PETpla.net Insider 07+08 / 2015

CAPPING CLOSURES 32 PET planet insider Vol. 16 No. 07+08/15 www.petpla.net a strong connection between the neck and the closure (with the exception of the PCO 1881; see chapter 4.10). A second feature of these necks is to allow a gradual release of pres- sure when the consumer opens the caps. This is accomplished by verti- cal grooves in the threaded part of the sure and bottle neck. The pres- sure escapes as soon as the top seal is broken once the closure is unscrewed less than half a turn. At that time, at least 1.5 thread turns are still engaged, preventing the cap from being blown off (Fig. 4.2). The neck styles available today are Alcoa, BPF, and PCO. Over the years designers have success- fully reduced the amount of material needed in the neck without jeopard- ising functionality. The popularity of PCO necks can be explained by the fact that it is about 0.5g lighter than an Alcoa or a BPF neck (Fig. 4.3). Modern neck finishes are about 2g lighter than the ones used 10 or 15 years ago. For once, the neck support ring (NSR) underneath the threaded part has been reduced in thickness as it became apparent that even a ring that is less than 2mm in thickness can safely guide preform and bottle through conveyors and blow moulding machines. The area between threads and NSR has also been lightweighted over the years, and further weight reduction improvements are under way. 4.2.2 Neck finishes for still water bottles The requirements for still water bottles are considerably less than they are for CSD bottles. As there is no pressure buildup inside the bottle and water does not spoil easily, the cap must only keep the contents safe from the environment and allow its easy opening. The weight reduction initia- tives that have resulted in super-light- weight bottles have also cut down the neck finish on these bottles to what seems to be the absolute minimum. There is no need for two thread turns, and most water bottles feature quick- close threads with two or three starts. The short thread engagement trans- lates into a short neck finish with an appropriate short cap, thereby saving material both ways. Thin neck walls and rim make for the lightest configu- ration possible. These caps close within half a turn and are very popular with consumers as the three neck starts allow easy closing and the short thread length guarantees easy opening (Fig. 4.4). 4.2.3 Neck finishes for hot-filled products Requirements for these necks and caps are quite different from the other two categories we examined so far. For once, most necks are 38mm or more. Historically, juice bottles had a wider neck when compared with water bottles, and brand owners would rather go with what consumers are used to. However, there is another reason that has a lot to do with PET processing. When bottles for hot-filling are blown in the so-called heat-set process, the stretch rod used is larger in diameter. This is because it conveys cooling air into the center of the mould during the cooling cycle of the blow process. The larger neck finish of the bottle allows more room for this function. As the bottle goes through the filling process, the neck undergoes sev- eral stages. First, it heats up possibly above the glass transition temperature of around 75°C. Above this tempera- ture PET becomes rubbery and starts to deform. Resins for the heat-set process are formulated to increase the glass transition temperature, and the actual temperature depends on both the filling temperature (82–95°C) and the time between filling and cooling in the cooling tunnel. At the same time the neck finish is exposed to relatively high temperatures, the bottle contents and headspace air expand, causing an overpressure inside the bottle. As the neck finish is not oriented and therefore amorphous in structure, it is ill-prepared for this stress. After cool- ing, a vacuum develops inside the bottle that could potentially collapse the neck. For all these reasons, neck fin- ishes of hot-fill bottles are heavier than non-hot-fill bottles. A 37mm neck finish may weigh as much as 10g to the underside of the NSR with wall thicknesses exceeding 2.5mm. It will depend on the overall package, fill- Fig. 4.3 Weights of modern neck finishes. (Table courtesy of Corvaglia Ltd). Fig 4.4