Variability of powder characteristics and their effect on dimensional variability of powder injection moulded components

Abstract

In this study, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel. Three lots of gas atomised and three lots of water atomised powders were characterised and used to make six batches of PIM compound. These compound lots were injection moulded using a cavity pressure transducer and screw position regulation controls. The moulded geometry was measured in the green state and sintered state for dimensional variability. The general findings are that gas atomised powder produce less dimensional variability than the water atomised powder from lot to lot, however, the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering. Also, the lot to lot variation in the powder characteristics, such as particle size and pycnometer density, have an effect on dimensional stability whereas variations in powder characteristics such as surface area, tap and apparent density, and chemistry have little effect on dimensional stability.     

Mechanical properties of metal injection moulded 316L stainless steel using both prealloy and master alloy techniques

Abstract

Stainless steel 316L MIM components can be made from either prealloyed powders or from master alloys blended with carbonyl iron powder. In this study these two techniques were compared using prealloyed and master alloyed gas atomised powders of ? 16 μm and ? 22 μm sizes. Four different compounds were prepared, characterised and injection moulded into tensile bars. The bars were compared for green strength, green defects, sintered strength and microstructure. The green components are stronger when carbonyl iron powder is used with the gas atomised master alloy. This material also seems to be less susceptible to moulding defects. The sintering strength of the material produced using the pre-alloyed powder was higher than the master alloyed prepared material. Little difference in mechanical properties existed between the materials fabricated from gas atomised prealloyed ? 16 μm and the ? 22 μm powders. Also, the viscosity of the mixtures was higher for the ? 16 μm material and the master alloy mixtures than for the –22 μm gas atomised prealloyed powders.     

Modelling of transient temperature fields during filling stage of powder injection moulding

Abstract

Numerical simulation of the powder injection moulding of the stainless steel feedstock was performed using finite elements/finite differences method. Influence of the process parameters on the temperature development in the moulded part was estimated. Frictional heating between the layers of the feedstock with different viscosity may cause the temperature rise to a level dangerous for the integrity of the feedstock. Both process conditions and position of the gate affect the temperature distribution within the part. The temperature homogeneity can be improved by optimisation of the mould design and process conditions.     

Sintering parameters and mechanical properties of injection moulded aluminium powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded aluminium powder. Gas atomised aluminium powder was injection moulded with wax based binder. The critical powder loading for injection moulding was 62·5 vol.-% for feedstock. Binder debinding was performed in solvent and thermal method. After debinding, the samples were sintered at different temperatures and times in high purity N₂. Metallographic studies were conducted to determine the extent of densification and the corresponding microstructural changes. The results show that gas atomised aluminium powder could be sintered to a maximum 96·2% of theoretical density. Maximum density, tensile strength and hardness were obtained when sintered at 650°C for 60 min.     

Powder extrusion moulding of 430L stainless steel thin tubes for porous metal supported SOFCs

Abstract

Thin and thick walled tubes made of 430L stainless steel for porous metal supported solid oxide fuel cells (SOFCs) were prepared by powder extrusion moulding (PEM). A gas atomised metal powder and a binder composed by 50?vol.‐% high density polyethylene and 50?vol.‐% paraffin wax were chosen. The most suitable powder loading for feedstock (68?vol.‐%) was determined by means of torque experiments and rheological measurements. Mixing temperature was selected taking into account the results from differential scanning calorimetry. The extrusion of tubes was carried out in a single screw extruder and two kinds of tubes with two different wall thicknesses were obtained. In order to investigate the homogeneity of feedstock, microstructure of green tubes was evaluated by SEM. Thermal debinding was performed on the basis of thermogravimetric analysis results. After sintering, tubes with good dimensional stability and without defects were obtained. Different wall thicknesses extruded were 150 and 500?μm.     

Characterisation of 316L powder injection moulding feedstock for purpose of numerical simulation of PIM process

Abstract

Viscosity, specific heat and thermal conductivity of the standard feedstock of 316L stainless steel have been measured under the typical conditions of a real powder injection moulding (PIM) process. The viscosity was measured in a wide range of shear rates at four different temperatures. The experimental viscosity data were fitted into the Carreau-Yasuda model. Both specific heat and thermal conductivity were measured in the temperature range that overlaps the recommended processing range for the studied feedstock. It has been shown that at high cooling rates the transition temperature of the binder material is shifted towards lower temperatures. Tabulated values of thermal conductivity and specific heat for the studied feedstock are presented. The obtained data can be used for numerical simulation of the powder injection moulding process.     

Modified metal injection moulding process of 316L stainless steel powders using thermosetting binder

Abstract

A modified metal injection moulding (MIM) process of 316L stainless steels powders using an acrylic thermosetting resin has been developed. Gas and water atomised 316L powders were used. In order to optimise the mixing and moulding steps, different volume fractions of the two components were investigated. Mixing of metal powder and binder was carried out at room temperature and immediately moulding was performed by pouring the slurry in the moulds. It was then heated at 90°C to permit the polymerisation and cross linking of the resin. Different heating cycles, rates, and atmospheres were studied by means of thermogravimetrical analysis. The data obtained were used to establish the best debinding cycle. The debound samples were sintered at different temperatures and high densities (98% of theoretical) were obtained. Materials in as moulded (green part), debound (brown part), and sintered conditions were examined by means of SEM.     

Microstructural and mechanical properties of injection moulded gas and water atomised 17-4 PH stainless steel powder

Abstract

This paper describes the microstructural and mechanical properties of injection moulded 17-4 PH stainless steel gas and water atomised powder. Gas and water atomised stainless steel powders were injection moulded with wax based binder. The critical powder loading for injection moulding were 62·5 and 55 vol.-% for gas and water atomised powders respectively. Binder debinding was performed using solvent and thermal method. After dedinding the samples were sintered at different temperatures for 1 h in pure H₂. Metallographic studies were conducted to determine to extend densification and the corresponding microstructural changes. The results show that gas atomised powder could be sintered to a maximum (98·7%) of theoretical density, and water atomised powder could be sintered to a maximum (97·08%) of theoretical density. Maximum tensile strength was obtained for gas atomised powder sintered at 1350°C. The tensile strength of the water atomised powder sintered at the same temperature was lower owing to higher porosity. Finally, mechanical tests show that the water atomised powder has lower mechanical properties than gas atomised powder.     

Optimisation of process conditions in powder injection moulding of microsystem components using robust design method Part 2 – Secondary design parameters

Abstract

Powder injection moulding (PIM) is a proper fabrication method for microsystem technology components. This paper studies the process control of PIM to create thin walled, high aspect ratio geometries, which can be easily found in microtechnology based electro chemical, mechanical and biological systems (MECS). The powder used in this study is gas atomised 316L stainless steel with a median particle size of 10 μm. The effects of reducing the thickness of high aspect ratio geometries on the secondary design parameters including the maximum wall shear stress, cooling time and standard deviations of the melt front velocity and areas are studied. The study shows process parameters including fill time, feedstock injection temperature, mould wall temperature and switchover position can be optimised using the Taguchi robust design method.     

Effect of nickel boride and boron additions on sintering characteristics of injection moulded 316L powder using water soluble binder system

Abstract

A new metal injection moulding system for 316L stainless steels has successfully been derived and tested. A mixture of small water atomised powder (average size 15 µm), larger gas atomised powder (average size 75 µm), and sintering additives has been coupled with a new water soluble binder system for economical powder injection moulding. The details for each process step and the effect of sintering additives are described. The binder system consists of poly (2-ethyl-2-oxazoline) as the leachable polymer, polyethylene as the backbone, and stearic acid as a surfactant and plasticiser. This binder system provides satisfactory mixture stability, excellent mouldability, and reasonably fast water leaching and thermal debinding rates. The optimum powder/binder compositions were determined using torque and capillary rheometry. Densification was by persistent liquid phase sintering through additives, such as nickel boride and boron. This 316L powder system was sintered to 7·9 g cm ^-3 (98·75% of theoretical) at 1285°C using nickel boride addition and at 1245°using boron addition. nickel boride additions are particularly effective at increasing the tensile strength and ductility. In contrast, the boron additions only increase the tensile strength and decrease ductility. Based on microstructure evaluations, this effect is traced to a continuous boride phase on the grain boundaries of the boron doped samples and a discontinuous boride phase on the grain boundaries of the nickel boride samples.     

PowderMet2009: prospects improving after automotive debacle

Abstract

A homogeneous powder and binder distribution in the green body in powder injection moulding (PIM) is important. In the present study, the mould filling model of PIM has been developed, based on the multiphase fluid theory, viscosity model of feedstock and powder-binder drag force model. The particle Reynolds number is influenced by the particle size and density, resulting in the different drag force between powder and binder. Furthermore, the varied velocity of binder and powder will be obtained with numerical calculation of the continuity equations, leading to the change of green body homogeneity. CFX was used to simulate the mould filling in PIM. The results showed that the homogeneity of green bodies was relative to the filling patterns, which varied with different powder densities. The powders were not suitable for PIM when the particle size was bigger than 20 μm, and the fine powders were beneficial to improve the homogeneity.     

Influence of injection moulding and sintering parameters on properties of 316L MIM compact

Abstract

This study elucidates the effects of key injection moulding and sintering factors on the dimensions and mechanical properties of 316L stainless steel metal injection moulded compact. Sintered parts of optimal quality can be produced by properly setting the process parameters. Taguchi method and principal component analysis are performed initially to elucidate and optimise the key control factors that affect the qualities of metal injection moulded compact. Next, a feasible process window is tested by observing the powder and binder distribution of green parts, for various control factors of injection moulding. Experimental findings show that, first, a proper injection speed facilitates mould filling during injection moulding and so improving the quality of sintered parts; second, temperature critically determines the rate of dimensional shrinkage, density and hardness of sintered parts; Third, optimal parameters setting can efficiently improve the quality of 316L metal injection moulded compact.     

Hot micro-embossing: effect of pressure on 316L metal parts

Abstract

The use of replicative processes has become strategic and critical in industry to produce precise, microscopically detailed metallic parts and devices via low cost manufacturing routes. Metal powder hot embossing is an emerging process that brings some advantages associated with the reduction of production costs relative to powder injection moulding (PIM). The technology involves four distinct steps: preparation of the selected feedstock material (powder and binder); hot embossing; debinding; and sintering. The effect of continuous pressure during the hot embossing step as a means of replicating microdetails in 316L stainless steel parts is examined. Dimensional accuracy, microstructure and mechanical properties of the parts produced were evaluated. For the configuration tested, the most promising results were achieved when processing at 180°C for 30 min at a pressure of 14 MPa.     

Metal injection moulding of bronze using thermoplastic binder based on HDPE

Abstract

In the present work, the injection moulding process of a Cu–10Sn bronze has been studied. Different formulations of binders based on high density polyethylene, paraffin wax and polyethylene glycol have been used. The optimisation of the metallic load is based on torque measurements and rheological studies. The optimum powder loading was 60 vol.-%. The moulding parameters are selected to obtain homogeneous specimens with three different geometries and without distortions. The green parts have an adequate strength for handling. The organic binder was eliminated by thermal debinding under N₂/10%H₂ atmosphere. The debinding process has been designed by means of thermogravimetrical analysis of binder and feedstock and considering the maximum heating rates at which the samples do not present cracks. The specimens were sintered at temperatures between 875 and 950°C in the same reducing atmosphere.     

Mechanical alloying of FeAl intermetallic powder for metal injection moulding process

Abstract

The Fe–48 at.-%Al powder used for powder injection moulding (PIM) was prepared by mechanical alloying in a high energy planetary ball mill and subsequent vacuum annealing. The effects of stearic acid (SA) as the process control agent on powder characteristics were investigated using X-ray diffractometer, laser particle size analysis and scanning electron microscopy. The dependence of solvent debinding efficiency of PIM feedstock on the powder characteristics was also studied. The results indicate that a low SA content for ball milling helps to prepare near spherical, coarse powder particles that exhibit good solvent debinding efficiency. On the contrary, a high SA content leads to thin layered, small particles with poor PIM solvent debinding efficiency. An intermediate SA content such as 1 wt-% makes the ball milled powder combine the advantageous characteristics for PIM process.     

Integrated filling,packing and cooling CAE analysis of powder injection moulding parts

Abstract

A coupled numerical analysis of the filling, packing and cooling stages of powder injection moulding (PIM) has been implemented. Finite element method/finite difference method methodologies were used in the filling and packing stages while Boundary Element Method (BEM) was used for the cooling stage. Using these methodologies, a numerical simulation program for the injection moulding process of PIM parts, PIMSolver was developed by taking into account the peculiar rheological behaviour of powder–binder mixtures. Specifically, the apparent slip phenomena at the mould wall and the yield stress were incorporated into the above analysis. The coupled analysis among the filling, packing and cooling stages was performed because the viscosity and slip phenomena of powder–binder mixture highly depend on temperature. In order to evaluate the significance of the coupled analysis and slip phenomena, several PIM experiments were performed using 316L stainless steel powders dispersed in a paraffin wax–polypropylene binder system. Using the examples of a U-shaped test specimen and an electronic package part, the importance of coupled numerical analysis for PIM parts and the significance of slip dependency of temperature during the coupled analysis were demonstrated.     

Netshaping concepts for tungsten alloys and composites

Abstract

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.     

Effect of atomisation medium on sintering properties of austenitic stainless steel by eliminating influence of particle shape and particle size

Abstract

Gas and water atomised 316L stainless steel powders with similar powder morphology and particle size were injection moulded and sintered. The results show that compacts prepared from the gas atomised powder exhibit higher density and tensile strength, whereas those prepared from the water atomised powder exhibit higher elongation, finer grain size and superior corrosion resistance. Chemical analysis shows that the water atomised powder has a higher Si and O content, and microstructural analysis of the sintered compacts reveals that SiO₂ particles disperse as a second phase in the compacts prepared from the atomised powder, which accounts for the property behaviour. Due to the presence of SiO₂, the porosity increases, whereas the pore coarsening and grain growth are inhibited. Besides, SiO₂ particles can also improve the passivation effect of stainless steel, and hence increase the corrosion resistance.     

Effect of Binder Composition on Rheology of Iron Powder Injection Moulding Feedstocks: Experimental Design

Abstract

A statistical analysis based on the McLean-Anderson design method was conducted to study the influence of each binder component on the flow properties of iron feed stocks for powder injection moulding. The formulations were composed of carbonyl iron powder (FE), polyethylene resin (PE), polyethylene wax (PEW), and stearic acid (SA). The viscosities of 15 different formulations were measured using a capillary viscometer. Fluidity and pseudoplasticity were evaluated and the effect of each component on these properties was analysed. The studied composition ranges of FE, PE, PEW, and SA were 90—92, 3—4·5, 2—4, and 0—1·5 wt—% respectively. It was found that lower iron powder content or higher binder content improved the fluidity of the feedstock. The relative effectiveness of binder ingredients in increasing fluidity is SA>PEW>PE and in enhancing pseudoplasticity PE > PEW > SA. Feedstocks with binder phases containing about 50 wt-%PE have the greatest fluidity with flow index n in the range 0·6—0·8. PM/0699     

Processing of Mn–Zn ferrites using mould casting with acrylic thermosetting binder

Abstract

In this paper, a simple manufacturing process for Mn–Zn ferrite powder is described, which can be considered as a modified powder injection moulding process. This method uses acrylic thermosetting resin as the binder. The moulding is carried out at room temperature by directly pouring the slurry (resin and ferrite) in the mould. The mixture is heated at the curing temperature (70°C) of resin to permit polymerisation and cross linking of the polymer. In order to optimise the moulding step, different volume fractions of powder with resin were mixed. The optimal powder load was 50 vol.-%. The best thermal debinding cycle was determined by means of thermo-gravimetric analysis. Sintering was performed according to oxygen partial pressure equilibrium curves at 1330°C for 3 h. Magnetic properties were compared with those obtained by uniaxial compacted parts.