Effects of wall slip on ZrO2 rheological behavior in micro powder injection molding

The effects of wall slip on the rheological behavior of ZrO₂ feedstock flowing through various channels were studied. As compared to conventional no-slip condition, a power-law wall slip model was established to simulate the filling behavior of ZrO₂ feedstock flowing through the mold. The effects of wall slip on pressure distribution of the micro spool mandrils during filling was compared with the injection molded micro components. Experimental results verified that the simulation including wall slip yielded better prediction for the pressure gradient as well as crack propagation for the micro components. Increasing mold temperature not only enhanced the feedstock temperature flowing through the micro channels, but also reduced the temperature difference of the micro components and likely the ensuing thermal stress as well as cracks. Powder-binder separation is more sensitive to the mold temperature variation than melt temperature variation. Defect-free micro spool mandrils were injection molded using the optimized parameters.     

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.     

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.     

Wall Slip of Linear Polymer Melts During Ultrasonic‐assisted Micro‐injection Molding

The wall slip of linear polymer melts under ultrasonic vibration is investigated by correcting the slip mechanism, and melt flow behaviors in ultrasonic‐assisted micro‐injection molding (UμIM) method are discussed. Based on the effect mechanism of ultrasonic vibration on the melt, theoretical models of the critical shear stresses for the onset of weak and strong wall slip during UμIM are established, and the change in rheological properties due to the onset of wall slip under ultrasonic vibration is experimental investigated by a built measurement system. The results show that the onset of weak and strong wall slip of the melt in micro cavity are promoted by ultrasonic vibration, which agree with the built theoretical models, and the melt filling capability in micro cavity is enhanced by reducing apparent viscosity and releasing shear stress of the polymer melt, which improves the molding quality of micro polymer parts via UμIM method. POLYM. ENG. SCI., 59:E7–E13, 2019. © 2018 Society of Plastics Engineers     

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。     

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。     

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.     

Wall slip of molten polymers

There is considerable experimental evidence that the classical no-slip boundary condition of fluid mechanics is not always a valid assumption for the flow of high molecular weight molten polymers. In fact, molten polymers slip macroscopically at solid surfaces when the wall shear stress exceeds a critical value. Moreover, for linear polymers there exists a second critical wall shear stress value at which a transition from a weak to a strong slip occurs. These two modes of slip (weak and strong) are due to flow-induced chain detachment/desorption at the polymer/wall interface and to chain disentanglement of the polymer chains in the bulk from a monolayer of polymer chains adsorbed at the interface. In this review, the two physical mechanisms of slip are discussed and validated on the basis of published experimental data. The slip velocity of molten polymers is a complex function and has been reported to depend on wall shear and normal stresses, temperature, and molecular characteristics of polymers (molecular weight and its distribution). Proposed slip models, static and dynamic, are also reviewed and their significance on the rheology and flow simulations of molten polymers is discussed.     

Wall slip of concentrated suspension melts in capillary flows

The effects of wall slip of concentrated suspension melts in capillary flows were investigated at elevated temperature. The modeled material is a mixture of polymer EVA (Ethylene Vinyl Acetate) and non-colloidal spherical powder (glass microspheres) with mean particle size within 53∼63 μm. The effect of particle concentration on wall slip was studied experimentally in a capillary rheometer. For suspensions with different particle loadings (35%, 40%, and 45% by volume), the slip velocity V_s increased with an increase of particle concentration at the same testing temperature. A master slip curve can be obtained by plotting slip velocity versus the product of wall shear stress and square root of particle concentration. As such, a new particle concentration-dependent slip model is proposed. A theoretical approach coupled with the new slip model and flow equation is employed to characterize the flow behavior of concentrated suspension in a capillary rheometer, with reasonable agreement obtained with experimental observations.     

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     

壁面滑移对两种聚合物熔体共挤出影响的数值研究

在壁面滑移的边界条件下,利用聚合物流体计算软件包POLYFLOW对两种熔体的二维等温共挤出进行了数值模拟.在两侧壁面滑移系数相同和两侧壁面滑移系数不同这两种情形下分别计算了共挤出流动的速度场、压力场、黏度场及剪切速率场,讨论了壁面滑移对共挤出流场、界面形状和挤出胀大的影响.模拟结果表明：当两侧壁面滑移情况相同时,滑移系数越大,界面偏移越大,熔体胀大率增大;当两侧壁面滑移情况不同时,滑移系数相差越大,界面偏移越大,滑移系数小的一侧熔体挤出胀大显著.     

气体在任意截面形状微尺度槽道中的滑移流动

利用正交函数法对气体在具有任意截面形状的微尺度槽道内的充分发展层流滑移流动特性进行了理论分析,获得了任意截面形状微槽道内的速度分布和流动阻力特性的分析解,并以矩形微槽为例分析了微槽截面上的速度分布和阻力特性.结果表明：随Kn数的增加,由于壁面处滑移流动的影响,气体流经微槽的流动阻力常数小于大尺度理论预测值;理论分析解的结果与实验结果吻合较好,表明在一定的Kn数范围内Navier-Stokes方程在考虑了速度滑移后可以描述微通道内的气体流动过程;正交函数法在微槽内滑移流动的分析中是可行的.     

流动小室制品微注塑过程的流痕形成问题研究

利用Moldflow模拟了流动小室的微注塑填充和保压过程,分析了不同尺寸参数的腔体中的压力和剪切速率分布。根据实验制品流痕仅存在于微结构处的特点,发现局部剪切增大导致的壁面滑移是流痕产生的主要原因,提出可通过减小注射速度、增加制品总厚或改变浇口位置的方法消除流痕。研究结果表明,局部微结构的存在可能会对微注塑制品的产品质量产生显著的影响。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

Application of the rheology of monodisperse and polydisperse polystyrenes to the analysis of injection molding behavior

Monodisperse and polydisperse polystyrenes of equal weight average molecular weight (M _w) are evaluated for melt flow behavior in an Instron capillary rheometer and for injection molding behavior in a 12 ounce in-line reciprocationg screw injection molding machine. The influence of molecular weight distribution on the shape of the flow curves is deconstrated over a wide range of shear rate and temperature. The influence is also reflected in injection molding behavior as defined by pressure to fill or flash the mold at a given melt temperature. Studies of capillary rheometer data for correlation with injection moling beghavior indicate poor agreement when low shear rate viscosity data are used. Good agrement is foun using high shear rate viscosity data in the range 10³ to 10⁴ sec^?1 Striking crossover points on melt rheology and injection colding area diagram curvs are found with the monodisperse and polydisperse polystyrenes of the same M These crossovers shift with melt temperature and make possible the determination of a “controlling shear rate” for the injection molding process. This is found to be 3500 sec^?1 for short shot and 6200 sec^?1 for flash with the ASTM test specimen mold used in this study.     

EXTRUSION AND LUBRICATION FLOWS OF VISCOPLASTIC FLUIDS WITH WALL SLIP

The simplest model flow which approximates the extrusion (shallow screw channels) and lubrication flow is the steady, laminar flow occurring between two infinitely long parallel plates i.e., the generalized plane Couette flow. Here we develop an analytical model of the generalized plane Couette flow of viscoplastic fluids. The deformation and flow behavior of viscoplastic fluids can be realistically represented with the Herschel-Bulkley constitutive equation, which we have utilized as the basis for the development of our analytical model. Furthermore, as also demonstrated here, the deformation behavior of viscoplastic fluids is generally complicated by the presence of wall slip at solid walls, which occurs as a function of the wall shear stress. The wall slip versus the wall shear stress behavior of viscoplastic fluids can be experimentally characterized using viscomelric flows, including steady torsional and capillary flows. Thus determined Navier's wall slip coefficient can then be utilized in modeling of processing flows. In our analytical model of the generalized plane Couette flow of viscoplastic fluids the Navier's wall slip boundary condition was included. This model should be an important engineering tool, which provides design expressions for the extrusion and lubrication flows of viscoplastic fluids, with or without wall slip occurring at the walls. @KEYWORDS:Extrusion, lubrication, flow, viscoplastic, slip.     

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.     

Microscopic studies of static and dynamic contact angles

Molecular dynamics simulations are used to test macroscopic theories for static and dynamic contact angles. Young's equation is verified by comparing observed static contact angles to angles calculated from the independently measured surface tensions between phases. Laplace's relation between the interfacial curvature and pressure is also checked. Both equations agree with simulation results within statistical errors. Hydrodynamic theories of dynamic contact angles are less well defined because they produce diverging stresses at the contact line between the solid and fluid interfaces if the usual no-slip boundary condition is assumed. Our simulations show that slip occurs within about two molecular diameters of the contact line, and that local hydrodynamics breaks down in the slip region. The slip results from large tangential stresses along the solid wall. A surprising result is that changes in the boundary condition for single-fluid flow at molecular scales produce dramatic changes in the dynamic contact angle.     

金属粉末注射成型工艺

将现代塑料注射成型技术引入粉末冶金领域，不但扩大注射成型技术的应用，同时也推动粉末冶金工艺的发展。详细讨论金属粉末注射成型工艺及其应用，对其材料选择，产品与模具的设计、注射成型工艺过程及烧结工艺过程进行了系统的分析，并展望了该工艺中金属粉末、粘结剂、注射成型装备和工艺过程数值模拟软件等方面的发展趋势。