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.     

金属注射成型专用粘结剂主体聚合物合成研究

对金属注射成型专用粘结剂的主体聚合物进行了研究 ,结果表明 ,用甲基丙烯酸甲酯与乙酸乙烯酯共聚 ,用偶氮二异丁腈作自由基引发剂 ,用硫醇作分子量调节剂可以获得较理想的粘结剂主体聚合物 ;并考察了各因素对聚合物产率和熔点的影响     

Gas-assisted powder injection molding: A study on the effect of processing variables on gas penetration

Parts of polypropylene and of a stainless steel powder feedstock were molded by means of gas-assisted injection molding in epoxy cavities made by stereolithography. The design of the experiment method using the Taguchi L₉ array was implemented to test the effect of gas pressure, gas delay time, shot size and melt temperature on gas penetration depth and residual wall thickness. Simulations were conducted and compared with direct experimentation. Simulation predicted that the shot size was the only significant factor when processing polypropylene and the powder metal feedstock. The experiment showed that shot size and gas delay time were significant when processing polypropylene; and shot size, gas pressure, and melt temperature were significant factors when processing the powder metal feedstock. The residual wall thickness could not be controlled by the processing variables used in this study as the S/N ratios calculated were very small.     

An analytical approach to relating powder characteristics and rheological properties of powder injection molding feedstocks

This study quantitatively analyzes the relationship between the material structure (powder characteristics) and flow properties of highly filled compounds. The model parameters are the powder particle-size distribution and its volume fraction in a compound. The simulated material structure is developed using the Hard Spheres Model, from which the structural functions in terms of the statistical distribution of inter-particle distances and the histograms of effective volumes of groups of close particles are calculated. A close similarity between simulated and real functions was obtained. The results obtained imply that numerical modeling might serve as a reliable and sensitive approach for analyzing the influence of powder particle-size distribution on the ability to process highly filled compounds.     

Fabrication of high-aspect-ratio micro piezoelectric array by powder injection molding

A powder injection molding process to fabricate a high-aspect-ratio piezoelectric microarray was developed. The reverse shaped sacrificial mold insert was developed by X-ray micromachining, and then insert-type injection molding was conducted to fabricate the piezoelectric microarray structure. For the micro-powder injection molding, rheological properties of the powder binder mixture were evaluated and analyzed. The measured flow behavior exponent was 0.44 and the flow activation energy was 83 kJ/mol⁻¹. Based on the analyzed rheological property, injection molding conditions were optimized. The rheological property and the injection molding conditions were crucial to ensure complete filling. Using the optimized conditions, two high-aspect-ratio piezoelectric microarrays were fabricated: (i) 20-μm patterns with 1:5 aspect ratio and (ii) 40-μm patterns with 1:10 aspect ratio.     

Rheological and thermal debinding behaviors of silicon nitride in powder injection molding

In powder injection molding process, it is important to analyze the rheological and thermal debinding behaviors of feedstock, because they can directly affect the final quality of products. Therefore, for the silicon nitride based feedstocks, the rheological and thermal debinding behaviors were investigated and compared between feedstocks prepared with the combination of two types of powders and sintering aid ratios. At first, the optimal solids loading for each feedstock was determined based on the results of the torque rheometer experiment. The viscosity of the feedstocks was measured using the capillary rheometer, and rheological properties were evaluated with the Power Law-Arrhenius model. Silicon nitride (JPN) feedstock with 5 wt% yttria and 2 wt% alumina (JPN+5Y2A) had the highest moldability index among all feedstocks. The powders with sintering aid ratio of 5 wt% yttria and 2 wt% alumina made the feedstocks have a high moldability index. Thermogravimetric experiment was also performed to analyze the pyrolysis behavior of the feedstocks, and the apparent activation energies for each feedstock were obtained using Kissinger method with the thermogravimetric analysis results. Based on the results, the master decomposition curve (MDC) was developed, and the utility of MDC for optimizing the thermal debinding process was verified.     

Enhancing piezoelectricity of PLZT ceramics by bismuth stearate coating via water-soluble defatted powder injection molding

Lead zirconate titanate-based piezoelectric ceramics (Pb_0·91La_0·06(Zr_0·58Ti_0.42)_0.975Nb_0·02O₃, PLZT) were prepared by water-soluble defatted powder injection molding using bismuth stearate as surfactant. The effects of bismuth stearate surfactant amount on the feed viscosity, water degreasing rate and electrical properties were studied. The results showed that the bismuth stearate coating reduces agglomeration between the PLZT powders and decreases the viscosity of the feed. During sintering, bismuth stearate is decomposed into bismuth oxide, which acts as sintering aid to increase the density of the sintered bismuth stearate-coated PLZT ceramics. Bi³⁺ dissolves into the perovskite crystal lattice, which increases the tetragonality factor and the content of tetragonal phase, and improves the dielectric, ferroelectric and piezoelectric properties of the bismuth stearate-coated PLZT ceramics. This study provides a novel modification method for the production of injection molded ceramics using a water-soluble binder system.     

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.     

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.     

纵论注射成型技术

以注射成型技术在橡胶行业的发展现状为依据,从工作原理、生产工艺、产品质量、生产效率等几个方面着手,对模压成型技术和注射成型技术进行了比较,阐述了注射成型技术的优越性。同时,针对推广注射成型技术遇到的困难,提出了注射成型技术在橡胶行业的推广过程中所需要解决的一系列问题。     

Microstructure evolution of 316L stainless steel micro components prepared by micro powder injection molding

Fabrication of micro components comprising of functional units of different dimensional order, ?100 μm, ?80 μm and ?60 μm, by micro powder injection molding is described in the paper. The microstructure is characterized for various microsize structures and substructures under different sintering conditions. The micro components are successfully molded, debound and sintered based on an in-house feedstock. Although sintered as an integral component, the microsize structure is at final stage sintering whereas the substructure is at intermediate stage sintering within the temperatures studied. The smallest microsize structures, ?60 μm, have the highest relative density. Higher surface area/volume ratio of the microsize structures is attributed to the advanced sintering. XPS results show that oxides on the debound micro components and consequent reduction products during hydrogen sintering attribute to the formation and progression of the observed dense layers.     

Kneading and molding of ceramic microparts by precision powder injection molding (PIM)

This paper investigates the kneading and formability of microparts made using alumina in micro‐powder injection molding. In this study, quality feedstock with uniform powder dispersion was achieved when optimum kneading process was performed. In addition, the thin microplates were successfully manufactured using a custom‐made injection machine. Shrinkage was significantly reduced in microspecimens when the mold temperature was increased to 70°C. The results of flow visualization were conformed to that of experiments in this study. A very important result for flow visualization and experiment was molten polymer filled the cavity by shortest period producing a least shrinkage in microparts. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 892–899, 2006     

过程参数对气辅成型的影响

本文基于Hele-Shaw模型，采用CAE技术，模拟了气辅注射成型中几个重要工艺参数：熔体温度、气体压力、延迟时间、熔体预注射量等对其成型的影响。并用流变学理论进行分析，揭示了各工艺参数对气辅注射成型的影响机理。     

Yttria stabilized zirconia formed by micro ceramic injection molding: Rheological properties and debinding effects on the sintered part

Micro Ceramic Injection Molding (μCIM) is a near net-shape process to produce smaller and intricate parts at a competitive cost. The application of nano-sized ceramic powder in μCIM has the advantages of fine grain size growth and good surface finish. However, the nano size effect causes agglomeration and low powder loadings, which result in defects during the μCIM process and in the sintered components. This study extensively investigated the debinding and sintering of yttria-stabilized zirconia (YSZ), as well as its rheological properties, using polypropylene (PP) as the primary binder and palm stearin as the secondary binder. 50 nm Yttria stabilized zirconia (YSZ) powders were mixed with palm stearin and PP at a powder loading of 37–43 vol%. The results of rheological studies showed that the feedstock had a dilatant flow characteristic and a viscosity of around 10–40 Pa s. Feedstock with 38 vol% powder loading had the lowest activation energy of 9.48 kJ/mol. The green part of the injected feedstock had flexural strength ranging from 13 to16 MPa, within which the feedstock with 43 vol% powder loading had the highest green density. Solvent debinding was carried out at three temperatures (50, 60, and 70 °C) using heptane. A large porous region was clearly identified at 70 °C compared with 50 °C. A debinding split furnace with argon gas was used to remove PP at 450 °C for 4 h. The debound samples did not shrink when 94%–98% of the binder system was removed. All debound samples sintered at 1350 °C and 41 vol% had the highest mechanical properties with hardness of 900 HV and a flexural strength of 400 MPa.     

Effect of sintering on the microstructure and mechanical properties of alloy titanium-wollastonite composite fabricated by powder injection moulding process

Composite biomaterials are in high demand in the medical field of today. The combination of bioactive wollastonite (WA) glass ceramic with the biocompatibility of alloy titanium (Ti6Al4V) could be a good candidate for implant applications. The rheological properties of Ti6Al4V/WA feedstock show a pseudoplastic behaviour with low activation energy. The feedstock was successfully injected as a green part with no defects. The green part was solvent debound for 6?h in heptane and thermal debound in an argon environment for 1?h. The brown part was successfully sintered at 1300?°C for 3?h with 5?°C/min heating and cooling rates. The average sintered density was 4.12?g/cm³; which is 97.5% from the theoretical density. The highest Young's modulus obtained was 18.10?GPa; which is in the range of human bone strength. EDX analysis shows that by increasing sintering temperature, the level of oxygen decreased. Cell viability test shown an absorbance increased with days increasing indicated that the cellular were proliferated on the composite Ti6Al4V/WA composite which also proved that the composite was non-toxic. This indicates that the Ti6Al4V/WA composite is suitable for bone implant applications.     

Investigation of stainless steel 316L/zirconia joint part fabricated by powder injection molding

The joint part of stainless steel 316L (SUS316L)/zirconia was investigated. The specimens were fabricated using the powder injection molding (PIM) with insert-injection molding. SUS316L, zirconia, and a mixture of SUS316L and zirconia were used as bonding materials. The processing conditions for PIM were developed and optimized to avoid defects in the final sintered part. The bonded parts of SUS316L/mixed layer and zirconia/mixed layer were fabricated without joint failure. However, warpage and contamination existed due to the different sintering behavior and remaining binders. To reduce the warpage, the solids loading of SUS316L were changed to 44 and 40 vol. %. The warpage was decreased by reducing the shrinkage difference between the SUS316L and mixed layer. The contamination was also reduced by increasing the powder bedding in the debinding and sintering process. The microstructures and chemical compositions were confirmed by a scanning electron microscope (SEM) and an energy dispersive X-ray spectroscope (EDS). Migration of the main components was observed at the interface and the main components were presented together at the whole part interface and interface between 2 materials in the mixed layer. It was confirmed that the joining was done with bonding of the same materials in each layer and diffusion bonding.     

气体辅助注射成型设备的构建

为促进气体辅助注射成型设备的国产比，介绍了在传统注射成型设备基础上构建该设备的关键技术，提出了气体辅助注射成型设备注气系统的组建及对注射机和注射模的要求，突破了传统注射成型的技术限制，可节约大量因引进进口设备所花费的资金。     

塑料制品注塑成型缺陷的成因分析

通过列举几种注塑成型过程中最为典型的几种缺陷,结合生产实际,从影响注塑成型制品质量的因素(产生的物理原因、注塑模具和制品结构、注塑工艺参数有关的原因、塑料材料)着手,分析了归纳注塑成型制品典型质量缺陷产生的原因,并提出了克服缺陷的具体改良措施。     

正交试验优选气辅成型工艺参数

为优化气辅成型工艺参数,采用单因素法考察工艺参数对气辅成型质量的影响,以熔体预注射量、熔体温度、模具温度、注气压力、延迟时间、注气时间为因素,气体穿透深度、最大气指幅度为评价指标,采用L25(56)正交试验设计优选气辅成型工艺参数为:熔体预注射量85％,熔体温度240?℃,模具温度40?℃,延迟时间4?s,注气压力3?...     

注塑工艺参数对制品残余应力和收缩的影响

注塑成型工艺参数对制品的最终残余应力和收缩有着直接的影响。基于线性黏弹性模型模拟计算了注塑成型过程中由温度和压力引起的残余应力和收缩。以无定型材料PS和ABS为例,系统地研究了不同成型工艺条件下平板制件的最终残余应力和收缩,并和实验结果进行对比验证。结果表明：在流动方向上无定型材料的收缩基本保持不变,残余应力沿壁厚分布的形状也基本相同,但流动末端处的应力值稍大于流动入口处;保压压力是影响制品收缩的关键因素,提高保压压力和注射温度可以降低制品的最终收缩,而模具温度对收缩的影响较小。