PETpla.net Insider 11 / 2014

40 PET planet insider Vol. 15 No. 11/14 www.petpla.net about 100–120 bar on the screen of the machine (see Figs 3.6 and 3.8). 3.2.2 Injection Speed and Time Another point to note here is that injection time cannot be dialed in as well even though this time is displayed on the screen. As pressure and speed are interrelated, here are the factors that affect injection time:  Speed setting  Pressure setting  Material viscosity  Temperature This gives the operator excellent but only indirect control over the injec- tion time. PET, like all other plastics, behaves mostly like a non-Newtonian liquid, i.e., its viscosity (its resistance to flow) changes with the shear rate. Shear rate is controlled by the injec- tion speed. At very high shear rates PET would be oversheared and turn to a yellow/brown color. Therefore, the process window for injection speed is limited, and the operator has to take into account that changes to the injec- tion speed will lead to changes in the material viscosity. Manufacturers recommend a fill rate of 10–12 g/s, and the graph in Fig. 3.9 is based on this rate while taking into account the fact that the rate needs to increase for heavier preforms to prevent the melt from freezing up during injection. There are usually three, four, or five settings for injection speed along the way the screw or shooting pot takes to push the required material into the cavi- ties. The operator determines where the screw or shooting pod stroke starts, and the screw will end its travel when it comes to rest after injecting the material. The starting point of the screw is called shotsize, a slightly misleading name as we will see. Typi- cal values for PET injection machines are 120–200mm. In most cases, the actually required amount of resin is less than the maximum and can be calculated as a ballpark figure using this formula: SS = W ×4/(D2 ×π×density) SS = shotsize in centimeters W = weight of one preform × cavitation in grams D = screw diameter in centimeters Density(melt) = 1.15 g/cm3. The operator should add 5–10 mm to the so calculated value because it is not advisable to let the screw or shooting pod bottom out at the end of the stroke. The small distance between the end of the stroke and the end of the barrel is called the cush- ion and is usually kept at around 3–5 mm. Longer cushions are possible but increase residence time. Some machines offer closed-loop cushion control where a cushion value is set and the machine determines the cor- rect shotsize to achieve it. In order to get to the suggested injection time values, the operator has to try different speed settings. Most machines offer three to five different ones. Speed settings can be con- trolled in two ways:  In an open-loop setting they are in percentage. Open loop means that there is no feedback between the settings and the actual speeds. The hydraulic valve opens at the set percentage and a certain speed is achieved.  In a closed-loop system they are in millimeters per second. Here a feedback loop measures the actual speed and adjusts the valve to achieve the set speed. In a closed-loop system values at the screen may look like this:  Screw speeds: 15 40 15 mm/s  Screw positions: 25 55 65 mm In this three-point example shot- size is 65mm. The screw will then move from 65 to 55mm at a speed of 15mm/s, from 55 to 25mm at a speed of 40mm/s, and at 15mm/s thereaf- ter. These values will then result in a certain injection time, and the opera- tor can manipulate them to get to the desired time. Figure 3.8 The hydraulic force of 100 works on an area that is five to seven times the screw (or shooting pod) area. The force at the end of the screw is multiplied by the same factor as the difference in areas. 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