PETpla.net Insider 01+02 / 2015

PREFORM PRODUCTION 37 PET planet insider Vol. 16 No. 01+02/15 www.petpla.net 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 * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 3.2. Copyright Elsevier 2012 pressures will require longer dwell times. A good starting point is 0.5 s. After the dwell timer has timed out the shut-off nozzle that is situated between the extruder and the hot runner closes allowing transfer (shoot- ing pot system) or recovery (recipro- cating system). 3.2.6 Cooling time At the same time that the screw decompresses the melt, the cooling timer starts. During cooling time the preform cools down further so that:  It can be properly ejected.  Gate crystallinity is minimized.  The preform shrinks away from the cavity to allow easy removal from the cavity when the mold opens. Minimum cooling time is about 0.5 s to give the valve stems enough time to properly close. Recommended cool- ing times depending on pre-form wall thickness are on the graph in Fig. 3.12. 3.2.7 Gating Most modern tools control the opening to the cavities with a cylin- drical piston called valve stem. This stem is connected to a pneumatic cyl- inder that moves it back into the open position and forward into the closed position (Fig. 3.13). Molten material surrounds the valve whose housing (the nozzle tip) is temperature controlled by either a percentage heater or a thermocouple. The valve stem must be open during injection, hold, and decompression but is otherwise controlled by timers that may delay the opening and clos- ing actions. The valve stem open delay timer can be activated from a number of cycle steps such as mould open (before ejection) or mold starts to close (after ejection). While the valve stem is in the closed, forward position, it touches the cold mould wall. If it stays there too long it may cool down too much and the material around it will crystallize. The valve stem is therefore opened well before it has to be open, i.e., when injection starts. When the valve stem is opened too early, molten material may enter the cavity as drool while the mold is open. This is especially a danger with low IV resins, with insufficient decom- pression, or when high temperatures have to be used. The valve stem close delay timer is triggered from the end of hold time. The timer needs to be set as a minimum, the same as the pullback dwell time, to allow the pressure to dissipate. A short delay time cools the preform nub sooner, whereas a longer time gives the nozzle tip heater time to transmit heat into the nub area. The operator has to judge from the appearance of the gate which way is better. The goal is to achieve the least amount of gate crystallinity and properly form the preform nub. The maximum time that can be entered for this timer is 0.4s shorter than cooling time. This will ensure that the valve stem has enough time to close before the mold opens. Process conditions such as melt temperature, injection and hold pressures, as well as the resin all have to be taken into account when adjusting this timer. Figure 3.12 Recommended cooling time versus preform wall thickness Figure 3.13 The vertically shown valve stem opens and closes material access to the cavity at the bottom. (Diagram courtesy of Mold-Masters). Figure 3.14 Three-station servo robots carry water-cooled take-out plates with three times the number of injection cavities. (Photograph courtesy of SIPA)