Bietresato, Marco (2008) Sviluppo di un ambiente per la prototipazione virtuale del processo di stampaggio ad iniezione di materie plastiche. [Tesi di dottorato]
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In this thesis a new approach for the Robust Design of the injection moulding process has been developed and successfully applied to two industrial cases. This approach makes use of several tools (numerical simulations, heuristic and stochastic methods: FEM, DoE, RSM, Monte Carlo) integrated together in a sort of environment, the Virtual Prototyping Environment (VPE), able to overcome the limitations of the traditional numeric approach, which is, on the contrary, based on the intensive but exclusive use of FE models.
In fact, the FE simulation presents the main advantage that the experimentation and the optimization activities require neither the manufacturing of the tooling system nor the utilization of the production system but it suffers the presence of important limitations due to the simplification of reality made in the virtual transposition of a system and to the deterministic nature of solving algorithms. The consequences of these limitations are respectively: (i) the need to adjust some parameters of the model in order to fit prediction with experimentation, and (ii) the impossibility to take in account fluctuations of process conditions.
On the contrary, the Virtual Prototyping Environment manages to perform an optimal product and process design, considering also the stochastic variability present in all the manufacturing processes and thus integrating Concurrent Engineering with Robust Design.
As an effective use of this Environment presupposes a realistic model of the process, the activity of
model calibration is fundamental. The tuning procedure here presented limits the experimental activity on the injection moulding machine to a short utilization without any die (discharging tests).
The numerical simulation of the evacuation of polymer from the cylinder through the nozzle leads to the definition of the junction losses coefficients of Bagley’s correction. The discharging test thus completes the VPE by releasing it from the need to have specific dies for the identification of an
optimal and robust combination of process parameters.
In this work the following points are analysed:
· The injection moulding process of filled polymers, the injection machine and its subsystems (plasticizing unit, feeding system, moulds);
· The problem of variability in the manufacturing processes, with special reference to the plastic injection moulding process (in particular: real values of process parameters,
· The actual approaches for the experimentation and optimization of the injection process (“physical” and “virtual” approaches) and the proposed solutions for the search of a robust process set-up.
The collection, study and critical analysis (advantages, disadvantages) of the solutions outlined in the Literature have led to:
· identify the possible tools to work with FEM, necessary for a virtual transposition of the injection process which keeps in account its variability too;
· define the architecture of the Virtual Prototyping environment for the injection process in which the different tools necessary for its implementation are integrated (FEM – Moldflow Plastic Insight 6.1, DoE, RSM – Minitab 14 e Design Expert 7, Monte Carlo Method – Crystal Ball 7);
· implement a new FEM tuning procedure that limits the experimental activity on the injection moulding machine to a short utilization without any die (it is based on the use of
machine pressure profiles obtained during some air-injected shots: the “machine discharging tests”).
Having formalized an application procedure for this VPE, divided into six steps, the proposed Environment has been applied and validated in two industrial cases, different for product complexity, moulded material and, above of all, aims of the study:
· evaluating the robustness of a production set up on fixed nominal values and, in case, identifying a more precise adjustment (part in production: tub rear cover for a washing
machine - PP 20% glass fibre filled);
· exploring different process settings through a simulation plan with the aim of locating a robust optimum (new part to be produced as a substitute for a previous one, aluminium diecast: motor cover – PA66 20% glass fibre filled).
This Virtual Environment demonstrates to fit to both the presented cases,
· giving forecasts of the output lined with production surveys about non-conformity percentages and placement compared with the acceptability limits,
· permitting to find the settings able to produce the parts with near 0% scraps, in the former case, or to reduce non-conformities on the basis of the estimated response distributions, in the latter case.
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