PETpla.net Insider 03 / 2015

PREFORM PRODUCTION 45 PET planet insider Vol. 16 No. 03/15 www.petpla.net Experiments have shown that the boundary layer, shown here in light-gray, is about 0.25mm thick. In a 2.3-mm-thick body wall this leaves only 1.8mm for the material to flow through. In order to reduce complica- tions, typically a preform of this type will have the same gate wall thickness as the body and the injection speed is controlled to move the material for- ward at a constant speed. When the melt front reaches the thinner neck area, the pressure behind it increases while the pressure in front decreases as in the case of any other liquid when faced with an orifice. Peak injection pressure will happen at this point, and values of 125 bar (1800 psi) may be neces¬sary to overcome the restric- tion. This is against the recommenda- tion of a maximum of 100 bar (1450 psi) but unavoidable in this situation. A few measures can assist in injecting into these thin channels:  Use of low IV resins: As shown in Chapter 2.3, the lower the IV, the lower the melt viscosity and there- fore the lower the force required during injection.  Increase temperatures to lower the IV of any material: This often does not work well because with higher temperatures, higher shear is also encountered.  Increase the cooling water temper- ature especially during start-up.  Increase clamp tonnage to hold the clamp closed during higher pressure hold times. This will lead however to more tool wear. Setting the transition point with these preforms can be a challenge. When it is set too high, underfills will occur, which may be difficult to see (when not all preforms have them) or may lead to machine stoppages because of problems in the postmold cooling area. When it is set too low very high pressures may be neces- sary, which may lead to shear stress. Operators are well advised to carefully control this parameter taking all ele- ments into account. 3.6 Acetaldehyde in PET Bottles Acetaldehyde (AA) is a natural sweetener that is present in all citrus fruits and is often used as a sweet- ener in beverages. It is also a by- product of heating PET, especially heating it to melt temperature. Its sig- nificance in the PET industry relates to the production of water bottles. Still water taste is very sensitive to even small AA concentrations, whereas the sugar content of carbonated drinks and juices masks any flavor contrib- uted by AA com¬pletely. Producers of preforms for other beverages may still monitor AA content as a way of keep- ing track of the maintenance state of screw and bar¬rel, because when these parts wear, shear stress and AA creation increase. 3.6.1 AA Creation In the initial stages of PET resin manufacture, AA level may be as high as 150 ppm. What happens is that – OH end groups combine with water, glycol, or oxygen that may come in contact with the resin at this stage to form AA whose formula is CH3CHO. During solid-state polycondensa¬tion the material is heated and AA is removed by nitrogen gas to a level of <1ppm. Drying is again critical to AA generation during preform injection molding as moisture present in the resin will not only break the molecular chains but also create AA. A material temperature of 165°C (329 °F), as measured at the extruder throat of the injection machine, has been proven as optimal to minimize AA generation. Residence time and temperature are the other crucial factors (Fig. 3.18). The relationship between resi- dence time and AA level is linear. For the preform molder this means that extruder size and preform weight should be closely matched. One intriguing detail of PET preform manufacture is that cycle times do not vary significantly depending on weight but rather on wall thickness. Two preforms with the same wall thickness where pre-form A is twice the weight of preform B may differ in cycle time by only 1 or 2 s. This means that the residence time of the lighter preform B is almost twice as long as that of preform A with cor- responding effect on AA content. Over the past few years water bottles especially have become lighter with the negative effect that their resi- dence time has increased if they are produced in the same extruders as they were before lightweighing. The relationship between AA gen- eration and temperature is exponen- tial (Fig. 3.19). Figure 3.18