Heat Set PET 101
PET is a semi-crystalline thermoplastic, which softens at approx. 76°C (what is called “Glass Transition”). Above this temperature, the material becomes elastic, and can be formed, a property utilized effectively in the Stretch Blow Molding process.
Due to its glass transition at approx. 76°C, PET is initially unsuitable as a bottle material for a hot-filling process above this temperature, since deformations may occur: Firstly, the bottles shrink, since they “remember” their previous shape (namely the preform), and secondly they collapse under internal pressure, a typical phenomenon during the cool-down period after hot filling.
For this reason, the hot-fill PET bottles feature, what are called "Vacuum Panels", which compensate for the negative pressure (vacuum) produced during the cooldown period without the bottle collapsing. In a way, these Vacuum Panels are used for "Designed Collapse", so that the bottle does not deform in undesired manner.
During the "Stretching" part of the production process for PET bottles, the material crystallizes. We talk here of Stretch-Induced Crystallization. In the standard process (often known as "Cold Set Process"), the material is frozen in this state at the mould wall, which is chilled. Inner stresses are then retained, and lead to reshrinking, particularly at heat-up. If, however, the material is heated still further after being stretched, it undergoes ‘After Crystallization’ (known as thermally induced crystallization). The stresses in the material are then decreased, thus reducing the tendency towards reshrinking. The increased crystallinity gives the material significantly enhanced thermal stability. The glass transition temperature and the rigidity increase – and we call this process “Heat-Setting”.
Producing Hot-Fill PET Bottles
This improvement in PET’s properties, thanks to "After Crystallization", is utilized in order to produce Hot-Fill PET bottles. The performs are here, in contrast to the standard stretch blow molding process, not blown into a cold mould, but into one heated up to as much as 160°C.
To enable the bottles to be removed from the hot moulds however, they have to be cooled. For this purpose, compressed air is blown through bores in the tubular-shaped Stretching Rod, which cools the bottle from the inside known as a flushing-air process.
The heating time for the preforms is longer, since hot-fill PET bottles are always manufactured from heavy preforms, so as to obtain increased stability.
In the heat-set process, the bottle’s characteristics depend even more closely on the process settings and the choice of preform than is the case in the standard process. In particular, the temperature of the product during filling and the time during which this temperature has to be maintained, are crucial factors. The higher the thermal stress, the higher the degree of crystallinity required. In order to obtain a very high degree of crystallinity, a long crystallization time in the blowing mold is needed. Consequently, the more crystallinity is required, the lower the station’s output will be.
PET container for Hot Filled Applications
The container's body side wall is rigidized against radical and longitudinal vacuum distribution so that paper labels can be applied to the container. In addition, the amorphous threaded mouth of the container is rigidized by gussets molded into the container at the junction of the mouth and body portion of the container to resist deformation when the container is capped. Finally, a bulbous vacuum deformation area in the container is provided adjacent the container mouth.
Questions about Heat Set PET?
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