Further improvements in the manufacture of PET bottles
Netstal-Maschinen AG, Näfels, Switzerland, is promising further improvements in the manufacture of PET bottles. The Swiss injection molding machine producer has continued to develop the cooling system of its PET-Line range. In this system, which is marketed under the Calitec brand, compressed air is used to place the interior of the preform under light pressure. This prevents shrinkage of the warm molding since it is uniformly pressed onto the cooling sleeve and also shortens the post cooling phase. A rubber ring mounted on a piston rod is pushed into the preform orifice and pneumatically squeezed in order to seal its interior. This allows the preform to be uniformly and intensively cooled without any interruptions, which has been shown to improve part quality.
The patent pending Preblow process now takes this a step further. As investigations by Netstal experts have demonstrated the base area of preforms can be preblown under the influence of compressed air right after the injection molding process. This process step increases the surface area of the base section, which in turn means a lower wall thickness in this critical area ("LightBase"). This effect allows the base to be more efficiently heated during the later blow molding process when the preforms are blown into bottles.
Since it is not possible to injection mold such a thin wall preform base processors are often faced with an accumulation of material in the sprue area. As the sprue area is in the middle of the preform base it is also furthest from the side mounted IR lamps in the blow molding machine. If the excess material in this area is not sufficiently warmed – radiant energy flux decreases with the square of the distance – the PET there will not stretch correctly and can even become somewhat brittle. According to Stefan Bock, Head of Application Technology PET-Systems at Netstal, preblow means that these problems are now a thing of the past. He regards the larger blow molding process window and the further reduced cycle times through reduced accumulation of material in the base as a significant innovation in the manufacture of PET bottles.
Bock indicated that an initial trial with 1 liter juice bottles had shown it was possible to achieve a weight reduction of 1 g in the base. During preblowing of the base the material was distributed to the bottle body becoming part of the PET bottle wall following the blow molding process. He added that by enlarging the tool core this material could be saved without losing stability. In a system making a 15.8 g preform for still mineral water Netstal were able to reduce material usage by at least 0.5 g per bottle. For 128 cavities at a cycle time of around 7.2 seconds this represents a saving of some 255 t of PET per annum.
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