PETpla.net Insider 08 / 2017

BOTTLE MAKING 36 PET planet Insider Vol. 18 No. 08/17 www.petpla.net Please order your copy at the PETplanet Insider book shop: www.petpla.net/book3 Stretch Blow Molding Second Edition by Ottmar Brandau € 130,00 320 pages © Copyright Elsevier 2012 Figure 3.3 One electric servo drives all moulds on this machine. Photo cour- tesy of W. Amsler Equipment Inc. Looking at Fig. 3.1, preforms follow the mandrel track counter- clockwise around the machine from the feeding station through the oven section. Equilibration happens in the turnaround section before entering the five-cavity blow clamp. Five mandrels are shuttled at the same time into the blow mould where the preforms are stretched and blown at the same time. All blow cavities are located side by side. On some machines, both mould halves move with a common cylinder and other machines fea- ture individual cylinders. In most machines, preforms travel upside down and the stretch rods are engaged from the bottom. After blow- ing, the three bottles are shuttled to the take-out station where they are turned upright onto a conveyor belt (Fig. 3.4). Stand-up conveyors are common for the range of outputs pos- sible with these machines. Figure 3.4 Stand-up conveyors are most commonly used at the end of linear machines. Photo courtesy of W. Amsler Equipment Inc. While these machines also offer a low entry level into the PET market, they do have some serious draw- backs. The most serious one is the way preforms go through the oven system. It is nearly impossible to have all preforms spend exactly the same amount of time inside the ovens as they are being indexed. This becomes more problematic as cavitation increases. Furthermore, the heat an infrared lamp emits is not even along its length. The centre of the lamp emits usually 10–20% more heat through higher temperatures than the ends. The result of both circumstances is that preforms are not evenly heated up, and tem- perature differences as high as 8°C (14°F) between preforms in a four-cav- ity machine are not unusual. This, in turn, leads to differences in bottle wall distribution. Some of these differences can be improved on if each cavity has a separate set of blow valves but not all machines are equipped with this feature. Some machines feature oven tracks where preforms run parallel to each other with lamps in between. These work better but are still not at the level of continuous motion machines that is discussed later in this book. Most machines also space the preforms in the same pitch as the blow cavities. This leads to an uneconomi- cal use of oven power as the lamps heat up a large amount of air. Another downside of these machines is the rotating movement of first the preforms (upside down) and then the bottles (right side up). The mounted devices with three cylinders each (up/down, forward/backward, turn in/turn out) can be a maintenance issue that has plagued many companies. The simple mandrel design is easy to repair but does not give the same security in holding the preforms concentric, as per the more sophis- ticated designs of different types of machines. This is because the mandrel outside diameter has to be about 0.02 mm (0.0001″) smaller than the lower tolerance dimension of the neck inside diameter; however, many preforms are larger as the tolerance range allowed is about +/−0.008mm (0.003″). In the worst-case scenario, preform necks are larger (0.17wmm (0.007″)) than the mandrels and, as a result, preforms wobble and are heated unevenly on their way through the ovens.