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.     

Effects of mold dimensions on rheological of feedstock in micro powder injection molding

Micro powder injection molding (µPIM) differs from conventional powder injection molding (PIM) with respect to the effects of the mold dimensions. In this study, the rheological properties of the feedstock (mixture of carbonyl iron powders and binder) in molds with different diameters were analyzed by numerical simulation based on the powder-binder two fluid model. Viscosity, temperature, shear strain rate, velocity and the flow distance of the feedstock varied for different molds, but did not just reduce by proportion when the mold was smaller which would lead to many defects in green parts. The inner wall of the mold that was the key factor influencing the filling step in µPIM needed more studies.     

Torque rheometry and rheological analysis of powder–polymer mixture for aluminum powder injection molding

Selection of desired powder–polymer mixture (feedstock) formulation is a key factor in manufacturing perfect parts via powder injection molding. In the present study, feedstock characteristics of an aluminum-based powder were investigated by torque rheometry and rheological analyses. Several binders containing various amounts of polypropylene (PP), paraffin wax (PW), and stearic acid (SA) were selected for torque mixing and viscosity evaluation. Then, feedstocks consisting of 54, 58, 62, and 66 vol. % solid contents were prepared with modified binder. Feedstock flow behaviors were investigated regarding the rheological parameters such as mixing torque, viscosity, flow behavior index, flow activation energy and moldability index. It was found that increasing solid loading from 54 to 62 vol. % led to improved rheological behavior. This improvement was not observed in high solid contents, i.e., 66 vol. %. Based on experimental results, the optimized binder composition (60PW,35PP,5SA vol. %) and the optimum powder loading (62 vol. %) were selected as the best formulations for injection of aluminum powder. These values are supported by critical powder volume concentration measurements deduced from the oil absorption method. The resulting aluminum molded green parts with no defects exhibited the straightforward injection molding process of selected feedstock.     

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.     

Influence of injection temperature and pressure on the properties of alumina parts fabricated by low pressure injection molding (LPIM)

The quality of ceramics parts made by powder injection molding (PIM) method is influenced by a range of factors such as powder and binder characteristics, rheological behavior of feedstock, molding parameters and debinding and sintering conditions. In this study, to optimize the molding parameters, the effect of injection temperature and pressure on the properties of alumina ceramics in the LPIM process were thoroughly studied. Experimental tests were conducted on alumina feedstock with 60 vol% powder. Injection molding was carried out at temperatures and pressures of 70–100 °C and 0.1–0.6 MPa respectively. Results showed that increase in injection temperature and pressure and the resulting increase in flow rate leads to the formation of void which impairs the properties of molded parts. The SEM studies showed that injection at temperature of 100 °C results in evaporation of binder components. From the processing point of view, the temperature of 80 °C and pressure of 0.6 MPa seems to be the most suitable condition for injection molding. In addition, the effects of sintering conditions (temperature and time) on the microstructure and mechanical properties are discussed. The best final properties were found using injection molding under the above stated conditions, thermal debinding and sintering at 1700 °C during 3 h.     

ZrO2陶瓷注射成形的流变特性

以加入3％molY2O3的ZrO2粉末为原料,选用聚苯乙烯（PS）、乙烯和醋酸乙烯酯共聚物（EVA）、聚酰胺蜡（NEW）、聚乙烯蜡（PEW）和硬脂酸（SA）为粘结剂体系制成注射成形的喂料,使用XLY—Ⅱ型的毛细管流变仪测定喂料的流变参数,通过线性回归分析．计算出非牛顿指数和粘流活化能。实验表明：喂料的粘度随着温度的升高以及剪切速率的增大而减小,具有较好的充模性．呈假塑性流体,在三种组分的喂料中,Z3配方表现出良好的流动性和较小的温度敏感性．适合陶瓷注射成形。     

Numerical and experimental study of injection step,separation, and imbalance filling in low pressure injection molding of ceramic components

In recent years, widespread use of advanced ceramics in vast areas of the industries such as medical technology and automotive industries has led to the growth of the advanced ceramics market. Due to the difficult machining processes and the difficulty of forming complex parts, manufacturing of ceramic components has been restricted. Powder injection molding can be used to overcome these restrictions. In this paper, SiC feedstock was injected at various temperatures and flow rates. Numerical and experimental results of injection step and imbalance filling were studied. As well as the separation of powder and binder was simulated and verified by experimental tests. Thermogravimetric analyzer and rotational rheometer were employed to evaluate the separation in the green parts and rheological properties of the prepared feedstocks, respectively.     

Micro powder injection moulding of alumina micro-channel part

A feedstock consisting of submicron alumina powder and a formulated binder, was developed to fabricate alumina micro-channel part by micro powder injection moulding. During small scale-mixing, the mixing torques of feedstocks with four different powder loadings were used to establish a suitable powder loading. The thermal and rheological properties of the selected feedstock were examined and used to establish conditions for large scale mixing, debinding and injection moulding. The micro-channel parts were pressureless sintered at different temperatures. The results showed that the moulded, debound and sintered micro-channel parts had good shape retention. The dimensions of the micro-channel part changed with the different processing steps. High densification of the micro-channel parts was achieved at sintering temperatures of 1350 °C and above. Above 1350 °C, the grain grew significantly with increasing the sintering temperatures and thus it led to a decrease in the microhardness.     

Flow behavior of unsaturated polyester resin—Microsized 17‐4PH stainless steel powder—Feedstocks

For the realization of stainless steel parts carrying either surface relief microstructures or possessing dimensions in submillimeter‐range the different variants of micro injection molding are suitable replication techniques, followed by thermal postprocessing like debinding and sintering. With respect to warpage‐free sintered parts the initial solid load in the feedstock should be as large as possible. A reduction of the binder viscosity enables a reduction of the feedstock viscosity; hence, larger solid loads can be achieved. Using an unsaturated polyester binder, diluted with different amounts of the reactive thinner styrene, feedstocks with a solid content up to 46 vol% were realized. The influence of the filler on the feedstock viscosity, the maximum accessible load and the flow activation energy was investigated intensely. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Powder Processing, Rheology, and Mechanical Properties of Feedstock for Fused Deposition of Si3N4 Ceramics

Fused deposition of ceramics (FDC) is a technique in which green parts are fabricated directly from CAD designs. The feedstock for FDC is a 1.778 mm diameter filament that requires a low viscosity and high column strength. This study explores the powder processing science, as well as the rheological and mechanical properties required for a successful FDC feedstock material. GS44 Si₃N₄ powders were dispersed in RU9 binder using oleyl alcohol (OA). The viscosity of the RU9/OA/Si₃N₄ mixture was measured as a function of temperature, solids loading, and OA concentration. The mechanical properties of the filament feedstock were evaluated in compression to establish FDC process limits. The feedstock material shows a shear thinning behavior with OA acting mainly as a plasticizer. The viscosity of GS44-filled RU9 decreases with temperature, and increases with solids content. At 185°C and 55 vol% loading, the viscosity was found to be in the range of 49–7 Pa·s for a corresponding shear rate of 70–1128 s⁻¹. This was sufficiently low for FDC. Based on pressure requirements for FDC extrusion (Δ P ), and maximum sustainable stress without buckling by the filament (σ_E), it has been found that for successful FDC of RU955, 1.1Δ P < σ_E.     

聚乳酸基注射陶瓷喂料的流变特性

使用聚乳酸、乳酸预聚物为粘结剂体系,与ZrO₂粉末制成注射陶瓷喂料。使用毛细管流变仪对陶瓷喂料的流变特性进行测试,通过线性回归分析,计算出非牛顿指数和粘流活化能。实验表明:在ZrO₂质量分数为83%—85%时,喂料的粘度随着温度的升高以及剪切速率的增大而减小,呈假塑性流体,具有较好的充模性。在四种陶瓷喂料中,3号喂料在170℃和36774Pa剪切应力条件下,其非牛顿指数n和粘流活化能E分别为0.36和41kJ·mo^-1,其综合流变性能最好。     

不同粘结剂含量对金属喂料流动性能的影响

通过密炼机混炼出不同比例金属粉末与粘结剂的金属喂料,对不同粘结剂含量的金属喂料进行喂料流动性能对比实验,对比金属粉末含量对喂料熔融指数的影响及不同金属粉末含量在不同温度下表观粘度随剪切速率的变化,得到最佳注塑成型实验所用的最佳喂料配方。结果表明,随着粘结剂含量的增加,金属喂料的流动性能一直提高。在达到金属粉末质量分数92.6%之前,熔融指数急剧上升,当达到92.6%后,熔融指数缓步上升。     

Ink development for the additive manufacturing of strong green parts by layerwise slurry deposition (LSD-print)

Obtaining dense fine ceramics by the binder jetting additive manufacturing process is challenging. A slurry-based binder jetting process, such as the layerwise slurry deposition (LSD-print) process, can enable the printing of dense ceramic parts. This work describes a procedure to develop and qualify a suitable ink to manufacture silicon carbide green parts by LSD-print. Not only the printability but also the compatibility of the ink with the powder bed and the effect of the binding agent on the properties of the green parts are considered. Both aspects are important to obtain high green strength, which is necessary for printing large or thin-walled parts. Characterization methods, such as rheological and surface tension measurements, are applied to optimize three selected inks. The interplay between ink and powder bed is tested by contact angle measurements and by comparing the biaxial strength of cast and additively manufactured specimens. Out of the three binding agents tested, a polyethyleneimine and a phenolic resin have a high potential for their use in the LSD-print of silicon carbide green bodies, whereas a polyacrylate binding agent did not show the required properties.     

Novel fabrication of more homogeneous water-soluble binder system feedstock by surface modification of oleic acid

Compared to conventional debinding process of wax-based binders used in ceramic injection molding, polyethylene glycol-based binders (water-soluble binders) with its high efficiency and environmental acceptability have appeared as a good alternative. However, water-soluble binder feedstock still presents some problems such as powder–binder separation or particle segregation which limits injected parts’ final properties. In the present work, we have successfully prepared the more homogeneous water-soluble feedstock with lower shear viscosity by a prior ball milling treatment to induce a small quantity of oleic acid to the surface of zirconia powders before the mixing process. Also, the surface modification mechanism of oleic acid to zirconia powders has been systematically discussed. With the modified powder, as-leached part with less agglomeration and phase separation has been fabricated, which suggests a novel modification method for fabricating injection molded ceramic parts by using water-soluble binders.     

A numerical model to predict the powder–binder separation during micro-powder injection molding

The micro-powder injection molding (micro-PIM) process has the potential to bridge the gap between the design and manufacturing of micro-components that are often used in small and handy devices. Numerical modeling helps to analyze and overcome various difficulties of micro-PIM. In the present work, a numerical model is developed to predict the powder–binder separation (a common defect in PIM and especially severe in micro-PIM) during the injection of an alumina feedstock. A powder–binder separation criterion is proposed dealing with applied injection pressure and friction force between the powder and binder. An indirect comparison of feedstock travel time between two locations is used to validate the model. The predicted segregation from the simulated result is supported by a qualitative experimental measurement. The developed model can be used to optimize injection parameters to get a defect-free product.     

Rheological behavior of zirconia feedstock flowing through various channels considering wall-slip

The existence of wall slip for ZrO₂ feedstock flow in micro powder injection molding was investigated based on capillary rheometer experiments using dies of three dimensions. A power law function was derived by data fitting to determine the wall slip velocity based on which numerical simulation was carried out to explore the influence of wall slip on micro injection molding. Experimental results indicate that the feedstock is less sensitive to temperature fluctuation at higher shear rates. Power-law model can provide higher accuracy than the modified Cross model to depict the rheological behavior of the feedstock in capillary flows with different channels. Numerical simulation results show that in case of steady flow higher dynamic viscosity of the feedstock and higher pressure losses of the flow appeared when the wall slip boundary was included as compared to no-slip assumption in micro powder injection molding. This is because that when the wall slip boundary was included the shear rate distribution of the feedstock was lower than that of the feedstock assuming no-slip boundary.     

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.     

Development of fibrillar morphology in immiscible PP/PS blends under shear flow

The formation dynamics of fibrillar morphology in dilute immiscible polypropylene (PP)/polystyrene blends under simple shear flow is investigated using optical‐shear technique. Two strategies in generating fibrillar droplets under shear flow, namely temperature quench and shear jump, are studied. It is found that the shear‐induced deformation of PP droplets is closely related to the total shear strain and changes of rheological properties of components during the temperature quench or shear‐jump process. The shape evolution of fibrillar droplets under shear flow displays large deviation to the prediction of affine deformation theory based on Newtonian fluids and that of three deformation models, which consider the viscoelastic properties of components. The possible effect of droplet coalescence, breakup, and interfacial slip on the deviation between the experimental data and the prediction values for droplet deformation are discussed. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Development of the powder reaction moulding process

BACKGROUND: The powder reaction moulding process uses a reactive monomer as carrier and binder for the moulding of metal or ceramic powders. De‐binding is achieved using thermal depolymerisation which is followed by sintering to give the finished component. Binder can be recovered for re‐use. RESULTS: Moulding compounds, with various powder volume fractions, have been prepared using stainless steel, silicon nitride and alumina with n‐butyl cyanoacrylate as binder, and the stability of the compounds established. Rheological properties of the compounds have been measured using both pressure flow and drag flow methods. Compounds are strongly pseudoplastic. Comparison of experimental results with theoretical models, describing suspension flow behaviour shows that experimental maximum volume fractions are close to the theoretical volume fraction of 0.42 for silicon nitride, 0.68 for alumina and 0.7 for stainless steel. Differential scanning calorimetry and thermogravimetry have been used to simulate de‐binding and show a rapid loss of binder through depolymerisation. Post‐sintering porosity of the ceramic materials is high but this is thought to arise from the low pressure moulding techniques used. Porosity of the stainless steel mouldings is much lower. CONCLUSIONS: The results validate the powder reaction moulding idea and demonstrate applicability to three widely different powder materials. Copyright © 2008 Society of Chemical Industry