Roll fed flotation/impingement air ovens and related thermoforming systems for corrugation-free heating and expanding of gas impregnated thermoplastic webs

• US 9,427,903 B2
• Filed: 07/09/2012
• Issued: 08/30/2016
• Est. Priority Date: 04/14/2008
• Status: Active Grant

• Current Assignee: Dart Container Corporation (Robert Charles Dart)
• Original Assignee: Dart Container Corporation (Robert Charles Dart)
• Inventors: Branch, Gregory, Nadella, Krishna
• Primary Examiner(s): Grun, Robert J
• Application Number:
  US13/544,763
• Publication Number:
  US 20120267814A1
• Time in Patent Office: 1,513 Days
• Field of Search: 264/50, 264/544
• US Class Current: 1/1
• CPC Class Codes: B29C 44/20 for articles of indefinite length
• CPC Class Codes: B29C 44/3446 Feeding the blowing agent
• CPC Class Codes: B29C 44/3453 Feeding the blowing agent to solid plastic material
• CPC Class Codes: B29C 51/082 by shaping between complementary mould parts
• CPC Class Codes: B29K 2067/00 Use of polyesters or derivatives thereof, as moulding material
• CPC Class Codes: B29K 2067/003 PET, i.e. poylethylene terephthalate
• CPC Class Codes: B29K 2105/04 cellular or porous
• CPC Class Codes: B29K 2105/041 Microporous

Abstract

Disclosed herein are roll fed air heated flotation ovens and related thermoforming systems, assemblies, and machines that enable the corrugation-free expansion of a gas impregnated thermoplastic web passing through an oven chamber (without use of a pin-chain assembly), as well as to related methods. In an embodiment, a gas impregnated thermoplastic web is conveyed and expanded through an elongated air heated oven chamber, wherein the elongated oven chamber includes a plurality of downwardly directed heated air nozzles positioned at regular intervals along and within the upper portion of the oven chamber, and a plurality of upwardly directed heated air nozzles positioned at regular intervals along and within the lower portion of the oven chamber, but staggered apart from the downwardly directed heated air nozzles such that the gas impregnated thermoplastic material web undulates in an up and down wavelike fashion, thereby minimizing sagging, bagging, puckering, and/or buckling of the web.

Claims

1. A method for generating microcellular bubbles in a thermoplastic material, the method comprising:

conveying a thermoplastic material impregnated with a gas, through an oven chamber, wherein the thermoplastic material is continuously conveyed through the oven chamber;

blowing air through at least two upwardly directed nozzles and at least two downwardly directed nozzles, toward the gas impregnated material inside the oven chamber, with the blown air, heating the gas impregnated material, causing microcellular bubbles of gas in the material to nucleate and grow inside the material, while the material moves through the oven chamber, with only the blown air, suspending the material while the material moves through the oven chamber and with only the blown air, directing the material to undulate through at least two complete undulations as the material moves through the oven chamber and while microcellular bubbles nucleate and grow inside the material wherein impinging heated air streams from the at least two upwardly directed nozzles and at least two downwardly directed nozzles cause heating and uniform expansion of the gas impregnated material such that corrugation-free expansion of the gas impregnated material is enabled;

accumulating in an enclosure of an accumulation system where the enclosure includes an interior, a first nip roller that is disposed in the interior for tensioning the gas impregnated thermoplastic material as the gas impregnated thermoplastic material moves through the oven chamber;

heating the gas impregnated thermoplastic material in the enclosure with a heater; and

advancing the gas impregnated thermoplastic material accumulated in the enclosure via another nip roller included in the interior of the enclosure.

2. The method of claim 1 wherein conveying the gas impregnated thermoplastic material through the oven chamber includes pulling the material through the oven chamber.

3. The method of claim 1 wherein conveying the gas impregnated thermoplastic material through the oven chamber includes the material moving in substantially the same direction through the oven chamber.

4. The method of claim 1 wherein blowing air through at least two upwardly directed nozzles and at least two downwardly directed nozzles includes blowing air through a first upwardly directed nozzle, a first downwardly directed nozzle, a second upwardly directed nozzle, and a second downwardly directed nozzle, such that as the gas impregnated material moves though the oven chamber:

the material is first urged up by air blown from the first upwardly directed nozzle, then, the material is urged down by the air blown from the first downwardly directed nozzle,

then, the material is urged back up by the air blown from the second upwardly directed nozzle,

then, the material is urged back down by the air blown from the second downwardly directed nozzle.

5. The method of claim 1 wherein heating the gas impregnated material includes heating the material to its glass transition temperature.

6. The method of claim 1 wherein directing the material to undulate as the material moves through the oven chamber includes directing the material to form two peaks and two troughs as the material moves through the oven chamber.

7. The method of claim 1 wherein the material leaves the oven chamber before the microcellular bubbles have finished growing.

8. The method of claim 1 further comprising cooling the material immediately after the material leaves the oven chamber, causing the microcellular bubbles of gas in the material to stop growing inside the material.

9. The method of claim 1 wherein the material leaves the oven chamber after microcellular bubbles have stopped nucleating inside the material.

10. The method of claim 1 wherein the material leaving the oven chamber moves at a speed that is 1.5 times faster than the speed of the material entering the oven chamber.

11. The method of claim 1 wherein the material leaving the oven chamber had a width that is 1.5 times longer than the width of the material entering the oven chamber.

12. The method of claim 1 wherein the material leaving the oven chamber has a relative density that is 20%.

13. The method of claim 1 further comprising thermoforming the material after the material leaves the oven chamber.

14. The method of claim 1 further comprising:

advancing, via the another nip roller, material accumulated in the enclosure toward a thermoforming system; and

thermoforming the advanced material.

15. The method of claim 14 wherein advancing material accumulated in the enclosure includes intermittently advancing material toward the thermoforming system.