Plastic and nonmetallic article shaping or treating: processes – Direct application of fluid pressure differential to... – Starting material is nonhollow planar finite length preform...
Reexamination Certificate
2000-04-26
2003-04-29
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of fluid pressure differential to...
Starting material is nonhollow planar finite length preform...
C264S909000, C264S230000, C264S235000, C264S234000, C264S3420RE, C264S547000, C264S548000, C264SDIG007, C425S387100
Reexamination Certificate
active
06555047
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved apparatus and method for forming polyethylene terephthalate container-shaped or tubular articles with flat surfaces or sharply defined contours, which are dimensionally stable up to relatively high temperatures and to the articles made thereby. In particular, the invention relates to tubular belts and open-ended containers having superior dimensional, thermal and optical properties.
BACKGROUND OF THE INVENTION
The prior art relating to the molecular orientation and heat-shrinking processes of thermoplastic saturated linear polymers, such as polypropylene, polyethylene or polyethylene terephthalate (“PET”), is extensive. It is well know in the art that films or tubes of unoriented thermoplastics may be heated to their orientation temperature and “stretched” in order to “orient” the linear polymeric chains. Such orientation greatly increases the strength of the material in the direction of stretching. By simultaneously or serially stretching a film of unoriented linear polymer in two directions perpendicular to each other, a material of consistent superior properties in all directions is obtained. Such products are referred to as being biaxially oriented. Biaxially oriented thermoplastics have many desireable properties including increased tensile strength and elastic modules.
There are two general categories of thermoplastics that are capable of orientation. The mono-1-olefins, such as polyethylene and polypropylene, are crystalline polymers. Other thermoplastics, most predominant among these being PET, are crystallizable polymers. Crystallizable polymers can be produced in an amorphous or non-crystalline solid state capable of being transformed into a crystalline form through heating to temperatures above the orientation temperature of the material. The length of time required to crystallize crystallizable polymers is dependent on the temperature and the degree of crystallinity required. Oriented then crystallized polymers have significantly enhanced thermal dimensional stability over crystalline polymers because of their heat-setting abilities.
The temperature employed in heat-setting a crystallizable polymer defines the maximum temperature to which the product may subsequently be heated without causing the polymer to relax toward its unoriented shape.
In the case of PET, the optimal orientation temperature range in which biaxial stretching occurs is between 80° C. and 110° C. U.S. Pat. No. 2,823,421 of Scarlett, for example, describes a method for orienting an amorphous film of PET 3.25 times its original longitudinal width at a temperature between 80°-90° C. The temperature of the film is then raised to between 95°-110° C. before it is transversely stretched. The resultant biaxially oriented film is then heat-set at a temperature in the 150°-250° C. range.
Although raising the temperature of oriented PET during heat-setting will “set” the form of the film, unless restrained by some means such as tenting frames, molds or air pressure, the film tends to shrink significantly during the heat-setting process. Oriented crystalline polymers will also shrink upon heating.
The heat-shrinking characteristics of oriented crystalline and crystallizable polymers is exploited by this invention to form products with unique characteristics. For either group of polymers, the shape an article is conformed to during heat-shrinking is maintained by the article after it is cooled to room temperature. A crystalline polymer will lose its shape when heated above its orientation temperature, while crystallizable polymers may be heat-set to temperatures above its orientation temperature but below its melting point.
Heat-shrink tubing for the insulation of electrical connections is well know in the prior art. Another example of a process used to capitalize on this property, the heat-shrinking of polyvinyl chloride, a crystalline polymer, for the purpose of placing a hard plastic coating on photoflash lamps, is described in U.S. Pat. No. 4,045,530 of Reiber. U.S. Pat. No. 2,784,456 of Grabenstein describes the use of bands of PET, a crystallizable polymer, to seal bottles containing beverages and foods by heat shrinking the bands over the bottle and cap juncture. Neither of these patents discloses the use of the heat-shrinking process in order to mold the shape of an article to be later used independent from the coated substrate.
Crystallizable polymers, such as PET, also may be heat-set in a non-oriented form. Raising the temperature of amorphous PET above its orientation temperature range will “set” the form of the object, producing a strong, hard but somewhat brittle material. Heat set unoriented PET is milky white and translucent and will retain its physical structure on heating to temperatures in the 200° to 250° C. range.
Due to the excellent strength characteristics of oriented plastics, there are a substantial number of commercially available products composed of these materials. For example, the commonly used two liter bottles of carbonated drinks are generally made of oriented PET.
Patents describing processes and apparatus for the efficient production of open ended containers made of biaxially oriented thermoplastics are numerous. See, for example, U.S. Pat. No. 4,711,624 of Watson; U.S. Pat. Nos. 4,381,279, 4,405,546 and 4,264,558 of Jakobsen; U.S. Pat. Nos. 4,563,325 and 3,532,786 of Coffman; and U.S. Pat. Nos. 3,412,188 and 3,439,380 of Seefluth.
The most frequently described method for forming containers utilizes a combination of injection molding and blow-forming. According to these procedures, a solution of molten thermoplastic is injection molded into a mold to form a parison or pre-form. Typically, the parison is removed from the injection mold and placed in or surrounded by a female mold. The temperature of the parison is brought into the orientation temperature range, at which time it is blow-molded into a female mold in order to biaxially orient the thermoplastic and give it its final shape.
There are several advantages in utilizing this two-step process. The portion of the parison that will be used as the neck of the container may be injection molded to contain intricate structure such as the ribbing required for a screw-on cap. This neck portion can be positioned so that its shape is retained during the blow-molding.
Once shaped, the blow-molded container may be cooled to room temperature to retain its shape. If a crystallizable polymer is used, the container may be heat-set to higher temperatures prior to cooling. If heat setting is desired, a positive pressure must be maintained in the container to prevent shrinkage during heating. For an example of this general type of apparatus and method see U.S. Pat. No. 4,108,937 of Martineu.
Another series of patents describes the plug-forming of thermoplastic sheets. Blow-forming a sheet requires that a sheet of thermoplastic material be clamped over a mold, heated to its orientation temperature and then conformed to the mold by the action of positive pressure. In plug-molding, a male form is used to assist in the conformational process. U.S. Pat. No. 4,420,454 of Kawaguchi describes a method of plug-molding followed by blow-molding to produce biaxially oriented containers.
A commonly employed method for the production of thermoplastic containers, particularly for use in the food industry, is referred to as thermoforming. Thermoforming is the formation of an article by manipulation of thermoplastic material at a temperature above its flow temperature but below its melt temperature.
In many of these systems, the process begins with a blank of thermoplastic material. The temperature of the blank is elevated to near its melt temperature and then forged into a disc-shaped preform. The peripheral edge of the preform is the incipient rim of the final container, which is rapidly cooled after forging while the bulk of the preform remains at an elevated temperature. The preform is then subjected to thermoforming and the thermoplastic attains the desired final shape. Preforms c
Fortex, Inc.
Lee Edmund H.
Silbaugh Jan H.
Swanson & Bratschun LLC
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