Facoltà: Ingegneria Gestionale - classe 10
Collocazione: 5939
Autore: Cozzi Pierluigi Angelo
Data: 01/10/2007
Titolo: THE INJECTION MOULDING PROCESS FOR PET. CRITICAL POINTS AND SOME IDEAS TO IMPROVE THE PROCESS EFFICIENCY.
Relatore: Locati Giancarlo
Correlatore: Brun Alessandro
Autorizzazione per la consultazione: NO
Le tesi si possono consultare unicamente in sede
Nowadays plastic components are everywhere, some time they are also replacing traditional materials like steel and glass. In this thesis I have dealt with the production of PET bottles, a successful substitute for glass. The aim of the thesis was to explore some critical points and possible solutions for the main production processes problems. In particular, I have been analyzing the production of Polyethylene Terephthalate (PET) preforms obtained by the injection moulding process. The most critical point is the water content of PET pellets. PET is a hygroscopic polymer, consequently PET pellets needs to be dried before the injection moulding phase and it must be guaranteed that they are moisture free when the melt polymer is injected into the mould cavity. A level of 50 ppm of water in the polymer is considered safe. Since the PET preform is subsequently subjected to a biaxial blowing, it needs to be free of any defect. To do so, the injection phase needs to be as accurate as possible and kept under control. By using Fick’s mass transport laws it has been possible to reach the conclusion that a two steps drying plant, a vacuum drier followed from a dehumidifying drier, could be a new suitable solution to obtain a well dried polymer in a short time and with the right quantity of final moisture. Concerning the control of the accuracy of the injection process, the conclusion was that it is more suitable to have a pressure control sensor located on the nozzle. The study of the statistical distribution of the samples weight revealed that the more the sensor is close to the mould cavity the more the measure is accurate and the process stable. From the other side, a cost analysis revealed that a sensor located on the nozzle is the best option. The difference in quality between the nozzle sensor and the cavity sensor is small, but to have the sensor in the cavity requires equipping all the moulds with cavity sensors, while the sensor in the nozzle doesn’t need to be changed all the time like the one in the mould.
