PETpla.net Insider 08 / 2014

PREFORM PRODUCTION 30 PET planet insider Vol. 15 No. 08/14 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 purpose the neck inserts are mounted on slides that are often cam driven. Cores and cavities are always water- cooled, whereas neck inserts may or may not be water-cooled. Injection moulding of preforms is different from other forms of injection moulding as the preform wall is relatively thick, injection pressures are relatively low, and the injection speed is low to pre- vent shearing of the material. We begin injection with the tool closed, forming an empty cavity as in Fig. 2.13. Fig. 2.13: Empty cavity The material enters the cavity through the gate. Despite the rela- tively low injection pressure, the mate- rial pressure may bend the injection core to one side and cause what is known as “core shift” with the result- ing 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 (Fig. 2.14). Fig. 2.14: Start of injection As the hot material hits the cold mould walls, the resin in direct contact with the wall freezes off and forms a boundary layer. The material in this layer will not change during injection. Its thickness restricts the mold chan- nel and is one reason why a minimum wall thickness must be maintained in the preform gate area (Fig. 2.15). Fig. 2.15 Boundary layer builds 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. The air that is present in the mould cavity must have an escape path. Otherwise, trapped air would lead to sink marks in the preforms. Four to eight vents approximately 0.001–0.0015mm deep 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 is improved 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 moving a polishing stone back and forth on the longitudinal axis of the core (Fig. 2.16). Fig. 2.16: Injection continues At this point in the injection pro- cess the cavity has been filled (Fig. 2.17). 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 exclu- sively. The point at which this occurs is called the transition or switchover 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 centre of the melt stream. This is nec- essary to avoid sink marks (Fig. 2.17). Fig. 2.17: Injection cavity full 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 this reason. In single-stage stretch blow moulding the warmer gate area limits the processability of the preform as the temperature is a result of wall thickness and injection parameters and therefore not directly controllable. (Fig. 2.18) Fig. 2.18: Cooling time When problems with a particular preform arise, designers should be aware of the various aspects of the injection moulding process and drying parameters and should first ensure that preforms were processed cor- rectly before making changes to the shape of the preform. * This article was published in Bottles, Preforms and Closures, Ottmar Brandau, Chapter 2.4. Copyright Elsevier 2012