The drying of a virgin PET resin and reground PET flakes at temperature levels of 160-180oC to below 0.005% moisture content is essential for the production of amorphous multilayer polyester bottles. Polyester is an effective desiccant. The water absorption depends on a relative humidity, residence time, temperature, and dimension of the flakes.
When flakes containing moisture are heated up to the melting temperature, hydrolytic degradation occurs lowering a viscosity ofthe melt that results in enhanced ability of preforms/bottles to crystallize (milky appearance).
The dryer and the hopper for the virgin PET chips are of standard design. The unit for the reground PET flakes is of almost the same design but it needs an internal agitator (propeller) which prevents the amorphous PET flakes from sticking together. To be exact this is not a problem inherent in the highly stretched bottle sidewalls (these flakes do not show this property). The problem is rather that ground-up flakes come from non-stretched portions of PET bottles, i.e. the neck or bottom.
Above the glass transition temperature (74oC) the flakes will stick together because of adhesional forces. It is therefore necessary to reduce a contact time between the individual flakes and keep them constantly moving in a hopper to avoid the above mentioned sticking process and bridge building in the hopper. This is carried out by the agitator installed inside the hopper. The flakes coming
from less stretched bottle regions, i.e. the neck and bottom recrystallize during the drying process. Crystallized flakes will not stick together.
The dried reground PET flakes are fed via a feeding-extruder into the throat of the satellite injection (2nd and 3rd) unit of the multilayer single stage machine.
An extruder feeder is necessary because PET flakes are bulky and cause problems when they are fed by gravity into the screw of the injection unit.
Co-injection molding of virgin and reground PET flakes
The virgin PET as well as the reground PET flakes are melted inside the separate injection units by means of external electrical heating of the injection barrel and the applied shear forces of the screw, driven by a hydraulic motor (see Figure 1).
The PET melt, either coming from virgin PET or from PET flakes, is accumulated in the head-space of the barrel head and released into the hot runner (melt-channel system) under high injection pressure and at a certain melt-stream velocity. The melt is kept in a molten stage, inside the hot runner system because of the electric heating systems. The two melt-streams (virgin and reground PET) are kept, up to the injection nozzle, in separate hot runner channels.
At the end of the hot-runner channels, at the gate area of the injection cavity, two nozzles are installed like a double tube or as one tube with a second smaller tube inside it.
A portion of a virgin PET melt, forming the outside layers, is first injected into the preform cavity. Under controlled pressure another melt-portion coming from the reground PET is injected into the core of the virgin PET melt cake. Subsequently both injection units inject further melt (from virgin and reground PET) simultaneously during the cavity fill process. This three phase filling process results in a preform made from the following layers: inside/middle/outside (virgin PET/regrind PET/virgin PET).
During the cavity filling process, the layers do not mix together because the individual melt-layers have a high melt viscosity and are not subjected to a turbulent flow.
The adhesion between the individual preform layers is as good as if the layers were welded together and formed monolayer preform (made out of one melt stream).
Thermal conditioning is the next step of the multilayer preform production. The purpose of thermal conditioning of a given preform is to provide the necessary temperature distribution in a preform. After leaving the injection mold and undergoing cooling process, the preform has a cross-sectional temperature distribution of an upside-down U-shape, which means that the middle of the preform wall shows higher temperatures than the two outside skin-layers.
Since PET stretch properties are influenced by the temperature level above the glass transition point of PET, the preform requires more even temperature distribution, otherwise the middle layer will stretch at a different rate from the skin-layers. Thermal conditioning can be carried out by allowing the preform to equilibrate before stretch-blow-molding or by applying thermal energy from the outside by moving the heater-pots around the preform and/or by allowing a heated core rod to plunge into the center of the preform, thus influencing the thermal profile of a preform wall from the inside.
After thermal conditioning is accomplished, the preform is transferred into the blow mold of the stretch-blow-molding station. Here the preform is axially stretched by using a stretch rod and circumferentially inflated by air pressure, to match the shape of the blow mold. The final bottle is cooled down due to the contact heat losses on the metal surface of the blow mold.
The stretch-blow-molding process leads to a biaxial orientation of the macromolecules resulting in better mechanical properties and lowering the gas permeation of bottles.
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