Process analysis of the forming process during blow moulding of rubbers and the resulting component quality

• Main Author: Ruano Bocaletti, P. (Pamela)
• Format: Dissertation
• Language: English
• Published: University of Oulu 2018
• Subjects: Industrial Engineering and Management
• Online Access: http://urn.fi/URN:NBN:fi:oulu-201802281272; http://nbn-resolving.de/urn:nbn:fi:oulu-201802281272

Silicone rubbers are inorganic polymers whose structure contains repeating silicon-oxygen groups. Among their properties are high resistance to temperature, ultraviolet light and oxidation. Silicone rubbers offer little stability against deformation and tears very fast, which makes it, on its uncured state, not suitable for extrusion blow moulding. To improve the blowability of the rubbers, a precuring step is used. A goal of this thesis is to better understand the inflation process of precured silicone rubbers. In addition, it is aimed to produce hollow bodies and examine their part quality. The blow moulding process was studied in free and confined inflation. First, free parison inflation profiles were studied to determine the parameters that have an influence on the inflation process. The blowing pressure, parison temperature, pre-curing degree, die gap of the extrusion head, wall thickness and the shore hardness of the material for the solid silicone rubber Addisil 20XXE were studied on this first stage. Results show that inflating the parison after the extrusion is key for the process. Parison in the studied range also benefit from lower pressures and a smaller die gap. The study of confined parison inflation profiles show that it is possible to use the extrusion blow moulding process to produce hollow bodies of silicone rubber using the parameters determined on the first stage of the research. First, confined inflation was studied with glass bottles. Afterwards, bottles were produced and cured using a heated tool. The part quality of the bottles formed were studied. Aspects such as surface quality, wall thickness distribution and quality of the weld seams were studied. Results show that the surface quality is highly dependent on the quality of the parison. The study of the wall thickness distribution shows that the centering of the die gap and the position of the parison when inflating have a big influence on the resulting distribution. The weld seams proved to fully seal and cure. These results apply mostly for parisons 32 cm long or shorter. Longer parisons could need higher inflation pressures. An important step for further research is the automation of the blow moulding process, in order to reduce significant sources of error. The results of this thesis allow further research to focus on improving the quality of the hollow bodies and the stability of the process.