It is also important that the temperature of the preforms being fed into the reheat blow machine be relatively constant. Typically, temperature changes should be made in small increments allow at least 5 minutes for the machine to equilibrate before assessing the effect of the change. The temperature of the preform during the reheat blow molding process can be controlled by adjusting settings at various points in the reheat blow machine. Therefore, it is critical that the natural stretch ratio be surpassed slightly during the stretching process. It is here that the highly desired increases in physical properties are maximized. This is known as the strain-hardening region (E). Once the material has been stretched past its natural stretch ratio (NSR) (D), a dramatic increase in force is required for additional stretching to occur. This is followed by a region of continued stretching, where the amount of force remains constant (C). If stretching continues past the yield point (B), however, the material will start to stretch and become thinner, causing permanent deformation. Here PET behaves somewhat like an elastic material: if it is stretched only a small amount and released, it will shrink back to its original size. As force is applied to the material, there is a region where very little stretching occurs (A). Orientation is a physical alignment of the polymer chains in a regular configuration.Ī typical stretching sequence for PET is shown in Figure 1.

Second, with the stretching comes orientation and a dramatic improvement in physical and barrier properties. Stretching allows thinner, more uniform sidewalls and thus less expensive containers. There are two major reasons for stretching PET.

In the two-stage process, preforms are injection molded, stored for a short period of time (typically 1 to 4 days), and blown into containers using a reheat-blow (RHB) machine. In the one-stage process, preforms are injection molded, conditioned to the proper temperature, and blown into containers-all in one continuous process. There are two distinct stretch blow molding techniques. Biaxial orientation provides enhanced physical properties, clarity, and gas barrier properties, which are all important in products such as bottles for carbonated beverages. This process involves the production of hollow objects, such as bottles, having biaxial molecular orientation.

Preform blow molding