with reference to fig1 container 10 , which is shown in the form of a thermoplastic bottle , comprises a base wall 11 and an opening 12 opposite said base wall and has , on wall 9 , a fold line ( foldable region ) 13 disposed on a medial or axial plane . this fold line 13 is obtained by modifying the physical structure of the material from which the bottle is formed , for example , modifying it mechanically as will be described below in greater detail with reference to fig5 and 6 . depending upon the shape of the container , and especially its size relative to said opening , it may be produced either by thermoforming a flat preform , or by blow molding or stretch blow molding a hollow preform . in order to achieve a highly mechanically resistant container of reduced mass , thermoplastic material such as pet is advantageously used , a material whose physical structure can range from amorphous to crystalline . a highly mechanically resistant intermediate structure retaining the transparency of the material can be attained by submitting the material to considerable bi - axial elongation while the container is being formed , for example by stretch blow molding a preform and / or submitting the material to thermofixation after formation , which increases the degree of crystallization . the physical structure of the material is modified only in the narrow zone 13a ( enlarged in fig1 ) of bottle wall 9 where fold line 13 will be located . this structural modification , which in this example consists of transforming the intermediate , transparent structure of the material utilized into a crystalline , opaque structure , can be obtained by mechanical or thermal action or by irradiating zone 13a , for example , with a laser beam . modification of the physical structure also refers to a purely thermal or physio - chemical modification , such as , for example , transforming the reticulation connections of the polymer chains . when mechanical action is utilized , zone 13a undergoes slight supplemental stretching in a direction perpendicular to the fold line , either during stretch blow molding of the preform or thereafter , and advantageously at a lower temperature than the temperature at which stretch blow molding takes place . when thermal action is used , after the container has been formed , the synthetic material is hardened by a crystallization process during which zone 13a is raised to a temperature higher than the temperature at which formation takes place . this can be accomplished , for example , by temporarily placing one filament heated by an electrical current in the immediate proximity of the container wall along the fold line . irradiating zone 13a with particles , photons or suitable chemical agents also hardens the material in this zone by reticulation of the polymer chains comprising the material . the structural transformations of the material in zone 13a make it harder and more fragile than the rest of wall 9 , but maintain at least the same tensile strength . thus , the tensile strength of the wall in zone 13a is not diminished , despite the fact that it is thinner . resistance measurements have demonstrated this property . in actuality , it is only 10 % thinner . this ensures the integrity of the bottle &# 39 ; s resistance to shocks as well as to the pressure of the liquid it holds . however , tensile strength and compression forces caused by bending the wall during folding are intensified in the folding zone 13a by a factor equal to the cube of the ratio of the thickness of wall 9 outside said zone to its thickness inside said zone . therefore , when the wall is folded , it breaks neatly and only in the fold zone , which is harder and more fragile than the rest of the wall . by virtue of the hardened , more fragile fold line the bottle is easily flattened ( fig2 ) when empty and can be stored in a trash can or other waste receptacle without occupying a large amount of space . pet bottles presently in use are notoriously difficult to bend , as they have no fold line . it is actually very difficult to crease the walls of a container made of these materials as they also have a sort of shape memory , i . e ., they tend to resume their original shape after having been creased . as shown in fig3 and 4 , the fold lines may be shaped differently , as they depend upon the particular container shape . in the case of container 20 shown in fig3 a longitudinal fold line 21 is disposed on each side of the bottle in a median or axial plane . there might also be two other longitudinal fold lines 21 disposed in a median plane perpendicular to the plane passing through fold lines 21 . in addition , a first transverse fold line 22 is located near the base wall of the bottle and a second transverse fold line 23 is located near the bottle opening . in the case of bottle 30 shown in fig4 one longitudinal fold line 31 is provided on each side of the bottle . each of the fold lines is subdivided into two oblique lines 32 disposed near the base wall of the bottle and two other oblique lines 33 near the opening . for any given bottle , any possible combination of fold lines may be provided , such as , for example , one or more longitudinal fold lines 31 in combination with either two or more oblique lines 32 , or two or more oblique lines 33 . fig5 and 6 are schematic illustrations giving examples of one particular embodiment of the method of manufacturing containers according to the invention . this type of container is currently made by stretch blow molding a preform initially consisting of a preform which has been previously injection molded or compression molded . during a first phase the preform temperature is raised to a range in which the theology of the material is conducive to reorienting the molecular chains of the polymer and thereby reinforce it mechanically ( partial microscopic crystallization ). next the preform is elongated in a bi - axial direction by stretch blow molding . for this purpose the preforms are introduced into a mold 40 consisting of two shells 41 and 42 defining a molding cavity 3 shaped like the container to be molded . the broken lines in fig5 show a preform 44 which is inflated during the stretch blow molding process . the preform passes through an intermediate stage 45 shown by solid lines , during which compressed air is injected according to the arrow 46 , through an injection nozzle 47 attached to the preform . the pressure of the air blowing on the container is maintained or increased so as to keep it firmly applied to the surface of cavity 43 . in the final phase of the stretch blow molding process , shown in fig6 when the container has at least partially attained its ultimate dimensions 48 , in order to create the fold line , the mold is opened slightly , that is , shells 41 and 42 are separated , thereby creating a space 50 between the shell edges . since pressure inside the blow molded container is considerable , opening the mold causes supplemental stretching of the container wall along the line of the cavity opening at the junction of shells 41 and 42 and perpendicular to said line , thereby causing the material to crystallize within a narrow zone corresponding in width to said space 50 , which may range from several tenths of a millimeter to several millimeters , and thus creates the fold line . this supplemental stretching takes place at nearly the same temperature as the mold surface temperature , which is lower than the temperature of the preform during stretch blow molding . to reinforce tensile strength of the container , supplemental stretching and thermofixation can be performed simultaneously at a relatively high temperature , which may be approximately 100 ° c . this is actually the temperature at which recyled bottles are cleaned and beverages are pasteurized . thermofixation is accomplished by temporarily raising the container temperature after stretch blow molding it in a first mold so as to develop crystallization without rendering it opaque , that is , a lesser degree of crystallization than that attained during the supplemental stretch blow molding in the fold zone . it should be understood that in the example described , the fold line is a horizontal fold line disposed along a median plane of the container formed . in order to create fold lines disposed in several planes , one need merely make a mold comprised of several shells which can be separated in pairs during the final phase of the blow molding process . additionally , note that the method can be used not only for bottles , but also for other types of packaging such as , for example , containers for solid substances such as powders , granular material , tubes for paste such as food products , cosmetics , toothpaste , industrial paste such as grease , cleaning agents , etc . many products in these categories are now packaged in metal or synthetic tubes which are difficult to bend and tend to resume their original shape . this problem can be resolved by applying the method of the invention to create a fold line in such packages . therefore , such synthetic packaging material has general applications and may gradually replace conventional metal or plastic tubes .