PETpla.net Insider 09 / 2014

PREFORM PRODUCTION 36 PET planet insider Vol. 15 No. 09/14 www.petpla.net pressure.) However, to make sure that PET can enter the cavity without too much shear stress, a ratio of 72–80% is used in most cases. This means that a preform with a 3mm body wall thickness will have a gate wall thickness of 2,16–2,34mm. The exception to this rule is a preform with a body wall thickness less than 2,5mm. These have become quite popular with lightweight water bottles as a lower preform wall thick- ness reduces cooling requirements and speeds up cycle times. There is a minimum gate wall thickness of about 1,8–2mm, and gate to body wall thickness ratios of up to 100% may become necessary for these preforms. Two-stage process blow moulding All preforms are at the same tem- perature when they are fed into the blow moulding machine. In the heating section they spin on mandrels while infrared heaters bring them to blow- ing temperature. This virtually guar- antees that all preforms are at the same temperature circumferentially. One limitation of the process is that, in order to protect the neck finish, the first lamp in the oven section has to have a certain distance from the neck finish that is protected by a sheet metal shield. Therefore, the preform section below the neck support ring cannot be heated to the degree that proces¬sors would want to. The material in this sec- tion forms the shoulder of the bottle, where less material is needed, and it is beneficial for overall weight savings to pull material out of this area. Preform designs for the two-stage process take this into account by bringing the thin- ner neck section down to a point where material is then needed to form the wider body of the bottle. When reheating preforms in a good blow moulding machine, knowl- edgeable processors can actually bring the inside temperature of the preform to a higher level than the out- side, achieving the optimal tempera- ture profile through the wall. They do this by using high lamp settings and enough venting air in the oven to keep the oven temperature below blow- ing temperature. A longer equilibra- tion time after heating is also helpful for this purpose. This is beneficial because the inner surface material of the preform has to stretch further than the outer one. By changing lamp heater outputs, oven fan cooling, and preform throughput, operators can precisely adjust the temperature to an optimal level. For these reasons high blow up ratios up to twelve are possi- ble in the two-stage process, resulting in very good properties such as car- bonation retention for CSD bottles. Single-stage process In this process preforms and bot- tles are manufactured in the same machine. After injection moulding, the preforms stay in the neck inserts and are shuttled to a conditioning station in some machines or directly blown in other machines. At this point in time the preform has only partially cooled down from the melting temperature of around 250°C (482°F). The cool- ing of the preform comes from inside (injection core) and outside (injection cavity) during injection, resulting in the centre of the preform wall staying warmest. This residual heat makes preforms from the single-stage pro- cess on average warmer than their two-stage counterparts. Because the last part of the pre- form to receive hot material is toward the gate area, this part of the pre- form is always warmer than the area underneath the neck. Processors are therefore limited in how they can process these preforms. Designers compensate for this in two ways. They use a lower gate to body wall thickness ratio, usually between 58% and 66%. This reduces wall thickness in the gate area, allowing faster cooling. They also tend to have the greatest body wall thickness underneath the neck if possi- ble. Quite contrary to intuition, a thicker wall in the PET preform in this process leads to a thinner wall in the corre- sponding bottle wall thickness. This is true because a thicker wall retains more heat and will then subsequently stretch so much more that a thinner bottle wall. Nonetheless, when the blow moulding process is the slave of the injection process, process capabili- ties diminish. Another limitation of the single- stage process is viscous heating. To understand this process we have to examine what happens inside the molten resin during injection. PET flows through the barrel, hot runner channels, and nozzles in a laminar fashion like honey flowing through a squeeze bottle nozzle. This flow is characterized by the highest shear rates occurring at the channel walls, whereas there is much less shear at the centre directly adjacent to the runner walls as the speed drops to zero (Fig. 2.19). Figure 2.19 Friction just off the channel walls causes viscous heating Shear deformation causes internal friction between adjacent entangled polymer chains, which results in shear heating. As a result of the laminar flow there is an elevated temperature in the ring-shaped area just off the chan- nel wall (Fig. 2.20). Figure 2.20 Calculated temperature spike in area adjacent to channel wall Most hot runners for single-stage machines are designed in a way that one large channel diverts into two smaller channels that come in at 90° (Fig. 2.21). Figure 2.21 When one channel inter- sects at 90°, the result is a skewed tem- perature profile.