WO1997003808A1

WO1997003808A1 - Method and equipment for biaxially stretching a primary sheath

Info

Publication number: WO1997003808A1
Application number: PCT/FR1996/001105
Authority: WO
Inventors: Denis Bonnel
Original assignee: Les Plastiques De La Deome
Priority date: 1995-07-18
Filing date: 1996-07-16
Publication date: 1997-02-06
Also published as: FR2736861A1; EP0839086A1; FR2736861B1; AU6618096A

Abstract

A method is provided wherein the neck (C) of a secondary bubble (S) is radially expanded using a mass of heating liquid (52) contained in a primary sheath (S) and flowing from a vertical downleg (14a) to a vertical upleg (14c) thereof, and the expansion of the neck (C) of the secondary bubble (S) is controlled by adjusting the amount of internal heating liquid (52) and balancing the level of the external heating bath (31) and the level of the internal bath. The equipment comprises a supply (50) of heated liquid (52) connected via a pipe to a nozzle (55) for at least temporarily passing through the wall of the vertical upleg (14c) of the sheath (14b) in the biaxial stretching station, and delivering an amount of liquid into said leg (14c), the horizontal leg (14b) and the other vertical leg (14a) to control the radial expansion of the secondary bubble. The equipment further comprises means (44) for balancing the level of the external hot bath (31) and the level of the liquid (52) inside the neck (C) of the secondary bubble (S).

Description

Method and installation for ensuring the bi-stretching of a primary sheath

The invention relates to the manufacture of synthetic material films stretched in two perpendicular directions, respectively longitudinally and transversely, from a primary plastic sheath coming from an inflation extrusion installation.

The usual known processes consist, in a first phase, of extruding a tube of small diameter, but thick, or a first bubble, of cooling, then of flattening the primary sheath thus obtained, and, in a second phase, of heating the primary sheath by circulating it downhill in an oven disposed between two draft scarves, the downstream one of which rotates faster than the upstream one, and injecting into this sheath a pressurized gas forming a secondary bubble ensuring transverse stretching. The multiplication by four or five of the diameter of the primary sheath, and the elongation provided by the speed differential between the scarves, provide a stretching, respectively transverse and longitudinal, of the wall of the sheath. The film thus obtained, commonly known as bi-oriented, has better qualities, in particular in shrinkage.

The secondary bubble is inflated while it is suspended in a vertical oven, bringing it locally to the softening temperature of its constituent material, the control of the heating means and the internal pressure of the bubble must be very precise, failing which the bubble deforms and causes variations in the thickness of the film, and even a production stop.

In practice, this manufacturing technique is very delicate to use industrially, and its application is limited. This is all the more unfortunate since this technique makes it possible to produce bi-oriented films at lower cost, since it requires less investment than the transverse stretching technique by means of jaws.

FR-A-2 445 212 discloses an installation in which the primary sheath is heated by circulating it in a bath of hot fluid, and its re-inflation is carried out by a gas under pressure while it is moved from bottom up.

This technique makes it possible to bring the neck of the secondary bubble, that is to say the part separating the primary sheath from the secondary bubble, to a uniform temperature, but does not solve the problems of instability of the bubble.

In EP-A-410 792, the primary sheath moving from top to bottom, is heated, upstream of the secondary bubble, by hot fluid introduced into the bottom of the primary bubble, and constantly renewed. This technique does not provide any solution to the instability of the secondary bubble which, in a traditional way, is inflated by a gas under pressure.

Finally, FR-A-2 019 129 discloses an installation in which the primary sheath is cooled at the extruder outlet, internally, by a liquor of cooling, circulating between it and a sleeve and falling into the bottom of the primary bubble, and, externally, by passing through a tank, surrounding the aforementioned sleeve and containing a cooling liquor constantly renewed. This technique makes it possible to obtain a primary sheath which is less crystallized and whose subsequent bi-axial stretching is easier, but does not provide any solution for improving the stability of the secondary bubble.

The object of the invention is to provide a manufacturing process eliminating the constraints imposed by controlling the pressure of the gas in the secondary bubble, and improving the stability of this bubble. This method is of the type in which the primary plastic sheath describes a U-shaped trajectory with a descending vertical branch, a horizontal branch, and an ascending vertical branch, trajectory during which said sheath is heated by passage through a liquid bath hot.

According to the invention, the method consists in carrying out the radial expansion of the neck of the secondary bubble by means of a mass of heating liquid contained in the primary sheath and going from its descending vertical branch to its ascending vertical branch, and to control the expansion of the secondary bubble neck by adjusting the quantity of internal heating liquid and by equalizing the level of the external heating bath with that of the internal bath. With this process, the quantity of fluid contained in the secondary bubble acts permanently, by its weight, to ballast and hold the cervix and force it to expand, and by its hydrostatic character, like a liquid calibrator automatically regulating the variations of sections or shapes of the cervix. In other words, the radial expansion, which traditionally is ensured by an internal gas pressure, difficult to control, is ensured here by a hydrostatic pressure, of low value and compensated by the hydrostatic pressure of the fluid of the external heating bath. As a result, the formation of the secondary bubble neck, and consequently the radial expansion, are ensured with a very low pressure, generally between 0.050 and 0.1 kg per cm ² , which is favorable to the obtaining the mechanical and optical characteristics of the plastic film.

In one embodiment, the method consists in dividing the external heating bath of the primary sheath into several superimposed zones, and in supplying these zones, independently of one another, with independent heating liquids, and so that the temperature of the zones increases from the entry of the sheath into the external bath to the formation zone, from the neck of the secondary bubble, then decreasing from this zone to the exit from the bath.

This heating mode provides a gradual rise in temperature in the primary sheath and the neck, followed by progressive cooling, but above all, it allows precisely regulate the temperature in each of the zones and thus eliminate, among the causes of instability, variations in the temperature of heating of the primary sheath.

Advantageously, the method also consists in heating the horizontal branch of the primary sheath by circulating it in a bath of hot liquid, independent of the external heating bath of its ascending vertical branch and brought to a temperature lower than that of this bath.

This technique makes it possible to give the primary sheath, before it passes through the reheating bath, a first supply of calories which reduces the quantity of calories which must be supplied in order to bring it to its softening temperature. In other words, this process makes it possible, with the same traveling speed of the primary sheath, to reduce the length, and in this case the height of the reheating bath necessary to bring it to its softening temperature, and consequently, reduce the mass of liquid to be heated, and maintain a constant temperature.

The invention also relates to an installation for ensuring the bi-stretching of a primary sheath.

This installation is of the type comprising an inflation extrusion assembly forming a primary sheath, means for cooling this sheath, return means leading the primary sheath to a bi-stretching station itself comprising an upstream pulling scarf. and a downstream pulling scarf, and return means causing the primary sheath to describe a U-shaped trajectory during which it passes through a hot heating bath from which it emerges by a vertical and ascending trajectory.

According to the invention, the installation comprises:

a reservoir of heated liquid connected by a pipe to a nozzle capable of crossing, at least temporarily, the wall of the ascending vertical branch of the primary sheath at the bi-stretching station, to pour into this branch, into the horizontal branch, and in the other vertical branch, a quantity of liquid, determining the radial expansion of the secondary bubble,

- And means for making the outside hot bath level coincide with the liquid level inside the neck of the secondary bubble. The retractable nozzle can be permanently inserted into the secondary bubble to compensate for variations in the level of the internal coolant, but it is generally preferable for it to pass through the wall of this bubble only during the adjustment phases. level, to allow the outside air trapped in the secondary bubble to be used as a complementary means of pressurizing the internal bubble. In one embodiment of the invention, the ascending vertical branch of the sheath passes axially through a hot bath containment tank, divided into at least two lower compartments, and into at least three upper compartments by horizontal partitions each comprising a circular central opening whose diameter is greater than the diameter that must have, in this zone, respectively, the primary sheath, the neck and the secondary bubble, while each of the compartments is connected, by a continuous supply circuit, to an independent tank containing a heating liquid maintained at constant temperature. This simple structure makes it possible to obtain heating zones at different temperatures, perfectly controlled and guaranteeing the shape stability of the bubble neck, therefore the thickness of the film and the operating conditions of the installation.

Other characteristics and advantages will emerge from the description which follows with reference to the appended schematic drawings showing by way of nonlimiting example an embodiment of this installation.

Figure 1 is a side view in partial section schematically showing an embodiment of the installation when it is in operation.

Figures 2, 3 and 4 are side views with partial section of the installation, each illustrating one of the phases of its start-up. As shown in FIG. 2, the installation according to the invention comprises a first drawing station A and a second bi-drawing station B.

Station A is, in known manner, composed of an extruder 2 discharging through a die 3 on a calibration sleeve 4, or in an inflation extrusion station. The tubular sheath 5 thus obtained is flattened by passage between two rollers 6a - 6b and cooled by soaking in a cooling tank 7. Rollers 8 return the primary sheath P thus obtained to the bi-stretching station B.

In a known manner, this station comprises an upstream pulling scarf 9 and a downstream pulling scarf 10, and deflection rollers 12 and 13 causing the primary sheath to describe a U-shaped trajectory comprising a vertical descending branch 14a, a horizontal branch 14b , and an ascending vertical branch 14c. The cylinders of the downstream scarf 10 rotate with a peripheral speed greater than that of the rollers of the upstream scarf 9, in the ratio corresponding to the desired longitudinal stretching of the secondary sheath S.

At the exit from the drawing station, the secondary sheath S is flattened and directed, by deflection rollers 15, on a station D ensuring its rewinding on spools 16. According to the invention, and as shown in more detail in FIG. 1, the bi-stretching station B is composed of a lower preheating tank 20 carrying the return rollers 12-13 of the horizontal branch 14b of the primary sheath P, and containing a bath 22 of heating fluid . This tank is connected by a circuit 23-24, with pump 25, to a reservoir 26 provided with means 27 for maintaining the temperature of the liquid 22. These temperature maintenance means comprise a temperature sensor 28, a heating rod 29 and a control unit 30.

The tank 20 communicates with a chimney 32 extending vertically around the vertical downward branch 14a of the primary sheath P. It supports, around the vertical ascending branch 14c of the same primary sheath, a tank 33 for retaining a heating bath 31. More specifically, this tank has a lower section 33a and an upper section 33b surrounding the primary sheath P and the formation zone respectively. neck C of the secondary bubble S. The section of tank 33a is divided by a horizontal wall 34 into two compartments 36, 37, while the upper section 33b is divided by three horizontal partitions 35 into three upper compartments 38, 39, 40. This figure clearly shows that each of the partitions 34 and 35 is provided with a circular central opening, respectively 34a and 35a, the diameter of which is greater than the corresponding zone of the primary sheath P and of the corresponding part of the neck C of the bubble respectively. . Each of the lower compartments 36, 37 and upper 38, 39 and 40 is connected, by an independent circuit generally designated by 41 and provided with pumps 42, to an independent reservoir, respectively 36a, 37a, 38a, 39a and 40a. Each tank contains a heating liquid which is maintained at a constant temperature by means identical to those 27 previously described. The bowl 33 communicates with the tank 20 through an opening 51 which is bordered by a lip seal 43 cooperating with the periphery of the primary sheath P. It is associated with a means, such as a pump 44, the conduit of which suction 45 plunges into the tank, and whose exhaust duct 46 plunges into the tank 20.

This installation also includes a reservoir 50 containing a preheated liquid 52 and communicating, via a pipe 53 with pump 54, with a nozzle 55, deposited vertically at least above the level of filling NR. This tip is of the pivoting or retractable type, so as to be able to occupy an erasing position, and a working position in which it intersects the trajectory of the wall of the bubble, and more precisely, crosses this wall to come and pour the liquid. 52 in the ascending vertical branch 14c of the primary sheath, or in the bubble S.

The primary sheath P, constituted for example by a linear polyethylene tube with an external diameter of 150 mm and a thickness of 0.8 mm, is drawn between the different guide and return rollers from the cooling station 7 to upstream scarf 10. Subsequently, and as shown in FIG. 2, the sheath 52 is first filled with the fluid in its branches 14a, 14b and 14c. This filling is carried out by introducing the nozzle 55 into the primary sheath, and by pouring the liquid by means of the pump 54 until the level, in the two vertical branches 14a, 14b is clearly above the top of the tank 33 and reaches for example the value NR represented in FIG. 2. This liquid has the effect of opening the primary sheath which until now has been flattened. The fluid is discharged at a temperature of the order of 30 °, so as to communicate its calories to the sheath to begin its preheating. In this phase, the speed of travel of the sheath is reduced and of the order of 7 m / min. Thereafter, and as shown in FIG. 3, the tank 20, which until then was empty, is filled with the liquid 22 coming from the tank 26 at a temperature of the order of 60 °. This calorific contribution also contributes to the preheating of the primary sheath P passing through this tank. At this stage, the ascending vertical branch 14c of the primary sheath P rising in the containment tank 33 forms a cylindrical body which comes into contact with the seal 43 between the tank and the tank.

The pressure of the fluid in the lower part of the primary sheath P above the rollers 13 is of the order of 0.5 kg per cm ² , for a primary sheath having a diameter of 150 mm containing a column of water of 5 m.

In the following phase shown in FIG. 4, each of the compartments of the tank 33 is filled with liquids brought to a temperature of 60 °, then each of the compartments is brought to its operating temperature. Thus compartment 36 is set at 70 °, compartment 37 at 100 °, compartment 38 at 120 °, compartment 39 at 120 °, and compartment 40 at 100 °.

It will be noted that the temperature of the compartments increases from the first compartment 36, lower and upstream, to the first compartment 38 for forming the neck C of the bubble S, then decreasing from this zone.

It is obvious that the number of lower and upper compartments can be increased in number and diameter, depending on the diametrical dimensions of the final bubble. However, it seems necessary that the lower compartments are at least two in number, and that the upper compartments are three in number in order to obtain satisfactory thermal regulation.

As soon as the temperature of the compartments 38 and 39 reaches the softening temperature of the material of the primary sheath P, the fluid 52 placed inside the latter causes, by its weight, the radial expansion of the sheath and the formation of the neck C visible in FIG. 1. It follows that the upper level of the fluid 52 descends from its position NR to a position ND (FIG. 4) which is lower than the level of the heating fluid 31 contained in the tank 33. Simultaneously , the diametral increase of the sheath, by formation of the bubble S, causes an elevation of the level in this tank.

The final calibration is carried out by reintroducing the tip 55 into the secondary bubble S so as to increase the quantity of fluid 52 placed in the neck C until the desired diameter of the bubble is obtained, this diameter being for example from four to five times greater than the diameter of the primary sheath. It will be noted that, by effect of gravity, the level of the fluid contained in the descending vertical branch 14a of the primary sheath automatically adjusts to the level of the liquid contained in the bubble C and deforming the ascending vertical branch 14b. The final adjustment is completed by actuating the pump 44 to bring the level of the liquid contained in the tank 33 to the same height as the level of the fluid placed inside the bubble C.

When these starting operations are completed, the pulling scarf 10 is brought to its final speed, for example 40 m / min, so as to ensure the longitudinal stretching, at the same time as the internal fluid ensures the radial expansion of the bubble.

The normal operation of this installation makes it possible, from a bilinear polyethylene tube having a diameter of 150 mm and a thickness of 0.8 mm produced by station A, to obtain, at bi-stretching station B, a tubular sheath having a diameter of 1100 mm and a thickness of 0.015 mm, stretched, transversely and longitudinally and therefore having excellent qualities for retraction.

It appears from the above that, with the method and the installation according to the invention, the quantity of fluid 52 contained in the secondary bubble 34 acts permanently, by its weight, to ballast and hold the base of the bubble and force it to expand radially. By its hydrostatic character this fluid calibrates the neck 34, pressing on a fluid of the same density, and of density greater than air. As a result, this calibration is carried out under better conditions than when it is carried out by an internal gas pressure opposing an external gas pressure.

All the forces necessary for the radial expansion of the bubble, which in current systems are exerted by an internal air pressure, are here replaced by the weight of the internal fluid which exerts on the neck of the bubble only very low pressure of the order of 0.05 kg per cm ² .

In operation, the nozzle 55 is removed, so that, when the levels of the interior and exterior fluids are balanced in the zone 33, the pressure inside the secondary bubble S is close to zero. This can cause wrinkles when flattening the sheath.

To remedy this, as shown in FIG. 1, the frames 70 of the flattening cylinders 72 are articulated at 73 around parallel horizontal axes, and are associated with means making it possible to bring them one of the other. These means can be constituted, as shown for the right frame in FIG. 1, by a counterweight 74, or as shown for the left frame, by a jack 75 exerting a substantially constant force on the corresponding frame. These means make it possible, by pivoting the two frames together, to reduce the volume of the bubble S, and consequently, to increase the pressure of the atmospheric air contained in this bubble. This increase in pressure, which is of the order of 0.05 to 0.1 kg per cm ² , is small but is sufficient to avoid the formation of folds during the flattening of the sheath.

Unlike an existing bi-stretch installation pulling the bubble longitudinally downwards, heated by a reheating oven placed at the top, which requires continuously adjust the bubble heating, failing which, if the neck is too hot, it can move and cause wrinkles, and if it is not hot enough, require greater gas pressure can lead to the bursting of the bubble, the method according to the invention ensures this expansion only by the weight of the fluid contained in the bubble which has a value of a few kilograms and is very stable. Under these conditions, if the sheath is too heated, it will also deform more radially, which has the effect of reducing the pressure communicated to it by the internal liquid. This can cause wrinkles, but no instability of the cervix. Likewise, in the event of insufficient heating, the cervix is formed less, the level of liquid in the bubble rises to, by its weight, automatically recalibrate the cervix.

With this process, a temperature difference of the order of 5 to 10 ° from the softening point of the material can be accepted, without this having any consequences on the quality of the final film or on the stability of the bubble. .

The installation makes it possible to bi-stretch or bi-orient very common materials on the market, such as standard linear polyethylenes of grade 1 and density 0.920, but it can also bi-stretch other mono or multilayer materials, such as polypropylene, polyethylenes and all other thermoplastics.

Advantageously, the chimney 32 is provided with safety means 76, such as a rotary knife capable, in the event of production stoppage, of incising the lower part of the descending vertical branch 14a of the primary sheath P to rapidly empty the fluid 52 contained in this branch, but also in the ascending vertical branch 14c. It is obvious that under these conditions the tank 20 has a capacity allowing it to accommodate, in addition to the normal volume of the fluid 22, the entire volume of the fluid 52, and the entire volume of the fluid contained in the reheating tank 33 This provision ensures the safety of personnel working around the installation.

The heating fluid can consist of water, oil or any other liquid or viscous product resistant to a temperature of at least 150 ° C, non-combustible and considered to be harmless to humans. Optionally, additives are added to this liquid to prevent it from wetting the walls, inside and outside, of the bubble C. It may be an acrylic coating in aqueous phase or any other heat-sealable acrylic varnish.

Claims

1. Method for ensuring the bi-stretching of a primary plastic sheath describing a U-shaped trajectory with a descending vertical branch (14a), a horizontal branch (14b), and an ascending vertical branch (14c), trajectory during which said sheath is heated by passage through a bath of hot liquid, characterized in that it consists in carrying out the radial expansion of the neck (C) of the secondary bubble (S) by means of a mass of heating liquid (52 ) contained in the primary sheath (S) and going from its descending vertical branch (14a) to its ascending vertical branch (14c), and in controlling the expansion of the neck (C) of the secondary bubble (S) by adjusting the quantity of internal heating liquid (52) and by equalizing the level of the external heating bath (31) with that of the internal bath.

2. Method according to claim 1, characterized in that it consists in dividing the external heating bath (31) of the primary sheath (P) into several superimposed zones (36 to 40), and in supplying these zones, independently l '' from each other, by independent heating liquids, and so that the temperature of the zones increases from the entry of the sheath (P) in the external bath (31) to the zone of formation of the neck (C) of the secondary bubble (S), then decreasing from this area to the exit of the bath (31).

3. Method according to any one of claims 1 and 2 characterized in that it consists in heating the horizontal branch (14b) of the primary sheath (P) by circulating it in a bath of hot liquid (22), independent of the external reheating bath (31) of its ascending vertical branch (14c) and brought to a temperature lower than that of this bath.

4. Installation for bi-stretching a primary sheath of synthetic material comprising an extrusion assembly (A) forming a primary sheath (P), means (7) for cooling this sheath, and return means leading the sheath to a bi-stretching station (B) comprising, itself, an upstream pulling scarf (9) and a downstream pulling scarf (10), and return means (12, 13) describing to the primary sheath (P) a U-shaped trajectory during which it passes through a hot reheating bath, from which it emerges by an ascending vertical trajectory (14c) characterized in that it comprises:

- A reservoir (50) of heated liquid (52) connected by a pipe to a nozzle (55) capable of crossing, at least temporarily, the wall of the ascending vertical branch (14c) of the sheath (14b) at the bi station stretching, to pour into this branch (14c), into the horizontal branch (14b), and into the other vertical branch (14a), a quantity of liquid, determining the radial expansion of the secondary bubble, - And means (44) for making the external hot bath level (31) coincide with the liquid level (52) inside the neck (C) of the secondary bubble (S).

5. Installation according to claim 4 characterized in that the ascending vertical branch (14c) of the sheath passes axially through a holding tank (33) of the hot bath (31), divided into at least two lower compartments (36, 37), and in at least three upper compartments (38, 39, 48) by horizontal partitions (34, 35) each comprising a circular central opening (34a, 35a) whose diameter is greater than the diameter which must have, in this zone, respectively , the primary sheath (P), the neck (C) and the secondary bubble (S), while each of the compartments (37 to 40) is connected, by a circuit (41) with continuous supply, to an independent tank (36a to 40a) containing a reheating liquid maintained at constant temperature.

6. Installation according to all of claims 4 to 5 characterized in that the tank (33) is carried by a tank (20) which, crossed by the horizontal branch (14b) of the primary sheath (P), carries means deflection (12, 13) of the descending vertical branch (14a) and of the ascending vertical branch (14c) of this sheath, and contains a bath (22) of heating fluid connected, by a circuit (23, 24), to an independent reservoir (26) provided with means for maintaining the liquid at constant temperature.