For the following discussion on the stretching/blowing behavior of PET bottles, we will assume that the preform is well designed and that it has been properly heated and conditioned. The preform design should ideally be based on the dimensions of the finished bottle and the stretching characteristics of the specific PET grade to be used. When blowing bottles, it is critical to start with a correctly designed preform to acquire the desired physical properties. Planar Stretch (PSR) => HSR x ASR = 10 to 15īLOWING/STRETCHING BEHAVIOR OF PET BOTTLES Hoop Stretch Ratio (HSR) => bottle diameter/ average preform diameter (OD+ID/ 2) = 4.0 to 5.0Īxial Stretch Ratio (ASR) => bottle body length/ preform body length (not including un-stretched neck) = 2.5 to 3.0 Preform design should be such that the following parameters are maintained. Drawing of the bottle with all dimensions, including wall thicknesses. These are called the hoop stretch ratio (HSR) and the axial stretch ratio (ASR).Ģ. Since a preform is stretched in both hoop (diameter) and axial (length) directions, data for both are needed. Empirical data, obtained through experiments, on how much a specific PET must be stretched to reach its natural stretch ratio. One of the most critical aspects of producing high-quality PET bottles is the design of the preform.ġ. In cases where preforms have been stored in unusually cold or hot conditions, allow the temperature of the preforms to reach the normal preform temperature before feeding them into the machine. To eliminate the need to adjust the temperature of the machine to compensate for varying preform temperatures, maintain a constant preform storage temperature. Preforms that are colder than the machine is set up for will tend to yield bottles that are pearlescent preforms that are hotter than the machine is set up for will tend to yield bottles that are hazy.
![pet bottles preform pet bottles preform](https://5.imimg.com/data5/WW/AN/MY-2996200/bopp-tape-500x500.jpeg)
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.
![pet bottles preform pet bottles preform](https://ueeshop.ly200-cdn.com/u_file/UPAM/UPAM944/2006/products/10/8c982f3cf9.jpg)
![pet bottles preform pet bottles preform](https://i.pinimg.com/736x/e1/5a/78/e15a78285381035df0751b2fc946b390.jpg)
In the one-stage process, preforms are injection molded, conditioned to the proper temperature, and blown into containers-all in one continuous process.
![pet bottles preform pet bottles preform](https://www.beverage-filling-machine.com/photo/pl527710-heat_shrink_cap_seal_tamper_evident_band_pre_form_for_jar_bottle_wine_bottle.jpg)
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.