Powder-binder separation in injection moulded green parts

For powder injection moulding (PIM) the ceramic powder is mixed with a thermoplastic binder system to achieve an injectable feedstock. In contrast to injection moulding of polymeric components, the binder must be removed after the shaping step before sintering the ceramic part to full density. During the mould filling process shear forces act on the blend that might cause separations of powder particles and binder. In this case polymer films form at the mould surface and at internal interfaces which induce microstructural defects in the debinded part. In particular for multi-component parts this effect is critical since binder films in the joining zone weaken the bonding strength between the two components that might even lead to delamination.For detecting binder separations within the injection moulded bulk material and at joining zones of two-component parts the microstructure of green samples has been studied. Since conventional machining techniques like grinding and polishing modify the original structure, e.g. when particles are pulled out of the matrix and binder smears onto the surface, a special ceramographic method for the preparation of cross-sections was applied. This approach bases on broad ion beam techniques and enables the simultaneous polishing of hard ceramic particles and soft polymer molecules without destroying the structure or producing a relief at the surface. In the analysed samples binder accumulations were found along flow lines, at weld lines, at boundaries of so-called dead water regions and at the interface of two-component parts.     

Kinetics and equilibrium of Eco-debinding of PZT ceramics shaped by thermoplastic extrusion

Thermoplastic extrusion can present an alternative shaping process to a well-established powder pressing or tape casting method for lead zirconate titanate (PZT) ceramic sensor structures, if a complex debinding step can be achieved in an economic way.In this study PZT ceramic parts were made by thermoplastic extrusion using ethylene vinyl acetate and paraffin as a binder. Samples were thermally debinded according to different programs with and without a powder bed. A modified second order decomposition kinetics model is proposed which is especially suitable for describing partial debinding. Also a novel simplified model for wick-debinding is proposed to evaluate the contribution of a capillary extraction mechanism.Results show that isothermal debinding leads to partial debinded parts with a well-defined equilibrium residual binder, which is a function of a time-temperature program. We demonstrate that 90% of the binder can be removed at only 200 °C, even for a multicomponent binder system.     

Effect of binder on the structure and mechanical properties of lightweight bubble alumina ceramic

The effect of binders such as ammonium aluminum sulphate, phosphoric acid and composite binder on the properties of lightweight bubble alumina ceramic was studied. The composite binder was composed of ammonium aluminum sulphate and phosphoric acid. Ammonium aluminum sulphate solution can improve compressive strength of alumina bubbles effectively but can not improve that of lightweight bubble alumina ceramic due to the fewer nano-alumina powders in situ decomposed of ammonium alumina sulphate. Trans-ball fractures occurred in thermal shock test. Phosphoric acid solution can improve compressive strength of alumina bubble ceramic because of promoting sintering properties of aluminum phosphate in situ produced by phosphoric acid and alumina component during sintering but decrease that of alumina bubbles. Along-ball fractures occurred in thermal shock test. The composite binder combined with the advantages of ammonium alumina sulphate and phosphoric acid and improved the compressive strength of both alumina bubbles and lightweight bubble alumina ceramic, and effectively reduce the amount of the binders and lower the product cost. At the sintering temperature of 1700 °C, with composite ammonium alumina sulphate and phosphoric acid as binder, the density of lightweight bubble alumina ceramic was between 1.20 and 1.60 g/cm³, and the compressive strength was 18-42 MPa.     

Nonreinforcing filler–elastomer systems. I. Experiments based on model systems

Five methods were modified to test for interactions between binder and filler of composite solid propellants. Methods based on uncured binder or model compounds were rate of solution of binder from composite mixtures; centrifugal separation of binder from the filler; of composite mixtures; measurement of the contact angle between binder and filler; and adsorption of binder or model compounds by filler from solution. Stress–strain–birefringence was measured on cured binders containing small amounts of filler. In addition to the polymers used as binders and the model compounds, three physical forms of aluminum, and the additives tris [1-methylaziridinyl] phosphine oxide (MAPO) and a polysebacate of methyl-N-diethanolamine were included in the study. The filler was ammonium perchlorate in all experiments. The polysebacate was an effective adhesion improver in the polyurethane–ammonium perchlorate composite. MAPO was not as effective in the system polybutadiene–ammonium perchlorate. Fibrous forms of aluminum result in a weaker propellant then does powdered aluminum. Nonfunctionally terminated polymers were poorer in adhesion, contact angel, and adsorption tests compared with carboxyl- and hydroxyl-terminated types. The nonpolymeric model compounds yielded inconclusive data.     

Scaling Analysis of the Effect of Binder Content and Binder Distribution on the Gas Permeability of Porous Green Ceramics

Two models have been developed to describe how variations in binder content and binder distribution influence the specific surface and the gas permeability of porous media. In the Core-Shell model, a shell of binder surrounds each ceramic particle, and the specific surface increases with decreasing volume fraction of binder. In the Multi-Sphere model, binder particles occupy the interstices between the ceramic particles, and the specific surface generally decreases with decreasing volume fraction of binder. The variation in the permeability with binder content predicted by each model is different as compared with the case of a constant specific surface.     

Vibrational molding of ceramic concretes. Molding systems and the regularities of the process

Conclusions We developed a vibrational molding process for obtaining ceramic concretes using rigid mixtures and metallic molds. The as-formed semifinished products are characterized by a fairly high mechanical strength. A siliceous (silica-based) ceramic concrete having a porosity of 12–16% and an ultimate compressive strength of 20–25 N/mm² was obtained. The mechanism of structure evolution of the semifinished products was identified; it is determined by the interaction between the phases and fixing (orientation) of the liquid phase of the highly concentrated binder suspensions due to the molecular forces of the filler grains.The decisive effect of the binder on the mechanical properties of the ceramic concretes was established. On decreasing the binder content of the ceramic concretes from 39 to 12%, their specific strength increases by 4 times.Translated from Ogneupory, No. 6, pp. 8–14, June, 1993.     

工业纯铜陶瓷/渗铝复合涂层的制备及耐蚀性

以相同的陶瓷骨料(由TiO2、A12O3和Zn组成),在纯铜基材上分别制备热化学反应法陶瓷涂层和热化学反应陶瓷/渗铝复合涂层,通过X射线衍射和扫描电镜研究了2种涂层的组成和结构,并比较了两者的性能差异.结果表明,热化学反应陶瓷/渗铝复合涂层在热固化过程中除了产生机械和化学结合外,还产生了冶金结合,其致密性、结合强度均较...     

Binder stabilization and rheology optimization for vat-photopolymerization 3D printing of silica-based ceramic mixtures

Enhancing inlet gas temperature in aero/gas turbines to reduce their carbon-footprint, has led to a strive for better performing inlet cooling mechanism of the turbine blades. The internal cooling of the blades is made by ceramic cores in their casting process, but conventional ceramic molding has long reached its maximum possible geometrical complexity, hence shedding light on 3D printing of these cores. The objective of this study is to develop low-viscous, fully stabilized, commercially viable ink for vat-photopolymerization of silica-based ceramics. This paper investigates the best dispersion type and amount for different formulated monomer mixtures, and explains the best correlation between viscosity, solid loading, binders, dispersants, peeling forces and mechanical properties, and offers an optimized mixture to avoid the common ceramic printing issue, namely crack propagation of cores during sintering. Among five dispersant agents, the SOL20, SOL24 and FA4611 exhibited better performance than other dispersion agents, and the optimum concentration level for each binder and dispersant agent was ensured through sedimentation test. Their dispersion capability and long-term stability were further investigated to designate the best dispersion agent for each binder system. Further verification was made by sedimentation study of the samples at 40 °C for 40 days and reducing the superficial area of the used powder mixture. According to the result of the rheology analysis, the best dispersions were achieved using SOL20 for the loaded binder mixtures of M1 and M4, SOL24 for M3and FA4611 for M2. The instability of M1 and M2 with their respective dispersant agent was coordinated through the thixotropic agent of TX/2, and complete stabilization and near-Newtonian behavior were achieved. However, the research showed that the addition of TX/2 to fully stabilized M4 and M2 suspensions negatively impacts the mixtures’ rheological behavior from near-Newtonian to shear-thickening. In the final stage of this study, peeling forces, sintering and three-point bending tests were conducted to determine the final formulated suspension to print ceramic core components. M4 and SOL20 combination was selected for SiO₂-ZrSiO₄ loading and dispersing, respectively. The impact of solid loading between the range of 58 and 65 vol% on the rheological behavior of the final suspension and the mechanical properties of sintered bodies were investigated to assign an optimum solid rate. The adequate strength on sintered and degree of viscosity for ceramic vat-polymerization processing was achieved at 58 vol%. Lastly, a validation study is conducted by printing a complex ceramic core model by a commercial LCD hobby printer. This validation shows the significance of this study to scale up the manufacturing of complex-shaped ceramic cores and to revolutionize the sector, by printing inexpensive and readily available irregular-shaped (non-atomized) ceramic powder, using the most cost-effective LCD printers (non-specialized expensive ceramic printers).     

Effect of porosity on the elastic properties of porcelainized stoneware tiles by a multi-layered model

Porcelainized stoneware represents a leading product in the world market of ceramic tiles, thanks to its relevant bending strength (with respect to other classes of tiles) and extremely low water absorption: these properties derive from its really low content of residual porosity. Nevertheless, an accurate investigation of the cross section of a porcelainized stoneware tile reveals a non-uniform distribution of the residual pores through the thickness, which results in a spatial gradient of properties. Porcelainized stoneware, therefore, may be looked at as a functionally graded material. In the present research, commercial porcelainized stonewares were analysed in order to define the effect of the residual porosity and its spatial distribution on the mechanical properties of tiles. Polished cross sections of porcelainized stoneware tiles were investigated by optical and scanning electron microscopy in order to define the content and distribution of residual pores as a function of distance from the working surface. For each porcelainized stoneware, the local elastic properties of the ceramic matrix were measured by a depth-sensing Vickers micro-indentation technique, then the so-obtained microstructural images and elastic properties were used to model the stoneware tile mechanical properties. In particular, the cross section of each tile was described as a multi-layered system, each layer of which was considered as a composite material formed by a ceramic matrix and residual pores. The elastic properties of each layer were predicted by applying analytical equations derived from the theory of composite materials and, as a new approach, by performing microstructure-based finite element simulations. In order to validate the proposed multi-layered model and identify the most reliable predictive technique, the numerical results were compared with experimental data obtained by a resonance-based method.     

Extraction of binders from green ceramic bodies by supercritical fluid: influence of the porosity

Organic additives can be extracted from extruded or injection-moulded ceramic parts by using supercritical fluids leading to defect free green parts in a short time. A model previously defined, that predicts kinetics of extraction of solid binder molecules by solubilization and diffusion of solubilized species has been modified with a corrective term taking into account the capillary migration of a liquid organic phase according to the microstructure (pore size distribution, tortuosity) of the green sample and to the characteristics of the liquid phase (viscosity, surface tension). An analytical expression of this corrective term is proposed, for a paraffin binder and for three alkanes which are representative of its molecular distribution. The corrective term is related to the length scale over which the liquid binder flow occurs and to its linear velocity. The modified model allows the prediction of the kinetics of extraction of a given paraffin binder, by using the effective diffusion coefficient of the binder, determined with large pore size green samples and a capillary factor, calculated from binder and microstructure characteristics of the green part.     

Extrusion of Metal–Ceramic Composite Pipes

Metal–ceramic composite pipes were prepared through simultaneous extrusion of different pastes by a multi-billet extrusion method. ZrO₂ and stainless steel powders were chosen, and an aqueous solution of water-soluble polymer, hydroxypropyl methylcellulose (HPMC), was used as binder. The maximum extrusion pressure and the minimum amount of binder required reached their lowest values when the mixing fraction of ZrO₂ powder was 0.4. The minimum amount of binder for forming the outer layers was 4%–5% higher than that for inner layers, even for the same powder. It was possible to decrease the binder content and broaden the extrudable range of the binder content by means of mixing coarse and fine powders.     

Performance modulation and 3D printing parameters optimization of implantable medical tricalcium-silicate/polyetherimide composite

Tricalcium silicate (C3S)/polyetherimide (PEI) stents are manufactured through an additive manufacturing process using binder jetting. The key issues of C3S/PEI composite ceramic slurry and additive manufacturing process parameters are discussed in detail. Firstly, the low-temperature auxiliary sintering temperature of the sample was determined, and the influence of PEI content on the compressive strength and bending strength before and after sintering was studied. The sintering temperature and optimal PEI content are 340 °C and 10 wt%. Under this PEI content, the flow rate change during the printing process of the slurry was measured, and a C3S/PEI composite slurry suitable for binder jetting additive manufacturing was obtained, and it had excellent mechanical properties. The effect of the parameters of the binder jetting additive manufacturing process on the molding quality of the C3S/10PEI composite ceramic slurry was studied. The effect of the printed layer height on the deposition line width and height was explored, resulting in a selection rule for the printing layer height using nozzle diameters. The influence of the number of layers of the printed sample on the height and line width of the sample is studied. Under the condition that the height of the printing layer is 80% of the nozzle diameter and the hot air assisted drying, the maximum error of the forming size is only 3.13%. Finally, the biocompatibility and cell adsorption effect of the scaffold were studied, and it was found that the C3S/PEI scaffold, which was additively manufactured by binder jetting and sintered at low temperature, had good biological properties.     

Local Flow Behavior of Ceramic Slurries in Tape Casting, as Investigated by Laser–Doppler Velocimetry

The local flow behavior in a tape-casting unit for processing ceramic slurries was investigated by laser–Doppler velocimetry (LDV). The most remarkable feature of the LDV system used was its high spatial resolution (24 μm) and temporal resolution (5 μs). A translucent Newtonian model fluid which, in the relevant processing window, at higher shear rates, exhibits a rheological behavior similar to that of a typical ceramic slurry, was chosen to determine the velocity distribution in the flow channel (i.e., in the tape-casting unit). The flow behavior was measured directly in several particular regions of the flow channel, which substantially consisted of a double doctor-blade assembly. The velocity distribution obtained below the forming doctor blade showed the strong influence of the casting speed on the flow behavior of the model fluid. Furthermore, the measured flow behavior was analyzed, based on the assumptions of fluid dynamics. Below the doctor blade, the flow behavior was considered to be a combination of pressure flow and drag (Couette) flow. The flow rates from the applied hydrostatic pressure and external drag forces were shown to be additive. A comparison between the measured velocity profiles and the model calculations showed excellent agreement. A flow reversal (secondary flow), never before measured quantitatively, was detected between the two inserted rectangular blades. Apparently, secondary flows influenced the surface properties of the cast green tapes.     

Strength of Green Ceramics with Low Binder Content

Acrylic-based polymers are common binders that impart high green strength (>2 MPa) at low concentrations (<5.0 vol%). Strength at low binder concentrations may be determined by chemical bonding at the ceramic–polymer interface. We have studied the binding mechanisms as a function of ceramic surface chemistry using a cross-linkable binder, which is based on a soluble poly(acrylic acid) (PAA, MW = 60 000) and glycerol. The cross-linked PAA binder system has been integrated into a solid freeform fabrication process, which provides a means of fabricating very reproducible green bodies, including SiO₂, TiO₂, Al₂O₃, multicomponent oxides, and non-oxides, with uniform density and composition. The ceramic parts contain only 2.5 vol% binder (solids basis), which increases the strength of the ceramic systems by at least a factor of 8 while the strength of Al₂O₃ components increases by a factor of ∼24 (0.3 to 7.6 MPa). Addition of the binder improves the toughness of the ceramic bodies by an order of magnitude with SiO₂ representing the largest relative increase (2.8 × 10⁻³ to 4.4 × 10⁻² MPa·m^1/2). The mechanical properties are dictated by two binding mechanisms: binder adsorption and mechanical interlocking. High green strengths result from adsorption of the binder onto the ceramic surface whereas toughness is enhanced by poor adhesion of the binder to the ceramic surface.     

粘结剂配比对干法制备ZrO2陶瓷轴承造粒粉性能的影响

为探究粘结剂中聚乙烯醇、聚乙二醇、羧甲基纤维素钠的配比对干法制备ZrO2陶瓷轴承造粒粉性能的影响,配制6份不同配比的粘结剂溶液,采用旋转流场式干法造粒机制备ZrO2陶瓷轴承造粒粉。搭建颗粒级配检测平台对造粒粉粒径分布进行分析,借助Carr流动性指数法分析造粒粉流动性。结果表明:聚乙烯醇对造粒粉颗粒级配及流动性影响最大,聚乙二醇次之,羧甲基纤维素钠最小。当粘结剂溶液中聚乙烯醇:聚乙二醇:羧甲基纤维素钠配比为3:6:1时,干法制备ZrO2陶瓷轴承造粒粉的有效颗粒占比为81.6%,流动性指数为81,造粒粉级配均匀、颗粒流动性好。     

2-D simulation of wick debinding for ceramic parts in close proximity

In wick debinding the binder phase is removed from an injection moulded ceramic part by the capillary action of a wicking powder. The binder is withdrawn in the liquid phase in the form of a front, and fronts originating from parts in close proximity may collide, thereby reducing the potential for the binder to be extracted by the capillarity of the wicking powder. The model presented by Somasundram et al. (2008) is employed here to simulate isothermal debinding of cylindrical parts located in close proximity, using the level-set method to track the progress of binder fronts. Two-dimensional cases were simulated using a commercial finite element solver and the results are compared with preliminary experimental results which show the potential of the model and demand further experimental investigation. A simplified model is also presented which encapsulates the main features of the detailed model for simple geometries.     

Thermoplastic Green Machining for the Fabrication of a Piezoelectric Ceramic/Polymer Composite with 2-2 Connectivity

A piezoelectric ceramic/polymer composite with 2-2 connectivity was fabricated by thermoplastic green machining. A thermoplastic body, consisting of 60 vol% lead zirconate titanate ceramic particles (PZT) and 40 vol% thermoplastic binders, was computer numeric controlled-machined, creating periodic channels in the green PZT body. Following thermal treatment (binder burnout and sintering), a 25 vol% array of 147 μm thin PZT slabs with an aspect ratio of seven separated by 442 μm channels was fabricated. The channels were infiltrated with epoxy resin, in order to fabricate the PZT/epoxy composite with 2-2 connectivity. This novel process was evaluated in terms of the machinability and sinterability of the thermoplastic PZT compound. Also, the electromechanical properties of the PZT/epoxy composite were measured.     

Compaction behaviour of agglomerated alumina powders

The effects of agglomerate properties, such as the binder type, binder content, moisture level, and agglomerate size, on a model compaction process was investigated by using green density-pressure interrelationships for a range of agglomerated alumina powders. The model compaction process involved single ended nominal uniaxial stress transmission in a cylindrical die. The influences of the sample aspect ratio, die wall lubrication, and compaction rate were also investigated. Two types of water soluble polymeric agents, a poly(vinyl alcohol) (PVA) and a poly(ethylene glycol) (PEG), were used. It was shown that certain agglomerate properties have a strong influence upon the compaction behaviour of these ceramic powders. The extent of the compaction is enhanced by using agglomerates with a low agglomerate yield point. In the PVA system, the agglomerate yield points decreased with increasing moisture content. The compaction behaviour of the agglomerates showed a rate dependency, that is, the compaction is retarded with increased pressing rate. The green densities of the compacts prepared in the unlubricated die were lower than those of the compacts prepared in the lubricated die due to the higher wall frictional forces operating in the unlubricated die.     

Guar gum: A novel binder for ceramic extrusion

Ceramic honeycomb extrusion is a technique capable of attaining high strength, porous ceramics. However, challenges prevent the realisation of its potential. These include the design of an intricate honeycomb die and the formulation of an extrudable paste. The present study addresses the latter by using guar gum (GG) as a binder. GG was rationally selected because hydrogels thereof exhibit strong shear-thinning and high stiffness properties, which are required for extrusion. Rheological analyses demonstrated ceramic pastes with similar qualities were achieved, with hydroxyapatite (HA) used as the model ceramic. The shear stiffness modulus of HA pastes was determined as 8.4 MPa with a yield stress of 1.1 kPa. Moreover, this was achieved with GG as the sole additive, which further facilitates the overall fabrication process. The binder extraction notably occurred at relatively low temperatures when other high molecular weight polymers demand temperatures above 1000 °C; therefore the latter precludes the use of ceramics with low sintering onset. The process culminated in a porous HA scaffold with similar porosity to that of a commercial HA graft, but with higher compressive strength. Lastly, the study notes that the biological and water-soluble properties of GG can broaden its application into other ceramic fabrication processes.     

The effect of primary particle surface energy on agglomeration rate in fluidised bed wet granulation

The effect of primary particle surface wettability by a binder solution on the rate of agglomeration in a fluid-bed top-spray granulation process was investigated. A model system consisting of hydrophilic and hydrophobic spherical primary particles with a narrow size distribution, and an aqueous solution of hydroxy propyl-cellulose (HPC) as binder, was used. The surface energy of the primary particles was measured by inverse gas chromatography (IGC) and their wettability was characterised by static and dynamic contact angle. Granulation was carried out in a desktop fluid-bed granulator and the resulting granule size distribution and granule microstructure were analysed. The hydrophobic particles gave a wider granule size distribution (larger maximum granule size) than hydrophilic ones under otherwise identical conditions, and the granules were notably rounder and more compact. However, the fraction of un-granulated fines was also higher in the case of hydrophobic primary particles. SEM analysis of granule microstructure revealed that the hydrophilic particles were coated by the binder solution, which left a smaller amount of binder available to form bonds at particle contacts. On the other hand, all of the binder was found to form solid bridges in the case of hydrophobic primary particles. A population balance model was used to explain the observed granulation behaviour.