h) that works well on thick and thin sections with excellent shape retention.

The last step of the PIM process is sintering. Sintering is one of the oldest human technologies, originating in the prehistoric era with the firing of pottery. After the 1940s sintering has been studied fundamentally and scientifically leading to remarkable developments in sintering science. Nowadays, sintering can be used for the fabrication of all kinds of parts, including

Sintering is a thermal treatment that transforms metallic or ceramic powders into bulk materials with improved mechanical strength that in most cases have residual porosity. Sintering is performed at temperatures below the melting temperature of the major constituent in the metal or ceramic powder, generally within 70 to 90% of the melting point (Lame *et al*, 2003). The temperature inside the sintering furnace is high enough to start the recrystallization process of the metal or ceramic particles, but low enough so that the particles remain unmelted. At such temperatures, the particles recrystallize into each other

without chemical reactions. Lower temperatures minimize defects and distortions. Faster than thermal debinding (around 6 h) Soft binder allows warpage, poor dimensional control and relatively slow process (up to 60 h). If a wick is used problems to separate part

Solvent hazard, chemical handling and environmental concern (unless water soluble binder is used). Expensive equipment if using supercritical extraction. Drying before sintering required if using

Possible hazards with acid catalysts and decomposition products. Exhaust products must be treated properly to

from it.

liquid solvent.

prevent health and environmental hazards.

**Technique Key Features Advantages Disadvantages** 

**5.4 Comparison between debinding processes** 

Slowly heat green part

temperatures with a continuous sweep gas to remove binder.

Green part is placed in a solvent in gaseous or liquid state to extract binder via dissolution.

Heat green part in atmosphere

containing catalyst to depolymerize binder and sweep away monomers. Binder goes from solid to gas.

causing them to fuse together (Boljanovic, 2010).

Table 3. Comparison between three major debinding techniques.

powder-metallurgical parts and bulk ceramic components (Kang, 2005).

techniques are summarized in Table 3.

to melting or degradation

**Debinding** 

Thermal

Solvent

Catalytic

**6. Sintering** 

of the catalytic process, since the rate of diffusion of monomers is high due to the small size of their molecules (Clemens, 2009). Furthermore, the small molecules generated have a high vapor pressure, which greatly minimizes the potential for capillary condensation and allows thick part sections to be debound (Krueger, 1996).

Polyacetal-based binders depolymerizes catalytically under acidic conditions yielding formaldehyde, a direct solid to gas transition (Fig. 12). Temperature and catalyst concentration play a key role in determining the rate of debinding, as well as the particle size of the powder and geometry of the molded part. This type of debinding is performed below the melting temperature of POM, generally between 110 and 150 C. The use of these relative low temperatures prevents the formation of a liquid phase and thus prevents deformation of the "green part" due to gravitational distortion or stress relaxation (Fu *et al*, 2005). Also the internal gas pressure is low, which minimizes the danger of crack formation and propagation. For POM, 100% nitric acid (HNO3) is the most suitable catalyst. Even though nitric acid is a strongly oxidizing agent, its anhydrous form does not react with most of the commonly used metal powders (Krueger *et al*, 1993).

Fig. 12. Decomposition of POM in the presence of nitric acid.

The debinding time depends on the quantity of catalysts and temperature used. An increase of these two factors can shorten the time of depolymerization. However, there are some limitations as to how much temperature and catalyst content can be increased, not only due to possible damage to the molded part but also due to health concerns. As previously mentioned, POM decomposes into formaldehyde, which due to its toxicity has limitations in the allowable quantity (0.1 kg/h) and concentration (20 mg/m3) that can be present in the working environment (Goyer *et al*, 2006). The exhaust from the debinding oven must be treated in two steps to get rid of the toxic bi-product of the depolymerization of POM, which are nitrogen dioxide (NO2) and formaldehyde (CH2O). First the exhaust is burned in a reducing atmosphere (no oxygen and rich in nitrogen) at a temperature of 600 C, transforming nitric dioxide into nitrogen gas (N2). The second step consists of burning in an oxidizing atmosphere at 800 C to transform formaldehyde into water and carbon dioxide (Torralba *et al*, 2011). If properly treated, the exhaust fumes coming out of the debinding oven does not represent a health hazard.

It is important to mention that binders based on POM usually have a backbone polymer which is not susceptible to catalytic debinding. Such backbone polymer helps retain strength and shape stability in the "brown part". However, sintering cannot begin in the presence of this backbone polymer and thus a thermal treatment between 200 and 600 C is applied to the part prior to the start of the sintering process (Tandon, 2008).

of the catalytic process, since the rate of diffusion of monomers is high due to the small size of their molecules (Clemens, 2009). Furthermore, the small molecules generated have a high vapor pressure, which greatly minimizes the potential for capillary condensation and allows

Polyacetal-based binders depolymerizes catalytically under acidic conditions yielding formaldehyde, a direct solid to gas transition (Fig. 12). Temperature and catalyst concentration play a key role in determining the rate of debinding, as well as the particle size of the powder and geometry of the molded part. This type of debinding is performed below the melting temperature of POM, generally between 110 and 150 C. The use of these relative low temperatures prevents the formation of a liquid phase and thus prevents deformation of the "green part" due to gravitational distortion or stress relaxation (Fu *et al*, 2005). Also the internal gas pressure is low, which minimizes the danger of crack formation and propagation. For POM, 100% nitric acid (HNO3) is the most suitable catalyst. Even though nitric acid is a strongly oxidizing agent, its anhydrous form does not react with most

The debinding time depends on the quantity of catalysts and temperature used. An increase of these two factors can shorten the time of depolymerization. However, there are some limitations as to how much temperature and catalyst content can be increased, not only due to possible damage to the molded part but also due to health concerns. As previously mentioned, POM decomposes into formaldehyde, which due to its toxicity has limitations in the allowable quantity (0.1 kg/h) and concentration (20 mg/m3) that can be present in the working environment (Goyer *et al*, 2006). The exhaust from the debinding oven must be treated in two steps to get rid of the toxic bi-product of the depolymerization of POM, which are nitrogen dioxide (NO2) and formaldehyde (CH2O). First the exhaust is burned in a reducing atmosphere (no oxygen and rich in nitrogen) at a temperature of 600 C, transforming nitric dioxide into nitrogen gas (N2). The second step consists of burning in an oxidizing atmosphere at 800 C to transform formaldehyde into water and carbon dioxide (Torralba *et al*, 2011). If properly treated, the exhaust fumes coming out of the debinding

It is important to mention that binders based on POM usually have a backbone polymer which is not susceptible to catalytic debinding. Such backbone polymer helps retain strength and shape stability in the "brown part". However, sintering cannot begin in the presence of this backbone polymer and thus a thermal treatment between 200 and 600 C is applied to

thick part sections to be debound (Krueger, 1996).

of the commonly used metal powders (Krueger *et al*, 1993).

Fig. 12. Decomposition of POM in the presence of nitric acid.

the part prior to the start of the sintering process (Tandon, 2008).

oven does not represent a health hazard.

#### **5.4 Comparison between debinding processes**

The major differences, advantages and disadvantages between the three types of debinding techniques are summarized in Table 3.


Table 3. Comparison between three major debinding techniques.

### **6. Sintering**

The last step of the PIM process is sintering. Sintering is one of the oldest human technologies, originating in the prehistoric era with the firing of pottery. After the 1940s sintering has been studied fundamentally and scientifically leading to remarkable developments in sintering science. Nowadays, sintering can be used for the fabrication of all kinds of parts, including powder-metallurgical parts and bulk ceramic components (Kang, 2005).

Sintering is a thermal treatment that transforms metallic or ceramic powders into bulk materials with improved mechanical strength that in most cases have residual porosity. Sintering is performed at temperatures below the melting temperature of the major constituent in the metal or ceramic powder, generally within 70 to 90% of the melting point (Lame *et al*, 2003). The temperature inside the sintering furnace is high enough to start the recrystallization process of the metal or ceramic particles, but low enough so that the particles remain unmelted. At such temperatures, the particles recrystallize into each other causing them to fuse together (Boljanovic, 2010).

Powder Injection Molding of Metal and Ceramic Parts 85

order to harden sintered parts, a controlled cooling rate must be carried out in a separate section of the sintering furnace. Cooling is done in a protective atmosphere in order to prevent oxidation of sintered parts. Dissociated ammonia and nitrogen-based atmospheres are commonly used, however vacuum atmospheres are also used for stainless steel and tungsten parts for example. The cooling rate is critical since the mechanical properties of the sintered

PIM is a powerful process for the manufacturing of parts with complex geometry. It combines the design benefits of thermoplastic injection molding and the efficiency of powder metallurgy. Due to its capability is expected that PIM will growth in importance. However the current state of the art does not allow for PIM to be widely used and therefore there is a need to optimize the process to increase its efficiency and productivity, as well as the quality of the final parts. For instance, there is still room for improving the performance of binders as to obtain excellent flowability in the molten state while having high mechanical properties in the solid state. Optimization can also be brought into the powder design as to obtain the most adequate particle size distribution and size ratios for multimodal powders. Also, it has been shown that using nanoparticles brings many benefits into the PIM process, but handling procedures of these materials and their processing to avoid agglomeration is still far from optimal. In summary, PIM technology still offers a

Ahn, S., Park, S.J., Lee, S., Atre, S.V. & German, R.M. (2009). Effect of powders and binders

Aggarwal, G., Smid, I., Park, S.J. & German, R.M. (2007). Development of niobium powder

Auzene, D. & Roberjot, S. (2011). Investigation into water soluble binder systems for powder

Boljanovic, V. (2010). Powder metallurgy. In *Metal Shaping Processes: Casting and Molding,* 

Chan, T.Y. & Lin, S.L. (1995). Effects of stearic acid on the injection molding of alumina.

Clemens, F. (2009). Thermoplastic extrusion for ceramic bodies. In *Extrusion in Ceramics-*

on material properties and molding parameters in iron and stainless steel powder injection molding process. *Powder Technology*, Vol.193, No.2, (July 2009), pp.162-169,

injection molding. Part II: Debinding and sintering. *International Journal of Refractory Metals & Hard Materials*, Vol.25, No.3, (May 2007), pp. 226-236, ISSN 0263-4368. Arburg. (2009). Powder Injection Moulding (PIM) – Production of complex moulded parts

from metal and ceramic. Arburg GmbH + Co KG, Lossburg, Germany, Available from: http://www.arburg.de/com/common/download/WEB\_522785\_en\_GB.pdf

injection moulding. *Powder Injection Moulsding International*, Vol.5, No. 1, (March

*Particulate Processing, Deformation Processes, Metal Removal*. Industrial Press Inc.,

*Journal of the American Ceramic Society*, Vol.78, No.10, (October 1995), pp. 2746-2752,

*Engineering Materials and Processes,* Handle, F. (Ed.), Springer-Verlag, , ISBN 978-

parts is affected by the phase transformation of the material (Boljanovic, 2010).

broad field of opportunities for improvement through applied research.

**7. Conclusion – Closing remarks** 

**8. References** 

ISSN 0032-5910.

ISSN 1551-2916.

2011), pp.54-57, ISSN 1753-1497.

ISBN 9780831 133801, New York, USA, pp. 75-106.

3540271000, Berlin, Germany, pp. 295-311.

During sintering, solid-state atomic diffusion takes place, followed by recrystallization and grain growth. When the temperature exceeds one half to two thirds of the melting temperature of the powder material, significant atomic diffusion occurs and some chemical changes may happen on the surface of the particles, such as the vaporization of chemically bounded water. As temperature keeps increasing, thermolysis occurs which is a process that burns out the organic components such as remaining binder, dispersant, etc. During sintering there is a great deal of particle movement and mass transport. It has been identified that there are at least six different mechanisms for mass transfer involved in sintering, which are surface diffusion, evaporation-condensation, grain boundary diffusion, lattice diffusion, viscous flow, and plastic flow. These mechanisms lead to growth of necks between particles and thus increase the strength of the consolidated powders. However some mechanisms also lead to shrinkage (more than 10%) and densification. Surface diffusion is the mechanism that produces surface smoothing, particle joining and pore rounding, but not volume shrinkage. If the material has high vapor pressure, sublimation and vapor transport produce the same effects as surface diffusion. Diffusion along the grain boundaries and through the lattice produces both neck growth and volume shrinkage. Bulk viscous flow plays an important role in densification when a wetting liquid is present, while plastic deformation is important when a mechanical pressure is applied (Kang, 2007).

Fusing of the metallic particles during the sintering process has been observed using synchrotron microtomography as shown in the figures below for copper particles. It is clearly observed that during sintering the particles get closer to each other as interparticle necks grow and the porosity is reduced. From room temperature (Fig. 13A) to 1000 C (Fig. 13B) there is no significant shrinkage but the neck formation causes small displacements and rotations of the particles that lead to a different particle packing. As a result of this rearrangement, some pores decrease in size but others increase. After sintering for 120 min at 1050 C, the interparticle necks have grown up and it is harder to distinguish individual particles (Fig. 13C). Finally, (Fig. 13D) after the part was sintered for an additional 100 min and cooled down to room temperature, most pores have vanished but some large pores remain (Lame *et al*, 2003).

Fig. 13. Non-compacted copper particles at different stages of the sintering process, (A) at room temperature before sintering, (B) after heating to 1050 C from room temperature in 45 min, (C) at 1050 C after 120 min of sintering, and (D) after sintering for 220 min at 1050 C and cooling to room temperature in 15 min (Lame *et al*, 2003).

Depending on the size of the part and the material used, the sintering time can vary. For small parts such as bushings, the average time varies from 1 to 1.5 h. For average-size ferrous parts, the sintering time can be 3 h. However tungsten parts can have a sintering time of up to 8 h. In order to harden sintered parts, a controlled cooling rate must be carried out in a separate section of the sintering furnace. Cooling is done in a protective atmosphere in order to prevent oxidation of sintered parts. Dissociated ammonia and nitrogen-based atmospheres are commonly used, however vacuum atmospheres are also used for stainless steel and tungsten parts for example. The cooling rate is critical since the mechanical properties of the sintered parts is affected by the phase transformation of the material (Boljanovic, 2010).

#### **7. Conclusion – Closing remarks**

PIM is a powerful process for the manufacturing of parts with complex geometry. It combines the design benefits of thermoplastic injection molding and the efficiency of powder metallurgy. Due to its capability is expected that PIM will growth in importance. However the current state of the art does not allow for PIM to be widely used and therefore there is a need to optimize the process to increase its efficiency and productivity, as well as the quality of the final parts. For instance, there is still room for improving the performance of binders as to obtain excellent flowability in the molten state while having high mechanical properties in the solid state. Optimization can also be brought into the powder design as to obtain the most adequate particle size distribution and size ratios for multimodal powders. Also, it has been shown that using nanoparticles brings many benefits into the PIM process, but handling procedures of these materials and their processing to avoid agglomeration is still far from optimal. In summary, PIM technology still offers a broad field of opportunities for improvement through applied research.

#### **8. References**

84 Some Critical Issues for Injection Molding

During sintering, solid-state atomic diffusion takes place, followed by recrystallization and grain growth. When the temperature exceeds one half to two thirds of the melting temperature of the powder material, significant atomic diffusion occurs and some chemical changes may happen on the surface of the particles, such as the vaporization of chemically bounded water. As temperature keeps increasing, thermolysis occurs which is a process that burns out the organic components such as remaining binder, dispersant, etc. During sintering there is a great deal of particle movement and mass transport. It has been identified that there are at least six different mechanisms for mass transfer involved in sintering, which are surface diffusion, evaporation-condensation, grain boundary diffusion, lattice diffusion, viscous flow, and plastic flow. These mechanisms lead to growth of necks between particles and thus increase the strength of the consolidated powders. However some mechanisms also lead to shrinkage (more than 10%) and densification. Surface diffusion is the mechanism that produces surface smoothing, particle joining and pore rounding, but not volume shrinkage. If the material has high vapor pressure, sublimation and vapor transport produce the same effects as surface diffusion. Diffusion along the grain boundaries and through the lattice produces both neck growth and volume shrinkage. Bulk viscous flow plays an important role in densification when a wetting liquid is present, while plastic deformation is important when a mechanical pressure is applied (Kang, 2007).

Fusing of the metallic particles during the sintering process has been observed using synchrotron microtomography as shown in the figures below for copper particles. It is clearly observed that during sintering the particles get closer to each other as interparticle necks grow and the porosity is reduced. From room temperature (Fig. 13A) to 1000 C (Fig. 13B) there is no significant shrinkage but the neck formation causes small displacements and rotations of the particles that lead to a different particle packing. As a result of this rearrangement, some pores decrease in size but others increase. After sintering for 120 min at 1050 C, the interparticle necks have grown up and it is harder to distinguish individual particles (Fig. 13C). Finally, (Fig. 13D) after the part was sintered for an additional 100 min and cooled down to room

temperature, most pores have vanished but some large pores remain (Lame *et al*, 2003).

Fig. 13. Non-compacted copper particles at different stages of the sintering process, (A) at room temperature before sintering, (B) after heating to 1050 C from room temperature in 45 min, (C) at 1050 C after 120 min of sintering, and (D) after sintering for 220 min at 1050 C

Depending on the size of the part and the material used, the sintering time can vary. For small parts such as bushings, the average time varies from 1 to 1.5 h. For average-size ferrous parts, the sintering time can be 3 h. However tungsten parts can have a sintering time of up to 8 h. In

and cooling to room temperature in 15 min (Lame *et al*, 2003).


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**4** 

Lovro Gorjan  *Jožef Stefan Institute,* 

*Slovenia* 

**Wick Debinding – An Effective Way of Solving** 

 *Research and Development Center for Ignition Systems and Electronics d.o.o.,* 

Powder injection molding (PIM) has been shown itself to be a successful shaping technique for producing complex-shaped ceramic, metal or cermet parts. The process starts with preparing a high solid loading suspension, where ceramic or metal powder is mixed with a thermoplastic material. At high temperature the suspension is fluid and can be injected into molds by applying a pressure. Inside the mold the suspension takes the shape of the mold and then cools below the melting point of the thermoplastic material and solidifies into a green body. After the molding cycle the green body consists of solid particles held together

The challenging and time-consuming operation in the powder-injection molding process is removing the binder from the green bodies prior to the sintering, without causing any deformation or cracks. The debinding process is difficult because green bodies contain relatively large amount of poorly volatile binder in the solid state, i.e. below the melting point. Debinding is usually achieved by slowly heating the green bodies, causing the binder to decompose and vaporize. This is the thermal debinding process. The difficulties are especially severe in low-pressure injection molding, since in this case the binder does not contain a backbone polymer that would hold the particles firmly in place during the debinding. Lowpressure injection molding (LPIM) is a variant of injection molding where relatively low pressures are used, typically less than 0.7 MPa, and the pressure is applied using compressed air instead of a screw (like in the more common high-pressure variant). The liquid medium in the feedstock is a low-melting-point wax, which is crucial for the low viscosity of the molten feedstock. The advantages of LPIM, in comparison with other ceramic injection techniques, include the lower cost of the molds, less die wear and less expensive and simpler equipment for the injection molding (Zorzi et al., 2003; Cetinel et al., 2010; Loebbecke et al., 2009; Gorjan et al., 2010). The method has also been shown to be appropriate for the shaping of

microcomponents (Cetinel et al., 2010; Bauer & Knitter, 2002; Wang et al., 2008).

However, an effective way of reducing the formation of defects in the process of binder removal exists. That is, to introduce an additional debinding step – debinding in a wicking embedment (Curry, 1975; German, 1987; Wei, 1989; Liu, 1999; Bao & Evans, 1991; German,

**1. Introduction** 

by the thermoplasic phase, which serves as a binder.

**Problems in the Debinding Process** 

 **of Powder Injection Molding** 

