Modeling of mass transfers in a porous green compact with two-component binder during thermal debinding

Mass transfers and phase changes of two-component binder in a porous green compact during thermal debinding process are modeled. The evaporation of low molecular weight (LMW) component and volatile fragments, the thermal degradation of high molecular weight (HMW) component, the capillary driven and pressure driven liquid phase transports, the binary diffusion in solutions, the convection and diffusion of gas phases, and the heat transfer in a porous medium are captured in the model. The model is validated with experimental data. The simulated results show that mass transfers during the early stage of thermal debinding are mainly due to capillary driven and pressure driven liquid transports. During the final stage of thermal debinding, both convective liquid and gas transports are important in binder removal. The developed model provides physical understanding of binder removal mechanisms that are essential for process optimization.     

Numerical and experimental investigations on thermal debinding of polymeric binder of powder injection molding compact

A mathematical model is established to describe the thermal debinding process of polymeric binder from a powder injection molding compact. The model takes into account of the thermal degradation of liquid polymer into liquid volatile fragment, the evaporation of liquid volatile fragment, the capillary driven liquid phase transport, the binary diffusion in solution, the convection and diffusion of gas phases, and the heat transfer in a porous medium. The proposed model is solved numerically based on a finite volume method and validated with experimental data. Based on the numerical results, the binder removal, the pressure buildup, the binder distribution, the mass transfers, and the removal mechanisms during thermal debinding are studied.     

Effect of microcrystalline wax on the solvent debinding of the Sr-ferrite ceramics prepared by powder injection molding

Powder injection molding (PIM) opens new possibilities to process complex Sr-ferrite components for magnetic applications. In the present study, new binder system with the addition of microcrystalline wax (MW) was used for the Sr-ferrite powder injection molding. The optimum binder composition was determined based on rheological measurement, mircrostructure observation by SEM, thermal change by DSC and debinding process. The results indicated that MW with 10 vol.% addition in the binder system containing high density polyethylene (HDPE), paraffin wax (PW) and stearic acid (SA) decreases the defects of the green parts after solvent debinding because the distribution of the binder system between the Sr-ferrite particles becomes more homogeneous. Using the proper binder system containing MW, the defect-free and dense Sr-ferrite ceramics can be prepared after solvent and thermal debinding and sintering. Based on the experimental results, the effects of MW microcrystalline wax on the solvent debinding of the Sr-ferrite ceramics were detailed discussed.     

Effects of the binder compositions on the homogeneity of ceramic injection molded compacts

In this study, the effects of different binder compositions on the homogeneity of injection molded ceramic compacts are discussed from the standpoint of the rheological properties of the feedstocks, density, density deviation, and flexural strength of the as-prepared compacts, and the microstructures of the as-prepared compacts, compacts after solvent debinding, and compacts after thermal debinding. Also the pore size distribution of the compact after thermal debinding was characterized to examine the distribution of the binders in the as-prepare compacts. The feedstock with 30% macromolecular binders attains the minimum power-law index n value at 160 °C, hence it is the most appropriate feedstock for injection molding. The optimal composition is feedstock B30, which is the most homogeneous at an injection temperature of 160 °C. The macromolecular binder-to-PW weight ratio of 3:7 allows best mixing of feedstock and more homogeneous binder distribution. B30 has the highest density, the lowest density deviation and the lowest deviations in flexural strength of the as-prepared compact. Compacts prepared with B30 have the most homogenous pore size distribution after thermal debinding, as determined from the sharp, narrow peaks on the pore size distribution curves.     

Phenomenological study and modelling of wick debinding

Wick debinding employs capillary suction (via a surrounding wicking powder) to remove the liquid binder phase from powder injection moulded parts (known as a compact). Experimental measurements of binder distribution within the compact during debinding highlight flaws in previous wick debinding models. The spatially uniform distribution of binder observed consistently during debinding indicates that it is removed in order of pore size regardless of location in the compact. A model is proposed which gives good agreement with 1-D experimental data of binder distribution. Key parameters of the model are the permeability of the wicking powder and the relationship between the capillary pressure, saturation and relative permeability of the compact.     

氧化锆陶瓷注射成型溶剂脱脂行为的研究

以氧化锆陶瓷粉体为研究对象,采用低毒性、低成本的煤油作为有机脱脂溶剂,研究了注射成型生坯的溶剂脱脂行为,探索了时间、温度、固相体积分数和粘结剂体系对脱脂速率的影响,分别讨论了各因素影响脱脂速率的机理.结果表明:脱脂速率随温度升高而升高,随时间的延长而降低;脱脂初期扩散是控制性环节,温度是影响反应速率的主要因素,脱脂后期溶解成为控制性环节,浓度差成为影响反应速率的主要因素;注射坯体固相体积分数越高,脱脂速率越慢,粘结剂最终脱除率越低;用部分植物油取代石蜡的蜡-油基复合型粘结剂注射坯体的脱脂速率快于传统蜡基型的注射坯体,最终脱脂率也高于传统型的蜡基注射坯体.     

Binder removal via a two-stage debinding process for ceramic injection molding parts

Debinding binders in two stages is critical to maintaining the shape of injected parts; the resulting decomposition affects the strength and rigidity of a structure. This study determines the optimal debinding process on the basis of a higher binder removal rate and the production of defect-free parts. The feedstock used was a combination of alumina–zirconia powder with a binder that consists of high-density polyethylene (HDPE), paraffin wax (PW), and stearic acid (SA). During the first stage, the injected parts were immersed in an n-heptane solution at 50 °C, 60 °C, 65 °C, and 70 °C to remove PW and SA. Binder weight loss was evaluated as a function of time. In the second stage, HDPE was removed by using thermal debinding. The results show that the optimum solvent debinding process runs for 16 h at 60 °C. The weight loss of the binder reaches 41.1% and results in the formation of defect-free parts. The binders are degraded at approximately 550 °C during thermal debinding. This degradation resulted in decomposition of nearly 96.9% of the binders. Low heating rates (1 °C/min to 2 °C/min) prevent defects from forming in the injected parts.     

Optimization of binder removal for ceramic microfabrication via polymer co-extrusion

The objective of this study is to assess the feasibility of solvent extraction (SE) for partial binder removal in the context of polymer co-extrusion with a thermoplastic binder component. Polymer co-extrusion is able to produce multilayered, functionally graded and/or textured structures in an efficient manufacturing process, but requires a polymer binder system with suitable flow characteristics. Traditionally, the binder is removed by thermal debinding (TD), which, however, is prone to form cracks or blisters, both of which are attributed to a lack of initial pore space that allows pyrolysis products to escape. The primary focus of this work is to demonstrate that a binder system with a high soluble binder content is suitable for conventional polymer co-extrusion and to document that a two-step binder removal process involving both SE and TD eliminates debinding defects. The overall fabrication process is documented for the extrusion of solid ceramic rods and co-extrusion of tubes, where alumina powder was batched with polyethylene butyl acrylate (PEBA) as backbone polymer and polyethylene glycol (PEG) as water soluble binder. SE for specimen with varying PEBA:PEG ratios was tested in water at three different temperatures for various times. The 1:1 mixture showed a PEG removal up to 80 wt.% of the original PEG content after 6 h extraction; after subsequent thermal debinding, rods and tubes sintered successfully without defects, demonstrating the viability of the process.     

Supercritical Carbon Dioxide debinding in metal injection molding (MIM) process

The conventional debinding process in metal injection molding (MIM) is critical, environmentally unfriendly and time consuming. On the other hand, supercritical debinding is thought to be an effective method appropriate for eliminating the aforementioned inconvenience in the prior art. In this paper, supercritical debinding is compared with the conventional wicking debinding process. The binder removal rates in supercritical CO₂ have been measured at 333.15 K, 348.15 K, and 358.15 K in the pressure range from 20 MPa to 28 MPa. After sintering, the surface of the silver bodies were observed by using SEM. When the supercritical CO₂ debinding was carried out at 348.15 K, all the paraffin wax (71 wt% of binder mixture) was removed in 2 hours under 28 MPa and in 2.5 hours under 25 MPa. We also studied the cosolvent effects on the binder removal rate in the supercritical CO₂ debinding. It was found that the addition of non-polar cosolvent (n-hexane) dramatically improves the binder removal rate (more than 2 times) for the paraffin wax-based binder system. This paper is dedicated to Dr. Youn Yong Lee on the occasion of his retirement from Korea Institute of Science and Technology.     

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

Al₂O₃-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.     

Theory and practice of rapid and safe thermal debinding in ceramic injection molding

In this paper, the dynamic process of thermal debinding was analyzed. The critical thickness of debinding was defined as the thickness at which the steps to control dynamics change from diffusion of gas products in liquid binder to diffusion or permeation in pores. Subsequently, the equation of critical thickness was deduced. The results show that the critical thickness of debinding is independent of the thickness of the green body, but mainly depends on the particle size, solid content, and binder composition. Moreover, larger particle diameter and higher solid loading will contribute to a greater critical thickness, which means that higher heating rate can be used in the initial stage of debinding. The prediction is in agreement with the experimental results. In addition, the effect of binder composition on the formation of cracks during thermal debinding was investigated by thermogravimetry combined with FTIR spectrometry (TG-FTIR) analysis of binders and thermogravimetric curves of feedstocks. It suggests that the feedstock containing polystyrene and acrylic resin is helpful for rapid and safe thermal debinding.     

Time-dependent properties of bimodal POM — Application in powder injection molding

Powder Injection Molding is applied for manufacturing complex and precise components from metal, ceramics or cemented carbide powder. It basically consists of mixing the powder and a polymeric binder, injecting this mixture in a mold with the desired form, debinding and then sintering. Among the debinding techniques applied, catalytic debinding of polyoxymethylene (POM) stands out due to the high debinding rates and low risk of cracking. In this work, the potential use of a bimodal POM-based material as the main binder component was evaluated by comparing its thermal and time-dependent properties to a standard monomodal POM. In addition, the potential optimization of this bimodal material was investigated by preparing five new formulations of bimodal POM with higher concentration of short polymeric chains (wax). This work has shown that macroscopic properties of POM are sensitive to the addition of these short chains. For instance, the magnitude of the complex viscosity for the commercial bimodal material was found to be more than 67% lower than for the monomodal POM in the whole range of frequencies studied. Finally, a new formulation of bimodal POM was suggested with 8% of added wax concentration (in weight), resulting in a material with very fine structure that has shown even better flowability than the commercial bimodal POM, without compromising its thermal and mechanical properties.     

Reaction moulding of metal and ceramic powders

A processing route for fabrication of products from hard metal and ceramic powders is described in which a reactive monomer such as ethylcyanoacrylate is used as carrier and binder with the opportunity of recovery through thermal depolymerization. A feasibility study has examined some of the main technical points including monomer–powder reactivity and rheology, debinding rates and compact green density. The stability of ethylcyanoacrylate with various powder volume fractions of silicon nitride and 316L stainless steel has been examined, with p-toluenesulphonic acid used as polymerization inhibitor, and it was found that mixes with higher powder volume fractions required higher levels of inhibitor. Assessment of the flow behaviour of several of the compositions showed them to be significantly non-Newtonian and the high viscosity of blends with a high loading of stainless steel suggests some premature polymerization. Measurement of green density of polymerized compacts demonstrated that high packing efficiency could be achieved. Thermogravimetry showed that rates of binder loss depended upon sample size and powder thermal conductivity.     

Water‐Soluble Binder System Based on Poly‐Methyl Methacrylate and Poly‐Ethylene Glycol for Injection Molding of Large‐Sized Ceramic Parts

Water debinding technique was applied in preparing injection molded large‐sized parts. The results showed that the origin of cracking and blistering defects was the swelling from polyethylene glycols of folded chain crystal. The combination of polyethylene glycols with molecular weight 600 and 4000 could ensure both debinding quality and green body strength. It was shown that higher than 20% of insoluble backbone binder was sufficient to resist the stress from swelling. For the compact with length sale of 3.14 mm (the ratio of volume to surface area), approximately 65% of polyethylene glycols were leached after 22 h at 40°C.     

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.     

Fabrication and characterization of silica ceramic compact prepared using Aloe-Vera mucilage as a binder

In this study, silica compacts were fabricated through a powder processing route at different compaction pressure, using Aloe-Vera (AV) mucilage as a binder. The silica compacts were prepared at 90, 100, and 110 MPa compaction pressure using 0%–16 wt% of AV binder. The optimum amount of AV binder was 14 wt% for both 90 and 100 MPa and 12 wt% for 110 MPa. The maximum achieved green density and green strength of silica compacts at the optimum binder amount were 62.3% and 4 MPa, respectively at 110 MPa compaction pressure. The green silica compacts prepared at 110 MPa compaction pressure exhibited a minimum porosity of 21% and maximum flexural strength of 15 MPa after sintering at 1400°C. The green silica compacts with the optimum amount of binder were strong enough for machining. The Fourier transform infrared spectroscopy analysis revealed the functional groups present in AV mucilage. The binder burnout characteristic of AV mucilage in the silica compact was determined by thermogravimetric analysis and differential thermal analysis. Additionally, AV gel acted as a binder and solvent simultaneously for ceramic compaction.     

陶瓷注射成型中粘结剂的热脱脂特性评价

脱脂是陶瓷注射成型工艺的制约性步骤.通过热分析的方法研究了PW+EVA+SA的热特性.通过计算的方法和实测的方法得到粘结剂整体的热失重曲线,发现两者较为接近,继而提出了一个定量评价粘结剂脱脂特性的物理参量--脱脂惯量.通过对脱脂惯量的计算发现PW78%+EVA21%+SA1%粘结剂体系具有最好的脱脂特性.     

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 internal binder on microstructure in compacts made from granules

This paper focuses on the influence of granule characteristics on the microstructure of the compact. Alumina granules were prepared with a poly-acryl acid (PAA) or a poly-vinyl alcohol (PVA) as binders by a spray-drying method. Observation with a liquid immersion method shows a significant difference in the binder distribution. Very uniform and non-uniform distributions were noted for the PAA and PVA binders, respectively. PVA binder segregated at surface and subsurface of the granules. The compression strength and deformation behavior were examined on a single granule with a micro-compression testing machine. The granule with the PAA binder has a low yield stress. In the forming process, the relative density of the compact body started to increase at low pressure. Homogeneous internal structures were noted in the green compact at all pressures examined. The granule with PVA binder showed higher yield stress. The internal structure of the compact was inhomogeneous, and large interstices were often observed between granules in the green compact also.     

Powder injection molding of complex-shaped aluminium nitride ceramic with high thermal conductivity

Aluminum nitride (AlN) is a promising material for heat sinks and microelectronic applications because of the advantages of high theoretical thermal conductivity, high mechanical strength, good electrical insulation, low dielectric constant and low thermal expansion coefficient. However, the difficulties in shaping complex-shaped parts with a high thermal conductivity have retarded the wide applications of AlN ceramic. Herein, we design a new binder system containing resin components and adopt the powder injection molding technology to fabricate complex-shaped AlN parts. After the debinding process, the special binder system would produce residual carbon, which could react with Al₂O₃ and result in decreasing oxygen impurity and forming the yttrium-rich aluminates. The yttrium-rich aluminates can accelerate the densification of AlN ceramic and fasten the oxygen on the triangular grain boundary, leaving the clean grain boundary beneficial for high thermal conductivity. The as-prepared AlN parts with complex shape possess a high thermal conductivity of 248 W m⁻¹ K⁻¹.