Current thermoplastic processing techniques involve high capital costs for moulds and
significant use of energy to melt or soften the materials. Single-step process cold
forming techniques, such as stretch forming, could be cost effective methods for
manufacturing large parts with shallow cross-sections from plastic sheet. The present
work is a preliminary investigation of a cold forming technique for polycarbonate.
The objective of this work is to characterize the bulk deformation behaviour of
polycarbonate using tensile tests and dome stretch forming tests. Two different
molecular weight polycarbonate sheets with 1.6 mm thickness were studied: (i) one with
Mw = 42,000 g/mol and (ii) the other with Mw = 52,000 g/mol. For the latter, 3.2 mm
sheets were also studied.
Tensile tests conducted at three different cross-head speeds, i.e., 2, 20 and 200 mm/min
showed very similar elastic and plastic deformation properties for the two molecular
weights. Correspondingly, the activation volumes at yield were almost identical. There
was also negligible difference in the thermophysical properties between the two materials
as found by differential scanning calorimetry.
Dome stretch forming tests were conducted on a metal forming machine. Specimens of
varying width were tested to give different strain states ranging from deep drawing to biaxial. The limiting dome height or the maximum level of stretch forming
iv
increases with specimen width. This is due to biaxial deformation which increases the
maximum strain. Forming limit diagrams (FLDs) were also constructed from the local
strains measured from printed fine circle grid patterns on the polycarbonate sheet
surfaces. The FLDs showed common general characteristics with metals except for a few
key differences. An area of very few data points was found to lie between the “safe
zone” and the “necked zone”. This void was referred to as the “unstable neck formation
zone”. It exists because of the large local increases in strain associated with the unstable
nature of polymer neck formation.
Much more study is required before polycarbonate can be cold formed at strains below
the unstable neck deformation. However, the materials and techniques used in this work
have demonstrated that the process can be viable for forming shallow large parts from
relatively thin thermoplastic sheet a as long as the local biaxial strains are less than 20%.
| Identifer | oai:union.ndltd.org:WATERLOO/oai:uwspace.uwaterloo.ca:10012/4176 |
| Date | January 2008 |
| Creators | Clark, Darren |
| Source Sets | University of Waterloo Electronic Theses Repository |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
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