PETpla.net Insider 06+07 / 2014

BOTTLE MAKING 40 PET planet insider Vol. 15 No. 06+07/14 www.petpla.net Natural stretch ratio (or natural draw ratio) – Bottles, performs and closures, part 12 Behaviour in the blow mould The stretch or draw ratio of a polymer is the ratio of the resulting length (in the direction of applied stress) to the original length. When PET is stretched, for example, during blow moulding, it reaches a point at which an increase in the force is required to continue stretching. The point at which the PET requires this extra force is called the natural stretch ratio (NSR) for a particular set of stretching conditions. PETplanet insider is publishing extracts from successive chapters of Ottmar Brandau’s “Bottles, Preforms and Clo- sures”, which was first published by hbmedia. A newly revised version is reissued under the Elsevier imprint. The NSR is reached when strain (or work) hardening occurs on the stress– strain curve for materials. Recall that before a material yields, once the applied force is removed, it can return to its original dimensions. Stretching beyond the yield point results in permanent deformation, and further stretching will result in fracture. In some materials, including PET, strain hardening can occur before fracture, which is essentially the aligning (or orienting) of the structural regions of the material in the direction of the applied stress, which can result in improved physical properties for the material.