US3704975A - Thermoplastic film slitting - Google Patents

Abstract

Process and apparatus for the scoring of thermoplastic sheet material, and the scored product thereof, wherein the sheet material is advanced relative to a polymeric filament scoring means at a differential speed effecting scoring.

Description

United States Patent I [151 3,704,975 Bunting, Jr. et al. 1 Dec. 5, 1972 [54] THERMOPLASTIC FILM SLITTING [56] References Cited 72 Inventors: William w. Bunting, 11., Wilming- UNITED STATES PATENTS ton, Del.; Robert E. Buskirk, Pennsvme, NJ; Jo n P og e t 2,623,586 12/1952 Volpl ..83/l7l M. Mahla, of Wi mi gto ll 1 Newman 3,504,067 3/1970 Trecek ..425/305 [73] Asslgnee P Nemm's'and 1,246,411 11/1917 Fontaine ..42s/s24 pany, W1lmmgton, Del. [22] Filed: April 7, 1971 Primary Exaniiner-H. A., Kilby, Jr. [211 App! NM 131,931 Att0rneyHarry J. McCauley 52 U.S. Cl. ..42s/30s, 83/6, 83/171, I [57] ABSTRACT 425/324, 425/385 Process and apparatus for the scoring of thermoplastic [51] Int. Cl. ..B29h 3/00 heet material, and the scored product thereof, [58] held of Search 30211305 3241 253319 1 wherein the sheet material is advanced relative to a polymeric filament scaring means at a differential speed effecting scoring.

5 Claims, 13 Drawing Figures PATENTEB 5 I973 SHEET 1 [IF 3 W WMMMM m,

PATENTED DEC 5 I973 SHEET 3 OF 3 r v 3 MM MZM g m a wmw EJE Y 3 1 THERMOPLASTIC FILM SLITTING BRIEF SUMMARY OF THE INVENTION Generally, this invention comprises the scored product and the process and apparatus for scoring thermoplastic sheet material comprising advancing the sheet material under tension around a curved surface to form a wrap angle, passing a tensioned polymeric yarn at a lesser wrap angle in contact with the sheet material and advancing the sheet material at a differential speed with respect to the yarnpreselected to score the sheet material.

DRAWINGS scored, shown in association with tear strips visually locating the score lines, 7

FIGS. 5A and 5B are schematic showings of a design of soda cracker package provided with a continuous score line according to this invention, and

FIGS. '6A and 6B are, respectively, schematic showings of a cigarette package and a cigar wrapping provided with continuous score lines according to this invention.

DETAILED DESCRIPTION The use of high-strength thermoplastic product packaging films has created a need for easy-opening facilities, such as tear strips or the like. However, these have not been entirely satisfactory, sometimes breaking under tension and at other times tearing in an undesired direction, so that the package contents spill or break.

This invention provides a solution to the problemby prescoring packaging film. If the score line is preselected to retain a relatively high percentage of the original film strength, packages of acceptable strength and other properties result but, at the same time, a film cleavage line is provided. in exact desired orientation with respect to the product package.

Referring to FIGS. 1, 2 and 3, there is shown a preferred embodiment of apparatus for scoring sheet material, such as packaging film, according to this invention.

Here a supply roll 10 of sheet stock 9 to be scored is mounted on an unwind spindle 11 and the sheet threaded through a tensioning roll set 12a, 12, 12b to a small diameter roller 15, where its course is generally reversed with arelatively large wrap angle, the supplementary angle of which is denoted min FIG. 3.

Scoring, in this embodiment, is conducted on the outside surface of the sheet material by a running polymeric monofilament 18 which is passed under ten sion in contact with the sheet 9 (in this case generally counter thereto, although concurrent operation is equally practicable) with the monofilament course controlled by yarn guides 20, the supply and take-up apparatus for the monofilament being omitted from the showing. The monofilament wrap angle around roller 15 is much smaller than the sheet material wrap angle, and the supplementary angle therefor is denoted B in FIG. 3.

' Sheet material 9 carrying score line 23 is then threaded through a tensioning roll set 25a, 25b, 25, 25c and thence to a power-rotated product reception roll 26. A weighted spring-biased roller 28 carried on a swing arm 29 pivoted on pin 30 bears against the score line 23 in the course of product wind-up, and the roll pressing action eliminates bead effects which can result from the scoring.

The depth of the score 23 obtained is a function of monofilament l8 denier, running speed, angle of wrap about roller 15 andthe monofilament tension for a given sheet material 9, as well as the sheet thickness, speed and its angle of wrap aroundroller 15.

Moreover, it has been found that scoring can be effected as sheet is being wound on a roll, such as take-up roll 26, FIG. 1, in which case monofilament 18 is trained over the surface of the sheet as the sheet is taken up on the roll. I

In addition, while this invention is effective'in scoring thermoplastic sheet material per se, the scoring of thermoplastic coatings such as a polyvinylidene chloride (saranlvapor barrier and protective coating overlaid on a cellophane base establishes a. score line along which the cellophane tears preferentially, thus obtaining the benefits-of scoringwithout actually scoring the base material. A tear-effective score is obtained even though the thermoplastic is coated on both sides of the celloohane base, so that the moisture barrier properties of at least one of the overlaid coatings thenremains intact.

Individual score cross-section specimens were appraised for both profile and depth by two methods: (I) potting in a plastic material, polishing and then examining microscopically and (2) wrapping a scored film over a sharp edge, such as a razor blade, with the specimen surface aligned with the sharp edge. Then the background edge presented a light-reflective surface in line with the score, which permitted measurements of score depths with the same accuracy as in method (1) but with much less labor and expense. Method (2) is accordingly preferred.

Scoring with a running monofilament contacting a sheet material surface under relatively high tension and high relative speed can generate considerable heat. X- ray analysis of scored specimens of certain films, particularly polyesters, revealed that polymer orientation at the score was considerably different from that of the surrounding film, indicating that high temperatures were generated in the score region causing changes in both the orientation and crystallinity of the film. It is known that degradation and doe-orientation has an effect on tensile strength and film tearability. Thus, teara bility appears to be not a function of score depth solely,

but has to take account of accompanying localized V degradation of the film also. This phenomenon is particularly evidentas regards Example 1 of the following Table l reporting tests on polyester films, where the measured tensile strength is seen to be appreciably lower than that to be expected for the remaining thickness underlying the score. It was noted that, as film speed increased, the loss in tensile strength approximated more closely that attributed to the depth of score.

All tensile tests hereinafter reported were made on an Instron test machine using-l inch wide X 3 inches long specimens and a jaw travel speed of 2 inches/min. An average of five specimens were used in each test. Care must be employed in specimen alignment, because a partially severed sample which is misaligned in the test apparatus separates partially due to tearing and not fully in tension, thereby giving lower readings. Tensile test data obtained is somewhat scattered, due in part to the above causes but also to variations in yarn and film thickness, which cause variations in the depth of score. Also, it has been found that scatter in tensile test data is reduced proportionately with score depth.

Scoring of 0.0005 inch gage polyester film showed a bead adhering to the score edge on the film surface. This appears to be film material plowed out of the score and left attached to the upper edge of the score. If the film is wound in a roll without regard for this bead, a corresponding bead is formed on'the package. The film unwound from the roll will be stretched at this location and a dimple effect appears along the score line, giving poor package appearance in the end use.

The use of pressing roll 28, FIG. 1, bearing down on score line 23 presses loose material, or loosely attached material, at the bead edge back into the score line. In this manner a film roll can be wound without a bead and film can be controllably scored to a preselected depth without dimpling due to localized stretching.

There is some indication that a scored film has a tendency to fold upon itself as it is wound on a roll, buckling at the score. This buckling can itself cause a bead; however, pressing roll 28 is completely effective in preventing folding action and thus gives a flat wound roll. If scored film is rewound, it is desirable to employ a pressing roll such as roll 28 to re-press the sheet material along the score line as the material is taken up on its new roll.

With some polymeric sheet material, notably polyethylene, it has been observed that substantially all material is carried away from the score line by the running monofilament, and therefore no bead problems are encountered with these films.

Scoring action was studied by the use of a trilobal (i.e., three leaf clover-like cross-section) monofilament yarn in the scoring of polyester film. It was found that the trilobal yarn rotated as it passed over the film surface, essentially rifiing the bottom of the score. Since the trilobal yarn has no more inherent twist than a round monofilament yarn, it is believed that the latter also rotates during scoring. This could explain bead formation in films such as polyesters, in that the monofilament possibly lifts out material from the bottom of the score and deposits it on the film surface adjacent the score line. That is, there is a wiping action of the film upon the rotating monofilament which reattaches removed material at the film surface.

The examples reported in the following Tables l-llI, inclusive, all employed a polyamide monofilament yarn scoring means running counter to the films to be scored. A stationary round cross-section sheet back-up support bar was utilized as a substitute for roller 15, FIG. 1.

The examples of Table I show the effect of film speed on tensile strength and score depth while holding all other process variables constant.

Thus, as the film speed is increased, the depth of score decreases. This shows that film speed is a controlling factor and can be used to control score depth under a given set of processing conditions.

Examples 1, 2 and 3 utilized k mil polyester films coated on both sides with polyvinylidene chloride coating to reduce moisture permeability and enhance sealability. Since the score is made from only one side, the full benefits of the coating on the other side remain.

The tensile strength tests reveal that loss in tensile strength is not exactly in proportion to score depth, especially for Example 1 where a relatively deep score was made in the k mil film.

The film speeds used in Examples 1, 2 and 3 are relatively low for film finishing operations. However, they are typical of film speeds used in packaging. Thus, film can be scored at relatively low film speeds if the scoring operation has to accommodate packaging machinery film speeds in the range of 50-500 ft./min.

One purpose of the tests of Table I was to verify that scoring and score depth control could be achieved at relatively low film speeds.

TABLE I Ex. 1 Ex. 2 I Ex. 3 Control Film Polyes Polye Polye polyester Film 0.0005 0.0005 0.0005 0.0005 in Thickness in. in. in. Film Speed 84 295 500 ft/min ft/min ft/min Bar Material Steel Steel Steel Bar Diameter 114 1/4 1/4 in. in. in. in. Yarn Semi Semi Semi dull dull dull nylon nylon nylon monomonomonofila fila fila ment ment merit Yarn Denier 15 15 15 Yarn Speed 3500 3500 350 yd/min yd/min yd/min Yarn Tension 25-30 25-30 25-30 g. g. Alpha 59 68 59 68 59 68 Beta 118 118 118 Tensile 0.36 lb 7.74 lb 8.90 lb 13.52 lb Strength Strength as 3 57 66 Per Cent of Control Sample Score Depth 0.00041 0.00025 0.00022 0.0 in.

in. in. in. Per Cent 82 50 44 Score Depth Film manufacturing plants operate at speeds of 1000 ft./min or higher. Accordingly, Examples 4 and 5, reported in Table II, demonstrate that, by adjusting variables such as bar diameter, yarn denier, yarn speed and angles of wrap over those of Table I, scoring can be achieved at relatively high film speeds.

Thus, Example 4 shows that scoring can be effected to the point of complete severance of the film at a film speed of 750 ft./min. However, by simply increasing the film speed to 1000 ftJmin. while retaining all other conditions unchanged, the score depth was reduced to a level of only 36 percent.

Decreasing the bar diameter from one-fourth inch in Examples 1-3 to one-sixteenth inch in Examples 4 and 5 essentially increases the yarn force per unit area of scoring which, in turn, permits scoring at higher speeds. Also, increasing the yarn running speed enables scoring athigher film speeds.

It will be noted that yarn denier was increased in Table 11 over Table I. Using larger deniers, more material has to be removed for a given score depth, so that, if the yarn denieris reduced under the scoring conditions of Table II, all other variables remaining unchanged, it is practicable to further increase film speed to achieve a given score depth. In addition, it is practicable to increase the yarn tension above the 30 gram level to bring more cutting force to bear on the film, which could permit increasing the permissible film speed for a given score depth.

TABLE II Example 4 Example 5 Film Polyester Polyester Film Thickness 0.0005 in. 0.0005 in. FilmSpeed 750 ft/min 1000 ft/min Bar Material Steel Steel Bar Diameter 1/16 in. 1/16 in. Yarn Semidull Semidull nylon nylon monofilament monofilament Yarn Denier 30. 30 Yarn Speed 4500 yd/min 4500 yd/min Yarn Tension 24-30 g. 25-30 g. Alpha. 75 75 Beta 90 90 Score Depth 0.0005 in. 0.00018 in. Per Cent Score Depth 100 .36

A comparison of conditions and results obtained in the scoring of polypropylene and polyethylene films is presented in Table III infra.

In Examples 6-8, inclusive, a very shallow angle of wrap of the monofilamentabout the support bar was employed to avoid complete severance of the film. For these films particularly, but for polyester films also, appropriate preselection of angle B efiects a control of score depth.

The tests of Table III confirm that scoring of polypropylene and polyethylene films is more readily accomplished than for polyester films,-and this is to be expected, because of the lower melting temperatures of the former.

In addition, the yarn speed was reduced to 2000 yds./min. for Examples 6 and 8 to achieve the reported score depths.

Comparing Example 7 with Example 6, an increase in film speed by a factor of almost three, all other variables except running monofilament speed being held constant, showed again that, as film speed increases, the depth of score decreases.

The scoring tests on polypropylene and polyethylene revealed no difficulties in the control of score depth by selective adjustment of the available parameters, and scoring of sheet materials at speeds in excess of at least 1000 ft./min., matching film plant finishing speeds, is perfectly feasible.

monofilament Yarn Denier 15 15 v 15 Yarn Speed 2000 yd/min 3500 yd/min 2000 yd/min Yarn Tension -30 g. 25-30 3. 25-30 g. Alpha 59 68 59 68 59 68 Beta 160 170 175 Score Depth 0.0007 in. 0.00036 in. 0.00078 in. Per Cent 70 36 65 Score Depth Tests indicate that, under a given set of filmoperating conditions and a given set of processing conditions for scoring, the preferred control of score depth will usually be by appropriate preselection of the angle B V comprise advancing sheet material at a velocity in the range of about 50.ft./min. to about 1000 ft./min. in contact with a running polymeric filament scoring means having a thickness in the range of about 1.7-7.0 mils drawn past the sheet material at a relative velocity with respect to the sheet material such as to remove a preselected score depth of material from the sheet material.

The term filament scoring means as employed herein and in the claims includes monofilaments of both round and multi-lobed cross-sections as well as yarns made by twisting together a plurality of monofilaments. Polyamides are especially preferred because of their high strength; however, a wide varietyrof other polymers can be employed, depending upon the particular requirements.

Referring to FIGS. 4A-4F', inclusive, articles packaged with scored films can employ distinctively colored tear strips 33in a variety of different positions relative to the scores 35 (or 35 cut in films 34.

Thus, where the tear strip function is exclusively to denote visually the locationof the tearable score lines, tear strips 33 are conveniently applied on the outside of the package adjacent the score line (FIG. 4A), or overlying the score line (FIG. 4C), or lying between a pair of score lines (FIG. 4E).

On the other hand, if the tear strip is required to assist in package opening, in the manner now practiced in the art, tear strips 33 are applied on the inside surface of the packaging film adjacent the product wrapped, so that the location can. be adjacent the score line (FIG. 4B), underlying the score line (FIG. 4D) or between a pair of score lines 35, 35', but on theinside of the package structure (FIG. 4F).

Sometimes the nature of the product itself dictates the preferred location of the package score line. Thus, for a double cracker package 37, such as that shown in FIGS. 5A and 5B, the score line 35 is ideally located overlying the breakgrooves of the crackers themselves. Score line 35 can extend around three faces of the package only, if opening legendry such as the advisory word press is printed on the package ends where compression readily breaks the package open along the score line. However, ready opening from both sides is preferred, and a continuous 360 score around the entire package cross-section is the best design.

v The simultaneous breaking of the cracker product with package opening is particularly advantageous, since the support afforded by adjacent crackers restricts severance to the exact cracker boundaries, without undesirable crumbling of the edges.

The package of FIGS. A and 5B is also especially attractive for the marketing of moist products, such as pet foods or the like, where quick product availability without contact with the hands is an advantage.

Some products, such as cigarettes or cigars, can be damaged by excessive pressure application in the opening and an ideal package in these cases embodies a pull tab 40 left as an integral extension of the package 41 enclosing the cigarette pack (FIG. 6A). Then, merely pulling the tab in prolongation with the package readily breaks open the 360 score line 42, which, in this in stance, is located flush with the forward shoulder of the package, although it can be disposed farther back if vancing said sheet material at a velocity in the range of about 50 ft./min. to about 1,000 ft./min., means drawing a polymeric filament scoring means having a thickness in the range of about 1.7-7.0 mils past said sheet material at a relative velocity with respect to said sheet materialin the range of about 1550 ft./min. to about 14,500 ft./min., means maintaining the relative position of said polymeric filament scoring means with respect to said sheet material such as to remove a preselected score depth of material from said sheet material, means collecting the scored product, and means smoothing upstanding material disposed along the edges of a sheet material score course into substantial planarity with the main body of said sheet material.

2. Apparatus for scoring sheet material according to claim 1 wherein said polymeric filament scoring means is a monofilament.

3. Apparatus for scoring sheet material according to claim 1 wherein said means maintaining the relative position of said polymeric filament scoring means with respect to said sheet material comprises a sheet material back-up support member formed to an arcuate profile in the region contacting said sheet material.

4. Apparatus for scoring sheet material according to claim 1 wherein said polymeric filament scoring means is fabricated from polyamide.

5. Apparatus for scoring sheet material according to claim 1 wherein said polymeric filament scoring means is substantially circular in cross-section.

Claims (5)

1. Apparatus for scoring thermoplastic polymeric sheet material comprising, in combination, means advancing said sheet material at a velocity in the range of about 50 ft./min. to about 1,000 ft./min., means drawing a polymeric filament scoring means having a thickness in the range of about 1.7-7.0 mils past said sheet material at a relative velocity with respect to said sheet material in the range of about 1550 ft./min. to about 14,500 ft./min., means maintaining the relative position of said polymeric filament scoring means with respect to said sheet material such as to remove a preselected score depth of material from said sheet material, means collecting the scored product, and means smoothing upstanding materiAl disposed along the edges of a sheet material score course into substantial planarity with the main body of said sheet material.

2. Apparatus for scoring sheet material according to claim 1 wherein said polymeric filament scoring means is a monofilament.

3. Apparatus for scoring sheet material according to claim 1 wherein said means maintaining the relative position of said polymeric filament scoring means with respect to said sheet material comprises a sheet material back-up support member formed to an arcuate profile in the region contacting said sheet material.

4. Apparatus for scoring sheet material according to claim 1 wherein said polymeric filament scoring means is fabricated from polyamide.

5. Apparatus for scoring sheet material according to claim 1 wherein said polymeric filament scoring means is substantially circular in cross-section.

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