**4.1 Material properties of MIM feedstock using nano-scale powder**

Further improvements on the quality of μ-MIM products are required in practical productions. In general the use of finer metal powders is one of solutions to improve the dimensional accuracy and surface roughness of μ-MIM products. It can be also expected to inhibit the grain growth by reducing the sintering temperature. On the other hand, sintering inhabitation by higher oxidation, lower packed density, higher viscosity and higher cost are some of drawbacks in MIM production. MIM feedstock is prepared using various particle sized metal powders from micro-scale to nano-scale and the effects of particle size on material properties of MIM feedstock were investigated.

The metal powders used for the experiments are five types of pure Cu powders as shown in Fig.28. The reasons for pure Cu powder is used as the research material are that Cu has a high thermal conductivity, thus it is expected to be used for microscopic structures with high-specific surface area but it is not easy to mass-produce the metallic parts with such a structure, besides Cu can be reduced easily by H2 gas because of the high Gibb's energy. The micro-sized Cu powder is a conventional material manufactured commercially by a wateratomization method (*d50*=8.2μm, *d50*=20μm) or wet-electrolytic method (*d50*=1.5μm,

line is 0.4µm deep and 9µm wide, which the size is equivalent to disappear after sintering. Micro-pillar structured parts were manufactured by LIGA/μ-SPiMIM process as shown in Fig.27. The SP-mold (Fig.27(b)) was prepared by injection molding PMMA polymer into Niform (Fig.27(a)). Although the defects that occurred during the injection molding such as weld line and rounded edge, SP-mold can be manufactured with low cost and high cycle time. MIM feedstock prepared varied particle diameter of stainless steel 316L powder were injection-molded in the SP-mold, the sintered parts (Fig.27(c)(d)) were obtained after debinding and sintering. As similar to sintered parts manufactured by Resist/μ-SPiMIM process, micro-parts with much higher quality in shape and surface could be obtained by

Fig. 27. SEM images and profiles of Ni-form, SP-mold and sintered parts manufactured by

Further improvements on the quality of μ-MIM products are required in practical productions. In general the use of finer metal powders is one of solutions to improve the dimensional accuracy and surface roughness of μ-MIM products. It can be also expected to inhibit the grain growth by reducing the sintering temperature. On the other hand, sintering inhabitation by higher oxidation, lower packed density, higher viscosity and higher cost are some of drawbacks in MIM production. MIM feedstock is prepared using various particle sized metal powders from micro-scale to nano-scale and the effects of particle size on

The metal powders used for the experiments are five types of pure Cu powders as shown in Fig.28. The reasons for pure Cu powder is used as the research material are that Cu has a high thermal conductivity, thus it is expected to be used for microscopic structures with high-specific surface area but it is not easy to mass-produce the metallic parts with such a structure, besides Cu can be reduced easily by H2 gas because of the high Gibb's energy. The micro-sized Cu powder is a conventional material manufactured commercially by a wateratomization method (*d50*=8.2μm, *d50*=20μm) or wet-electrolytic method (*d50*=1.5μm,

**4. Use of nano-scale powder in micro sacrificial plastic mold insert MIM** 

0 100 200 300

**4.1 Material properties of MIM feedstock using nano-scale powder** 

material properties of MIM feedstock were investigated.

*D50*=3μm powder

0 100 200 300

(d) Sintered part: *D50*=9μm powder

0 100 200 300

using fine powder better than coarse one.

SEM image Profile

LIGA/μ-SPiMIM process.

0 100 200 300

(a) Ni-form (b) SP-mold (c) Sintered part:

*d50*=0.30μm). On the other hand, the nano-sized Cu powder is an ultra-fine material produced by a radio-frequency thermal plasma method (*dBET*=0.13μm). The primary particles are completely different in size and shape from micro-sized Cu powder. As nanosized powder has a large specific surface, melt viscosity of the feedstock increases more significantly. Therefore it is a key technology for deriving the effectiveness of nano-scale powders to select the component of binder and its fraction of MIM feedstock. Multicomponent binder composed of polyacetal polymer and paraffin wax is used. The binder content is predicted by referring 35vol.% of the practical optimum binder content for Cu powder (*d50*=8.2μm) which are using in conventional MIM production. Tap densities of various sized Cu powders are shown in Fig.29(a). Tap density of Cu powder increases exponentially as the particle size of Cu powder increases. As for the binder content, it decreases when the particle size increase as shown in Fig.29(a). As the results, a finer


Fig. 28. Properties of Cu powders with various particle sizes.

Fig. 29. Properties of MIM feedstock with various particle sizes of metal powder; (a)Tap density and binder content; (b) Melt viscosity.

Micro Metal Powder Injection Molding 125

debinding and sintering process. Fig.32 shows the SEM images of green compacts and sintered parts produced by various sized Cu powders. As the particle size of Cu powder used decreases, the filling of the green compacts in molding was improved except for *d50*=0.13μm powder specimen. The decreasing of particle size results in a marked improvements of surface roughness, transcription and dimensional variation of sintered parts. Fig.33 shows SEM images of green compact and sintered part manufactured by using *d50*=0.3µm Cu powder. For the entire body of green compact, the feedstock was filled fully in fine pillar structure, but a slight deformation is visible in sintered part. This is considered to be due to large amount of binder, and further study on the decision of binder content in

(a) *dBET*=0.13µm (b) *d50*=0.3µm (c) *d50*=1.5µm (d) *d50*=8.2µm (e) *d50*=20µm

Fig. 32. SEM images of green compact and sintered part manufactured by using Cu powders

50μm

Fig. 31. Scheme of small direct mixing-injection molding machine.

particle agglomeration is needed for the quality improvement.

Green compact

Sintered part

with various particle sizes.

(a) Micro powder dispersed (b) Nano powder dispersed (c) Nano powder aggregated Fig. 30. Configuration interaction of binder in MIM feedstock with varied particle.

powder has a larger specific surface, the melt viscosity of the feedstock increases more significantly as shown in Fig.29(b). When the binder content is predicted from the equation based on the space rate estimated from tap density, and the calculation of the binder content which the spaces are filled up, were made sometimes it shows that the binder content is too high and the melt viscosity becomes much lower. This reason is assumed to be due to remarkable agglomeration of nano-sized particles as shown in Fig.30. Thus the melt viscosity of feedstock was tried to keep constant by changing binder content using the equation based on the space rate estimated from tap density.

#### **4.2 Molding machine for micro sacrificial plastic mold insert MIM**

It is often desirable to conduct a material parametric study using a very small amount of nano-sized powder. The use of relatively large amount of feedstock is required for a conventional injection molding machine. Therefore, a direct mixing-injection molding machine as illustrated in Fig.31 has been developed. This machine is small enough to be placed on top of a table, and it enables the mixing of metal powder and binders followed by injection molding, therefore it can achieve molding without pelletizing. The capacity of the furnace is 0.05cm3 in volume which is equivalent to the general size of a single feedstock pellet. A mixing is completed by a rotation of the plunger with 3mm in diameter. The mixing condition is basically controlled by furnace temperature, rotation speed and mixing time. The procedure of injection molding on this machine which is metal powder and binders are previously metered with balance and homogeneously mixed into the furnace. The feedstock is fully injected into the cavity at high speed when the plunger is pressed by compressed air with 0.1-0.85MPa. Therefore, this operation can achieve an accurate control on the volume of feedstock for each shot. In case of µ-SPiMIM process, resist film and NIL film is inserted into the mold cavity as shown in Fig.31(d) and (e), respectively. The green compact and SP-mold are ejected as one component.

#### **4.3 Effects of particle size in resist sacrificial plastic mold insert MIM**

In Resist/μ-SPiMIM process, the resist film made of PMMA polymer with numerous microholes was used as SP-mold. The feedstock was prepared using Cu powders with various particle sizes and the green compacts could be prepared with a high efficiency in experiment using a small amount of feedstock by a small molding machine introduced in previous section. The sintered parts with micro-pillar structure were obtained after

Confined region Dispersed particle Aggregated particle

(a) Micro powder dispersed (b) Nano powder dispersed (c) Nano powder aggregated

powder has a larger specific surface, the melt viscosity of the feedstock increases more significantly as shown in Fig.29(b). When the binder content is predicted from the equation based on the space rate estimated from tap density, and the calculation of the binder content which the spaces are filled up, were made sometimes it shows that the binder content is too high and the melt viscosity becomes much lower. This reason is assumed to be due to remarkable agglomeration of nano-sized particles as shown in Fig.30. Thus the melt viscosity of feedstock was tried to keep constant by changing binder content using the

It is often desirable to conduct a material parametric study using a very small amount of nano-sized powder. The use of relatively large amount of feedstock is required for a conventional injection molding machine. Therefore, a direct mixing-injection molding machine as illustrated in Fig.31 has been developed. This machine is small enough to be placed on top of a table, and it enables the mixing of metal powder and binders followed by injection molding, therefore it can achieve molding without pelletizing. The capacity of the furnace is 0.05cm3 in volume which is equivalent to the general size of a single feedstock pellet. A mixing is completed by a rotation of the plunger with 3mm in diameter. The mixing condition is basically controlled by furnace temperature, rotation speed and mixing time. The procedure of injection molding on this machine which is metal powder and binders are previously metered with balance and homogeneously mixed into the furnace. The feedstock is fully injected into the cavity at high speed when the plunger is pressed by compressed air with 0.1-0.85MPa. Therefore, this operation can achieve an accurate control on the volume of feedstock for each shot. In case of µ-SPiMIM process, resist film and NIL film is inserted into the mold cavity as shown in Fig.31(d) and (e), respectively. The green

Fig. 30. Configuration interaction of binder in MIM feedstock with varied particle.

equation based on the space rate estimated from tap density.

Lubricate layer

compact and SP-mold are ejected as one component.

**4.3 Effects of particle size in resist sacrificial plastic mold insert MIM** 

In Resist/μ-SPiMIM process, the resist film made of PMMA polymer with numerous microholes was used as SP-mold. The feedstock was prepared using Cu powders with various particle sizes and the green compacts could be prepared with a high efficiency in experiment using a small amount of feedstock by a small molding machine introduced in previous section. The sintered parts with micro-pillar structure were obtained after

**4.2 Molding machine for micro sacrificial plastic mold insert MIM** 

Fig. 31. Scheme of small direct mixing-injection molding machine.

debinding and sintering process. Fig.32 shows the SEM images of green compacts and sintered parts produced by various sized Cu powders. As the particle size of Cu powder used decreases, the filling of the green compacts in molding was improved except for *d50*=0.13μm powder specimen. The decreasing of particle size results in a marked improvements of surface roughness, transcription and dimensional variation of sintered parts. Fig.33 shows SEM images of green compact and sintered part manufactured by using *d50*=0.3µm Cu powder. For the entire body of green compact, the feedstock was filled fully in fine pillar structure, but a slight deformation is visible in sintered part. This is considered to be due to large amount of binder, and further study on the decision of binder content in particle agglomeration is needed for the quality improvement.


Fig. 32. SEM images of green compact and sintered part manufactured by using Cu powders with various particle sizes.

Micro Metal Powder Injection Molding 127

temperature, 2) Pressing and cooling to transcript Si-mold shape to PMMA film and 3) Demolding the film from Si-mold. Subsequently the μ-SPiMIM process is proceeded by three steps as shown in Fig.34(ii); 1) Injection molding of MIM feedstock into the SP-mold, and ejecting the green compacts and SP-mold as one component, 2) Removing the SP-mold and

The metal powder used for the experiments is nano-sized Cu powder which is manufactured commercially by a wet-reduction method (*d50*=0.7μm, tap density: 3.17g/cm3, specific surface area: 1.69m2/g). The solid loading is 50vol.% and melt viscosity of the feedstock is attained to 43.7 Pa-s. The feedstock composed of nano-sized Cu powder and oxymethylene-based binder was adequately prepared and molded into NIL film made of PMMA polymer with fine line-scan structures (5µm or 10µm in width and 10µm in height), and the molded parts (with a single length of 4mm) were sintered in a reductive gas atmosphere followed by solvent debinding of the films. The debinding and sintering condition was optimized by investigating the effects of sintering temperature and atmosphere gas on density, shrinkage, composition and profile accuracy with thermo-

Fig.35 shows the SEM images of green compacts, and sintered parts processed at 573K and 973K. In the green compacts with both L/S=5µm and 10µm, the feedstock has be seemingly filled into the micro channels completely, but the polymer binder builds up at the upper corners of microscopic structures. The sintered parts processed at 573K have many Green compact Sintered part, *Ts*=573K Sintered part, *Ts*=973K

Green compact Sintered part, *Ts*=573K Sintered part, *Ts*=973K

3) Debinding polymeric binders followed by sintering.

Cross-section

Surface

Cross-section

Surface

(a) In L/S=10µm structure

(b) In L/S=5µm structure

Fig. 35. SEM images of green compacts and sintered parts.

gravimetric analysis, carbon and oxygen analysis and SEM observation.

Fig. 33. SEM images of green compact and sintered part manufactured by using *d50*=0.3µm Cu powder.

### **4.4 Nano-imprint lithography (NIL) sacrificial plastic mold insert MIM**

Fig.34 shows the flow of NIL/μ-SPiMIM process. Thermal NIL technique is an application for the hot embossing technique but it can achieve higher resolution than conventional ones. The main features of NIL process are to create fine profiles with dimensional accuracy in nanometre order, and microstructures with several micrometers. However, the materials applicable to NIL process are limited only to polymers and glasses. Thus the combination process named as NIL/μ-SPiMIM which was used NIL process for manufacturing SP-mold in μ-SPiMIM process was proposed. As shown in Fig.34(i), thermal NIL process mainly consists of three steps; 1) Heating to soften PMMA film more than the glass-transition

Fig. 34. Flow of NIL/µ-SPiMIM process.

(a) Green compact (b) Sintered part

100μm

Fig. 33. SEM images of green compact and sintered part manufactured by using *d50*=0.3µm

Fig.34 shows the flow of NIL/μ-SPiMIM process. Thermal NIL technique is an application for the hot embossing technique but it can achieve higher resolution than conventional ones. The main features of NIL process are to create fine profiles with dimensional accuracy in nanometre order, and microstructures with several micrometers. However, the materials applicable to NIL process are limited only to polymers and glasses. Thus the combination process named as NIL/μ-SPiMIM which was used NIL process for manufacturing SP-mold in μ-SPiMIM process was proposed. As shown in Fig.34(i), thermal NIL process mainly consists of three steps; 1) Heating to soften PMMA film more than the glass-transition

**4.4 Nano-imprint lithography (NIL) sacrificial plastic mold insert MIM** 

Cu powder.

Fig. 34. Flow of NIL/µ-SPiMIM process.

temperature, 2) Pressing and cooling to transcript Si-mold shape to PMMA film and 3) Demolding the film from Si-mold. Subsequently the μ-SPiMIM process is proceeded by three steps as shown in Fig.34(ii); 1) Injection molding of MIM feedstock into the SP-mold, and ejecting the green compacts and SP-mold as one component, 2) Removing the SP-mold and 3) Debinding polymeric binders followed by sintering.

The metal powder used for the experiments is nano-sized Cu powder which is manufactured commercially by a wet-reduction method (*d50*=0.7μm, tap density: 3.17g/cm3, specific surface area: 1.69m2/g). The solid loading is 50vol.% and melt viscosity of the feedstock is attained to 43.7 Pa-s. The feedstock composed of nano-sized Cu powder and oxymethylene-based binder was adequately prepared and molded into NIL film made of PMMA polymer with fine line-scan structures (5µm or 10µm in width and 10µm in height), and the molded parts (with a single length of 4mm) were sintered in a reductive gas atmosphere followed by solvent debinding of the films. The debinding and sintering condition was optimized by investigating the effects of sintering temperature and atmosphere gas on density, shrinkage, composition and profile accuracy with thermogravimetric analysis, carbon and oxygen analysis and SEM observation.

Fig.35 shows the SEM images of green compacts, and sintered parts processed at 573K and 973K. In the green compacts with both L/S=5µm and 10µm, the feedstock has be seemingly filled into the micro channels completely, but the polymer binder builds up at the upper corners of microscopic structures. The sintered parts processed at 573K have many


(b) In L/S=5µm structure

Fig. 35. SEM images of green compacts and sintered parts.

Micro Metal Powder Injection Molding 129

manufacturing a single digit micrometer-sized structured part, but technical problems on quality such as high-densification and sintering by using nano-scale metal powders should be cleared in NIL/μ-SPiMIM process at this moment. In addition, evaluation methods and designing of micro devices are hoped with effectiveness of these micro-structure and a variety of metallic own properties. These μ-SPiMIM processes have a unique advantage on fabricating as a one component with macro-scale and micro-scale structures made from a variety of materials. Also the micro-scale open and closed porous structures can be formed in sintering parts. This is not accomplished by semiconductor processes and depositions methods. Therefore it is hoping to use advanced applications such as micro reactor and micro-patterned electrodes with catalyst activity for fuel cell and battery manufacturing or

In this chapter, a general characteristic of MIM process on materials and conditions for manufacturing of small metallic parts with high quality was described utilizing actual data, and the complex flow characteristics of MIM were introduced on two kinds of small components, such as micro gear and micro dumbbell specimen. Then the technical problems to be solved for micro-miniaturizing of MIM parts were addressed, and the effectiveness of sacrificial plastic mold in micro-MIM process was shown by citing some example productions of micro-structured parts. A variety of methods for fabricating of the sacrificial plastic mold such as rapid proto-typing, plastic injection-molding, LIGA process and nanoimprint lithography process, were introduced by showing the investigation results on the effects of metal particle size and processing conditions. The use of nano-sized metal powder was applied in micro MIM process inserted sacrificial plastic molds made of resist or nanoimprint lithography, the results that the decreasing of particle size improved the surface roughness and shape-transcription of sintered parts were shown obviously. In collusion, micro sacrificial plastic mold insert metal injection molding, named as μ-SPiMIM method has a great potential to solve technical problems occurring in the μ-MIM process, it can be produced precisely the 3 dimensional complex metallic metal components with single-digit

Author deeply appreciated for research foundation supports and understandings to President Dr. Shigeo Tanaka from Taisei Kogyo Co., Ltd., and great efforts of experimental works to many former graduated students from Osaka Prefectural College of Technology.

German, R.M. (1984), "Powder Metallurgy Science", Metal Powder Industry, ISBN 978-

German, R.M. and Bose, A. (1997), "Injection Molding of Metals and Ceramics", Metal

Löhe, D. and Haußelt, J. (2005), Microengineering of Metals and Ceramics: Part I and Part II.

*Advanced Micro & Nanosystem,* Vol.4, WILEY-VCH Verlag GmbH & Co. KGaA, pp.

micro-sensors for medical devices.

**5. Conclusion** 

micrometer structures.

**6. Acknowledgment** 

0918404602

Powder Industry, ISBN 978-1878954619

89, 127, 258, 276, 306, 354, 441, 464, ISBN 978-3527323784

**7. References** 

sub-micron pores, because both debinding and oxidizing have not been completed at the low temperature. Then the microscopic structures are kept in accurate shape. In general the sintered parts processed at higher temperature shrink more, and the corner of microscopic structures becomes dull correspondingly. However the sintered parts processed at 973K keep the edge sharpness under optimized debinding-sintering conditions. The green compacts have slight concave portions on the top face of microscopic structure, but it was attained to fill the feedstock into SP-mold with sufficiently high transcription. On the other hand, the sintered parts processed at 973K shrank 20% in both height and width, and became round at the top and bottom of corner portions. As the sintering temperature is raised, the shrinkage ratio increased remarkably up to 873K and further increased gradually. It was also seen that the shrinkage ratios of L/S=5µm structures are larger than that of L/S=10µm ones and the whole bodies.

The processability of a variety of μ-SPiMIM processes as above-described is summarized in Fig.36 in compared with the other precision processing and machining methods. A conventional rapid plot typing is difficult to manufacture SP-mold with micro-scale structure and high dimensional accuracy. Then RP/μ-SPiMIM method is not superior compared to micro machining such as micro-cutting, micro-EDM and micro-casting on the size of products, but it has a great potential to manufacture complex shaped parts such as micro-impeller shown in Fig.17. Further micro-miniaturization and surface quality improvement of rapid plot typing are required for μ-SPiMIM process, micro rapid plottyping using stereo-lithography and 3D-printing technology is a prospective method for manufacturing of fine structured SP-mold. On the other hand, LIGA/μ-SPiMIM and Resist/μ-SPiMIM are very hopeful combined methods for manufacturing metallic microstructured parts with high aspect ratios. The size possible for manufacturing is ranged from hundreds micrometers to tens micrometers. The problems are included high manufacturing cost and shape limitation of SP-mold. Furthermore, NIL techniques has a possibility for

Fig. 36. Processability of a variety of µ-SPiMIM processes.

manufacturing a single digit micrometer-sized structured part, but technical problems on quality such as high-densification and sintering by using nano-scale metal powders should be cleared in NIL/μ-SPiMIM process at this moment. In addition, evaluation methods and designing of micro devices are hoped with effectiveness of these micro-structure and a variety of metallic own properties. These μ-SPiMIM processes have a unique advantage on fabricating as a one component with macro-scale and micro-scale structures made from a variety of materials. Also the micro-scale open and closed porous structures can be formed in sintering parts. This is not accomplished by semiconductor processes and depositions methods. Therefore it is hoping to use advanced applications such as micro reactor and micro-patterned electrodes with catalyst activity for fuel cell and battery manufacturing or micro-sensors for medical devices.
