Papers by Author: António Sergio Pouzada

Abstract: Structural foams (SF) consist of a sandwich-like material composed by a cellular core and a solid skin. It is the ideal material solution for injection moulding of short production series of large size parts. The injection moulding of SF is a low pressure process, thus it is a viable alternative for light moulding tools. The right formulation of structural foams and the use of adequate processing conditions are important to optimize the performance of the plastics part. In this study the processing conditions (injection speeds and percentages of mould filling) and the amount of chemical blowing agent (CBA) added to polypropylene were assessed on injection moulding circular discs. A hybrid mould with epoxy/aluminium composite cavity was used to produce the parts. The structural foam performance was evaluated by rheological, morphological and mechanical tests.

Abstract: Microinjection moulding is one of the key technologies for the mass production of plastics microcomponents. Recently, significant effort has been made to test the limits of applicability of existent numerical codes for simulating the polymer flow at the microscale. However, the modelling precision in what concerns polymer flow in microimpressions depends on factors which may not be properly accounted for in the process simulation. In this study, a micropart with variable thickness was designed, and the moulding block fabricated and instrumented. Short shots and complete filling of the cavity were carried out and the flow front progress was subsequently evaluated. These data were also assessed numerically by 3D-finite element modelling. A flow simulation considering the polymer as incompressible was carried out to investigate how the mesh size and density affected the prediction of the flow field in the microimpression, using the same processing conditions of the experimental study. The reduction of the mesh size as well as the increase of the mesh density are consistent with better representativeness of the experimental flow front progress in the microimpression. Moreover, the weld line prediction also tends to be improved. This study suggests that the mesh adaption and domain discretization is important in numerical studies of the polymer flow at the microscale.

Abstract: The concept of hybrid mould combines the conventional techniques of mould manufacturing and Rapid Prototyping and Rapid Tooling, resorting to non-conventional materials for producing moulding blocks, e. g., epoxy resin composites. Composites based on an epoxy system with 15% weight fraction of short steel fibres (SSF) were considered adequate for improving the performance of moulding blocks. The epoxy/short steel fibre composite moulding blocks were produced by vacuum casting in silicone moulds. Polypropylene (PP) was mixed with a commercial PP masterbatch with 50% of nanoclay and injected in a hybrid mould under various processing conditions. These were chosen from a central composite design with 15 experiments. The moulding microstructure was assessed by polarized light microscopy and differential scanning calorimetry. The skin-core morphology was observed and suggested that the low thermal conductivity of the epoxy composite produces a thinner skin when compared to all-steel moulds. The nanoclay concentration was the variable with the most significant effect on skin thickness and crystallinity. The addition of 1 wt% nanoclay under certain processing conditions favours the formation of β-form spherulites and the increase of crystallinity.

Abstract: Metallic fibres in polymeric matrix are used for mouldings blocks of hybrid injection moulds improving the mechanical and thermal properties. This paper reports on the characterization of epoxy resin/short steel fibres (SSF) composites. The effect of the concentration of 2,4,6-tris (dimethylamino-methyl) phenol as accelerator in the epoxy system was evaluated by viscosity and dynamical mechanical analyses. The composites were characterized by compression and microhardness tests. The fibres dispersion into the epoxy matrix was analysed by optical microscopy. It was found that the best accelerator concentration for this type of composite was 5,0 parts by weight

Abstract: This study evaluates the influence of graphite and multi-wall carbon nanotubes on the mechanical and electric properties of cast epoxy resin. The epoxy resin based composites were prepared with various graphite and MWNCT content up to 5.0%. Specimens were characterized by DMA, SEM and electric resistivity tests. The observation of fracture surfaces showed a reasonable dispersion of graphite and MWCNT into the epoxy matrix. The graphite and MWCNT have almost the same effect in the electric conductivity of the epoxy composites at low content (0.2 and 0.5 %). The MWCNT composites seem to reach percolation at concentrations near 0.5 % whereas graphite composites reach it at 2%. Higher concentration of graphite and MWCNT have limited effect in the electric conductivity but reduces mechanical properties.

Abstract: Epoxide filled composites are being increasingly used for mouldings blocks of hybrid injection moulds. The filling is sought for improving both mechanical and thermal properties that are relevant for the mould performance. In spite of several works investigating the particulate filling of resins, there are only a few reports on fibre reinforcement. Composites based on an epoxy system with varying volume fractions of short steel fibres (SSF) were investigated. The mechanical properties were determined for each composite, and the topography of the fracture surfaces was analyzed by SEM. The mechanical properties of the epoxy filled composites were also compared to commercial particulate composites that are used for producing casting moulds. In spite of the SSF being more difficult to mix that the usual metal particulate fillers, it was found that the resulting composites show some improvement in the mechanical properties.

Abstract: Rapid tools for injection moulding are often produced by casting of epoxide filled composites. The moulding blocks obtained in this way are likely to undergo wear especially at the surfaces of the moulding cores during the ejection phase. In this study, tribological properties of epoxide composites containing different volume fractions of short steel fibres used in moulding blocks were assessed. Friction tests with the composites and moulded polypropylene were carried out with a prototype equipment that reproduces the actual ejection phase of injection moulding. The friction data were interpreted in terms of the roughness and compared with the microscopic features of the epoxy composite surface.

Abstract: Stereolithography is a rapid manufacturing process that builds objects layer-by-layer based on the photo polymerization of a liquid resin. Due to the good geometric precision, this process has been used in Rapid Tooling for injection moulding. These tools are suitable for the production of short runs of parts and prototypes, where without resorting to the manufacture of a conventional metallic mould. The mechanical and thermal properties of the stereolitography resins, used in the manufacture of hybrid moulds, differ substantially from those of the conventional metallic materials. Particularly they must be known for guaranteeing the structural integrity and the thermal performance of the mould. Also the chemistry behaviour of the resin in contact with the polymer is important when tribological aspects are involved, especially during the ejection of the moulding. In this work, the behaviour of hybrid moulds with SL Vantico 5260 resin moulding cores was assessed. Several thermoplastics (iPP, ABS, PET and PA 6.6) were moulded, and their mechanical, thermal and chemical properties considered and related to the performance of the moulding process. Also, the friction properties of the SL resin in contact with the moulded thermoplastic in moulding conditions were observed and considered in the context of the performance of hybrid moulds.

Abstract: In this study, the as-moulded shrinkage and pressure data are obtained experimentally and compared with numerical simulations. The mouldings were produced in polypropylene (PP). The effect of pressure on viscosity in the predicted pressure evolution was analyzed and also its influence on the shrinkage. The results show that the rise of holding pressure determines the reduction of the shrinkage. Also, it was observed that the pressure predictions are qualitatively in good agreement with the experimental data. However noticeable quantitative discrepancies can be observed when the effect of pressure on viscosity is not considered. If the effect of pressure on the melt viscosity is considered the deviation between predicted and the experimental pressure evolution is substantially reduced.

Abstract: The α to β phase transformation of PVDF through the stretching process at different temperatures was investigated. The optimum stretching conditions were studied and characterised by infrared spectroscopy and differential scanning calorimetry. The maximum β−phase content was achieved at 80°C and a stretch ratio of 5. Accompanying the phase transformation, a orientation of the polymer chains and a packing of the crystalline structure is observed. The stretch ratio does not significantly affect the degree of crystallinity.