PETpla.net Insider 11 / 2018

BOTTLE MAKING 20 PET planet Insider Vol. 19 No. 11/18 www.petpla.net Please order your copy at the PETplanet Insider book shop: www.petpla.net/book3 Stretch Blow Molding Third Edition by Ottmar Brandau € 130,00 374 pages © Copyright Elsevier 2017, 2012 5.2 Base mould Today, all base moulds feature a small recess like a well in the center. This allows some room for the protruding injection gate on the preform. Keep- ing the gate in the center of the mould is probably the most important task in the blow process because any devia- tion from the center leads to uneven wall thickness variation. The well catches the injection gate and prevents it from slip- ping as long as there is enough pressure from the stretch rod. In many custom applications the bottle bottom is thicker than is really needed but because of preform design or machine insufficiencies it ends up like that. In these cases it is often the cool- ing of the bottle bottom that controls the cycle time. Proper cooling is therefore crucial to come to a cost-effective solu- tion with fast cycles. Cooling lines should not be smaller than 6mm in diameter (unless of course there is no room in very small bottles) and the flow path should have no restrictions. High water supply pressure with low water return pressure are also helpful (Chapter 13, Section 13.2). 5.3 Making a mould Today’s mould-making process starts with a three-dimensional (3D) computer model of the container itself. Physical models may be made by a variety of processes, the most popular still being stereo lithography with 3D printing catch- ing up quickly because of the availability of low-cost printers. The model may be used to give marketing people a better “feeling” for a new container. Once approved, data of the computer model are then fitted in a new or existing mould base. At this point, shrinkage has to be added to the container dimensions. Polyethylene terephthalate (PET) shrinks approximately 0.08% but shrinkage is not uniform and it is the experience of the mould maker that determines how closely the capacity of the container matches specification. A variety of computer-aided design (CAD) / computer-aided manufacture (CAM) programs allow the creation of machine cutter paths that are down- loaded directly into high-speed machin- ing centers. Machine operators load and center blocks of aluminum of suitable size and special cutters, spinning at up to 30,000 r.p.m., move at a speed of up to 20m/min. The resulting cavity surface is already smooth to the eye but most mould makers add a high, mirror-like polish, which still requires skilled, manual labor. The use of sandblasted sur- faces that are common in other plastic processes has gained some ground as there is little difference in the appear- ance of the containers. Some mould makers then coat the cavity surfaces with various materials, often contain- ing nickel and Teflon, to give it abrasion resistance. 5.4 Venting Venting is another area where the experience of the mould maker becomes extremely important. Because PET fills the mould cavity during blowing, the air inside the cavity must be exhausted. For this purpose mould makers add a variety of vents. Compared to other processes, such as injection moulding or extru- sion blow moulding, PET is processed at a relatively low temperature in the RSBM process. Vent sizes are limited to 0.04mm (0.0015in.) in injection mould- ing but vents of up to 0.5mm (0.020in.) are used in RSBM with hole vents up to 1mm (0.040in.). All moulds have vents on the contact surface of the cavities. One mould half is typically completely recessed against the mould base by up to 0.20mm (0.08in.) or more commonly by 0.15mm (0.006in.). Base vents are also common and are accomplished by leaving the base insert to move 0.25–0.3mm (0.010–0.012in.) down- ward under the force of the stretch rod. The resulting ring-shaped gap between base insert and mold cavity allows air to escape. Hole vents up to 1mm are used in areas where air entrapment is sus- pected. Vents of this diameter may not show in areas where the material has stretched and consequentially strain- hardened but will show as small dimples where this is not the case. A common example of highly stretched material is the foot of a petaloid base for carbon- ated soft drinks (CSD) containers. Two small holes in each foot let air escape that might otherwise be trapped by the material flowing around it (Fig. 5.4). Figure 5.4: Hole vents up to 1mm in diam- eter can be successfully used as shown here in the panel area of a hot-fill bottle. Photo courtesy of Garrtech Inc. Another use of venting is to direct PET into hard-to-blow areas. In a highly oval bottle, for example, there is always the possibility of a ridge of higher wall thickness forming at the center of the narrow side of the container. Vent holes at the far side of the mould can attract PET to flow more quickly into these areas, thereby stretching out the preform walls close to the narrow side. A fine sandblast finish instead of the mirror- finish also helps to let the air move out of the mould. Due to low temperature in the RSBM process compared with uses in other processes, PET does not flow easily into small mould crevices. Minimum dimen- sions for female radii might be given as 0.8mm (1/32in.) but it will depend on the stretch ratio of the PET flowing toward it whether it will fill out or form a greater radius instead. Male radii should be double that amount especially when used in bases. Here a sharp radius may cause a crease in the material and open the door to stress cracking. Venting in these areas can be attempted to reduce the risk of air entrapment stopping the advance of the parison but more often than not they do not seem to have much effect. We will simply have to live with the fact that PET benefits from more generous radii in this process.