Plastic and nonmetallic article shaping or treating: processes – Recycling of reclaimed or purified process material – Of process trim or excess blanked material
Reexamination Certificate
2000-11-28
2004-11-09
Lee, Edmund H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Recycling of reclaimed or purified process material
Of process trim or excess blanked material
C264S210100, C264S210200, C264S151000, C264S553000, C264S552000, C264S544000
Reexamination Certificate
active
06814905
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to thermoforming and, more particularly, to a continuous process and apparatus for thermoforming polyesters into articles such as ovenable containers, food packaging trays, and the like.
BACKGROUND OF THE INVENTION
Continuous vacuum-forming devices for making containers from thermoplastic sheets typically utilize a continuous sheet of molten plastic which is extruded and vacuum-formed on a continuous belt or a rotating drum having a plurality of mold cavities. Many of these devices utilize residual heat from the extrusion process, thus avoiding the need to reheat the plastic sheet prior to thermoforming. It generally is considered desirable that heat-set articles such as ovenable containers have relatively uniform thermal crystallinity throughout, the article to provide adequate dimensional stability and impact resistance. For example, Demerest U.S. Pat. No. 5,614,228 describes a continuous rotary thermoforming apparatus in which a sheet of molten polyethylene terephthalate is extruded and vacuum-formed on a rotating drum having a plurality of mold cavities around its circumference. A hot oil circuit and electric heating elements are provided to impart different amounts of heat to different locations of the sheet during thermoforming. According to Demerest, additional heat is applied to portions of the articles that have a greater wall thickness to produce more uniform crystallinity throughout the article. The sheet is required to be tensioned and oriented during article forming to prevent the sheet from warping or otherwise distorting during cooling. Orienting the sheet also is said to result in articles having high impact resistance.
Several drawbacks exist with the type of thermoforming device described by Demerest. For example, a minimum amount of crystallinity, which is stated to be at least about 20%, must be obtained in the article to permit the article to be removed from the mold cavity without significant distortion. Thus, the device is not useful for applications where lower degrees of crystallinity may be desired in an article or a portion thereof. Moreover, the degrees of crystallinity actually obtained by using the Demerest apparatus typically are significantly higher than the stated minimum degree, and cannot be controlled effectively. Another drawback is that forming the sheet under tension results in distortion of the article after molding, which limits the ability of the apparatus to be used for many applications requiring especially high tolerances.
The device described in Demerest also is limited in terms of production speed. Following thermoforming, the articles undergo a series of cooling and drying steps prior to being separated from the mold cavities. The formed sheets are (again) tensioned to prevent distortion during separation from the mold. This type of procedure places severe limitations on production rates, especially for larger sized articles.
Dalgewicz U.S. Pat. No. 6,077,904 discloses a thermoforming process for preparing polyesters that are said to have improved impact properties, low oxygen permeability, and low dimensional shrinkage during heating. According to Dalgewicz '904, impact modifiers are dissolved into molten polyester to form a eutectic alloy. On slow cooling, the eutectic alloy is said to freeze to form a mixture of particles of the impact modifier embedded in a matrix of the polyester. By controlling the solidification of the melt, it is said that the size and distribution of precipitates of impact modifier from the melt can be controlled to permit control of the mechanical properties of the composition.
Dalgewicz '904 suffers from several drawbacks. For one, the thermoformed polyesters are extremely brittle, limiting their usefulness in many applications. The eutectic alloy formed requires the use of polyesters having a high initial intrinsic viscosity (I.V.), and also makes the polyesters more susceptible to thermal gradients upon the slow cooling. In addition, large 3D spheroids are developed in the polyesters, resulting in a high 3D morphology, which is undesirable in many applications. Yet another disadvantage of Dalgewicz '904 is that the required cooling rate is very slow. Slow cooling increases the overall time required for processing, which reduces efficiency and cost effectiveness.
Manlove U.S. Pat. No. 6,086,800 teaches a process and apparatus for continuously thermoforming articles. The apparatus has a plurality of mold facets, each of which has (i) a static upper mold facet section and (ii) a dynamic lower mold facet section to which a mold cavity is attached. The two-part mold facet defines a relatively deep mold, i.e., adapted to form deep-drawn articles. A thermoplastic sheet covers each mold facet and is held in place by a vacuum groove located on the upper mold facet section. The material is shaped by actuating an assist plug in combination with a controlled evacuation of air from the mold cavity.
Manlove also suffers from numerous drawbacks. For example, the static upper mold facet section is situated above the mold cavity. This means that the material is formed over the static upper mold facet and then into the lower mold cavity, resulting in poor mold definition. Also, the static upper and dynamic lower mold facet configuration substantially limits production speed and increases waste, i.e., results in larger amounts of unused “trim” that must be discarded or recycled. Further, the lack of proximity of the upper mold facet section relative to the lower mold facet section prevents the upper mold facet section from being an effective means for influencing the temperature of the thermoplastic material within the mold cavity, in particular the portion that is formed into the article. The mold facet configuration also encounters alignment difficulties at high temperatures due to thermal expansion, which effectively limits the device to low temperature applications.
Gartland U.S. Pat. No. 4,469,270 describes a discontinuous thermoforming apparatus having a mold for thermoforming a plastic article having a flange portion. Vacuum and/or pressurized gas is used to conform a sheet to the shape of the heated mold. External cooling means are provided to maintain a portion of the flange of the article at a temperature that is said to be insufficient to induce undesirable thermal crystallization. This portion of the article preferably has a degree of crystallinity of not more than 10% to improve adhesion of lidding films to the article. The remaining portions of the flange and the remainder of the article are said to preferably have the same average crystallinity.
It would be desirable to develop a continuous process and apparatus for thermoforming articles having excellent heat resistance and dimensional stability. It also would be desirable to develop a continuous process and apparatus for thermoforming articles that exhibit excellent stress relaxation and that do, not undergo appreciable distortion during cooling. It would be especially desirable to develop a process and apparatus capable of faster production times while substantially avoiding distortion, even for the production of larger sized articles.
SUMMARY OF THE INVENTION
According to one embodiment of the present invention, a continuous process for preparing a thermoformed article comprises extruding a thermoplastic layer through an extrusion die to form an extrudate in a substantially non-oriented state. The extrudate is contacted with a mold surface, such as a mold cavity (female mold) or a male mold. A stripper plate is disposed adjacent to the mold surface for controlling the temperature of proximate areas of the extrudate, e.g., the area that is formed into the flange portion of a container. The stripper plate optionally is also used for assisting in separating the articles from the mold surface by lowering the entire mold relative to the stripper plate. The extrudate remains in contact with the mold surface for a time sufficient to form the article. Ovenable containers and other artic
Bond John
Buff J. Charles
Dalgewicz Edward J.
Associated Packaging Enterprises, Inc.
Buff J. Charles
Lee Edmund H.
LandOfFree
Continuous process and apparatus for making thermoformed... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Continuous process and apparatus for making thermoformed..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Continuous process and apparatus for making thermoformed... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3323992