WO1996005046A1

WO1996005046A1 - Improved device for heat sealing film edges and methods relating thereto

Info

Publication number: WO1996005046A1
Application number: PCT/US1995/008951
Authority: WO
Inventors: Anthony Edward Umlah; Vaughn Carroll Galloway; Frederick Henry Fiesser; Robert Vincent Jeral
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1994-08-10
Filing date: 1995-08-01
Publication date: 1996-02-22
Anticipated expiration: 1997-02-10

Abstract

An improved heat seal roller (20a, 20b) design having a pair of convex heat sealing surfaces (40). The convex heat sealing surfaces cut the film (12) at the contact point (28) between the sealing surfaces and creates a bead seal upon the film above the contact point.

Description

TITLE

IMPROVED DEVICE FOR HEAT SEALING FTLM EDGES

AND METHODS RELATING THERETO

FIELD OF THE INVENTION

The present invention relates generally to packaging or similar-type machines which heat seal or weld films or film layers (or edges) together. More specifically, the present invention is directed to an improved heat seal roller design, having improved durability and accuracy.

BACKGROUND OF THE INVENTION Heat weldable or heat sealable films in general are well known. Typically, such films have broad softening points at temperatures above about 200°F, and when two or more such films are positioned together, heat and pressure softens (and sometimes partially or wholly melts) the films, causing them to fuse together. Broadly speaking, packaging machines are well known which heat weld plastic films together. An example of such a machine is discussed in U.S. 5,125,216 to Redaelli.

Film edges are often heat welded together by means of heat seal rollers. In a typical set-up, film edges move between counter-rotating heat seal rollers which provide heat and pressure against each side of the adjoining films or film edges.

Ideally, the heat seal rollers consistently- fovide uniform pressure and temperature against the film edges as they move between the rollers. Generally, this is accomplished by aligning the rotating heat sealing surfaces in a parallel configuration. Unfortunately over time, the positioning (particularly the axis of rotation) for one or both of the rollers can shift, even slightly, and this will often cause the heat seal surfaces to shift relative to each other. Oftentimes, this will detrimentally change the heat sealing performance of the rollers, and such misalignment can also cause uneven or premature wear. A need therefore exists in the art for a heat seal roller design which is durable and which can effectively accomodate relatively large changes in alignment over time. A secondary need exists in the art for greater accuracy in heat welding films, whereby the location of the weld is predictable and consistent. SUMMARY OF THE INVENTION The present invention is directed to an improved heat seal roller design. In the preferred embodiment, a pair of complimentary heat seal rollers counter-rotate in unison as film moves between the rollers. The preferred rollers have heat seal surfaces which have substantially identical, complementary convex configurations, whereby the outermost points of the convex heat seal surfaces are substantially in contact with one another.

In use, the film or film edges are moved between the counter-rotating heat seal rollers. The rollers are preferably biased together due to a spring, pneumatic pressure or the like, thereby causing the rollers to substantially contact at about the outermost point of the convex heat Seal surfaces. The film is generally cut at this contact point as the film moves between the heat seal rollers. Above and below this contact point, the films are heat sealed together. The film above the contact point forms the desired, durable bead seal, and the film below the contact point generally becomes scrap material.

Preferrably, the contact between the heat seal surfaces is merely a tiny point defined by the outermost point of each convex surface. Preferrably, the contact point defines a height and width in the range of about 100 to about 0.001 average film thicknesses, more preferably about 10 to about 0.1, yet more preferably about 5 to about 0.5 average film thicknesses.

An "average film thickness" as used herein, is intended to mean one half the thickness of the film(s) to be heat sealed between the rollers. For example, if a 1 mil film is heat sealed to a 3 mil film, then the "average film thickness" is intend to be 2 mils. Also, where four 1 mil films are heat sealed together, then the "average film thickness" is intended to mean 2 mils.

The contact point cuts the film as it moves through the heat seal rollers. Above the contact point, the films or fihn edges are heat sealed together, and this portion of the heat seal rollers is called the "heat seal region". The heat seal region preferrably defines an average distance between the two heat seal roller surfaces of preferably greater than about 2 average film thicknesses, and less than about 2.5 average film thicknesses, more preferably about 2.1 to about 2.4 average film thicknesses.

Preferably, the midpoint of the heat seal region lies above the contact point a distance no less than about 3 average film thicknesses and no greater than about 250 average film thicknesses (more preferrably about 7 to about 125, and yet more preferably about 15 to about 70 average film thicknesses). The heat seal region preferrably defines a height in the range of about 0.1 to about 500 average film thicknesses, more preferably about 1 to about 100 and yet more preferabably about 25 to about 75 average film thicknesses. Preferrably, the rollers rotate about axis of rotation which are substantially parallel to one another. However, if the axis of rotatioirbeeeme substantially non- parallel due to unwanted shifting in alignment, such as due to jarring, misuse or long term use, the convex configuration of the heat seal surfaces accomodates such misalignment, since the pivoting about the convex surface contact point generally is not detrimental to the performance of the heat seal rollers. Whereas conventional heat seal rollers diminish in performance and generally wear prematurely due to such misalignment, the roller design of the present invention is capable of accomodating such misalignment problems.

Preferably, the heat seal rollers of the present invention further comprise one or more flexible belts which move with (or drive) the rotational movement of the heat seal rollers. A separate belt can be used for each roller or one belt can accomodate both rollers. The belts are preferrably a soft, durable rubber like material, such as, lower modulous ethylene propylene diene rubber ("EPDM"), acrylo butyl styrene rubber ("ABS"), chloro sulfonated rubber or the like. For improved durability and resistance to adverse conditions (i.e., heat, friction, etc.), a belt grade fluoroelastomer is preferred, such as, VITON® brand fluoroelastomer available from E.I du Pont de Nemours and Company.

The belt or belts preferably ride along a grove or other guide mechanism and are preferrably driven by a drive mechnanism which is separate from the heat seal rollers. This is important, particularly where the heat sealing operation is frequently stopped and started. When the heat sealing operation is stopped, the heat seal rollers must quickly move apart away from the film; otherwise, the heat of the rollers upon the stationary filn would mek the film, thereby causing a flaw in the heat seal. However, once the heat sealing operation is stopped and the rollers moved apart, the film would otherwise be able to shift in the absence of the engagement between the belts and the film (the engagement between the belt and film can be very tight or quite loose, depending upon the coefficient of friction or otherwise the capability of the belt to maintain film alignment during heat seal roller operation).

By designing the belts to align and hold the film independant of the roller engagement of the film, the film is kept in alignment during stopping and starting. Otherwise, the film is likely to shift and a flaw in the heat seal is generally experienced. Such film shifting can cause unwanted failures along the weld line. Furthermore, environmental and costs concerns have intensified efforts to reduce scrap, and by more accurately controlling the location of the weld line, reduced scrap is possible. Whereas the roller's heat seal surface is generally a metal surface which typicaHy lacks effectiveness n sufficiently mamtainmg film alignment as it moves between the rollers, the belt provides a much more relieable engagement with the film to ensure predictable film alignment during machine operation. Hence, the belt(s) hold the film layers in place as they are heat sealed.

In yet another preferred embodiment, the belt not only firmly holds the film in place during heat welding, but also, the belt (or alternatively a separate belt) carries scrap film (film which lies below the heat weld line) away from the packaged article. In this embodiment, the heat welding operation involves a high pressure contact point between the heat seal rollers. At this high pressure contact point, the film is severed, leaving scrap to be taken away by the lower belt or belts. Preferrably, the belts move the severed scrap to a vacuum line which then removes the scrap and places it in a separate receptical for later disposal. Preferrably, this belt moves in cooperation with the movement of the rollers and film layers to provide a continuous process for removing scrap as the film is heat welded.

Scrap is preferrably accumulated on a reel by means of a take-up roller. The take-up roller preferrably comprises one or more protrusions which catch the dangling scrap strip and automatically causes the scrap strip to be reeled. The take up reel can be driven by any mechanism, but preferrably, a simple pneumatic motor is use, wherein the pneumatic motor is driven at a pneumatic pressure at the low end or below its design specification. At such low pressures, the motor has very little torque (which is generally inconsequential, since the reeling of the scrap generally requires very little torque) and therefore can easily accomodate changes in scrap rate without the need for complex mechanical clutches, speed control circuits or the like.

DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of the preferred heat seal roller mechanism of the present invention.

FIGURE 2 is a cut away perspective view along 2-2 of FIGURE 1. FIGURE 3 is a magnified detailed view of the heat sealing portion of the rollers which is encircled in FIGURE 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred heat seal mechanism of the present invention is shown generally at 10 in FIGURE 1. As shown in this FIGURE, two film edges 12 and 12a are aligned and are carried by friction engagement by lower belts 14a, 14b, 14c, and 14d and upper belts 16a and 16b. The belts assist in carrying the film edges between pinch rollers 18a and 18b.

The pinch rollers are biased toward one another, thereby providing friction engagement between the films and the upper belts. The film is engaged by the two upper belts, thereby providing reliable film alignment. Such film alignment is particularly advantageous, where the heat sealing operation is frequently stopped and started, because upon stopping the heat sealing operation, the heat seal rollers must be moved apart (otherwise the heat from the heat sealing surfaces would melt the film, thereby creating a heat seal flaw), and the films can shift in the absence of the film engagement between the upper belts.

As the film is aligned and moves past the pinch rollers, the film moves between heat seal rollers 20a and 20b. FIGURE 2 illustrates a cut away perspective view along line 2-2 in FIGURE 1. As can be seen by FIGURE 2, a product 22 is encompassed by a heat weldable film 24 and is moved along a conveyor belt 26.

The edges of the film have been brought around the product and aligned together as the film edges move between the heat seal rollers 20a and 20b. Upper belts 16a and 16b carry the film between the rollers. The heat seal rollers cut the film and the lower scrap portion is carried by lower belts 14a, 14b, 14c, and 14d. FIGURE 3 is a cut away magnified view of the heat sealing and cutting area of the heat seal rollers of FIGURE 2. As shown in FIGURE 3, upper belts 16a and 16b carry film edges 12 and 12a. The film edges are cut and heat sealed at the heat seal surfaces shown at 289a and 29b. The heat seal surfaces are convex and the outermost point of the heat seal surfaces tough at contact point 28. The heat seal rollers are heated above the heat sealing temperature of the weldable films 12 and 12a, such as, by electrical heating or the like. As the film passes between the heat seal rollers, the contact pomtbetween the rollers cuts the film layers. Film below the contact point becomes scrap, and film above the contact point is heat sealed (creating a bead seal) as shown at 40 in FIGURE 3. Preferrably, the contact between the heat seal surfaces is merely a tiny point defined by the outermost point of each convex surface. Preferrably, the contact point defines an height and width in the range of about 100 to about 0.001 average film thicknesses, more preferably about 10 to about 0.1, yet more preferably about 5 to about 0.5 average film thicknesses. An "average film thickness" as used herein, is intended to mean one half the thickness of the film(s) to be heat sealed between the rollers. For example, if a 1 mil film is heat sealed to a 3 mil film, then the "average film thickness" is intend to be 2 mils. Also, where four 1 mil films are heat sealed together, then the "average film thickness" is intended to mean 2 mils. The contact point 28 cuts the film as it moves through the heat seal rollers.

Above the contact point 40, the films or filrrr edges are heat-sealed together, and this portion of the heat seal rollers is called the "heat seal region". The heat seal region preferrably defines an average distance between the two heat seal roller surfaces of preferably greater than about 2 average film thicknesses, and less than about 2.5 average film thicknesses, more preferably about 2.1 to about 2.4 average film thicknesses.

Preferably, the midpoint of the heat seal region lies above the contact point a distance no less than about 3 average film thicknesses and no greater than about 250 average film thicknesses (more preferrably about 7 to about 125, and yet more preferably about 15 to about 70 average film thicknesses). The heat seal region preferrably defines a height in the range of about 0.1 to about 500 average film thicknesses, more preferably about 1 to about 100 and yet more preferabably about 25 to about 75 average film thicknesses.

The scrap film below the cut line is preferably moved by the lower belts into a vacuum line which pulls the scrap into the vaccuum line, sending the scrap to a waste receptical. Preferrably, the scrap is reeled on a take-up reel by a low torque motor capable of accomodating different scrap rates. A preferred such motor is a pneumatic motor operated at a sufficiently low pressure to enable different take-up speeds without the need for a mechanical clutch or pneumatic pressure adjustment.

Claims

WHAT IS CLAIMED IS:

1. A heat seal mechanism comprising: a pair of counter-rotating heat seal rollers, each having a convex heat seal surface, the heat seal rollers being biased together and being heated to a heat welding temperature appropriate for a film as the film is passed between the rollers, whereby the convex heat seal surfaces of the rollers provide a contact point between the rollers which cuts the film as it moves between the rollers and whereby the heat seal surface above the contact point heat seals the films together, thereby creating a bead seal.

2. A heat seal mechanism in accordance with Claim 1 wherein the contact point defines a height and a width in the range of about 100 to about 0.001 average film thicknesses, and a midpoint of a heat seal region above the contact point defines an average distance between the two heat seal roller surfaces of greater than about 2 average film thicknesses, and less than about 2.5 average film thicknesses, said midpoint lying above the contact point a distance no less than about 3 average film thicknesses and no greater than about 250 average film thicknesses and whereby the heat seal region defines a height in the range of about 0.1 to about 500 average film thicknesses.

3. A heat seal mechanism in accordance with Claim 2 further comprising a flexible belt which holds the film in alignment as it moves between the heat seal rollers.

4. A heat seal mechanism in accordance with Claim 1 whereby film below the contact point is severed as it moves past the contact point and belts move the severed film below to an appropriate waste receptical.

5. A heat seal mechanism in accordance with Claim 4 wherein the waste receptical comprises a take-up reel driven by a low torque motor.