US 2606856 A
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1952 R. c. HURREY ET AL 2,606,856
METHOD FOR HEAT SEALING Filed Sept. 6, 1946 5 Sheets-Sheet 1 Ross c. HURREY Mama 2 Maumauz E IN V EN TORS Aug. 12, 1952 R. C. HURREY ETAL METHOD FOR HEAT SEALING Filed Sept. 6, 1946 3 Sheets-Sheet 2 26 OscnLAToQ Fly. 2
1 A L 20 J FQTZ 11] 1 2 k /8 20 V J Ross 'C. HuRREY Home? nq oumsus VENTORS 1952 R/ c. HURREY ET AL 2,606,856
METHOD FOR HEAT SEALING Filed Sept. 6, 1946 5 Sheets-Sheet 3 Ross C. l-lunne'v Hang? P- 1 our!!! V EN TORS Patented Aug. 12, 195.2
METHQE F08, HEAT SEALING Ross liiurrey and Homer R. Montague,
' Washington, D. ,6.
Application September 6, 1946, Serial No. 695,090
This invention relates to the art or" protective bags, wrappers and the like, and more particularly to improvements in the method ratus by which the layers, sheets or webs constituting such bags or wrappers are sealed together.
It is an object of our invention to provide improved means for forming and sealing the closures of bags and wrappers by the use of heat. It is a further object to provide improved means and methods for the heat sealing of bags and wrappers which are already partially formed, and which are filled with the intended contents. Still a further object is to greatly simplify the apparatus and process required in such sealing operations, with concomitant savings in the costof the machines and reductions in the amount of maintenance thereof.
Another object of our invention is to reduce the time required to seal a bag or wrapper, whereby the speed of operation of filling and closing machines may be greatly increased, and the cost of the operations thereby reduced in comparison with prior art systems.
A still further object is to provide a method and apparatus for the use of high-frequency electric energy as the source of theheat used in such sealing operations, whereby the degree of control of temperatures that may be exercised is in creased. Our invention makes it possible to perform sealing operations of the type indicated with simpler equipment, at higher speeds and with greater reliability than when gas, electric resistance or other known types of heaters are used, and these improvements are, by the application of the principles disclosed herein, achieved at an actual reduction in the cost of sealing such mate-' rials, notwithstanding the relatively higher cost of high frequency power as compared with the sources of heat heretofore used in this art.
An additional object of our invention. is to provide a process and apparatus for the heat seal ing of bags or similar wrappers made of material which is not inherently heat scalable, but wh ch is coated or printed with an electrically polar heat scalable material on those portions of the surfaces which it is desired ultimately to seal together by application of heat. In connection with this object of our invention, it will be'under stood that the material itself from which the wrappers are made, generally constitutes a ther mal barrier when sealing is done by transfer of heat through'such material to reach the heat scalable substance, which greatly slows down the heat sealing process, and hence the rate at which and appa 2 bags or other wrappers can be handled by given machinery. I
A still further object of our invention is to provide a method and apparatus enabling the heat sealing of the mouths of filled multi-ply bags, at
process permits such sealing compound to be raised to its sealing temperature. I I The above and other objects and advantages of our invention will best be understood by reference to the following detailed description of certain preferred embodiments of the invention, taken in connection with the attached drawings, in which:
Fig. l is a schematic perspective view of a portion of a machine in accordance with the invention, as used for the heat sealing of filled bags,
Fig. 2 is a diagrammatic view of the electrical connections used with the machine of Fig. 1,
Fig. 3 is a partial view similar to Fig. 2 but illustrating the parts at a later phase of the operationcycle,
Fig. 4 is a view similar'to Fig. 3 buttaken at a still later phase, I i I Figs. 5 and 6 are schematic views of the operation of the structure of Fig. 1 at two different points in the cycle, 7 I I Fig. 7 is a perspective view of a somewhat modified form of the invention, and I Fig. 8 is a sectional view illustrating the rela tion between a pair of modified electrodes and a gussetted multi-wall sack such as a shipping sack.-
Conventional bag and wrapper heat sealing. machines, and machines for sealing the mouths of liner bags used inside pasteboard cartons, with which we are familiar, are alike in that the heat requirements of the adhesive material are supplied by thermal conduction through the mate rial, such as paper, glassine or the like, composing the bag, wrapper or liner which is to be sealed. Usually, such heat is obtained from one or more heated jaws (or members equivalent in function), which are arranged to squeeze together the plies of material to be sealed. After a suitable interval to allow the sealing material to melt or become tacky, said jaws are retracted, the conveyor carrying the bag or wrapper is advanced to a new position, and the joint between the plies is squeezed between a pair of members which are at a relatively low temperature, in order to cool only such material, but the paper or other support which carries it, as well as the relatively massive jaws or similar members which conduct the heat to such support. All of these parts are continuously losing heat by radiation, convection and the like to the atmosphere as well as to other machine parts. Further, the requirement of such systems that the heat pass through the support. of paper or the like imposes undesirable limitations on the operating speeds obtainable, since not all support materials are capable ofwithstanding high temperatures for the periodrequiredto melt the adhesive quickly, and "the in,-
mentioned, by the application of high frequencyheating; especially dielectric heating; our invention makes it possibleto supply-the desired quantity of heatdirectly to the heat sealing material without having to pass such heat through any other elements. To accomplish this, we establish a high frequency electric-field between electrodes lying preferably on either side of the mouth of the bag to be sealed, thefrequency of the field being chosen so as-to achieve a maxi-- mum practical amount of dielectric loss in the particular heat sealing material being used. In this way, we are enabled to heat the adhesive with a minimum of concomitant heating ofthe paper or the like on which it is coated, and with substantially 100% utilization of such heat as is developed.
The above discussion is applicable as well to materials which are inherently heat scalable ('e. g., viny-lite, pliofilm) by what maybe termed autogeneous welding, as to materials which are not themselves heat scalable but which are provided with heat scalable areas by the application to such areas of a layer of material of desired characteristics; examples of such coated wrapper materials, and of the appropriate coatingstherefor, are given in the patent to Bell, No. 2,237,327, dated April 8, 1941. In the latter cases, however, our process is more advantageous than in the former, for. the reason that it is possible to select a frequency which preferentially heats the. coated heat scalable substance as compared with its paper carrier..- so that the efi'iciency of application can be greater, and higher production speeds obtained, than in a case where the high frequency field heats the material of the wrap indiscriminately throughout its thickness, muchof such heat (that is, the heat not generated closely adjacent the surfaces to be sealed) being wasted insofar as the heat sealing is concerned. Such waste heat is in fact a positive detriment, as it operates to reduce. the. cooling 4 or settling time of the portions which have been sealed together.
In the case of multiwall bags, for example shipping sacks, the presence of several insulating layers of paper or like materials between the outside of the sack and the areas which must be sealed has effectively prevented the application of heat sealing techniques to such containers. In accordance with our process, these intermediate layersmay be by-passed insofaras heating an internal layer of scalable material is concerned, by so choosing the frequency of the oscillation. as to make the power loss, and hence heat generated, in the sealing material, much greater than the loss in the intermediate paper layers. Concurrently, the presence of the unheatedouter layers of paper enables faster cooling of the sealed material after the heating phase of the process has been concluded.
We have found that multi-wall shipping sacks maybe satisfactorily sealed in the above Way at a higher rate, and with. less bulky and complex equipment, than by any known commercially practicable method. In fact, we believe thatwe are the firsttopresent a process of commercially practicable speed for the heat sealing of multiwall sacksof heavy caliper paper, notwithstanding the fact that heat sealing of, such bags has long. been attempted without success by other means.
In the application, of the above principles-to the high speed heat sealing of protective bags, we have found that it is particularly desirable to utilize the metallic electrode members also'as the pressing jaws of the. sealing station. This applicationiis very advantageous, since such electrodes, may be, maintained at any desired low temperature without adversely efiecting their efficaoy as high frequency electrodes, and hence the, application of such jaws directly to thebag material after the heating phase of the cycle simultaneously freezes the sealing material and holds the bag layers together. Hence, it is feasible to perform all the sealing steps of heating the. adhesive, pressing the layers together, and cooling the joint, at a single station, as compared with the two or more stations of the prior art. Great savings in machine complexity and in machine time per sealed bag are thus obtained. 7
Referring now to Fig. 1 of the drawings, we have shown the sealing station of a machine adapted to seal filledv protective bags such as the bag it, which is provided with an internal band [2 of heat scalable material adjacent its open mouth I4, and is inserted in a carton i5 which will ultimately be, closed about the sealed bag. It will be understood that bag it) has already been filled, by hand or by apparatus not shown. A conveyor I6 is provided to move the bag [0 into sealing position and away from such position, means being provided to start and stop such conveyor in, timed relation to the other parts to be described. Such means are. well known inthe art, and hence we have omitted a description. thereof since these form no part of our present invention.
A pair of metallic electrodes I8, 20 are so mounted with respect to the path of bags on conveyor I6. as to be reciprocable toward-and away from 'a bag on such conveyor, as indicated by arrows in Fig. 1. Each electrode is connected, as shown in Fig. 2, by conductors 22, 24 to a Source of high frequency power such as the oscillator 26, which is powered from the, supply mains by conductors 28. In order to limit the application of the high frequency power to the electrodes to those periods in which it is required, we have indicated a control means such as a mercury switch 3%) of known type, mounted for rotation by a shaft 34 having a vane 36 at its end. A switch control member 38 is mounted on one electrode 20, and comprises two arms 46 and 42, lower arm 40 being straight and upper arm 42 being somewhat longer and provided with a latch-like element 44 pivoted thereto as at 45 and hanging down so as to contact the upper extremity of vane 38 of switch 3s. The parts are so adjusted that as, or shortly after, electrode 29 begins to move toward bag (see Fig. 3), latch element 44 strikes the upper end of vane 36, rotating switch 3!) to initiate the flow of powor to the electrodes. Switch 3t remains in its rotated position as electrode 28 continues to move inward, until, just before the electrodes touch the bag material, lower arm 45 strikes the lower end of vane 36 (Fig. l) and rotates switch 3i! to the power off position. The remainder of the motion of electrodes l8, 2% operates to squeeze the joint between the layers of bag Hi, and since such electrodes are cool, the sealing material is simultaneously frozen to complete the seal. It will be understood that electrodes [8 and 20 are normally approximately at room temperature, since no heat is applied thereto, but if conditions require, they may be finned or cooled by other means to any temperature desired, e. g., by the use of a circulating refrigerant. l
Upon the retraction of electrodes 18 and 20, switch 30 does not operate to turn the power on again, since latch element 44 slipsup over the end of vane 36 in its retrograde movement, and electrode 28 continues its reverse movement to its rearward position in preparation for the next cycle.
It will have been observed from the above de scription that power is drawn from oscillator during only a small portion of the complete cycle of electrode motion. In actual operation this portion may be as small as one-tenth of the total cycle in some applications, which enables the use of an oscillator whose continuous power delivery rating is much smaller than would be required if the heating power were drawn continuously. All that is necessary is to design the oscillator, according to well known principles, so that it can handle short surges of power up to ten or more times its continuous power rating, so long as the long-time average of power drawn is within such rating. By this means, a smaller and less costly oscillator may be employed than would be the case if the oscillator design were such that its instantaneous power rating were no higher than its continuous rating.
When sealing bags of the type illustrated in Fig. 1, it may be desirable to stretch the mouth of the bag just prior to the application of the sealing pressure, in order to insure smooth con tact between the two layers of bag material, and to prevent wrinkles therein. We have illustrated in schematic fashion elements which perform such a function, comprising a vertically reciprocable element 45 which carries two crank arms 48, 50 pivoted thereon as at 52, 54, and biased toward each other by a spring 55. Stop pins so are mounted on element 46 to, maintain the crank arms normally in the position shown in Fig. 1. A pair of pins 52, 54 are secured on a fixed part 66 of the machine, and ar so located that, when element 46 moves downwardly, crank arms 48, 53 are rotated by reason of their contacting the pins 82 and $5, and
hence the power end of each is swung outwardly. Pivotally secured to the lower end of each crank is a two armed lever (68, ll) respectively), connected through one of its arms and a spring (12, M respectively) to the crank arm. Stop pins E6 on the lower end of each crank arm 68 and 50 serve to maintain the levers 68 and I0 normally in the position shown.
As element 46 moves downwardly and crank arms 48, 5!] swing outwardly, the lower ends of levers 68 and 7G engage within the bag mouth and draw it out until the front and rear surfaces of the bag material lie close together. Just or slightly before, jaws or electrodes 13 and iii reach a point at which they contact the bag material, the lower ends of levers 68 and it slip out of contact with the bag, and springs 72 and it snap the levers into alignment with crank arms 43 and which are now in such a position that neither they nor levers 58, I0 interfere with the pressing of the bag mouth between the electrodes. As electrodes l8 and 20 are retracted, element 68 moves upward, the bag moving forward on its conveyor, and the parts ultimately resume their Figure 1 position.
Figs. 5 and 6 diagrammatically show the relative positions of the element 46, crank arms 48 and 5G, and levers 68 and 10, prior to engagement of the electrodes with the bag, and immediately after the levers have slipped out of the bag mouth.
In the form of the invention shown in perspective in Fig. 7 of the drawings, a conveyor IE0 is provided, which carries cartons such as IE2 each containing a filled bag its with a heat scalable band I93 around its inner mouth surfaces. The conveyor 80 in this embodiment moves continuously rather than step-by-step, and conveys the bag mouths between a pair of electrode rails Hi8 connected to a high-frequency source H!) by any convenient means. The rails its are substantially spaced apart at the entrance end, but converge until they are only a fraction of an inch apart at the bag position indicated in the figure. In other words, the electrode rails themselves operate to force the walls of the bag mouth towards one another, the gussets being collapsed rather than extended as in the previously described embodiment. High frequency heating occurs all along the path of the bag between the rails I08, and as the bag mouth passes out of the field between these rails, it is received between a pair of belts I I2, I M mounted to travel in a horizontal plane by means of rollers H6. The arrangement of the belts is such as to form a pinching entrance portion i it! which narrows until the belts are substantially in contact, and at which point they serve to compress the bag mouth to complete the seal, the sealing material cooling as the sides of the bag mouth are held in contact during passage between such. belts.
It will be understood that belts H2, lit are at room temperature, or maintained at an even lower temperature by cooling means, whereby the dhesive material is rapidly set and the bag emerges from between the belts with the mouth completely sealed.
We have found that it is necessary to maintain the voltage supplied by the oscillators at a rather low value in order to prevent voltage breakdown of the material of which the bags or wrappers are made, since the separation of the electrodes in-either ofthe above' embodiments becomes very small indeed at some points in either process. We have found that, While it is therefore diflicult to obtaina sufficient power dissipation in the heat scalable materials to heat them to the softening or sealing temperatures, a desirable speed of sealing'can be obtained even at the necessarily low voltage, by raising the oscillator frequency to a point above that at which dielectric heating is usually carried on.
A frequency in excess of 150 megacycles per second is very desirable, and-permits the small volume of heat scalable material per bag to be heated as required by an instantaneous high frequency power of the order of 500 Watts, corresponding to a heating period of one-tenth the total operating cycle, and an average oscillator rating of only 50 watts. No difiiculty is experienced in obtaining this quantity of power at the frequencies indicated, by use of suitable present day high frequency tubes and cirouit'designs. The
above data are basedupon the sealing of bags having a sealing area of approximately eleven square inches, at the rate of sixty bags per minute.
The above descriptions relate principally to the sealing bags or similar wrappers consisting principally of a single layer of wrapper material coated interiorly with a layer or localized area of dielectrically lossy thermoplastic adhesive. Since, as already pointed out, a proper selection of oscillator frequently permits suchan adhesive to be heated to its melting point with a minimum of waste of heat in the support layers themselves, it is clear that a wrapperor bag consisting of a plurality of plies of paper orthe like may be sealed in a similar manner. The advantages to be gained'by the use of our invention in sealing such materials are even greater than in the case of single-ply materials, since attempts to heat an internal adhesive to its melting point by conduction through a plurality of paper plies require so much time as tomake such a process commercially impractical. In accordance with our invention, the energy required for this application is little if any greater than in the case of a single-ply material, since at the proper oscillator frequency only 'a trivial amount of the energy is converted into'heat Within the paper layers themselves.
The above considerations apply as well Where the multi-ply bag has only a single internal layer or localized area of heat scalable material, and where a layer or stripe of such adhesive is located also between the plies of the bag material. The latter situation arises where it is desired, in one operation, not onlyto seal the mouth of the bag but to seal together the plies of the bag material adj acent'the mouth, for greater strength or improved appearance.
The utilization of the improved methods above described enable a maximum amount of heat to be generated at the point of actual use, rather than by conduction through the support materials. The speed with which bag or wrapper sealing operations may be carried on, however, depends also to some extent upon the rate at which heat can be removed from the sealing material once it has been melted, and the joinder of the support layers, such as paper, accomplished. This is because it is necessary that the thermoplastic material be substantially solidified before pressure upon the seal can be released. Such a limitation on operation speed is not, of course, as serious as the limitation arising from the of the process it is only required that the ma-- terial be cooled from its melting point to a temperature slightly below that pointin other Words, the heat required to be extracted prior to release of pressure is only a fraction of that required to be applied to melt the adhesive originally. Nevertheless, it is often desirable to accelerate the cooling process, particularly where it has become, due to the use of our novel heating arrangement, the limiting factor on machine speed. This can be accomplished very satisfactorily by simply providing the electrodes or sealing jaws with means for circulating a coolant or refrigerant there through, or even by providing heat exchange fins upon such electrodes to utilize to a maximum the cooling effect of the ambient atmosphere. 7 I
In multi-ply work, as suggested above, it is very likely that the process speed will be limited by the cooling time of the adhesive, rather than by its heating time, since the latter is not restricted by Q the necessity of utilizing heat conducted through the multiple layers of wrap material. Under these circumstances, we prefer to use an adhesive which has a higher melting point than that ordinarily used for single-ply materials. This may slightly increase the time required for the heating phase of the cycle, but such an increase will be more than offset by the reduction in cooling time to the set point of the adhesive, yielding an over-all saving in the process time required. This is because of the fact that the temperature gradient between the adhesive and its surroundings, at the commencement of the cooling phase, will be greater for high meltingpoint adhesives, whereby the time required for the adhesive to drop to its setting temperature is reduced.
In using the techniques of the present invention in connection with the sealing of large multi-ply sacks or bags intended for rough usage, such as shipping sacks for the transportation of coffee, cement, chemicals or other commodities in quantities of the order of pounds or more, it is clear that the saving in time as against sealing by conductive means becomes much greater than in the case of a relatively few plies of relatively thin materials. A typical shipping sack may be formed of four, six or more pliesof kraft paper of sixty pounds basis weight, and hence the thickness of the bag mouth, particularly at the gusseted edges, may approach a quarter of an inch. Since in a bag formed of six-ply paper there are 24 plies adjacent the gussets but only twelve plies at the central portion, there will be a considerable diiference in thickness, as indicated in Fig. 8. As shown in that figure, such a condition may be compensated for by shaping the faces of electrodes H8, 220 so as to flare outwardly at their ends, providing a greater spacing between electrodes at points adjacent the gussets. This construction enables uniform pressure to be applied throughout the face of the closure notwithstanding the variations in thickness which result from the gussets.
The electrodes illustrated in Figure 8 also show the provision of internal channels 222 adapted to the circulation of a suitable coolant, which maintains such electrodes at a desired low temperature. By this means, cooling of the outer plies of the shipping sack commences immediately upon contact with the electrodes, and indeed cooling of the outer plies may go on simultaneously with the generation of sealing heat between the two innermost surfaces of the closure. This type of operation enables a perfect seal to be obtained in a minimum of operating time.
It will be understood that heavy duty multiwall sacks or bags such as those referred to above will be provided with a suitable mechanical type of closure securing means, such as a sewed closure. for mechanical strength, the heat seal at the bag mouth being provided principally to insure a moisture-proof and sift-proof seal. If the sewed closure is completed prior to the forming of the heat seal, the heat sealing material will effectively seal around the sewn threads and block this possible avenue of leakage at the closure. However, it is quite possible to complete the heat seal before the filled bag is mechanically secured.
We are aware that many changes and modifications may be made in the arrangements and processes described herein, and we therefore do not wish to be restricted to the precise exemplary details recited. In particular, we are aware that the principles of local conversion of electrical energy to heat may be extended to the use of high frequency magnetic fields, as opposed to dielectric fields, and that such an extension has particular applicability to the sealing together of materials whose meeting surfaces comprise layers of metallic foil bearing a heat sealable material thereon, e. g., in the case of a foil-lined bag or wrapper provided with an internal area of thermoplastic adhesive. In such a case, the presence of the electrically conductive metal layer effectively prevents the use of an electric field for sealing, as such layer completely shields the thermoplastic material. However, the use of a magnetic field enables heat to be generated within the metal layer itself due to well known eddy-current effects, and such localized heat readily effects the melting of the adhesive coated thereon. As in the previous application, the melting of the adhesive is in no wise impeded by the presence of numerous outer layers of paper or other thermally insulating materials comprising the wrapper, since here, again, the heat required is generated substantially at the point of use rather than requiring to be conducted through the outer layers of the bag or Wrap material.
We also wish it to be understood that ourinvention is applicable to other operations than the sealing of filled bags or wrappers, in that its priciples may very well be applied in an obvious manner to the sealing together of any layers of heat sealable material having the proper dielectric characteristics. For example, the invention may be utilized in the bottoming of formed tubes to form the normally closed end of an empty bag, such as by forming a tube of a suitable heat sealable material, cutting off a length of such tube, flattening said length, and either effecting a simple fin-type closure of one end, or folding the flattened tube back upon itself and sealing the two walls of the tube together and to an outer surface of said tube in one and the same operation, as in the Well known Simplex bag machine.
In using the term heat sealable material herein, we intend to include not only materials which are inherently heat sealable, such as the examples mentioned earlier in this specification, on one or both sides, but also materials such as paper, glassine or the like which are rendered heat sealable by the application of a localized or a distributed coating of a heat sealable material such as those mentioned in the Bell patent referred to above.
In view of the many changes and modifications which may be made in the invention without departing from the spirit thereof, we do not wish to be restricted to the precise details disclosed, but desire to include within our invention all such changes and modifications as come within the scope of the appended claims.
1. The process of sealing together thin layers of heat-sealable, dielectric materials, comprising supporting said layers in face-to-face relationship between a pair of electrodes, exposing said layers to the action of a high-frequency electrical field of at least megacycles per second between said electrodes for a sufficient time to raise said layers to a sealing temperature, then discontinuing said electrical field and immediately thereafter pressing said layers together to seal the same to one another.
2. The process of sealing together thin layers of heat-sealable material, comprising supporting said layers between a pair of thermally conductive electrodes maintained below the sealing temperature of said material, and out of contact therewith, establishing a high-frequency dielectric field between said electrodes for a sulficient time to raise said layers to a sealing temperature, then discontinuing said electrical field and immediately thereafter pressing said layers into intimate contact between said electrodes to seal the layers to one another.
3. A method for sealing together layers of heat sealable materials, comprising placing said layers in facing relationship, subjecting said layers to a high frequency field of at least 150 megacycles per second for a length of time sufllcient to soften said heat sealable material, removing said high frequency field, and forcing said softened layers into intimate contact to thereby bond said layers together.
ROSS C. HURREY. HOMER R. MONTAGUE.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 2,276,994 Milinowski Mar. 17, 1942 2,354,714 Strickland Aug. 1, 1944 2,401,991 Walton June 11, 1946 2,432,412 Hacklander Dec. 9, 1947 2,465,102 Joy Mar. 22, 1949 2,477,040 Brown et al July 26, 1949 FOREIGN PATENTS Number Country Date 573,518 Great Britain Nov. 23, 1945