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.     

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.     

聚甲醛的生产和应用

简要介绍了聚甲醛的特性、生产工艺及广泛用途;分析了我国聚甲醛的生产现状、生产企业及发展前景;并对未来聚甲醛行业的发展提出了建议。     

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     

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.     

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.     

Development of mechanical and visible transparency properties of spark plasma sintered spinel fabricated by powder injection molding of MgAl2O4 nanoparticles

This research aims to investigate the density, hardness, fracture toughness, and infrared and visible transmittance of spark plasma sintered (SPS) spinel discs fabricated through the powder injection molding (PIM) method. These properties were compared with the sample directly SPSed without the PIM process. For this purpose, initially, a feedstock was prepared with 80 wt% spinel nanopowder and 20 wt% binder. The results revealed that the hardness and fracture toughness of the SPSed spinel disc sintered at 1400 °C were greater than those of the spinel sample without PIM treatment. Also, for the PIMed sample and then SPSed sample, the level of infrared and visible transmittance was ~10% greater than for the SPSed spinel nanopowders.     

Role of primary amine in polyoxymethylene (POM)/bentonite nanocomposite formation

Polyoxymethylene (POM)/organo-modified bentonite nanocomposites are successfully prepared by melt intercalation method of which a primary ammonium salt is an effective surfactant, as evidenced from improvement in mechanical and gas barrier properties. Nanocomposite structures analyzed by XRD and TEM show mixed nanostructure of flocculation and exfoliation (flocculated/exfoliated nanocomposite) when primary ammonium-treated bentonite is used, whereas the quaternary ammonium-treated bentonites induce the mixture of intercalation and flocculation (intercalated/flocculated nanocomposite). The incorporation of organo-modified bentonite gives an effect on crystallization by generating numerous nucleating sites, especially in the case of bentonite with primary ammonium surfactant. The nanocomposites obtained exhibit improvement in flexural strength, flexural modulus, and elongation at break. The thermal degradation temperature is decreased by 40 °C, whereas the oxygen barrier is increased by 50%, as compared to neat POM.     

Spinning wheel powder feeding device — fundamentals and applications

A spinning wheel powder feeding system has been developed as a conveying mechanism to feed fine particle aggregates on a laboratory scale. An example of a use of this conveying mechanism is with a transport tube reactor, since the reactor only provides a few seconds residence time to react the powder. Methods to shear the powder mechanically, as opposed to using a high gas velocity, are developed as to not reduce the available residence time in the reactor. The objective is to feed a powder at the smallest particle aggregate size possible rather than a large particle aggregate size generated by an upstream feeding device, and to achieve such dispersion using minimized gas flow. Statistical results show that the spinning wheel alone is able to reduce the mean aggregate size of the Particle Size Distribution (PSD) and when a minimal amount of gas is added to the system the PSD is reduced further. In addition, a fundamental model employing a discrete particle aggregate breakage equation combined with a Monte Carlo method has shown that the spinning wheel feeding system is able to consistently reduce particle aggregate size.     

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.     

粉末注射成型技术的研究和应用

介绍了粉末注射成型技术的概念和工艺,包括金属粉末注射成型(MIM)、陶瓷粉末注射成型(CIM),讲述了黏结剂的使用发展情况;阐述了粉末注射成型专用注射成型机的特点,PIM粉末注射专用塑化系统,PIM专用料斗滑块装置,镶套式机筒座等,介绍了PIM技术在诸多行业中的应用,3C行业,医疗行业,汽车行业等,并展望了PIM研究的发展方向。     

Reproducibility of flow properties of microcrystalline cellulose — Avicel PH102

Earlier work showed that a batch of microcrystalline cellulose (MCC, Avicel PH102) exhibited minimum acceptable flow properties required for successful high speed tableting. The aim of this work was to assess the suitability of using Avicel PH102 as a reference indicator of minimum powder flow by examining the reproducibility of flow properties of seven batches of Avicel PH102. Powder flow properties have been characterized in terms of flow function, flow factor, cohesion, bulk density, effective angle of internal friction, angle of linearized yield locus, and angle of internal friction at steady-state flow. Results show that the measured flow function is independent of operator. Batch-to-batch flow functions are not statistically different at 95% confidence level suggesting acceptable reproducibility. These results corroborate that Avicel PH102 may be used as a reference powder for predicting flow performance of a new formulation during high speed tableting.     

Effect of powders and binders on material properties and molding parameters in iron and stainless steel powder injection molding process

It is essential to study and optimize multiple objective functions such as binder system design, feedstock, part geometry, mold design, and processing conditions in order to develop a successful powder injection molding process. A powder with different combinations of binder systems and a binder system with different combinations of powder systems were investigated with a combined experimental and simulation study. First, an experimental rheological study was performed to evaluate the influence of the powder/binder combinations on the rheological behavior and thermal stability of carbonyl iron and stainless steel powder injection molding (PIM) feedstocks. Second, based on the characterization of the feedstock, the simulation study revealed that the pressure-related parameters such as wall shear stress, injection pressure, and clamping force were mainly dependent on the binder system and not much on the powder characteristics, in the range of particle attributes studied. Third, to the temperature-related parameters such as melt front temperature difference and cooling time, binder selection is more critical than powder selection. Fourth, for the velocity-related parameter, maximum shear rate, the selection of both powder and binder system is critical in control. It is demonstrated that the simulation study is essential in the development stage for successful PIM.     

铸铁表面的不锈钢粉末喷涂技术

介绍了铸铁表面喷涂不锈钢粉末的预加工、喷涂工艺及喷涂后加工,并对涂层的性能做了测试,取得了较好的试验效果     

Fracture behavior of carbon/epoxy laminated composite reinforced by iron powder

The fracture behaviors of a newly developed iron-powder reinforced carbon/epoxy laminated composite are investigated in this paper. Three kinds of DCB (double cantilever beam) specimens (without iron powder, with iron powder and with iron powder in a magnetic field) were prepared by the ASTM D 5528-94a. For the third DCB specimen, the unidirectional laminas were stacked with iron powder spread evenly on each lamina’s surface. This process was performed in a magnetic field to keep the iron powder standing along the out-plane direction. From the test data of Instron 5567, the fracture toughness, G _I , was calculated by using the compliance calibration method for each of the three kinds of specimens. The calculated fracture toughness shows that the iron powder effectively disturbs the progress of fiber branching between the laminates and provides a good stitching to the in-plane laminates during the fracture.     

The effect of powder characteristics on pressure sensitivity of powder injection moulding compounds

Pressure effect on the rheological behaviour of powder-polymer compounds intended for powder injection moulding was determined. A single-piston capillary rheometer modified by addition of a second chamber with a restricting needle valve generating backpressure increasing the pressure in the melted material during the flow through the die was employed to determine pressure sensitivity coefficients. The results obtained for three compounds varying in the characteristics of powders used confirmed that compounds at the loading level close to the maximum packing are more sensitive to pressure than polymer binder. It is shown that pressure sensitivity coefficients of these materials are strongly dependent on powder characteristics — particle size and particle size distribution. The highest coefficient (32.9 GPa⁻¹) was found for compound containing broad particle size distributed powder having a perceptible portion of small particles.     

Preparation of SBA-15 extrudates: Evaluation of textural and mechanical properties

SBA-15 powder (P) was shaped into cylindrical extrudates (Ex) by compounding with additives such as bentonite (binder), methylcellulose and tetraethylorthosilicate (TEOS). The extrudates of SBA-15 were characterized by XRD, N₂ adsorption and thermogravimetric analysis. The orderly growth of SBA-15 is evident from the XRD patterns. The surface area and pore volume of SBA-15 (P) were around 884 m²/g and 1.1 cm³/g, respectively. The decrease in surface area and pore volume were observed for SBA-15 (Ex) in comparison to SBA-15 (P). This may be due to partial blocking of pore entry and surface coverage of additives during shaping of extrudates. The mechanical strength of SBA-15 (Ex) was examined by vertical crushing method. The effect of additives on the mechanical strength of SBA-15 (Ex) was examined by varying the composition. The results indicate linear increase of mechanical strength with increase in the content of bentonite and methylcellulose, but non-linear response with increase of TEOS and water. Compounding additives have been established to improve the mechanical strength of the extrudates.     

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     

Binder jetting additive manufacturing of silicon carbide ceramics: Development of bimodal powder feedstocks by modeling and experimental methods

A limitation of binder jetting additive manufacturing is the low density of fabricated parts. Mixing powders with different sizes is a promising approach to increase powder bed packing density and, hence, printed part density. However, in previous studies mixed powder feedstock was prepared by trial and error method. In this research, both modeling and experimental methods were used to prepare the bimodal powder feedstocks. Analytical packing model was introduced for irregular powders. A bimodal powder was prepared by mixing two different-sized silicon carbide powders (i.e. coarse and fine) using ball mill, and their tap densities were measured. Silicon carbide plates were printed using the coarse and bimodal powders by a commercial binder jetting system. Results showed that the modeling method could predict the tap density of bimodal powders with high accuracy. The printed parts from bimodal powder achieved higher green densities than those from the unimodal powder.