PETpla.net Insider 10 / 2014

PREFORM PRODUCTION 44 PET planet insider Vol. 15 No. 10/14 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 2.6. Copyright Elsevier 2012 A display of a temperature with- out set point is the temperature of the incoming resin as measured just above the extruder throat. A tempera- ture of about 165°C (329°F) is optimal for PET processing Practical Guide to Injection Moulding of Preforms The percentage of power the controller puts out to the heater band. For example, if this value is 40%, the heater band is on for 4s and then off for 6s. The controller will use a value that is best suited to keep the heater band at the set point. This is regulated by a so-called proportional, integral, and derivative (PID) loop, and all con- trollers use some form of this control programme (fig. 3.7). A typical temperature profile starts at 270°C (518°F) at the feed zone and increases to 285°C (545°F) toward the extruder nozzle. This can be used for most PET applications. Temperatures over 300°C (572 °F) are not recom- mended as they will lead to burning the material if the residence time becomes too long. The extruder cannot be started until all heaters are at the set point and the soak timer has timed out. Most machines have a protection against this “cold-start” condition. Screw rpm should be chosen at the lowest level that still guarantees that the extruder is not holding the machine back from cycling. This can be checked by watching the end of screw turning (called recovery) and stopping the time until transfer or injection occurs. If it is more than 1 or 2s, screw rpm can be lowered. On machines with “short” screws, i.e. with L/D ratios below 22, a higher screw speed may be required to assure proper melting. Back pres- sure is hydraulic pressure that is acting against the return movement of a recovering screw. Without it the screw may slip under the material without melting it. Because PET is so shear sensitive, selected adjustments should be below 20 bar (300 psi). If it is too low, bubbles may appear. These are made up of air. Air is of course present inside the material pellets and must be pushed out of the extruder throat as the material moves forward. Too low a back pressure may make it too easy for the material to move for- ward without squeezing out the air. Figure 3.6 Mixing in stages is the advantage of barrier screws. (Diagram courtesy of Barr Inc). Figure 3.7 A variety of parameters are available for temperature control. Numbers in white are for display only. (Picture courtesy of Netstal-Maschinen AG).