PETpla.net Insider 06 / 2017

BOTTLE MAKING 28 PET planet Insider Vol. 18 No. 06/17 www.petpla.net Figure 2.11 Injection 3 Figure 2.12 Injection 4 Figure 2.13 Injection 5 We begin injection with the tool closed, forming an empty cavity as shown in Fig. 2.8. Material enters the cavity through the gate (Fig. 2.9). Despite the rela-tively low injection pressure, the material pressure may bend the injection core to one side and cause what is known as ‘core shift’ with the resulting preform wall thickness becoming uneven. This is especially true for thin cores (below 17mm) but may also happen for standard ones when guide bushings are worn out, for example. As the hot material hits the cold mould walls, the resin in direct contact with the wall freezes off and forms a boundary layer (Fig. 2.10). The material in this layer will not change during injection. Its thickness restricts the mould channel and is one reason why a minimum wall thickness must be maintained in the preform gate area. As more material enters the cavity, the boundary layer expands along the length of the preform. Its thickness stays the same as long as hot mate- rial is flowing through (Fig. 2.11). The air that is present in the mold cavity must have an escape path, otherwise, trapped air would lead to sink marks in the preforms. Four to eight vents with depth of approximately 0.001– 0.0015mm are machined into the face area of the preform neck, allowing air to vent to the outside. Sink marks are also prevented and the flow of material improves by giving cores a finish in the direction of material flow rather than radially. This is achieved by special machinery that turns the cores while simultaneously moving a polishing stone back and forth on the longitudinal axis of the core. At this point, the cavity has been filled during the injection process (Fig. 2.12). The added resistance causes the hydraulic pressure to increase and it is here that the machine needs to be switched from injection to hold or packing pressure. This can be done by using the actual pressure as the setting to trigger the hold pressure but, for PET, a position-based trigger has proven to be more consistent and is therefore used almost exclusively. The point at which this occurs is called the transition or switch-over point and can be dialed in on the screen. During the hold phase mate- rial that is now starting to shrink as it cools is replaced through the still- open center of the melt stream (Fig. 2.13). This is necessary to avoid sink marks. During cooling time, the material cools quickly and shrinks onto the core in the process. It is notewor- thy that the gate area of the preform always stays warmest as it is the last part of the preform to receive hot material. Most preform defects such as cloudiness are located here, for that reason. In single-stage stretch- blow molding, the warmer gate area limits the pro¬cessability of the preform as the temperature cannot completely be dialed in but is a result of wall thickness and injection param- eters. When problems with a particular preform arise, designers should be aware of the various aspects of the injection molding process and drying parameters and first ensure that pre- forms were processed correctly before making changes to their shape. 2.5 Behavior in the blow mould Natural Stretch Ratio (or Natural Draw Ratio) The stretch or draw ratio of a poly- mer is the ratio of the resulting length (in the direction of applied stress) to the original length. When PET is stretched, for example, during blow molding, it reaches a point at which an increase in the force is required to continue stretching. The point at which the PET requires this extra force is called the natural stretch ratio (NSR) for a particular set of stretching conditions. The NSR is reached when strain (or work) hardening occurs on the stress–strain curve for materials. Recall that before a material yields, once the applied force is removed, it can return to its original dimensions. Stretching beyond the yield point results in perma- nent deformation, and further stretching will result in fracture. In some materi- als, including PET, strain hardening can occur before fracture, which is essen- tially the aligning (or orienting) of the structural regions of the material in the direction of the applied stress which can result in improved physical properties of the material. The design of a PET preform is such that during stretch blow, the optimum orientation is achieved just as the stretched walls meet the mould. This point occurs just beyond the NSR. Proper stretching results in longer shelf life and less gas perme- ability, e.g., higher carbon dioxide retention for soda. Overstretching results in a “pearlescent” appearance to the bottle signifying microcracks (fracture) and excessive deformation. A resin with a low IV has a higher NSR than a resin with high IV. The polymer chains in a low IV resin are shorter and, therefore, less entangled and can be easily stretched more than those in a high IV resin. In the high IV resin, chain entanglement limits the amount of stretch; this is, similar to trying to pull one end from a tangled ball of string where the knots limit the length that can be pulled out. For this reason, preform designs differ when considering low IV or high IV PET. The figures in the following sec- tions illustrate the material stretching in the blow mould without relating to actual data. Strain (elongation) is plot- ted on the horizontal axis and the cor- responding stress on the vertical axis. To obtain these data, a heated test strip of PET might be pulled on a spe- cial machine that records the pulling force and the elongation of the strip. In the RSBM process, the stretch