US 3673041 A
Description (OCR text may contain errors)
June 2 7, 1972 G. SCHULZ ETAL 3,673,041
HEAT SEALER June 27, 1972 l G, SCHULZ ETAL 3,673,041
HEAT SEALERv Filed Sept. 8, 1970 2 Sheets-Sheet 2 le' le l /9 /10 I l-/e United States Patent O 3,673,041 HEAT SEALER Gerald L. Schulz, Hollistou, and Raymond T. Mansur, Framingham, Mass., assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 8, 1970, Ser. No. 70,167 Int. Cl. B32b 31/00, 3/ 00; C09j 5/00 U.S. Cl. 156-306 3 Claims ABSTRACT F THE DISCLOSURE Heat sealing opposed thermoplastic surfaces of open ended pouches, whose sealing surfaces are likely to be contaminated with liquids or fatty substances is accomplished by forcing a transversely radiused, heated sealing bar against the opposed unsealed sheets of the pouch which are supported by a resilient anvil, thereby squeezing the contaminants out of the sealing area before the sheets fuse together and in those cases where solid particulate contaminants are found in the sealing area utilizing steam flushing to clean the sealing surfaces prior to sealing.
The invention described herein may be manufactured, used and licensed by or for the Government for govemmental purposes without the payment to us of any royalty thereon.
BACKGROUND OF THE INVENTION This invention relates broadly to a method and apparatus for forming a fused juncture between opposed thermoplastic sheets, lms or coated surfaces. More particularly, this invention relates to a method and apparatus for heat sealing wherein the opposed surfaces are contaminated with aqueous or oily liquids or fatty substances.
Closure seals of flexible packaging materials are 0btained |by fusion or heat sealing of the inner thermoplastic layer of the opposed sheets of material.
Such sealing is accomplished by pressing the opposed layers between rigid, heated bars or jaws under pressure until the layers have fused. The sealing jaws, typically, have either flat or serrated surfaces.
In recent years, flexible packaging has made tremendous inroads into the food packaging field. This impact is manifested by the increase in the boil-in-bag and frozen food markets featuring flexible packaging. However, the nonfrozen, shelf-stable analogue of the boil-in-bag, i.e., thermally processed food in a flexible package, remains as one area of great potential application. During the past decade, a considerable amount of progress has been made in the development of llexible packages for thermoprocessed foods. Flexible packaging materials are now available which are capable of withstanding the rigors of retorting; processing techniques have been developed and proven; and levels of extractable substances from processed materials are well within the established safety limits. Storage stability and resistance of flexible packages to damage have been found to be more than adequate.
One major problem area remaining, however, is the large number of package failures which are obtained in flexible packaging applications, due principally to defective closure seals. The primary cause of such defective closure seals is the presence of occluded matter in the closure seal area. Efforts to prevent contamination of the seal surfaces have not been successful. Since present methods require the filling of packages at high speeds through a relatively small opening, positive prevention of contamination resulting from the splashing of product material from a lling horn to the package surfaces has not been attainable, even with the most sophisticated filling equipment available. Contamination of the sealing surfaces of the flexible packaging material by particles and fibers, as well as liquid and soft fatty contaminants can result in a defective closure seal. Detection of particulate contamination, although diflicult and unreliable, is possible by visual inspection of the seal surfaces prior to sealing. Visual detection of small amounts of liquids or fatty substances, however, is virtually impossible. In view of the likelihood of seal surface contamination and the dilliculty in perceiving this contamination, it is essential that there be developed a technique that Will provide a positive heat seal in the presence of liquids, fatty'substances or both on the seal interface surfaces.
SUMMARY A positive heat seal of interface surfaces contaminated with liquids and/or fatty materials is obtained according to the present invention by employing a transversely radiused sealing bar in conjunction with a resilient anvil. Such a device functions to physically force liquid or soft fatty contaminants from the seal area before fusion of the layers occurs.
BRIEF DESCRIPTION OF THE DRAWING FIG. l is side view in elevation of a heat sealing device according to the present invention.
FIGS. 2 and 3 are transverse cross sectional views of prior art heat sealing apparatus and FIG. 4 is a plan view of a seal obtained with the apparatus of FIGS. 2 and 3.
FIGS. 5, `6, 8, and 10 are transverse cross-sectional views depicting stages in the heat sealing of pouches with the apparatus of FIG. l.
FIGS. 7, 9, and 11 are plan views of the pouch seal area at various stages of the sealing process.
DESCRIPTION OF THE PREFERRED IEMBODIMENT With reference to the drawing, there s illustrated in FIG. 1 an embodiment of this invention which will produce a positive heat seal between opposed layers of thermoplastic material whose seal surfaces have been contaminated with liquids or soft, fatty substances. The apparatus shown in FIG. 1 is conventional except for the sealing bar and the anvil material. A flat base 10, supports a right angled bracket 11, one leg of which extends upwardly from said base and the other leg being parallel with said base. Air cylinder 12 is attached to said bracket and pneumatically forces rod 13 under pressure toward and away from said base. Attached to the free end of the rod is the heat sealer 14 consisting of a heater block 1S and a sealer bar 0r jaw 16. The heater block portion contains a number of electrical resistance cartridge heaters 17 which produce the desired level of heat within the block, which heat is conducted to the sealer jaw. The sealer jaw, as is shown in FIGS. l and 5, is transversely radiused along its entire length to form a rounded bearing surface 16'. A resilient anvil 19 seated in an anvil support 18 is located on the base directly below the heat sealer and limits the downward movement of the sealer jaw when the air cylinder lis. activated.
yIn attempting to heat seal opposed surfaces of thermoplastic material contaminated with liquid or soft fatty materials, as are commonly found in food products, with conventional heat sealing equipment the contaminants tend to be trapped within the seal area weakening the seal. FIG. Z diagrammatically illustrates a conventional heat sealing device having a jaw 20 extending down from the heater block Z1 which jaw has a ilat bearing surface 20' and a rigid `anvil 23 spaced from said jaw. The bearing surface of the jaw is parallel with the bearing surface of the anvil. Two opposed sheets 24 of thermoplastic material having a liquid 25 contaminant coating the seal area are positioned on the anvil 23 and, in FIG. 3, the
heated jaw 20 is brought against the vsheets 24 with suiiicient pressure and for sufficient time to seal the sheets together. As shown in FIG. 4, there is trapped within the seal area 26, pockets of liquid material 25, which will reduce the strength of the seal or may even produce a failure in vthe seal.
'I'he sequence of events and resulting seal produced in accordance with the instant invention are shown in FIGS. to 11. Two sheets =30 of thermoplastic material comprising the open end of -a pouch vare positioned over the resilient anvil 19. The inner surfaces of the sheets are coated -with a liquid contaminant 31 in the area to be fused or sealed together. As the heated, transversely radiused jaw 1-6 is brought into initial contact with the sheets 30, as in PIG. 6, a hair line seal 32 is produced across the width of the pouch opening as shown in FIG. 7. Asy the jaw continues its downward motion as in FIG. 8, the top surface of the anvil 19 yields under the pressure and the squeegee action of the heated jaw against the yielding anvil forces the contaminant 31 away from the center of the seal 32 as depicted in FIG. 9. 'I'he nal step is shown in FIG. 10 where the anvil has yielded substantially under the force of the jaw. This position is maintaineduntil suicient heat has been transferred to fuse the lm surfaces 30 and form the continuous seal G2 shown in FIG. 11.
To obtain the desi-red contaminant-free it is necesl sary that the anvil material be resilient, iLe. capable of undergoing elastic deformation under the pressure of the transversely radiused jaw. Elastomers, such as silicone rubbers, having a durometer (Shore A) within the range of 55 to 80 have the desired physical characteristics for this application but greater durability of the anvil is obtained when` the elastomer has a durometer within the range of-from about 65 to about 75. It is also necessary that the sealing jaw be curved to provide the proper wiping or squeegee action. The jaw preferably varies in width from about 1A: to about 1/2 inch and has a transverse radius varying 1A to 1/2 inch.
The pressure, temperature and dwell times necessary to accomplish the seal will vary with the materials being sealed and such conditions are well-known in the art. The following examples illustrate the results obtained by the practice of this invention.
Example I To determine the effects of liquid and soft fatty substance contamination on seals produced according to this invention, the seal interface surfaces of pouches were coated with each type of contaminant and sealed at the optimum conditions established for each of the materials. A second set of samples, prepared in the same manner, was sealed in a at bar sealer. Seal strength values were measured on an Instron tensile tester, using 1/2 inch wide specimens cut from the closure seal of test packages. The
loading rate (crosshead speed) used was ten inches per minute.
In this example the transversely radiused sealing bar (also referred to as the curved bar) had a width of inch and a radius of M: inch and the anvil was a silicon rubber material having a durometer of 72. The control sealer had a at 1 inch wide bar, opposed by a silicone rubber having a durometer of 57. The packaging materials used were (l) a 0.003 inch modified polyolefin- 0.00035 inch aluminum foil-0.0005 inch polyester and (2) a 0.003 inch high density polyethylene-0.00035 inch aluminum foil-0.0005 inch polyester. The sealing conond. The fatty substance used as a contaminant in the seal areas was margarine.
Table 1 shows the average seal strength values in pounds per inch of seal width obtained with at and curved bar Sealers. Under ideal conditions, i.e. clean seal surfaces sealed at optimum conditions, the at bar seals were slightly stronger than those made on the curved bar sealer. When the seal surfaces were contaminated with water or grease, the strength of the flat bar seals dropped to less than the minimum acceptable strength of 10 pounds per inch, while those made on the curved 'bar sealer showed considerably less strength loss and were all above the 10 pound minimum. v
Pouches made from the two packaging materials em ployed in Example I were lled with a mixture of beans and tomato sauce and sealed on the flat and curved bar sealers at the conditions set forth in Example I. Prior to sealing, a coating of sauce from lthe product was spread evenly over the entire seal interface surfaces of each pouch. Excess air was removed prior to sealing. Some of the sealed packages were heat processed at 250 F. for. 30 minutes. Internal pressure burst tests were conducted on retorted and unretorte'd packages. Pressurization was accomplished with a hypodermic needle through a sealant patch on the center of each pouch. During pressurization, the pouches were restrained between two rigid plates to limit expansion to one inch. A pressure increase of 1 p.s.i.g. per minute was used. Table 2 shows the average burst strength values of pouches with clean and contaminated curved bar and at bar closure seals. There were no appreciable diiferences between clean and contaminated packages when sealed on the curved bar sealer of this invention. 'Ihe packages sealed on the flat bar were weakened by contamination to the Point that they could not be fully expanded by pressurization before leakage occurred.
This invention also contemplates the usecof a steam flush as an, adjunct to the curved -bar sealing to reduce the headspace gas volume in the pouch and to remove fibrous and particulate material from the seal area. The curved bar sealer makes it possible to employ a steam flush since a positive seal is produced in the presence of moisture on the seal surface interfaces. Seal surfaces. of pouches contaminated with ground beef in barbecue sauce can vbe cleaned by flushing with steam for 21/2 seconds. A steam nozzle, adapted to direct steam at an angle of 45 degrees to the sides of the pouch, has been employed to distribute steam over the entire seal surface. Residual gas volumes resulting from steam flushing of pouches were found to be well below the maximum allowable headspace gas volume of 6 cc. Visual examination of pouches prior to retorting showed no defective seals and all test packages survived retorting with no visible evidence of closure seal degradation. Pressure tests of retorted pouches showed an average burst pressure of 12.9 p.s.i.g. which compares favorably with values obtained previously from both ilat and curved bar seals on clean packages.
Although several embodiments and examples of the invention have been described herein, they are intended to be merely illustrative, and various modilications can be made therein without departing from the spirit and scope of the invention as delined in the following claims.
1. A method of forming a fused juncture between two superimposed flexible sheets having opposed thermoplastic surfaces, said surfaces being susceptible to the presence of Huid contaminants in the area to be sealed, which comprises applying heat and pressure to the area of said sheets which are to be fused together by causing a transversely radiused, heated sealing bar to press said sheets against an anvil having a resilient surface to squeeze any iluid con- References Cited UNITED STATES PATENTS 3,035,381 5/1962 Hosso 156-515 2,638,963 5/1953 Frederick et al 156-515 3,078,201 2/1963 Christie 156-306 2,941,575 6/1960 Malmberg et al 156-515 2,712,343 7/1955 Stanton 156-583 3,017,315 1/1962 Doyle 156-583 DOUGLAS I. DRUMMOND, Primary yExaminer U.S. Cl. X.R.