While this book will not detail every aspect of the injection moulding of PET preforms we will give on overview of some aspects that are critical to designers.

Drying of PET

Because PET is hygroscopic it must be dried before it can be injected. The maximum amount of water that can be in the resin when it is in the extruder throat is 50 ppm. This residual moisture will react with the PET in the extruder and lead to an acceptable drop of 0.03 to 0.04 in IV. Higher moisture levels will lead to much higher IV drops, rendering the material unsuitable for the application.

Three moisture levels, three resulting IV results. A material of 0.82 IV will be reduced to an IV of 0.68 when processed with o moisture content of 200 ppm.

The correct dying parameters are a combination of time and temperature at certain airflow. Modern dryers are able to generate the required air flow of 4 m3/h per kg per hour (1 cfm per lb per hour). Under these conditions, processors must calculate or determine by practical experiment the optimum residence time of the resin in the hopper for a given job. To do this practically, a handful of colour pellets is placed on top of the resin in the hopper with the time noted. The colored 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 centre of the hopper traveling up to 20% faster. Therefore a median residence time must be chosen. Once this residence time has been established the proper drying temperature can be chosen as from the graph on the next page.

Maximum drying temperature is 171°C (340°F). Higher temperatures lead to oxidation, which shows up as a yellowing of the resin.
Improper drying and the resultant drop in IV change the inflation behaviour of the preform in that the preform will inflate under lower pressure because the natural stretch ratio is greater. In turn this will lead to less orientation and weaker bottles. Preform designers should know this connection in case problems arise during production that are all too easily blamed on preform design.

Injection of Preforms

We will not discuss the melting and visco-elastic flow of the material in the extruder barrel as they do not pertain as much to the preform 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.

The various components of a typical injection mould.

Injection moulds consists of the male core, the female cavity and the neck inserts. 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 ore always water- Cooled, neck inserts may or may not be, 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 prevent shearing of the material.

We begin injection with the tool closed forming on empty cavity as above.

Material enters the cavity through the gate. Despite the relatively 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 17 mm) but may also happen for standard ones when guide bushings ore 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. 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 material 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. 4 to 8 vents approximately 0.001 to 0.0015 mm 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 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 simultaneously moving a polishing stone back and forth on the longitudinal axis of the core.

At this point in the injection process the cavity has been filled. 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. During the hold phase material that is now starting to shrink as it cools is replaced through the still open centre of the melt stream. This is necessary to avoid sink marks.

During cooling time material is now cooling quickly and shrinking onto the core in the process. It is noteworthy 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 moulding the warmer gate area limits the processability of the preform as the temperature cannot completely be dialed in but is a result of wall thickness and injection parameters.

When problems with a particular preform arise designers should be aware of the various aspects of the injection moulding process and drying parameters and first ensure that preforms were processed correctly before making changes to the shape of the preform.