PETpla.net Insider 05 / 2021

BOTTLE MAKING 20 PETplanet Insider Vol. 22 No. 05/21 www.petpla.net tip has moved to a position very close to, but not touching, the inside of the preform base. The locking is often checked with the help of a switch at 69 degrees (Fig. 7.4) . Figure 7.4 Mould assembly at the start of the blowing process. Stretch rod engages Both blow nozzle (if present) and stretch rod start moving at 51 °C. The preblow pressure may start as soon as the blow nozzle is fully down but there are some delays before full preblow pressure is actually acting on the preform. For example, the blow valve has a certain delay because mechani- cal parts in the valve must physi- cally move before air can actually pass through. The air has to cross the distance between the valve and the cavity and the closer the valve is mounted the faster the process can start. Once inside the cavity, the inrushing air will have slightly decreased in pressure and it is a fraction of a second before pressure is built up again. These delays com- bined, take approximately 0.05– 0.15 s, mostly dependent on the distance between blow valve and cavity. Stretch rod action may be delayed to allow pressure to enter at the same time as the stretch rod moves. Depending on the preform temperature the stretch rod may stretch the gate section of the pre- form first because of mechanical stress acting on this surface before reaching other parts of the preform (Fig. 7.5) . Figure 7.5 The stretch rod engages the lower part of the preform and may pull it out before stretching other parts. Preblow engages As the stretch rod travels further preblow pressure reaches its maxi- mum value of 5–20 bar (70–290 psi). The rubber-like state of the polyethylene terephthalate (PET) will now be inflated into a bubble at the weakest point. This is usu- ally one third to halfway down the heated length of the preform. With properly designed preforms, this yielding phase (Chapter 3, Section 3.2) will come to an end before the bubble reaches the mould walls, thus allowing room for the material to orient more later. As the mate- rial extends and reaches the end of its natural stretch ratio the ensu- ing molecular orientation increases the mechanical strength of the stretched areas. Now areas border- ing on the already stretched part are weaker and will therefore start to extend. If the preform is at the same temperature throughout the walls, the bubble will develop from the top down (Fig. 7.6) . Figure 7.6 Preblow pressure creates a bubble not unlike the way a balloon inflates. The stretch speed is an impor- tant aspect of proper bottle blowing. Too low a speed can lead to pre- forms cooling down to a temperature approaching the glass transition tem- perature where they will not stretch evenly. Higher speeds also improve molecular orientation. Stretch rod at base insert Figure 7.7 When the stretch rod has fully extended the bubble has grown to fill most of the cavity. As the stretch rod races toward the base insert of the mould, more and more sections of the preform start yielding, thereby enlarging the bubble. The speed of development and the extent of the bubble depend on the material’s intrinsic viscosity (IV) and temperature. (Fig. 7.7) . The stretch rod reaches the end position at approximately 90 degrees in the cycle depending on the length of the bottle. It is advantageous in some cases to hold the preblow pressure for another 0.05–0.1 s allowing the bubble to extend further down and so pushing more mate- rial to the outer rims of the bottom of the bottle. For many other bottles it is best to trigger the high pressure as soon as the stretch rod is fully extended. On some machines there is a special timer called “tempori- sation” which delays the onset of high-pressure blow. Other machines feature simply an adjustable switch indicating the stretch being fully extended, and otherwise rely on the slight delay of the start of blow for the reasons described earlier. The preform is now ready for orientation (Fig. 7.8) . Figure 7.8 The bubble can be extended further by slightly delaying the onset of high- pressure blow. High-pressure blow Blowing may start as early as 95 degrees or as late as 120 degrees depending on the air delay or the amount of temporisation. The impact of air at 40 bar (580 psi) pressure is dramatic: it takes only around 0.02 s to change the bubble into a fully formed bottle. Air pressure forces the material against the cooler walls of the mould cavity. The material must be cooled below the glass transition temperature and another benefit of running cooler preforms is that the required cooling is minimised, allowing faster cycle times. It should be noted that the stretch rod has to hold the preform gate firmly in the center of the base insert. Any deviation will result in uneven bottle wall thickness. Some linear machines use the air pressure to the stretch rod cylinder as speed control. Reducing