PETpla.net Insider 06 / 2017

BOTTLE MAKING 26 PET planet Insider Vol. 18 No. 06/17 www.petpla.net preform design and process condi- tions. (Higher crystallinity levels are achieved in the heat-set process, see Chapter 7.) Figure 2.4 Different states of PET pre- sent in each bottle The finished bottle will have amorphous portions in the neck and gate area where the bottle was not stretched, oriented portions in the side walls, and sometimes thermally crystallised portions around the gate, a common preform defect that cannot be corrected during blow molding (Fig. 2.4). 2.3 Drying of PET Because PET is hygroscopic, it must be dried before it can be injected. The maximum amount of water in the resin when in the extruder throat is 50ppm. This residual mois- ture will react with the PET in the extruder and lead to an acceptable drop of 0.03–0.04 in IV (Fig. 2.5). Higher moisture levels will lead to much higher IV drops, rendering the material unsuitable for the application. Figure 2.5 Three moisturelevels, three resultingIV results.Amaterialof 0.82 IV will be reduced to an IV of 0.68 when processed with a moisture content of 200 ppm. The correct drying parameters are a combination of time and tem- perature at a certain airflow. Modern dryers are able to generate the required airflow of 4m 3 /h per kg per hour (1cfm per lb per hour). Under these conditions, processors must calculate or determine, by a practical experiment, the residence time of the resin in the hopper for a given job. To do this practically, a handful of color pellets is placed on top of the resin in the hopper with the time noted. The coloured pellets will eventually show up in the preforms and the time can then be measured. Depending on the position of the resin in the hopper, drying times differ, with the resin in the center of the hopper traveling up to 20% faster. Therefore, a median resi- dence time must be chosen. Once this residence time has been established, the proper drying temperature can be chosen, for example, from the graph in Fig. 2.6. The maximum value of drying tem- perature is 171 °C (340 °F). Higher temperatures lead to oxidation, which manifest as a yellowing of the resin. Figure 2.6 Drying time must be chosen to match the residence time of the resin in the hopper. Improper drying and the result- ant drop in IV change the inflation behavior of the preform in that the preform will inflate under lower pres- sure, because the natural stretch ratio is greater. In turn, this will lead to less orientation and weaker bottles. Preform designers should be aware of this connection in case problems that are all too easily blamed on preform design, arise during production. 2.4 Behavior in the injection mould We will not discuss the melting and visco-elastic flow of the mate- rial in the extruder barrel as they do not pertain as much to the pre- form design. But the injection part is important for designers to understand because of the particular opportunities and process limits, as well as possible defects that will then affect the blown bottles. Figure 2.7 Various components of a typical injection mould Injection moulds consist of the male core, the female cavity, and the neck inserts (Fig. 2.7). The latter have to move during ejection of the part to release the undercuts created by the thread beads. For this purpose, they are mounted on slides that are often cam-driven. Cores and cavities are always water-cooled; neck inserts may or may not be. Injection mould- ing of preforms is different from other forms of injection moulding as the preform wall is relatively thick, injec- tion pressures are relatively low, and the injection speed is low to prevent shearing of the material. Figure 2.8 Empty cavity Figure 2.9 Injection 1 Figure 2.10 Injection 2