PETpla.net Insider 10 / 2020

BOTTLE MAKING 39 PETplanet Insider Vol. 21 No. 10/20 www.petpla.net thermocouple is mounted and how many lamps are used. If the lamps clos- est to the thermocouple are at a high percentage, the reading will be higher just because of this and not because the temperature is actually higher. If the thermocouple was mounted some distance away from the lamps, let us say, somewhere in the exhaust duct it would give a more reliable reading of the temperature we are interested in when it comes to preform heating but would then not give us a good picture of the heat inside the oven. In short, this reading should be regarded with caution. There are two features a blow machine must have so that air cooling can be effec- tive:  a thermocouple in one oven measuring the oven temperature and  a variable speed motor for the air exhaust fan. The thermocouple must be protected from lamp radia- tion and be in the air stream, cooling the preforms, not in the one cooling the lamps. The fan should effectively blow warm air out of the system so the oven tempera- ture can be controlled under a variety of process conditions. The reading of this thermo- couple must be taken with caution. Depending on how high in the oven it is mounted and whether the top lamps are on or not, readings can differ by a significant amount. The reading is useful to check for differences once a suitable oven temperature has been established but it is not possi- ble to give a recommendation that fits all circumstances. Equilibration time is another way of evening out differences in temperature through the preform wall. Equilibration happens when preforms leave the oven sec- tion on the way to the blow moulding station as well as between ovens. Time spend in equilibration depends on the physical length of track between heating and blow- ing and the throughput rate. Due to more exposure to the environ- ment outside the ovens, outer preform walls tend to cool down more than inner walls thus contributing to the optimal temperature profile. Some machines have extra space in the heating section to allow ovens be moved for optimal equilibration. It is the lack of sufficient equili- bration time on some small, linear machines that often leads to inferior bottle quality. 6.5 Optimal preform temperature Now that we have established the optimal temperature profile within the preform wall, it is time to consider the overall temperature of the preform. PET has a temperature process window from 90 to 115°C. Higher temperatures cause the material to crystallise. This becomes first visible as cloudiness or haze when www.krones.com The future is circular Can plastic packages be sustainable? Sure, if they’re resource-economically produced and efficiently recycled. KRONES has the right tech- nologies for this. Harness them, and become part of a closed-cycle economy that really pays off – for manufacturers, consumers and the natural environment alike.