PETpla.net Insider 05 / 2018

BOTTLE MAKING 32 PET planet Insider Vol. 19 No. 05/18 www.petpla.net Another new approach is heating with laser beams. Beams created by diode lasers are emitted in the NIR spectrum directed very precisely onto the various areas of the preform with limited convectional heat. This oven system can be linear or circular as shown in Fig 4.7. Figure 4.7 Circular oven with laser heat- ing. Picture part of patent #8, 330, 290. Lamp control There are a variety of control mechanisms present depending on machine manufacturer and year of manufacture. Modern machines allow adjustment from control screens while older machines use a number of con- trol devices. Voltage regulators control all lamps from 0 to 220V. On some control screens this is expressed as a percentage from 0 to 99%. Machines differ in how many voltage regulators are in use (Fig. 4.8). Figure 4.8 Modern machines offer lamp control right on screen. Diagram cour- tesy of KHS Corpoplast. In addition there is often a master module allowing the adjustment of all lamps in one oven up to the maxi- mum voltage. Individual lamps can also be switched off completely either right on the screen or by pulling the plug in the control cabinet (see chapter 10 on how these features are used in the process). Most machines monitor current ow to the lamps and warn opera- tors if the current falls below a lower threshold because of lamp burnout or cable-breakage. Fan cooling Besides cooling the lamps and reflectors oven systems must also provide preform cooling (chapter 6). Designers use air blowers for this task exclusively and machines differ in the number, size, location, and control of these. Simpler methods are fans with fixed speed motors while more sophisticated machines offer variable speed motors to control air flow. In the latter case thermocouples in one oven provide a means of correlating the effect of the fan to the oven environment (see chapter 6, section 6.4 for pos- sible problems). Close-looping of fan speed and oven temperature may or may not be available and is not always advised. Consider this sce- nario: Preform temperature is set to 90°C and oven temperature to 85°C, both being close looped. Let us say the plant temperature increases during the day as is common in the summer months in Northern cli- mates. The effect on preform and oven temperature is not the same though. If the preforms become warmer the machine regulates the lamps down. This leads to a drop in oven temperature. If this drop is larger than the increased air tem- perature the fan will decrease its speed leading to increased preform temperatures and the lamps will be further decreased to compensate. The two regulatory circuits would basically fight each other unless the increase in environmental tempera- ture change affects both parts by the same percentage, which could actually lead to overregulation when the lamp output drops and the fan speed increases at the same time. It is therefore often better to close- loop the preform temperature only and control oven temperature manu- ally. Choose at least three times the number of cavities as the moving average for lamp control, that is, on a 20-cavity machine it should be at least 60. Choose a small propor- tional factor (the rate at which the lamp output will be changed) as well, typically 0.3. This will avoid overregulation (fig. 4.9). Figure 4.9 Air ow in oven with pre- forms right side up. Diagram courtesy of Adphos Group. 4.2 Transfer functions Rotary machines One variable speed DC or AC motor drives all machine functions through a combination of gears, chains, or linkages, ensuring that all physical movements are synchronised whether preforms are in the machine or not (except the stretch rod). Brakes and clutches on the main wheels prevent overloads or unwanted move- ments (spinning) during a sudden stop. An electronic rotary indicator signals the main drive position to the programmable logic controller (PLC) during each rotation. The PLC man- ages all nonsynchronised machine actions such as blow air. A hand crank allows manual machine rotation for adjustment or clearing of preforms. Transfer func- tions vary a great deal from machine to machine. Fig. 4.10 will give an idea of the components involved.