PETpla.net Insider 10 / 2014

42 PET planet insider Vol. 15 No. 10/14 www.petpla.net Screens on injection machines of different manufacturers differ greatly in their representation of basically the same parameters. Some are more text based, whereas others use graphics to display relationships. The experienced processor will intuitively be able to make sense of different dis- plays, whereas a novice may have dif- ficulties in understanding an unfamiliar operator interface (OI) to another. The concepts explained in this chapter are usable on all machines, from the crud- est single-line OI to the most sophisti- cated graphical interface. It will be up to the reader to recognise them in the various formats. (Fig.3.1) 3.1 Extrusion and screw control The first task of the injection machine is to transform the dried resin pellets into a homogeneous melt that can be injected into the tool. Three extrusion systems are used for the production of PET preforms:  Standard reciprocating screw whereby the screw recovers, then pushes the material out  Two-stage (“P”) system, that consists of a top-mounted screw that recov- ers and then transfers the molten material into a shooting pot that sub- sequently injects the material  Two extruders feeding a common shooting pot The advantage of systems with a shooting pot is that the screw has a longer portion of the cycle time to recover. With reciprocating screws the screw can only recover after hold and decompression times as it is the screw that injects and holds the pressure. In a shooting pot system the screw transfers molten material to the shooting pot under low trans- fer pressure, and then immediately starts recovering. This allows the use of smaller screws for the same overall output, which is beneficial for PET because shear stress increases with the square of the screw diameter and is therefore harder to control with larger-diameter screws. The resin that enters the extruder throat is a mix of crystals and amor- phous parts. In order to melt the resin the extruder must  heat and soften the amorphous fraction and  melt the crystalline fraction. All crystals must be melted because unmelted crystals would act as nuclei (starting points) of crystallisation in the preforms. This has to be avoided as the goal is to have the preform completely amorphous (see Chapter 2.2). The melting of the resin is accomplished mainly by transferring the mechanical energy of the rotating screw into shear energy. By rubbing the spherulites against each other and against barrel and screw, the extruder brings on the necessary shear heat for melting. 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 actu- ally overheats the barrel and must be cooled down to maintain the tem- perature that is selected. This usually happens at the end of the barrel in the so-called metering zone of the screw. Most of the heat (about 70%) comes from pellet inlet temperature (dryer) and friction (screw and barrel). Most screws used in PET process- ing are called barrier screws. Like all screws, barrier screws are manufac- tured with three distinct areas (fig. 3.2):  Feeding  Compression or transition  Metering Each screw consists of a number of flights that are organised around a changing root diameter. The L/D ratio is calculated by dividing the flighted length by the screw diameter (fig. 3.2). This parameter allows easy classifica- tion of a screw in terms of residence time. Short L/D ratios are for materi- als that are sensitive to degradation when exposed to heat for a long time, whereas long ratios are for materials that need the extra residence time to completely melt. Typical ratios for PET screw are 22:1 to 24:1 with shorter screws being used in some single- stage machines. The compression ratio is calcu- lated by dividing the flight metering height by the flight feeding height. The compression rate is calculated by dividing length of the transition zone by the value that arises when the metering root depth is subtracted from the feed root depth. Figure 3.1 Only thorough understanding of the moulding process allows opera- tors to continually mould high-quality preforms. Figure 3.2 Standard screws are common for many injection-moulded parts. (Dia- gram courtesy of Barr Inc)