Why CSD bottle design different between water bottle
The first plastic bottle appeared on the market in attempt to replace glass for CSD application is in 1978 by Coca-Cola company. Before that, carbonated soft drinks are packaged by glass container, which posts two major disadvantages: breakage when dropped and too heavy for transportation. Therefore plastic counter parts were looked into, and PET was the chosen material, and for a good reason: transparency, light weight, and shatter resistant.
The first PET bottle for CSD application comes in two parts, the PET bottle with round base and a base which glued at the bottom for up-right standing. However, this solution requires two separate manufacturing processes and the base cup is made of different material from the body, which poses a recycling issue. It is until the early 90’s when the three-piece mold technology was introduced. The integrated base PET bottle became economically feasible and revolutionized rigid packaging for the entire beverage industry.
(Extended reading: How a beverage vendor selects the suitable PET blow molding machine)
PET is the ideal replacement material for glass due to its’ physical property after been stretched, since the random molecular structure would be in a more aligned state after being stretched at a specific ratio (2.5-3 times for example). At this ratio, PET underwent a plastic deformation, and when processed under a dual-axis forming procedures such as stretch blow molding process, the material is oriented at two directions making it stronger while improving the CO2 retention.
What makes CSD bottle different from water bottle is its’ ability to withstand internal pressure before bursting. For CSD application, bottle needs to be able to withhold up to 10 bar of pressure before failure (some stated average of 8.6 bar). This is achieved by the petaloid form of the base, since this form would greatly increase the surface area at the bottom, meaning the base gets stretched out more and able to sustain higher pressure.
There are several factors known in industry which might affect the bottle performance: (i) PET molecular orientation, (ii)degree of crystallinity at the base, (iii)PET IV value, (iv) Stretch ratio, (v) Stretch speed, (vi)Stretching temperature. The more orientated that material at the base means the higher degree of crystallization.
Higher IV value also desired for the process, if preform lost too much IV value due to non-optimum injection processing parameter, and the dropped of the IV value as resulted. It would affect the bottle performance. Higher stretch ratio and speed will also increase the orientation. Higher stretch temperature decreases the molecular orientation when stretching. All these play an important role in the ability for the bottle to withhold pressure and must be taken into consideration when processing them.
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