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Although hot embossing is gaining popularity in the replication of micro-structures, it needs a breakthrough improvement on cycle time reduction before it can become a mass-production process. Previously, the authors developed a Rapid Thermal Response (RTR) molding process for enhancing the quality of conventionally molded parts. In the current study, this technology was adopted to hot embossing micro-structures. In the paper, RTR hot embossing machine setup and mold construction were described, and the results of fatigue test and RTR embossing were presented. Microstructures with characteristic dimension of 2?m were successfully replicated with substantially reduced cycle time. The fatigue test result indicated that this embossing technology is durable and reliable for microscale feature replication.
Recent progress in computer-aided polymer processing analysis demonstrates the need for accurate description of the material behavior under the conjugated effect of applied stress and temperature. In this work, we are interested in the characterization of circular thermoplastic membranes, ABS and HIPS thermoforming grade, under biaxial deformation using the bubble inflation technique. Hyperelastic (Mooney-Rivlin, Ogden) models are considered. First, the governing equations for the inflation of a flat circular membrane are solved using a dynamic finite element model (triangular membrane elements), and there after, a neuronal algorithm is employed to determine the materials constants. Moreover, the influence of the Mooney-Rivlin and Ogden constitutive models on the thickness and the stress distribution in the thermoforming sheet are analysed.
Hot embossing is an effective method for transferring micro-features in mold to plastic film or plates. Improving uniformity of pressure and reducing cycle time are constant challenges with MEMS or NEMS applications. This paper reports development of a rapid heating and uniformly pressing system for micro hot embossing. Direct fluids are used as working media. The seal film/mold/substrate stack is placed in a closed chamber. Then the fluid is introduced into the chamber for heating and pressing the stack. Micro patterns in the mold can be successfully replicated onto the substrate. Perfectly uniform embossing pressure throughout whole area can be achieved. The cycle time is less than 30 seconds.
Closed mold reactive liquid composite molding processes such as resin transfer molding (RTM) and any of its variations such as vacuum-assisted resin transfer molding (VARTM), Seemann Composite Resin Infusion Molding Process (SCRIMP), etc, are environmentally friendly alternatives to open mold processes, which have been traditionally employed to form large composite parts. However, in most cases, in order to improve and/or protect the part surfaces, gel coating is required. The gel coat is applied with the mold open, which releases harmful volatile organic compounds (VOCs) to the environment, partially compromising the benefits of the closed mold processes. In-mold coating (IMC) is an attractive alternative to gel coating to eliminate VOCs. IMC is a coating operation performed by injecting a coating material onto the surface of the substrate with the mold closed. The coating flows by compressing the substrate under pressure. In the present work, we develop mathematical models to predict the filling and packing stages of IMC for RTM substrates. These models include the effect of the compressibility of the substrate and mold.
A kinetic model of the vulcanization process of high consistency rubber (HCR) and liquid silicone rubber (LSR) was developed. The exothermal vulcanization process was measured with a differential scanning calorimeter. Viscosity was measured with a cone and plate rheometer. A computer program fit coefficients to the experimental data. Values for activation energy and fitted rate coefficient were found using both nth order polynomial and autocatalytic models. A kinetic model of vulcanization will help manufacturers understand and optimize their production processes.
In low temperature composite manufacturing processes, a major concern for material suppliers and fabricators is how to control the resin gel time and cure time and how to achieve a high final conversion with low residual volatile organic chemicals. In this study, a cobalt promoter catalyzed dual-initiator system is used to control the reaction rate and resin conversion of unsaturated polyester (UP) resins. A mechanistic kinetic model is developed to predict the reaction kinetics with dual initiators. This model can be utilized to simulate both isothermal and dynamic reaction rate and conversion profiles. It can also be used to predict the effect of promoter contents on UP resins cured at low temperatures.
The automotive industry requires parts with high strength and low weight, and if the application requires it, surface quality. These requirements have led to investigate the use of carbon fibers as a reinforcement alternative to the widely used glass fibers. Our previous studies compared the performance of glass and carbon fibers in unsaturated polyester based sheet molding compounds (SMC) for non-structural applications. These showed that there are compromises between the performance of physical properties, cost (incurred by adding the more expensive carbon fibers), and consistency (i.e. variability). In this work, we investigated the effect on physical properties of SMC structural parts when there is a mixture of carbon and glass fibers. Special considerations in the analysis and the implementation of these experiments are discussed.
Polymer-layered silicate nanocomposites(PLSN) as nanometer scale reinforcements offers an interesting alternative for the modification of polymer matrix properties with really great improvements in their mechanical, thermal and physical properties. The biphenyl epoxy(BPE)-phenol aralkyl novolac (so called xylok resin, XK)-montmorillonite (MMT) hybrid PLSN were newly synthesized via indirect melt process using pre-intercalated XK-MMT PLSN to avoid the fatal disadvantages of storage stability in conventionally synthesized epoxy PLSN due to the reaction between the epoxy resins and the organic group in the MMT. This storage stability is one of the important properties for the commercial uses such as epoxy molding compounds (EMC). To develop a novel formulations for the semiconductor packaging which have the good storage stability, we prepared pre-intercalated XK-MMT PLSN as the first step, and then we synthesized BPE-XK-MMT PLSN using these pre-intercalated XK-MMT PLSN. In this work we studied the effects of the MMT with different organic groups. Also we investigated the evidences of the power ultrasonication effects on the nano-scale structure. The x-ray diffractometer results shows the intercalated or exfoliated PLSN which were characterized by conducting differential scanning calorimeter, dynamic mechanical analyzer, thermo gravimetric analyzer, universal testing machine, and impedance analyzer. Moreover we could figure out the optimum contents of the MMT from the agglomeration due to the higher loading of the MMT.
Recycled poly(ethylene terephthalate) (R-PET) was chain extended with pyromellitic dianhydride (PMDA) in an industrial scale twin-screw reactive extrusion system. Reactive extruded recycled poly(ethylene terephthalate) (RER-PET) samples at different PMDA concentrations were characterised in terms of rheological properties; thermal transitions and crystallinity. The results confirm the increase in molecular weight with an increase of PMDA concentration, and the formation of branching at concentrations above 0.25 wt.% PMDA. Structural changes due to PMDA addition affect the Tm, Tc and the crystallinity; however, no significant change was observed for the Tg.
The effects of annealing time and temperature on the transient shear rheology of a thermotropic liquid crystalline polymer are reported. During flow startup, the first shear stress maximum and shear stress minimum was found to depend on annealing time, temperature, whereas the second shear stress maximum, a true overshoot, depends on the temperature, but not on annealing time. In situ rheo-optical characterization revealed that the evolution of the melt texture depends on the annealing time and temperature. We confirm that the threaded texture leads to the generation of the local shear stress maximum and shear stress minimum during shear flow startup.
The SER Universal Testing Platform marks a revolutionary breakthrough in the field of physical material characterization technology. Designed as a detachable fixture for a rotational rheometer host system, the SER design incorporates dual windup drums that ensure a truly uniform extensional deformation during uniaxial extension experiments. Although originally developed as an extensional rheometer to be used for the rheological characterization of uncured polymers, this remarkably versatile miniature test platform can be used in characterizing a host of physical properties on a variety polymer melts and solid state materials over a very wide range of temperatures and kinematic deformations and rates. This single instrument is capable of converting a conventional rotational rheometer host system into a single universal testing station capable of performing experiments from extensional melt rheology to solids tensile, tear, cut fracture, peel/adhesion and friction testing all within a controlled environment. Experimental results demonstrating some of these testing capabilities are presented for polyethylenes of varying macrostructure.
Quick tests of elastomer compounds are usually done at low shear rates far from processing. A rheological characterisation of thermo sets under curing conditions like in the later processing is so far not possible.A new device, the RCR" a further development of the "Rheovulkameter" concept is described. The selectivity is compared to a standard Mooney and rheometer measurement for elastomer compounds. A model to estimate entrance pressure loss to characterize elongation behaviour from the simple extrusion test is introduced and compared to capillary rheometer data. Finally the determination of rheological parameters of thermo set material under process like conditions is introduced."
Knowledge of the elongational viscosity and its influence on the extrusion process is becoming more and more important. In order to determine how far the quality of extruded films can be deduced by measurements of the shear and elongational viscosity, an offline system, a twin bore capillary rheometer with a zero die, and a shear-elongation die as an online system have been used. The shear-elongation die has been integrated into the film extrusion line. The film quality is characterized by the film thickness profile for PE-LD and PE-LLD.Furthermore, a comparison between the rheometers will follow, giving information of the effective measuring ranges and the applicability.
Creep experiments have been applied to probe the zero-shear viscosity ?0 of molten entangled polyolefin chains directly and precisely in a constant-stress rheometer. Use of these ?0 data, with precise weight-average molecular weight Mw data, has enabled detection of long-chain branching (LCB) levels in polyethylene to as low as 0.006 branches/1000C (carbons). Approaches are discussed to address several complications to this relationship, such as sample stabilization, residual stress, polydispersity, and short-chain branching, to interpret LCB levels in polyolefin materials.
We have investigated the shape of storage modulus (G') and loss modulus (G) of complex fluids under large amplitude oscillatory shear (LAOS) flow. However as the strain amplitude increases the stress curve becomes distorted and some important information may be smothered during data processing. Thus we need to investigate the stress data more precisely and systematically. In this work we have obtained the stress data using high performance ADC (analog digital converting) card and investigated the nonlinear response of complex fluids Diverse and unique stress patterns were observed depending on the material system as well as flow environment. It was found that they are characteristics of the material system."
Melt compounding was employed to prepare nanocomposites of exfoliated organophilic montmorillonite (o-mmt) clay dispersed in a maletaed poypropylene (PPgMA) and PPgMA compatibilized composites of clay and polypropylene (PP). Several grades of PPgMA of different melt flow indices (MFI) were analyzed for the effectiveness of melt exfoliation of organoclay. The extent of clay exfoliation in the nanocomposites was confirmed by X-ray diffraction and transmission electron microscopy. The thermal effect on the rheology and compounding was also investigated. The shear viscosities of the PPMA compatibilizers are highly dependent on the processing temperature. The experimental results indicated that the high mixing temperature causes easier diffusion of polymer into clay galleries, and more complete wetting of clay stacks, however, the mixing torque exerted on the clay particles becomes lower. Thus the mixing temperature had to be varied according to different grades of PPMA in order to achieve desirable level of torque and yield extensive exfoliation of organoclay in the nanocomposites.
Nanoindentation techniques are increasingly being used to probe the mechanical response of polymers. In contrast to traditional engineering materials (i.e., metals and ceramics) to which indentation techniques have most often been applied, the characterization of polymers by a single modulus or hardness values is generally of limited value, particularly because polymers behave in a viscoelastic fashion. Additionally, polymers often exhibit nonlinear behavior at relatively small levels of strain, and their responses to tension, compression, and shear can be quite different. Thus, a number of challenges exist to applying nanoindentation methods to polymeric materials. In this paper, the use of nanoindentation to characterize polymeric materials is presented and discussed, including both quasi-static and dynamic methods.
In this paper, numerical simulation of the hot embossing process with non-isothermal embossing conditions (i.e. the initial temperature of the polymer substrate is the room temperature) was carried out to observe the flow patterns of thin PMMA films into micro cavities. Different thicknesses of PMMA films, from 1 ?m to 400 ?m, were used in the simulation. It was found that, as the thickness of the PMMA film reduced the filling mechanism varied. For PMMA films with a thickness above 100 ?m, the polymer flow climbed along the wall of the heated die, and then compressed downward and squeezed outward. In contrast, for a smaller thickness of less than 50 ?m, the flow was uniform and the wall climbing flow was absent. This size effect was explained using the temperature distribution of the polymer substrate during the embossing process. For a thickness above 100 ?m, the high temperature zone was localized in the vicinity of the die wall, and consequently localized wall climbing flow resulted.
The fluid dynamics of channel geometries for liquid state materials characterization in microfluidic devices are investigated. A pressure driven microchannel device is sought that has an adjustable flow type, approximating the function of the four-roll mill. In particular, classes of channel flows in which the full range of linear flows (extension, shear and rotation) can be approximated in the neighborhood surrounding a stagnation point are investigated using finite element flow simulation and flow classification criteria. A class of flow geometries is identified which makes use of opposing, laterally offset fluid streams that produce a stagnation point in the center of the geometry.
During the last three decades a progress has been made in modeling of the die swell by the introduction of the so-called first normal stress difference and shear stress. Initially developed by Tanner, the model has undergone several improvements or alterations, leading to the development of new formulations. The purpose of the present investigation is to review the formulation of the die swell models proposed by Tanner, Bagley and Duffy, Mendelson and Finger, White and Roman, Vinogradov and Malkin, Macosko and Kumar et al. Next, an alternative formulation is proposed, which does not appear to exhibit mathematical defects, and an attempt is made to explore its modeling performance by comparing the predictions with the experiments of low density polyethylene, polypropylene and polystyrene under a steady shear mode over the wide range of shear rates and processing temperatures.
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