Zhi Qiang Cheng

Viscous Behaviours of Feedstocks for Micro MIM

The viscous behavior of feedstocks plays the crucial role in Micro MIM. It affects directly injectability of the components and finally quality of the sintered component, because of the possible segregation induced by injection. The studies on viscosity of the feedstocks are realized by a series of the tests, in which a torque rheometer and a capillary rheometer are employed. The effects of binder composition, powder size, temperature and powder loading in volume on viscosity of the feedstocks are investigated. The mixtures of three kinds of binder composition, mixed with 5µm or 16µm 316L stainless steel powders, are evaluated. The best binder composition is determined by comparison of the viscous behaviors among the self-mixed feedstocks and the commercial one. It results in the suitable ranges of heating temperature and powder loading in volume for the feedstocks. The critical powder loading in volume is determined by a series of the capillary tests with the gradual increase of powder loading. These works provide the valuable reference for the research on binder composition and the process of micro metal injection molding.

A Streamline-Upwind Model for Filling Front Advection in Powder Injection Moulding

The filling process of powder injection molding is modeled by the flows of two variably adjacent domains in the mold cavity. The feedstock is filled into the cavity while the air is expelled out by the injected feedstock [1]. Eulerian description is adopted. The filling patterns are determined by the solution of an advection equation, governed by the velocity field in both the feedstock flow and air flow [2]. In the real physics, the advance of filling front depends mainly on the flow of feedstock that locates behind the front. The flow of air in front of the injected material plays in fact no meaningful effect. However, the actual algorithm for solution of the advection equation takes equally the importance for both the flow of viscous feedstock and that of the slight air. Under such a condition, the injection flow of feedstock in simulation may be misdirected unrealistically by the velocity field in the air portion of the mold cavity. To correct this defect, an upwind scheme is proposed to reinforce the effect of upwind flow and reduce the effect of downstream flow. The present paper involves the investigation of an upwind algorithm for simulation of the filling state during powder injection molding. A Petrov-Galerkin upwind based method (SUPG) is adopted for numerical simulation of the transport equation instead of the Taylor-Galerkin method in previous work. In the proposed implementation of the Streamline-Upwind/Petrov-Galerkin (SUPG) approach. A stabilization method is used to prevent oscillations in the convectiondominated problems. It consists in the introduction of an artificial diffusion in streamline direction. Suitable modification of the test function is the important issue. It ensures the stable simulation of filling process and results in the more realistic prediction of filling patterns. The implementation of upwind scheme in mould filling state simulation, based on an advection equation and the whole velocity field of feedstock and air flow, makes the prediction of filling evolution mostly dependent on the flow effect of injected material. This new procedure reveals its effectiveness for complicated filling flows with front joining. It can be furthermore adapted to the prediction of powder segregation effect in injection process. It involves the solution of two coupled advection equations for evolution of the powder and binder volume fractions that should be dominated only by the upwind flows of each phase. The numerical results obtained by the proposed approach are compared with experimental ones. The efficiency of the proposed approach is approved.

Simulation of micro injection moulding with emphasis on the formulation of feedstock viscosity : use of non-equilibrium molecular dynamics for the determination of viscosity of multi-body fluid

The need for prediction of shear viscosity of fluid in particle charged‐Micro Injection Molding at mesoscale, by modelling a whole system particle‐polymer with inter‐dependencies, permits to establish a more realistic feedstock viscosity formulation. The applicability of non‐equilibrium molecular dynamics (NEMD) is investigated for the determination of shear viscosity of melts composed of particles/polymers in microcavities. NEMD is used to simulate planar Poiseuille flow of metallic particle‐polymer melt. Simulations are carried out using molecular dynamics simulation package ESPResSo. The variation of viscosity face to temperature is in agreement with theoretical results. Simulations are compared to experiments. The equivalent viscosity formulation is tuned according to NEMD simulation results, and implemented in a MIM solver built up by the authors. MIM simulations are compared to previously implemented simulations using another equivalent viscosity formulation based on experiments for the case of mono...

Particle‐polymer Interactions Analysis For Metal Powder Injection Molding Feedstock: A Molecular‐dynamics Simulation With Dissipative Particle Dynamics Using ESPResSo

The applicability of Dissipative Particle Dynamics is investigated for the rheological study of melts composed of particles/polymers in microcavities for micro injection moulding. The need for prediction of rheological parameters of fluid in Particle charged‐Micro Injection Molding is of great importance so far as this would give crucial information concerning, e.g., spatial arrangement of polymeric chains and metallic particles and possible segregation effects. Complex fluids like polymer melts have macroscopic behaviour that greatly depends on their microstructure. The dynamics of a complex fluid is fundamentally affected by its microscopic structure. A complex fluid is no longer completely described by the Navier‐Stokes equation, but described by so‐called Fokker‐Planck equations. A fluid is represented by interacting particles. These particles are allowed to move in a continuous space. Each particle carries certain information pertaining to the flow like the volume of the fluid, or only the mass of the fluid. The equations of motion can also be defined through Newton’s laws of motion as in molecular dynamics. Simulations are carried out using molecular dynamics simulation package ESPResSo.

Development of Metal/Polymer Mixtures Dedicated to Macro and Micro powder Injection Moulding : Experiments and Simulations

Important research tasks at ENSMM/LMA are concerned for the development of mixtures of fine powders associated to polymer binders dedicated to the powder injection moulding (PIM) and to the powder injection micro‐moulding (μPIM) in accordance with many works already carried out with different feedstock suppliers dedicated to the macro‐components. These research tasks are completed with the simulations of injection and sintering for solid state diffusion for to validate the mumerical models.

Multiphysic coupling and full cycle simulation of microwave sintering applied to a ceramic compact obtained by ceramic injection moulding

ABSTRACT Microwave sintering represents the coupling of multiple physical phenomena. It involves the distribution of electromagnetic fields, heat generation by electromagnetic effects, heat conduction in the material, and evolution of the densification in the sintered components. This paper describes the mathematical models and the numerical methods used to simulate the complex sintering process. Simulation results are provided for the prediction of shrinkage and evolution of the relative density of the sintered materials. A full cycle simulation of the microwave sintering process have been realized on the COMSOL Multiphysics finite element software platform. This work provides an important approach to studying the process of microwave sintering. The simulation results for sintering submicron zirconia powders are compared with experimental results in terms of the relative densities of the sintered material.

Progressive Failure Analysis of Pipeline Repaired by Composite Wrapping

Composite overwrap systems have been widely used to repair damaged pipelines. Its effectiveness has been proven by many researches and engineering applications. However, the research on progressive failure mode of the repaired structure has not been reported. In the present paper, finite element method with Hashin failure criteria is developed to realize the progressive failure analysis. The predicted burst pressure is in good agreement with the burst experiment. Different from widely-reported failure progress in Composite Overwrapped Pressure Vessels (COPV), the progressive failure analysis for the defected pipeline overwrapped by composite reveals very different failure stages: stable failure propagation and rapid failure propagation. The identification of critical pressure between these two stages is valuable in composite reparation design for the defected pipeline.

Modified Method for Simulation of the Filling States at the End of Injection Molding Process

In order to improve the accuracy of numerical simulation for injection molding process, a modified method for outlet condition was introduced. As the feedstock is regarded as incompressible fluid, the filling ratio should be a linear one with respect to time. But there remains a persistent trouble in previous researches that the linearity is not respected when the filling front approaches near the outlet boundary. The problem is caused by lack of adequate treatment on the outlet boundary. To remedy this defect, the present paper deals with the modeling and realization of suitable condition on outlet boundary for solution of the whole filling process. A simple straight channel mold was taken as an example to prove the proposed simulation method. The result shows that this modified method can suppress the distortion phenomenon and can be valid to realize the correct simulation for the filling of incompressible viscous flow at the ending stage. This long-term filling problem was finally solved.