PETpla.net Insider 06+07 / 2014

BOTTLE MAKING 42 PET planet insider Vol. 15 No. 06+07/14 www.petpla.net The design of a PET preform is such that during stretch blow, the optimum orientation is achieved just as the stretched walls meet the mould. This point occurs just beyond the NSR. Proper stretching results in longer shelf life and less gas perme- ability, e.g., higher carbon dioxide retention for soda. Overstretching results in a “pearlescent” appearance of the bottle, signifying microcracks (fracture) and excessive deformation. A resin with a low IV has a higher NSR than a high-IV resin. The polymer chains in a low-IV resin are shorter, therefore, less entangled and can be easily stretched more than a high-IV resin. In the high-IV resin, chain entanglement limits the amount of stretch; similar to trying to pull one end from a tangled ball of string where the knots limit the length that can be pulled out. For this reason, preform designs differ when considering low- or high-IV PET. The figures illustrate the material stretching in the blow mould without relating to actual data. Strain (elon- gation) is plotted on the horizontal axis and the corresponding stress on the vertical axis. To obtain these data a heated test strip of PET might be pulled on a special machine that records the pulling force and the elongation of the strip. In the reheat stretch blow moulding (RSBM) pro- cess the stretch rod and blow air pro- vide the stress needed to transform the preform into a bottle. The top right figure in the diagrams and the follow- ing figures indicate the stage of the preform in the blow mould. Elastic deformation The first stage is elastic deforma- tion. Here the material stretches but will retract if the stress is removed. This is similar to the way that metals behave, but the shape of the curve is slightly different. Looking at a preform, this stage can be compared with the stretch rod starting to push on the pre- form. If the stretch rod were retracted, the preform would shrink back almost to its original length (Fig. 1). Yielding The second stage is yielding. With no increase in stress, the material “gives” elongating easily. This is what happens in the blow mould when the primary or preblow air partially inflates the preform. The preform will continue to inflate until it reaches the NSR, after which higher stress is needed to achieve further elongation (Fig. 2) The third stage is called strain hardening. Applied stress levels have to increase exponentially in order to force the material to stretch further. At this point in the blow process high- pressure air enters the preform and forces it to stretch from a bubble to the blow cavity walls, where it rapidly cools down. It is during the strain- hardening phase that the material achieves orientation (Fig. 3). Relevant parameters IV, temperature, and copolymer content play a role in determining how far the material stretches during yielding and what force is required to stretch it further. Temperature condi- tioning allows the operator to improve the blow moulding process by making certain parts of the preform hotter or colder and changing the way in which they will stretch. The objective of preform design (or selection) and blow moulding processes is to properly match up the NSR of the preform under the blow moulding conditions with the design stretch ratios of the preform–bottle combination. Because the strain-hardening phase of the process is so impor- tant for bottle performance, correct preform design, temperature profile, and blow air timing are all necessary Please order your copy at the PETplanet insider book shop: https://www.petpla.net/books Bottles, Preforms and Closures A Design Guide for PET Packaging Second Edition by Ottmar Brandau € 115,00 180 pages © Copyright Elsevier 2012 to guarantee the best bottle. If the inflated preform reaches the bottle mould during the preblow phase, ori- entation does not occur to a sufficient degree and the finished bottle might fail any number of tests. * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 2.3., Copyright Elsevier 2012 Figure 1: Elastic deformation occurs when the stretch rod starts moving Figure 2: At the yielding plateau no further stress is required for additional strain Figure 3: High blow pressure forces the material to strain harden