text: If I understand your question: Wrinkles can generally form when the material is in compression - it is a form of buckling, so try looking up 'buckling' or instability as a topic.  Plastics are not isotropic and exhibit different properties in different directions.  (Metals, in contrast, can be isotropic, but are subject to Poisson's ratio and exhibit different behavior depending on "plane stress" or "plane strain" conditions.)\n\nPlastics for the most part are not true crystalline solids; they are viscoelastic materials with properties of directionality depending on previous processing.  For most all plastics there will be some degree of creep under tensile stress.  Relaxation of the tensile stress allows the elastic component to retract in the opposite direction of the applied tensile stress, but the plastic component (the permanent set or "visco" component) won't contract.  Meanwhile, the stress direction at 90° to the tensile stress has likely experienced less stress and so has a larger or more elastic component.  What this means is that as the tensile component exceeds the yield point, the cross direction has not, and may have a tendency to buckle in the cross direction.  It also depends on how you're holding onto the plastic when stretching (biaxial or uniaxial?).  \n\nSome plastics have a strong tendency to align their molecules when stretch, and become more 'crystalline' in nature.  You can see this effect by 'stress whitening' when stretching a plastic grocery shopping bag made from HDPE.\n\nThis is kind of a complicated materials engineering question requiring a little more detail into the stress state, the exact material, even the environmental conditions play a role (how warm is the ambient, humidity level, etc.)\n\nHowever, for the most part, try looking up 'buckling' because that topic may give you more insight into why the wrinkles form.