PETpla.net Insider 11 / 2019

BOTTLE MAKING 28 PET planet Insider Vol. 20 No. 11/19 www.petpla.net 5.2 Base Mould Today, all base moulds feature a small recess like a well in the center. This allows some room for the pro- truding injection gate on the preform. Keeping the gate in the center of the mould is probably the most important task in the blow process because any deviation from the center leads to uneven wall thickness variation. The well catches the injection gate and prevents it from slipping 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 cooling of the bottle bottom that controls the cycle time. Proper cooling is therefore crucial to come to a cost-effective solution 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 catching up quickly because of the availability of low-cost print- ers. 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. Poly- ethylene 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 crea- tion of machine cutter paths that are downloaded directly into high-speed machining centers. Machine operators load and center blocks of aluminium 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 labour. The use of sandblasted sur- faces that are common in other plastic processes has gained some ground as there is little difference in the appearance of the containers. Some mould makers then coat the cavity surfaces with various materials, often containing 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 pro- cesses, such as injection moulding or extrusion blow moulding, PET is processed at a relatively low tempera- ture in the RSBM process. Vent sizes are limited to 0.04mm (0.0015 in.) in injection moulding but vents of up to 0.5mm (0.020 in.) are used in RSBM with hole vents up to 1mm (0.040 in.). 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 (0.08 in.) or more commonly by 0.15mm (0.006 in.). Base vents are also common and are accomplished by leaving the base insert to move 0.25–0.3mm (0.010–0.012 in.) down- ward under the force of the stretch rod. The resulting ring-shaped gap between base insert and mould 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 carbonated soft drinks (CSD) con- tainers. 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 diameter 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 dimensions for female radii might be given as 0.8mm (1/32 in.) 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 crack- ing. 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 gen- erous radii in this process. 5.5 Stretch Rod In two-stage moulding stretch rods are typically made of solid Stainless Steel with sizes 9–16mm (3/8–5/8 in.). Stretch rod diameters are often lim- ited by the neck size and larger rods should always be used when the neck size allows it. This facilitates the exact placing and holding of the gate in the center of the mould. A downside of a large stretch rod diameter is that the