PETpla.net Insider 04 / 2018

BOTTLE MAKING 29 PET planet Insider Vol. 19 No. 04/18 www.petpla.net also often pushed outward to heat the gate area of the preform from above. In this case the lamp may be rotated in order that it points downward. Another design feature is the use of tubes with a higher linear watt density in the first (or the first two) row of lamps. Watt- ages range from 1000 to 3500W. Figure 4.2 Easy access to the infrared lamps is another important design fea- ture. Courtesy of Krones AG. The stack of lamps must also be adjustable in the vertical direction to allow them to be aimed at the exact location in preforms that may vary with the type of neck finish used (Fig. 4.2). Opposite the lamps are the reflec- tors. These are, of course, made from highly reflective material and contribute up to 40% of preform heating. Polished aluminium has proven to be the most economical material to use. To prevent oxidation, reflectors must be cooled either by air or water and both methods are in use. Tarnished reflectors must be replaced or resurfaced, otherwise energy consumption will increase and/ or bottle quality will suffer. Protecting the neck finish from infrared heat is another crucial part of oven design. Polyethylene tere- phtalate (PET) starts getting soft at temperatures above 65°C (150°F). If necks soften they are subject to deformation under the impact of hoop stress, that is, the force of the air pressure against the inner neck walls or through handling inside the machine. Neck finishes are protected with one or several metal shields that are either air or water-cooled. Hoop stress increases with the diameter of the neck. Therefore widemouth bottles are exposed to higher hoop stress. Air cooling may be sufficient for a 28mm neck but water cooling may be necessary to protect a neck of 63mm. Figure 4.3 An infrared temperature sensor is mounted before the blowing section monitoring preform tempera- ture. At the end of the oven track, just before preforms enter the blow sta- tion or blow wheel, a single infrared thermocouple provides information on preform skin temperature (Fig. 4.3). It should be noted that the thermocouple by its nature cannot provide information on tempera- tures underneath the preform skin. Depending on preform thickness and equilibration time the sensor might report the same temperature for very different preform condi- tions. Since it is only monitoring one specific location, the reading will only change when the particular lamp or lamps that are affecting this spot are changed. This can lead to confusion and operators should take note which lamp is affect- ing the part of the preform that the sensor is monitoring. However, the reading is very useful in monitor- ing the process once it has been established and machines allow the setting of upper and lower toler- ances and will alert operators to deviations. Some machines also close-loop the preform temperature with the lamp output thus reducing the impact of changing environmen- tal conditions. Because of com- plex interrelationships, algorithms employed on some older machines do not always result in the most stable process and many operators choose not to use this option. product development caps & closures design high cavitation moulds multi-component moulds closure production systems after sales service competent – professional – inspiring Dr.-Walter-Zumtobel-Straße 9 | 6850 Dornbirn | Austria | www.z-moulds.com CHINAPLAS April 24th-27th meet us at booth 2B53 Mould at CHINAPLAS 28mm 1881 FlipTop @KraussMaffei at booth 2T01