Volume 26, Issue 5

The impact-resistant capacity of reinforced concrete (RC) structures can be rationally upgraded by mixing polyvinyl alcohol (PVA) short fibers into concrete. The static and dynamic post peak residual tensile strength of the concrete that occurs because of the bridge effects of PVA short fibers. In this paper, to establish a simple impact response analysis method for RC structures under impact loading, a method that treats PVA short fibers mixed into concrete as shell elements of PVA plates with an equivalent tensile strength to the post peak residual tensile strength of the concrete is proposed. The applicability of the method has been investigated by conducting a three-dimensional elastoplastic finite element analysis using LS-DYNA code and comparing the results with the falling-weight impact test results for RC beams. From this study, it was confirmed that the crack distributions and the failure mode of RC beams mixed with PVA short fibers can be predicted and that the effects of mixing PVA short fibers on improving the impact resistant capacity can be estimated by using the proposed numerical analysis method.

The wide range of applications associated with nanocomposites is due to their improved properties when compared to conventional composites. In this study, commercial epoxy-based powder coatings were formulated with 2 and 4% (w/w) of an organically modified montmorillonite (OMMT) by incorporation in the melt state (extrusion). These composites were characterized by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and X-ray diffraction (XDR). The thermal behavior and stability of the epoxy nanocomposites were studied by TGA and DSC. The nanoparticle structure within the polymer matrix was analyzed by XRD and TEM. A predominantly exfoliated structure was observed by TEM, which was confirmed by XDR analysis. The study demonstrated that the nanoclay increases the glass transition and crosslinking temperatures and also enhances the thermal stability of the coating.

This study proposes a simple technology using electromagnetism to control the direct hot imprint resist of a nickel mold. Combining the present gas-assisted hot imprint molding technology, electroforming technology, and self-designed heatable electromagnetic plate for even control and progressive pressuring technology, this study used electromagnetic nickel mold direct hot imprint to replicate microstructures, in order to make the molding technology and application of micro/nanoimprint more mature. This study first used gas-assisted hot embossing method to replicate structures on polycarbonate (PC), this PC film with microstructures is cast into nickel molds by electroforming. This nickel mold was used as the mold for magnetic embossing to imprint hot plastic polymethylmethacrylate (PMMA). The result showed that through gas-assisted hot embossing molding and electroforming, molds with high costs and complicated production processes can be massively replicated, and the replication precision is good. Through electroforming, the casting duration of magnetic nickel molds can be shortened and costs can be effectively lowered. Moreover, an electromagnetic plate was used to evenly control the direct hot pressure imprint resist, which is an advantage of the production process for micro structures.

High frequency induction heating is an efficient means of rapidly heating a mold surface by means of electromagnetic induction. It was recently applied to the rapid heating of injection molds for a range of purposes. To implement high frequency induction for rapid mold heating, the heating conditions have to be set appropriately to obtain the desired range of mold temperatures. In the present study, a rapid mold heating system is developed. It consists of a high frequency power supply, induction coils, a robot system, temperature sensors and controllers. The mold surface temperature is measured using the temperature sensors, and this data is then fed back to the controller so as to control the heating time. This fully automated induction heating system with feedback temperature control is then implemented for the injection molding of a transparent part with multiple holes, resulting in great improvement in the surface appearance as well as the optical properties of the molded part.

The bowing distortion of polypropylene (PP) film during transverse direction (TD) stretching with a tenter was newly predicted with consideration for heated air flow. The distribution of the heat transfer coefficient on film was calculated by the results of air flow analysis in the tenter in order to obtain a more realistic heat transfer condition than in previous works based on an impinging jet model and a wall jet model. Bowing distortion after stretching was calculated by applying the distribution of heat transfer coefficient obtained from air flow analysis under the assumption that the deformation of film obeys an elastic-plastic rule. Another bowing distortion was also calculated by applying an impinging jet model and a wall jet model in order to evaluate the applicability of the distribution of heat transfer coefficient obtained from air flow analysis. The authors experimentally measured the bowing distortion and the film thickness after stretching using a pilot plant. It has been shown that the calculated results based on the distribution of heat transfer coefficient obtained from air flow analysis are closer to the experimental results than previous works based on an impinging jet model and a wall jet model, particularly at faster line speed. It is revealed that the distribution of heat transfer coefficient has a strong influence on the temperature of film, and the difference of the predicted temperature causes a difference in the thickness change in film and longitudinal stress. The authors concluded that the difference in thickness change in film and longitudinal stress brought about the difference in bowing distortion.

The replication accuracy of ultra-high molecular weight polyethylene (UHMWPE) was investigated by using injection molding (IM) and injection compression molding (ICM). The mold insert was fabricated by using a stainless-steel etching method. Both the injection molded part and the injection compression molded part were observed under a microscope to compare the quality and accuracy of the replication. A high performance surface profiler was used to measure the microstructure profile. For the mold insert, the injection molded parts and injection compression molded parts were measured. The experimental results show that the UHMWPE can fill in microcavities by using IM and ICM technology. The height and shape of microstructure were influenced by the IM process parameters. For an injection molded part, the injection velocity was the most significant factor. This study also revealed that the ICM method was a more stable microstructure processing method than that of the IM. However, for better replication accuracy, both the IM and ICM processes require parametric optimization.

In this study we explore the use of solid-state nucleation in pellets as a means to decouple cell nucleation from cell growth in extrusion. This is achieved by feeding the extruder with gas-saturated PLA pellets. Cell nucleation occurs, similar to that of the well-studied solid-state batch foaming process, when gas-saturated pellets are crushed and heated in the feed section of the extruder. The polymer melt consequently contains the nucleated cells. Further, the gas content in the pellets provides a means to deliver a controlled amount of blowing agent for cell growth at the die exit. It was found that gas concentrations in partially desorbed pellets offers greater control of part density and foam morphology compared to that of fully saturated pellets at 2 MPa. Using Design-of-Experiment statistical methods it was found that gas concentration in the polymer was the most significant variable influencing foam density, whereas the extrudate cooling method was found to control cell size.

This study is analyzed variations of ultimate strength, friction coefficient and wear mass loss that depend on the injection molding techniques during the blending of short glass fiber (SGF) and polytetrafluoroethylene (PTFE) reinforced polycarbonate (PC) composites. A hybrid method including response surface methodology (RSM) and back-propagation neural network (BPNN) integrating simulated annealing algorithm (SAA) are proposed to determine an optimal parameter setting of the injection molding process. The specimens are prepared under different injection molding processing conditions based on a Taguchi orthogonal array table. The results of eighteen experimental runs were utilized to train the BPNN predicting ultimate strength, friction coefficient and wear mass loss. Simultaneously, the RSM and SAA approaches were individually applied to search for an optimal setting. In addition, the analysis of variance (ANOVA) was implemented to identify significant factors for the injection molding process parameters and the result of BPNN integrating SAA was also compared with RSM approach. The results of optimal parameters of injection molding process for the ultimate strength of x-direction and y-direction based on BPNN/SAA approach were increased 3.12%, and 6.18%, respectively.

Co-rotating twin-screw extruders are widely used compounding machines. They are mainly configured based on extensive experience and iterative approaches to optimise output and composite quality. The visualisation technology developed in the EU-project PEPTFlow allows visualisation of composite flow in twin-screw extruders under realistic processing conditions by tracking radioactive tracer particles in the polymer melt, using a specially developed camera system. This new approach allows polymer flow to be studied in different screw elements and screw configurations under realistic compounding conditions at normal temperatures and melt pressures. The paper presents the latest developments in the camera systems as well as the different ways to use and interpret the results. Detailed analysis of residence times and residence time distributions for standard compound screw elements, like kneading discs, conveying elements and reverse elements are presented. In addition for a better understanding of the flow field inside twin-screw extruders, numerical particle tracking is done. The Stokes equation, using XFEM method, are solved and the numerical RTD's (residence time distribution) are compared for various screw designs.

This study investigates the influence of melt and process parameters on the quality of water-assisted injection molded tubes. The influence was determined by the aid of a design of experiments (DOE) for both a HDPE and a PP resin. This design was carried out on two different cavities, in which the residual wall thickness (RWT) and the part weight were set as response functions. This was combined with a visual inspection of the part surface to detect correlations in the occurrence of part defects such as fingering and double wall. It was found that the water injection delay time and the water volume flow rate have the largest influence on the RWT and part weight. Moreover, the shear viscosity of the applied material determines the width and shape of the water bubble and consequently the RWT and part weight. In addition, process parameters such as melt temperature, water hold time and water volume flow rate as well as melt parameters such as shear viscosity have an influence on the formation of part defects.

Unlike gas-assisted injection molding, subsequent penetration occurs on the radial direction after water displacing polymer melt in cavity due to the incompressibility and fast cooling of water in water-assisted injection molding. To simulate this second penetration, a dimensionless method was employed to simplify the governing equations for melt flow. Based on the reduced model, a formula was derived via mathematical inference to calculate the second penetration velocity and distance. In order to catch the huge temperature gradient at water-polymer interface, both polymer and water thermal conduct problems were founded and coupled at the interface. The finite element method was employed to solve the melt flow problem, and the finite difference method was used to discretize the energy equation. To verify the validity of the proposed model and algorithm, experiments were performed to measure the displacement of the water–melt interface for various melt temperatures and water pressures. The experimental results indicate that the primary penetration theory is not accurately on predicting residual wall thickness, whereas the combined primary and second penetration method significantly improve the accuracy.

In France, bitumen has been used since four decades as a container material for low/intermediate activity and long lifetime radionuclides. Industrial Bituminized Waste Products (BWP) are produced by continuous extrusion of 60 wt.% bitumen and 40 wt.% salts, in which a very small fraction is radioactive. These incorporated salts are, in majority, of two types: soluble or insoluble. During storage, the organic matrix subjected to irradiation produces gases, which can give rise to BWP swelling in the container and then to a possible overflow. The extent of gas production is directly linked to the degree of dispersion of the salts into the bitumen matrix. To quantify this dispersion, the effect of extrusion operating conditions, such as feed rate and screw speed, have been studied using a laboratory scale co-rotating twin screw extruder. BWP rheological characterization shows the development of a pseudo-solid plateau at low frequency and a change in the linear viscoelastic domain when screw speed and feed rate are varied, indicating a modification of the dispersion state. Samples characterization with environmental scanning electron microscopy (ESEM) confirms in the case of insoluble and soluble salts a change in the number of agglomerates, accompanied by an improvement of the dispersion at high screw speed but less remarkable with the feed rate. A Carreau-Yasuda law with a yield stress is proposed to describe the rheological behaviour of these materials. The effects of volume fraction and maximum volume fraction on viscosity are described using a Krieger-Dougherty equation.

The aim of this article was to determine the effect of the dicumyl peroxide (DCP) content on thermal and mechanical properties of polylactide (PLA). Reactive extrusion of the PLA and DCP blends was performed. The DCP content varied from 0.2 to 1.0 wt.%. The extruded samples were characterized by the Fourier transform infrared spectroscopy (FTIR), analyses of gel content and swelling degree, thermogravimetry (TG), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), and tensile and impact strength tests. It was found that DCP caused crosslinking of PLA as well as contributed to formation of low-molecular weight products of decomposition and degradation processes. These products caused plasticization of PLA, which led to a decrease in the glass transition temperature. An increase in tensile strength and decrease in impact strength were observed as the DCP content increased.

We propose in this short note a rapid and efficient way to calculate the average residence time in a ko-kneader. Two experiments are enough to define the two parameters of the model. The proposed expression has been thoroughly validated by comparison with numerous experimental results obtained in various conditions of screw speed, feed rate and screw profile.