Micro-powder Injection Moulding Research Articles

Debinding of Yttria-Stabilised Zirconia/Bimodal Stainless Steel 316L Bi-Materials Produced through Two-Component Micro-Powder Injection Moulding.

The fabrication of bi-material micro-components via two-component micro-powder injection moulding (2C-µPIM) from 3 mol% yttria-stabilised zirconia (3YSZ) and micro/nano bimodal stainless steel 316L (SS 316L) powders has received insufficient attention. Apart from this, retaining the bonding between ceramic and metal at different processing stages of 2C-µPIM is challenging. This study investigated the solvent and thermal debinding mechanisms of green bi-material micro-parts of 3YSZ and bimodal SS 316L without collapsing the ceramic/metal joining. In this research, feedstocks were prepared by integrating the powders individually with palm stearin and low-density polyethylene binders. The results demonstrated that during the solvent debinding process, the palm stearin removal rate in the bi-materials composed of 3YSZ and bimodally configured SS 316L feedstocks intensified with an increase in temperature. The establishment of interconnected pores in the solvent-debound components facilitated the thermal debinding process, which removed 99% of the binder system. Following sintering, the debound bi-materials exhibited a relative density of 95.3%. According to a study of the microstructures using field emission scanning electron microscopy, an adequate bond between 3YSZ and bimodal SS 316L was established in the micro-part after sintering. The bi-material sintered at 1350 °C had the highest hardness of 1017.4 HV along the joining region.

Moldability and Solvent Debinding of Hydroxyapatite Micro-Part Processed through Micro-Powder Injection Molding

The development of the micro-powder injection molding (µPIM) process from the powder injection molding (PIM) process has been prompted by the demand of the worldwide market to produce micro-sized components. The need for µPIM-processed components is currently rising across a range of industries, including automotive, aerospace, food, biomedical, electronics, and telecommunications. In the current research work, homogeneous HA feedstock with a powder loading of 57 vol.% was prepared by mixing HA powder particles with palm stearin and low-density polyethylene (LDPE) binders at a mixing temperature of 150 °C for 6 h. Defect-free injection molded or green micro-sized components of HA were produced by employing injection pressure, injection time, mold temperature, and melt temperature of 12 bar, 5 s, 110 °C, and 180 °C, respectively. When mold temperatures less than 110 °C were used, short shot defects were frequently observed in green specimens. After solvent debinding at 60 °C for 50 min, 82.2% of the palm stearin was removed from the green part. No difference in dimension between the solvent debound part and the green part was noticed. An open-pore structure developed in the solvent debound HA microcomponent is helpful for eliminating the insoluble LDPE binder during the thermal debinding phase.

Comparison between Micro-Powder Injection Molding and Material Extrusion Additive Manufacturing of Metal Powders for the Fabrication of Sintered Components

Original compositions based on iron micro-powders and an organic binder mixture were developed for the fabrication of sintered metallic elements with micro-powder injection molding (µPIM) and material extrusion additive manufacturing of metal powders (MEX). The binder formulation was thoroughly adjusted to exhibit rheological and thermal properties suitable for µPIM and MEX. The focus was set on adapting the proper binder composition to meet the requirements for injection/extrusion and, at the same time, to have comparable thermogravimetric characteristics for the thermal debinding and sintering process. A basic analysis of the forming process indicates that the pressure has a low influence on clogging, while the temperature of the material and mold/nozzle impacts the viscosity of the composition significantly. The influence of the Fe micro-powder content in the range of 45–60 vol.% was evaluated against the injection/extrusion process parameters and properties of sintered elements. Different debinding and sintering processes (chemical and thermal) were evaluated for the optimal properties of the final samples. The obtained sintered elements were of high quality and showed minor signs of binder-related flaws, with shrinkage in the range of 10–15% for both the injection-molded and 3D printed parts. These results suggest that, with minor modifications, compositions tailored for the PIM technique can be adapted for the additive manufacturing of metal parts, achieving comparable characteristics of the parts obtained for both forming methods.

Micro-Injection Molding and Debinding Behavior of Hydroxyapatite/Zirconia Bi-Materials Fabricated by Two-Component Micro-Powder Injection Molding Process.

The micro-scale joining of two different materials using two-component micro-powder injection molding (2C-µPIM) is an intriguing technique. The formation of defects in bi-materials at different processing stages makes this technique challenging. This study presents the fabrication of defect-free bi-material micro-parts containing hydroxyapatite (HA) and 3 mol% yttria-stabilized zirconia (3YSZ) via 2C-µPIM. Critical powder volume concentrations (CPVCs) of 61.7 vol% and 47.1 vol% were obtained for the HA and 3YSZ powders, respectively. Based on the CPVCs, the optimal loadings for the HA and 3YSZ powders were selected as 60 vol% and 45 vol%, respectively. The HA and 3YSZ feedstocks were prepared by separately mixing the optimal powder contents with low-density polyethylene (LDPE) and palm stearin binders. The feedstocks displayed pseudoplastic behavior, and the lowest ranges of viscosity for the HA and 3YSZ at a temperature of 180 °C were 157.1-1392.5 Pa·s and 726.2-985.5 Pa·s, respectively. The feedstocks were injected to produce green HA/3YSZ micro-sized components. It was found that a solvent debinding temperature of 70 °C removed 60.6% of the palm stearin binder from the sample. In the thermal debinding stage, the open channels that formed in the bi-material sample's solvent debound at 70 °C and contributed to the removal of 93 to 95% of the binder system. When the debound bi-materials were sintered at 1300 °C, the highest relative density of 96.3% was obtained. The sintering operation revealed a linear shrinkage between 13 and 17% in the sintered HA/3YSZ micro-parts.

Physical Properties of Sintered Stainless Steel 17-4PH Micro-Part Processed by MicroPowder Injection Molding

Micro-powder injection molding (μPIM) is a modification of powder injection molding (PIM) process and a globally recognized manufacturing process route that can be used largely to produce sophisticated micro-sized components using a wide range of metals and ceramics. The demand of μPIM process is currently increasing in various applications in telecommunication, electronics, aerospace, biomedical, and automotive industries. In this research work, sintering at three different temperatures between 1250 °C and 1350 °C at a heating rate of 10 °C/min with a dwelling period of 3 h on micro-injection molded and debound (solvent and thermal) micro-sized components of stainless steel 17-4PH (SS 17-4PH) was carried out. After the sintering operation, defect-free SS 17-4PH micro-specimens were achieved. The relative density, which is referred to as an important physical property of SS 17-4PH micro-parts, increased substantially from 95.3% to 97.5% when the sintering temperature was enhanced from 1250 °C to 1350 °C. The maximum shrinkage of 12.9% was observed in micro-sized specimens at the sintering temperature of 1350 °C. After the completion of sintering process, the findings revealed that SS 17-4PH micro-parts can be produced successfully on the grounds of μPIM process employing the SS 17-4PH feedstock with powder loading of 69 vol.%.

Simulation of Micro-Powder Injection Molding Using a Dynamic Mesh Approach

Abstract Powder injection molding (PIM) is capable of manufacturing complex small components out of metals and ceramics. With increasing market demand micropowder injection molding (micro-PIM) is improving rapidly which calls for improved numerical models of different stages of PIM. The injection stage is quite important as different defects may occur during injection that cannot be eliminated in the subsequent processes. Injection pressure and velocity are critical controlling parameters to manufacture a defect-free component. However, in most of the simulations, either injection pressure or injection velocity is taken as a constant value, while in reality, the injection molding of the piston moves at different speeds, and injection pressure and velocity change accordingly. In the experimental part of the present work, the piston movement is tracked using a high-speed camera which was mimicked in mold filling simulation of an alumina feedstock, considering the Dynamic Mesh approach. The solution of the full Navier–Stokes equation in an Eulerian-multiphase framework is used to simulate the mold filling where the secondary phase is air that is displaced by moving melt front. Additionally, a scalar equation was used to account for the liquid fraction of the feedstock. It is further demonstrated that the proposed model provides insight into the fluid-dynamic aspect of the mold-filling process.

Sintering Behavior of Bi-Material Micro-Component of 17-4PH Stainless Steel and Yttria-Stabilized Zirconia Produced by Two-Component Micro-Powder Injection Molding Process.

In this research, we investigated the influence of the sintering temperature on the physical and mechanical properties of micro-sized bi-material components of 17-4PH stainless steel and 3 mol% yttria-stabilized zirconia fabricated using a two-component micro-powder injection molding (2C-μPIM) process. First, 17-4PH and zirconia powders were separately mixed with binders to obtain feedstocks, which were then injection-molded into the dumbbell shape, followed by the binder extraction process. Subsequently, the debound micro-specimens were subjected to sintering between 1250 °C and 1350 °C for 3 h. Per the observations of the microstructures using scanning electron microscopy (SEM), a strong bond between metal and ceramic in micro-sized 17-4PH/zirconia components was formed when the sintering temperature exceeded 1300 °C. The maximum relative density of 99% was achieved when the bi-material micro-part was sintered at 1350 °C. The linear shrinkage increased from 9.6% to 17.4% when the sintering temperature was increased from 1250 °C to 1350 °C. The highest hardness value of 1439.6 HV was achieved at 1350 °C along the bi-material bonding region. Moreover, a maximum tensile strength of 13.7 MPa was obtained at 1350 °C.

Bi-Material Micro-Part of Stainless Steel and Zirconia by Two-Component Micro-Powder Injection Molding: Rheological Properties and Solvent Debinding Behavior

From the micro-powder injection molding (μPIM) process, a two-component micro-powder injection molding (2C-μPIM) process has evolved due to the growing demand for multi-functional micro-components in avant-garde applications. 2C-μPIM technology provides the opportunity to conjugate distinct materials within one part. Stainless steel (SS 17-4PH) and 3 mol.% yttria-stabilized zirconia (3YSZ) are characteristically recognized for their corrosion resistance and high hardness. In this work, the obtained critical powder volume concentration (CPVC) of SS 17-4PH and 3YSZ powders were 71.7 and 47.1 vol.%, respectively. Solid loadings of 2 and 3 vol.% less than the CPVC were considered as the optimal content for both powders. Feedstocks were obtained by mixing SS 17-4PH and 3YSZ powders with a binder system comprised of palm stearin (PS) and low-density polyethylene (LDPE). The rheological behaviors of the prepared feedstocks were assessed to figure out the feedstocks having the best rheological properties. The feedstocks of SS 17-4PH and 3YSZ with powder loadings of 69 and 44 vol.% were eventually injected to produce bi-material micro-parts. The optimal solvent debinding temperature of the green bi-material micro-part was then investigated, and it was found that 73.3% soluble binder was removed when bi-material was immersed in acetone at 70 °C for 40 min.

Effects of Nanopowder Addition on Rheological Properties of Feedstock for Micropowder Injection Moulding Process

Micropowder injection moulding (μPIM) is an ideal alternative for miniaturization of parts because of its ability to produce complex micro-geometries at low manufacturing cost. Utilizing bimodal powder mixtures in feedstock can increase packing density and densification of parts fabricated via μPIM. This study investigated the feedstock consists of nano-micro 316L stainless steel powder-and polyethylene glycol-based binder system. The flowability of different feedstocks was evaluated using rheological parameters, including critical powder volume concentration (CPVC), melt viscosity, activation energy, and rheological index. Results showed that mixing nanopowders with micropowders increases CPVC from 67.66 vol.% (pure micropowder) to 78.33 vol.%. The nano-micropowder feedstock showed viscosity below 45 Pa·s and shear rate in the range of 102 s−1 to 105 s−1, which are suitable for the μPIM process. The determined flow index values ranged from 0.25 to 0.76, and the reduced n values at high temperature with the addition of nanopowder indicated a possible increase in shear-thinning behavior. A defect-free microsample was obtained at an injection temperature of 85°C with sintering at 1200°C. Using the nano–micro bimodal powders, the hardness of the obtained samples also increased from 182 HV to 221 HV, with strength of 501 MPa, which is higher than that of the sample obtained with the use of micropowder only (435 MPa).

Rheological study of alumina feedstock for a micro-powder injection moulding application

Rheological analysis of an alumina feedstock is carried out to understand its flow behaviour. The alumina powder is mixed with indigenous binder system (combination of low-density polyethylene and paraffin wax) to prepare the feedstock. The prepared feedstock behaves like shear thinning material. Its temperature and time-dependent rheological character are analysed. Structural stability and spontaneous change of viscosity of the feedstock are checked during the shear jump. Effects of powder size and powder loading on the feedstock rheology are also tested. Injection of micro-channels is carried out with the prepared alumina feedstock followed by successful debinding and sintering. Around 97% of sintered density is achieved at 1540 °C sintering temperature with a soaking time of 30 min. The three-dimensional optical images confirm the shape retention of the micro-channel samples. These findings could be useful to choose the appropriate feedstock for different product development having micro-features.

Fabrication of Micro-Parts with High-Aspect Ratio Micro-Hole Array by Micro-Powder Injection Molding.

The present study investigated high-aspect ratio micro-hole array parts which were made by ZrO2 micro-powder with different particle sizes and micro-powder injection molding technology. It analysed the influence of particle sizes on feedstock, debinding and sintering of ceramic nozzles with multi-micro-holes. The forming quality of ceramic nozzles with multi-micro-holes was discussed in this paper. The results show that the two mixed ZrO2 feedstocks have fine uniformity. The average deviation of the feedstock made with 200 nm powder was −2%, and the average deviation of the feedstock made with 100 nm powder was −7.1%. The sample showed certain sintering characteristics which provided better strength (11.10 MPa) to parts after debinding. The linear shrinkage and the density of the two powder samples at different sintering temperatures increased as the sintering temperature increased. If the temperature continued to increase, the linear shrinkage and the density decreased. The highest hardness and flexural strength values of the ZrO2 sample with 200 nm powder used were: 1265.5 HV and 453.4 MPa, and the crystalline particle size was 0.36 μm. The highest hardness and flexural strength values of the ZrO2 sample with 100 nm powder used were: 1425.8 HV and 503.6 MPa, and the crystalline particle size was 0.18 μm. The ceramic nozzles with multi-micro holes shrunk to nearly the same axial, radial and circumferential directions during sintering. After sintering, the roundness of ceramic micro-hole met the user requirements, and the circular hole had a high parallelism in the axial direction. The micropore diameter was 450 ± 5 μm, and it was possible to control the dimensional accuracy within 1.5% after sintering. The study presented a superior application prospect for high-aspect ratio micro hole array parts in aerospace, electronics and biomedicine.

Developments in micro- and nano-defects detection using bacterial cells

This paper describes improvements to the Nondestructive Testing (NDT) technique recently proposed, based on the use of bacterial cell suspensions to identify micro- and nano-surface defects. New bacterial strains were used with magnetic fields to improve bacteria mobility. Different materials and defect morphologies were tested, including nanoindentation defects, micro-powder injection moulding components and micro-laser welding. Nanoindentations with 0.6µm depth and 5.3µm side length were successfully detected. Bacterial cells allow identifying different topographic attributes of the surfaces, such as roughness. Cracks of about 0.5µm wide and 10µm depth in a reference test block Type 1 were successfully detected.

Fabrication of cemented tungsten carbide components by micro-powder injection moulding

Micro-powder injection moulding (micro-PIM) is an advanced net-shaping process for the fabrication of metal and ceramic complex micro-components. Cemented tungsten carbide (WC–Co) hardmetal is known for its high hardness and wear resistance in various applications. Micro-PIM is a new alternative manufacturing technique for hardmetal micro-parts. In this work, the fabrication of WC–Co components via a micro-PIM process was studied. A fine WC–10Co–0.8VC (wt.%) powder was mixed with a binder system consisting of paraffin wax, low density polyethylene and stearic acid. A micro-component was injected at low pressure using a semi-automatic injection moulding machine. The injection temperature was determined from the rheological investigation of the feedstock. The binder extraction was carried out in solvent and thermal debinding methods under an argon atmosphere. Thermal gravimetric analysis was used to confirm the removal of the soluble binder from the green part. The sintering process has been performed within a temperature range of 1330–1450°C under vacuum. After sintering, a density of 94.5% theoretical density was obtained, which is a reasonable value. The micro-components showed length shrinkage between 16 and 22% and good surface quality and hardness values when compared with conventional powder metallurgy. This research shows that micro-PIM is able to produce small WC–Co components with properties comparable to conventional powder metallurgy.

Mechanical and functional properties of Invar alloy for μ-MIM

Micrometal injection moulding (μ-MIM) is a promising alternative for fabricating micro parts. Low coefficient of thermal expansion Fex–Ni1−x alloys are suitable to meet high dimensional stability with temperature keeping acceptable mechanical properties. However, these alloys are sensible of elements contamination during the debinding and sintering stages, and their dimensional stability and mechanical properties could be affected. The present work studies the effect of using combinations of debinding and sintering atmospheres in the micropowder injection moulding of Invar 36 feedstocks based on cellulose acetate butyrate binders. Microtensile specimens were successfully injected. Densification and microstructure at different conditions were linked with mechanical and physical properties of the parts. The processing conditions, and thus, the residual C, O and especially H influence severely these properties.

Creating movable interfaces by micro-powder injection moulding

This paper presents a novel in situ technique to produce articulated components with high-precision, micro-scale movable interfaces by micro-powder injection moulding (μPIM). The presented process route is based on the use of micro-scale sacrificial layer between the movable subcomponents which is eliminated during the debinding step, creating a dimensionally-controlled, micro-scale mobile interface. The fabrication technique combines the advantages of micro-powder overmoulding, catalytic debinding and sintering. The demonstrated example was a finger bone prosthesis joint consisting of two sub-components with an interface between components of 200μm in size. The geometries of the sub-components were designed such that they are inseparable throughout the process whilst allowing them to move relative to each other after the debinding stage. The components produced showed the feasibility of the process route to produce readily-assembled meso-, and potentially micro-, scale articulated systems.

Mixing Behavior and Flowing Property in Micro-Powder Injection Molding of Carbonyl Steel Feedstock

A feedstock consisting of carbonyl steel powder and multi-component binder was developed for powder injection molding (PIM), and the mixing behavior of feedstock was investigated under different powder loading, different temperature and different binder systems. The thermal and rheological properties were investigated. The results showed that the torque of feedstock could reach a steady state in s short time of 90 min when the loading powder was 52%. The torque of feedstock at 160°C was lower than that at 150 °C before the heating time was less 90 min. The torque of feedstock could reach steady state level in a short time of 90 min when the content of SA was 10%. SA served as surface-active agent that reduced the mixture viscosity and increased the solids content of the mixture by creating an interfacial bridge between the powder and the binder. The viscosity of feedstock decreased with increasing shear rate and temperature because the agglomerations between particles were eliminated owing to the movement.

Fabrication of hollow, 3D, micro-scale metallic structures by micro-powder injection moulding

Architectures of current micro-scale metallic structures are unable to meet some requirements of micro-component designers, particularly when a design calls for truly three-dimensional internal micro-geometries. Such complex geometries are particularly important for emerging microsystem applications such as high-performance microfluidics for analytical applications, or micro-engines for power generation. Here we report on a strategy by which miniaturized metallic structures with 3D cavities can be fabricated. We show results for a particular form of the strategy in which fabrication of sub-millimetre cavities with radii of 150μm is achieved in 316L stainless steel. Fabrication of the cavities is achieved by a method which uses micro-polymer insert moulding, micro-powder over-moulding, catalytic debinding and sintering. This work establishes for the first time the feasibility of micro-moulding based fabrication routes for the production of 3D internal geometries in miniaturized metallic structures.

Fabrication of ceramic micro-scale hollow components by micro-powder injection moulding

Rapid developments in microsystem technologies demand ceramic microcomponents of increasing geometrical complexity. State-of-the-art microfabrication routes of ceramics are either limited in geometrical complexity and/or high volume capabilities. This paper presents a process route by which ceramic microcomponents with relatively complex three-dimensional architectures could be realised by a high-volume technique. The proposed strategy, in which yttria-stabilised zirconia was implemented, combines the capabilities of insert-micromoulding, powder micro-overmoulding, catalytic debinding and sintering. The produced architectures demonstrate the capability of the technique to combine the high performance of ceramic materials with the dimensional accuracy and mass manufacturability of powder micromoulding.

REPRINT OF: High current liquid metal ion source using porous tungsten multiemitters

We recently developed an indium Liquid-Metal-Ion-Source that can emit currents from sub-μA up to several mA. It is based on a porous tungsten crown structure with 28 individual emitters, which is manufactured using Micro-Powder Injection Molding (μPIM) and electrochemical etching. The emitter combines the advantages of internal capillary feeding with excellent emission properties due to micron-size tips. Significant progress was made on the homogeneity of the emission over its current-voltage characteristic as well as on investigating its long-term stability. This LMIS seems very suitable for space propulsion as well as for micro/nano manufacturing applications with greatly increased milling/drilling speeds. This paper summarizes the latest developments on our porous multiemitters with respect to manufacturing, emission properties and long-term testing.

High current liquid metal ion source using porous tungsten multiemitters

We recently developed an indium Liquid-Metal-Ion-Source that can emit currents from sub-μA up to several mA. It is based on a porous tungsten crown structure with 28 individual emitters, which is manufactured using Micro-Powder Injection Molding (μPIM) and electrochemical etching. The emitter combines the advantages of internal capillary feeding with excellent emission properties due to micron-size tips. Significant progress was made on the homogeneity of the emission over its current–voltage characteristic as well as on investigating its long-term stability. This LMIS seems very suitable for space propulsion as well as for micro/nano manufacturing applications with greatly increased milling/drilling speeds. This paper summarizes the latest developments on our porous multiemitters with respect to manufacturing, emission properties and long-term testing.