Method and Apparatus for Heat Sealing a Plurality of Sets of Continuous Plastic Strips Together by Radio Frequency Energy

This invention relates to the plastic fabricating art, and more particularly to a unique radio frequency powered press and method for simultaneously processing a plurality of sets of continuous plastic strips to seal the strips of each set together and for forming a tear seal before indexing the sets forwardly and repeating these operations.

Radio frequency powered sealing apparatus has been proposed heretofore for heat sealing a single set of superimposed strips of plastic sheeting together including means for forming a tear seal contiguous to the heat seal to facilitate separation of a finished item from stock material. A great variety of finished articles of manufacture have been sealed together utilising this heat sealing technique but the equipment and methods employed heretofore have been restricted in various respects. For example, only a single item is commonly processed by prior equipment including means for indexing strip material past a heat sealing station. The U.S. Pat. No. 2,758,631 granted to Peterson et al on Apr. 14, 1956 discloses apparatus having these features. Techniques have also been proposed for forming plastic cushion covers using heat sealing equipment in which the bottom and top plastic covers are heat sealed to the lateral edges of a gusset strip. The gusset strip is formed into a ring of appropriate size and this is assembled along the interior of a sealing die ring, the edges of the gusset being overturned and suitably anchored in place along the exterior of this die. This complex and cumbersome assembly is then placed between the press platens and energized with radio frequency energy to seal the components together. Thereafter the entire assembly is removed and the plastic product is detached following which other plastic components are assembled to the fully detached die and inserted in the press for the next sealing operation.

Attempts have been made to increase the size of the platens to permit multiple items to be sealed simultaneously in side-by-side relation as disclosed for example, in the U.S. Pat. No. 4,043,858 to Dantowitz. However, problems are encountered unless the individual items are relatively small. This is because of difficulties encountered in obtaining uniform distribution of electrical energy and sealing effectiveness in larger sized platens. In consequence, a satisfactory seal may be obtained in one area but an unsatisfactory seal in another area which of course results in a commercially unacceptable product. The uneven heating can also cause malfunctioning of the equipment and sticking of the plastic to the die components which results in costly loss of production time and may require removal of the dies for servicing and refinishing. In larger die assemblies it is also much more difficult and costly to achieve and maintain critically essential precise parallelism between the die and the cooperating platen.

The foregoing and numerous other shortcomings, limitations and disadvantages of prior heat sealing methods and equipment are avoided by the present invention by means of which production output has been nearly doubled using smaller equipment and substantially less power. These results are achieved with a unique unitary die assembly having a double faced floating die mounted between and normally out of contact with an upper and a lower platen. This assembly is mounted on the bed of a typical C-frame press with the upper and lower platens connected to the terminals of a suitable radio frequency generator. A plurality of sets of superimposed continuous strips of plastic are suitably aligned and advanced crosswise of the upper and lower faces of the center die by suitable indexing equipment. Closure and energization of the die assembly provides a plurality of items of manufacture during each operating cycle of a few seconds duration. Blowers serve to direct a cooling medium over the die at a rate to maintain the die components below the melting temperature of the plastic to provide highly effective and satisfactory heat sealing without risk of the dies adhering to the plastic material. The dies on each face of the center platen may be designed to produce either single or multiple items and each preferably includes means for automatically ejecting the finished product from the die lightly attached to the strip stock by a weak tear seal.

Accordingly, it is a primary object of this invention to provide a unique radio frequency powered apparatus and a unique method for simultaneously heat sealing a plurality of sets of plastic strips together;

Another object of this invention is the provision of apparatus for automatically indexing a plurality of sets of continuous plastic strips attached to the opposite faces of a double faced die energized by radio frequency energy to seal the strips of each set together in unison along a pre-selected area of each set of strips;

Another object of the invention is the provision of a radio frequency powered heat sealing die assembly featuring a floating double faced intermediate platen located between a pair of platens for use in simultaneously sealing separate sets of plastic sheets together;

Another object of the invention is the provision of a unique heat sealing die assembly insertable as a unit in a power press designed to process separate sets of plastic sheets and including means for circulating a cooling medium in contact with the sealing die;

Another object of the invention is the provision of radio frequency powered heat sealing apparatus utilizing a double faced sealing die floatingly supported between upper and lower platens electrically isolated therefrom with each face of the sealing die equipped with sealing dies in side-by-side relation and cooperating to heat seal a plurality of components opposite each face of the die during each operating cycle.

These and other more specific objects will appear upon reading the following specification and claims and upon considering in connection therewith the attached drawing to which they relate.

Referring now to the drawing in which a preferred embodiment of the invention is illustrated:

FIG. 1 is an elevational diagrammatic view showing an illustrative embodiment of our improved heat sealing die in open position with two sets of continuous plastic strips threaded therepast between supply equipment and take up indexing equipment;

FIG. 2 is a horizontal cross sectional view taken along the broken line 2--2 on FIG. 1;

FIG. 3 is an elevational view on an enlarged scale of the die assembly per se with portions broken away and in cross section to show structural details;

FIG. 4 is a fragmentary cross sectional view showing details of the sealing die, the hole blanking dies and the resilient ejector plate;

FIG. 5 is a fragmentary cross sectional view on a substantially enlarged scale taken along line 5--5 on FIG. 2 showing details of the combined sealing and tear seal die; and

FIG. 6 is a perspective view of a typical plastic envelope formed by the illustrated sealing die and designed for mounting in a ring binder.

Referring more particularly to FIGS. 1 and 2, there is shown an illustrative embodiment of the invention sealing die assembly, designated generally 10, mounted in a conventional C-frame pneumatic press 11. This press is provided with a pneumatic cylinder 12 having a piston 13 provided with a header plate 14 suitably secured to the upper platen 17 of the die assembly as will be described more fully presently. The sealing die assembly 10 rests on the base or bed 15 of the press 11.

Die assembly 10 is shown on an enlarged scale in FIG. 3 and includes an upper platen 17, a lower platen 18, and an intermediate platen formed in two halves 19a and 19b suitably rigidly and detachably secured together by means not shown. The intermediate platen is floatingly supported on a plurality of non-conductive guide posts 20, 20, fixedly supported near the corners of the lower platen 18. Stops 21 secured to the upper ends of posts 20 limit the upward movement of the intermediate platen under the action of compression springs 23, the upper ends of which bear against non-conductive washers 24. The center platen is made in two halves 19a, 19b to simplify both initial manufacturing operations and servicing operations for self-apparent reasons.

Each of the two halves of the center platen 19a, 19b includes a combined heat seal and tear seal die 26 formed from good conductive material, such as copper or brass, and shaped lengthwise thereof in the configuration of the heat sealing operation to be performed. As herein shown by way of example, each of the dies 26 is generally rectangular in configuration and sized to form an envelope 30 open along one end edge 31 (FIG. 6). Die strips 26 are approximately one inch wide and their outer lateral edges are accurately shaped as is well known in this art with a heat sealing land 28 and a tear seal forming knife edge 29. The knife edge 29 lies in a plane spaced outwardly from the heat sealing land 28 and substantially cuts through the plastic stock when the die is closed against the stock. In accordance with conventional practice, the newly formed plastic items remain attached to the sheet stock as the sets of plastic sheets are indexed out of the sealing station but are readily separated from the stock in a hand separating operation.

As clearly appears from FIG. 2, the three platens are sized to accommodate at least a plurality of dies to make two or more items from each set of strips during each operating cycle of the sealing station. Thus as shown in that Figure there are two sets of die strips 26 on each face of the floating center platen and these are preferably arranged so that the adjacent parallel sides are either immediately adjacent one another or actually merged. For example, the thickness of the die strip is there shown as one and one half times the thickness of the remainder of these strips. The tear seal forming knife edge 29a is located intermediate the heat sealing lands 28a, 28a. Dies 26 are suitably fixed to their respective platens 19a, 19b as by mounting lugs 32. These lugs may be soldered or separably attached to die strips 26 and each includes a screw for detachably securing the lugs to one of the associated center platens.

A non-conductive ejector 35 is supported within each of die strips 26 on resilient rubber bosses 36 long enough to support the outer surface of the ejector in a plane outwardly of the plane of the heat sealing lands 28. The peripheries of the ejectors lie closely adjacent the sealing lands 28 and are effective as the die opens after a sealing operation to separate the plastic strips from the sealing lands.

Many items heat sealed by the die assembly 10 require one or more holes to receive loose leaf binder rings or the like. To meet this need each of the center platens is provided with blanking punches 38 having tubular outer ends sharpened with a cutting edge 39 to blank out discs of plastic. These discs pass inwardly along the tubular passages 40 which open through the sidewall of punches 38.

The adjacent surfaces of the upper and lower platens 17 and 18 are equipped with a layer of non-conductive material such as a sheet of Mylar or phenolic 42, 43 thereby electrically isolating all components of the floating center platen 19a, 19b from the upper and lower platens 17 and 18.

Referring now to FIGS. 1 and 2, the accessories employed in guiding a plurality of sets of continuous plastic strips 44, 45 past the opposite faces of the floating center platen will be described. The supply reels of the continuous plastic strips are not shown but will be understood as located to the left of FIG. 1. As best appears from FIG. 2, each of these sets of strips includes a narrow strip 44 and a slightly wider strip 45 and appropriate to make the transparent plastic envelope shown in FIG. 6. The strips of each set are channeled over suitably supported rollers 47, 48 into separate aligning channels of a suitable strip aligning device 50. Such aligners are well known and include separate passages for strips 44, 45. Each strip is accurately held in a particular position relative to the other by means of adjustable side guide plates 51, 52 lying in parallel vertical planes. The strips of each set then pass over rollers 53, 54 positioned to support the strips of each set in direct contact on a plane intermediate the sealing dies 26 and the adjacent surface of one of the upper and lower platens 17, 18. Guide rollers 53, 54 cooperate with and serve the same purpose as a companion set of rollers 55, 56 on the right hand side of the sealing station.

Thereafter the two sets of strips 44, 45 pass over other rollers 57, 58 and 59 which cooperate to bring both sets of strips into contact with one another at the entrance end of an automatic indexing mechanism 60. This indexing mechanism may be of any known construction and typically includes a power driven conveyor belt 61 mounted beneath a pair of cooperating clamps 62, 63 functioning alternately to press both sets of strips into contact with belt 61 to index the strips to the right. Thus clamp 62 moves downwardly to press both sets of strips against the conveyor belt and moves forwardly with belt 61 until the material has been indexed sufficiently to initiate the next heat sealing operation of die 10. Clamp 63 then clamps the material downwardly against the stopped conveyor belt as clamping mechanism 62 returns to its initial position shown in FIG. 1. The finished products issuing from the indexing device 60 are thereafter readily separated by hand from the stock material.

Another important accessory includes means for cooling die assembly 10 and particularly the floating center platen. This comprises a motor driven fan 70 having air distributing ducts 71 positioned to direct jets of cooling air over the die assembly from a position providing a minimum of interference with operation and servicing of the die assembly.

Die assembly 10 is energized from any suitable radio frequency generator 80 having one of its output leads 81 grounded and the other lead 82 connected to the upper platen 17, the lower platen 18 being grounded through the machine to complete the electrical circuit. An output of 750 to 1,000 kw suffices to form at least four 8.times.11 ring binder sized envelopes 30 during each heat sealing cycle. The die is closed in known manner by ram 13 operating at a pressure of about 80 psi. The press cycles about 15 to 20 strokes per minute to provide at least one finished envelope per second each having an open inlet 31 crosswise of its upper end. No heat sealing of this end occurs because the narrow strip 44 is fed through the heat sealing station with the edge forming inlet 31 spaced inwardly from the top edge of dies 26 as viewed in FIG. 2.

While the particular method and apparatus for heat sealing a plurality of sets of continuous plastic strips together by radio frequency energy herein shown and disclosed in detail is fully capable of attaining the objects and providing the advantages hereinbefore stated, it is to be understood that it is merely illustrative of the presently preferred embodiment of the invention and that no limitations are intended to the detail of construction or design herein shown other than as defined in the appended claims.