FR3053905A1 - METHOD FOR MANUFACTURING PLANAR MATERIAL IN PLASTIC MATERIAL FOR THE MANUFACTURE OF PACKAGING BAGS AND MACHINE FOR CARRYING OUT SAID METHOD - Google Patents

Abstract

The method comprises the steps of: - using an extrusion / inflation machine comprising three extruders (2a, 2b, 2c) and a single die (3) capable of coextruding a sheath (5), and comprising, at a distance from the die (3), rollers (6) for gripping / pulling the sheath (5); - Providing for the introduction into the extruder (2b) for extruding a central layer (20b) of the wall of the sheath (5), an expansion agent; determining the gap of the die (3) at a value of between 1 mm and 2.5 mm, the inflation rate of the sheath (5) at a value between 1.2 and 4 and the extrusion speed the sheath (5) and the rotational speed of the nip rolls (6) at a ratio of 1: 4 to 1:40; - Coextruding the sheath (5) to a total thickness of between 1 mm and 2.5 mm; - Inflation of the sheath (5) and longitudinal stretching of the sheath so as to obtain, after inflation and stretching, a sheath (5) whose wall has a thickness of between 35 and 60 microns.

Description

® FRENCH REPUBLIC

NATIONAL INSTITUTE OF INDUSTRIAL PROPERTY © Publication number: 3,053,905 (to be used only for reproduction orders)

©) National registration number: 16 56850

COURBEVOIE © Int Cl ⁸ : B 29 C 47/06 (2017.01), B 29 C 44/24, B 29 K23 / 00

A1 PATENT APPLICATION

©) Date of filing: 18.07.16. (© Priority: (© Applicant (s): LEYGATECH Simplified joint stock company - FR. @ Inventor (s): BROUILLAT JEAN-MATTHIEU. ©) Date of availability of the request: 19.01.18 Bulletin 18/03. ©) List of documents cited in the preliminary search report: See the end of this booklet (© References to other related national documents: ©) Holder (s): LEYGATECH Simplified joint-stock company. ©) Extension request (s): (© Agent (s): LLR.

METHOD FOR MANUFACTURING A PLASTIC MATERIAL IN PLASTIC MATERIAL FOR THE MANUFACTURE OF PACKAGING BAGS AND MACHINE FOR CARRYING OUT SAID METHOD.

FR 3 053 905 - A1

The method includes the steps of:

- use an extrusion / inflation machine comprising three extruders (2a, 2b, 2c) and a single die (3) capable of co-extruding a sheath (5), and comprising, at a distance from the die ( 3), pinch / draw rollers (6) of the sheath (5);

- Provide for the introduction into the extruder (2b) intended to extrude a central layer (20b) of the wall of the sheath (5), of an expansion agent;

- determine the air gap of the die (3) at a value between 1 mm and 2.5 mm, the inflation rate of the sheath (5) at a value between 1.2 and 4 and the extrusion speed of the sheath (5) and the speed of rotation of the pinch / draw rollers (6) in a ratio of between 1: 4 and 1:40;

- carry out the co-extrusion of the sheath (5) with a total thickness of between 1 mm and 2.5 mm;

- Carry out the inflation of the sheath (5) and the longitudinal stretching of this sheath so as to obtain, after inflation and stretching, a sheath (5) whose wall has a thickness of between 35 and 60 microns.

The present invention relates to a method of manufacturing a flat material of plastic material intended for the manufacture of packaging bags, and a machine for implementing this method.

For packaging all kinds of products, for example shopping, it has been common to use, in recent decades, plastic packaging bags intended for single use, these bags being made of a film of a thickness of the order of 15 microns.

These types of bags have become much less desirable these days, or are even being banned, due to the spread of plastic in the environment resulting from its use.

It therefore becomes desirable to produce resistant and reusable packaging bags, in a thickness of between 35 and 60 microns.

It would be conceivable to simply increase the thickness of the planar material used up to 35 to 60 microns; however, the use of the additional material necessary to make this material is hardly ecological, and the increase in cost is hardly acceptable from an economic point of view.

It would also be conceivable to load said flat material with particles of recycled products, so as to obtain the required thickness, these products being inexpensive; the grammage of the planar material would however be high, of the order of 48 to 53 g / m ² , which would be a significant practical constraint.

There is therefore no satisfactory process for obtaining a planar material of the aforementioned thickness, and the present invention aims to remedy this shortcoming.

To this end, the process which it concerns comprises the steps consisting in:

- Use an extrusion / inflation machine comprising three extruders ending in a single die capable of carrying out the co-extrusion of a sheath, and comprising, at a distance from the die, pinch / draw rolls of the extruded sheath, said extruders being able to extrude polyolefins;

- provide for the introduction into the extruder intended to extrude a central layer of the wall of the sheath, of an expansion agent producing, at the level of the extruder and / or the die, an expansion of the material intended to constitute this central layer;

- determine the air gap of the die at a value between 1 mm and

2.5 mm;

- determine the inflation rate of the sheath to a value between 1.2 and 4;

- determine the speed of extrusion of the sheath and the speed of rotation of the pinch / draw rollers so that the ratio of these speeds is between 1: 4 and 1:40, so that the machine produces a longitudinal stretch of the sheath simultaneously with the inflation of this sheath;

- Introducing said blowing agent into the extruder intended to extrude said central layer from the wall of the sheath;

- Carry out the co-extrusion of the sheath by means of the three extruders so that this sheath has a wall formed by two non-expanded lateral layers and from said expanded central layer, this wall having, at the outlet of the die, a total thickness between 1 mm and 2.5 mm;

- Carry out the inflation of the sheath and the longitudinal stretching of this sheath so as to obtain, after inflation and stretching, a sheath whose wall has a thickness of between 35 and 60 microns.

It will be understood that, by the term air gap, we mean the space between the punch and the body of the die.

It will also be understood that by the term inflation rate, reference is made to the ratio between the radius of the sheath after inflation and the radius of this sheath before inflation.

The invention thus consisted in designing to co-extrude a sheath with two unexpanded lateral layers and in an expanded central layer and in carrying out a simultaneous radial and longitudinal stretching of this sheath at the outlet of the die so as to obtain the thickness of sought wall, between 35 and 60 microns.

In doing so, the inventor took the opposite view of a certainty that those competent in the manufacture of sheaths or films for an application to the manufacture of packaging bags, according to which it is not conceivable to produce a sheath or a film of small thickness, namely of the order of 50 to 60 microns, including an expanded layer. Indeed, it is well known that with this kind of thickness, the micro-bubbles produced by the expansion are larger than the desired thickness for the sheath, which results in that part of the bubbles that the expanded layer punctures on the surface of the layer. This results in major defects in extrusion, profile and printability of the sheath, which is moreover in the form of a kind of lace than in the form of a sheath.

By designing to carry out a co-extrusion of three layers of polyolefins, including an expanded central layer and two unexpanded side layers, the inventor discovered that the two side layers, in addition to the possibility of obtaining the resistance characteristics, appearance, solderability and printability required for the sheath obtained, also allowed for containment and control of the expansion of the material of the central layer, and that the simultaneous radial and longitudinal stretching of this sheath at the outlet of the die allowed '' obtain the desired wall thickness, between 35 and 60 microns, without affecting the structure of the wall of the sheath thus obtained.

It appears that this sheath is resistant, although made up of a reduced quantity of material due to the expanded structure of its central layer, and that it has smooth external faces, at least one of which is capable of being heat-sealed and the the other is suitable for printing. The sheath thus obtained has a mass of the order of 35 g / m ² for a thickness of 50 microns

The inventor further conceived that it was necessary for the blowing agent to carry out the expansion within the extruder intended to produce the central layer, so that the expansion is carried out, if not completely, all at least most of the time, when the material passes through the extrusion die, so that good control of the expansion of said central layer is obtained.

The method may include a final step of longitudinal cutting of the sheath, in order to obtain a film.

This sheath or this film constitutes the planar material intended for the manufacture of packaging bags.

Preferably, the determinations of the air gap, the inflation rate and the ratio of the speeds of extrusion of the sheath and rotation of the pinch / pull rollers are such that each lateral layer of the wall of the sheath has, after inflation , a thickness of at least 4 microns.

These determinations are preferably such that each lateral layer of the wall of the sheath has, after inflation and longitudinal stretching, a thickness of 10 microns and that the central layer has, after inflation and longitudinal stretching, a thickness of 30 microns.

According to one possibility, the blowing agent is carbon dioxide or nitrogen, and the step of introducing this gas into the extruder is carried out by adding gas inside the screw chamber that comprises this extruder, upstream from the outlet of this chamber, so that the screw produces an intimate mixture of this gas and the molten plastic material inside the chamber.

According to another possibility, the blowing agent is a chemical additive releasing carbon dioxide in the event of a rise in temperature and the operation of introducing this additive into the extruder is carried out by adding the additive to the inside the screw chamber that this extruder comprises, upstream of the exit from this chamber, so that the screw produces an intimate mixture of this gas and of the molten plastic present inside the chamber.

The blowing agent may in particular be a mixture of citric acid and sodium bicarbonate dosed at 1% releasing carbon dioxide in the event of a temperature rise, and the operation of introducing this additive into the extruder is made by adding this mixture inside the screw chamber that this extruder comprises, upstream of the exit from this chamber so that the screw produces an intimate mixture of this mixture and the molten plastic present inside the room.

The process can include:

the use of an extrusion / inflation machine including, in addition to the three aforementioned extruders, at least one additional extruder ending in the die, and

- When performing co-extrusion, the co-extrusion of at least one additional layer on the face of at least one side layer which is opposite to said central layer.

The invention also relates to an extrusion / inflation machine comprising, in a manner known per se, extruders ending in a single die capable of carrying out the co-extrusion of a sheath, and comprising, at a distance from the die, rollers pinching / pulling the extruded sheath, said extruders being able to extrude polyolefins; the machine further includes means for adjusting the die air gap, means for adjusting the inflation rate of the extruded sheath and means for adjusting the speed of rotation of the nip / pull rollers; according to the invention, the machine comprises:

- three extruders ending in said single die;

a means of introduction, into the extruder intended to extrude a central layer of the wall of the sheath, of an expansion agent producing, at the level of the extruder and / or the die, an expansion of the material intended to constitute this central layer.

When the blowing agent is a gas, said introduction means may be in the form of a pipe opening into the screw chamber that includes said extruder; when the blowing agent is a chemical additive, said introduction means can be simply the opening through which the material to be extruded is introduced into said extruder.

Preferably, the lateral layers and the central layer of the sheath are made of polyethylene; they can also be made of polypropylene.

The plastic film is obtained by longitudinal cutting of the sheath.

The invention will be clearly understood, and other characteristics and advantages thereof will appear, with reference to the appended schematic drawing, representing, by way of nonlimiting example, an embodiment of the extrusion machine which it concerned.

Figure 1 is a very schematic and simplified view, partially in section, of an extrusion / inflation machine allowing the implementation of the method according to the invention; and FIG. 2 is a side view, on a greatly enlarged scale, of a portion of the wall of a sheath obtained by the machine according to FIG. 1.

FIG. 1 represents the extrusion / inflation machine 1, comprising three extruders 2a, 2b, 2c, a co-extrusion die 3, a guide 4 making it possible to direct an extruded sheath 5 towards a pair of pinch / draw rollers 6 of this sheath 5, and a coil 7 for winding the sheath thus produced.

Each extruder 2a, 2b, 2c includes a heated chamber 10, into which opens a hopper 11 for supplying material to be extruded, and a screw 12, also heated, for pushing the material to be extruded towards the die 3. The extruders 2a , 2b, 2c are connected to this die by pipes 13.

Among these extruders 2, those referenced 2a and 2c are lateral with respect to that referenced 2b, which is therefore central; this central extruder 2b comprises a pipe 15 opening into its chamber 10, which allows the introduction of a blowing agent into the material it is intended to extrude.

The die 3 has an extrusion ring having a circular shape and consequently producing the tubular sheath 5, and is equipped, at the level of the internal zone of this ring, with nozzles 18 for diffusing an inflation gas from the sheath 5 produced by machine 1.

The extrusion die 3 is subdivided into three respective conduits each supplied by one of the extruders 2a, 2b, 2c and comprises a punch for adjusting the air gap of this die, this air gap being adjustable between 1 and

2.5 mm.

The three extruders 2a, 2b and 2c open out using the lines 13 in the die 3.

The extruders 2a, 2b and 2c make it possible to co-extrude, at the outlet of the die 3, a sheath 5 whose wall visible in FIG. 2 is formed of two side layers 20a, 20c unexpanded and of said expanded central layer 20b , the lateral layers being produced by the extruders 2a and 2c respectively and the central layer being produced by the extruder 2b.

The flow rate of the nozzles 18 is adjustable so that the inflation rate of the sheath 5 is between 1.2 and 4, this rate being the ratio between the radius of the sheath 5 after inflation and the radius of this sheath before inflation .

The pair of rollers 6 makes it possible to pinch the sheath 5 thus formed so as to close the internal volume of this sheath and to allow it to inflate. The extrusion speed of the sheath 5 and the rotation speed of these rollers 6 are adjustable so that the ratio of these speeds is between 1: 4 and 1:40. These rollers 6 thus also produce a longitudinal stretching of the sheath 5 simultaneously with the inflation of this sheath.

According to a practical example, the diameter of the extrusion ring is 200 mm; the material is extruded at the outlet of the die 3 at a rate of 170 kg / h; it is sought to obtain a sheath 5 having a width (that is to say a flat width) of 1140 mm; the air gap of the die 3 is adjusted to a total thickness of 1.4 mm, dividing into extrusion openings of the lateral layers each having a thickness of 0.280 mm and an extrusion opening of the central layer having a thickness 0.840 mm; the inflation rate is set to 3.63 and the pull rate (that is to say the ratio between said speeds) is set to 9.39.

Each hopper 11 is supplied with polyolefins in the form of granules, using dosers. These granules can in particular be different polyethylenes. In the extruders 2a and 2c for extruding the lateral layers, pellets of medium density polyethylene (MDPE) and linear low density polyethylene (LLDPE) are fed. These granules can include additives and / or catalysts but do not contain blowing agents. Other shaping additives can be added to help shape the material. The material obtained is distributed homogeneously using a spiral mandrel allowing better distribution of the material over the entire circumference of the circularly shaped die.

In the extruder 2b, granules of low density polyethylene and a blowing agent are introduced by means of the hopper 11 and a blowing agent is introduced into the chamber 10 through line 15. This blowing agent is an endothermic additive releasing carbon dioxide in the event of a rise in temperature, in particular baking soda and citric acid, dosed at 1%. The screw 12 then produces an intimate mixture of this blowing agent and of the molten plastic material inside the chamber and between the turns of the screw. The extruder 2b and the screw 12 are heated to a temperature of, for example, between 160 and 220 ° C. The expansion of the central layer 20b begins in the extrusion chamber 10 and ends in the die 3.

The machine thus configured realizes a simultaneous radial stretching and longitudinal stretching of the sheath 5, making it possible to obtain, after these stretchings, a sheath 5 having a wall whose total thickness is between 35 and 60 microns.

Each lateral layer 20a, 20c ultimately has a thickness of the order of 10 microns and said expanded central layer 20b has a thickness of the order of 30 microns.

In FIG. 2, the circles of the central layer 20b represent the bubbles of carbon dioxide trapped in the central layer and surrounded on either side by the lateral layers 20a and 20c.

The invention thus provides a method of manufacturing a resistant sheath, having a thickness of between 35 and 60 microns. The sheath thus obtained has a mass of the order of 35 g / m ² for a thickness of 50 microns. By longitudinal cutting, it provides a film of plastic material intended for the manufacture of packaging bags.

It goes without saying that the invention is not limited to the method described above by way of example but that it includes all the technical equivalents and variants of the means described as well as their combinations. Thus, it would be possible to use a chemical additive in place of a gas to achieve the expansion of the material constituting the central layer, in which case this additive would be introduced at the level of the hopper 11 and its mixture with said material. would be done upstream of screw 12.

Claims (11)

1. A method of manufacturing a flat material of plastic material intended for the manufacture of packaging bags, characterized in that it comprises the steps consisting in: - use an extrusion / inflation machine comprising three extruders (2a, 2b, 2c) ending in a single die (3) capable of co-extruding a sheath (5), and comprising, at a distance from the die (3), rollers (6) for pinching / pulling the extruded sheath (5), said extruders (2a, 2b, 2c) being able to extrude polyolefins; - provide for the introduction, into the extruder (2b) intended to extrude a central layer (20b) of the wall of the sheath (5), of an expansion agent producing, at the level of the extruder (2b) and / or of the die (3), an expansion of the material intended to constitute this central layer; - determine the air gap of the die (3) at a value between 1 mm and 2.5 mm; - determine the inflation rate of the sheath (5) to a value between 1.2 and 4; - determine the speed of extrusion of the sheath (5) and the speed of rotation of the pinch / draw rollers (6) so that the ratio of these speeds is between 1: 4 and 1:40, so that the machine produces a longitudinal stretching of the sheath (5) simultaneously with the inflation of this sheath; - Introducing said blowing agent into the extruder (2b) intended to extrude said central layer from the wall of the sheath (5); - Co-extruding the sheath (5) by means of the three extruders (2a, 2b, 2c) so that this sheath (5) has a wall formed by two lateral layers (20a, 20c) which are not expanded and from said expanded central layer (20b), this wall having, at the outlet of the die (3), a total thickness of between 1 mm and 2.5 mm; - Carry out the inflation of the sheath (5) and the longitudinal stretching of this sheath so as to obtain, after inflation and stretching, a sheath (5) whose wall has a thickness of between 35 and 60 microns. 2. Method according to claim 1, characterized in that it comprises a final step of longitudinal cutting of the sheath (5), for obtaining a film. 3. Method according to claim 1 or claim 2, characterized in that the determinations of the air gap, the inflation rate and the ratio of the speeds of extrusion of the sheath (5) and rotation of the rollers (6) of pinching / pulling are such that each lateral layer (20a, 20c) of the wall of the sheath (5) has, after inflation, a thickness of at least 4 microns. 4. Method according to claim 3, characterized in that the determinations of the air gap, the inflation rate and the ratio of the speeds of extrusion of the sheath (5) and rotation of the pinch / draw rollers (6) are such that each lateral layer (20a, 20c) of the wall of the sheath (5) has, after inflation and longitudinal stretching, a thickness of 10 microns and that the central layer (20b) has, after inflation and longitudinal stretching, a thickness 30 microns. 5. Method according to one of claims 1 to 4, characterized in that the blowing agent is carbon dioxide or nitrogen, and in that the step of introducing this gas into the extruder (2b) is produced by adding gas inside the screw chamber (10) that this extruder (2b) includes, upstream of the exit from this chamber, so that the screw (12) mixes intimate of this gas and of the molten plastic inside the chamber (10). 6. Method according to one of claims 1 to 4, characterized in that the blowing agent is a chemical additive releasing carbon dioxide in the event of a rise in temperature and the operation of introducing this additive into the extruder (2b) is produced by adding the additive inside the screw chamber (10) that this extruder comprises, upstream of the exit from this chamber, so that the screw (12) achieves a intimate mixture of this gas and the molten plastic present inside the chamber (10). 7. Method according to claim 6, characterized in that the blowing agent is a mixture of citric acid and sodium bicarbonate dosed at 1% releasing carbon dioxide in the event of a rise in temperature, and in that the operation of introducing this additive into the extruder (2b) is carried out by adding this mixture to the interior of the chamber (10) with screws that this extruder comprises, upstream from the outlet of this chamber (10 ) so that the screw (12) produces an intimate mixture of this mixture and of the molten plastic present inside the chamber. 8. Method according to one of claims 1 to 7, characterized in that it comprises: the use of an extrusion / inflation machine including, in addition to the three aforementioned extruders, at least one additional extruder ending in the die, and - When performing co-extrusion, the co-extrusion of at least one additional layer on the face of at least one side layer which is opposite to said central layer. 9. Extrusion / inflation machine for implementing the method according to one of claims 1 to 8, including extruders (2a, 2b, 2c) ending in a single die (3) capable of carrying out co-extrusion a sheath (5), and comprising, at a distance from the die (3), rollers (6) for pinching / pulling the extruded sheath (5), said extruders (2a, 2b, 2c) being able to extrude polyolefins, the machine further including means for adjusting the air gap of the die (3), means for adjusting the inflation rate of the extruded sheath (5) and means for adjusting the speed of rotation of the rollers (6) pinching / pulling, characterized in that it comprises: - three extruders (2a, 2b, 2c) ending in said single die (3); a means of introduction, into the extruder (2b) intended to extrude the central layer (20b) of the wall of the sheath (5), of an expansion agent producing, at the level of the extruder (2b ) and / or the die (3), an expansion of the material intended to constitute this central layer (20b). 10. Sheath (5) made of plastic material obtained by the method according to one of claims 1 to 8, characterized in that the lateral layers (20a, 20c) and the central layer (20b) of the sheath (5) are polyethylene or polypropylene. 11. Plastic film obtained by longitudinal cutting of the sheath (5) according to claim 10. 1/1