Frederik Desplentere

Variability of flax fibre morphology and mechanical properties in injection moulded short straw flax fibre-reinforced PP composites

The influence of compounding and injection moulding on the initial variability and morphology of short straw flax fibres is determined and the mechanical properties for the injection moulded fibre reinforced composites are measured. It is found that the composition of the straw flax, flax fibre bundles and woody parts, together with the cutting process strongly affects the initial fibre morphology and its variability. In the initial fibres, small particles as well as long fibres with large width were found. A filter was used to reject the fibres with an aspect ratio below 15 before calculating statistics because these fibres have a negligible contribution to the composite reinforcement. After processing, the initial fibre length and width decrease strongly (−38% to −66% for length and −22% to −72% for width). Also, the variability is affected resulting in a standard deviation shifted towards lower fibre lengths and widths (−55% for length and −71% for width). The improvement of mechanical properties of the flax compound compared to the pure matrix material for the injection-moulded samples is found to be similar to the results for compounds with further processed flax fibres such as scutched and hackled fibres. An increase of tensile strength by 20% was found, for stiffness the increase is in the order of 50–70%. This indicates that despite the very large variability of the initial straw flax fibres and the strong changes of the variability in each processing step, a compound is obtained with improved mechanical properties.

Thermal modelling of normal distributed nanoparticles through thickness in an inorganic material matrix

Nanoscale materials showing superior thermal properties have raised the interest of the building industry. By adding these materials to conventional construction materials, it is possible to decrease the total thermal conductivity by almost one order of magnitude. This conductivity is mainly influenced by the dispersion quality within the matrix material. At the industrial scale, the main challenge is to control this dispersion to reduce or even eliminate thermal bridges. This allows to reach an industrially relevant process to balance out the high material cost and their superior thermal insulation properties. Therefore, a methodology is required to measure and describe these nanoscale distributions within the inorganic matrix material. These distributions are either random or normally distributed through thickness within the matrix material. We show that the influence of these distributions is meaningful and modifies the thermal conductivity of the building material. Hence, this strategy will generate a...

Two-component injection moulding of thermoplastics with thermoset rubbers: Process development

Two-component injection moulding is a manufacturing process for combining polymers with different properties in a single injection moulding process. The process is typically used to combine thermoplastics with another thermoplastic or with a thermoplastic elastomer to create colour differences or hard and soft areas respectively. The present study aims at the development of a two-component injection moulding process for the combination of a thermoset rubber and a thermoplastic. Currently products that consist of those two materials (e.g. wheels, syringes and other products with gaskets) are made by assembling separate components. Implementing the two-component injection moulding technique for these products will result in better interphase properties, savings on rubber and avoiding costs related to the assembly process. A technological challenge is posed by the fact that injection moulding of rubbers and thermoplastics is fundamentally different. The injection moulding of a rubber requires a heated mould ...

A Combined Experimental and Modelling Approach towards an Optimized Heating Strategy in Thermoforming of Thermoplastics Sheets

Abstract Determining the operational settings for the heating equipment in thermoforming is still mainly done by trial and error as well as personal experience. Depending on the type of IR heating equipment, these settings can be the consumed electrical power or the desired temperature of the heating elements. In this study, a workflow is developed, applied and validated to characterize the IR heating equipment and to determine the optimal heating strategy. The workflow starts with an on-site equipment/machine characterization, which takes all machine and environment parameters into account. This approach results in the optimal heater setting and heating duration in order to obtain a through thickness temperature distribution which lies within a predefined forming range. The proposed methodology is universally applicable as it can deal with different types of sheet material and thicknesses. Moreover it can be applied to any type of IR heating element (halogen, metal foil, ceramic or quartz). Moreover, the methodology can easily be implemented in an industrial environment. Additionally, an estimate for the thermal efficiency of halogen heater equipment can be determined.

Processing biobased polymers using plasticizers: Numerical simulations versus experiments

In polymer processing, the use of biobased products shows lots of possibilities. Considering biobased materials, biodegradability is in most cases the most important issue. Next to this, bio based materials aimed at durable applications, are gaining interest. Within this research, the influence of plasticizers on the processing of the bio based material is investigated. This work is done for an extrusion grade of PLA, Natureworks PLA 2003D. Extrusion through a slit die equipped with pressure sensors is used to compare the experimental pressure values to numerical simulation results. Additional experimental data (temperature and pressure data along the extrusion screw and die are recorded) is generated on a dr. Collin Lab extruder producing a 25mm diameter tube. All these experimental data is used to indicate the appropriate functioning of the numerical simulation tool Virtual Extrusion Laboratory 6.7 for the simulation of both the industrial available extrusion grade PLA and the compound in which 15% of p...

Experimental and Computational Analysis of the Heating Step during Thermoforming of Thermoplastics

The present study addresses the difficulties in heating thermoplastic sheets for ther-moforming applications. In industrial environments, the sheets are heated in a contact free method by means of convective hot air ovens and infrared radiation. In this study the temperature evolution at the outer surface as well as the core of thermoplastic sheets as a function of time is measured by means of thermocouples. These measurements reveal significant through thickness temperature dif-ferences which need to be resolved before high quality products can be made. The temperature dif-ferences can be decreased by decreasing the radiative power. This is however not acceptable in in-dustry since it lowers the number of produced parts per unit of time.In order to gain insight in the time-temperature relationship during the heating phase, a finite differ-ence model is developed. The model clearly shows the constantly changing through thickness tem-perature distribution and can be used as a tool by the thermoforming industry to optimize the pro-duction process.

Computational Modeling of Thermal Phenomena in Nanomaterials for Building Applications

By adding low thermal conductive particles to conventional building materials, it is possible to lower the total thermal conductivity of the new building composite by almost an order of magnitude. These porous materials, such as aerogel, have very interesting thermal properties. In this article, firstly, the structure of an aerogel is described and several heat transfer mechanisms with respect to the nanoscale are listed. Secondly, the thermal conductivity of the solid spherical aerogel backbone, with different interfacial contact areas, is analytically calculated. Based on these calculations, thermal flux simulations are provided, to investigate the size effect of this interfacial contact area. Thirdly, a cubic unit cell approach of solid backbone and air voids is described to examine the difference in gaseous and solid conductivity and their individual influence. The values for the conductivity for each phase were calculated analytically. In the fourth place, the total thermal conductivity of this nanomaterial is predicted by use of simulations. The influence of several particle diameters and gaseous void dimensions are included in three clarifying cases. The predicted aerogel conductivities can be used in a microscale model for building insulation simulations. Finally, in the last section, a defect in the solid backbone is introduced to investigate its influence on the total thermal conductivity.

Characterisation of the mechanical behaviour of a polyurethane elastomer based on indentation and tensile creep experiments

This research focuses on the determination of the mechanical properties of a viscoelastic polyurethane material with 2 different measuring techniques on 2 different length scales. Instrumented Indentation Testing (IIT) was used to test the material on a micro scale while tensile creep experiments characterised the macro scale material behaviour. All experimental data were processed by means of a fitting procedure based on the standard linear solid material model. The experiments were performed with different loading rates and hold values. The developed fitting procedure proved to be applicable to analyse the experimental data on both length scales. FEM was used to coordinate the applied strains of both measuring techniques. A comparison between the results originating from the experiments with both techniques indicated a stiffer material response on the micro scale (up to 4x). The more complex strain field inside the material during indentation compared to the uniform tensile loading on macro scale is responsible for this large discrepancy. For this reason comparing the results of IIT with tensile creep results should be done with great care.

Modelling the elastic response of a polyurethane open cell foam based on a minimal surface energy approach

Within the present study the elastic response of a flexible open cell polyurethane foam was studied by means of experimental compression test and finite element (FE) modelling. The compression tests revealed a pronounced sample size effect which was taken into account using an analytical model. In order to eliminate the sample size and damage effects, a minimal sample size of at least 50 times the cell size was necessary in the case of the flexible foam. Surface evolver software was used to model the open cell foam structures. The FE unit cells are based on the well-known Kelvin cell and the more complex Weaire-Phelan cell topology. In both cases the cross sectional shape of the cell edges was completely determined by the minimization of the surface energy. The thus build FE-models possess a good resemblance to real open cell foam structures. The influence of relative density and shape anisotropy on the elastic properties of the cellular structures was analysed using the FE-models.

Influence of the power law index on the fiber breakage during injection molding by numerical simulations

In predictive engineering for polymer processes, the proper prediction of material microstructure from known processing conditions and constituent material properties is a critical step forward properly predicting bulk properties in the finished composite. Operating within the context of long-fiber thermoplastics (LFT, length > 15mm) this investigation concentrates on the influence of the power law index on the final fiber length distribution within the injection molded part. To realize this, the Autodesk Simulation Moldflow Insight Scandium 2013 software has been used. In this software, a fiber breakage algorithm is available from this release on. Using virtual material data with realistic viscosity levels allows to separate the influence of the power law index on the fiber breakage from the other material and process parameters. Applying standard settings for the fiber breakage parameters results in an obvious influence on the fiber length distribution through the thickness of the part and also as funct...