Formability of Polycarbonate

Polycarbonate;Formability;Mechanical Engineering

Clark, Darren

University of Waterloo

关键词: Polycarbonate;    Formability;    Mechanical Engineering;   

Others  :  https://uwspace.uwaterloo.ca/bitstream/10012/4176/1/Darren%20Clark%20Thesis%20Final%20Revised.pdf

瑞士|英语

来源: UWSPACE Waterloo Institutional Repository

【 摘 要 】

Current thermoplastic processing techniques involve high capital costs for moulds andsignificant use of energy to melt or soften the materials. Single-step process coldforming techniques, such as stretch forming, could be cost effective methods formanufacturing large parts with shallow cross-sections from plastic sheet. The presentwork is a preliminary investigation of a cold forming technique for polycarbonate.The objective of this work is to characterize the bulk deformation behaviour ofpolycarbonate using tensile tests and dome stretch forming tests. Two differentmolecular weight polycarbonate sheets with 1.6 mm thickness were studied: (i) one withMw = 42,000 g/mol and (ii) the other with Mw = 52,000 g/mol. For the latter, 3.2 mmsheets were also studied.Tensile tests conducted at three different cross-head speeds, i.e., 2, 20 and 200 mm/minshowed very similar elastic and plastic deformation properties for the two molecularweights. Correspondingly, the activation volumes at yield were almost identical. Therewas also negligible difference in the thermophysical properties between the two materialsas found by differential scanning calorimetry.Dome stretch forming tests were conducted on a metal forming machine. Specimens ofvarying width were tested to give different strain states ranging from deep drawing to biaxial. The limiting dome height or the maximum level of stretch formingivincreases with specimen width. This is due to biaxial deformation which increases themaximum strain. Forming limit diagrams (FLDs) were also constructed from the localstrains measured from printed fine circle grid patterns on the polycarbonate sheetsurfaces. The FLDs showed common general characteristics with metals except for a fewkey differences. An area of very few data points was found to lie between the ;;safezone” and the ;;necked zone”. This void was referred to as the ;;unstable neck formationzone”. It exists because of the large local increases in strain associated with the unstablenature of polymer neck formation.Much more study is required before polycarbonate can be cold formed at strains belowthe unstable neck deformation. However, the materials and techniques used in this workhave demonstrated that the process can be viable for forming shallow large parts fromrelatively thin thermoplastic sheet a as long as the local biaxial strains are less than 20%.