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Contenu archivé le 2024-06-10

Injection molding of hollow components

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Simulation methodologies were developed, in the case of the sintering modelling, or adapted, for the injection moulding behaviour of ceramic and metallic feedstock, in order to provide sophisticated user friendly and customisable tools for the process set-up. A brand new code was developed to simulate all sintering diffusion mechanism and, thus, the shrinkage of both metallic and ceramic injection moulded parts. After the characterisation of binders and feedstocks, suitable FEM-based methodologies were used to study the best injection conditions and mould parameters. Reliable results were achieved for the evaluation of macroscopic shrinkage behaviour and residual stresses of sintered 3D real geometry components thanks to the development of a FE model. Then these methodologies have been applied to the design and the manufacturing of the project mock-ups, in order to check their reliability.
The injection moulding process has been developed to manufacture hollow parts with the lost core technique. Different partners have performed co-injection tests and selected, with the contribution of the modelling results, the best fusible alloy and the best operating conditions to be used in mock-ups manufacturing.
Solvent and thermal cycles for the removal of the low fusible cores and of the polymeric binder have been selected, in order to avoid pollution of the injected components and of the working furnaces. Optimal protective powder beds were identified in order to minimise the residual content of binder in the components before sintering.
The best thermal cycle in order to minimise shrinkage and cracks and maximise final density of parts has been identified both for metallic and ceramic parts. Particular care has been adopted, taking into account the very thin sections of the hollow parts. Moreover, the pressure-less sintering of ceramic materials without a protective atmosphere has been developed using alumina refractory cases. The appropriate sintering aids composition was defined and different sintering powder bed composition tested, so that to demonstrate the feasibility of the approach based on RCP sintering. Suitable conventional sintering cycles in protective atmosphere were developed to provide a back-up solution.
A specific, flexible binder system formulation was developed, especially dedicated to the lost core technique. It has been adapted during the research development to the different demands encountered during injection, core removal and debinding tests. It ensures a good compatibility with the low fusible alloys used to obtain the hollow geometry and it has been positively tested from the point of view of flow behaviour with the high-pressure injection of the project mock-ups. Different binder compositions were tested in order to obtain optimal results in terms of viscosity and debinding properties, accomplishing a binder system which is suitable both metallic and ceramic powders.
The current know-how on PIM technology has been implemented in the design stage, so as to develop specific guidelines to design mechanical complex components to be injection moulded with ceramic and metallic powders. Then, these guidelines have been applied to the design of mock-ups and for the selection of real parts.

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