The last of this series describes the importance of developing strain-hardened crystals during blow molding to produce bottles with optimum properties. Techniques commonly used to measure the amount of crystallinity in the bottle are discussed. The concept of the Natural Stretch Ratio (NSR) of PET is reviewed and how it must be considered when designing and blowing preforms. The concept of heat-setting is introduced in order to reduce the stress that develops in a blown bottle. Lastly a number of common myths pertaining to PET bottles are discussed.