FR2467675A1 - Use of hot air jets to seal incipient cracks in packaging films - to suppress crack propagation from residual stress zones near weld junctions - Google Patents

Abstract

In process for wrapping a series of articles between two layers of wide thermoplastics film which are then welded along their borders and then between successive packs before being mechanically trimmed, the welded zones are subjected to hot air jets to fuse and seal any incipient splits induced alongside the weld zones, esp. at the corners where the transverse and longitudinal welds intersect. Esp. for use with 'SERIENPACKER' or 'PACKAUTOMATEN' machines.

Description

The present invention relates to methods and apparatus according to which successive objects, generally i'denti'ques ~ between them and of any shape, are one after the other tightly wrapped in a plastic film, preferably , but not necessarily heat shrinkable.

An example of this type of machine is that sold by the firm BECK + CO under the name "SERIENPACKER" or "PACKAUTOMATEN".

 In this machine, the objects to be packaged, such as for example books, are led between two parallel strips of very thin heat-shrinkable films, in simultaneous unwinding, and of a width greater than that of these objects.

In the anterior zone of the machine, the two films are applied respectively to the two opposite faces of the articles to be packaged, and their longitudinal edges are brought together against each other by rollers and / or conveyor belts forming along the series of objects with two margins defining a sort of tubular sheath in which said objects are trapped.

 In the central zone of the machine, almost simultaneously, two series of welding tools intervene, each determining the welding of the two films and the cutting of the pair of welded films.

 A first tool, in the shape of a horizontal knife, acts transversely and isolates each object from the next, while separating it.

A couple of others, in the form of rollers, act simultaneously on each marginal zone, by enclosing each object by welding and by cutting the margin which is preferably collected by winding on a coil, to prevent it from coming lose in the machine mechanism.

 Although this machine performs the above series of operations very correctly, it may however happen that tear initiations arise in the connection zones between the transverse welds and the abovementioned longitudinal welds.

 Indeed, the marginal zones of the films are sQufl1ses at the time of longiatudinal welds to transverse stresses such as the intervention of the welding tool created triangular notches which 'are likely to then determine tear incidences in these stress zones.

 Although they do not represent any defects in the packaging proper, the formation of these notches nevertheless causes drawbacks in the very operation of the machine.

Indeed, at the point where the wrapped objects are collected individually, the abovementioned marginal bands must be collected, so as to prevent them from piling up irregularly around the machine, risking accidentally interfering with mechanical parts. of it.

For this, each marginal strip is wound under tension, forming a wafer which is collected at regular intervals.

 Under the effect of this tension, it happens that these bands break at the location of one of these tear primers, and stop coming to wind on this cake. On the machines in question, to avoid crowding the free end of the strip, it is necessary to stop the advance of the machine as soon as such a break occurs, to allow the user to carry out the repair and re-roll operations.

 This stopping and the duration of these operations seriously slow down the rate and the flow of the machine, which is a serious drawback.

This drawback, which was quite rarely encountered with conventional plastic films, becomes a much greater risk with ultra-thin films, of the order of 10 to 15 microns, the use of which is increasingly recommended, given the much lower cost price of these packages, having regard to the lower quantity of material used, and the much more aesthetic appearance of these films. This is why it had become important to eliminate this drawback.

To this end, the invention consists in a simple arrangement of these machines, making it possible to remedy these drawbacks by neutralizing the primers of # tear as they arise. To this end, the invention consists, in a machine of aforementioned type in which a succession of objects are enclosed between two continuous plastic films, by two longitudinal marginal welds, in the direction of said unwinding and two transverse welds perpendicular to the previous ones, between the successive objects to neutralize the incipient tears likely to arise at the points where the ends of the transverse welding tool meet the longitudinal welding zones, by directing towards these points jets of air heated to a temperature close to the softening point of the film material.

It is known in fact that, when one proceeds to such heating on a film having a hole in a tear, the material delimiting this hole or this tear undergoes a beginning of fusion on the periphery of said hole by forming a bead which reinforces this periphery thus preventing the tear from spreading further.

 It is this phenomenon which occurs at each tear initiation caused by the transverse welding tool, so that, at the time of the separation of the marginal strips and of their winding under tension, the incipient tears no longer risk being propagate by determining the breakage of the bands and the stopping of the machine, so that the abovementioned drawbacks are eliminated.

 It is this phenomenon which is exploited according to the invention, in the immediate vicinity of the ends of the transverse welding tool, either at the point of intervention of this tool, or immediately after.

 For this purpose, hot air jets are constantly projected at the points of the longitudinal zone where the end of the transverse tool must intervene, and it is at the time of this intervention that the advance of the couple of films trapping the objects being interrupted for the duration of the welding, the impact time of these hot air jets becomes sufficient to cause local melting of the film material. Whether the initiation of tearing is caused at this point or just before, this fusion has the effect of neutralizing this initiation, so that at the time of the tensioning of the marginal bands, the risk of ruptures of these banks has become practically no.

To explain the invention more fully, reference will be made to the following description, corresponding to the appended drawing which schematically represents the implementation of the method according to the invention.

In this drawing, there is shown in L a succession of books intended to be packed in a machine of the aforementioned type, between two films P1 and P2 taking place simultaneously and parallel to one another by applying respectively to the upper faces and lower books L.

In the known machine, the longitudinal edges, that is to say parallel to the direction of movement of the films are joined together by known devices designated in the drawing by S1 and 52 such as rollers or conveyor belts forming two flat edges B1 and B2 delimiting a kind of seed
2 tubular G, in which the books L are trapped.

A transverse welding tool then intervenes so as to separate in the sheath G volumes each containing a book L, and delimited by a transverse weld T.

 Welding tools (not shown, but similar in appearance to the S1-Sz rollers) then intervene to weld the marginal areas and at the same time cut them off by isolating each object L completely conditioned.

 It is at the ends A of this weld T that the aforementioned tear primers can then occur, and which should be neutralized using the method according to the invention.

In fact, the material of the films P1 and P2, when it is joined to form the edges B1 and B2, is subjected to a stress, and the welding tool forming the weld
T meets this material subject to constraints by risking bursting this material at locations A.

 According to the invention, at the location of points A, and practically at the location of the tool causing the weld T, are arranged two hot air blowing nozzles, only one of which is shown at C, directing on each point A jet of air heated to a temperature such that the material of the films P1 and P2 is softened when the strips P1 P2 stop for the intervention of the transverse tool, so as to create, at the locations of the tear primers A, beads of material reinforcing these primers, by virtue of the aforementioned phenomenon.

it is easy to control this phenomenon experimentally, for example by inserting a heated pin into a film such as P. It can be seen that this perforation by a heated pin of the film P produces only a single hole while if the pin does not was not heated, this perforation would immediately determine a tear when the film was subjected to any tension.

Of course, although reference has been made in the foregoing to a particular type of machine currently on the market, the principle of the invention applies to all types of machine in which an identical or similar phenomenon is encountered, it that is to say the formation of holes or of tearing incidences at the location where a tool comes into contact with a film of heat-shrinkable material subjected to a tensile stress.

 The invention is therefore not limited to the embodiment described and shown in the above example, but on the contrary will cover all variants.

Claims (4)

R E V E N D r c A T Z O N S 1. Method for neutralizing the usccptil rupture primers  to be born in plastic wrap under the effect of the intervention of a tool on this film  in an area where said film is subject to a contra.  tension, process characterized in that, at this point, is directed onto the film a jet of gas heated to a  temperature close to the softening point of said  film, 2. Apparatus for implementing the method according to the claim 1, characterized in that it consists of the ego  a nozzle directing on a plastic film a jet of air heated to a temperature close to the point of softening of this film to the point where a primer  tear is likely to arise in this  film under the effect of the intervention of a tool on the  said film subjected to a tension constraint. 3. Machine according to claim 2, of the type in which a succession of objects is enclosed between two films of  plastic material in continuous flow, by two welds marginal longitudinal and two transverse welds  between successive objects, in which, with a view to neutra read the primers for tears likely to arise  sance at the point where the ends of the weld tool tran s- side meets longitudinal weld zones, machine characterized in that it has projection nozzles air heated to a temperature close to the softened point film material, these air jets being direct managed towards said points where are likely to be born  sance these tear primers.  4. Machine according to any one of claims 1 and 2,  characterized in that the temperature of the hot air is: such  that the jet of hot air sent on the film by the seats causes their material to melt when said film  marks a corresponding stop time ctio. from o * ~:  risking creating a tear.