Page 42 - Plastics News January 2018

FeAtures
FEATURES

their measurement of tensile yield stress of stretched Results and Discussion
PET material , for a PET bottle to have a high mechanical Thickness Distribution and Stresses in Carbonated
properties, its stretch ratio should be higher than the Bottle
hardening point which corresponds to a ratio of initial
thickness to final thickness (t0/ t1) of 1.6. They observed Thickness distribution of the carbonated PET bottle used
that the structural weakness of the bottom was related on this study is seen in figure 4. This distribution has
to an abrupt change of the thickness between the center been obtained through an injection stretch blow molding
region and the region nearby. simulation program. To be able to get this thickness
distribution by means of this simulation program, the
There are generally two kinds of cracking shapes at the
bottom of the bottle. The first is in the radial direction version of the above bottle that was optimized by
obtaining minimum stress distribution at the bottle base,
(Fig.1a), which begins from the bottom center and goes
towards the outside. The second is circumferential was used as mould. The process parameters such as stretch
rod movement during stretching stage and pressure change
direction (Fig 1b) where cracking arises at some distance
from the bottom center and progresses circumferentially. during all process are the two of the most important
factors which affect the thickness distribution, but the
Lyu et al.1 have said that there are three parameters at
the bottom, which affect the cracking mostly and they graphical results of these two parameters will be discussed
in another paper.
have optimized the petaloid bottom by modifying these
three parameters, based on measurements of effective As can be seen in figure 4, the thickness distribution is
stresses at the bottom of the bottle. These parameters are not uniform all around the bottle. The thickest region is
foot length, valley width, clearance as seen in figure 2. the petaloid shaped base, which is around 2-3 mm thick.
The other regions except for the base of the bottle. It can
They have found that the circumferential cracks are less be assumed to have uniform thickness between 0.3 and
at the valley in the cases of large clearance, large foot 0.4 mm and there is abrupt change in thickness between
length and narrow valley width. This study took these sidewall and central region at the base, therefore, the
parameters into consideration as well, but there are many fact that there is cracking problems at the base may be
more design parameters than those mentioned above, anticipated. Also, thickness distribution at the base is
which affect the cracking at the base. Geometrically, the circumferential. At this region, it may not be possible to
weakest region is the valley, because the highest principal achieve a uniform distribution because of its geometrical
stress has occurred at this region.
structure but, reducing the stresses in this region can
Experimental Setting the Material Properties be possible by modifying process parameters. Simulated

Material properties for Virgin PET such as the Young’s stresses in the petaloid based bottle of 1500 ml. are shown
Modulus, Poisson Ratio, and Density that were used in in figure 6, 7.
the stress analyzes with CATIA V5 R14 are given in table1. Based on the simulation results, it can be said that the
Bottle Design Used maximum stress on the bottle is seen on the sidewall and
the minimum stress is seen at the foot section of the base.
The bottle design which was used for stress analyses The reason why the minimum stress is at the foot section
is shown in figure 3. The bottle has a volume of 1.5 lt. of bottle base is due to the fact that the stress analyzes
and its thickness distribution is not even. The values were carried out at the position where the bottle stands
of thickness are approximately 3mm and 0.3mm at upright. That is why, bottom of the bottle touches to the
the bottom of the bottle and sidewall of the bottle, base. Hence, the minimum stress occurs at this region.
respectively. However, the thicknesses change depending However, at the valley region, the stress is relatively high
on the process parameters of injection stretch blow to the other region near the center of bottle base.
molding (ISBM), especially, temperature distribution on
the preform is crucially important. The maximum stress is observed at the sidewall of the

Plastics News Januar y 2018 42