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

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

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 (Nishiyabu et al., 2007).

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

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

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

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

Micro Metal Powder Injection Molding 117

Micro Electric Discharge

P/M

**Taniguchi)**

**Processing**

 **(by**

Rapid Prototyping

Fig. 19. Situation of MIM process in road map on micro-nano processing.

**Processing Limitation**

**Processing**

**3D**

Lithography

Inter-atomic or lattice spacing(0.3nm)

超精密加工限界(谷口)

**Micro**・**Nano**

**Ultra-Precision**

**2D**

Process Resolution (Dimensional tolerance) nm

nm

µm

mm

Electric Discharge

**Micro**・**Nano**

Society of Mechanical Engineers (JSME), which was originally surveyed by Prof. Hata. It is obvious from this road map that the process resolution is becoming progressively more accurate from micro-scale to nano-scale with the passing of the year by two or three dimensional micro-nano processing such as several lithography techniques, nano-imprint, micro electric discharge and micro-rapid prototyping and so on. By using these three dimensional micro-nano processing's to make the molds, the dimensional tolerance of P/M and MIM is becoming higher and it is even more so high for µ-MIM. However, most of these ultra-precision processing's is capable of applying for polymers, not for metals. Therefore, it is suggested that the use of plastic mold in MIM process is effective for the manufacturing of

Year

1960 2007 2030

3D Lithography

µ-MIM

2D 3D

Micro Rapid Prototyping

Nano Imprint FIB EPL

3D Cutting

MIM

fine mold in addition to providing a high performance in filling and easy handling.

In producing of the higher precision parts, it is necessary to use the molds that are more precise than conventional molds manufactured by cutting and electric discharging. LIGA ((Lithographie,Galvanoformung,Abformung) is the abbreviation of German words for lithography, electroforming and molding, which is one of the micro-processing techniques applied for semiconductor production such as Micro-Electro-Mechanical-System (MEMS) or Micro System Technology (MST). LIGA is mainly consisted of 3 processes; the first is irradiation process to transcript the mask shape to resin, namely resist, the second is developing process to remove unnecessary part of the resist, and to make negative structure of the desired body, the third is molding process by electro-forming metal structure in the resist. The main features of LIGA process are to create the fine profiles with dimensional accuracy in nanometre order, and micro-structures with high aspect ratio. However, the materials applicable to LIGA are limited, and the process is not suitable for mass-production because of its high cost. The manufacturing method of micro sacrificial plastic mold insert metal injection molding (µ-SPiMIM) combined LIGA process, namely LIGA/μ-SPiMIM process has been proposed to solve specific problems involving the miniaturization of MIM. As shown in Fig.20, the μ-SPiMIM method inserted ultra-fine molds which were fabricated

**3.4 Combination of LIGA and MIM process** 

Fig. 17. Examples of metal parts manufactured by SPiMIM process.

Fig. 18. Effects of mold material on filling behaviour in zigzag spring specimen.

considered because the plastic mold cools the feedstock much slower than metallic one. This is due to the low thermal conductivity of plastic and heat generated by friction between viscous flowing feedstock and plastic mold. In consequence of high viscosity of the feedstock, there is no rapid increase in plastic mold. Therefore, it can be expected to easily fill the viscous feedstock in narrow cavity of micro parts. In particular, with injection molding of micro pillar-structured parts, it is effective to use plastic mold.

#### **3.3 Effectiveness of plastic in mold fabrication for μ-MIM**

The products shown in section 3.1 cannot easily be manufactured by conventional MIM process, but they are not so tiny parts. Therefore, the SPiMIM method was applied into µ-MIM process by using finer tooling production method as below. Fig.19 shows the situation of MIM process overwritten in a road map on micro-nano processing developed by Japan

Fig. 17. Examples of metal parts manufactured by SPiMIM process.

D

B

C

Fig. 18. Effects of mold material on filling behaviour in zigzag spring specimen.

0 5 10 15 20 25 30 Injection pressure (MPa)

molding of micro pillar-structured parts, it is effective to use plastic mold.

**3.3 Effectiveness of plastic in mold fabrication for μ-MIM** 

A

B

Filling volume (%)

considered because the plastic mold cools the feedstock much slower than metallic one. This is due to the low thermal conductivity of plastic and heat generated by friction between viscous flowing feedstock and plastic mold. In consequence of high viscosity of the feedstock, there is no rapid increase in plastic mold. Therefore, it can be expected to easily fill the viscous feedstock in narrow cavity of micro parts. In particular, with injection

SP-mold Metallic mold

C

D

The products shown in section 3.1 cannot easily be manufactured by conventional MIM process, but they are not so tiny parts. Therefore, the SPiMIM method was applied into µ-MIM process by using finer tooling production method as below. Fig.19 shows the situation of MIM process overwritten in a road map on micro-nano processing developed by Japan

Fig. 19. Situation of MIM process in road map on micro-nano processing.

Society of Mechanical Engineers (JSME), which was originally surveyed by Prof. Hata. It is obvious from this road map that the process resolution is becoming progressively more accurate from micro-scale to nano-scale with the passing of the year by two or three dimensional micro-nano processing such as several lithography techniques, nano-imprint, micro electric discharge and micro-rapid prototyping and so on. By using these three dimensional micro-nano processing's to make the molds, the dimensional tolerance of P/M and MIM is becoming higher and it is even more so high for µ-MIM. However, most of these ultra-precision processing's is capable of applying for polymers, not for metals. Therefore, it is suggested that the use of plastic mold in MIM process is effective for the manufacturing of fine mold in addition to providing a high performance in filling and easy handling.

#### **3.4 Combination of LIGA and MIM process**

In producing of the higher precision parts, it is necessary to use the molds that are more precise than conventional molds manufactured by cutting and electric discharging. LIGA ((Lithographie,Galvanoformung,Abformung) is the abbreviation of German words for lithography, electroforming and molding, which is one of the micro-processing techniques applied for semiconductor production such as Micro-Electro-Mechanical-System (MEMS) or Micro System Technology (MST). LIGA is mainly consisted of 3 processes; the first is irradiation process to transcript the mask shape to resin, namely resist, the second is developing process to remove unnecessary part of the resist, and to make negative structure of the desired body, the third is molding process by electro-forming metal structure in the resist. The main features of LIGA process are to create the fine profiles with dimensional accuracy in nanometre order, and micro-structures with high aspect ratio. However, the materials applicable to LIGA are limited, and the process is not suitable for mass-production because of its high cost. The manufacturing method of micro sacrificial plastic mold insert metal injection molding (µ-SPiMIM) combined LIGA process, namely LIGA/μ-SPiMIM process has been proposed to solve specific problems involving the miniaturization of MIM. As shown in Fig.20, the μ-SPiMIM method inserted ultra-fine molds which were fabricated

Micro Metal Powder Injection Molding 119

(a) Resist film (b) Sintered part

(c) Micro fluidic chip sintered

*D50*=3μm powder

Arithmetic average surface roughness, *Ra* (μm) 0.17 0.43 1.40

0 100

30μm

Height, *h* (μm) .

Fig. 22. SEM images, profiles and roughness of Ni-forms and sintered parts manufactured

(c) Sintered parts: *D50*=9μm powder

50μm

100μm 100μm

Height, *h* (μm) .

0 100

38μm

100μm

50m

Fig. 21. Resist film and sintered parts manufactured by Resist/μ-SPiMIM process.

(a) Ni-form (b) Sintered parts:

1mm

µm

100μm

µm

(i) Square

(ii) Cylindrical

column

by Resist/μ-SPiMIM process.

Height, *h* (μm) .

0 100

55μm

column

by LIGA process, namely LIGA/µ-SPiMIM process, was proposed (Nishiyabu et al, 2007). Two types of SP-molds with fine structures such as 1) PMMA resist and 2) PMMA mold injected into Ni-electroform which is a typical LIGA process, are named as "One-step transcription method" and "Two-steps transcription method", respectively. In one-step transcription method, PMMA resist is used for SP-mold. Therefore, this is very good for transcription, because original shaped resist is used as SP-mold, and out-gassing is very easy in injection molding. However the cost for producing PMMA resist is extremely high. In two-steps transcription method, Ni-electroform is used only for injection molding of SPmold, it is preferable for mass-production with cost efficient, but the quality reduction of transcription due to the twice injection molding operations is a main issue.

Fig. 20. Flow of two types of transcription methods in LIGA based μ-SPiMIM Processes.

#### **3.5 Manufacturing by resist/µ-SPiMIM process**

To demonstrate the possibility of Resist/µ-SPiMIM process, authors tried to manufacture multi-pillar structures assuming a micro-fluidic device, such as micro-reactor and micromixer as shown in Fig.21 (Nishiyabu et al, 2007). The resist film shown in Fig.21(a) was used as SP-mold. The dimension of a cylinder is 200µm in height and 50µm in diameter. The sintered part with 30-40% shrunk dimension after sintering was obtained as shown in Fig.21(b). It is also shown in Fig.21(c) that micro fluidic chip made of stainless steel 316L can be fabricated with relative ease by Resist/µ-SPiMIM process. Though the aspect ratios of these structures are not so high, it is very difficult to mold such a fine structure with conventional injection molding using metallic mold. Due to the fragility of green compact and fine structures with high aspect ratio, the de-molding is extremely difficult, because the surface area of multi-pillar structures increase exponentially with increasing in number of

by LIGA process, namely LIGA/µ-SPiMIM process, was proposed (Nishiyabu et al, 2007). Two types of SP-molds with fine structures such as 1) PMMA resist and 2) PMMA mold injected into Ni-electroform which is a typical LIGA process, are named as "One-step transcription method" and "Two-steps transcription method", respectively. In one-step transcription method, PMMA resist is used for SP-mold. Therefore, this is very good for transcription, because original shaped resist is used as SP-mold, and out-gassing is very easy in injection molding. However the cost for producing PMMA resist is extremely high. In two-steps transcription method, Ni-electroform is used only for injection molding of SPmold, it is preferable for mass-production with cost efficient, but the quality reduction of

transcription due to the twice injection molding operations is a main issue.

Fig. 20. Flow of two types of transcription methods in LIGA based μ-SPiMIM Processes.

To demonstrate the possibility of Resist/µ-SPiMIM process, authors tried to manufacture multi-pillar structures assuming a micro-fluidic device, such as micro-reactor and micromixer as shown in Fig.21 (Nishiyabu et al, 2007). The resist film shown in Fig.21(a) was used as SP-mold. The dimension of a cylinder is 200µm in height and 50µm in diameter. The sintered part with 30-40% shrunk dimension after sintering was obtained as shown in Fig.21(b). It is also shown in Fig.21(c) that micro fluidic chip made of stainless steel 316L can be fabricated with relative ease by Resist/µ-SPiMIM process. Though the aspect ratios of these structures are not so high, it is very difficult to mold such a fine structure with conventional injection molding using metallic mold. Due to the fragility of green compact and fine structures with high aspect ratio, the de-molding is extremely difficult, because the surface area of multi-pillar structures increase exponentially with increasing in number of

**3.5 Manufacturing by resist/µ-SPiMIM process** 

(c) Micro fluidic chip sintered

Fig. 21. Resist film and sintered parts manufactured by Resist/μ-SPiMIM process.

Fig. 22. SEM images, profiles and roughness of Ni-forms and sintered parts manufactured by Resist/μ-SPiMIM process.

Micro Metal Powder Injection Molding 121

The effects of injection molding condition on defects were investigated. Fig.25 shows the depth of weld line and width of short filling developed in SP-molds fabricated under various injection speeds and mold temperatures, but the material temperature and holding pressure were held constant. It is obvious from these graphs that the both sizes of weld line and short filling decreased as injection speed and mold temperature increased. As the results of the manufacturing of SP-molds under the maximum injection speed and highest mold temperature, the size of the defects was reduced as shown in Fig.26. The size of weld

(a) (b) Fig. 25. Effects of injection speed and mold temperature on molding quality of SP-mold; (a)

0

40

80

Short filling, *bs* (μm).

120

160

Size of weld line 4µm deep, 90µm wide 0.4µm deep, 9µm wide

0 100 200 300

Fig. 26. SEM images and profiles of SP-molds prepared before and after optimization of

*vi*=10mm/s, *TM*=60C *Tr*=195C, *Ph*=30MPa

A

(a) Origin condition (b) Optimized condition

A

A' A'

*vi*=150mm/s, *TM*=90C *Tr*=250C, *Ph*=160MPa

0 100 200 300

0 50 100 150 Injection speed, *Vi* (mm/s)

 TM=30℃ TM=50℃ TM=70℃ TM=90℃ *Tr*=250℃ *Ph*=160MPa

*T <sup>M</sup>* =30℃ *T <sup>M</sup>* =50℃ *T <sup>M</sup>* =70℃ *T <sup>M</sup>* =90℃

Depth of weld line; (b) Width of short filling.

0 50 100 150 Injection speed, *Vi* (mm/s)

 TM=30℃ TM=50℃ TM=70℃ TM=90℃ *Tr* =250℃ *Ph*=160MPa

*T <sup>M</sup>* =30℃ *T <sup>M</sup>* =50℃ *T <sup>M</sup>* =70℃ *T <sup>M</sup>* =90℃

Molding condition

0

4

8

Weld line depth, *dw* (μm).

12

16

SEM image

A-A' Profile

molding condition.

pillars. However, the de-molding is not required in µ-SPiMIM process, therefore this process possesses great advantages in producing the tiny parts with micro-structures. Fig.22 shows SEM images, profiles and roughness of Ni-forms and sintered parts manufactured by Resist/μ-SPiMIM process using varied particle diameter of stainless steel 316 powder. The shape accuracy and surface roughness of sintered parts manufactured using finer powder is considerably improved compared to coarse ones and it is close to Ni-forms. It is also obvious to be able to manufacture metallic parts smaller than SP-mold.

#### **3.6 Defects in molding in LIGA/µ-SPiMIM process**

LIGA/μ-SPiMIM process requires two steps transcription, which is 1) plastic injection molding for SP-mold, and 2) metal powder injection molding for green compact. Thus, it is very important for obtaining accurate green compact to manufacture precisely SP-mold. However, the defects such as the development of weld line and short filling often occurred around pillars of molded parts in plastic injection molding. In order to reduce those defects to the scale of having no influences after sintering, the molding conditions for manufacturing the SP-mold should be optimized and also the effects of metal particle size and processing conditions on the shrinkage, transcription and surface roughness of sintered parts should be investigated. The specimen for SP-mold with numerous micro-pillars is shown in Fig.23. The size of square column is 150μm on a side and 200μm in height. In case of improper molding conditions, defects such as weld line and short filling caused by injection molding of SP-mold as shown in Fig.24(a). As the result, the sintered parts with unwanted shape such as convex line and rounded edge as shown in Fig.24(b) were obtained.

Fig. 23. Geometry of SP-mold with numerous micro-pillars.

Fig. 24. Defects caused by injection molding of SP-mold; (a) SP-mold; (b) Sintered part.

pillars. However, the de-molding is not required in µ-SPiMIM process, therefore this process possesses great advantages in producing the tiny parts with micro-structures. Fig.22 shows SEM images, profiles and roughness of Ni-forms and sintered parts manufactured by Resist/μ-SPiMIM process using varied particle diameter of stainless steel 316 powder. The shape accuracy and surface roughness of sintered parts manufactured using finer powder is considerably improved compared to coarse ones and it is close to Ni-forms. It is also

LIGA/μ-SPiMIM process requires two steps transcription, which is 1) plastic injection molding for SP-mold, and 2) metal powder injection molding for green compact. Thus, it is very important for obtaining accurate green compact to manufacture precisely SP-mold. However, the defects such as the development of weld line and short filling often occurred around pillars of molded parts in plastic injection molding. In order to reduce those defects to the scale of having no influences after sintering, the molding conditions for manufacturing the SP-mold should be optimized and also the effects of metal particle size and processing conditions on the shrinkage, transcription and surface roughness of sintered parts should be investigated. The specimen for SP-mold with numerous micro-pillars is shown in Fig.23. The size of square column is 150μm on a side and 200μm in height. In case of improper molding conditions, defects such as weld line and short filling caused by injection molding of SP-mold as shown in Fig.24(a). As the result, the sintered parts with unwanted shape such as convex line and

(a) (b)

A A'

A-A'

0.5mm

0.15mm

0.2mm

Fig. 24. Defects caused by injection molding of SP-mold; (a) SP-mold; (b) Sintered part.

18mm

1mm

obvious to be able to manufacture metallic parts smaller than SP-mold.

**3.6 Defects in molding in LIGA/µ-SPiMIM process** 

rounded edge as shown in Fig.24(b) were obtained.

Gate

Fig. 23. Geometry of SP-mold with numerous micro-pillars.

A A'

13mm

The effects of injection molding condition on defects were investigated. Fig.25 shows the depth of weld line and width of short filling developed in SP-molds fabricated under various injection speeds and mold temperatures, but the material temperature and holding pressure were held constant. It is obvious from these graphs that the both sizes of weld line and short filling decreased as injection speed and mold temperature increased. As the results of the manufacturing of SP-molds under the maximum injection speed and highest mold temperature, the size of the defects was reduced as shown in Fig.26. The size of weld

Fig. 25. Effects of injection speed and mold temperature on molding quality of SP-mold; (a) Depth of weld line; (b) Width of short filling.


Fig. 26. SEM images and profiles of SP-molds prepared before and after optimization of molding condition.

Micro Metal Powder Injection Molding 123

*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

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

density 0.66g/cm3 2.10 g/cm3 3.50 g/cm3 4.80 g/cm3 4.81 g/cm3

surface 5.1 cm2/g 1.6 cm2/g 0.65 cm2/g 0.101 cm2/g 0.125 cm2/g

(a) (b)

20

30 40

50

Binder content,

Melt viscosity, *η* (Pa・s)

0

0.01 0.1 1 10 100 Paricle size, *d* (μm)

20

40

60

80

100

*Vb* (vol%)

60 70

80

Fig. 29. Properties of MIM feedstock with various particle sizes of metal powder; (a)Tap

method Water atomization method

Wet reduction

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

density and binder content; (b) Melt viscosity.

0.01 0.1 1 10 100 Paricle size, *d* (μm)

Production method

> SEM image

> > Tap

Specific

0

1 2

3

Tap density, *ρTAP* (g/cm3

)

4 5

6

Radio frequency plasma method

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 using fine powder better than coarse one.

Fig. 27. SEM images and profiles of Ni-form, SP-mold and sintered parts manufactured by LIGA/μ-SPiMIM process.
