Moldability Index Research Articles

Rheological Analysis of Zirconia-Hydroxyapatite with Bi-Modal System of Binders; Low-Density Polyethylene and Palm Stearin

The two component micro-powder injection molding (2C-μPIM) process has evolved from μPIM process because of the increasing demand for multi-functional micro-components applications. In this research work, the selected materials to fabricate micro-sized bi-material parts are zirconia (ZrO2) and hydroxyapatite (HA). ZrO2 is chosen for structural integrity and bio-inert, while HA is mainly chosen for bio-active properties. The reason of employing the multi-component binders is to ensure the flowability of the feedstock. Feedstock rheological characteristics needs to be carefully investigated to avoid any undesirable and inhomogeneous mixture between powder and binder. A common binder system which is comprised of palm stearin and low-density polyethylene (LDPE) were mixed with individual ZrO2 and HA powder particles to prepare for ZrO2 and HA feedstocks. Typically, the feedstocks were obtained ZrO2 and HA powders independently with a binder ratio of 60 wt.% of palm stearin and 40wt.% low-density polyethylene (LDPE). The mixing was carried out in Brabender mixer. Before mixing, critical powder volume percentage (CPVP) analysis was carried out to determine the optimal powder loadings required to prepare the ZrO2 and HA feedstocks. In this research work, the obtained CPVP of ZrO2 and HA powders were 47.0 and 59.0 vol.%, respectively. Based on CPVP analysis, six feedstocks with optimal powder loadings of 43, 44 and 45 vol.% for ZrO2 and 54, 55 and 56 vol.% for HA were prepared. The rheological analysis involving viscosity, shear rate, flow behavior index, activation energy and moldability index was investigated using capillary rheometer. Based on the obtained rheology result, it shows that the overall shear rate and viscosity are within the 2C-μPIM process recommended range. All tested composition shows pseudoplastic behavior. The results of the study found that ZrO2 and HA with optimal powder loadings of 55 vol.% and 44 vol.% have good rheological properties compared to feedstocks with other powder loadings. This is because both materials meet the criteria of good rheological properties which are low viscosity, high shear rate, flow behavior index less than one, low activation energy and high moldability index.

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.

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.

Effect of Powder Shape and Size on Rheological, Thermal, and Segregation Properties of Low-Pressure Powder Injection Molding Feedstocks

Optimization of molding parameters at different filling stages requires that the characteristics of low-viscosity feedstocks must be properly known. In this study, the influence of the particle size (3, 7, and 12 μm) and powder morphology (gas- and water-atomized) was quantified using different feedstock formulations based on a 17-4 PH stainless steel powder, all combined with the same binder system. The specific heat capacity, viscosity, and segregation levels were assessed using a differential scanning calorimeter, a rotational rheometer, and a thermogravimetric analyzer, respectively. It was then shown that the feedstock prepared with the gas-atomized powder exhibits a higher specific heat capacity value when compared to a water-atomized powder (i.e., Cp varying from 0.6 to 0.3 J/g K, respectively). It was also shown that the feedstock viscosity profiles and the intensity of segregation both depend significantly on the powder shape and size used in the feedstock formulation. For short processing times (e.g., < 1 min spent in molten state), a feedstock formulated with coarse gas-atomized powder can be considered to be the best candidate for an injection process because its viscosity and segregation potential are relatively low (with a viscosity varying from 0.5 to 2 Pa s, and a volume fraction of powder close to 60 vol.%). Following this, the moldability index was used to predict that coarser water-atomized powder (i.e., 12 versus 7 μm) will not have a significant effect on the molding properties, and can be seen as a potential means of decreasing shrinkage during sintering without impacting the injection properties. For very long processing times, the magnitude of segregation remains insignificant for feedstock formulated with fine powder (i.e., a volume fraction of powder remaining close to 60 vol.%). However, this important gain (i.e., no segregation) comes with a significant increase in viscosity varying from 2 to 10 Pa s, resulting in a decrease in the molding properties.

Rheological and thermal debinding behaviors of silicon nitride in powder injection molding

Abstract In powder injection molding process, it is important to analyze the rheological and thermal debinding behaviors of feedstock, because they can directly affect the final quality of products. Therefore, for the silicon nitride based feedstocks, the rheological and thermal debinding behaviors were investigated and compared between feedstocks prepared with the combination of two types of powders and sintering aid ratios. At first, the optimal solids loading for each feedstock was determined based on the results of the torque rheometer experiment. The viscosity of the feedstocks was measured using the capillary rheometer, and rheological properties were evaluated with the Power Law-Arrhenius model. Silicon nitride (JPN) feedstock with 5 wt% yttria and 2 wt% alumina (JPN+5Y2A) had the highest moldability index among all feedstocks. The powders with sintering aid ratio of 5 wt% yttria and 2 wt% alumina made the feedstocks have a high moldability index. Thermogravimetric experiment was also performed to analyze the pyrolysis behavior of the feedstocks, and the apparent activation energies for each feedstock were obtained using Kissinger method with the thermogravimetric analysis results. Based on the results, the master decomposition curve (MDC) was developed, and the utility of MDC for optimizing the thermal debinding process was verified.

Multi-Objective Optimization of Injection Molding Process for Determination of Feasible Moldability Index

A multi-objective optimization of injection molding for determination of feasible moldability index was investigated in this study. The feasible moldability indices were calculated by implementing Taguchi method and Fuzzy Analytic Hierarchy Process with the order performance by its similarity to the ideal solution technique. Major process parameters of the injection molding like melt temperature, gate design, filling and cooling times were selected as variables to minimize common manufacturing defects like shrinkage, warpage and short shot. The analytic hierarchy process was used to set initial weights of variables and seriousness of each defect was evaluated by fuzzy decision-making process. Finite Element Analysis of the injection molding process was performed for selected work piece using polypropylene material. Results showed that it is more convenient to calculate several sets of variable parameters with high moldability indices, rather than focusing on single configuration with the highest moldability index. It is clear that the combination of Taguchi with TOPSIS and FAHP gives more applicable alternatives with high moldability indices, if the solution with highest moldability index in not practical.

Fabrication of pressureless sintered Si3N4 ceramic balls by powder injection molding

Pressureless sintered Si3N4 ceramic balls were fabricated by powder injection molding technology. Yttria and alumina as a sintering aid were added to improve the sintered density. The mixing was performed with twin-screw extrude mixer to fabricate the homogeneous feedstock. The rheological behavior of Si3N4 feedstock was analyzed with Power Law-Arrhenius model and moldability index. With the optimized injection molding parameters, green parts without defects were obtained. A two-step debinding process was applied to remove the binders in the green parts. Solvent debinding was firstly conducted in n-hexane at 50 °C for 5 h. The optimized thermal debinding was secondly conducted in air atmosphere based on TGA experiment results. The sintering was conducted in nitrogen atmosphere at 1750 and 1800 °C for 4 h. Embedding powder (Si3N4:BN = 1:1) was used to avoid dissociation of silicon nitride. The balls with 5 mm diameter were finally fabricated with no defects. The relative density and mechanical properties including 3-point bending strength, Vickers hardness and fracture toughness had a higher value at the sintering temperature of 1800 °C than at 1750 °C. It indicates that the Si3N4 ceramic balls with high physical and mechanical properties can be successfully fabricated by optimized PIM process.

Comparative study of nanoparticle effects on feedstock behavior for injection molding

ABSTRACTRheological properties of bimodal feedstocks containing nano- and micro-particles were studied. To examine nanoparticle effects, five feedstocks were prepared with the nanoparticle contents from 0 to 75 vol. %. Each feedstock was formulated with the optimal amounts of the powders, determined from the critical solids loadings of the powders. Viscosity measurement of the feedstocks was performed using a capillary rheometer. From the result, the flow behavior index and flow activation energy were obtained as representative rheological parameters. The moldability index for each feedstock was also calculated and compared. Some bimodal feedstocks showed lower viscosity than microfeedstock due to the roller bearing effect of the nanoparticles. The rheological parameters indicated the bimodal feedstocks were more desirable for mold filling except for 75% nano-bimodal feedstock. The results revealed 25% nano-bimodal feedstock showed the highest moldability index and was the most desirable feedstock to prepare a green body via nano/micro-bimodal powder injection molding.

Effect of binder composition on rheological behavior of PMN-PZT ceramic feedstock

An investigation of the effect of binder composition on rheological behavior of lead magnesium niobate–lead zirconate titanate (PMN-PZT) ceramic feedstock is presented in this paper. Rheological behavior is the key factor for manufacturing a micro-sized structure with high aspect-ratio. Each binder composition was categorized into three groups to analyze the effect based on the variation of ratio within filler (first), within backbone (second), and between filler and backbone (third). The critical solids loading for each feedstock was evaluated using a torque rheometer. To compare the net effect of binder composition, all the feedstocks were mixed with the solids loading of 45 vol%. The effects of binder composition on rheology were analyzed using a capillary rheometer with different shear rates and temperatures. The main rheological parameters, such as viscosity, shear sensitivity, and temperature sensitivity, were closely related to the molecular weight difference in binder composition. First, the viscosity increased as the ratio of binders with higher molecular weight increased for each group. Second, the sensitivity of viscosity on shear rate was enhanced with the increase of polyethylene (PE) in the second and backbone in the third, respectively. Third, the sensitivity of viscosity on temperature was lowered as the amount of binders with higher molecular weight increased for each group. Based on the experimental results, the moldability index as a function of viscosity, shear sensitivity, and temperature sensitivity was calculated. The feedstock with the largest amount of filler showed the best rheological property for powder injection molding.

Optimization of injection molding process based on fuzzy quality evaluation and Taguchi experimental design

This paper presents the multi-objective optimization in injection molding. The aim of this study is to determine the best alternative for the best moldability index and better alternatives which are close to the best alternative for an injected part in injection molding technology. Consideration of Taguchi and fuzzy analytic hierarchy process (FAHP) with technique for the order performance by its similarity to the ideal solution (TOPSIS) offers an efficient use approach to evaluate different objectives. Three common defects which reduce the final quality of the injected parts are short shot possibility, shrinkage rate and warpage. The selected material for injection is polypropylene. L18 orthogonal array of Taguchi is selected and for gaining accurate results, Finite Element Analysis (FEA) is applied. To validate the simulation result, the experiment is carried out for two circular flat plates with thickness of 1mm. Finally, the highest moldability index is related to trial 2 for both simulation and experimental results. This study outcome shows instead of having one trial number which has the highest moldability index, there are 10 trial numbers with high moldability index between 0.6–1 for simulation results and 5 trial numbers with high moldability index between 0.6–1 for experimental results. It is clear, by using Taguchi with TOPSIS and FAHP if the best alternative is not possible to maintain, there are other alternatives which are very close to the best alternative with high moldability index. The main advantage of the proposed method is to increase the number of alternatives which in turn increases the moldability of the final product in injection molding.

Development of Hypereutectic AlSi Alloy Powder Injection Molding Feedstocks by Rheological Analysis

The comprehensive properties of a feedstock have a critical influence on the powder injection molding process. Proper feedstock with homogeneous structure, favorable flow characteristic, and moldability is the prerequisite for obtaining a final part with excellent comprehensive properties. The objective of the present work was to develop an optimal feedstock for fabrication of hypereutectic AlSi (20 wt.%) alloy parts by the powder injection molding technique. For this purpose, micron‐sized hypereutectic AlSi (20 wt.%) alloy powder was mixed with different amounts of a binder which consisted of 35 wt.% high‐density polyethylene, 62 wt.% carnauba wax, and 3 wt.% stearic acid. The binder contents of the feedstocks were in the range from 13 wt.% to 21 wt.%. The influences of binder content, shear rate, and temperature on the rheological behaviors of feedstocks have been investigated via a capillary rheometer. The feedstock with 21 wt.% binder exhibited a variable flow behavior and was culled. The rest of the feedstocks showed a pseudoplastic behavior. Comprehensive analysis of rheological parameters such as the flow behavior index, yield stress, flow activation energy, and the general moldability index, the feedstock with 17 wt.% binder exhibited the best rheological performance and favorable moldability. The molded part with 17 wt.% binder had constant density, good shape retention, and stiffness as well as homogeneous distribution of the powder and binder. After solvent debinding, the debound item showed a homogeneous porous structure which is suitable for the subsequent thermal debinding and sintering processes.

Analysis of the rheological behavior of Fe trimodal micro-nano powder feedstock in micro powder injection molding

The rheological behavior of Fe trimodal powder feedstock for μ-PIM was investigated using torque rheometer analysis. The feedstock was prepared by mixing Fe micropowder (~4μm) and nanopowder agglomerates containing discrete particles with sizes of ~300nm and ~30nm, with low viscosity wax-based binders. During the feedstock mixing, a significant increase in mixing torque was observed at powder content above 72vol% in the feedstock, indicating the critical powder loading. The results obtained from the torque rheometer experiment at selected powder loadings of 66–74vol% were used as fundamental rheological parameters including flow behavior index (n), flow activation energy (E), and general moldability index (αstv). In addition, the influence of shear rate, temperature, and powder loading on feedstock viscosity was evaluated. It was found that the feedstock with 72vol% powder loading exhibited the best rheological behavior with the highest moldability parameter and homogeneous microstructure. Due to the lack of research on the trimodal system in the field of powder metallurgy and powder technology, this paper provides the possibility of applying the trimodal powder mixture to a near net-shape production technique such as PIM.

Influence of nano powder on rheological behavior of bimodal feedstock in powder injection molding

Rheological behavior of feedstock, which highly affects the condition of green body, is substantial issue in powder injection molding process. In this paper, the influence of nano powder contents in bimodal feedstock on solids loading and rheological behavior has been investigated. The bimodal powders were fabricated with 100nm and 4μm sized 316L stainless steel powders, and mixed with wax-based binder system. The critical solids loading for each powder was measured by torque rheometer. In order to analyze net effect of nano powder ratio, all feedstocks were formulated as solids loading of 42vol.%. Feedstock homogeneity was evaluated by rotational rheometer. Capillary rheometer test was conducted to measure the viscosity of feedstock with different temperatures, shear rates and nano powder ratios. Rheological parameters, like flow index, flow activation energy and moldability index, were calculated based on the capillary rheometer test. The results indicated the critical solids loading and feedstock homogeneity decreased as nano power ratio increased in bimodal feedstock. Rheological parameters were also affected by nano powder ratio, and the optimal mixing ratio of nano powder for moldability index was investigated as 12vol.%.

Influences of Restaurant Waste Fats and Oils (RWFO) from Grease Trap as Binder on Rheological and Solvent Extraction Behavior in SS316L Metal Injection Molding

This article deals with rheological and solvent extraction behavior of stainless steel 316L feedstocks using Restaurant Waste Fats and Oils (RWFO) from grease traps as binder components along with Polypropylene (PP) copolymer as a backbone binder. Optimal binder formulation and effect of solvent extraction variables on green compacts are being analyzed. Four binder formulations based on volumetric ratio/weight fraction between PP and RWFO being mixed with 60% volumetric powder loading of SS316L powder each as feedstock. The rheological analysis are based on viscosity, shear rate, temperature, activation energy, flow behavior index, and moldability index. The optimal feedstock formulation will be injected to form green compact to undergo the solvent extraction process. Solvent extraction variables are based on solvent temperature which are 40 °C, 50 °C, and 60 °C with different organic solvents of n-hexane and n-heptane. Analysis of the weight loss percentage and diffusion coefficient is done on the green compact during the solvent extraction process. Differential Scanning Calorimeter (DSC) is used to confirm the extraction of the RWFO in green compacts. It is found that all binder fractions exhibit pseudoplastic behavior or shear thinning where the viscosity decreases with increasing shear rate. After considering the factors that affect the rheological characteristic of the binder formulation, feedstock with binder formulation of 20/20 volumetric ratio between PP and RWFO rise as the optimal binder. It is found that the n-hexane solvent requires less time for extracting the RWFO at the temperature of 60 °C as proved by its diffusion coefficient.

The Influence of Sewage Fat Composition on Rheological Behavior of Metal Injection Moulding

Influence of sewage ratio or Fat Oil Grease (FOG) on the feedstock rheological characteristic for optimal binder formulation in metal injection moulding is evaluated besides Polypropylene (PP) as a backbone binder. Powder loading of 62% of water atomised SS316L being used here to determine the possibility of the best binder formulation which could be optimised for optimal powder loading base on rheological characteristic analysis. Two binder formulations of PP to SF being selected here are 60/40, 50/50 and 40/60 accordingly with the powder loading of 62% each binder formulation. The analysis will be base on viscosity, shear rate, temperature, activation energy, flow behaviour index and moldability index. It is found that from rheological result views, binder with composition of 60/40 and 50/50 exhibit pseudoplastic behaviour or shear thinning where the viscosity decrease with increasing shear rate. For 40/60 binder ratio is not suitable since the behaviour of the flow indicates dilatants behaviour. After considering all the criteria in terms of flow behaviour index, activation energy, viscosity and mouldability index, binder with ratio of 60/40 is evolve as a good selections.

Torque rheometry and rheological analysis of powder–polymer mixture for aluminum powder injection molding

Selection of desired powder–polymer mixture (feedstock) formulation is a key factor in manufacturing perfect parts via powder injection molding. In the present study, feedstock characteristics of an aluminum-based powder were investigated by torque rheometry and rheological analyses. Several binders containing various amounts of polypropylene (PP), paraffin wax (PW), and stearic acid (SA) were selected for torque mixing and viscosity evaluation. Then, feedstocks consisting of 54, 58, 62, and 66 vol. % solid contents were prepared with modified binder. Feedstock flow behaviors were investigated regarding the rheological parameters such as mixing torque, viscosity, flow behavior index, flow activation energy and moldability index. It was found that increasing solid loading from 54 to 62 vol. % led to improved rheological behavior. This improvement was not observed in high solid contents, i.e., 66 vol. %. Based on experimental results, the optimized binder composition (60PW,35PP,5SA vol. %) and the optimum powder loading (62 vol. %) were selected as the best formulations for injection of aluminum powder. These values are supported by critical powder volume concentration measurements deduced from the oil absorption method. The resulting aluminum molded green parts with no defects exhibited the straightforward injection molding process of selected feedstock.

Mixing Homogeneity and Rheological Characterization for Optimal Binder Formulation for Metal Injection Moulding

Mixing homogeneity and the feedstock rheological characteristic for optimal binder formulation in metal injection moulding is evaluated between Polypropylene (PP) and Sewage fat (SF) or Fat Oil Grease (FOG). Difference powder loading of SS316L also being used here to determine the possibility of the best binder formulation selected could be optimised for optimal powder loading base on rheological characteristic analysis. Two binder formulations of PP to SF being selected here are 60/40 and 70/30 accordingly with the powder loading of 60% and 55% for each binder formulation. The analysis will be base on viscosity, shear rate, temperature, activation energy, flow behaviour index and moldability index. It is found that rheological result shows all the two binder formulations with both powder loading exhibit pseudoplastic behaviour or shear thinning where the viscosity decrease with increasing shear rate. Feedstock viscosity also decrease with increasing temperature indication of suitability for moulding. Results from all the analysis conducted shows that the volumetric powder loading of 60% with binder volumetric of 60% for PP and 40% for SF contributes significant stability and suitability for optimum powder loading.

Analysis of the Rheological Behavior and Stability of Inconel 718 Powder Injection Molding (PIM) Feedstock

In PIM process, the molding stage is a critical step for the fabrication of sound parts without cracks and distortions. So, this step requires specific rheological behavior. Rheological analysis can be made to quantify the stability of the PIM feedstock during molding process. In this study, an experimental rheological study has been performed to evaluate the influence of palm stearin on the rheological behavior and stability of Inconel 718 powder injection molding (PIM) feedstock. The effects of shear rate and temperature were investigated via capillary rheometer method. The viscosity and shear rate of the feedstock were measured at various range of temperature and shear rate across the L/D = 1 capillary rheometer. The results show that the formulation binder system PSPE 7030 is the best feedstock to be injection molded because it has moderate value of flow sensitivity index, lower value for flow activation energy and higher value for mold ability index. Besides that, the increasing of PS content in the binder system has found reduce viscosity and value of flow activation energy and increase value of mold ability index of feedstock.

Rheological Characterization of Water Atomised Stainless Steel SS316L for Micro MIM

This paper investigates the performance of feedstock characteristics for micro metal injection molding (μMIM) by using optimum power loading variation and rheological characterization. The study has been emphasized on the powder and binder system in which stainless steel SS316L powder are mixed with composite binder, which consists of PEG (Polyethelena Glycol), PMMA (Polymethyl Methacrilate) and SA (Stearic Acid) by variation of powder loading concentration. The rheology properties are investigated using Shimadzu Flowtester CFT-500D capillary rheometer. As the geometry of water atomised stainless steel powder are irregular shape, therefore it is expected significant changes in the rheological results that can influence the microcomponent, surface quality, shape retention and resolution capabilities. The optimization of the μMIM rheological properties as a function of stainless steel powder loading concentration are evaluated by flow behavior exponent, activation energy and moldability index. From the results, it shows that 61.5%vol contributes a significant stability over a range of temperature and the best powder loading from a critical powder volume percentage (CPVP) and rheological point of view.