Key Engineering Materials Vols. 651-653

Abstract: Metal injection moulding (MIM) has over the past decade established itself as a competitive manufacturing process to produce in large quantities small precision components with complex shape which would be costly to produce by alternative methods. In this process, during the injection phase, segregation appears in the feedstock and defects will be appears in the component during the sintering. To limit this effect, during decades a vast variety of binder systems have been developed. Binder systems are formulated as a mixture of different organic or inorganic substances with several functions. Binder system has the main commitments of giving the necessary rheological behavior to the feedstocks for injection moulding to transport the powder particles into the mould cavity and the cohesion of the green part.In this paper a study of the behavior of binders based on polyethylene glycol (PEG) and feedstocks based on superalloy A-286 powder were investigated by Fourier Transform InfraRed spectroscopy (FTIR) coupling with rheology. This methodology permits to compare the rheological behavior of the materials with the thermal behavior of the binder at a temperature close to the temperature of injection. Analyzes was made for the mixing and injection process. It is shows relationship between interactions and miscibility of polymer and influences on the rheological and mechanical behavior. All methodologies revealed no chemical interactions between the powder and the binder. This result shows the formulation of binder for the superalloy A-286 is not homogenous and the PEG is degraded under mixing and injection process conditions of this kind of formulation of feedstock.

Abstract: Thermal flow tests were performed on steamed bamboo powder using capillaries that were processed under different conditions in order to investigate the effect of the die surface state on the fluidity of the woody powder. The capillaries were processed by wire-cut electric discharge machining, reaming or drilling, and the arithmetic average roughness (Ra) varied from 0.5 to 2.5 μm. The bamboo powder was first steamed at 200 °C for 20 min, and its particle size was then controlled using different mesh screens. The thermal flow temperature was set at 200 °C. The results indicated that the flow behavior improved with increasing particle size. For the capillaries processed by WEDM, the flow rate for samples with particle sizes of 75~150 and 150~300 μm decreased with increasing Ra. On the other hand, when reaming or drilling was used to process the capillaries, the flow rate was almost independent of Ra, regardless of the particle size.

Abstract: Several polymers can be combined in one multilayer structure by reactive coextrusion. Tie-layers are often used to compatibilize adjacent layers and may reduce or suppress interfacial instabilities and defects in multilayer coextrusion flow. However, a new additional defect defined as “grainy” defect can be observed. In our best of knowledge, no study in literature has been dedicated to understand its origin. The phenomena are quite complex due to the coupling of the effects of flow and the physico-chemical mechanisms at the interface. The aim of this work is to understand the relationship between the instabilities and defects encountered in multilayer coextruded films and the role of the copolymer formed in-situ between tie and barrier layers. Polyamide 6 (PA6) and ethylene-vinyl alcohol copolymer (EVOH) were used as barrier layers sandwiched in polypropylene (PP) with or without tie-layer based on polypropylene grafted maleic anhydride (PP-g-MA). Influence of process parameters and nature of the polymer pair on the generation of “grainy” defect has been assessed and related to the rheological and the physico-chemical properties of layers. These experiments showed that this defect appeared mainly in the compatibilized EVOH system and could be distinguished from the usual coextrusion instabilities. Interfacial properties between tie and barrier layers have been investigated. Shear stress relaxation experiments have been carried out on reactive tie/barrier bilayers. Due to the interphase generated in-situ, the relaxation behavior was altered by extending the relaxation time. Investigation of interfacial morphology highlighted that the copolymer architecture significantly affected the interface/interphase development and interface roughness. Hence, relationships between relaxation process, interfacial morphology and copolymer structure were correlated with the generation of grainy defects in coextrusion.

Abstract: The packaging of liquid products is conventionally realized by using two production stages, which are the stretch blow molding and the filling. In the stretch blow molding process, hot polyethylene terephthalate (PET) preforms are inflated by pressurized air into a cavity to form plastic bottles. In a follow-up process, these packages are filled by a separate machine with the desired liquid product. In contrast to that, liquid-forming combines the blowing and filling stages by directly using the liquid product to form a plastic bottle. Through this substitution, two main challenges arise. Firstly, there are significant inertia effects through the liquid mass, leading to additional reaction forces and a spatially inhomogeneous pressure distribution inside the preform. Secondly, the heat transfer between preform and fluid is drastically increased. Because of this cooling effect, a specific combination of forming speed as well as initial preform and liquid temperatures is necessary to avoid thermally induced preform rupture. This is based on the fact that the formability of PET rapidly declines below its glass transition temperature (T_g). Consequently, a process control requires the knowledge of how the process parameters influence the preform cooling. In this paper, a numerical simulation of the liquid-forming process (LF) is introduced including the preform cooling during forming. In addition, the strain-dependent self-heating effect of PET is implemented. Process experiments under different parameter combinations are conducted using simplified bottle geometry. Through a comparison of the results from experiments and from simulation, the influence of process parameters on the temperature drop and thus on thermally induced failure is determined. In this way, process understanding and control are increased.

Abstract: A transverse isotropic viscous model accounting for the anisotropy exhibited in fiber-reinforced composite is integrated in the numerical platform of the software Rem3D®. Simulations under various mechanical loading are tested for volume fiber concentrations of 3.5% and 14.7%. Equivalent stresses and equivalent strain rate deformations given by the software were compared to the ones predicted by the model, finding very good agreements. As a second point developed on this paper, we comment on the slip condition between Die/Punch tool with the composite under compression. We noticed that the variation of the viscosity value on a small layer between the Die/Punch tooland the composite affects the nature of the contact. A viscous friction is then formulated as a technique to set slip/no-slip contact condition. We found that the slip condition is recovered at lower values of the viscosity in the interface Die/Punch with the reinforced composite, whereas the no slip condition stated for higher viscosity values.

Abstract: Functional surface effects on plastic components are a matter of current research activities. To make these surface effects usable on an industrial scale, new processes have to be developed. Forming processes such as injection moulding are currently used to produce functional surfaces, but machine and tool costs are considerably high. Therefore, it is of high importance to analyse to what extent the thermoforming process can be applied to manufacture micro-structures.The so called In-Mould-Graining (IMG) process is a promising variant of the thermoforming enabling the manufacture of micro-structures. The advantage of the IMG process is, that the structure is formed to the final shape of the part. If the structure on the film surface is set before the forming, the conversion may cause a deformation of the structure. However, the forming accuracy of the IMG process is limited by the rapid cooling of the film at the cold mould surface. A significant time interval between the heating and the forming leads to a cooling of the film, especially at its surface and thereby reduces the forming accuracy in the area of the grains.In this paper it is investigated to what extent the moulding accuracy of the IMG can be increased by the use of a variothermal mould heating. To achieve the required forming accuracy, only the surface layer of the micro-structured region of the cavity is heated by induction before the contact between film and cavity occurs. At the same time, the entire mould is cooled with a conventional water cooling system. The analysis of the process and formed parts shows, that especially the mould temperature during the forming process has a significant impact on the forming accuracy. A forming accuracy of a 50 µm high micro-structure of more than 95 % can be achieved. Furthermore, the contact of angle of a water droplet on a micro structured thermoformed surface can be increased up to + 145°.

Abstract: The main purposes of this work are the technical investigation of polymer melt interactions of a well-designed PE/EVA blend with an azodicarbonamide foaming agent and the evaluation of the final characteristics of the foamed products. The investigation is mainly focused on the material characterization of the PE/EVA blend with Differential Scanning Calorimetry, taking samples from the virgin materials and from the injection moulded parts.

Abstract: For materials formed in the rubbery region eg. PET in the stretch blow moulding process, the normal assumption is that the material is incompressible. In this paper the validity of this assumption is challenged by conducting a series of experiments that measure the volumetric strain under different strain and temperature histories. Experiments have been conducted on a biaxial testing machine instrumented with lasers for measuring the strain through the thickness in combination with and digital image correlation for measuring the in plane strain for PET stretched in uniaxial and biaxial deformation between 90°C and 100°C. Results will be presented that show that the Poisson’s ratio for PET can vary between 0.4 and 0.55 depending on the test conditions. It is concluded that the values measured greater than 0.5 are due to the strain induced crystallinity that occurs with PET during the biaxial deformation process.______________________________________________________________________

Abstract: This paper deals with a numerical and experimental investigation on the influence of residual stresses on fatigue crack growth in AA2024-T3 friction stir welded butt joints. An integrated FEM-DBEM procedure for the simulation of crack propagation is proposed and discussed. A numerical FEM model of the welding process of precipitation hardenable AA2024-T3 aluminum alloy is employed to infer the process induced residual stress field. The reliability of the FEM simulations with respect to the induced residual stresses is assessed comparing numerical outcomes with experimental data obtained by means of the contour method. The computed stress field is transferred to a DBEM environment and superimposed to the stress field produced by a remote fatigue traction load applied on a friction stir welded cracked specimen. Numerical results are compared with experimental data showing good agreement and highlighting the predictive capability of the proposed method. Furthermore, the influence of the residual stress distribution on crack growth is evidenced.

Abstract: Full-field measurement methods have emerged in the last years and these methods are characterized by directly providing displacement and strain fields for all points over the specimen surface. Thus, the design of heterogeneous tests can be performed for material parameter identification purposes since the inhomogeneous strain fields can be measured. However, (i) no defined criterion yet exists for designing new heterogeneous tests, (ii) it is rather difficult to compare and rate different tests and (iii) a quantitative way to define the best test for material behavior characterization of sheet metals has yet to be proposed. Due to this, the goal of this work is the development of a global indicator able to assess mechanical tests. The proposed indicator quantifies the strain state range, the deformation heterogeneity and the strain level achieved in the test, based on a continuous evaluation of the strain field up to rupture. This global indicator was applied to rank some classical tests, such as uniaxial tensile, simple shear, plane strain and biaxial tensile tests. These tests were carried out numerically by reproducing the virtual behavior of DC04 mild steel. A constitutive model composed by the non-quadratic Yld2004-18p yield criterion combined with a mixed isotropic-kinematic hardening law and a macroscopic rupture criterion was used. The performance of the tests was compared with the indicator and a ranking was established. The results obtained show that biaxial tension is the test providing more information for the mechanical behavior characterization of the material. It was also verified that plane strain test presents a better performance than simple shear and uniaxial tensile tests.