Page 43 - Plastics News January 2018

FeAtures

bottle, but this is the strongest section of the bottle. So, 7mm for foot length, valley width and clearance,
the bottle is safe in this region of the bottle. Although the respectively and this bottle’s maximum stress due to
stress at the Page 2 of 6 valley near the bottom centre internal gas pressure is 9.88 e+6 N/m2 (Figure 6). The
seems to be less than that of the sidewall region, as the stress value of the optimized PET bottle whose dimensions
yield stress of the valley is less than that of the sidewall, were calculated by Echip7 is 8.61 e+6 N/m2 (Figure 7). This
which is around 5 e+7 N/m2, the weakest region of value is within the lower and higher limitations defined
the bottle is the valley. So, the stress at the valley, by Echip7. So the optimized bottle stress is 12.68 % less
namely maximum stress on the bottle should be lowered than the current bottle stress.
by modifying these three parameters mentioned above. Conclusion

For a bottle of 1.5 lt. whose thickness is 2mm, the In this study we optimized the petaloid base of the CSD
maximum Von Mises Stresses are 9.86 e+6 N/m2 and 1.48 bottle in order to prevent cracking since the cracking
e+7 N/m2 at the gas pressure of 4 e+5 N/m2, 6 e+5 N/ problem has occurred in this section of the bottle. In the
m2, respectively. According to these results, the maximum previous studies, it has been defined that there are three
stresses at the petaloid base for both gas pressures are parameters which affect the cracking at the petaloid base
lower than the yield stress of the material. Hereby, of the bottle most; these are foot length, clearance and
the base of the bottle seems to be sound against these valley width. These three parameters were therefore
internal pressures applied by the carbonated soft drinks. modified by changing their numerical values through the
However, there are still cracking problems observed in Echip 7. The optimization process was carried out by
industry. So, it is very important to improve the design of measuring Von Mises stress values at the bottom of the
the petaloid shaped base and to achieve uniform thickness bottle by means of Catia V5 R14 program and concluded
all around the bottle.
that the highest maximum stress occurred at the sidewalls
Optimization process of the petaloid bottom and where sects with the sidewalls.

The optimization process of the petaloid bottom was As a result,, the optimum dimensions of the three
carried out modifying of the three parameters which are parameters were found by the Echip 7 and the bottle was
foot length, clearance, valley width shown in figure 2. hereby redesigned. All stress distributions with respect to
As the range of modification for each parameter is very various design parameters are shown in figure 8, 9, 10.
large, trial numbers was determined by Echip7design of It can be said that small foot length, small clearance and
experiment and optimization software program. medium valley width are the best combination for a sound
petaloid base design.
For each parameter, the lower limit, upper limit and step
range were entered to the program (Table2). Hereby On the other hand, valley width does not have any
the trial numbers and consequently, the simulation considerable effect on the stress occurrence at the
time were successfully reduced to a manageable size. petaloid base of the bottle.
Through the use of the same program, the optimum Future Work
dimensions were obtained by obtained results. The
graphical results are seen in figure 5. The design variables Apart from the geometrical structure of the petaloid base,
and their numerical values are shown in table 2 and the no doubt, there is also a certain effect of the process
results are given in figure 5. As seen on figure 5, the parameters on the cracking mechanism.. Therefore, the
optimum values are 22.45 mm, 12.95 mm, 1mm for foot process parameters need to be optimized as well the
length, valley width and clearance, respectively. geometrical structure of the petaloid base. Verification
of the newly designed petaloid base’s resistance against
As a result of these optimum dimensions, the maximum cracking should be carried out.. On the other hand, it will
stress on the bottle is 8.2 e+6 N/m2 . The dimensions of be useful to conduct a morphological analysis of the PET
the bottle currently used in industry are 29 mm, 16.17mm,
bottle at points of concern.

43 Januar y 2018 Plastics News