After heating, the preform in the initial amorphous state becomes a high elastic state, like rubber. After biaxial stretching, the orientation of the macromolecule chains leads to crystallization. After blowing, the material is as hard as a solid, so a state transition occurs. The state transition is due to the critical change limit. Within the range of hardening strengthening limits, plastic fluidity has a highly bi-directional characteristic, increasing the internal stress of PET. ◎ strain hardening coefficient due to the final stretch should always exceed the strain hardening limit, in order to get solid response, thus affecting the induction of crystallization, to ensure that the body wall thickness uniformity. Once close to the strain hardening limit, the strain increases as a power exponent. In fact, the onset of strain hardening depends on the maximum strain value, ie the inherent strain rate limit (λn). Sidel's free bottle blowing study through the preform proves that λn depends on the intrinsic viscosity of the material and the preform temperature. In addition, the cylindrical preform is more easily stretched in the radial direction than in the axial direction, i.e., the inherent strain rate in the radial direction is greater than the intrinsic strain rate in the axial direction, resulting in axial orientation, while the axial orientation depends on the material properties Viscosity.
Self-regulating action of PET preform The stress distribution on the preform causes the orthotropic expansion of each part of the preform. This extension is determined by the stress-strengthening factor. In the drawing rod and high pressure under the joint action, the preform has just begun to deformation, the thinnest
The weak link is the hottest or the thinnest place in the wall, from which the first to start the deformation. The intensity is locally increased when the strain strengthening limit is large, since induced crystallization is produced. Once the deformed area is self-regulated beyond the strength of the undeformed area, the undeformed area begins to deform along the moving "bubble boundary". This expansion is called self-regulating. Although the self-regulating effect only occurs when the strain strengthening limit is reached, it controls the thickness of the bottle wall. The biaxial orientation of PET preforms as described above is that the biaxial orientation of the molecules is the result of the biaxial stretching of the plastic under certain conditions. In the geometry, the preform has a specific biaxial rate; the different parts of the preform temperature distribution. In order to produce a bottle of a specific performance in the industry, it is necessary to control the drawing speed and the cooling rate. ◎ Material Properties The material of the preform must be disordered (low crystallinity) to ensure proper orientation in biaxial stretching. In addition, since the high-viscosity polyester has a higher anisotropy (orthotropic orientation) than the low-viscosity polyester before the critical stretching limit (before degradation occurs) under the same conditions, the intrinsic viscosity of the material Must exceed the required value of orientation.