Meaning
Experimental measurement of the deformation of a thin polymer sheet under pneumatic pressure is used to analyze the biaxial stretch behavior of soft materials. During bubble inflation testing, a circular clamped diaphragm deforms into a hemispherical bubble as pressurized air is introduced from one side.
Deformation Measurement
Optical tracking systems monitor the crown of the expanding hemisphere to record changes in radius and thickness over time. Using these measurements from bubble inflation testing, engineers calculate the true stress and true strain at the apex where the deformation is balanced biaxial. These data are essential for modeling the high-strain regions of polymer sheets during thermoforming processes.
By analyzing the rate of expansion, researchers can determine the extensional viscosity under conditions that replicate actual factory floor operations.
Material Property
Extractive flow of molten polymer requires deep understanding of the melt strength at high extension rates. Standard melt flow index values do not capture the behavior of the melt during rapid extension, which makes bubble inflation testing an indispensable method for evaluating material suitability. It evaluates the resistance to localized thinning, preventing blow-outs during the manufacturing process.
Melt Strength
Process stability during sheet forming relies heavily on the extensional viscosity of the polymer. A lack of strain hardening during bubble inflation testing indicates that the polymer will thin out and rupture under low stretching forces. This behavior correlates directly with part defects such as unequal wall thickness in the thermoformed cup.
