Powder-binder Mixture Research Articles

Feasibility study on piston-based feedstock fabrication with Ti-6Al-4V metal injection molding feedstock

The use of feedstocks from metal injection molding (MIM) for the additive manufacturing (AM) of green parts, which are then debound and sintered in a process called shaping, debinding, and sintering (SDS), is promising in terms of production costs of metallic parts. However, in order to use the cost-efficient AM technique fused filament fabrication (FFF) for SDS, powder-binder mixtures known for MIM feedstocks must be adapted to filament requirements resulting in adjustments to debinding and sintering. In contrast to FFF, screw-based material extrusion is capable of processing already available MIM feedstocks, but machine costs are high due to complex print heads. In this work, a new process called piston-based feedstock fabrication (PFF) is developed for processing already available MIM feedstocks at comparable costs to FFF. First, the state of the art is reviewed highlighting the potential of piston-based material extrusion for its usage in SDS. Based on the review, an extruder module is developed which is integrated into a kinematic system. Experimental studies are performed to validate the developed PFF printer. As material, a Ti-6Al-4V MIM feedstock is used. Thresholds for piston speed (0.175 mm/min), extrusion temperature (80 °C), and nozzle diameter (0.4 mm) are determined to ensure a viscosity that allows to control the extrusion process via steps per mm. With these thresholds it is found that a constant extrusion process can be established in a filling range of the cylinder up to 155 mm. Finally, the performance of the PFF system is evaluated in terms of nozzle geometry, print speed, and reproducibility showing that reproducible green part properties are achieved at a maximum speed of 8.18 mm/s while using a tapered FFF nozzle.

Role of elongational viscosity of feedstock in extrusion-based additive manufacturing of powder-binder mixtures

The 3D printing of metals and ceramics by the extrusion of a powder/thermoplastic binder feedstock is an extrusion-based additive manufacturing (EAM) technique and has received significant interest. EAM feedstocks are generally characterized by their shear viscosity. A quantitative comparison with the shear flow data, through an estimation of the Trouton ratio, indicates that the extensional viscosities are three orders of magnitude greater than their shear flow viscosity at a comparable shear rate obtained in three different high loaded polymers retained for this study. This experimental study addresses the unsolved issue of the role of elongational viscosity in the modelling of EAM of highly viscous melts. The study was conducted using three feedstocks with a water-soluble binder and high powder loading. The different powder materials used for this study are stainless steel, alumina and zirconia. Initially, the rheological properties of the feedstocks were assessed using capillary rheometers. A pressure drop model based on the shear and elongational components of the viscosity was proposed to predict the extrusion pressure during capillary tests. The model was adapted to develop a specific EAM machine, namely, an EFeSTO, equipped with a pellet extrusion unit. Experimental EAM tests were conducted, and the pressure drops were analytically predicted and experimentally measured. A total of 31 different combinations of extrusion velocities, nozzle diameters, 3D printed shapes and materials were tested through a total 184 experimental runs. The model predicts well the experimental pressures for the steel feedstock, whereas it underestimates the pressure for the two ceramic feedstocks owing to their different thermal properties. The results of this study clearly demonstrate that the pressure, and therefore the material flow during the EAM processes of viscous materials, cannot be modelled well without considering the elongational viscosity.

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.

Microstructure and physical and mechanical properties of micro cemented carbide injection moulded components

Micro cemented carbide (WC–Co) injection moulded components were fabricated using a powder injection moulding (PIM) process. At first the WC–Co powder was mixed with a binder system. Powder-binder mixture as called feedstock is then moulded into desired shape and followed by debinding process. After debinding, the samples were sintered at a temperature range between 1370°C and 1450°C for 3min and 8min. The effects of sintering processing parameters on the microstructure and the physical and mechanical properties of the components were investigated. According to field emission scanning electron microscopy (FESEM) observations of the microstructure, tungsten carbide (WC) particles are homogeneously distributed and have excellent bonding with the cobalt (Co) metallic matrix. The sintering cycles at 1410°C with an 8min holding time lead to optimise physical and mechanical properties. This sintering cycle indicated low porosity among the sintered parts. The relative density and hardness of micro WC–Co injection moulded components increased with the increase in sintering temperature and the best relative density obtained was 97.57% of the theoretical density at 1410°C with an 8min holding time. The highest hardness value of 1645HV was obtained at a sintering temperature above 1410°C. However, the highest transverse rupture strength (TRS) of 1120MPa was achieved at 1410°C with an 8min holding time, and the TRS decreases with a further increase in the sintering temperature.

Experimental study on moldability and segregation of Inconel 718 feedstocks used in low-pressure powder injection molding

Moldability and segregation of feedstock are linked to the rheological behavior of the powder-binder mixture. In this study, the impact of binders on viscosity and segregation of feedstocks was investigated. The experiments were conducted on several feedstocks obtained by mixing Inconel 718 powder with paraffin wax-based binder systems. The viscosity of feedstocks was measured by a rotational rheometer while the segregation within green parts was evaluated using a thermogravimetric analyzer. It was demonstrated that the variation in solid loading within a molded part can be measured with a sensitivity of at least ±0.25 vol% of powder. The results indicated that the predominant powder-binder separation appears clearly at the top and the bottom of the molded part. It was also shown that the viscosity profiles of feedstocks and the intensity of segregation depends significantly on the binder constituents used in feedstock formulation. The mixture containing only paraffin wax produced the best trade-off between high moldability and low segregation for an injection process requiring an extended time range between injection and solidification of the part (e.g. up to 10 min). For a short processing time (e.g. <1 min spent in molten state), the feedstocks containing paraffin wax with stearic acid or small amount of ethylene vinyl acetate can be also considered as good candidates for LPIM process because their viscosity and segregation potential are relatively low.

Influence of segregation on rheological properties of wax-based feedstocks

The rheological behavior of wax-based feedstocks has a direct impact on successful mold filling for parts produced by low-pressure powder injection molding. During a rheological test, segregation may occur within low-viscosity feedstocks, leading to errors in results, and to an improper evaluation of the flowability of powder-binder mixtures. The segregation occurring during rheological tests is not generally considered when measuring the viscosity profiles of such mixtures. In this study, the impact of testing time, testing protocol (increasing or decreasing shear rate), polymer degradation, powder deagglomeration, and temperature on the rheological properties of feedstocks was investigated. The segregation occurring during rheological testing produces a meaningful effect in low-viscosity feedstocks. Because the segregation is driven by the time spent in the molten state, the effects of segregation on viscosity were visible, particularly at the end of a long rheological test. During a short rheological test, the pseudoplastic effect occurring in wax-based feedstocks was only attributable to the binder molecules' orientation with respect to the flow. In this respect, and contrary to the information reported in literature, it was demonstrated that the dilatant effect observed was in fact more likely due to segregation occurring during the rheological test, rather than as a result of the elimination of the preferential layer formed during the pseudoplastic regime. Because the rheological properties of low-viscosity feedstocks were significantly influenced by the test duration, a rheological test duration of a few minutes must be used to precisely quantify the flowability of such feedstocks.

Low-pressure powder injection molding using an innovative injection press concept

In low-pressure powder injection molding, the injection presses are equipped with an interconnecting pipe in which the segregation of feedstocks may occur during the dead time of the process. This segregation phenomenon limits the use of low-viscosity feedstocks leading to a decrease in the capacity to produce more complex-shaped components. The full moldability potential of the process is still limited by the injection method proposed in the commercial machines. A new injection concept was developed to increase the moldability of powder-binder mixtures, while avoiding segregation within the injection press. This innovative injection system consists of eliminating the interconnecting pipe and injection valve by using a sliding platform concept which was adapted for the transportation of the feedstock from the container to the mold cavity. Validation of the injection system was performed using three wax-based feedstocks. Injection profiles obtained with two different molds were correlated with the rheological behavior of the feedstocks, and thermogravimetric analyses were used to quantify the segregation within the system. An experimental simulation of the segregation within the machine confirmed that the injection method avoids segregation for a feedstock exhibiting a viscosity as low as 0.1 Pa·s. The best moldability properties were obtained with the feedstock containing paraffin wax and stearic acid.

Segregation Measurement of Inconel 718 Feedstocks Used in Low-Pressure Metal Injection Molding

Low-pressure metal injection molding (LP-MIM) is an advanced manufacturing technology where a wax-based feedstock is injected into a complex shape before densification heat treatments. Feedstock is generally designed to minimize segregation, maximize flowability, maximize the strength of the molded component, maximize the solid loading potential and ease of debinding. In this study, the emphasis is placed on the evaluation of the effect of segregation on different wax-based Inconel 718 superalloy feedstocks used in LP-MIM. In powder metallurgy, particle or phase segregation generates a fluctuation of the particle distribution in powder-binder mixtures from point to point. Such demixing generally occurs before or during the injection process, and can lead to the formation of defects such as cracks, distortions or heterogeneous shrinkage of the sintered parts. Different wax-based feedstocks were poured in cylindrical hot molds (95°C), maintained in molten state for 1 minutes or for 60 minutes, and rapidly cooled to room temperature. The specimens were then extracted from the top and bottom regions of each cylindrical part. A thermogravimetric analysis technique was used to measure the volume fraction of powder at these two locations in order to quantify the degree of segregation in green parts. The best candidate feedstocks minimizing segregation are the mixtures containing only paraffin wax, or those containing paraffin wax and ethylene vinyl acetate combined. An increase in the time spent in the molten state and the use of beeswax or stearic acid promote the powder-binder separation of feedstocks.

Fabrication of high-aspect-ratio micro piezoelectric array by powder injection molding

A powder injection molding process to fabricate a high-aspect-ratio piezoelectric microarray was developed. The reverse shaped sacrificial mold insert was developed by X-ray micromachining, and then insert-type injection molding was conducted to fabricate the piezoelectric microarray structure. For the micro-powder injection molding, rheological properties of the powder binder mixture were evaluated and analyzed. The measured flow behavior exponent was 0.44 and the flow activation energy was 83kJ/mol−1. Based on the analyzed rheological property, injection molding conditions were optimized. The rheological property and the injection molding conditions were crucial to ensure complete filling. Using the optimized conditions, two high-aspect-ratio piezoelectric microarrays were fabricated: (i) 20-μm patterns with 1:5 aspect ratio and (ii) 40-μm patterns with 1:10 aspect ratio.

Rheological study of feed stock for NiTi alloy molded parts

A rheological behaviour of the powder-binder mixture is one of essential analysis upon to success of Metal Injection Moulding (MIM) process. The purpose of this experimental work is to investigate the rheological behavior of feedstock containing mixtures of elemental Ni and Ti powders mixed with composite binder of palm stearin (PS) and polyethylene (PE) binder system. An equiatomic Ni-Ti (50-50) ratio was used in the present work for all formulations owing to excellent shape memory behaviour. The experimental rheological result indicated that all the feedstocks exhibited pseudo plastic flow behaviour; viscosity decreasing with temperature and shear rate. Increasing the powder loading resulted in higher viscosity, particularly at the low-range of shear rate. Owing to pseudo-plastic flow, it was found that the feedstock prepared exhibit promising rheological properties, thus resulting successfully injection moulding at an optimum temperature of 130°C.

3D screen printing for the fabrication of small intricate Ti-6Al-4V parts

For the production of complex shaped titanium parts huge advances have been made by powder metallurgy (P/M) during the last years. However, when it comes to miniaturized parts with intricate inner structures like closed channels without entrapped powder and high aspect ratios conventional P/M methods are stretched to their limits. To remedy this shortcoming, Fraunhofer IFAM Dresden uses 3D screen printing, a modified screen printing process based on a powder-binder mixture which is printed layer-by-layer to the desired shape followed by debinding and a conventional sintering step. Especially for small parts a productivity of several million parts per year is achievable. The objective of this work is to demonstrate the possibility to manufacture complex designs based on titanium including thin walls down to 120 μm. Therefore, pre-alloyed gas atomized Ti-6Al-4V powder was used. Analysis of part accuracy, resulting porosity, impurity level and mechanical properties were performed.

FEEDSTOCK FLOW CHARACTERIZATION AND PROCESSING OF POROUS NITI BY METAL INJECTION MOULDING (MIM)

Porous NiTi alloy with a nearly three-dimensionally interconnected pore structure has been successfully fabricated by a transient liquid phase sintering following the metal injection moulding (MIM) process, using a mixture of Ni and Ti elemental powders. The elemental Ni and Ti powders mixture was mixed with a binder system, comprised mainly polyethylene-glycol (PEG) in an alternative technique using a speed mixer, principally incorporating a dual asymmetric centrifuge (DAC). The powder-binder mixture was then characterized using a capillary rheometer at various temperatures and shear rates. It was found that the feedstock exhibited pseudo-plastic behaviour, which is favourable for the MIM process. A temperature range of 120oC - 130oC was considered as the optimum operating condition for the injection moulding processing. The parts were moulded into cylindrical shapes, leached in warm water (60oC for 10 hours), thermally debound in argon and subsequently sintered in a vacuum furnace at four different temperatures ranging from 950oC to 1250oC. All samples underwent expansion in both diameter and height after water leaching and sintering. The XRD results showed that increasing the sintering temperature resulted in a major fraction of the B2 NiTi phase due to phase homogenization and subsequently decreased the amount of secondary phases such as NiTi2 and Ni3Ti. Besides that, the formation of the transient liquid phase during sintering enabled major fraction of pores to be developed with porosity and average size of 39 - 45% and 100 - 120 μm, respectively. The porous parts produced have a great potential to be used as an implant in biomedical applications.

Segregation measurement of powder injection molding feedstock using thermogravimetric analysis, pycnometer density and differential scanning calorimetry techniques

In this study, three measurement techniques were experimentally compared to quantify the effects of segregation on powder injection molding feedstock. In a powder metallurgy process, particle or phase segregation generates a fluctuation of the particle distribution in powder–binder mixture from point-to-point. Such demixing occurs generally before or during the injection process and can lead to the formation of defects such as cracks, distortions or heterogeneous shrinkage of the sintered parts. Thermogravimetric analysis, pycnometer density and differential scanning calorimetry were used to measure respectively the mass loss after binder burnout, the density and the enthalpy of fusion on several feedstocks with different solid loadings. It was demonstrated that the variation in solid loading can be measured with a sensitivity of at least ±0.5vol.% of powder using the TGA and PD techniques only. It was also shown that the thermogravimetric analysis and the pycnometer density results are independent of feedstock formulation and can be obtained without the use of a calibration curve. The thermogravimetric analysis and the pycnometer density measurement are complementary and well-adapted experimental methods to measure the effects of segregation on powder injection molded green components.

Impact of binders on viscosity of low-pressure powder injection molded Inconel 718 superalloy

Rheological behavior of powder-binder mixture has a direct impact on the successful mold filling for parts obtained from powder injection molding. In this study, the impact of binders on rheological properties of feedstocks was investigated. The experiments were conducted on several feedstocks obtained by mixing of Inconel 718 powder with wax-based binder systems. Their rheological and thermal properties were investigated using rotational rheometry and differential scanning calorimetry techniques, respectively. It was demonstrated that a large amount of ethylene vinyl acetate (EVA) should be added to paraffin wax (PW) to produce a thickening effect of the mixture. At low shear rate, the mixing of high-viscosity paraffin waxes with a low-viscosity PW produces a similar thickening effect denoted with the EVA. In addition, the feedstocks containing only PW demonstrate a pseudoplastic behavior. It was also shown that an addition of only 1 vol% of stearic acid (SA) in PW generates an important decrease in viscosity, and further increases of this constituent induce no effect on rheological behavior. From a rheological perspective, the best candidate feedstocks are the mixtures containing PW and SA while feedstocks based on PW, beeswax or containing a small amount of EVA could be also considered as good.

Micro-powder injection molding of cemented tungsten carbide: feedstock preparation and properties

Micro-powder injection molding (μPIM) is an advanced net-shaping process for manufacturing metal, ceramic and carbide complex micro-components. The hardmetal ceramic-metal composite cemented tungsten carbide (WC–Co) is known for its high hardness, reasonable toughness, and high abrasion and wear resistance in various applications. This study focused on the characterization and evaluation of WC–Co powder as a feedstock through μPIM. A WC–10wt.% Co powder was mixed with a paraffin wax (PW) and low density polyethylene (LDPE) binder to obtain the feedstock. The powder-binder mixture was prepared in 49–51 vol.% powder loadings to investigate the effects of the powder content on the feedstock properties. Differential scanning calorimetry and thermogravimetric analysis were conducted to determine the mixing, injection molding, and de–molding stage temperatures. The flow behaviors of the feedstock were evaluated using a capillary rheometer. Finally, the mechanical properties and density of the molded and sintered components were evaluated. This experimental work reveals that the as-received WC powder requires de-agglomeration using sufficient ball milling to achieve an acceptable level of powder loading. The green parts were successfully molded using feedstocks with 49 and 50vol.% powder loadings. The results indicate that the WC–10wt.% Co feedstock with a 50vol.% powder loading provides a sufficient compromise between moldability, green density and stiffness. After debinding and sintering, the findings indicate that a micro part can be successfully fabricated through the μPIM process using the WC–10wt.% Co feedstock.

Feedstock Characterization of Elemental Nickel and Titanium Powders Mixture for Metal Injection Moulding Process

The success of metal injection moulding (MIM) process is significantly influenced by the homogeneity level of the powder-binder mixture (feedstock). This paper highlights some initial characterization of the feedstock containing elemental Ni and Ti powders mixture, (50.8 at.% Ni/ 49.2 at.% Ti) mixed with Palm stearin-based binder system. The feedstock was prepared using an internal mixer, HAAKE Rheomix at a temperature of 160°C for 2 hours. The feedstock was then characterized by Differential Scanning Calorimetry (DSC), Thermogravimetric (TGA), Scanning Electron Microscopy (SEM) and Capillary Rheometer. All the results obtained were analyzed and discussed for further injection moulding process.

The effects of material attributes on powder–binder separation phenomena in powder injection molding

Abstract The composition of the powder–binder mixture (feedstock) used in powder injection molding (PIM) plays a crucial role in fabricating defect-free parts. Powder–binder separation in the feedstock during mixing or molding is responsible for defects such as voids, cracks, or distortion in the sintered part. This study introduces an inter-particle spacing parameter, δ , based on particle size ( D ), particle volume fraction ( ϕ ) and maximum particle fraction ( ϕ m ). The derived parameter is investigated using feedstocks prepared from three different metal powders. Based on comparisons with experimental data from this study as well as reports in the literature, the inter-particle spacing parameter, δ , proves to be a useful predictor of powder–binder separation. The newly proposed parameter assists in designing and developing feedstocks to manufacture components by PIM.

Rheological behavior of magnetic powder mixtures for magnetic PIM

Powder injection molding (PIM) is a promising manufacturing technology for the net-shape production of small, complex, and precise metal or ceramic components. In order to manufacture high quality magnets using PIM, the magneto-rheological (MR) properties of the PIM feedstock, i.e. magnetic powder-binder mixture, should be investigated experimentally and theoretically. The current research aims at comprehensive understanding of the rheological characteristics of the PIM feedstock. The feedstock used in the experiment consists of strontium ferrite powder and paraffin wax. Steady and oscillatory shear tests have been carried out using a plate-and-plate rheometer, under the influence of a uniform magnetic field applied externally. Rheological properties of the PIM feedstock have been measured and characterized for various conditions by changing the temperature, the powder fraction and the magnetic flux density.

Investigation of the Moldability Parameters of PEG Based Steatite Feedstocks by Powder Injection Molding

Abstract In powder injection molding (PIM) barrel temperature, injection pressure and injection rate are important parameters for the molding of powder-binder mixtures. Improperly selected parameters lead to undesired problems such as separation of the powder-binder mixture and formation of collapses and cracks on the structure of the molded parts. In this study, steatite feedstocks based on PEG (polyethylene glycol) binders were injection molded into rectangular shape, ordered thickness with steps and zig-zag shaped mold cavities. The effects of the temperature and pressure of injection, rate of filling, temperature of the mold wall and geometry of the mold cavities were investigated. The optimum molding parameters of the feedstocks for the zig-zag shaped mold were determined to be an injection pressure of 80 to 140 MPa, a barrel temperature of 190 to 230°C, an injection rate of 5–25 · 10−6 m3 s−1 and a mold wall temperature of 32°C.

Influence of powder morphology on properties of ceramic injection moulding feedstocks

The powder–binder-mixture is the most critical aspect in the injection moulding processing chain, because the feedstock properties determine the success of the following steps, i.e. excellent flowability in the injecting phase and high green strength during debinding. Therefore, nearly spherical particles which pass each other easily in the feedstock stream are advantageous. The applicable particle size is limited in order to reach high solids loadings which are connected with the specific powder surface. According to this, in this study a very unfavourable powder was introduced into a feedstock. The investigated β-alumina powder consists of plate-like primary particles and has a specific surface of 25 m 2/g. During mould filling the particles orient in the feedstock stream which affects the debinding and sintering behaviour and is the source of defects. The influence of the plate-like shape on defect formation is demonstrated in comparison to spherical-shaped particles having the same specific surfaces.