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machined into the blow mold bottom. This allows room for the small injection vestige at the end of the preform and pre- vents the preform from slipping. • Bent stretch rod. Stretch rods are typically very sturdy at diam- eters of around 0.5 in. and do not easily bend. But lightweight water bottles require stretching out the entire base, and often rods are slimmed down at the ends to allow for a smaller con- tact area and cooling efect on the preform. Or, in the case of bottles with necks as small as 20 mm for cosmetic applications, the thick rods just won't ft into them. Thin rods can bend much more easily, and even thicker rods have been known to bend occasionally. In any case, a bent rod will easily be recognized as it will skew the gate always in the same direction while other defects occur more randomly. • Preform is bent before entering the blow mold. This is an issue that occurs more frequently in single-stage stretch-blow molding and has a diferent cause there. In two-stage it may happen when the preform wall thickness is uneven by more than 0.004 in. This leads to uneven heating—i.e., the thinner side gets hotter, and this side may then shrink more than the cooler side between the preforms leaving the ovens and the blow mold. In that case, the stretch rod hits the preform of- center and transports it to the blow mold in the same way. In single–stage stretch-blow there may be another problem besides possible wall-thickness diferences—non-uniform heat distribution in the preform. This is because viscous heating creates a ring of hotter material inside the molten plastic. When the runner is typically divided into two streams, hotter material is pushed more to the back than the front and this can often be measured in uneven wall thickness. UNEVEN HEATING OR COOLING It often bafes processors when the gate is in the center of a round bottle but the walls show diferences of 0.004 in. or more. This usually indicates that one side of the preform is cooler than the other. The warmer side stretches more and so thins out. In single- stage processes, this is quite common, as noted above, but can also happen in two-stage stretch-blow. It could be that air is blowing on the preforms after they stopped rotating. I have also seen that when preforms do not spin while in the oven system, heat from the oven metal heats up the side of the preform that is It often baffes processors when the gate is in the center of a round bottle but the walls show differences of 0.004 in. or more. This usually indicates that one side of the preform is cooler than the other. turned towards it. A thermal camera is helpful here to detect heat diferences and locate their sources. SMALL STRETCH RATIOS For the self-leveling efect described above to work, preforms must be stretched in both the vertical and hoop direction. Minimum ratios are 2:1 in the vertical and 4: 1 in the hoop plane. But design limitations, especially for small bottles below 12 oz, or the process itself (these numbers are already at the realistic max- imum for single-stage) may prevent designers from implementing large enough stretch ratios. As a result the material cannot fully stretch out the cooler parts and they stay thicker. In many cases preforms are purchased that have the right neck fnish and weight but are not necessarily designed for the partic- ular application they are used for. This can also lead to improper stretch ratios in all or part of the bottle. ABOUT THE AUTHOR: Ottmar Brandau has been working in the plastics industry since 1978 and is the president of Apex Container Tech Inc., Wasaga Beach, Ont. His latest book, The Rapid Guide to Perfect PET Bottles, describes 30 common problems and their solutions. It can be found at blowmolding.org/shop. Contact: (705) 429-1492; info@blowmolding.org; blowmolding.org. @plastechmag 33 Plastics Technology B L O W M O L D I N G