The general stretch blow molding
procedure of the PET bottle is shown in here. In the first step, the preform of the PET bottle is produced by injection molding, where the space between the metal core and the injection mold is filled with melted PET. After the cooling and heat adjusting process, the preform is enclosed inside the blow mold and then stretched with a metal rod (step 2). In the third step, PET is deformed, by the combination of stretching load and air pressure, to the shape of the mold. Finally when the bottle is sufficiently cooled inside the mold, it is ejected(step4).
The simulations were performed for three different cases. In the first case, the preform was blown without stretching. In the second case, the preform was initially stretched by a rod to the bottom of the mold, and after finishing this process, air pressure was applied. In the third case, the blowing pressure was applied before the stretching rod reached the bottom, the material was inflated in the surface normal direction while being stretched in the longitudinal direction. These different forming procedures resulted in different material deformation modes. The simulated deformation shapes of the different cases are shown blow:
In case one, during the early stage of the process, the preform tended not to extend in the longitudinal direction, but inflated in the radial direction. It deformed like a balloon, and due to the constraint of the surrounding mold, most of the material got into contact with the tool in this early stage. More than 70% of the material contacted 40% of the total area of the mold surface. Because the node of the parison was assumed to stick to the mold surface after contact, the remaining 30% of material should govern the further deformation and be distributed to the rest of the mold surface. This resulted in serious thinning of the thickness.
Figure (1 a). Blowing without stretching (step a)
Figure (1 b). Blowing without stretching (step b)
In case two, in the stretching stage, obvious shrinkage was observed in the radial direction, and when stretching was finished, the total length of the parison became more than two times the initial length., It is interesting to see that in the blowing stage bulging started at the top and bottom ends and the two inflated ends joined in the final stage of forming. In an actual forming process, under a certain condition, a folded line can be observed on the product. This deformation mode might be the reason for the generation of that folded line.
Figure (2a). Stretching stage (step a)
Figure (2b). Blowing stage (step b)