**Micro Metal Powder Injection Molding**

Kazuaki Nishiyabu *Kinki University, Japan* 

#### **1. Introduction**

104 Some Critical Issues for Injection Molding

Wang J.; Liu G.; Xiong Y.; Huang X.; Tian Y. (2008). Fabrication of ceramic microcomponents

Wei T. S.; German R. M., US Patent number 5028367: Two-stage fast debinding of injection molding powder campacts, filed December 1989, published July 1991 Wright J. K.; Edirisinghe M. J.; Zhang J. G.; Evans J. R. G. (1990). Particle Packing in Ceramic

Zorzi J. E.; Perottoni C. A.; Da Jornada A. H. (2003). A new partially isostatic method for fast

14, No. 9 (October 2008), pp. 1245-1249, ISSN 09467076

57, No. 24 (August 2003), pp. 3784 – 3788, ISSN 0167-577X

1990), pp. 2653-58, ISSN 0002-7820

and microreactor for the steam reforming of ethanol, *Microsystem Technologies*, Vol.

Injection Molding, *Journal of American Ceramic Society*, Vol. 73, No. 91 (September

debinding of low-pressure injection molded ceramic parts, *Matterials Letters*, Vol.

Powder injection molding (PIM), which encompasses metal powder injection molding (MIM) and ceramic powder injection molding (CIM) is a net-shape process for the manufacturing of high volume and high precision components for use in a variety of industries. The micro-miniaturization of dimension and structures in MIM is facing with various technical problems, such as incomplete filling to narrow cavity, failure in demolding of fragile green compacts, and deformation in debinding and sintering process. Therefore micro MIM (μ-MIM) process is a more sophisticated process for tiny metal components and micro structured parts. This chapter introduces a general flow of MIM process, the material properties of the feedstock and focuses on the unique phenomena in the micro injection molding and the filling behaviour. A flow simulation of micro gear and micro dumbbell tensile specimen will be carried out and the flow pattern by short shot test and internal pressure measured will be compared to the simulation results. The production method of micro sacrificial plastic mold insert MIM (μ-SPiMIM) process has been proposed to solve drastically the specific problems involving the miniaturization of MIM parts. The sacrificial plastic mold (SP-mold) is prepared by injection-molding polymethylmethacrylate (PMMA) polymer into Ni-electroform. Micro-sized stainless steel 316L powder feedstock is injectionmolded into the SP-mold which consists of micro multi-pillar structures. The effects of metal particle size and processing conditions on the quality of molded and sintered parts are evaluated. For the higher quality of μ-SPiMIM process, the feedstock composed of nanosized Cu powder and oxymethylene-based binder is adequately prepared and molded into PMMA films with fine line-scan structures which are prepared by nano-imprint lithography (NIL) technique. From the evaluation results on the effects of particle size of metal powder and processing conditions toward the high precision of sintered parts, it is concluded that the μ-SPiMIM process has a great potential to produce precisely the complex metallic parts with fine micro-structures.

#### **1.1 What is metal powder injection molding (MIM)?**

Metal powder injection molding (MIM) is a manufacturing method that combines traditional powder metallurgy (P/M) with plastic injection molding as shown in Fig.1. Over the past decade it has established itself as a competitive manufacturing process for small precision components that would be costly to produce by alternative methods. It can be used to produce comparatively small parts with complex shapes from almost all types of materials such as metals, ceramics, inter-metallic compounds, and composites (German, 1984). Recently MIM

Micro Metal Powder Injection Molding 107

Impeller

MIM

**1mm 10mm**

Cam **5mm**

Connector

Difficult fabricating fine metallic mould

Use of plastic mould

Excellent filling Easy handling

Sintered part : Smooth surface Good transcription

Slow solidification

Trade-off

Lower thermal conductivity

> Free demoulding

Easy thermal decomposition

10-6 10-5 10-4 10-3 10-2 10-1 1 m

Size of products or structures

Miniaturization in dimension and shape of MIM parts

Higher oxidation

Fig. 2. Dimensional tolerance versus size of MIM Products.

10-8 Planetary-gear


100μm

Reactor

Difficult full filling to fine cavity

Use of finer particle size of metal powder

Higher resistance

High specific surface area High material

Lower tap density Higher viscosity

Green compact: Reduced density

Higher binder content

cost

10-3

10-4

10-5

Dimensional tolerance

m

10-6

10-7

Fig. 3. Technical problems and its solutions to miniaturize MIM parts.

Poor filling Poor mouldablity Poor sintering

Higher Remaining carbon

sintered parts. In Fig.3, the solutions for filling of viscous feedstock into the fine mold and fabricating of fine mold are shown in addition to the effectiveness and the disadvantages. The use of finer powder is essential to fill the feedstock into several tenth and a few micronsized cavity. However, nano-sized powder has extremely high specific surface area, thus the tap density is very low and the viscosity of the feedstock increases remarkably. It is also

Sintered part: Reduced strength

Fig. 1. Flow of metal powder injection molding (MIM) process indicated by bold line.

has been studied not only for hard metals, but also for materials such as titanium, copper and aluminium (German and Bose, 1997). Unlike in the case of P/M, MIM requires mixing metal powders with a large amount of polymeric binder. Afterwards the organic constituents are removed in a debinding step using solvent extraction or pyrolysis. The brown body is held in the molded form by only metal powder after debinding. The mixing and debinding of polymer with metal powder is a very unique and an original process of MIM. Thus it is very effective for manufacturing higher functional metal parts to apply this unique process.

#### **1.2 Micro-miniaturization techniques by MIM**

Fig.2 shows some typical examples of commercial MIM products, such as connector, impeller, cam, planetary-gear set and micro-reactor. In these conventional MIM products, as the size become smaller, the dimensional accuracy becomes higher as it falls within a few tenth micrometers. As the size of MIM products decreases much further, the dimensional accuracy is hoping to ensure within a few micrometers. In practical productions, however, it cannot be achieved easily. The production method of these tiny metallic parts which have micro-size and micro-structure are called µ-MIM, which is capable of manufacturing the micro-structured parts such as micro-pillars (Löhe and Haußelt, 2005).

#### **1.3 Technical problems and solutions to miniaturize MIM parts**

The μ-MIM process is very useful for the manufacturing of micro-sized and microstructured metallic parts, but it is facing with various technical problems in each process. For example, it is difficult to fill feedstock completely into a narrow cavity and to demold fragile green compacts from a metallic mold in injection molding process. A careful handling is also required in the debinding and sintering processes. There are many other technical problems such as measuring of the density, the shape and mechanical properties of

**Plastic Injection Molding Powder Metallurgy**

Polymer Metal Powder

Mixing

Fig. 1. Flow of metal powder injection molding (MIM) process indicated by bold line.

unique process.

**1.2 Micro-miniaturization techniques by MIM** 

Injection

Molded part

Compound

has been studied not only for hard metals, but also for materials such as titanium, copper and aluminium (German and Bose, 1997). Unlike in the case of P/M, MIM requires mixing metal powders with a large amount of polymeric binder. Afterwards the organic constituents are removed in a debinding step using solvent extraction or pyrolysis. The brown body is held in the molded form by only metal powder after debinding. The mixing and debinding of polymer with metal powder is a very unique and an original process of MIM. Thus it is very effective for manufacturing higher functional metal parts to apply this

Brown body

Debinding

Sintering

Pressing

Compact

Sintered body

Fig.2 shows some typical examples of commercial MIM products, such as connector, impeller, cam, planetary-gear set and micro-reactor. In these conventional MIM products, as the size become smaller, the dimensional accuracy becomes higher as it falls within a few tenth micrometers. As the size of MIM products decreases much further, the dimensional accuracy is hoping to ensure within a few micrometers. In practical productions, however, it cannot be achieved easily. The production method of these tiny metallic parts which have micro-size and micro-structure are called µ-MIM, which is capable of manufacturing the

The μ-MIM process is very useful for the manufacturing of micro-sized and microstructured metallic parts, but it is facing with various technical problems in each process. For example, it is difficult to fill feedstock completely into a narrow cavity and to demold fragile green compacts from a metallic mold in injection molding process. A careful handling is also required in the debinding and sintering processes. There are many other technical problems such as measuring of the density, the shape and mechanical properties of

micro-structured parts such as micro-pillars (Löhe and Haußelt, 2005).

**1.3 Technical problems and solutions to miniaturize MIM parts** 

Fig. 2. Dimensional tolerance versus size of MIM Products.

Fig. 3. Technical problems and its solutions to miniaturize MIM parts.

sintered parts. In Fig.3, the solutions for filling of viscous feedstock into the fine mold and fabricating of fine mold are shown in addition to the effectiveness and the disadvantages. The use of finer powder is essential to fill the feedstock into several tenth and a few micronsized cavity. However, nano-sized powder has extremely high specific surface area, thus the tap density is very low and the viscosity of the feedstock increases remarkably. It is also

Micro Metal Powder Injection Molding 109

(a) (b) Fig. 5. Melt viscosities of MIM feedstock with various fractional metal powders; (a) 316L

1

1 10 100 1000 10000

Shear rate (/s)

2μm,40vol% 2μm,50vol% 2μm,60vol% 2μm,65vol%

10

100

1000

Melt viscosity (Pa・s)

10000

100000

fraction of the metal powder was varied from 40 to 65%. It is obvious that the melt viscosity of the feedstock increases remarkably as volume fraction of metal powder increases, and

The feedstock used for MIM is generally prepared by mixing metal powder and binders with a twin screw extruder and kneader. A highly-homogenized feedstock is significant to manufacture the high quality of sintered parts, but it is not easy due to a big difference in specific gravity between metal powder and binders. The homogeneity of the feedstock was evaluated with the coefficient of variation (CV). Fig.6 shows the CV values of binder weight contained in a pellet of feedstock which was prepared with various mixing volumes such as 100cc, 1000cc and 40,000cc in a lot. The experimental result shows that the variation of

fine powder makes the melt viscosity of feedstock much higher than the coarse one.

**2.2 Effects of capacity of mixing machine and injection molding machine** 

binder content can reduce significantly with decreasing of the mixing volume.

Fig. 6. Variation of binder weight in a pellet prepared with various mixing volumes.

Fig.7. shows the distributions of weight of green compacts prepared by two types of injection molding machines with varied capacities. Micro injection molding machine

1 10 100 1000 10000 100000 Mixing volume (cc)

powder (*D50*=10μm) feedstock; (b) 316L powder (*D50*=2μm) feedstock.

1 10 100 1000 10000

0.00

0.01

0.02

CV

0.03

Shear rate (/s)

1

10

100

10μm,40vol% 10μm,50vol% 10μm,60vol% 10μm,65vol%

1000

Melt viscosity (Pa・s)

10000

100000

susceptible to oxidation and relatively high production cost. Therefore these properties result in adverse affect for the production and the utilization. The fluidity of the feedstock is improved generally by increasing the binder content, but it makes lower the quality and the mechanical property of the sintered parts because the density of green compact become lower and the remaining carbon content increases. On the other hand, it is not easy to fabricate precisely a metallic mold, but a plastic is more superior for processability of the fine mold. In addition, a plastic mold is much superior for filling of the feedstock because its low thermal conductivity which delays solidification of melted materials. Moreover it is not necessary to demold a green compact from a plastic mold which can be decomposed thermally in debinding process. As a result, if there is a solution to the problem in sintering and injection of finer metal powder into plastic mold, a sintered part with smooth surface and good transcription can expect to be manufactured by MIM process. Therefore the tradeoff problem needs to be solved by any innovative technologies.

#### **2. Metal powder injection molding for small components**

#### **2.1 Material properties of MIM feedstock**

The metallic powders used for MIM process are plain and low alloy steels, high speed steels, stainless steels, superalloys, intermetallics, magnetic alloys, hardmetals, and titanium and so on (Osada et al, 2007). Among them, stainless steels are commonly used. Fig.4 shows distributions of particle diameter of 316L stainless steel powder (10μm and 2μm in mean diameter, Epson Atmix Co., Ltd., PF-20J, PF-2F) produced by water-atomization method. Fine powders sintered more readily than coarser ones, but there is a number of limiting factors. The metallic powders are compounded with wax and polymeric binder by highpressure kneader and are granulated by plunger-type extruder. The least possible amount of binder should be used, but an appropriate volume fraction of binder to powder exists. In industrial practice, the ratio varies from about 0.5 to 0.7.

Fig. 4. Distributions of particle size of stainless steel powders used for MIM feedstock; (a) 316L powder (*D50*=10μm); (b) 316L powder (*D50*=2μm).

Fig.5 shows the melt viscosity of the feedstock with various fractions of 316L stainless steel powders with different mean particle sizes, which was measured by a capillary graph. The binder used for the feedstock was polyacetal polymer and paraffin wax. The volume

susceptible to oxidation and relatively high production cost. Therefore these properties result in adverse affect for the production and the utilization. The fluidity of the feedstock is improved generally by increasing the binder content, but it makes lower the quality and the mechanical property of the sintered parts because the density of green compact become lower and the remaining carbon content increases. On the other hand, it is not easy to fabricate precisely a metallic mold, but a plastic is more superior for processability of the fine mold. In addition, a plastic mold is much superior for filling of the feedstock because its low thermal conductivity which delays solidification of melted materials. Moreover it is not necessary to demold a green compact from a plastic mold which can be decomposed thermally in debinding process. As a result, if there is a solution to the problem in sintering and injection of finer metal powder into plastic mold, a sintered part with smooth surface and good transcription can expect to be manufactured by MIM process. Therefore the trade-

The metallic powders used for MIM process are plain and low alloy steels, high speed steels, stainless steels, superalloys, intermetallics, magnetic alloys, hardmetals, and titanium and so on (Osada et al, 2007). Among them, stainless steels are commonly used. Fig.4 shows distributions of particle diameter of 316L stainless steel powder (10μm and 2μm in mean diameter, Epson Atmix Co., Ltd., PF-20J, PF-2F) produced by water-atomization method. Fine powders sintered more readily than coarser ones, but there is a number of limiting factors. The metallic powders are compounded with wax and polymeric binder by highpressure kneader and are granulated by plunger-type extruder. The least possible amount of binder should be used, but an appropriate volume fraction of binder to powder exists. In

(a) (b) Fig. 4. Distributions of particle size of stainless steel powders used for MIM feedstock; (a)

1 10 100

 Frequency (%) Cumulative frequebcy (%)

Particle diameter, *d* (μm)

Cumulative frequency (%)

Cumulative frequency (%)

Frequency (%)

Fig.5 shows the melt viscosity of the feedstock with various fractions of 316L stainless steel powders with different mean particle sizes, which was measured by a capillary graph. The binder used for the feedstock was polyacetal polymer and paraffin wax. The volume

off problem needs to be solved by any innovative technologies.

industrial practice, the ratio varies from about 0.5 to 0.7.

316L powder (*D50*=10μm); (b) 316L powder (*D50*=2μm).

1 10 100

 Frequency (%) Cumulative frequebcy (%)

Particle diameter, *d* (μm)

Frequency (%)

**2.1 Material properties of MIM feedstock** 

**2. Metal powder injection molding for small components** 

Fig. 5. Melt viscosities of MIM feedstock with various fractional metal powders; (a) 316L powder (*D50*=10μm) feedstock; (b) 316L powder (*D50*=2μm) feedstock.

fraction of the metal powder was varied from 40 to 65%. It is obvious that the melt viscosity of the feedstock increases remarkably as volume fraction of metal powder increases, and fine powder makes the melt viscosity of feedstock much higher than the coarse one.

#### **2.2 Effects of capacity of mixing machine and injection molding machine**

The feedstock used for MIM is generally prepared by mixing metal powder and binders with a twin screw extruder and kneader. A highly-homogenized feedstock is significant to manufacture the high quality of sintered parts, but it is not easy due to a big difference in specific gravity between metal powder and binders. The homogeneity of the feedstock was evaluated with the coefficient of variation (CV). Fig.6 shows the CV values of binder weight contained in a pellet of feedstock which was prepared with various mixing volumes such as 100cc, 1000cc and 40,000cc in a lot. The experimental result shows that the variation of binder content can reduce significantly with decreasing of the mixing volume.

Fig. 6. Variation of binder weight in a pellet prepared with various mixing volumes.

Fig.7. shows the distributions of weight of green compacts prepared by two types of injection molding machines with varied capacities. Micro injection molding machine

Micro Metal Powder Injection Molding 111

(a) (b)

the evaluation results, it was clarified that the accuracy class of the ultra-compact planet gear have not come up to that of ground precise gear which is equivalent to around five. However it is reviewed that the accuracy class of ultra-compact gear is around seven which

Computational fluid dynamics (CFD) analysis can provide valuable information to mold designers and manufacturers (Nishiyabu et al., 2008). Flow simulation of micro-planetarygear manufactured by MIM process was done using MoldexTM software. The finite element model shown in Fig.9 is used. In total, the cavity meshes and mold base meshes included approximately 2.3million elements. Whilst the material for the analysis has the material properties of MIM feedstock, it is not currently possible to accurately model all of the complex flow characteristics of MIM feedstock, such as layer slip, compressibility and jetting. The material in the analysis has similar flow characteristics to a heavy plastic. Most commercial injection-molding CFD programs cannot accurately create micro-parts model using standard machine settings as they have unsuitable values for shot weights and flow rates etc. From the flow simulation results shown in Fig.10, it is clarified that the filling time is approximately 1.8ms for spur-runner part but only 0.2ms for cavity one. Also the filling of gear teeth is at the end of flow, and is hard to apply high pressure into fine wall of teeth. Extra care must be taken with runner shapes and gate locations in micro-analysis due to the

Fig. 8. Micro-planetary-gear manufactured by MIM process; (a) Planetary-gear set;

is an acceptable level for general-purpose applications from a practical point of view.

(b) Planet gear.

exceptionally small and thin cavity shapes.

**1mm**

(b) Micro-gear part.

(a) (b)

Fig. 9. Micro-planetary-gear manufactured by MIM process; (a) Runner and spur part;

2,310,575 elements 440,188 nodes

Fig. 7. Distributions of weight of green compacts prepared by injection molding machines with varied capacities.

(Battenfeld GmbH, Microsystem50) has 50kN clamping force and is smaller than conventional one with 400kN (Nissei Plastic Industrial Co., Ltd., PN40-2H). The diameter of injection plunger in micro injection molding machines is 3mm which is much smaller than that of 19mm in conventional one. Minimum volume for fabricating of micro injection molding machine is 120mm3 which is much smaller than the conventional one. It is found that small capacity injection molding machine can be reduced the variation in weight of green compacts as compared to conventional one.

#### **2.3 Filling behavior and flow simulation**

#### **2.3.1 Micro gear**

In recent years, some advanced micro-manufacturing processes and the micro-sized gears made of metals and some advanced ceramics were demonstrated (Löhe and Haußelt, 2005). The micro-planetary gear motors made of Ni-Fe and Ni-based bulk metallic glasses were developed by X-ray lithography, electro-deposition and injection molding method (Ishida et al., 1995). However, micro-sized gears made of general-purpose durable materials are demanded for miniaturization and reliability improvement of products, and also the manufacturing is aiming to achieve a high economical efficiency for industrial needs. Authors have studied the tribological properties of micro-gear manufactured by MIM process and were evaluated quantitatively, thus the wear mechanisms were clarified (Kameo et al., 2006) and the accuracy of the ultra-compact planet gear was also evaluated by measuring the variation in dimensions of the gear teeth with digital image analysis (Nishiyabu et al., 2008). Fig.8 shows the figures of the micro-planetary gear composed of three types of gearwheels manufactured by μ-MIM process and the dimension of the planet gear (module: m=0.07mm, number of teeth: z=24). The materials used for producing the ultra-compact gears are stainless steel 17-4PH water-atomized powder (D50=2μm) and oxymethylene-based binders. The volume fraction of powder in the feedstock is 60%. The feedstock was injection-molded using high-speed injection molding machine (FANUC Ltd., S-2000i 50A). The green compacts were debound at 600ºC for 2hrs in N2 gas, and sintered at 1150ºC for 2hrs in Ar gas. Also the sintered parts were age-hardened at 480ºC for 1hrs. From

molding machine (CV=0.006)

Micro injection (5kN)

Fig. 7. Distributions of weight of green compacts prepared by injection molding machines

Conventional injection molding machine (40kN)

0 10 20 30 40 50 Shot number

(CV=0.019)

(Battenfeld GmbH, Microsystem50) has 50kN clamping force and is smaller than conventional one with 400kN (Nissei Plastic Industrial Co., Ltd., PN40-2H). The diameter of injection plunger in micro injection molding machines is 3mm which is much smaller than that of 19mm in conventional one. Minimum volume for fabricating of micro injection molding machine is 120mm3 which is much smaller than the conventional one. It is found that small capacity injection molding machine can be reduced the variation in weight of

In recent years, some advanced micro-manufacturing processes and the micro-sized gears made of metals and some advanced ceramics were demonstrated (Löhe and Haußelt, 2005). The micro-planetary gear motors made of Ni-Fe and Ni-based bulk metallic glasses were developed by X-ray lithography, electro-deposition and injection molding method (Ishida et al., 1995). However, micro-sized gears made of general-purpose durable materials are demanded for miniaturization and reliability improvement of products, and also the manufacturing is aiming to achieve a high economical efficiency for industrial needs. Authors have studied the tribological properties of micro-gear manufactured by MIM process and were evaluated quantitatively, thus the wear mechanisms were clarified (Kameo et al., 2006) and the accuracy of the ultra-compact planet gear was also evaluated by measuring the variation in dimensions of the gear teeth with digital image analysis (Nishiyabu et al., 2008). Fig.8 shows the figures of the micro-planetary gear composed of three types of gearwheels manufactured by μ-MIM process and the dimension of the planet gear (module: m=0.07mm, number of teeth: z=24). The materials used for producing the ultra-compact gears are stainless steel 17-4PH water-atomized powder (D50=2μm) and oxymethylene-based binders. The volume fraction of powder in the feedstock is 60%. The feedstock was injection-molded using high-speed injection molding machine (FANUC Ltd., S-2000i 50A). The green compacts were debound at 600ºC for 2hrs in N2 gas, and sintered at 1150ºC for 2hrs in Ar gas. Also the sintered parts were age-hardened at 480ºC for 1hrs. From

with varied capacities.

**2.3.1 Micro gear** 

green compacts as compared to conventional one.

0.006

0.007

0.008

Weight of green compact (g)

0.009

**2.3 Filling behavior and flow simulation** 

Fig. 8. Micro-planetary-gear manufactured by MIM process; (a) Planetary-gear set; (b) Planet gear.

the evaluation results, it was clarified that the accuracy class of the ultra-compact planet gear have not come up to that of ground precise gear which is equivalent to around five. However it is reviewed that the accuracy class of ultra-compact gear is around seven which is an acceptable level for general-purpose applications from a practical point of view.

Computational fluid dynamics (CFD) analysis can provide valuable information to mold designers and manufacturers (Nishiyabu et al., 2008). Flow simulation of micro-planetarygear manufactured by MIM process was done using MoldexTM software. The finite element model shown in Fig.9 is used. In total, the cavity meshes and mold base meshes included approximately 2.3million elements. Whilst the material for the analysis has the material properties of MIM feedstock, it is not currently possible to accurately model all of the complex flow characteristics of MIM feedstock, such as layer slip, compressibility and jetting. The material in the analysis has similar flow characteristics to a heavy plastic. Most commercial injection-molding CFD programs cannot accurately create micro-parts model using standard machine settings as they have unsuitable values for shot weights and flow rates etc. From the flow simulation results shown in Fig.10, it is clarified that the filling time is approximately 1.8ms for spur-runner part but only 0.2ms for cavity one. Also the filling of gear teeth is at the end of flow, and is hard to apply high pressure into fine wall of teeth. Extra care must be taken with runner shapes and gate locations in micro-analysis due to the exceptionally small and thin cavity shapes.

Fig. 9. Micro-planetary-gear manufactured by MIM process; (a) Runner and spur part; (b) Micro-gear part.

Micro Metal Powder Injection Molding 113

The conditions of injection molding for micro dumbbell specimens were examined with various injection pressures and speeds, but the other conditions such as holding and back pressure, injection and holding time, metered value, molten material temperature and mold temperature were constantly-applied. The results of the experiments using *D50*=10μm and *D50*=2μm 316L stainless powder feedstock in constancy of metal powder fraction of 50vol.% are shown in Fig.12(a) and (b), respectively. These diagrams show the filling behaviour. In case of 10μm powder feedstock, the maximum injection pressure was limited at 150MPa because of the high melt viscosity, while no limitation existed for 2μm powder feedstock. In case of 2μm powder feedstock, the short shot phenomenon was observed in the wide range of injection pressure and speed. In either case, the short shot was likely to occur at lower injection speed, while flash is significant at higher injection speed, because the melt viscosity of feedstock depends on shearing rate as shown in Fig.5. With this preliminary examination, it is concluded that the suitable injection pressure and speed are 20-70MPa and 300-

400mm/s for 10μm powder feedstock while 70MPa and 400mm/s for 2μm one.

The finite element model of micro-dumbbell with spur-runner parts as shown in Fig.13 was used for flow simulation. The cavity pressure profiles obtained by numerical analysis and

(a) (b) Fig. 12. Dependence of injection molding conditions on filling behaviour in micro dumbbell specimens molded using varied particle size feedstock; (a) *D50*=10μm powder feedstock; (b)

*D50*=2μm powder feedstock.

Fig. 13. FE model for micro-dumbbell with spur-runner parts.

Fig. 10. Filling state of micro-planetary-gear with spur-runner part.

#### **2.3.2 Micro-dumbbell tensile specimen**

As the size of MIM parts decrease, the test specimen is also necessary to downsize appropriately for actual evaluation. Authors (Nishiyabu et al., 2005) used micro-dumbbell tensile specimen with 0.1mm wide in narrowest portion as shown in Fig.11. The reason why the dumbbell shape with large volumes of clamps at the ends was attached is to investigate the performance of filling and the change of internal pressure when it was molded with an in-line screw type injection molding machine. The jigs for clamping are also manufactured by MIM, and the specimen can be observed by scanning electron microscope (SEM). The motion of deformation and damages occurred on the surface is recorded.

(a) Setting-up and photograph of micro dumbbell specimen with tensile jigs

 (b) Detail geometry of micro dumbbell specimen and the sintered part Fig. 11. Geometry of micro dumbbell specimens.

Filling time (10-3s) 1.8

0.9

0

As the size of MIM parts decrease, the test specimen is also necessary to downsize appropriately for actual evaluation. Authors (Nishiyabu et al., 2005) used micro-dumbbell tensile specimen with 0.1mm wide in narrowest portion as shown in Fig.11. The reason why the dumbbell shape with large volumes of clamps at the ends was attached is to investigate the performance of filling and the change of internal pressure when it was molded with an in-line screw type injection molding machine. The jigs for clamping are also manufactured by MIM, and the specimen can be observed by scanning electron microscope (SEM). The

(b) 1.896 ms (c) 2.033ms

(b) 1.987ms

Fig. 10. Filling state of micro-planetary-gear with spur-runner part.

motion of deformation and damages occurred on the surface is recorded.

(b) Detail geometry of micro dumbbell specimen and the sintered part

Fig. 11. Geometry of micro dumbbell specimens.

(a) Setting-up and photograph of micro dumbbell specimen with tensile jigs

**2.3.2 Micro-dumbbell tensile specimen** 

(a) 1.824

The conditions of injection molding for micro dumbbell specimens were examined with various injection pressures and speeds, but the other conditions such as holding and back pressure, injection and holding time, metered value, molten material temperature and mold temperature were constantly-applied. The results of the experiments using *D50*=10μm and *D50*=2μm 316L stainless powder feedstock in constancy of metal powder fraction of 50vol.% are shown in Fig.12(a) and (b), respectively. These diagrams show the filling behaviour. In case of 10μm powder feedstock, the maximum injection pressure was limited at 150MPa because of the high melt viscosity, while no limitation existed for 2μm powder feedstock. In case of 2μm powder feedstock, the short shot phenomenon was observed in the wide range of injection pressure and speed. In either case, the short shot was likely to occur at lower injection speed, while flash is significant at higher injection speed, because the melt viscosity of feedstock depends on shearing rate as shown in Fig.5. With this preliminary examination, it is concluded that the suitable injection pressure and speed are 20-70MPa and 300-

The finite element model of micro-dumbbell with spur-runner parts as shown in Fig.13 was used for flow simulation. The cavity pressure profiles obtained by numerical analysis and

400mm/s for 10μm powder feedstock while 70MPa and 400mm/s for 2μm one.

Fig. 12. Dependence of injection molding conditions on filling behaviour in micro dumbbell specimens molded using varied particle size feedstock; (a) *D50*=10μm powder feedstock; (b) *D50*=2μm powder feedstock.

Fig. 13. FE model for micro-dumbbell with spur-runner parts.

Micro Metal Powder Injection Molding 115

The result in Fig.15 shows that one of the reasons to be considered is difference in filling between simulation and short shot. When the flow simulation considered the effects of material gravity and cavity gas pressure (Fig.5(b)), the results could be simulated as a jetting phenomenon and it could not be identified the unstable filling state such as snake phenomenon (Fig.5(c)). Thus it is considered that the pressure is resulted to reduce

Fig.16 shows the flow of sacrificial plastic mold insert MIM (SPiMIM) process which is basically divided into three steps; 1) manufacturing of SP-mold, 2) injection molding of MIM feedstock into SP-mold insert, and de-molding the green compact and SP-mold insert part as one component, handling to the debinding-sintering process, and finally 3) debinding to eliminate the SP-mold and polymeric binder followed by sintering process. Therefore, the µ-SPiMIM process has great potentials to improve the filling, de-molding and handling, and to produce the tiny parts with 3 dimensional complex shapes and fine structures. The SPmolds used in this process can be manufactured by several types of methods such as injection molding, machining, rapid-proto typing, hot-embossing and lithography and so on

**3. Metal powder injection molding for micro pillar-structured parts** 

**3.1 Concept of sacrificial plastic mold insert MIM (SPiMIM) process** 

Fig. 16. Flow of micro sacrificial plastic mold insert MIM (SPiMIM) process.

Insert Handle

(ii) Injection molding of MIM

typing is used to make the closed mold to manufacture a micro-impeller.

**3.2 Differences in filing behavior of metallic mold vs. plastic mold** 

Fig.17 shows some examples of SPiMIM products such as an optical fibre connector, a zigzag spring and micro impeller. These shapes cannot be manufactured easily because they has hollow, under-cuts and external screw and very narrow portions. The core and mold insert are made of polymethylmethacrylate (PMMA) polymer by conventional plastic injection molding for optical fibre connector and a zigzag spring, while micro rapid plot-

(iii) Debinding and Sintering

The differences of mold materials on the filling behaviour were investigated by flow analysis using commercial plastic injection molding simulation software (MoldflowTM, MPI ver.4.0). The main difference in property of mold materials is thermal conductivity, which of steels (46.2W/mK) is 230 times higher than plastics (0.2W/mK). From the analytical results in zigzag spring specimen as shown in Fig.18, it was confirmed that filling of feedstock could be accomplished with lower injection pressure by using plastic mold. This is

significantly in the experiment.

(Nishiyabu et al., 2007).

(i) Manufacturing of SP-mold

experiment are shown in Fig.14. The far gate side is applied much lower pressure than the near gate. The results agree qualitatively, however it has large differences quantitatively between the simulation and the experiment.

Fig. 14. Cavity pressure versus process time.

Fig. 15. Comparison of filling between simulation and short shot results.

experiment are shown in Fig.14. The far gate side is applied much lower pressure than the near gate. The results agree qualitatively, however it has large differences quantitatively

> Near gate side Far gate side

Max cavity pressure

(a) Simulation (b) Experiment

Cavity pressure, p (MPa)

0

0 0.1 0.2 0.3

Near gate side Far gate side

Process time, t (s)

Max cavity pressure

30

60

90

120

150

Fig. 15. Comparison of filling between simulation and short shot results.

between the simulation and the experiment.

Fig. 14. Cavity pressure versus process time.

Process time, t (s)

0 0.1 0.2 0.3

0

30

60

Cavity pressure,p (MPa)

90

120

150

The result in Fig.15 shows that one of the reasons to be considered is difference in filling between simulation and short shot. When the flow simulation considered the effects of material gravity and cavity gas pressure (Fig.5(b)), the results could be simulated as a jetting phenomenon and it could not be identified the unstable filling state such as snake phenomenon (Fig.5(c)). Thus it is considered that the pressure is resulted to reduce significantly in the experiment.
