Patent Description:
Machines for the production of bags in polyethylene or biodegradable film provide in the majority of cases for processing on several tracks in order to increase productivity thereof.

The processing cycle involves starting from a reel of flattened tubular film (previously produced by an extrusion machine) of a width equal to a multiple of the width of the bags to be produced.

According to the machine model and the format of the bags to be obtained, the number of tracks can vary from <NUM> to <NUM> or even more.

In order to work on several tracks, the reel is positioned on a reel holder, also called an unwinder, which controls unwinding thereof by means of an axial or tangential motor drive.

The tubular film then enters an apparatus called a cutter/sealer equipped with blades heated to a temperature that can vary from <NUM> to <NUM> degrees (depending on the type and thickness of the film being processed).

Each blade, coming into contact with the plastic film, causes the longitudinal cutting thereof and at the same time, through the effect of the temperature of the blade, creates a longitudinal weld seam.

Substantially, if a cutter/sealer blade is positioned at the centre of the film being processed, it will divide the film into two parts, at the same time creating a weld seam in the areas of film that slide in contact with the blade. In this way two strips of tubular film are generated. Therefore, from a flattened tube-shaped film, two flattened tubes will be obtained, each of which will have a weld seam on one side, as schematised in the accompanying <FIG>, where the dots represent the weld seams that are formed on the film after cutting.

Similarly, if two cutter/sealer blades are positioned, the tube will be divided into three parts, as schematised in <FIG>, and so on.

After cutting and longitudinal welding, the film enters an apparatus known as gusseting machine, in which the side gussets of the bag to be produced are formed. To perform this operation, compressed air is injected manually into the tube before start-up of the machine. In this way tubes are created, which in jargon are called "balloons", one for each track being processed, where the air that has been injected remains if no leaks take place.

During processing, the film slides inside the gusseting machine driven by a pair of calenders. During this forward movement, the air remains inside the gusseting machine, i.e. the tube, in that the pair of calenders, which are rubber-coated, crush the tube, preventing the air from escaping.

The critical point of processing with the cutter/sealer device is the longitudinal weld.

Normally, control of the mechanical tightness of this longitudinal weld is carried out by the operator:.

During processing, it can happen that the film being processed becomes thicker and therefore requires a higher welding temperature, or that the cutter/sealer blade becomes soiled and no longer forms a regular weld. If the operator is not careful to check with a certain frequency the quality of the weld, the machine can produce considerable quantities of product that is not well welded and therefore a reject.

The prior art is based on photocells that control the diameter of the balloons of the gusseting machine. When the balloon of the gusseting machine deflates as a result of a breakage of the weld, the photocell stops the machine. This system has two weak points:.

<CIT> discloses a flat bottom plastic film bag made by a process wherein a blown film tube is continuously slit sealed into a plurality of tubes which are reinflated, then collapsed to form opposed double pleat folds, followed by corrugation of the side and end walls of the tube, formation of a transverse crease defining the fold line between the bottom and sidewalls and, finally, formation of gusset welds, the transverse bottom seal and perforation or cutoff to form the individual bags.

<CIT> discloses a method and apparatus for forming tubes of thermoplastic film.

<CIT> discloses a process and apparatus for testing the tightness of seam joints of synthetic resin sheets by means of vacuum.

The object of the invention is to eliminate the disadvantages of the prior art described above.

More particularly, an object of the invention is to provide a device that ensures reliable and constant control of the longitudinal weld of the bags, allowing immediate intervention in the event of a defective weld and avoiding rejects of material.

Another object of the invention is to provide such a device which is simple and inexpensive to manufacture.

These and other objects are achieved by the device for automatic control of the weld according to the invention that has the features of the appended independent claim <NUM>.

Advantageous embodiments of the invention are disclosed in the dependent claims.

Substantially, a machine for the production of plastic film bags according to the invention comprises: a station for unwinding a flattened tubular film; a cutter/sealer station, wherein at least one hot blade in contact with the plastic film causes longitudinal cutting and simultaneously, through the effect of the temperature of the blade, the formation of longitudinal weld seams, obtaining at least two flattened tubular films; a station of gusseting machines, i.e., apparatuses for the formation of the side gussets of the bags to be produced, in which each flattened tube takes on the shape of a balloon filled with compressed air with at least one longitudinal lateral weld; in which at least one feeler device is placed at each balloon for the automatic control of a corresponding longitudinal weld, simulating the pressure exerted by the operator's finger against the balloon of the gusseting machine and immediately signalling the possible rupture of the weld.

Further features of the invention will be made clearer by the following detailed description, referred to one of its embodiments purely by way of non-limiting example, illustrated in the accompanying drawings, in which:.

Before describing in more or less detail the device for the automatic control of the longitudinal weld on plastic film bags according to the invention, the production cycle of these bags is again described in brief with reference to the partial diagram in <FIG>.

It starts from a reel <NUM> of flattened tubular film <NUM> positioned on a reel holder <NUM>, from which the film is unwound by means of an axial <NUM> or tangential motor drive <NUM>.

As will be seen in the successive <FIG>, <FIG>, <FIG>, the machine shown works on three tracks, whereby the width of the flattened tubular film <NUM> is such that three bags of the desired width can be formed simultaneously from it.

The tubular film <NUM>, after having passed over a series of drive rollers, enters the cutter/sealer station <NUM>, where two hot blades come into contact with the plastic film, causing longitudinal cutting thereof and, at the same time, through the effect of the temperature of the blade, the formation of longitudinal weld seams <NUM> (see <FIG>), obtaining three flattened tubular films <NUM>.

In the longitudinal cutting and sealing station <NUM>, along the path of the flattened film, a zone <NUM> is also provided, where the two overlapping film layers are kept separate to avoid possible adhesion.

After cutting and longitudinal welding, each flattened tubular film <NUM> into which the original film has been divided enters a station <NUM> of the gusseting machines, i.e. apparatuses for the formation of the side gussets of the bags to be produced.

To carry out this operation, before start-up of the machine, compressed air <NUM> is manually injected inside the tubular film of each track being processed, in this way creating a tube <NUM> which in jargon is called a "balloon". In a manner in itself known, the compressed air is injected by forming a hole in the film <NUM>, which is subsequently closed.

During processing, the film slides inside the gusseting machine driven by a pair of rubber-coated calenders <NUM>, which crush the tube, preventing the injected air from escaping.

In <FIG> the balloon <NUM> has been shown with rhomboidal shape in section or longitudinal view, however a shape closer to the reality of the balloons <NUM> is that shown in <FIG> in longitudinal and transverse view, respectively.

Substantially, a balloon <NUM> has a lower section <NUM>, wherein it begins to be formed, with trapezoidal shape, with gradual reduction in width, in transverse view and triangular or conical in longitudinal view, followed by a perfectly tubular intermediate section <NUM>, which we can define as cylindrical, and ending with an upper section <NUM> again with trapezoidal shape, with gradual increase in width, in transverse view and triangular or conical in longitudinal view, until reaching the width of the flattened tube at the calenders <NUM>.

The gusseting machine comprises, in a manner in itself known, a pair of partitions <NUM>, made up of a series of transverse partitions <NUM>, arranged to form the upper section <NUM> conical in a longitudinal direction to the machine, so as to determine a flattening of the film incoming on the calenders <NUM>. On the two sides of the balloon, thus in a transverse direction to the machine, where at least one weld is present, again at the upper section <NUM>, respective triangular plates <NUM> (see <FIG>) are provided, which enter the space between the partitions <NUM> at an adjustable depth, in order to determine the side gussets of the bags to be produced, during the forward movement of the film.

At the exit from the station <NUM> of the gusseting machines, the flattened film goes to other transverse cutting and sealing stations, not shown in <FIG>, to form bags of the type required.

The device for automatic control of the longitudinal weld on plastic film bags according to the invention is now described, made up of a feeler device, shown in various views in <FIG>, and denoted as a whole by reference numeral <NUM>.

The feeler device <NUM> has to simulate the pressure exerted by the operator's finger against the balloon of the gusseting machine and immediately indicate any rupture of the weld.

A feeler device is installed on the machine for each weld to be checked. Therefore, in the specific case of processing on three tracks, with formation of three balloons, the central balloon will have two longitudinal welds at the respective sides, while the two side balloons will have one single lateral weld in that the other side of the tubular film is closed at the start.

A feeler device <NUM> is composed of a sliding pad <NUM> hinged to a base <NUM> and pushed against the corresponding balloon <NUM> by a piston <NUM> with pressure that can be regulated as a function of the type and thickness of the film used (therefore of its consistency). The pad <NUM> is positioned precisely in the area of the balloon where the corresponding longitudinal weld is present.

A position detector <NUM> is applied on the piston by means of which the position of the pad <NUM> is known at all times, position that is determined by the diameter of the balloon <NUM> against which the pad is pushed.

The base <NUM> of the feeler device is mounted so that it can be adjusted, from above or from below, on a guide bar <NUM>, shown in section in <FIG>.

During processing, the pad <NUM> of the feeler device <NUM> is subject to continuous oscillations caused by the sliding of the film and, above all, by the low consistency of the balloon <NUM>, which counters only thanks to the slight pressure of the internal air.

Specially designed software does not take into account the oscillations or even the slight deflation of balloon <NUM> caused by the natural leakage of air between the upper welding calenders <NUM> but stops the machine if the movement of the feeler device is too sudden, which indicates that some part of film has passed that is not properly welded and, through the slight force applied, the weld has failed.

<FIG> show the positioning of the feeler devices <NUM> at the balloons <NUM>.

As mentioned, in the specific case, four feeler devices <NUM> are provided, two mounted above and two below the guide bar <NUM>, in positions that can be adjusted on it as a function of the diameter of the balloons <NUM>, in such a way that the sliding pads <NUM> are inserted in the spaces between two adjacent balloons, coming into contact with respective longitudinal welds of the tubes.

More particularly, the upper feeler devices <NUM> are arranged to come into contact with their pads <NUM> with the left side welds of the respective balloons <NUM>, while the lower feeler devices <NUM> come into contact with their pads <NUM> with the right side welds of the respective balloons <NUM>.

Therefore, with the central balloon, which has two longitudinal welds, an upper and a lower feeler device will be engaged, while with the two side balloons an upper and a lower feeler device will be engaged, respectively, as can be seen in particular in <FIG>.

The provision of continuous monitoring of the state of inflation of the balloons <NUM> with the device according to the invention allows for real-time feedback on the operating status of the machine, avoiding false machine downtimes or wastage of material due to missed stops, as often takes place according to the prior art.

From what has been disclosed above the advantages of the device for the automatic control of the longitudinal weld on plastic film bags according to the invention are clear.

Claim 1:
Machine for the production of plastic film bags comprising:
- a station for unwinding of a flattened tubular film (<NUM>);
- a cutter/sealer station (<NUM>), where at least one hot blade in contact with the plastic film causes longitudinal cutting and, at the same time, through the effect of the temperature of the blade, the formation of longitudinal weld seams (<NUM>), obtaining at least two flattened tubular films (<NUM>);
- a station (<NUM>) of gusseting machines, i.e. apparatuses for the formation of the side gussets of the bags to be produced, wherein each flattened tubular film (<NUM>) takes on the shape of a balloon (<NUM>) filled with compressed air with at least one longitudinal lateral weld;
characterised in that at each balloon (<NUM>) at least one feeler device (<NUM>) for the automatic control of a corresponding longitudinal weld is placed, simulating the pressure exerted by the operator's finger against the balloon of the gusset machine and immediately signalling the possible break of the weld.