Our coke bottle testing concluded that if pumping air (as opposed to CO2) there was no bang, the plastic heated and expanded until it quite quietly and undramatically released its contents at around about 200psi. Quite an anti climax when youre wearing ear defenders and cowering in the adjoining room!

The only problem I would envisage would be how to find a use for a holey plastic bottle that had formed itself to the shape of the inside of your robot

Tim