PETpla.net Insider 09 / 2014

PREFORM PRODUCTION 39 PET planet insider Vol. 15 No. 09/14 www.petpla.net As you can see from Fig. 2.22, there is a tendency for the cooler mate- rial from the centre of the channel to flow toward the far side of the inter- secting channel, whereas the hotter material flows to the near side. While intermixing and subsequent tempera- ture homogenization is happening, it is in many cases (depending on hot runners used, material properties, temperature, pressure, speed, and shot weight) not enough to prevent the hotter resin from being injected into the side of the cavity that faces the back of the machine, whereas cooler material is injected in the front. Because PET is very sensitive to even small tempera- ture fluctuations, processors often fight uneven bottle walls with thinner sides toward the parts adjacent to the back of the machine (Fig. 2.22). Figure 2.22 Hotter material may be pushed to the nearer side of the sec- ondary runner. Uneven heat of the melt also causes the melt to move at different speeds into the cavities. This can be established by doing a so-called “short shot” where an insufficient amount of material is injected (or the injection is interrupted at some point). It can then be clearly seen if all cavities are fill- ing at the same speed. Uneven filling means that the heat profile in each pre- form is slightly different, which always results in wall thickness differences of the blown bottles (Fig. 2.23). Figure 2.23 Actual infrared photograph of preforms after injection. Large side- to side temperature variation is clearly visible. There is however a way to improve on some of the inadequacies of this process. I should mention here that uneven filling is also a problem in two- stage injection moulding. Although it does not affect bottle properties, uneven filling and with it cooling can lead to gate and packing problems in some cavities. This is because the material in the cavities that are filled first cools down quicker and increases in viscosity earlier compared with the late-filling material. This may lead to a situation where the fast-filling cavities are overpacked and the slow-filling ones are underpacked. The preforms from the former may be hard to blow, and preforms of the latter could show flow lines or sink marks. The degree to which this happens and how much preforms will be affected depends on the particular preform geometry and process conditions but will be a factor when the fastest cycle time is the pri- mary goal. One way to improve on this is to change the way the melt changes direction in the hot runner. Instead of changing in hard right angles, a brazed, rather than drilled, hot runner plate allows the gentle curving of the directional changes, leading to less of a melt separation by temperature. In brazing the hot runner plates, two individually milled plates are fused together after milling, allowing curved melt paths (Fig. 2.24). Figure 2.24 Lower temperature vari- ations and more equal flow are the advantages of brazed hot runner plates that allow curved paths. (Diagrams courtesy of Mold-Masters) For cavitations other than multi- ples of two, the path width has to be adjusted to achieve a similar result. In practice, these manipulations often work well for a certain gram weight but fail to deliver balanced flow with others. In PET injection moulding for the two-stage process, only certain cavitations are used in order to prevent a departure from a naturally balanced design. In most hot runners here, one flow is divided into three and that is the reason for the 48, 72, 144, and so on cavitations. In the one-stage process, just about any number of cavitations between one and 16 has been built. This is because converters want to use a given machine frame to the maxi- mum cavitation available for a bottle application. This has forced hot runner manufacturers to come up with differ- ent approaches to still deliver good flow behavior (Fig. 2.24). An excellent way of measuring the performance of any hot runner is to perform a short shot. The operator interrupts the injection about midway or dials in the shotsize so that only half of the resin necessary to fill the cavities is available. The machine is then stopped and the half-injected preforms are numbered for identifica- tion and weighed. It is not unusual to find a 50% variation at the 50% injec- tion mark, i.e., preforms may weigh between 14 and 21 g, for example. This uneven development leads to dif- ferent cooling times and incorporated stresses in the preforms. Rounded- path hot runners have proven to significantly ameliorate this situation (Figs 2.25 and 2.26). Figure 2.25 Equal-length paths to each cavity are critical for balanced hot run- ners. (Drawing courtesy of Huangyan Yusheng Polyester Mold Factory) Figure 2.26 Very even injection develop- ment is apparent by the even length of the shortened preforms. (Picture cour- tesy of Mold-Masters)