WO1985001252A1

WO1985001252A1 - Method of edge bonding sheets and resulting assemblies

Info

Publication number: WO1985001252A1
Application number: PCT/US1984/001464
Authority: WO
Inventors: Gerald Leroy Hurst
Original assignee: Hurst G L
Priority date: 1983-09-14
Filing date: 1984-09-13
Publication date: 1985-03-28
Also published as: EP0156892A1; AU579837B2; EP0156892A4; GB2157228A; GB2157228B; MX163413B; DE3490429T1; GB8511814D0; AU3435984A; JPS61500063A

Abstract

The object of the invention is to form a high strength seal between facing sheets of thermoplastic material along their edges, which may be formed as a stack of sheets. The improved process providing the high strength seal comprises forming a stack (12) of heat-sealable sheets (14) with nonheat-sealable layers (16) interposed at predetermined locations, pressing the sheets (14) and layers (16) against platen (18) by compression means (20) while exposing edge portion (22) to heat source (24) across the thickness of stack (12) to form a high strength seal (90) between the edge portion (22) of the heat-sealable sheets (14).

Description

METHOD OF EDGE BONDING SHEETS AND RESULTING ASSEMBLIES

RELATED APPLICATION

This is a continuation-in-part of application Serial No. 317.414. filed October 27, 1981.

FIELD OF THE INVENTION

This invention relates to manufacture of bondad assemblies and more particularly to bonding sheets of heat-sealable thermoplastics or of sheets containing heat-sealable thermoplastics portions. In another aspect, this invention relates to the manufacture of seams for bonding together sheets of thermoplastic material which are characterized by an increasing strength gradient in the direction of the periphery of the seam. In another aspect, this invention relates to a method for producing multiple envelopes or multiple sealed edges in a single heating application.

BACKGROUND OF THE INVENTION

The bonding or welding together of thermoplastic sheets to form a variety of articles is well known. Thermoplastic sheets commonly used in industry for heat-sealable operations include rubber hydro- chlorides, copolymers of vinyl chloride and vinyl acetate, polyethylene, polyvinylidine chloride, ethylene vinyl acetate copolymers and the like. These materials have conventionally been sealed or welded together by many techniques such as thermal heated mass or heated rollers sealing, dielectric impulse sealing, ultrasonic sealing or solvent sealing.

The heated mass method encompasses placing sheets of heat-sealable materials between a die and a support surface or matching die. Heat is then ccmiπonly applied through one or both dies at a y„-*—* sufficient temperature and for a sufficient period of time to bond the two sheets together. The dies may be of any desired shapes such as rollers or platens which transfer the thermal impulse as disclosed in U.S. Patent No. 2,730,161 issued to Langer on January 10, 1956. U.S. Patent No. 4,113,169 issued to Carlisle on September 12, 1978 discloses a method for producing containers by bonding two heat-sealable sheets together between flat platens.

Ultrasonic sealing is accomplished in a similar manner by placing sheets of heat sealable material between an ultrasonic probe and a backup plate. Thereafter the material is subjected to ultrasonic waves which heat and seal the layers together.

Impulse and dielectric sealing are accomplished by utilizing electric current. Impulse sealing is accomplished by pressing two sheets of heat-sealable material between a resistance surface and a support surface and then passing electric current through the resistance surface to produce heat. In the dielectric method, two sheets of heat-sealable material are placed between opposing conductive probes which act as the plates of the capacitor and the thermoplastic material serves as the dielectric. High frequency alternating current is applied to the plates causing the film to heat to sealing temperatures by the varying electric field. This dielectric technique is limited to films with suitable dielectric properties such as polyvinyl chloride.

These methods are all subject to the limitation that very few seals can be produced in a single application of heat. The inherent limitation of «»■•«• these methods is their application of thermal, electric, or ultrasonic energy from the active platen into the material. The heat or other energy is applied to the heat-sealable material through the die for the purpose of bonding together those portions of the sheets pressed between the dies. The material adjacent to that portion of the material held between the dies functions as a heat sink dissipating the heat applied through the dies. Thus, the thicker the heat-sealable material between the platens becomes, the"greater heat is dissipated throughout the remaining portion of material. Thus, a large number of sheets cannot be suitably sealed because sufficient heat cannot be transferred to those sheets furthest from the dies.

An additional limitation of these methods is, that the dies and platens used in the practice of these methods must be made of special materials, and machined to exact specifications and thus are expensive. Additionally, these dies are suitable for producing only a single bond width. Thus, a number of individual dies with varying bond widths are required if more than one width of bonding is to be produced.

U.S. Patent No. 4,055,452 to Carlisle discloses the use of a heated cutter blade to seal a stack of thermoplastic material when the stack is being cut by fusion. The resulting seal has a width only moderately greater than the film thickness which has been thickened.

In another attempt to produce thermoplastic material bonds in unusual shapes, U.S. Patent No.

2,730,161 discloses an indirect heat sealing ?~~ technique. A heated plate or roll is formed in the desired shape. A smooth transfer element contacts the heated shape to obtain a thermal pattern thereon. The thermal pattern is then vertically impressed on a pair of thermoplastic films to form a bond having the desired shape.

The present invention provides a method for simultaneously bonding together a multiplicity of heat-sealable sheets, a series of grouped sheets bonded together or a series of separate envelopes from sheets bonded together in a desired shape. The present invention may be practiced with inexpensive equipment which need not be fabricated by anyone with expertise in the field. The present invention provides a method whereby varying seam widths may be produced with a single die. Finally, the method of the present invention allows for generally superior bonding and increased production rate. SUMMARY OF THE INVENTION

According to the present invention, a method for bonding together a multiplicity of heat-sealable sheets or for producing a multiplicity of grouped heat-sealable sheets bonded together is provided which comprises stacking sheets one upon another such that the heat sealable portions and nonheat-sealable portions of the stack are oriented in a predetermined fashion, pressing the stack of sheets between dies defining a preselected pattern and effective to obtain intimate contact between sheets in the stack along the preselected pattern. The edges of the sheets between the dies are pressured while exposed to a heating source in a direction generally perpendicular to the exposed edges, and the exposed edges of the stack or portions thereof are heate _i& . a temperature and for a period of time sufficient to bond the sheets together as desired. It has been discovered that the width of the seal or seam formed is a function of the temperature applied, the duration of heating and the area of the heat-sealable material under pressure and is substantially greater than a sheet thickness as obtained herein. Further, the seal structure between heat sealed sheets includes a diffuse transition region between the unsealed and sealed zones, with the sealed zone having substantially no unsealed zones of^"* discontinuity therein.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the nature and objects of the present invention will be apparent from the Detailed Description taken in conjunction with the accompanying Drawings in whic s

FIGURE 1 is an exploded perspective view of a stack of heat-sealable and nonheat-sealable sheets;

FIGURE 2 is a side cross-sectional view of the- stack illustrated in FIGURE 1 edge trimmed and pressed between two dies;

FIGURES 3a and 3b are cross-sectional views of two seals produced by the method of the present invention;

FIGURE 4 is an exploded perspective view of a folded sheet having one heat-sealable side and one nonheat-sealable side folded to form a stack; ._{Λ^:ή m}

FIGURE 5 is a perspective view of a compressed stack formed by pressing an alternating stack between two dies; FIGURE 6 is a perspective view of the stack illustrated in FIGURE 5 trimmed to correcpond to the perimeter of the dies;

FIGURE 7 is an exploded perspective of a group of sheets with one heat-sealable side and one ^•' nonheat-sealable side arranged so as to allow production of a multiplicity of gusseted envelopes;

FIGURE 8 is a perspective view of a gusseted envelope made in accordance with the method of the present invention; FIGURE 9 is a perspective view of a die suitable for use in practice of the present invention;

FIGURE 10 is a side view of a mandrel wound with film; FIGURE 11 is a perspective view of a form in a preselected shape for cutting a wound film clamped therein;

FIGURE 12 is a side view of a cut stack of films clamped in an edge clamping and perpendicular heating appratus; and

FIGURE 13 is a pictorial characterization of a seal formed in accordance with the present invention and a seal formed in accordance with prior art.

>

DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, a method for making multiple seals or for the multiple sealing of grouped layers of a heat-sealable material in-a single heating cycle is provided to achieve a ^"plurality of sheets bonded together, or a multiplicity of grouped sheets bonded together, or a multiplicity of envelopes. The present invention is suitable for bonding together films or sheets of heat-sealable material. These films or sheets may be laminates combining a heat-sealable film with one or more other heat-sealable films, nonheat-sealable films, aluminum foil, papers or other substrates. A film, as defined by the Modern Plastics Encyclopedia, McGraw-Hill, Inc., 1977, is a flat section of thermoplastic resin or a regenerated ".?~^* cellulosic material which is very thin in relation to its length and breadth and has a nominal thickness not greater than 0.25 mm. These same materials in similar configurations but greater thickness are classified as sheets. Films and sheets may be used alone, in combination with another thermoplastic through coating or coextrusion, or as a laminate in conjunction with paper, aluminum foil or other film. As used therein, "sheet" or "sheets" shall mean a film or sheet of thermoplastic material, or laminates thereof. The method of this invention is illustrated schematically in FIGURES 1 and 2, and 4- 7; while exemplary welded products formed by this method are illustrated in FIGURES 3a, 3b and 8.

FIGURES 1 and 2 schematically illustrate one embodiment of the method of this invention wherein separate sheets are bound together at their edges. FIGURE 1 is an exploded perspective view of a stack 12 of heat-sealable sheets 14 with nonheat-sealable layer 16 interposed at predetermined locations (illustrated in FIGURE 1 as between paired sheets 14). FIGURE 2 is a side cross-sectional view which shows the stack 12 of FIGURE 1 which is edge trimmed and resting upon platen or die 18 and pressed against platen 18 by compression means 20 so that the edge portion 22 of sheets 14 and layers 16 in stack 12 are held in intimate contact and are exposed to heat source 24. Heat is then applied from a heat source 24 at a temperature generally uniform across the thickness of stack 12 and for a period of time and at an angle effective to bond the edge portion 22 of the heat-sealable sheets 14 together.

Preferably the edge portion 22 of the sheets__*λ_4 and layers 16 in the stack lie in a common plane. This may be accomplished either by aligning the edges of the sheets 14 and layers 16 in registry upon stacking, or alternatively by compressing the stack 12 and trimming away the excess material extending beyond platen 18 and compression means 20. In one embodiment of the present invention, the edge of the stack 12 is generally coincident with the edges of the platen 18 and the compression means 20. This assures that the edge portion of heat-sealable sheets 14 and nonheat-sealable layers 16 are held in intimate contact with each other to prevent thickening, wrinkling or shrinking. If the edges of the sheets 14 extend beyond the compression means 20 and platen 18, a seal may still be formed; however, the seal generally lacks uniformity because the sheets extending beyond the platen 18 and compression means 20 tend to expand and separate from one another and thus do not readily melt into each other when heated. In addition, if the heat-sealable sheets 14 are positioned so that the edge portions 22 are not held firmly together, the sheets 14 tend to shrink away from the heat source when heat is applied, causing a thickened and non-uniform seal.

The heat is applied generally perpendicular to the edge portions 22 of the stack 12 and at a temperature greater than the tacking point of the material comprising the heat-sealable sheets 14 and less than the melting point or decomposition temperature of the nonheat-sealable material 16. It has been discovered that transient temperatures, when the heat is supplied by hot air well in excess of the normal decomposition range of the nonheat-sealable-⁴-*¹ layers 16, can be tolerated because the compressed sheets 14 and layers 16 act as a block heat sink and the edges are prevented from shriveling away from the heat under the pressure applied. This results in only the very extreme edges of the sheets 14 and layers 16 being affected by overheating without adverse effect to the inner seal strength. This behavior is in sharp contrast to the experience with conventional heat sealing methods wherein the inner edge of the seals and the adjacent film may be severaly damaged by overheating.

At least sufficient pressure is applied to the stack edges during heating to prevent thickening of the seam. Thus, as the heat-sealable material melts, the pressure forces edge portion 22 of the sheets 14 to flow into each other to produce a uniform seam, while the pressure prevents the edge portion 22 of the sheets from thickening and shrinking away from the heat.

FIGURES 3a and 3b illustrate two possible seals made by the method of the present invention. FIGURE 3a illustrates the eight sheets 14 of FIGURES 1 and 2 which were bonded together at their edge portion 22 to form paired sheets 26. Dotted lines 28 indicate the area where sheets 14 are bonded together which is a substantially continuous bond of heat-sealable thermoplastic material. While FIGURE 3a shows only four paired sheets 26 formed from the eight sheets 14, illustrated in FIGURE 1, the number of sheets 14 which may be bonded together is not limited to any particular number but only by the size of the equipment available. Paired sheets 26 were formed by ^'"* the application of heat to the edge portion 22 of ⁴-'-' stack 12 illustrated in FIGURE 2. When heat is applied to the side of the stack 12, the edge portions 22 of heat-sealable sheets 14 melt into each other and form paired sheets 26. Interposed nonheat- sealable sheets 16 prevent the paired sheets from bonding to each other. Thus, a multiplicity of paired sheets 26 may be produced by increasing the height of the stack 12 by alternating paired heat- sealable sheets 14 between nonheat-sealable sheets 16.

^• FIGURE 3b illustrates two laminants 32 comprised of four heat-sealable sheets 14 bound at their edge portions 22. Laminants 32 are possible by interposing a nonheat-sealable layer between every i fourth heat-sealable sheet 14 in stack 12, rather than every second sheet. The nonheat-sealable layer prevents the bonding together of laminants 32 upon the application of heat but does not interfere with the bonding of the sheets 12 comprising laminants 32. It is apparent that by adjusting the location of the nonheat-sealable layers, the number of heat- sealable sheets bonded together may be varied.

The width of the heat seals produced by the present invention is dependent upon the temperature applied, the duration of heating, and the area of the. heat-sealable material under pressure, but is substantially greater than a film thickness. FIGURES 3a and 3b are, thus, not to scale wherein the length of seals 28 would be substantially greater than any one thickness of film. Where compression means 20 (FIGURE 2) is relatively narrow, for example 1/8 inch, the width of the heat seal is at least partially regulated by the width of the die over_t>^, moderate range of heating times and temperature because effective bonding occurs only in those areas where pressure is applied. Effective bonding may also be achieved under relatively prolonged heating in areas adjacent to the inside margin of the pressure ridges, away from the -- heat source, provided that the sheets of the stack are maintained in^"effectively the same alignment a's that provided by -the pressure ridges or platens.

However, film stack areas outside the outer margin of the.pressure ridge or platen and adjacent to the heat source will tend to thicken and exude heat-sealable material which in turn tends to bond the stack elements together to a degree dependent on the amount of heat applied and the method of trimming used. If a hot wire or hot die is used for trimming operations, this undesireable bonding and swelling

URJ^^ fa -^ ^I tends to increase greatly. The normally nonheat- sealable layers may also thicken and cause additional bonding between stack elements. Thus, it is generally necessary, when fusion cutting methods are used, to remove those portions of the outer periphery of the seal which are thickened in order to separate the stack elements once the cutting or trijnming is complete.

When wide dies are used, for example 3/8 inch, , the width of the heat seals produced by the present invention⁴ is virtually independent of the width of the sheet area under compression by the die, but the seal width varies primarily as a function of the applied temperature and duration of heating. If a relatively short heating period is utilized, the width of the seal may be relatively independent of the width of the die. Thus, bonded sheets having^" various seal or seam widths may be produced with the same apparatus. Typically, strong heat seals of 3-6 mm width may be produced with a dwell time of 1-6 minutes with the seal width varying approximately in proportion to the .. square root of the heating time at a given edge temperature. The seal strength tends to vary across the width of the seal, reaching a maximum strength at the outer edge and this strength gradient endows the seal with the desirable ability to yield partially under relievable stress without actually rupturing. In another aspect, the present invention may be used to produce a multiplicity of envelopes in a single heating step. The process is essentially the same as described above with minor variations. FIGURE 4 shows an exploded perspective view of a sheet 34 comprised of one side 36 of heat-sealable material (hereinafter "heat-sealable side") and the other side 38 comprised of nonheat-sealable material (hereinafter "nonheat-sealable side"). As 5 , illustrated, the sheet 34 is folded such that the heat-sealable side 36 in the first fold section 40 lies against the heat-sealable side 36 in the second fold section 42. The nonheat-sealable side 38 of the second fold 42 opposite heat-sealable side 36 makes

10 contact with nonheat-sealable side 38 of the sheet in the third fold section 44. Thus, by utilizing one sheet, it is possible to obtain paired heat-sealable sides of the sheet separated by paired nonheat- sealable sides of the sheet, thereby creating the

15 same effect as the stacked layers 16 and sheets 14 illustrated in FIGURE 1. By using a sheet 34 with a heat-sealable side 36 and a nonheat-sealable side^* 38, it is possible to eliminate the need for inserting the separate nonheat-sealable layers 16 between the

20 heat-sealable sheets 14 as discussed above in reference to FIGURES 1 and 2.

The folded sheet or sheets (one sheet may be folded-on top of a previous folded sheet) are pressed as shown in FIGURE 5 between two dies 46 of desire,d

25 shape. (Bottom die not shown.) Preferably, the folded sheet 34 extends beyond the edges of the dies 46. The folded sheet 34 is then trimmed to approximate the perimeter defined by dies 46 as illustrated in FIGURE 6. This trimming step creates

30. a trimmed stack 48 of individual sheets from the folded sections of sheet 34. Heat is then applied to the stack by heat source 50 at a temperature and for a duration sufficient to form the desired bond between the heat-sealable sides 36 of the sheets in trimmed stack 48. FIGURE 6 illustrates two substantially circular dies 46 with a small rectangular protrusion 52. An envelope may be formed 5 by applying heat to all sides of the trimmed stack 48 with the exception of side 54. The result is a multiplicity of circular envelopes with a stem corresponding to rectangular protrusion 52 with an open portion in the stem corresponding to side 54 of 10 trimmed stack 48 through which a desired substance such as d gas or liquid may be injected. This stem may then later be sealed.

Although a circular envelope is illustrated, any desired shape may be obtained. A rectangular or _j_2 square envelope could be obtained by utilizing a die of a desired rectangular or square shape. The same method is followed to produce other shaped envelopes. The material is stacked and trimmed, and heat is then applied. For example, heat may be ₂_ applied to only three sides of a rectangular stack, thus producing a three sided envelope with one open side. The stacked envelopes are removed from the dies and separated into individual envelopes which are characterized by an outside layer of nonheat- ~_r sealable material and inside layers of heat-sealable materials. Thereafter, a product may be inserted into the package and the remaining side of the envelope sealed. Alternatively, all four sides of the stack could be sealed. This procedure produces _Q completely sealed envelopes, which may thereafter be cut in half to produce two stacks of envelopes sealed on three sides with an open side for insertion of a product or item. Those skilled in the art will recognize that many different types of folding techniques can be utilized in accordance with the present invention, such as illustrated in FIGURE 7 which is an exploded perspective view of a stack formed by another^' fold method. The sheets in FIGURE 7 are characterized by a heat-sealable side 60 and a nonheat-sealable side 62 and are stacked in such a manner as to achieve the desired shape of an envelope or container. FIGURE 7 illustrates a method of stacking sheets so as to produce a multiplicity of gusseted bags in a single heating operation by using multiple sections 64 to form a stack 66. Sections 64 are comprised of a flat sheet 68 of laminant with heat-sealable side 60 and nonheat-sealable side 62. The second portion of section 64 comprises two folded sheets 70 with ._Λii-wl heat-sealable side 60 and nonheat-sealable side 62. These folded sheets 70 are folded in such a manner that the nonheat-sealable side 62 is folded onto itself. Folded sheets 70 are positioned on sheet 68 such that the edges of both sheets 70 opposite the fold line are adjacent to opposite sides of sheet 68. Over these two folded sheets 70 is a third sheet 72 whose heat-sealable side 60 contacts one of the folded heat-sealable sides 60 of each of the folded sheets 70. Nonheat-sealable side 62 of the sheet 74 is oriented so that it contacts nonheat-sealable side 62 of sheet 72 which begins the next succeeding section 64 of stack 66. By sealing sides A, B and C of the stack 66, a multiplicity of gusseted bags of the type illustrated in FIGURE 8 may be produced.

FIGURE 8 illustrates a gusseted bag 76 formed when the stack illustrated in FIGURE 7 is pressed between two platens and heated on three sides, two of which sides are coincident with the edges of the folded sheets 70 opposite the fold line. The result is a gusseted bag 76 with open end 78 (shown in phantom) through which products may be inserted and the bag later sealed.

Those skilled in the art will appreciate that the stack 66 illustrated in FIGURE 7 may be assembled such that its edges extend beyond the dies used for heat sealing. It will be appreciated that trimming of the stack to the die configuration may be done either before or after the stack is placed between the pressure dies. The stack is trimmed prior to heating in accordance with a method of the present invention. Trimming the stack after placing the

^'-*^■ stack between the dies is beneficial since alignment of the edges of the folded sheets 70 with the edges of sheets 68 and 72 prior to compressing the stack is not now necessary, because trimming of the stack achieves that result and thus allows for the saving of labor.

FIGURE 9 illustrates a platen or die 46. suitable for use in the present method. Die 46 is illustrated as substantially circular with a protruding rectangular portion 52 although it may be of any desired shape. Die 46 is illustrated in use in FIGURES 5 and 6. The die 46 has a frame St) and preferably a raised ridge 82 at the periphery. The ridge 82 of the die 46 may be made of a material having low thermal conductivity. The frame 80 may _ also be constructed of a material of low thermal conductivity. Low thermal conductivity or insulating material for the ridge 82 of the die 46 contacting the stack of sheets is desirable to form uniform seals throughout the stack. A die made of high thermal conductivity material is not desirable because heat applied to the side of the stack and the die would tend to heat the entire frame of the die, thereby increasing the heat transferred to the sheets adjacent to the dies, tending to make wider seals for sheets closer to dies 46 than those sheets in the center of the stack. In the extreme case, a die having high thermal conductivity would tend to laminate the sheets lying close to the die together over their entire area rather than merely bond them together at the edge with a seam, as a result of the heat transferred over the entire area of the sheets from the heated frame.

Ridge 82 on the die is provided in order to —— concentrate the pressure applied to the stack to the edges of the stack. This pressure assures close contact of the area to be sealed and has the advantage of preventing the edges of the stack from swelling and shrinking away from the heat source as the heat-sealable sides soften on heating. The applied pressure also eliminates or minimizes the wrinkling which produces weakened or irregular seals. A flat, ridgeless die or platen would undesirably distribute the pressure over the entire area of the stack with possible seal defects from the expansion of heated air and shrinkage of the stack edges away from the heat source as the heat-sealable layers soften or melt. As illustrated in FIGURE 9, ridge 82 need not extend the full perimeter of frame 80, and may be emitted preferably on those portions 84 of the die 46 which correspond to areas of the stack which are not to be sealed. Ridge 82 is preferably constructed of material which is heat resistant, such as silicone rubber. Generally, harder materials of low thermal conductivity materials such as wood or phenolic board may be used for the construction of the dies,-frames or raised peripheries. The successful use of non- resilient ridge materials is believed to result from a number of factors, including a contribution from the resilient sheet materials and the latent tendency of the heat seal layer to thicken. These factors assist in} maintaining generally continuous pressure over the seal area to accommodate small irregularities in the die materials. Those skilled in the art will recognize that any material of low thermal conductivity which prevents the excessive dissipation and conduction of heat may be utilized.

The heating means may be provided by a variety" of means for directing heat in a direction generally toward the exposed edges to be sealed and may include radiation, friction, convection, forced air, or direct contact with a heated element. Thus, the exposed edge portions are heated in a uniform manner. As discussed above, varying seal widths may be produced with the same apparatus according to the method of the present invention. The width of the seal is a function of the applied temperature, the duration of heating and the area of the sheets under pressure. The width of the seals produced by a die such as die 46 with a wide raised ridge 82 is primarily a function of the temperature and duration of heating. Thus, unlike prior art, a variety of platens corresponding to the desired seal width is not necessary. Typically, strong heat seals of 3-9 mm width may be produced with a dwell time of 0.5 to 20 minutes and preferably of 1-9 minutes at a temperature greater than the tacking point of the heat-sealable sheets or portion thereof and less than the melting point or decomposition temperature of the nonheat-sealable layer. This temperature is readily determined by knowing the composition of the materials comprising the sheet or may be determined easily by experimentation. In generally all cases, however, the seal width is substantially greater than a film thickness.

Generally, the heat is applied uniformly across the thickness of a stack. However, applied heat may vary in temperature and/or time along the length of the edge, particularly in areas of curvature.

Concave areas generally require additional heat i ...n..p_»_»u•t due to increased heat dissipation in the stack, while convex shaped areas require less heat input.

Although the drawings have illustrated the present invention in the form of bonded sheets and circular envelopes, one skilled in the art will recognize that virtually any desired shape may be sealed by this method. For example, this method would be suitable for making a donut shaped envelope, balloon or inner tube type device by providing a donut shaped platen and cutting the stack to conform to the platen or by stacking separate sheets cut in the desired shape, such as a donut, and sealing the sheets in a predetermined manner in accordance with the method of the present invention.

EXAMPLES The following examples are presented to exemplify and illustrate the present invention to those of ordinary skill in the art and are not intended to limit the subject invention in any manner_*

EXAMPLE 1 Two hundred forty sheets of 0.025 mm film made of biaxially oriented polyamide resin coated with ethylene-vinyl acetate were arranged to form a stack containing 120 sheet pairs with the coated surfaces of ethylene-vinyl acetate facing one another. The stack was clamped between two platen assemblies made

I of a 13 c_π square of 2 cm thick plywood frame which was lined along the inner edges around the perimeter with a 6 mm thick and 12 mm wide strip of silicone rubber. Once the stack was pressed between the two platens, it was trimmed with a saber saw to conform to the perimeter of the platen assemblies. Hot air_^ from a 750 watt industrial hot air blower was passed over the trimmed sides of the stack and platen assemblies at a temperature of approximately 220^βC for a period of 1 minute. The sides of the stack were chilled with a damp cloth prior to removal from the platen assembly. The resulting seals were approximately 4 mm wide, without any substantial thickening of the seal and exhibited a yield strength of 870,000 dynes/cm. Heat seals made with the same film on a commercially heated platen sealer as known in the prior art were tested for comparison and found to have a yield strength of 420,000 dynes/cm.

EXAMPLE 2 The procedure of Example 1 was repeated using

1,600 sheets, except the height of the stack exceeded 40 mm. A circular saw rather than a saber saw was used for trimming. The resulting seals had the same properties as those in Example 1 and samples of seals taken from the top, middle and bottom of the stack were indistinguishable.

EXAMPLE 3 5 The procedure of Example 1 was again repeated

_.substituting 0.025 mm coated polyester film for the coated polyamide film with essentially the same results.

EXAMPLE 4 0 The procedure of Example 2 was repeated except that the heating time was increased from 1 to 2 minutes. Essentially the same results were obtained except that the resulting seal was approximately 6 mm, representing an increase in the width of the seal 5 of about 2 mm.

EXAMPLE 5 ....--__,

The procedure of Example 1 was repeated using a 46 cm diameter circular platen constructed similarly to the platen in Example 1. Thirty sheets of coated o film were used. The stack was heated to 170^βC for 6 minutes. The resulting envelopes are essentially identical to commercially produced envelopes of the same film and dimensions except that the heat seals of the commercial product possess greater variations 5 in width. This greater variation in the width of the commercial seal appears to be a result of inaccurate secondary cutting die alignment.

EXAMPLE 6 Two films of heat-seal coated or laminated 0 material are wound together on a flat mandrel to produce a stack of double film having heat-sealable surfaces together. As depicted in FIGURE 10, mandrel 110 has been used to form the stack of wound sheets 112. Sheets 112 comprise at least a pair of films having facing heat-sealable surfaces.

Referring now to FIGURE 11, there is seen a pictorial characterization of a stack of wound sheets 112 between shaped forms 114. Shaped forms 114 are of cardboard or a more heat resisting material, which is cut to the external form of a desired envelope or other assembly. Clamps 116 hold the stack of wound , sheets 112 between forms 114 for cutting. A conventional hot wire cutter can be used to cut away xcess material, conforming wound sheets 112 with shaped forms 114, as shown at 118. The cutting process is completed to form a stack of sheets having an external form corresponding with shaped form 114. Cutting of wound sheets 112 may also be done with a conventional steel rule die or with a heated die, if an automated facility is desired. By way of example, the power required to cut the film by fusion is about 2-3 watt seconds (Joules) per mil-inch of cut for polyester/polyethylene or polyaraid/polyethylene laminants.

As shown in FIGURE 12, shaped stack 125 is placed within frame 120 for sealing the exposed edges of stack 125. Frame 120 may be generally in the pattern of the desired shape, although any frame configuration may be used, as hereinafter discussed. Insulation 122 is placed within frame 120 and insulation 122 has an internal surface in the desired pattern shape and size. Heating foil 124 is placed along the shaped internal surface of i insulation 122. Heating foil 124 may be conveniently formed from stainless steel foil and is typically connected to a power supply effective to obtain stack edge temperatures within a range as discussed for

Examples 1 and 5, above.

Shaped stack 125 is placed within edge heating foil 124 and platen 126 is placed over shaped stack 125. Platen 126 includes raised shapes 128 which

.conform to the selected pattern. Raised shapes 128 apply pressure to the exposed edge portions of shaped stack 125 as hereinabove discussed. The desired pressure may be easily supplied by clamps 130 which may be any conventional clamp mechanisms, such as the i "C" - clamp configuration depicted in FIGURE 12, or spring-type clamps or other means for applying a moderate pressure, generally less than about 20 psi, to shaped stack 125. Edge heating foil 124 may supply a power output of about 100 watts per heated foot, depending on tlje thickness of the stack. By way of example, energy was supplied at the rate of 400 watts over a period of eight minutes when sealing a stack of material in balloon shape having a periphery of about 8 feet. The sealed stack, may have all of the edges sealed together by heat-sealable material. If so, the -> desired patterns may be separated by cutting or abrading the stacked edges to remove the fused material.

As hereinabove described, the seal between facing layers of thermoplastic material is formed relatively slowly, e.g., over 1-10 minutes, while pressure is being applied to the edges of the film stack over which the seal is being formed. The resulting seals are quite unexpected. In the first instance, the pressure applied by the platen, as hereinabove discussed, acts to maintain the facing sheets in intimate contact and to prevent shrinkage and other flow of the material while heating. The applied pressure further prevents thickening of the seal. As a result, the seal thus formed has a width which is substantially greater than the thickness of .a sheet of thermoplastic material. Typically, a seal width of 25-200 times the thickness of a thermoplastic sheet may be obtained.

It has also been noticed that seals formed in accordance with the above examples exhibit a strength gradient, wherein increasing force is required to separate the seal as the separation moves from the seal interior to the external edge of the seal. In FIGURE 13 there is shown a pictorial representation prepared from actual micro-photographs comparing a seal formed in accordance with the •....•_*__. present invention and a seal formed in accordance with teachings of the prior art. In a first difference, seal 90 according to the present invention has a diffuse transition zone between unsealed zone 92 and sealed zone 94. Thus, the contact between facing thermoplastic sheets forms over some distance beneath the sheets, which may be in the range of 0.1 to 10 mm. It is believed thai: the diffuse transition zone 96 effects the separating strength gradient and the absence of separation zones 108, representing discontinuous, sealing provides the increased seal strength noted herein. By contrast, prior art seal 98 exhibits a sharp dividing line 106 forming the transition between unsealed zone 102 and sealed zone 104.

Yet another unexpected result is the absence_.of separation zones, or islands, in sealed zone 94. In the prior art seal 98, sealed zone 104 has many separation zones 108, exhibiting discontinuous sealing between the facing thermoplastic sheets. Seal 98 in the prior art typically results where the seal is formed by the application of high temperatures along edge portions of the seal for relatively short periods of time and without the application of pressure along the seal periphery.

Claims

1. A seal between two sheets each having at least one side of thermoplastic material comprising: a transition zone of about 0.1 to 10 mm separating an unsealed zone between said thermoplastic material of said sheets and a sealed zone between said thermoplastic material of said sheets, said transition zone providing a relatively continuous transition from said unsealed zone to said sealed zone; and said sealed zone being substantially uninterrupted by areas where thermoplastic material of said sheets forming said sealed zone remains unsealed.