Heat sealer

BACKGROUND OF THE INVENTION

This invention relates broadly to a method and ap- 30 paratus for forming a fused juncture between opposed thermoplastic sheets, films 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. 35

Closure seals of flexible packaging materials are obtained 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 40 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 45 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 50 in the development of flexible 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 55 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 60 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 65 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 filling horn to the package surfaces has not been attainable, even with the most sophisticated fill- 70 ing 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 difficult 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 difficulty 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. 1 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. 1.

FIGS. 7, 9, and 11 are plan views of the pouch seal area at various stages of the sealing process.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the drawing, there is 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 15 and a sealer bar or 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. 1 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 is activated.

In 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. 2 diagrammatically illustrates a conventional heat sealing device having a jaw 20 extending down from the heater block 21 which jaw has a flat 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

3,673,041

4 heated jaw 20 is brought against the sheets 24 with sufficient 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 the seal.

The sequence of events and resulting seal produced in accordance with the instant invention are shown in FIGS. 5 to 11. Two sheets 30 of thermoplastic material comprising the open end of a pouch are 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 16 is brought into initial contact with the sheets 30, as in. FIG. 6, a hair line seal 32 is produced across the width of the pouch opening as shown in FIG. 7. As 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. The final step is shown in FIG. 10 where the anvil has yielded substantially under the force of the jaw. This position is maintained until sufficient heat has been transferred to fuse the film surfaces 30 and form the continuous seal 32 showninFIG.il.

To obtain the desired contaminant-free it is necessary that the anvil material be resilient, i.e. 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 to about Vi inch and has a transverse radius varying W to Vi 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 flat bar sealer. Seal strength values were measured on an Instron tensile tester, using Vi inch wide specimens cut from the closure seal of test packages. The 5⁵loading 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 Mt inch and the anvil was a silicon rubber material having a durometer of 72. The control sealer had a flat 1 inch wide bar, opposed by a silicone rubber having a durometer of 57. The packaging materials used were (1) a 0.003 inch modified polyolefin0.00035 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 conditions for both materials with the flat bar sealer were: temperature of 410° F., pressure 40 p.s.i.g., and dwell time of 1 second.

With the curved bar sealer, the modified polyolefin was sealed at a temperature of 380° F., a pressure of 30 p.s.i.g. and a dwell time of 1 second and the high density polyethylene material was sealed at a temperature of 420° F., pressure of 30 p.s.i.g. and a dwell time of 1 second. 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 flat and curved 5 bar sealers. Under ideal conditions, i.e. clean seal surfaces sealed at optimum conditions, the flat 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 10 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.

TABLE 1 1 ¢- --------------------------------------------------------------------------------·............

Seal strength, pounds per inch of seal width

Flat Curved

Seal condition bar bar

High density polyethylene-alum.

foil-Mylar:

Clean_____-_____ —- — -------- 13.112.6

Water.— -______________ 5.411.3

Margarine---------.------- 8.712.4

Modified polyolefin-alum foilwaier___________________ o. vav. o

Margarine 6· 714.3

Example H

Pouches made from the two packaging materials em°^u ployed in Example I were filled 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 the product was spread _3g 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 unretorted packages. Pressuriza40 tion 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 con45 taminated curved bar and flat bar closure seals. There were no appreciable differences between clean and contaminated packages when sealed on the curved bar sealer of this invention. The packages sealed on the flat bar were weakened by contamination to the point that they ⁵⁰ could not be fully expanded by pressurization before leakage occurred.

TABLE 2

Burst strength, p.s.i.g.

Unre- ReSealer Sealsurface torted torted

High density polyethylene-alum.

foil-polyester:

Flat bar______________Clean_________

Do___________________Contaminated_____

Curved bar„_______________Clean------Do._.__________Contaminated_____

Modified polyolefin-alum. foil-polyFlatbar._:_______—____- Clean..----Do____________________Contaminated_____ „ _ Curved bar________________Clean---------- ⁰⁰ Do.._____:__________- Contaminated_____

12.810. δ

12.310.0

13.010.8

11.010.8

11.311.3

11.511.2

This invention also contemplates the use of a steam flush as an adjunct to the curved bar sealing to reduce the headspace gas volume in the pouch and to remove fibrous 70 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 be 75 cleaned by flushing with steam for 2Vi seconds. A steam

3,673,041 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 flat 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 modifications can be made therein without departing from the spirit and scope of the invention as defined in the following claims.