Powder Injection Moulding Feedstocks Research Articles

Influence of powder characteristics on the behaviour of PIM feedstock

This work aims to analyse the impact of powders which are not conventionally intended for powder injection moulding (PIM) and how their characteristics influence the behaviour of the feedstock during mixing. Tests were performed with different alumina powders using the same binder system. The results show that mixing has a strong impact on the packing density of powders inside the feedstock, while the deagglomeration of powders makes it possible to achieve high critical powder volume concentrations (CPVCs) equal to or greater than 58vol%. The CPVC depends on the deagglomeration efficiency. The agglomeration state – especially cohesion of the agglomerates – has an influence on the CPVC. The comparative study of mixing torques shows that the grain size and surface area of powders have a major impact on the mixing behaviour of the feedstock. During the implementation of powders, variabilities in the homogenisation of the powder/binder system and in deagglomeration are achieved as a result of powder agglomeration. It was demonstrated that the powders in this study perfectly satisfy the criteria imposed by the mixing process although they are not intended for PIM.

Powder Injection Moulding of Alumina Using PEG/PVB Binder Systems

Powder injection moulding (PIM) is a process that is suitable for a fabrication of small and complex shape components. It consists of 4 main steps: feedstock preparation by mixing powder and binder, injection moulding of the prepared feedstock into the desired mould, removal of the binder and finally sintering to obtain materials with specific properties. In this study, powder injection moulding of alumina (Al2O3), using polyethylene glycol (PEG) based binder systems, was investigated. PEG is soluble in water; therefore, the use of organic solvents required for debinding of wax-based binder system can be avoided. PEG with a molecular weight of either 1500 or 4000 was used as a major constituent together with polyvinyl butyral (PVB) as a minor component. Stearic acid was also added during feedstock preparation to act as a lubricant. After mixing the powder with the binder, a variety of Al2O3 feedstocks were injected into the moulds. The mouldings were prepared by a laboratory-scaled plunger-typed machine. Debinding was carried out using a combination of solvent extraction and thermal debinding. Water leaching tests were performed at 30 and 50 °C to study PEGs removal rate. The pyrolysis of PVB was completed during ramping up of the mouldings to the sintering temperature. The mouldings were subjected to sintering at 1500 °C in air. It was found from the study that PEG/PVB binder systems can be used for the preparation of alumina powder injection moulding feedstocks. Specimens retained their shapes during and after leaching of the PEGs.

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.

Rheological Analysis of Microminiature Powder Injection Molding (μPIM) Feedstock

A rheological analysis has been performed to evaluate the characteristics and behaviors of Microminiature Powder Injection Molding (μPIM) feedstocks. The feedstocks comprised of 316L stainless steel powder and water-based binder components. Feedstocks formulations with powder loading of 59% to 63% were prepared and investigated. In these formulations, the binder system consists of 65% Polyethelena Glycol (PEG), 25% Polymethyl Methacrilate (PMMA) and 10% Cellulose Acetate Butyrate (CAB) based on the weight fraction. The influences of rheological behaviors such as flow activation energy (E), Power-Law exponent (n), viscosity (η) and temperature (T) of the SS316L/PEG/PMMA/CAB feedstocks are analyzed and discussed. Results show that all of the feedstocks exhibited the pseudo-plastic flow behavior. The homogenous feedstock at 61 vol. % demonstrated the most satisfactory rheological properties for μPIM with the lowest flow activation energy, Power-Law exponent, n < 1 and moderate viscosity values. It was chosen to perform the injection molding process. Micro components have been replicated successfully by using this selected feedstock.

Mechanical alloying of FeAl intermetallic powder for metal injection moulding process

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.

Experimental and theoretical methods for optimal solids loading calculation in MIM feedstocks fabricated from powders with different particle characteristics

The aim of this work is to investigate the influence of the particle characteristics of powder to the mixing stage of the powder injection moulding process to identify the best procedure with which to evaluate the optimal solids loading for feedstock fabrication. To perform the present study, powder blends that are made from three bronze and four Inconel 718 powders, with different particle size distributions and morphologies, are mixed with a binder system that is based on polyethylene and wax. Powder–binder blends are prepared by varying the powder content to study how the powder characteristics affect the optimal solids loading. In addition, a new method is presented for determining the optimal solids loading in powder injection moulding feedstocks through the activation energy calculation of powder–binder blends that are prepared with different powder contents. This novel method is compared with the optimal loading values that have been determined by other ways as torque rheometry, determination of the tap-real density ratio and through rheological models.

Influence of powder particle size distribution on rheological properties of 316 L powder injection moulding feedstocks

The rheological behaviour of highly filled polymer systems used in powder injection moulding (PIM) technology influences strongly the final properties of the products. The study of the influence of particle size distribution on the rheological properties of 316 L stainless steel feedstocks is presented. Two kinds of gas-atomized powders were employed with d 90 of 8 and 26 μm of particle size. Five feedstocks were prepared at powder loadings of 50, 55, 60 and 65 vol.%, employing both starting powders and their own mixtures in volume ratios 25/75, 50/50 and 75/25. The binder was composed by 50 vol.% of high density polyethylene and 50 vol.% of paraffin wax. Feedstocks were prepared in a torque rheometer with a pair of roller rotor blades and rheological measurements were performed in a capillary rheometer. The effects of the powder loading and different particle size distributions on the rheological properties of feedstocks were investigated. Different rheological parameters, like melt viscosity, power flow index and activation energy were determined. A relationship between powder characteristics and rheological behaviour was established.

Low viscosity feedstocks for powder injection moulding

Powder injection moulding (PIM) carried out with the use of low viscosity feedstocks offers numerous benefits for manufacturing small complex shape parts. Unlike typical high pressure metal injection moulding (HPIM) viscous feedstocks, soft tooling can be employed for prototyping and small volume manufacturing. Compared to HPIM, there are very few studies on the rheology of low viscosity feedstocks. The objective of this paper is to clearly determine, using a statistical method, optimal models which define viscosity as a function of three parameters: shear rate, temperature and solid loading for low viscosity feedstocks. With the statistical method employed, it was found that the models of Herschel–Bulkley, Arrhenius, and Maron and Pierce can be used respectively to effectively model each of the three parameters stated previously. Moreover, the combination of these three models in one global model is proposed to predict the combined effect of the three parameters on low viscosity PIM feedstocks.

Flow and void formation in powder injection moulding feedstocks made with PEG/PMMA binders Part 2 – Slip band model

Flow and deformation of powder injection moulding (PIM) feedstocks are considered to occur in similar ways to those observed in materials containing substantial amounts of clay and water, i.e. by a slip band mechanism. A slip band model for PIM feedstocks has been postulated, which allows flow behaviour and void formation to be explained qualitatively. For feedstocks, the slip bands are assumed to be layers of mobile liquid, which, for polyethyleneglycol/polymethylmethacrylate (PEG/PMMA) binders, are considered to be mainly PEG. It is proposed that some of the slip band liquid is drawn from between the randomly densely packed particles between slip bands to form voids. It is considered for given moulding conditions that increasing the apparent viscosity of a feedstock, by decreasing the volume fraction of binder in slip bands, reduces voidage. This can be achieved by increasing the nominal powder loading of a feedstock and/or increasing the effectiveness of the PMMA in holding the particles together more strongly.

Flow and void formation in powder injection moulding feedstocks made with PEG/PMMA binders Part 1 – Experimental observations

Mixtures of 94 wt-% tungsten carbide (WC) and 6 wt-% cobalt (Co) powders with different particle sizes have been formed with binders composed of polyethyleneglycol (PEG) and polymethylmethacrylate (PMMA), in some cases with the addition of stearic acid (SA) as a lubricant. The presence of voids has had to be invoked to explain why, when the same moulding conditions were used, the as moulded and as leached densities varied with binder composition and in some cases, why the former were lower than expected from the proportions of binder components and solids used to prepare the feedstocks. For a given nominal binder content (vol.-%), calculated on the assumption of zero voidage, and using constant moulding conditions, it appears that the actual voidage tended to decrease as PMMA progressively replaced PEG, and to increase as SA was introduced to replace PEG. The former change to the binder composition should, with constant binder content and moulding conditions, increase the apparent viscosity of the feedstock, while the latter should reduce it. A slip band model is postulated which allows flow behaviour and void formation to be explained qualitatively. This is further developed in Part 2 of this paper. Data from previous studies using PEG/PMMA binders with stainless steel powders are included as these can now be given a qualitative explanation with the slip band model.

스테인레스계열(17-4PH, 316L, 440C) 분말을 이용한 Dental Scaler Tip 분말사출금형 개발

Powder injection molding(PIM) is widely used for many parts in the field of automotives, electronics and medical industries, due to the capability of net shaping for complex 3-D geometry. Powder injection mold design for the dental scaler tip, a component of medical appliance, was presented. In comparison with conventional machining process, powder injection molding has many advantages, specially in price and dimensional stability, for molding dental scaler tip which has complex geometry. Both product design and mold design for dental scaler tip were presented. A PIM feedstock was made of stainless series(17-4PH, 316L, 440C) powder and a wax based binder. The 'rapid mold' concept was applied to manufacture the various forms and materials of dental scaler tip including vibration characteristics.

Development of Dental Scaler Tip Mold with Powder Injection Molding Process

With the capability of net shaping for complex 3D geometry, powder injection molding (PIM) is widely used for automotive parts, electronics and medical industry. In this study, an ultrasonic dental scaler tip produced by machining process was redesigned for the PIM process. An injection mold was designed and machined to produce the dental scaler tip by the PIM process. The mold design was aided by CAE analysis. A PIM feedstock was made of SUS316L powder and a wax based binder. The filling balance in the mold was checked by a short shot test with LDPE and the PIM feedstock. Production capability of the PIM process for the dental scaler tip was examined with the mold. Although there were minor problems such as a discoloration around the gate and a flashing at the air vent, the PIM process turned out to be an excellent substitute for machining process to manufacture the ultrasonic dental scaler tip.

A new binder for powder injection molding titanium and other reactive metals

This paper describes the development of a new aromatic-based binder for powder injection molding (PIM) reactive metals, such as titanium, zirconium, niobium, tungsten, and molybdenum. Detailed processing results are discussed for titanium alloy (Ti-6,4) compacts fabricated using the new binder system. Because of the choice of the binder constituents, thermal removal is readily accomplished at low temperatures and short times via vacuum sublimation. In this way the binder can be cleanly extracted from the green part prior to sintering to minimize the amount of residual carbon left in the final component. Rheological measurements indicate that powder loadings in the PIM feedstock as high as 67 vol.% could be achieved using the new binder system, while still maintaining low mixing torques and injection molding pressures. Results from chemical analyses conducted on the Ti-6,4 powder before injection molding and the Ti-6,4 compacts after final sintering demonstrate no significant increase in carbon content due to processing. The raw powder contained 210 ppm carbon, while the as-sintered, PIM formed compacts exhibited 217 ppm carbon content.

Analysis of the rheological behavior and stability of 316L stainless steel–TiC powder injection molding feedstock

An experimental rheological study has been performed to evaluate the influence of TiC addition on the rheological behavior and stability of 316L stainless steel powder injection molding (PIM) feedstock. The effects of TiC concentration, solid loading, shear rate and temperature were investigated via capillary rheometer method. The stability of feedstocks was evaluated quantitatively using “instability index”, which describes the threshold beyond which the variation of viscosity becomes unacceptable for PIM purposes. The results show that the rheological behavior of PIM feedstocks highly depends on the blend composition. The addition of TiC particles to the stainless steel powder increases the viscosity of feedstock at relatively low shear rates, i.e. <500 s −1. Furthermore, the feedstock instability increases, particularly at higher solid loading. Nevertheless, with increasing shear rate and temperature, the viscosity decreases and the instability of feedstock improves. At relatively high shear rates, i.e. >2000 s −1, the viscosity of composite feedstocks was lower than that of the mono-component SS feedstock due to the better particle packing efficiency. This article presents the rheological behavior of bimodal powder mixture of stainless steel and TiC powders prepared for PIM application. The influences of blend composition, i.e. the TiC concentration and solid loading, and the processing parameters, i.e. temperature and shear rate, are addressed.

Investigation of rheological behaviour of 316L stainless steel–3 wt-%TiC powder injection moulding feedstock

The rheological behaviour of powder injection moulding feedstock comprising of 316L stainless steel and 3 wt-%TiC powders was studied using a capillary rheometer. The flowability and the sensitivity of viscosity to shear rate and temperature of the feedstock were investigated and compared with those of the binder system and the 316L SS PIM feedstock. The general rheological indexes were examined through relevant equations and the influence of TiC addition on the mouldability of the 316L SS feedstock was determined. It was found that all the feedstocks are basically pseudoplastic but the values of flow behaviour index n are influenced by the TiC addition, the solid volume fraction and the temperature. In this context, the results show that TiC addition plays an important role in rheological parameters, i.e. TiC particles decrease the viscosity of PIM feedstock and this effect is more pronounced in the system comprising higher solid volume fraction. Furthermore, the flow activation energy decreases with the introduction of TiC particles, in particular at high shear rates. In other words, the addition of ceramic particles would ease the feedstock flow and suppress the sensitivity of the feedstock to strain rate and temperature variations, if a proper moulding temperature is applied. From the viewpoint of mouldability, the optimum PIM condition for the SS/TiC composite powder containing a wax based binder system was found at 55% solid volume fraction and 70°C. At this condition, the viscosity of feedstock is low enough to fulfill the requirements of a medium pressure, injection moulding process.

Use of a Naphthalene-Based Binder in Injection Molding Net-Shape Titanium Components of Controlled Porosity

We have recently developed a naphthalene-based binder system for use in powder injection molding (PIM) of ceramic and metallic materials. The use of a binder that can be removed via sublimation offers several unique advantages relative to the typical thermoplastic and/or thermoset binders employed in PIM. One of these is that essentially no volume change takes place during debindering. This offers a relatively facile method of introducing porosity into a net-shape part of potentially complex geometry. In the study described in this paper, the effects of powder loading and subsequent isostatic compaction on the size and amount of porosity in the components produced by this technique were investigated. In general, it was found that the amount of porosity is inversely proportional to the initial concentration of metal powder in the PIM feedstock. Likewise, average pore size displays a similar relationship with powder loading.

Rheological properties of feedstocks prepared with steatite powder and polyethylene-based thermoplastic binders

In this study, rheological properties and behaviors of binder formulations and powder injection molding (PIM) feedstocks, used in the ceramic industry, were investigated. For this purpose, various binder formulations and PIM feedstocks were prepared. In these formulations, polyethylene (PE) and three waxes (carnauba, bees wax and paraffin) were used with steatite powder loading. A 50% (v/v) fraction of feedstocks was steatite powder with 8.15 μm average particle size. Steatite powder and binder formulations were mixed as dry in a Turbula shaker/mixer for 30 min. To achieve a good disperse mixing, twin screw extruder was also used. The rheological properties of the resulting binder formulations and feedstocks were characterized by a capillary rheometer. In the rheometer, temperature was varied from 120 to 180 °C and pressure was varied from 12.3 to 124.7 kPa. From the capillary rheometer flow curves, the flow behavior index ( n) parameters for the binder formulations and PIM feedstocks were determined to be less than 1. The apparent shear rates ( γ ̇ a ) of binder formulations and PIM feedstocks were determined to be in the range of 22.6–1005 s −1. For the above apparent shear rates and shear stresses, the apparent viscosities ( η a) were calculated as in the range of 23.8–1105.2 Pa s. The melt flow indexes (MFI) were determined under various shear stresses ( τ w=12.3–124.7 kPa) and found to be in the range of 4–231 g per 10 min. Effect of temperature onto apparent viscosities ( η a) of feedstocks were also studied, and flow activation energies under various shear stresses were determined to be within the range of 59–182 and 25–99 kJ/mol, for the binder formulations and feedstocks, respectively. Further, the thermal behavior and removal of binders from the feedstocks were determined by DTA–TG analysis, and the removal the binders from feedstocks by thermal treatments were achieved.

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

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.

Characterization of powder injection molding feedstock

Powder injection molding (PIM) is a cost-effective technique for producing small, complex, precision parts in high volumes. PIM consists of four main processing steps: mixing, injection molding, debinding and sintering. To reduce the design-to-manufacture cycle time for injection molding, simulation software could be used. To have a good understanding of the PIM process and to provide the necessary data for simulation, characterization of the material is essential. This paper presents the characterization of PIM feedstock consisting of 91 wt.% M2 high speed steel (HSS) powder and 9 wt.% PAN250 polymer binder. The mechanical properties (Young's modulus, Poisson's ratio, in-plane shear modulus), thermal properties (coefficient of thermal expansion (CTE), softening point, transition temperature, specific heat, thermal conductivity) and rheological property of the feedstock were established. The CTE of the molded feedstock in three perpendicular directions differed significantly. Except for the specific heat, the mechanical and thermal results showed that the feedstock's properties were generally closer to a polymer rather than a metal. Rheological results exhibited pseudoplastic or shear thinning flow behavior, where its viscosity decreased with increasing shear rate. The feedstock viscosity also decreased with increasing temperature and was found to be suitable for molding.

Low-pressure injection moulding of stainless steel powders

Powder injection molding (PIM) is a cost-effective technique for producing small, complex, precision parts in high volumes. PIM consists of four main processing steps: mixing, injection molding, debinding and sintering. To reduce the design-to-manufacture cycle time for injection molding, simulation software could be used. To have a good understanding of the PIM process and to provide the necessary data for simulation, characterization of the material is essential. This paper presents the characterization of PIM feedstock consisting of 91 wt.% M2 high speed steel (HSS) powder and 9 wt.% PAN250 polymer binder. The mechanical properties (Young's modulus, Poisson's ratio, in-plane shear modulus), thermal properties (coefficient of thermal expansion (CTE), softening point, transition temperature, specific heat, thermal conductivity) and rheological property of the feedstock were established. The CTE of the molded feedstock in three perpendicular directions differed significantly. Except for the specific heat, the mechanical and thermal results showed that the feedstock's properties were generally closer to a polymer rather than a metal. Rheological results exhibited pseudoplastic or shear thinning flow behavior, where its viscosity decreased with increasing shear rate. The feedstock viscosity also decreased with increasing temperature and was found to be suitable for molding.