Baesso, Ruggero (2008) Numerical and experimental investigation of the polymer injection forming process. [Tesi di dottorato]
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It has been recognized that certain performance functions are best handled by one kind of material such as metals, and that other functions are best handled by another kind of material such as polymers. This suggests that a union of two or several kinds of different materials will give synergistic results. End products that perform better than would have been possible if only one material were used, are the expected results.
Generally speaking, there are two kinds of composites: one with a matrix reinforced by particulate, fibres, etc. such as metal-matrix and polymer-matrix composites; and another one termed here as macro-composites, manufactured with materials having different properties. As for the latter, metal/polymer combined macro-composite components are representative of a unique combination of two kinds of different material used to achieve improved quality of the products, which have been used in a wide variety of applications in electronics, automotive and aerospace industries, etc. This work refers to macro-composite components consisting of one sheet-metal outer panel with a polymeric core material behind it.
However, the techniques for manufacturing such kinds of components are few and are usually time-consuming and labour intensive, resulting in a very expensive production. In most cases, the metal part and the polymer part are first fabricated separately by sheet metal forming processes and injection moulding respectively, and then they are joined together mainly by adhesion bonding. In this approach, good dimensional tolerance of both sheet metal and plastic parts is required for matching of the outer surface of the plastic part to the inner surface of the sheet metal part. It is not easy to achieve this when the bonding surface is complex and it is difficult to obtain a stable adhesion bonding interface.
In this Ph.D. dissertation, a new technique to manufacture the above mentioned metal/polymer macro-composite components is presented. In this process, the injected polymer melt from the injection machine forces the sheet metal blank to deform according to the contour of the mould and the space between the formed sheet blank acts as the moulding cavity of the polymer melt. As the melt cools down, it adheres to the surface of the formed sheet blank. The process, known in the academic and industrial world as “Polymer Injection Forming (PIF) process” is covered by a world patent by the English company Corus, which is an international company, providing steel and aluminium products and services to customers worldwide.
The aim of this Ph.D. dissertation consists on increasing the scientific knowledge about the Polymer Injection Forming process by means of both a numerical and an experimental approach. Several aspects of this new manufacturing process have been studied:
(i) the influence of the main injection moulding process parameters on the sheet metal formability has been experimentally investigated according to the Design of Experiments (DOE) method. This task required the design and set-up of a special injection mould in order to carry out experimental tests of the Polymer Injection Forming process. The experimental process was designed so that industrial operating conditions were approximated as closely as possible.
(ii) A combination of the finite element method (FEM) and Finite Volume Method (FVM) has been used to simulate the simultaneous forming process of both the plastic part and the sheet metal blank. The adopted multi-physics numerical approach allowed to investigate both state variables of the polymeric melt within the mould cavity and the deformation features, thickness distribution and evolution of plastic strains of the sheet metal.
(iii) The mechanism of adhesion bonding between the polymer and the surface of the formed sheet blank and the influence of the main injection moulding process parameters on the adhesion quality has been experimentally investigated according to the Design of Experiments method. The adhesion strength between the plastic and the metal part was measured by means of a custom shear test. This task required the design and set-up of special tools for clamping and testing the macro-composite part, which was used as specimen.
(iv) A software for the monitoring and control, in particular of the switch-over point, of the Polymer Injection Forming process has been developed in order to optimize the process and maximize its repeatability. Several experimental tests were carried out in order to study the influence of different switchover methods on both part quality and process variability. The efficiency of the proposed control algorithm was tested comparing the results obtained by the traditional control strategies with the ones obtained by the developed control method.
The work presented in this thesis was carried out at the DIMEG Labs, University of Padua, Italy, from January 2005 to December 2007, under the supervision of ing. Giovanni Lucchetta.
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