Low-pressure Injection Moulding Research Articles

A novel low-pressure injection molding technique for fabricating anode supported solid oxide fuel cells

A low pressure injection molding (LPIM) technique is successfully developed to fabricate porous NiO–YSZ anode substrates for cone-shaped tubular anode-supported solid oxide fuel cells (SOFCs). The porosity and microstructure of the anode samples prepared with different amount of pore formers are investigated through the Archimedes method and SEM analysis. Experimental results show that with 15 wt.% paraffin as plasticizer, porosity of the NiO–YSZ substrates sintered at 1400 °C is proportional to the amount of graphite as pore former, and proper porosities can be obtained with or without 5 wt.% graphite. NiO–YSZ/YSZ/LSM–YSZ single cells are assembled and tested to demonstrate the feasibility of the LPIM technique. At 800 °C, with moist hydrogen (75 ml min−1) as fuel and ambient air as oxidant, the cell with the anode substrate fabricated with 5 wt.% pore former shows a maximum power density of 531 mW cm−2, while the cell without any pore former, 491 mW cm−2. Two of the single cells (without graphite) are applied to assemble a two-cell-stack which gives an open circuit voltage of 1.75 V and a maximum output power of 5.32 W, at operating temperature of 800 °C.

Ceramic micro parts. Part 1: How thermal debinding can be utilized to enhance surface finish and mechanical properties

Thermal debinding represents the most critical processing step of powder injection moulding (PIM) of ceramics. Defects such as cracks and pores might be caused, when the process cannot be controlled properly. Considering low-pressure injection moulding (LPIM), however, thermal debinding opens up the possibility to enhance the mechanical properties of ceramic micro parts. In this study, the unique effect of surface defect healing is presented, which takes place during debinding. It enables improved surface finish and results in increased mechanical strength and reliability. As a model, 3Y-TZP micro bending bars with dimensions of 200 μm × 200 μm × 1200 μm were selected. It could be revealed that thermal debinding can be utilized to increase the characteristic 3-point-bending strength up to 3235 MPa with Weibull modulus of 21.4. This result corresponds to macroscopic bending strength of 1727 MPa, which can be achieved only by exhausting fabrication methods and surface post-processing.

Ceramic micro parts. Part 2: Process-related factors influencing surface finish and shape retention during thermal debinding

Thermal debinding opens up the chance to improve the surface finish of ceramic micro parts. This improvement can be ascribed to the unique effect of surface defect healing. It results in outstanding mechanical properties and was already presented in the first part of this paper. 3Y-TZP micro bending bars were selected as a model and fabricated by a modified option of low-pressure injection moulding (LPIM). The effect of surface defect healing, however, suffers from severe reproducibility of surface finish and shape retention. There are several process-related influencing factors, which have a great impact on the evolution of micro part properties during debinding. With this study, it is intended to give a comprehensive summary about the cause and effects of important process-related factors. Thereby, it is aimed to provide the required background in order to manage the fabrication process and to take advantage of the effect of surface defect healing.

Efeito do processamento em misturas de alumina/ligantes orgânicos usadas na moldagem por injeção em baixa pressão

A moldagem por injeção em baixa pressão (MIBP) é uma técnica que já vem sendo empregada na produção de peças cerâmicas com formas e geometrias complexas. A homogeneidade da mistura de ligantes orgânicos e pós cerâmicos é um fator determinante que deve ser controlado para minimizar a formação de imperfeições no processamento de feedstocks para MIBP. Defeitos típicos de processamento por MIBP, como bolhas de ar e aglomerados, geram gradientes de densidade nas misturas que, após conformação, possuem poucas possibilidades de remoção. Essas imperfeições comprometem o desempenho dos produtos obtidos por essa técnica. Este trabalho está focado na avaliação dessas heterogeneidades e como elas podem ser correlacionadas com a variação da densidade aparente e com o comportamento reológico dessas misturas. Para tanto, aluminas submicrométricas, como recebida e desaglomerada, foram adicionadas a uma mistura fundida de ligantes a base de parafinas, ceras e aditivos e processada em dois tipos diferentes de misturadores, com e sem o auxílio de vácuo. Foi observada a presença de aglomerados existentes na alumina como recebida, possivelmente gerados durante a etapa de calcinação. Também foi observado que o tipo de misturador e a aplicação ou não de vácuo durante a etapa final do processamento têm grande influência no tempo de mistura necessário para reduzir a viscosidade do feedstock para a injeção.

Impact of powder morphology on quality of low-pressure injection moulded reaction-bonded net shape oxide ceramics

Abstract Low-pressure injection moulding is a very efficient process for net shape manufacturing of ceramic micro parts. In order to obtain sintered ceramic specimens without shape distortion or damages, density gradients in the green bodies have to be avoided. Especially feedstocks with a solid loading near the critical powder volume content often tend to segregate the binder while flowing. However, the value of critical powder content can be significantly influenced by particle size, particle size distribution and particle morphology. This paper compares two powder mixtures of identical chemical compositions with different specific surfaces and morphology and evaluates their workability for low-pressure injection moulding. The aim of this paper is to identify the influence of morphology on feedstock rheology as well as on accuracy, mechanical properties and microstructure of net shape manufactured reaction-bonded zircon ceramics.

Low pressure injection moulding mass production technology of complex shape advanced ceramic components

This paper summarises and reviews the industrial manufacturing experience of ceramic components with custom designed complex shapes using low pressure injection moulding technology during a number of years. This technology is successfully used for manufacturing advanced ceramics with different compositions and for different applications. A production level is achieved with hundreds of pieces/day or week or higher. The major principles of this technology are reviewed based on the extensive processing experience. Some processing features, which affect the quality of ceramics and processing yield, are pointed out, particularly for industrial processing. Semi- and automated equipment for low pressure injection moulding technology providing high productivity are described.

Influence of Sintering Temperature on the Mechanical Properties of Alumina Springs

Low‐pressure injection molding (LPIM) was used to produce helical alumina springs. Three sets of springs were sintered at 1550°C, 1600°C and 1650°C to observe the effects on spring constant and fracture stress. Sintered alumina springs were obtained with densities ranging from 94.0% to 97.5% of the theoretical limit. Spring constants were measured from room temperature up to 1100°C. Fracture stress data were analyzed according to Weibull statistics and the method of maximum likelihood. Upon increase of sintering temperature from 1550°C to 1650°C, the spring constant and the Weibull characteristic strength of the alumina springs increases by 15% and 32%, respectively. On the other hand, sintering temperature has a negligible influence on Weibull's modulus. This is because internal bubbles and surface defects introduced in the production stage of the ceramic springs — more than the reduction in porosity with increasing sintering temperature — are critical in determining the reliability of the ceramic springs.

Prototype tooling for producing small series of polymer microparts

Layer-based manufacture of three-dimensional functional models and concept prototypes play a crucial role in the early assessment and verification of product designs. Any change of design, which is frequently needed during product development, causes the development costs to rise significantly. However, there is a limited set of materials available for layer-based manufacturing, especially for producing polymer microcomponents. Therefore, for fabricating small batches of microcomponents in the required material it is necessary to develop rapid tooling process chains, e.g. for low-pressure injection moulding. This paper investigates the moulding performance of µSL inserts as a function of tool geometry in combination with the effects of process factors. Condition monitoring techniques applied to the µ-IM process have been carried out to provide information that can be used to optimize the process. It was found that small batches of parts with fine details and relatively high aspect ratio structures can be produced and that reduced pressure loads and in particular the decrease of the µSL inserts layer thickness can extend the tool life.

Electrochemical Performance of Cone-Shaped Tubular Anode Supported Solid Oxide Fuel Cells Fabricated by Low-Pressure Injection Moulding Technique

Low-pressure injection moulding (LPIM) technique was successfully developed to fabricate porous NiO-YSZ anode substrates for cone-shaped tubular anode-supported solid oxide fuel cells. A single cell, NiO-YSZ / YSZ/ LSM-YSZ was assembled and tested to demonstrate the feasibility of the technique applied. The single cell provided a maximum power density of 795 mW/cm2 at 850oC with the effective area of the cathode 1.21cm2, using moist hydrogen(75ml/min) as fuel and ambient air as oxidant. The observed open-circuit voltages (OCV) was closed to the theoretical value and the scanning electron microscope (SEM) results revealed that the microstructures of the anode substrate and the cathode layer were porous and the electrolyte film was dense.

Electrochemical Performance of Cone-Shaped Tubular Anode Supported Solid Oxide Fuel Cells Fabricated by Low-Pressure Injection Moulding Technique

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Powder injection moulding of metallic and ceramic micro parts

Industrial use of micro components is determined by the availability of efficient manufacturing techniques. While micro injection moulding of plastic is common practice, metal and ceramic powder injection moulding (PIM) still is under development. High-pressure and low-pressure injection moulding methods complement each other ideally, covering the entire spectrum from prototype to large-scale production. With high-pressure PIM, micro gear wheels with diameters <300 μm can be fabricated using LIGA mould inserts. Densities between 97 and 99% of the theoretical values are achieved. Apart from oxide ceramics, metal materials like copper or powder metallurgical steels like 17-4PH or 316L are often applied. Multi-component injection moulding requires less mounting steps and, hence, offers decisive advantages for effective production of interesting material combinations like electrically conductive/insulating or hard/ductile. Studies relating to the fabrication of immobile as well as mobile shaft-wheel components were performed. Other activities focussed on in-mould labelling with foils containing ultra-fine particles to improve surface quality and detail accuracy. Low-pressure injection moulding allows for the manufacture of small series within 1–4 weeks at low cost. However, the process has features which are not compatible with high-pressure PIM. Although use of a low-viscous feedstock is associated with various benefits, low-pressure injection moulding has not met with acceptance in micro moulding.

Partial wick-debinding of low-pressure powder injection-moulded ceramic parts

Al 2O 3-based green bodies were shaped using low-pressure injection moulding. The binder content and the binder distribution during the thermal debinding inside a wicking embedment were analyzed. A distinct trailing front, which separates the binder-lean and binder-rich regions, was observed. This kind of binder distribution forms suddenly, after the moulded piece is heated above the melting point of the binder and is then cooled down. Mechanisms that can explain the observations are presented. The non-uniform binder distribution is explained by a capillary extraction of the binder with two different mobilities, which depend on the size of the pores inside the moulded piece. A sudden loss of binder at the beginning of the debinding process is the result of exudation, caused by a large thermal expansion of the binder as it melts. During cooling, the binder solidifies, which significantly affects the binder distribution due to a contraction of the binder.

Influence of dispersant, storage time and temperature on the rheological properties of zirconia–paraffin feedstocks for LPIM

The dependence of the rheological properties of zirconia–paraffin feedstocks for low-pressure injection moulding (LPIM) on binder composition, storage time and temperature was investigated. Feedstocks with varying amounts of dispersant were fabricated to work out the required quantity for a monolayer of dispersant molecules on zirconia particles. Experimental results revealed that a shear rate dependent characteristic of the viscosity against the amount of dispersant exists. The observations were compared with calculated values according to an adsorption model, which overestimated the required quantity of dispersant to form a monolayer. Feedstocks stored at elevated temperature for several days exhibited a time-dependent decrease of the yield stress and viscosity, which is supposed to be caused by physical or chemical interactions among zirconia and the dispersant. Increasing working temperature and decreasing solids loading were found to significantly decrease the yield stress as well as the viscosity. Flow activation energies and flow indices were calculated and compared with literature. This study shows that the dispersant used in this investigation has a remarkable influence especially on the time-dependent flow behaviour of zirconia–paraffin feedstocks that affects further processing and reproducibility.

Osseointegration of zirconia and titanium dental implants: a histological and histomorphometrical study in the maxilla of pigs

The purpose of the present study was to histologically compare the bone tissue responses to surface-modified zirconia and titanium implants. Threaded zirconia implants were produced using a new low-pressure injection moulding technique and thereafter surface treated by acid etching. Titanium implants with the exact shape and surface treated by sandblasting and acid etching (SLA) served as controls. Fifteen adult pigs received both implant types in the maxilla 6 months after extraction of the second and third incisors. The animals were sacrificed after 4, 8 and 12 weeks and 30 implants with surrounding bone were retrieved. Histological evaluation showed osseous integration for both materials. Zirconia implants revealed mean peri-implant bone density values of 42.3% (SD +/- 14.5) at 4 weeks, 52.6% (SD +/- 5.7) at 8 weeks and 54.6% (SD +/- 11.5) at 12 weeks after implantation, whereas Ti-SLA implants demonstrated mean values of 29% (SD +/- 10), 44.1% (SD +/- 18) and 51.6% (SD +/- 8.6) at corresponding time intervals. With respect to the bone-implant contact ratio, the mean values for zirconia ranged between 27.1% (SD +/- 3.5) and 51.1% (SD +/- 12.4) and for Ti-SLA, it ranged between 23.5% (SD +/- 7.5) and 58.5% (SD +/- 11.4). For the parameters investigated, no statistically significant differences between both types of implants could be detected at any time point. No statistical difference between implants could be demonstrated with any of the methods used. The limited number of animals per group, however, does not allow to conclude that there is no difference in osseointegration between the two types of implants, although the data tend to suggest such a trend.

Optimization of feedstock properties for reaction-bonded net-shape zircon ceramics by design of experiments

For the manufacturing of reaction-bonded ceramic microparts by low-pressure injection moulding, feedstocks with an optimized rheological behaviour are required. In order to evaluate the main influences on the rheological properties of feedstocks and their possible interactions, various compositions were systematically tested in the frame of design of experiments (DoE). For this purpose, ZrSi 2, ZrO 2, Al 2O 3, and MgO, and two different paraffins were used as starting materials. The influences of powder volume content, Zr/Si ratio, binder composition and processing temperature on the flowability of the feedstocks were observed in this line of experiments. A four-factorial fully fractional CCC-model was used. Finally the reliability of the computed statistical model was experimentally verified by means of two compositions, whose rheological behaviour has been predicted by the software.

Manufacturing of Net-Shape Reaction-Bonded Ceramic Microparts by Low-Pressure Injection Molding

Manufacturing of Net-Shape Reaction-Bonded Ceramic Microparts by Low-Pressure Injection Molding

Rheological properties of alumina feedstocks for the low-pressure injection moulding process

The low-pressure injection moulding process (LPIM) allows near-net shape manufacturing of ceramic microcomponents. Feedstocks used in this process have to fulfil different requirements. They have to show good flowability for the machining but they also need to have high solids content to reduce shrinkage during sintering. This work focuses on the flow behaviour of alumina feedstocks with high solids content, based on two different binder systems and two different alumina powders. By varying particle size, organic binder system, and solids content, the rheological properties of the feedstocks were modified significantly. The material parameters such as viscosity, the derived relative viscosity, and yield stress were investigated systematically. The Herschel–Bulkley model turned out to be suitable for the description of the rheological behaviour of these ceramic feedstocks. The dependency of the relative viscosity on solids content could be well described by models like Krieger–Dougherty, the Eilers, and the Quemada models, while the best results for the maximum packing factors were obtained with the Janardhana Reddy et al. model.

Apparent viscosity prediction of alumina–paraffin suspensions using artificial neural networks

A neural network model has been developed for the prediction of apparent viscosity of alumina–paraffin suspensions used in low-pressure injection moulding (LPIM) process. The model is based on a three-layer neural network with a backpropagation-learning algorithm. The training data were collected by the rotational viscometry followed by a nonlinear regression. The network is trained to predict the values of power-law model parameters suitable to describe non-Newtonian fluids. A comparison between experimental values and those predicted by the neural network shows a good coincidence. The approach helps to reduce the amount of experiments required to determine these constants in practice.

A New Dewaxing Method and its Effect on the Properties of Low-Pressure Injection Molded Ceramics

A new dewaxing method for low-pressure injection molded ceramics is presented. Supercritical extraction with carbon dioxide was used to remove paraffin wax from the ceramic green parts. The composition of organic additives for low-pressure injection molding feedstock and the extraction condition for the green parts were investigated. Moreover, the properties of sintered ceramic samples dewaxed by supercritical carbon dioxide were compared with those by thermal dewaxing. The results show that the new binder system containing 50wt% paraffin wax, 35% bee wax and 15% stearic acid fulfills the requirements of both low-pressure injection molding feedstocks and supercritical dewaxing, where the feedstock has high fluidity, low viscosity and quick solidification. The efficient extraction condition for supercritical dewaxing from the green parts is at 30MPa pressure and 45°C. Under this condition, defect free ceramic green parts can be obtained. Dewaxing methods have significant influence on the properties of sintered parts. The mechanical properties of the sintered sample can be improved by supercritical dewaxing. With this method, the bending strength of sintered samples (σ = 331.6 MPa) is higher than that obtained by thermal treatment (σ = 312.3MPa). The sintered samples dewaxed by supercritical CO2 have shown the property of higher density and less distortion compared to the thermal dewaxing method. Moreover, with supercritical extraction the dewaxing time can be reduced to about one tenth of the time required by thermal dewaxing.

Low-Pressure Injection Molding Processing of AISI T15 High Speed Steel Powders

Low-pressure powder injection molding was used to obtain AISI T15 high speed steel parts. The binders used were based on paraffin wax, low density polyethylene and stearic acid. The metals powders were characterized in terms of morphology, particle size distribution. The mixture was injected in the shape of square bar specimens to evaluate the performance of the injection in the green state, and then sintered. The samples were injected under the pressures of 0.4, 0.5 and 0.7MPa and at temperatures varying from 110 to 150°C aiming the optimization of the process. The results of the variation of injection pressure were evaluated by measuring the density of the green parts. Debinding was carried out in two steps: first, the molded part was immersed in heptane to remove the major component of the binder and then heated to remove the remaining binder. A second step debinding and sintering were performed in a single step. This procedure shortened considerably the debinding and sintering time.