Blow Mold Heating Zones for PET Container Crystallization
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Solution Overview
Problem
Large returnable containers molded with polyester resin, such as polyethylene terephthalate (PET), face challenges in achieving both heat resistance and transparency, with existing methods either compromising on heat resistance or transparency due to issues like whitening from spherulitic crystallization and strain from stretch orientation.
Innovation Solution
A method involving multiple blow molding steps and heat treatment steps with separate heating zones for the shoulder and body, using a thick preform and optimizing stretch ratios and temperatures to remove strain and prevent whitening, while maintaining top load resistance and transparency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a polyester resin (PET) is used to mold a large returnable container instead of polycarbonate, then product cost is reduced and transparency is improved, but heat resistance deteriorates
Solution Approach 1:
The patent applies parameter changes by controlling the crystallization temperature and degree of crystallinity of the polyester resin through specific heat treatment parameters. By heating the container to 80-120°C and maintaining it for 1-48 hours, the resin achieves optimal crystallization that provides heat resistance (withstanding 60-70°C washing) while maintaining transparency and mechanical properties.
Solution Approach 2:
The patent utilizes phase transitions of the polyester resin, specifically the crystallization process. The heat treatment causes the amorphous polyester resin to transition into a crystalline structure, which enhances heat resistance. The controlled crystallization at specific temperatures transforms the material properties without compromising transparency or increasing cost.
2Strength
If a thick preform is used for large container molding, then top load resistance is improved, but whitening due to spherulitic crystallization occurs during heat treatment
Solution Approach 1:
The patent resolves this contradiction by precisely controlling heat treatment parameters: temperature (80-120°C) and time (1-48 hours). This controlled parameter regime allows the thick-walled container to crystallize uniformly without forming large spherulites that cause whitening. The specific temperature range ensures gradual crystallization that maintains optical clarity while achieving the required strength.
Solution Approach 2:
The patent applies preliminary action by performing stretch orientation before heat treatment. The preform is stretched to high ratios (axial 2-10x, radial 1.5-5x) during blow molding, which orients the polymer chains. This preliminary orientation prevents random crystallization that would cause whitening, allowing controlled crystallization during subsequent heat treatment while maintaining transparency and strength.
3Stability of the object's composition
If a high stretch ratio is applied during blow molding, then strain is removed during heating, but stress whitening occurs in the body
Solution Approach 1:
The patent uses parameter changes by optimizing the combination of stretch ratio and heat treatment conditions. By controlling the heat treatment temperature (80-120°C) and time (1-48 hours), the patent allows strain relaxation without excessive crystallization that would cause whitening. The parameters are tuned to achieve strain removal while maintaining optical properties.
Solution Approach 2:
The patent applies preliminary action through controlled stretch orientation during blow molding. The preform is stretched to specific ratios (axial 2-10x, radial 1.5-5x) before heat treatment, which orients polymer chains in a controlled manner. This preliminary orientation enables subsequent strain removal during heat treatment without causing stress whitening, as the chains are already aligned and can relax uniformly.
4Temperature
If the shoulder area is heated at high temperature, then heat resistance is improved, but whitening occurs in the shoulder region
Solution Approach 1:
The patent applies local quality by recognizing that different regions of the container require different heat treatment intensities. The shoulder, being thicker and more prone to spherulitic crystallization, is heated at lower temperatures (80-120°C) compared to what might be applied to thinner sections. This localized temperature control prevents whitening in the shoulder while still achieving adequate heat resistance for the entire container.
Solution Approach 2:
The patent uses parameter changes by adjusting heat treatment temperature and time based on local wall thickness. For the thicker shoulder region, lower temperatures (80-120°C) and controlled time (1-48 hours) prevent excessive crystallization and whitening, while still achieving the required heat resistance. This parameter optimization resolves the contradiction between heat resistance and transparency in the shoulder area.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method effectively balances heat resistance and transparency in large returnable containers, enhancing their structural integrity and appearance by reducing strain and whitening, and preventing bisphenol A elution.
Implementation Method 1
a body zone of the blow mold is heated at a high temperature as compared with a shoulder zone of the blow mold, so that a strain that has occurred due to stretch orientation is removed
Implementation Method 2
the heating zones being insulated by an insulation section
Implementation Method 3
whitening due to spherulitic crystallization may occur in an area having a relatively large thickness (particularly the shoulder) when the blow-molded article is slowly cooled after heating
Data Source
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AI summary
A method of molding a large returnable container (40) includes a heat treatment step that includes subjecting a thick preform (10) formed of a polyester resin, or a primary blow-molded article (20) obtained by subjecting the preform to primary blow molding, to intermediate blow molding with heating to obtain an intermediate molded article (30) from which a strain that has occurred during blow molding has been removed, and a final blow molding step that includes subjecting the intermediate molded article that has shrunk to final blow molding with heating to obtain a large returnable container. The heat treatment step includes disposing the preform or the primary blow-molded article in a heat treatment mold (204), and pressurizing the preform or the primary blow-molded article by introducing high-pressure air into the preform or the primary blow-molded article so that a shoulder (22) and a body (24) obtained by blow molding close contact to a cavity surface (204A) of the heat treatment mold, and are subjected to a heat treatment, the shoulder (22) being heated at a low temperature as compared with the body (24).