Critical Solid Loading Research Articles

Development of High-Aspect-Ratio Soft Magnetic Microarrays for Magneto-Mechanical Actuation via Field-Induced Injection Molding.

Magnetorheological elastomers (MREs) are in demand in the field of high-tech microindustries and nanoindustries such as biomedical applications and soft robotics due to their exquisite magneto-sensitive response. Among various MRE applications, programmable actuators are emerging as promising soft robots because of their combined advantages of excellent flexibility and precise controllability in a magnetic system. Here, we present the development of magnetically programmable soft magnetic microarray actuators through field-induced injection molding using MREs, which consist of styrene-ethylene/butylene styrene (SEBS) elastomer and carbonyl iron powder (CIP). The ratio of the CIP/SEBS matrix was designed to maximize the CIP fraction based on a critical solids loading. Further, as part of the design of the magnetization distribution in micropillar arrays, the magnetorheological response of the molten composites was analyzed using the static and dynamic viscosity results for both the on and off magnetic states, which reflected the particle dipole interaction and subsequent particle alignment during the field-induced injection molding process. To develop a high-aspect-ratio soft magnetic microarray, X-ray lithography was applied to prepare the sacrificial molds with a height-to-width ratio of 10. The alignment of the CIP was designed to achieve a parallel magnetic direction along the micropillar columns, and consequently, the micropillar arrays successfully achieved the uniform and large bending actuation of up to approximately 81° with an applied magnetic field. This study suggests that the injection molding process offers a promising manufacturing approach to build a programmable soft magnetic microarray actuator.

Quantification of shape parameters of Co-Cr-Mo-Si multiphase compounds and their role in the processing of metal injection molding feedstocks

Particle shape is one of the most important powder attributes affecting the properties of multiphase compounds intended for metal injection molding. This work reports the processability of gas (GA) and water (WA) atomized Co-Cr-Mo-Si alloy related to quantitative analyses of particle shape using a newly developed algorithm based on the Euclidean distance mapping, which results are compared with those of commercially available dynamic image analysis. Both methods reveal similar values for the parameters that quantify the shape of the powders, such as circularity, aspect ratio, and symmetry. The alloys were mixed with wax/high-density polyethylene (50/50 wt%) binder using a double sigma mixer to produce homogeneous feedstocks. The critical solid loading and flow performance corresponding to the differences in powder shapes were derived from the torque and capillary rheometers, respectively, to obtain the optimum molding parameters. A stable and repeatable viscosity was observed for the GA feedstock, whereas the compound containing WA was prone to phase separation during flow. The defect-free sintered specimens were examined by metallographic analysis; the GA powder compound could be sintered to a maximum of 99.2 % of the theoretical density, whereas the irregular WA powder reached a maximum of 98.8 % of the theoretical density, and the carbon residue was found to be higher for the GA feedstock. The GA samples had a significantly higher ultimate tensile strength owing to their larger symmetry, which resulted in a lower porosity and stress concentration.

Design of Sustainable Aluminium-Based Feedstocks for Composite Extrusion Modelling (CEM).

Additive manufacturing (AM) has become one of the most promising manufacturing techniques in recent years due to the geometric design freedom that this technology offers. The main objective of this study is to explore Composite Extrusion Modelling (CEM) with aluminium as an alternative processing route for aluminium alloys. This process allows for working with pellets that are deposited directly, layer by layer. The aim of the technique is to obtain aluminium alloy samples for industrial applications with high precision, without defects, and which are processed in an environmentally friendly manner. For this purpose, an initial and preliminary study using powder injection moulding (PIM), necessary for the production of samples, has been carried out. The first challenge was the design of a sustainable aluminium-based feedstock. The powder injection moulding technique was used as a first approach to optimise the properties of the feedstock through a combination of water-soluble polymer, polyethyleneglycol (PEG), and cellulose acetate butyrate (CAB) wich produces low CO2 emissions. To do this, a microstructural characterisation was carried out and the critical solid loading and rheological properties of the feedstocks were studied. Furthermore, the debinding conditions and sintering parameters were adjusted in order to obtain samples with the required density for the following processes and with high geometrical accuracy. In the same way, the printing parameters were optimised for proper material deposition.

Development of stimuli-responsive flexible micropillar composites via magneto-induced injection molding and characterization of magnetic particle alignment

Magnetic soft composites have emerged as a promising soft actuator with a high degree of precision in the magnetic field stimuli system. But, their mass manufacturing strategies have yet to be developed, and most studies focus on small-scale near-net shaping production. Here, we first apply injection molding (which only takes a few seconds to shape) to fabricate the stimuli-responsive flexible micropillar composites with magnetic particle alignment design using an external magnetic field. The arrays exhibit magnetically anisotropic particle alignment up to 82.57 % along the longitudinal direction, showing a magnetic bending actuation response. We achieve a structural novelty of the magnetic micropillar with a high aspect ratio of up to 10 and pattern sizes of 50 μm via sacrificial LIGA insert mold and magneto-induced injection molding. For advanced mass production, the permanent metal mold is also applied to develop the micropillar composites based on the mechanical demolding approach; successful manufacturing is achieved by fabricating defect-free micropillar with 200 μm cylindrical pattern size and aspect ratio of 6. Further, the effect of powder volume fraction on the magneto-rheological behavior and corresponding magnetic performance is characterized in the injection molding process. The magnetic particle alignment trend is confirmed by the torque balance and the criteria of critical solids loading. Finally, we establish an injection molding process for magnetic soft composites, and verify the optimal powder fraction for the particle alignment.

Effect of powders on aluminum nitride components fabricated by PIM

Powder selection is essential to the success of powder injection molding (PIM). In this study, aluminum nitride (AlN) powder synthesized through carbon-thermal (C-R) reduction and direct nitridation (DN) were used as the raw material. These two kinds of powder were mixed with wax-based binder and the rheological properties of AlN feedstocks were examined. Then the green parts were fabricated by powder injection molding and sintered after solvent and thermal debinding. Finally, the appearance, microstructure and thermal conductivity of sintered AlN were inspected. It was found that the C-R powder has higher critical solid loading (60 vol%) than DN powder (58 vol%). Feedstocks viscosity decreased when the shear rate increased, indicating the presence of non-Newtonian pseudoplastic flow. The C-R powder also has better rheology properties with lower viscosity, higher melt flow rate, higher shear thinning, and higher activation energy. As for the performance after short-time sintering, the thermal conductivity of C-R AlN is 183 W·m−1·k−1 while the value of DN AlN is 130 W·m−1·k−1, and the bending strength of C-R AlN is 363 MPa while that of DN AlN is only 320 MPa.

Effect of interaction from the reaction of carboxyl/epoxy hyperbranched polyesters on properties of feedstocks for metal injection molding

The purpose of this study is to improve the properties of the feedstocks and shape retention of debinded parts by the reaction between 17-4PH stainless steel powders. Carboxyl-terminated hyperbranched polyester (CTHP) and epoxy-terminated hyperbranched polyester (ETHP) were used to treat the powders, and termed as CTHP-m and ETHP-m with carboxyl and epoxy group, respectively. Comparing with pristine, CTHP-m and ETHP-m, feedstock prepared from equal amount of CTHP-m and ETHP-m (CTHP-m/ETHP-m) possessed more excellent properties. The experimental results showed that the critical solids loading, flexural modulus, density and melt flow index of CTHP-m/ETHP-m feedstock were 63.8 vol.%, 2800 Mpa, 5.06 g cm−3 and 62 g/10 min, respectively, which were obviously higher than that of others. Also, the shape retention of CTHP-m/ETHP-m debinded parts was the best of all the samples. The improved properties of CTHP-m/ETHP-m feedstock were attributed to the powder interaction between CTHP-m and ETHP-m formed by the chemical reaction between epoxy and carboxyl group.

Rheological investigation and injection optimization of Fe–2Ni–2Cu feedstock for metal injection molding process

In this study, rheological behavior and optimized injection parameters for Fe–2Ni–2Cu feedstock in the metal injection molding process were investigated using the capillary and Taguchi methods. For this purpose, the critical solids loading obtained for the feedstock by analyzing various solids loading (56–64 vol%). In the next step, fundamental rheological characteristics of a homogenized feedstock, including flow behavior index (n), flow activation energy (E), and general moldability index (αstv), were studied. Optimization of the most important injection parameters, including injection speed, injection temperature, mold temperature, and holding pressure, was conducted applying the Taguchi method. According to the results of experiments, injection speed plays a significant role in increasing the density and flexural strength. Also, there are optimum points for injection temperature and mold temperature, while holding pressure does not significantly influence the density and flexural strength. Satisfying results have been shown in green density and flexural strength after the verification test. Rheological behavior analysis showed 60 vol% as critical solids loading, while the proper solids loading considered to be 58 vol% in order to make an easier flow during the injection for the feedstock. The flow behavior index of developed feedstock indicates the shear rate sensitivity was desirable since it shows less than one value. Also, flow activation energy, which shows the feedstock sensitivity to the temperature changes, came out to be 78.138 kJ/mol. Finally, The general moldability index was calculated to be 3.1302E-06 Pa−1 K−1 by using these parameters.

Injection moulding of porous MAX phase Ti3SiC2 without using space-holder

Ti3SiC2 is one of the most studied MAX phase due to its unique combination of metallic and ceramic like properties. In this work, Powder Injection Moulding (PIM) is proposed as a new processing route to manufacture porous MAX phase Ti3SiC2, without the need of a space holder. The main goal of this work is to broaden MAX phase application fields through powder injection moulding technology. In depth characterization of all process stages has been done to control porosity in the final parts. Self-synthesized powders were mixed with a multicomponent binder for the production of sustainable feedstocks. The binder selection was carried out considering the design of a process with the reduction of the carbon footprint. In this sense, the binder consists of a combination of water soluble and low CO2 emission polymers. Feedstocks produced exhibit good properties for injection moulding. Critical solid loading (52 vol%) and a set of debinding debinding conditions has been determined for a two-step process without producing any defects in the samples. In addition, sintering parameters have been adjusted to successfully achieve porous MAX phase samples.

Effect of 17-4PH stainless steel powders interaction on feedstocks

The objective of this work is to investigate the effects of 17-4PH stainless steel powders interaction on properties of feedstocks. γ-aminopropyltriethoxy silane (KH550), γ-(2,3-epoxypropoxy)propyltrimethoxy silane (KH560), and propyltrimethoxy silane (N313) were used to modify 17-4PH stainless steel powders, and described as KH550-m, KH560-m and N313-m with amino, epoxy and propyl group, respectively. The properties of feedstocks prepared from the pristine, KH550-m, KH560-m and N313-m were compared with that of equal amount of KH550-m and KH560-m (KH550-m/KH560-m). The experimental results revealed that the critical solids loading, density and melt flow rate of KH550-m/KH560-m feedstocks were 62.8 vol%, 5.24 g/cm3 and 51.42 g/10 min, respectively, which were higher than that of others. Besides, the debinded parts shape retention of KH550-m/KH560-m was also better than others. The good properties of KH550-m/KH560-m feedstocks originate from the strong interaction between KH550-m and KH560-m, which comes from the reaction and hydrogen bond between amino group and epoxy group.

Variation in particle size fraction to optimize metal injection molding of water atomized 17–4PH stainless steel feedstocks

This work reports on an essential effect of rather slight size differences in water atomized 17–4PH stainless steel powder on resulting microstructure and dimensional tolerances of metal injection molding (MIM) parts. The powders of round to irregular shapes prepared in three different powder volume fractions were admixed into the paraffin wax/HDPE (50/50) binder in a double sigma mixer. After determination of critical solid loading, the rheological, molding, and sintering performance of 66 vol% solid loading feedstocks was tested on a complex-shaped MIM component. The slight differences in size fractions are reflected also in the powder shape, which was quantified through sphericity factor and aspect ratio by dynamic image analysis. The results indicate an increase in viscosity, flow instability and injection pressure, lower sintered density with enhanced dimensional deformations for feedstocks having higher amount of coarser (10–20) μm fraction accompanied with higher shape irregularity determined from sphericity factor and aspect ratio.

The effect of debinding and sintering parameters on the mechanical and microstructural properties of Fe-2Ni metal injection molded compacts

This paper investigates the effect of debinding and sintering parameters on the density, ultimate tensile strength (UTS), hardness and grain size of the Fe-2Ni MIM compact. For this purpose, a wax-based binder system containing PE, PP, PW and SA was adopted for developing the feedstock. After optimizing the feedstock powder loading based on the critical solids loading and studying the homogeneity of feedstock, green parts were injected. The samples were then immersed in the n-hexane solution for different times to investigate the effect of debinding time on the weight loss of binder system. After observing the samples shrinkage behavior, sintering temperatures were selected and the samples were sintered at temperatures of 1050, 1125, 1200, 1275 and 1350 °C. Finally, the microstructure, density and hardness of the sintered samples were evaluated and the UTS of the samples was measured using the DIC technique. The results showed that immersion of samples for 12 h in n-hexane with 50 °C temperature applied as optimum parameters for solvent debinding. In addition, appropriate thermal debinding cycle was designed through the TGA analysis to produce flawless samples. It was observed that the density increases with the sintering temperature increasing, until where in the temperature of 1350 °C the relative density was 97.2%. UTS and hardness values of the sintered samples increased to (∼548 MPa) and (∼144 HV) for the sintered specimen at 1275 °C and decreased at higher temperature. This phenomenon relates to the substantial grain growth in the sintered sample at 1350 °C, as proved by the results of EBSD analysis. Finally, proper sintering condition for Fe-2Ni samples derived through the density, UTS, hardness and microstructural evaluations, so that sintering of the samples at 1275 °C under hydrogen atmosphere showed the optimum condition due to an excellent balance between the mechanical properties and grain size in the samples.

Rheological and Thermal Characterization of AISI 4605 Low-Alloy Steel Feedstock for Metal Injection Molding Process

In this study, rheological and thermal decomposition behavior of AISI 4605 MIM feedstock investigated using capillary and thermogravimetric analysis. For this purpose and in order to find the critical solids loading, the feedstock was prepared by compounding AISI 4605 micro powder (~ 4 µm) with an adopted multi-component wax-based binder system at various powder loadings (53–61 vol%). After preparing the feedstock with optimal solids loading, fundamental rheological characteristics of homogenized feedstock, including flow behavior index (n), flow activation energy (E), and general moldability index (αstv) were studied. Furthermore, thermal analysis of feedstock, including apparent debinding activation energy and master decomposition curve was investigated. The results showed that critical solids loading of feedstock is around 58 vol%. The flow behavior index of developed feedstock was acceptable, since it came out to be less than one. Also, flow activation energy which shows the sensitivity of material to temperature changes, came out to be 70.406 kJ/mol. Using these two parameters, general moldability index was calculated to be 2.3147E−06. Finally, the master decomposition curve of feedstock was constructed based on calculated apparent debinding activation energy and verified using decomposition curves at different heating rates of 2, 5 and 10 °C/min.

Study on Rheological Behavior and Mechanical Properties of PMN–PZT Ceramic Feedstock

In this research, rheological behavior and mechanical properties of lead magnesium niobate–lead zirconate titanate (PMN–PZT) ceramic feedstock have been studied. Rheological behavior and mechanical properties of feedstock are the critical factors to manufacture a micro-scale piezoelectric structure with high aspect ratio because they highly affect the complete-filling as well as demolding process. Each feedstock was divided into three groups based on the variation of binder composition within the filler, backbone, and entire binder system. The optimal solids loading was determined based on the measurement of the critical solids loading, and 2 vol% lower solids loading than the critical state was adopted with all the feedstocks. Rheological specification of the feedstock was evaluated by means of twin-bore capillary rheometer test under various shear rates and temperatures. Using rheological analysis, key parameters including viscosity, flow behavior index, and flow activation energy were acquired. General moldability index as a function of the key rheological parameters was calculated and analyzed, and the feedstock with the lowest solids loading exhibited the best rheological character. In the next step, the mechanical properties of green parts were evaluated by the triple-point flexure test. The flexural stress and elastic modulus were significantly affected by the solids loading rather than binder composition. In general, this paper explored both rheological behavior and mechanical properties of PMN–PZT feedstock for various binder systems and it provides valuable guidance for the manufacture of a high-end micro-sized piezoelectric structure with high aspect ratio.

Evaluation of powder loading and flow properties of Al2O3 ceramic injection molding feedstocks treated with stearic acid

Abstract Processability of ceramics via injection molding consists in reaching sufficiently high solid loading despite submicrometric size of Al2O3 ceramic particles with a correspondingly high specific surface area. High (critical) solid loading limits injection molding reflected by flow properties of the compounds. This paper is focused on the evaluation of the critical solid loading (CSL) parameter as well as reliable description of the flow properties. As surface active agents are usually incorporated in polymer binders, most often used stearic acid (SA) is combined with three different binders to achieve higher CSL. CSL is determined from evolution of mixing torque based on continuously increasing powder loading from 50 to 60 vol %. Flowability of Al2O3 feedstocks with a fixed powder loading (52 vol %) was analyzed with a capillary rheometer, and viscosity data were predicted by a master curve model representing a fixed functional structure with no adjustable (fitting) parameters. The only variables are shear rate and surface agent (SA) concentration. A deviation from the experimental data does not exceed an experimental error.

Rheological properties of titanium-hydroxyapatite with powder space holder composite feedstock for powder injection moulding

Metal-ceramic composites such as titanium-hydroxyapatite have been extensively studied as a new generation of implant material in medical application. The powder injection moulding (PIM) process is one the viable processing method in producing the titanium-hydroxyapatite composite which offers extreme geometry part at a lower cost of production. The final powder injection moulding (PIM) product was influenced by the feedstock rheological and thermal properties that related to the binder selection and the critical solids loading. The emphasis of this study was towards the rheological and thermal properties of Ti6Al4V-hydroxyapatite composite feedstock with the powder space holder using a low-density polyethene and palm stearin binder system. The critical solids loading for 90:10 of Ti6Al4V-hydroxyapatite with 20% of the space holder was selected for capillary rheometer tests. The solids loading was obtained at 73 vol.% with optimal solids loading in the range of 2–5% below the critical solids loading which were 68, 69 and 70 vol.%. All feedstocks exhibited shear thinning behaviour, with the shear rate and viscosity values in the range of the PIM feedstock. The work in this study found that the feedstock of 68 vol.% solids loading possessed the best rheological properties with a higher flow behaviour index.

Elucidating the influences of torque and energy in determining optimal loading

Abstract Critical solid loading is a composition where the particles are tightly packed without external pressure and the binder filled the entire spaces and voids between the particles. The aims of this study is to determine the critical powder volume concentration (CPVC) and to investigate the influence of the different powder loadings, mixing behavior and energy for magnesium metal injection molding feedstock. The present mixing study was conducted for magnesium (Mg) powder with different compositions of binder consist of paraffin wax (PW), high density polyethylene (HDPE), and stearic acid (SA). The result showed CPVC of Mg powder obtained was 69 vol.%. Result reveals that an increasing in the ratio of powder-binder will disturb the viscosity of the feedstock. The relative viscosity increased with increasing the powder loading and approach to infinite near to the critical solid loading. Besides, the processing temperature, totalized torque (TTQ) and energy involved in the mixing process also increased as powder loading increased.

Investigation on Feedstock Preparation for Micro-Cemented Carbide Injection Molding

Abstract This research was focused on mixing of submicron cemented carbide (WC-Co-VC) powder and binder. WC-Co-VC powder particle size and morphology were analyzed by laser diffraction and field emission scanning electron microscopy. The WC-Co-VC powder was kneaded with a paraffin wax based binder system. Based on critical solid loading, the feedstock with different solid loadings between 49 to 51 vol.% was prepared. Finally, the flow behavior of different feedstocks was investigated. Morphology of powder revealed that the particles of powder are slightly agglomerated and irregular in shape. The result of mixing indicted that the torque value increases as the solid loading increase from 49 vol.% to 51 vol.%. The feedstock exhibited homogeneity and the powder particles are homogenously coated with binder. The feedstock with solid loading of 51 vol.% is sensitive to temperature and showed high viscosity values. The feedstock with solid loadings of 49 and 50 vol.% had good compatibility and flow characteristics.

Thermoplastic processing and debinding behavior of NbC-M2 high speed steel cemented carbide

The aim of this study is to evaluate the thermoplastic processing on a new generation of hard metals based on NbC bonded with high speed steel. The conventional technique in producing cemented carbide parts is dry pressing with paraffin binder and post production. To overcome shape restriction in pressing and post production, thermoplastic shaping processes can be used. For this purpose, a previously known binder system for ceramic feedstocks containing paraffin wax and ethylene vinyl acetate was selected and used for the first time with cemented carbides. The stated organic binders were mixed with NbC-12 wt.% M2 high speed steel powder and feedstocks with solid loading content from 50 to 60 vol.% were obtained. The rheological behavior of the feedstocks as well as the critical solid loading content were assessed using a capillary rheometer. A model fitting approach was used for the first time in hardmetal feedstocks to estimate the critical solid loading. Experimental values were fitted to a model from Chong (1971) which is based on bimodally disperse powder mixtures and a critical powder loading of 70 vol.% could be calculated. However, based on the abrasion problems during mixing at high filler content, feedstock with 57.5 vol.% powder loading had the optimum processability characteristics, and was therefore selected for further shaping processes. Thermo gravimetric analysis on green samples was performed after shaping in order to develop an optimum thermal debinding profile. The thermal debinding program was adjusted with special attention on decomposition onset, maximum mass loss rate and decomposition offset temperatures. Initial trials using decomposition offset temperature as holding steps, resulted in fragmented samples after debinding. The final designed heating profile resulted in a consistent mass loss rate according to thermo gravimetric analysis which limited crack formation during debinding. Debinding under a forming gas (95N2-5H2) atmosphere using optimized heating cycles resulted in sound sintered bodies only when green parts were embedded in a powder bed during debinding.

Effect of nanoparticles in bimodal powder on physical and mechanical properties of powder injection molded parts

This paper presents investigation of how nanoparticles in bimodal powder that contains both nanoparticles and microparticles affect the physical and mechanical properties of injection-molded parts. As the starting materials, a pure-micropowder, and three different nano/micro-bimodal powders were prepared and mixed with a wax-based binder system. Addition of nanoparticles slightly decreased the critical solids loading and the feedstock homogeneity, and significantly increased the flexural strength and strain of green bodies. Addition of nanoparticles increased the density, decreased the grain size and provided superior surface finish of the sintered samples, and thereby improved their mechanical properties. The Vickers hardness of the samples increased as the nanoparticles content increased. Tensile strength was also improved by addition of small amounts of nanoparticles. However, the large passivation area of the nanoparticles raised the oxide content in the bimodal powder samples and degraded the strength and elongation. As a result, tensile strength was the highest in the sample fabricated from the bimodal powder that contained 12% nanoparticles.

Fabrication of micro-sized piezoelectric structure using powder injection molding with separated mold system

In this research, a powder injection molding process was developed to fabricate a micro-sized piezoelectric structure with high aspect-ratio. The separated mold system was prepared by the X-ray micromachining process. The optimal solids loading was determined to be 52 vol% considering the critical solids loading. To ensure a homogeneous powder-binder mixture, the mixing process was performed three times. For complete filling in micro-powder injection molding, rheological properties of the mixture were evaluated using a capillary rheometer with various shear rates and temperatures. The measured flow index n and activation energy E were values of 0.45 and 20.11 kJ/mol, respectively. Based on the rheological analysis, complete filling into the micro-cavity of the separated mold was achieved at the injection velocity of 140 mm/s. The green part was successfully demolded from the separated mold without showing any slanted or broken structures. For extraction of binders from the green part, optimal debinding conditions were determined based on the thermal decomposition behavior of binders and lead (Pb) loss measured by thermo gravimetric analyzer (TGA). After the sintering process was completed, the high-aspect-ratio piezoelectric microstructures were precisely fabricated.