BOTTLE MAKING PETplanet Insider Vol. 24 No. 05/23 www.petpla.net 26 particular position with respect to an oblong bottle shape. This requires an expensive device in two-stage moulding but is “free” in ISBM as the neck start can be chosen deliberately and the preforms are held in place between injection and blow without spinning. Vertical injection tooling leads to longer tool life because gravity does not try to pull tools off-centre as is the case with horizontal injection machines. Many custom bottles have special neck finishes for which it is hard to find preforms on the open market. In this case, a single-stage solution may be the most cost-effective solution as it is often cost-prohibitive to build and run injection tooling for preforms of low volumes. The inside of the preform is always hotter than the outside. This is of great advantage as the inside always has to stretch more than the outside (Chapter 6, Section 6.2). The reason why this is happing in single stage is that the cooling area on the core side is always smaller than on the cavity side and often with less flow (see Chapter 9, Section 9.6 for a more detailed explanation). This explains the ease of forming difficult container shapes. There are also a number of disadvantages mentioned as follows: Cycle times are relatively slow even when comparing the same thickness and weight preform moulded on a modern injection machine. Part of this has to do with the way the machine rotates requiring several tool movements that accumulate dead time. Long changeovers: Injection tooling is very cumbersome to remove as each injection core must be replaced individually. Machines are also difficult to access making it more timeconsuming to work on them. Each blow cavity requires one injection cavity. Because injection usually takes 2 to 3 times longer than blowing the blow section is idle for half to two thirds of the cycle time. In two-stage moulding it is much easier to match the output of an injection machine with that of a blow machine. For example, a 72-cavity injection machine running a 8s cycle time produces around 32,000pph. A blow machine with 16 cavities running at 2,000 bph per cavity will be the ideal candidate to match. Both machines can run at full capacity, a great advantage given capital and footprint limitations. As explained in greater detail in Chapter 9, Section 9.7 processors are not in complete control of the preform temperature profile limiting the uniformity of bottle wall thickness and reducing bottle quality (Fig. 8.4). Fig. 8.4 8.2 Machine controls Melting in Extruder The resin that enters the extruder throat is a mix of crystals and amorphous parts. In order to melt the resin the extruder must heat and soften the amorphous fraction and melt the crystalline fraction. By rubbing the pellets against each other and against barrel and screw the extruder generates the necessary shear heat for melting. All crystals must be melted or they will become nuclei (starting points) for crystallisation in the preform. Heat transfer from barrel through heater bands is only about 30%, may even be negative in some zones. Negative heat transfer would be the case when the temperature readout of an extruder zone is higher than the set point. In this case the friction inside the barrel is so high that it actually overheats the barrel and must be cooled down to maintain the temperature that is selected. This usually happens at the end of the barrel in the so-called metering zone of the screw. Most heat (about 70%) comes from pellet inlet temperature (dryer) and from friction (screw and barrel). The operator has control over the heats, the screw rotational speed, and the back pressure during screw rotation, which is called recovery. While temperature screens differ from machine to machine, they all convey the same information. They may show: the location of the heater band (usually going right to left) °C or °F the set point the actual temperature a display of a temperature without set point is the temperature of the incoming resin as measured just above the extruder throat. A temperature of about 165°C is optimal for PET processing the percentage of power the controller puts out to the heater band. For example, if this value is 40%, the heater band is on for 4s, then off for 6s. The controller will use a value that is best suited to keep the heater band at the set point. This is regulated by a so-called PID loop and all controllers use some form of this control program. A typical temperature profile starts at 270°C (518°F) at the feed zone and increases to 285°C (545°F) toward the extruder nozzle. This can be used for most PET applications. The extruder cannot be started until all heaters are at the set point and a soak timer has timed out. The machine heats have to be enabled either by a physical switch or by a switch on a screen before the machine starts heating. If a “soft start” feature is available and is selected heater bands heat up slowly. Soak time may be available on some machines and is the time between the moment when the last heater band has reached its set point and the moment the machine allows the extruder to start. Soak time is different for extruder and hot runner. A longer soak time does not harm the process but too short of a soak time may. Use 30min for the extruder and 15min for the hot runner. If a “standby” function is available it allows the dialing in of a second, lower set point. When the machine is expected to be down for longer than 30min, this feature is used to prevent material from burning without turning heats all together off. Older machines may not have a protection against a “cold start,” which is the (often accidental) turning on of the extruder before the heats are up and have had time to soak. This will usually break the screw at the thinnest point in the feed section. On these machines a note should be kept on the machine during the heating up process to indicate when it will be safe to start the extruder.
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