hand root portions 60 and side wall portions 54 and 56 undergo plastic deformation incident to axial expansion of the bellows such that the root portions expand or are drawn radially outwardly towards tank side wall portion 20 to lie in essential alignment with their adjacent crest portions at the completion of the expulsion operation in practice the sidewall of the fully extended bladder is not truly cylindrical but it is nonetheless possible to achieve product expulsion efficiencies in excess of 97 percent of course the size and number of convolutions required to achieve maximum expulsion efficiency will vary with tank size and geometry preferably the design of bellows 14 is such that its limiting mode of extreme extension is reached coincident with or shortly after initiation of the expulsion operation such that controlled plastic deformation of the convolution sidewall and root portions occurs through at least a substantial portion of the expulsion operation the constraint afforded crest portion 52 provides for controlled plastic deformation of the sidewall and root portions in a manner which avoids wrinkle induced puncture failure commonly encountered in a conventional bellows when extended substantially beyond its limiting mode of extreme extension also as a result of this controlled deformation the pressure required to fully extend the present bellows of a given wall thickness may be substantially reduced reference is now made to fig4 and 5 wherein two alternative forms of the present bellows construction are designated as 14a and 14b respectively and shown as being installed in a modified tank shell 12a.