Page 40 - Plastics News January 2018

FeAtures

Optimum Design of PET Bottle Bases Against Stress Cracking

While the petaloid bases provide stability to the bottles; stress cracking of the base during hot climates
occurs. In this study, dimensions of the petaloid base against stress cracking are optimized

Abstract cracking problems at the base due to geometric shape and
process parameters that affect the cracking phenomenon.
Petaloid shaped base for the carbonated soft drink bottles
made out of (polyethylene terephthalete) (PET) is quiet Therefore, there is a need to study these parameters by
modifying the petaloid shaped base and bottle production
common; and there are currently a few bottle designs
with slightly different petaloid base in the market. While process parameters such as blow pressure, temperature
distribution of preform at the blowing stage, velocity of
the petaloid bases provide stability to the bottles; stress
cracking of the base during hot climates occurs. In this the stretch rod, and total time of both the stretch and
the blowing stages.
study, dimensions of the petaloid base against stress
cracking are optimized via a FEA software package. Based First, the PET bottle to be studied upon and used in the
on the simulation results, a new design for the petaloid industry was drawn by CATIA V5 R14 on comply with it‘s
base and optimum process conditions for the production real dimensions. The wall thickness of the bottle was taken
of PET bottles are proposed. at three different values to be able to see the effects of
Introduction the thickness on the petaloid shaped base of the bottle;
and the thickness distribution throughout the bottle was
The use of the polyethylene terephthalete (PET) bottles also regarded as uniform although, in reality it is not
for carbonated soft drinks (CSDs) is fairly widespread1-7. uniform because only the effects of the changes on the
CSD bottles had previously consisted of two pieces. The geometric shape dimensions was desired to be analyzed.
first part is the blown section and the other part called Two different pressures as gas pressure were applied to
‘cap’ which was made of polyethylene is a separate section the inside surface of the bottle for each thickness.
for the blown bottle to be self standing. The two-piece
bottle use was sufficient for design purposes but it was not Von Mises stress values were considered on the test results.
Lyu et al.1 has used principal stresses but there is not much
satisfactory both for production costs and for production
efficiency, and there were no cracking problems at the difference between both units since the relation between
stress values and petaloid dimension changes are desired
bottom. The new one-piece bottle is made of only one
material, namely PET and it does not have a ‘cap’ section. to be obtained. Stress analyze numbers and configurations
were conducted by Echip, which is a design of experiment
Onepiece bottle has advantages like reducing material
consumption used for production and manufacturing and optimization software, is employed to determine the
number of trials and consequently the optimum values of
processes compared to the two-piece bottle. However,
some kind of cracking problems at the petaloid base of stress for the PET bottle.
the one-piece bottle has appeared. Lyu et al.1 have studied on the same volume of Page 1 of
6 the bottle except for its shape. They have used an even
There are not that many researches with regards to
cracking at the petaloid bottom of the PET bottles due thickness distribution on the sample but, in reality, the
actual thickness distribution is not even. They have made
to geometry of the petaloid base. Only Lyu et al.1 has
studied a similar project. The other researchers8-10 has their measurements at two pressures: 0.4 MPa. 0.6 MPa.
: And at three thicknesses: 0.35, 2.0, 3.36 mm by using
generally studied about this issue without considering
the geometric shape of the petaloid base and process commercial software, namely Abaqus (version 5.8-16).
These thicknesses are the average values of the sidewall,
conditions. But, at the present time, there are still
bottom section and preform, respectively. According to

Plastics News Januar y 2018 40