M. Sahli

Replication of microchannel structures in WC-Co feedstock using elastomeric replica moulds by hot embossing process.

Hot embossing is a net shaping process that is able to produce the micro-components of polymers with intrinsic and complex shapes at lower cost compared with machining and injection moulding. However, the emboss of hard metals, such as WC-Co, is more challenging due to their high thermal conductivity and ease of agglomeration. Thus, a WC-Co alloy mixed with a wax-based binder feedstock was selected. The formed feedstock exhibited pseudo-plastic flow and was successfully embossed (green part). Here, we developed a novel process that is used to replicate polymer microfluidic chips while simultaneously reducing the channel surface roughness of the mould insert, yielding optical-grade (less than 100 nm surface roughness) channels and reservoirs. This paper concerns the replication of metallic microfluidic mould inserts in WC-Co and the parameters associated with feedstock formation via a hot embossing process. A suitable formulation for micro-powder hot embossing has been established and characterised by thermogravimetric analyses and measurements of mixing torques to verify and quantify the homogeneity of the proposed feedstocks. The relative density of the samples increased with processing temperature, and almost fully dense materials were obtained. In this work, the effects of the sintering temperature on the physical properties were systematically analysed. The evolution of the metal surface morphology during the hot embossing process was also investigated. The results indicate that the feedstock can be used to manufacture micro-fluidic die mould cavities with a low roughness, proper dimensions and good shape retention. The shrinkage of the sintered part was approximately 19-24% compared with that of the brown part.

Experimental Analysis and Numerical Simulation of the Flow Behaviour of Thin Polymer Films during Hot Embossing

The geometrical and material properties of polymer components made by polymer hot embossing are largely affected by the embossing pressure and temperature, and depend on physical properties of considered polymers as well as on cavity shapes and roughness. The present paper is focussed on experiments and numerical simulations of the hot embossing processes consisting in the replication of polymers plates on engraved shapes in the metallic plates that are used for the tests. The numerical simulations of the embossing process are realized for 2D or 3D geometries, and provide the way to access to the pressure on the polymer substrate, as a function of the viscosity of the polymers under given pressure and temperature conditions. The ability of the process to fill the cavities depends on the viscoelastic and viscoplastic properties of the polymers, for the temperature and strain range considered. In the proposed analysis consisting of the micro‐indentation of the polymer plate, axisymmetric and 3D FE models ar...

Modeling, Identification and Simulation of the Sintering Stage for Micro-Bi-Material Components

This paper investigates the numerical simulation of the sintering stage by solid state diffusion during the metal injection molding process for micro-bi-material component based on a thermo-elasto-viscoplastic model. The physical parameters concerning very fine 316L stainless steel and copper powders with high volume loading contents involved in the sintering model have been identified in order to set up finite element simulations. The experimental tests have been carried out in a vertical dilatometer and the identification of the material parameters have been carried out with Matlab® platform software. Then in order to predict the shrinkage and relative density after densification, a solid state diffusion model for the sintering has been implemented in finite element software to perform the simulation of the sintering stage.

Simulation and Modeling of Sintering Process for 316L Stainless Steel Metal Injection Molding Parts

The metal injection molding (MIM) process allows the manufacturing of small and very complex metallic components. The metal injection molding processing combines the shaping capability of polymer injection molding with the large material variety of metals and ceramics. This paper discusses in detail the development of a numerical model capable of simulating micro-structural evolution and macroscopic deformation during sintering of complex powder compacts. A sintering model based on elastic–viscoplastic constitutive equations was proposed and the corresponding parameters such as bulk viscosity, shearing viscosity and sintering stress were identified from dilatometer experimental data. The constitutive model was then implemented into finite element software in order to perform the simulation of the sintering process. The numerical simulation methods being compared against results of the sintering experiments. The experimental data were obtained from sintering of 316L stainless steel powders.

Material characterisation to identify the rheological behaviour for powder injection moulding

Abstract For the purpose of powder injection moulding, a specific method is presented to identify the parameters for a combined rheological constitutive model. This model is a combination of three models used in powder injection moulding and accounts for the effects of the temperature, shear rate and particle size of the powder. The data obtained from experimental measurements were used for identification of the model parameters. The model with identified parameters could be used to realize accurate simulations.

Adhesion Strength Study of Silicone Rubber Compounds to Nylon 66

In recent years, multi-component injection molding has largely developed in the industries. During the manufacturing process, the critical challenge is in achieving an optimum adhesion between the two materials. This study examines the influence of curing kinetics on the interfacial adhesion strength between a silicone rubber and nylon 66 using a rotational rheometer. The evolution of adhesion strength of assembly during the curing behaviour of silicone rubber was measured by tensile test at different curing temperature and different curing time. The results showed a significant increase in adhesion was obtained while either the curing temperature or the curing time increased. In this study, the rheological property of silicone rubber fluid has been also determined by the rotational rheometer. In addition, the kinetics of the curing reaction of silicone rubber has been studied by differential scanning calorimeter.

Investigation on Replication of Microstructures on Flexible Substrates by Roll Embossing Process

The roll embossing is an efficient process to replicate micro‐patterns at continuous polymeric films. This process has been selected for its capabilities for manufacturing by rapid fabrication concepts high quality micro‐structures on polymeric thin films. The aim of this work is to develop experimental investigations and analyses of the proper embossing conditions (embossing velocity; material pressure and temperature) needed for the replication of micro‐structured parts with various geometries and sizes. In the present analysis, one propose a comparative that investigation considering filling rate, roughness and shape of embossed structures that have been realised and examined. The paper demonstrates that roll embossing shows proper replication facilities at the micro‐scale, to reproduce same geometrical dimensions and surface roughness than these corresponding to the cavities.

Modeling and Simulation of the Flow of a Thermoplastic Polymer during Filling of a Cylindrical Micro‐Cavity

This work is related to experiments and modelling concerning viscous polymer flow such as cyclo‐olefin polymer (COP) and cyco‐olefin copolymer (COC) arising in the hot embossing process in order to understand and predict the filling of microcavities. The simulation results are obtained for axisymmetric geometries. The filling time and the dimensions of polymer with the rheological and experimental process parameters are obtained. From the variations of the radius characterizing the squeeze flow of the polymers between plates with or without cavities, it is possible to relate the rheological properties (fluidity index, consistency, melt flow index and viscosity) to the aptitude of the polymers to reproduce the geometrical shape and surface asperities of a microstructured mould. The flow imposed to the polymeric material in shear or elongational mode was correlated to the rheological approach. This approach allows to better understand the compression of thermoplastic disks as well as the filling mechanism o...

Modelling and experimental determination of the replication of a cylindrical shape relief

In that paper, one uses the concepts of contact mechanics to describe the quality of reproduction of cylindrical cavity shape by hot embossing. It results in a negative replica of the initial shape. This model takes into account the deformation of the polymer material imposed by the forming process. These results point out the influence of experimental parameters (temperature, pressure) as well as those of the elastic modulus (Young’s modulus and Poisson’s ratio). The analytical data are compared with the metrology results obtained using a scanning mechanical microscope in two or three‐dimensional mode. The set of data enables a predictive approach of the engraving quality depending on the polymer mechanical properties. The final goal is to adapt this model to the hot embossing process.

On the use of hot embossing for the reproduction of the surface topography of mould microreliefs

Abstract This article describes the quality of reproduction of microcavities structured into mould substrates which were filled by the imposed flow of amorphous polymeric materials. The rheology of the selected materials (cyclic olefin copolymer, COC) depends on the experimental parameters (temperature, pressure) used for the hot embossing process. To support the experimental data, the polymers were qualified by their melt flow index, flow index and consistency. The efficiency of the filling procedure into microcavities with smaller and smaller size is described using the potentialities of a customized Scanning Mechanical Microscope (SMM).