Powder Injection Molding Process in Industrial Fields

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J. Jpn. Soc. Powder Powder Metallurgy Vol. 65, No. 9©2018 Japan Society of Powder and Powder Metallurgy

Review

Powder Injection Molding Process in Industrial Fields

Joo Won OH, Chang Woo GAL, Daseul SHIN, Jae Man PARK, Woo Seok YANG and Seong Jin PARK*

Department of Mechanical Engineering, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu, Pohang, Gyeongbuk, 37673, Republic of Korea.

Received February 9, 2018; Revised March 13, 2018; Accepted April 26, 2018

ABSTRACTPowder injection molding is a combined technique of traditional powder metallurgy and plastic injection

molding. Since injection molding step in the process enables to produce complex shapes, powder injection molding technique has high design flexibility and provides a chance to overcome limitations in the current manufacturing process. In this paper, the optimization of powder injection molding process was discussed. The process was roughly divided into 5 steps from solids loading determination to sintering, and the optimization approaches for each step was detailed. Thereafter, the industrial application of powder injection molding process was described with hydrophobic surgical forceps, soft magnet components, PZT array for ultrasound probe and PDP glass components. The process optimization was conducted for each product, and sound products were obtained with the process.

KEY WORDSapplication, powder injection molding, process optimization

1 IntroductionPowder injection molding (PIM) uses the shaping advantage

of injection molding but is applicable to metals and ceramics1). Unlike traditional powder metallurgy, the shape forming of the samples in PIM process is conducted via the injection molding technique. Therefore, 3-dimensional complex components can be easily produced by the process2). PIM process is also a near-net shaping technique with tight tolerance3). Such characteristics of PIM provide high material flexibility to the process because some critical post-processing, such as machining, can be omitted. For these reasons, PIM process is one of the most appropriate manufacturing processes for pats with micro-sized features4).

In order to develop a proper manufacturing process for a product, three factors, material, design and process, should always be considered to satisfy the market needs. Only when all the factors were well coordinated, proper products can be produced. Generally, the design, which implements the function of products, is given via precedent study. Thus, in many cases, material and process are taken into account for the process development. A high flexibility of material selection is advantageous to fabricate a product with the best performance because the material determines mechanical and physical properties. However, sometimes it is limited by the process. For example, machining is hardly applied to brittle or refractory materials. For this reason, PIM process, which has high

material flexibility, is one of desirable manufacturing processes to improve the product quality.

In this paper, a few examples of PIM process application in the industrial fields were provided. To develop the process, each step was optimized with experimental and simulation approaches. Thereafter, surgical forceps, magnetic components, PZT array for ultrasound probe and glass components were fabricated with the optimized condition.

2 Development of powder injection molding processPIM process is generally divided into 4 stages. However, far

more steps should be considered for the fabrication of sound products. Since most defects cannot be corrected in subsequent processing steps, optimization of each stage is essential. Fig. 1 is a systematic flow chart for developing the optimized PIM process.

Solids loading is a volumetric percentage of powder in a feedstock. High solids loading is desirable because low solids loading results in a high shrinkage of the samples5). It increases the difficulty of dimension control and may cause distortion during sintering. High solids loading also provides better properties with a higher density6). However, too high solids loading rather worsens the quality of samples1). A lack of binders will generate voids in feedstock and exposes the particles surface. As a result, a feedstock with too high solids loading shows high viscosity and low homogeneity. Therefore, the optimal solids loading should be determined before feedstock formulation. In the present works,

* Corresponding author, E-mail: [email protected]** The content of this article had been presented at JSPMIC2017.

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Joo Won OH, Chang Woo GAL, Daseul SHIN, Jae Man PARK, Woo Seok YANG and Seong Jin PARK

the critical solids loading was measured by a torque rheometer (Thermo scientific, HAAKE PolyLab QC Lab Mixer) to determine the optimal value. The critical solids loading is defined as the condition where the particles are packed as tightly as possible without external pressure and all space between the particles is filled with binder1). When the solids loading exceeds the critical value, the mixing torque rapidly increases due to a lack of binder. Therefore, the critical solids loading of a powder can be measured by observing a mixing torque change. Fig. 2 displays average mixing torques of 316L stainless steel with various solids loading. It was obvious that the slope of the graph suddenly changed at a certain point, and the result indicated the critical solids loading of the powder is 61 vol.%. This method was verified with density measurement7). The optimal solids loading is often determined as slightly lower than the critical value for process flexibility. Thus, 2% lower values were used as the optimal solids loading in the present works.

Feedstock homogeneity is an important factor in PIM process, since uneven density distribution in a green body can induce anisotropic shrinkage and large pores in the products9). Therefore, in many cases, the mixing procedure is repeated for several times to improve feedstock homogeneity. However, long mixing time reduces the process productivity and may cause feedstock

contamination1). Hence, a feedstock should be fabricated with the minimum number of mixing that achieves the maximum homogeneity. The feedstocks in this works were prepared via a twin-screw extruder at 160°C. The optimal amount of the powders were mixed with a wax-based binder system, which consisted of paraffin wax (PW), polypropylene (PP), polyethylene (PE) and Stearic acid (SA). The most desirable mixing time was determined from feedstock homogeneity, and it was evaluated by viscosity fluctuation with a capillary rheometer (Malvern, Capillary Rheometry Rosand RH7) in a constant shear rate. Fig. 3 shows the change of Si3N4 feedstock homogeneity with different mixing time. More homogeneous feedstock tends to have less viscosity fluctuation. The figure demonstrated that the viscosity and its fluctuation were decreased as mixing time increased. It implied exposed powder surface decreased and the powder distribution in the feedstock became more homogeneous. However, both of the values did not show significant improvement after 3 times mixing because it reached the maximum homogeneity with 3 times mixing. The optimal mixing time is varied depending on binder compositions and particle characteristic. For example, smaller particles require longer mixing time with strong agglomerations8). Thus, the mixing process should be optimized for each powder.

A failure to mold filling could induce defects, such as weld lines, air trap with jetting, flashing and separation, during the injection molding stage. Most defects formed during the stage become enlarged via subsequent debinding and sintering process9). Especially, multiple-cavity molds are wildly adopted in the industrial fields to obtain the high productivity, and larger cavity volume generally increases the defects possibility. Therefore, the understanding of mold filling behavior and optimization of process condition is critical to the success of PIM process. In the present study, a commercial injection molding software, Moldflow, was used to optimize the injection molding condition. To obtain the rheological properties of the feedstocks, the feedstock viscosity was measured by the capillary rheometer at various temperatures and shear rates. Material parameters for simulation were calculated with governing equations, such as Hele-Shaw, Cross-WLF and two-domain Tait PVT models, and the parameters were verified with

Fig. 1 Flowchart for developing powder injection molding process.

Fig. 2 Torque rheometer results of 316L stainless steel feedstock8).

Fig. 3 Mixing time dependency of feedstock homogeneity.

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a spiral mold test before conducting the simulation10). Thereafter, the optimization of the injection condition was performed with a sensitivity analysis. Once the condition was determined, the parts were injected by an injection molding machine (Sodick, TR30EH). With this procedure, defect-free green bodies were fabricated.

The samples in the debinding stage are the most vulnerable to defect formation because the porosity of the samples increases. Therefore, the debinding process goes with a slow heating rate, and as a result, it is often the most time-consuming process. However, a long processing time reduces the productivity and may cause powder oxidization11). The two-stage debinding process, which consisted of solvent and thermal debinding, can be a solution to overcome the problems because micro channels, generated by a dissolved filler binder, act as vapor paths for residual binders. In this works, 50°C n-hexane was used as the solvent. The samples were immersed in the solvent, and PW and SA were extracted from them. Fig. 4 presents an example of solvent debinding with Fe feedstocks. Most of PW and SA dissolved within 2 h, and the rates decreased as the time passed due to a diffusional time of solvent into the samples. The process time is highly affected by solvent, binder composition and particles. For example, Fig. 4 indicated a nano powder feedstock required more time to plateau than a micro powder feedstock. It also had the larger amount of remaining binders due to strong capillary force. The result implied a relatively slow heating rate should be applied to nano powder feedstock from low-temperature ranges even after long solvent debinding process. Heating cycle for thermal debinding was optimized based on the binder decomposition behavior, observed by a TGA (METTLER TOLEDO, TGA/DSC 1). Fig. 5 illustrates the decomposition behavior of 316L stainless steel feedstocks with the heating rate of 2°C/min in a hydrogen atmosphere. The binder system is decomposed a one or two groups depending on the molecular weight12). The graph also consisted of two sigmoidal graphs. The first sigmoid was decomposition behavior of PW and SA, and the second sigmoid was decomposition behavior of PP and PE. Since rapid decomposition of binders can result in a high-pressure gradient in samples, an isothermal step is recommended

below the temperature at which the decomposition rate shows the highest value. The 2 h isothermal steps were set at 50°C lower temperatures for PP and PE in this works. Although the effect of binder compositions is dominant in decomposition behavior of a binder system, it is sometimes influenced by particles like Fig. 6. In the graph, the decomposition behavior of feedstocks was varied by addition of nano powder. Thus, even if the binder system is the same, binder decomposition behavior should be analyzed for each feedstock.

Sintering is one of the key stages in PIM process as well as other powder metallurgy processes. During sintering, the densification of powder compacts is carried out by the thermally activated diffusion process. Therefore, it has high influences on the density and properties of the products14). In many cases, higher density is desirable because a sample with a high sintered density provides better properties, and it is normally obtained at a higher sintering temperature. However, too high temperature degrades some properties with large grains or loss of compositions, such as lead in PZT15). For this reason, optimization of sintering cycle needs to be conducted. Sintering behaviors of the samples were investigated with a dilatometer (NETZSCH, DIL 402 C). Since it can measure dimensional changes of a sample depending on the temperature, the density of the sample can be calculated with respect to the temperature with an assumption of isotropic shrinkage. The result also proves the necessity of isothermal holding. Fig. 6 depicts the temperature dependency of shrinkage rate for 316L stainless steel samples with different manufacturing processes. Fig. 6 (a) and (b) were shrinkage behaviors of samples prepared via PIM and die-compaction processes, respectively. Even though it was the same powder, sintering behavior was not consistent due to different sample preparation method. The shrinkage rates of the die-compacted samples at 1350°C were retained in the relatively high rates. It implied an isothermal holding would have a beneficial effect on the density increase. Whereas, the values of die-compacted samples were almost zero, and thus, isothermal holding may reduce the mechanical properties of the products by increasing the grain size without raising the density. Hence, in order to fabricate products with the best performance, sintering stage should

Fig. 4 Weight ratio of dissolved paraffin wax and stearic acid during solvent debinding13).

Fig. 5 Temperature dependency of remaining binder wt.% with the heating rate of 2°C/min7).

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be conducted with the optimal thermal cycle.

3 Application of powder injection molding process3.1 Hydrophobic surgical forceps

Micro pattern on the surface of surgical forceps increases the contact area with organs and provides a large friction force to prevent it from slipping. The patterns are traditionally fabricated by pattern rolling or cutting processes. Although the pattern rolling process provides high repeatability with tight tolerances, it has a limitation for producing small patterns with a high aspect ratio16). The cutting process also takes long manufacturing times and has a low productivity. In order to improve productivity and surface performance, PIM technology can be regarded as an alternative manufacturing technology with the advantages of mass production,

near net shape and shape complexity1,17). Laparoscopic surgery is a modern surgical technique that has several advantages, such as pain relief and reduction in recovery time, due to the small incision. Since this surgery is delicate from the perspective of a surgeon, it is important to secure a clear view of the surgical site. However, blood on the pattern of the forceps can interfere with the surgeon’s vision during surgery. In this regard, hydrophobic surface was also developed to prevent blood from being buried.

8 μm 17-4PH stainless steel powder (Atmix, Japan) was used in the present work. By considering the critical solids loading, the feedstock was formulated with 59 vol.% powder, and the mixing was repeated for 3 times to obtain the homogeneous feedstock. Thereafter, the mold filling simulation was conducted with Moldflow to optimize the injection molding condition and avoid defects. For the simulation, the viscosity of the feedstock was measured by the capillary rheometer. From the data, simulation parameters were calculated and applied for mold filling simulation. The result showed that the weld line appeared near the hold, at which the force applied, before the optimization as shown in Fig. 7. However, the amount of weld line was reduced by changing the conditions, such as mold and injection temperatures. After injection molding, the green bodies were firstly debound at 60°C in n-hexane for the optimum immersion time of 2 hours to remove PW and SA. Secondly, the thermal debinding was performed to decompose the remaining binders including PP and PE. The in-situ shrinkage was measured by the dilatometry experiment during sintering. The final shrinkage was about 16%. Sintering cycle was determined based on the result of the dilatometry data. A relatively low heating rate of 5°C/min was applied to the main densification range to minimize distortion19). The sintering was conducted in an H2 atmosphere at 1350°C for 1 h. Finally, the components of the surgical forceps were fabricated as shown in Fig. 8. In order to

Fig. 7 Weld line analysis: (a) before optimizing the process conditions (b) after optimizing the process conditions18).

Fig. 6 Sintering behavior of micro samples prepared by: (a) powder injection molded7) and (b) die-compacted14).

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develop a hydrophobic surface on the micro pattern of the surgical forceps, the surface needs to have a rough microstructure and low surface energy. Therefore, chemical etching was conducted for 6 h at room temperature to remove the passivation layer and form a rough surface. Thereafter, dried specimens were immersed in heptadecafluoro tetrahydrodecyl trichlorosilane solution (HDFS, Gelest, Inc.) with n-hexane for 3 h. Finally, the coated surface was rinsed with n-hexane and dried at room temperature. Fig. 9 shows the surface before and after surface treatment. It indicated the hydrophobic surface on the surgical forceps was successfully developed by surface treatment.3.2 Soft magnet for DC and low-frequency AC application

Soft magnetic materials are widely used in the motor, actuator, and sensor to improve the efficiency and controllability20,21). With the development of electromagnetic drives, the demand for complex shaped soft magnetic components increases20). In this regard, PIM is one of the most attractive techniques to fabricate soft magnets because several brittle materials, such as Fe-6.5Si alloy, can be easily formed into the complex-shapes. However, it is difficult to obtain near-full density via PIM due to relatively high binder contents in the feedstock. Since the relative density is a key characteristic that affects the saturation induction, near-full density is desirable for higher magnetic properties22). This problem can be overcome by adding Fe-17 at.% P powder, which has the lowest melting point, 1048°C, in Fe-P phase diagram. Formation of Fe-17 at.% P liquid phase activates the sintering process and enables to obtain near-full density at relatively low temperatures. Phosphorus is also reported to decrease coercivity and increase permeability23). For alternating current (AC) applications, reducing the eddy current loss is crucial since it increases proportionally to the square of the frequency of AC. For this reason, the eddy current loss should be minimized, and it can be reduced by lowering the electrical resistivity24,25).

As for the DC applications, 4 µm carbonyl iron (Fe) powder (BASF, Germany) and 5 µm Fe-17 at.% P (Fe-P) powder were used to fabricate sintered soft magnetic alloys. Fe powder was mixed with 8 wt.% Fe-P powder, and as a result, 2 feedstocks were prepared. The feedstocks were formulated with 57 vol.%

solids loading via 3 times mixing process. The feedstocks were injected into cylindrical samples to measure the magnetic properties. The injected samples were subjected to solvent and thermal debinding. The solvent debinding was performed in 50°C n-hexane for 12 h, and subsequently, the thermal debinding was carried out up to 800°C in a hydrogen atmosphere. Thereafter, the samples were sintered at the temperature range from 1300 to 1450°C. The Sintered density of the samples is listed in Table 1. The density significantly increased by Fe-P addition. However, collapse was also observed at 1450°C with the Fe-P added samples. Fig. 10 shows the effects of the sintering temperatures and Fe-P additions on the saturation values. As the temperature increased, the saturation polarization of the alloy increased. The saturation values also increased by adding Fe-P powder regardless of the temperature. As a result, the highest polarization value, 28299 Gauss, was obtained with Fe-P addition at 1400°C. Since the density was one of the key parameters for the magnetic saturation, a sample with higher density tends to provide a higher polarization value. Furthermore, the coercivity and remanence also decreased by Fe-P addition as shown in Fig. 11. From the result, it could be concluded that the magnet properties in DC applications increased by adding Fe-P powder.

Fig. 8 Components manufactured in the PIM process.

Fig. 9 Surface on the surgical forceps (a) before surface treatment (b) after surface treatment.

Table 1 Relative sintered of the samples with different sintering temperatures23).

Sintering temperature (°C) 1350 1400 1450

Without Fe-17 at.% P 98.5% 98.6% 99.3%With Fe-17 at.% P addition 99.1% 99.8% —

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For AC applications, 10 μm Fe-3.5 wt.% Si (Fe-Si) powder (Atmix, Japan) was used for low-frequency range with the additions of Fe-P powder. Fe-P powder was added to Fe-Si alloy to reduce the eddy current loss as the ratios from 0 to 8 wt.% with the 4% increments. The feedstocks were formulated with the powders of 58 vol.%, and the mixing process was repeated for 5 times. The toroidal-shaped samples were prepared to measure the core loss and saturation induction. The solvent debinding was performed under the same conditions as the samples for DC applications. Whearas, the thermal debinding was performed up to 950°C due to relatively large particle size. Thereafter, the samples were sintered at 1300°C. The result indicated that the liquid phase of Fe-P improved the density and saturation induction. The relative density of 99% and magnetic induction (B5000) of 1.78 T were obtained with 8 wt.% additions of Fe-P powder. Whereas, pure Fe-Si alloy had the relative density of 98% and magnetic induction (B5000) of 1.67 T only. Electrical resistivity also increased from 59 to 72 μΩ·cm. As a result, the eddy current loss reduced by 23% with the increase of the electrical resistivity as summarized in Table 2.3.3 PZT array for ultrasound probe

Piezoelectric ceramics has been attracting attention with its unique property, which is the energy transduction between mechanical strain and electrical charge. Utilizing these properties, this material can be applied to various applications, such as sensors, actuators, and transducers26-28). When it combines with a passive polymer and forms a 1-3 composite, it provides low acoustic impedance and

high electromechanical property29,30). In the present work, 1-3 type piezoelectric structures with high-aspect-ratio were successfully fabricated by the optimized PIM process with the sacrificial mold insert.

The feedstock with 45 vol.% powder was prepared by mixing Pb(Mg, Nb)O3-Pb(Zr, Ti)O3 [PMN-PZT] powder (Hayashi-chemic Co., Ltd., Japan) and the wax-based binder system. The sacrificial mold inserts with the pattern range from 25 to 150 μm were prepared via LIGA process at Pohang accelerator laboratory (PAL). Firstly, using the designed photomask (Fig. 12 (a)), X-ray gold mask, used to block a synchrotron X-ray, was developed with conventional UV-lithography technique as shown in Fig. 12 (b). Thereafter, the base PMMA was exposed to X-ray. As a result, micro-sized sacrificial mold insert was fabricated without any defects as displayed in Fig. 12 (c). Fig. 13 shows the entire image of the insert. In order to prevent the sacrificial insert from incomplete filling during injection molding, the rheological properties of the feedstock were evaluated with the Cross-Arrhenius power-law model. The feedstock showed a relatively high flow activation energy. Thus, the mold and injection temperatures were set to 65, 160°C to achieve complete filling. After molding, two kinds of demolding process were conducted. Firstly, a thermal demolding

Fig. 10 Effects of sintering temperature and Fe-17 at.% P addition on the magnetic polarization of sintered alloys: (a) without Fe-17at.% P, (b) with Fe-17 at.% P23).

Fig. 11 Magnetic properties of sintered alloys: (a) coercivity and (b) remanence23).

Table 2 Eddy current loss of the sintered alloy at 1300°C.

Sample Without Fe-17 at.% P

4 wt.% addition

8 wt.% addition

Eddy current losses (W/kg) 56.6 45.6 44.6

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technique was attempted to demold the sacrificial mold insert. The result showed each pillar was agglomerated because the binders also were simultaneously decomposed during demolding, as shown in Fig. 14 (a). To overcome the problem, solvent demolding method was conducted in 35°C acetone for 4 h to dissolve the only PMMA, leading to defect-free demolded parts as shown Fig. 14 (b). Thereafter, two-step debinding process was used to remove all the binders. As the first step, the solvent debinding was conducted to extract PW and SA in 45°C n-hexane for 12 h. The remaining binders were thermally decomposed with the slow heating rate of 0.5°C/min in a systematic thermal cycle determined by TGA experiment. During sintering, Pb evaporation was observed from 1350°C. Since it may lower the piezoelectric properties, the sintering temperature was set at 1300°C. Via the optimized PIM process, the fine-scaled piezoelectric microstructure was successfully fabricated as shown in Fig. 15. The final samples presented the same or better piezoelectric effect than a commercialized composite.

3.4 Glass componentsGlass is used in various regions, such as medical, chemistry, electrics

and so on. However, it is restricted to fabricate glass components having complicated shapes due to its brittleness. PIM can be one of the ideal alternatives for glass products because it produces complex shapes with economical costs compared to the conventional process1). In the present work, PIM process for glass materials was developed and optimized.

The plasma display panel (PDP) glass powder was used for the development of glass PIM process. The Powder was provided from Deajoo Electronic Materials Co., Ltd. in Korea. The main composition of the powder was SiO2-B2O3-Al2O3, and the melting point was 919°C. Since the powder was prepared via the milling process, it had an irregular shape with the mean particle size of 2 μm. Differential thermal analysis (DTA) was conducted to obtain the glass transition temperature (Tg) and crystallization point. The result indicated Tg and crystallization temperature of the powder were 614 and 707°C, respectively32). The homogeneous feedstock was prepared by mixing 56 vol.% powder and the wax-based binder system and injection molded under 160°C. In order to remove all the binders, both solvent and thermal debinding processes were adopted in the present work. Firstly, solvent debinding was carried out in 60°C n-hexane bath, and thermal debinding was performed up to 450°C. After debinding, the weight ratio of the binders was calculated, and it was verified that all the binders were removed. In order to optimize the sintering condition, sintering experiments were conducted with various temperatures and atmospheres. Firstly, Sintering was conducted at the temperatures from 625 to 800°C with 2 h holding in the air. Fig. 16 shows the temperature dependency of the sample density. The density increased as the sintering temperature increased up to 700°C. However, after the point, the value decreased due to the crystallization of the samples. As a result, the highest density, 92.4%, was obtained at 700°C. Thereafter, the effect of sintering atmosphere on the transparency was also investigated. The samples were sintered in air, argon and oxygen atmosphere, and it was found that the oxygen atmosphere provided the most transparent samples. Hence, the optimal sintering condition was determined as 700°C in an oxygen atmosphere. However, the relative density was not high enough for

Fig. 12 Procedure of the LIGA Process: (a) Photomask, (b) X-ray mask and (c) Sacrificial mold insert31).

Fig. 13 PMMA sacrificial mold insert31).

Fig. 14 Effect of demolding technique on micro array: (a) thermal demolding and (b) solvent demolding.

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glass component because high porosity decreased the transparency. For this reason, hot isostatic pressure (HIP) process was carried out. The components were thermally treated under 80 MPa and 700°C for 3 h, and the relative density rose to 99% with the process as Fig. 17 exhibits. To verify the soundness of the samples, thermal expansion analysis was conducted by the dilatometer since it was an important property for glasses33). The thermal expansion coefficient was 7.82 × 10−6°C−1, which was similar to the value of commercial PDP glass products. With the present work, glass PIM has shown the possibility of a promising alternative to current methods making glass products.

4 ConclusionPIM is promising manufacturing process for metals and ceramics.

Since process provides high design and material flexibility, PIM technique can be a solution to overcome some obstacles in conventional manufacturing processes. In this study, a development of PIM process was discussed. Many defects in PIM process tends to be enlarged as the process progressed, resulting in the low-quality products and requiring enormous post-processing. Hence, for PIM process development, the optimization of each step should be conducted with experimental and simulation approaches.● In mixing step, feedstock is prepared to offer flowability for

powder. The main factors of feedstock formulation are solids loading and feedstock homogeneity. High solids loading above the critical value results in too high feedstock viscosity and increases difficulties of mold filling. Whereas, the samples with a low solids loading may collapse during debinding and provides poor properties due to the low density. Thus, slightly lower value than the critical solids loading is a desirable solids loading condition. Uneven density distribution in green parts with inhomogeneous feedstock also causes shrinkage-related problems. Therefore, the feedstock homogeneity should

Fig. 15 Fabricated micro piezoelectric structure: (a) Pattern size: 40 μm, aspect ratio: 10 and (b) Pattern size: 20 μm, aspect ratio: 531).

Fig. 16 Effect of the sintering temperature on the sintered density of glass components32).

Fig. 17 Microstructures of glass samples: (a) Before HIP and (b) After HIP34).

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always be guaranteed.● Injection molding step determines the quality of green parts.

To produce high-quality products, sound green parts are essential, and green parts should be fabricated with proper injection condition. The large injection volume provides a high productivity but increases process challenges. Therefore, the process optimization should be conducted for mass production. However, the defects during the process are hardly identified before sintering. For this reason, the optimization of injection condition with simulation approach is desirable.

● The binders in green parts are removed in debinding step. Since residual binders can be cause of contamination during sintering, all the binder must be eliminated. Debinding is also one of the most time-consuming step in PIM process. Thus, debinding condition is directly influence the product quality and production rate, and it is important to find the optimal condition for minimal time without residual binders with optimization.

● Structural integrity for PIMed products results from sintering. Since sintering step highly affects mechanical and physical properties of the product, sintering is one of the most critical steps in PIM process. Densification behavior of each material is different, and even the same material also displays various sintering behavior depending on the condition, such as particle size, process types and atmosphere. Hence, sintering condition needs to be optimized by analyzing the densification behavior.

The paper also provided some examples of optimized PIM process for industrial application. Hydrophobic surgical forceps, soft magnet components, PZT array for ultrasound probe and PDP glass components were successfully fabricated by PIM process.

AcknowledgementThis work was supported by the National Research Foundation

of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2011-0030075).

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