FR2723865A1 - Device for conveying selectively plastic articles e.g. bottles for recycling - Google Patents

Abstract

A device for conveying selective articles, partic. plastic bottles or flasks, has an upwardly sloping guide channel (4) containing a system of two driven brushes (5, 6) placed opposite each other either side of the path of the bottles in the guide. The angle of inclination ( alpha ) of the guide, the separation distance of the brushes, the speed of rotation and the length and flexibility of the brush bristles determines the sort of articles having an external size (D) and weight less than a given value that will be carried up the channel by the bristles of the rotating brushes. The selective conveyor is associated with bottle compaction and comprising a heated pipe (13) inclined downwards, an exit door system (30) and then a system to flatten the softened bottles (14, 15).

Description

DEVICE FOR SELECTIVELY DRIVING ARTICLES, IN PARTICULAR
OF BOTTLES. AND EQUIPPED TREATMENT PLANT
OF SUCH A DEVICE
The present invention relates to the selective drive of articles, in particular according to their weight and / or their external dimensions. It relates more particularly to a device of this type intended to admit or reject bottles or bottles in collection and treatment installations. such items.

 The possibility of recycling used articles, such as plastic bottles or flasks, glass bottles ... depends on the sorting which can be done in order to group together articles of the same composition. Regarding the recovery of glass bottles, we now make available to the public specially provided skips. These are provided at their upper part with circular orifices allowing glass bottles to be introduced into the grab. It turns out, however, that through this same orifice it is possible to introduce other types of articles than glass bottles, such as bottles or plastic or aluminum bottles ... The problem therefore arises of selection at the entrance of such collection skips.

 The first goal set by the applicant is to propose a device for selecting articles according to their weight and dimensions, which is suitable for use in such collection facilities, without however this application being exclusive. .

 This object is perfectly achieved by the selective drive device for articles, in particular bottles or bottles, according to the invention. Typically said device comprises a channel for guiding the articles, inclined upwards, and at least one set of two rotary drive brushes; said brushes are placed opposite one another, on either side of the path of an article in the guide channel. In addition the angle of inclination of the guide channel, the spacing and the speed of rotation of the two brushes, as well as the length and the flexibility of the bristles of said brushes are determined so that only articles having given external dimensions and a weight less than a given weight value are entrained in the guide channel by the action of the bristles of the rotary brushes.

 It is the friction of the bristles against the opposite walls of the article during the rotation of the brushes, which will involve on said article sufficient force to on the one hand compensate for the effect of the article's own weight which tends to slide back into the guide channel and on the other hand to carry out the drive in front of the article. In order for this action of the bristles to be carried out, the first parameter resides in the dimensioning of the article relative to the spacing of the brushes. This dimensioning must be greater than the free space existing between the bristles of the brushes so that said bristles can bear and "brush" the two opposite faces of the article. For example for a plastic bottle having a diameter outside 84mm, the distance between the ends of the bristles of the two rotary brushes will be 78mm, ie a penetration of the bottle of the order of 3mm in each brush.

 The second parameter is the weight of the article. It is understood that the various elements entering into the structure of the drive device must be determined at the start depending on the application which is intended, for example the sorting between plastic bottles and glass bottles. In the latter case, for two bottles, one of plastic and the other of glass, having the same dimensions, it is mainly the angle of inclination of the guide channel and the speed of rotation of the brushes which will be decisive. so that the plastic bottle is entrained in the guide channel and not the glass bottle, due to the excessive weight of the latter.

 However, a particular problem arises with regard to plastic articles. Indeed the recycling of such articles must, for the good, also take into account the plastic material of which the article is made. For example, the recycling circuits for polyvinyl chloride are different from those for polyethylene.

 It is another object of the invention to propose a device of the aforementioned type which can differentiate plastic articles according to the material of which they are made.

 This is obtained by means of the device according to the invention in which the bristles of the rotary brushes are made of a material capable of being charged with electrostatic energy during friction on the surface of plastic articles, so that the speed of movement of an article given under the action of said brushes, inside the guide channel depends on the type of plastic material constituting the article.

 The Applicant has indeed noticed that, unexpectedly, the electrostatic energy charge caused by the friction of the bristles of the brushes on the plastic article also had an influence on the capacity of entrainment of these brushes in the guide channel and that this could cause appreciable variations in the speed of movement between articles differing only in the plastic material of which they are made.

 In particular, this effect is significant for differentiating plastic articles on the one hand from polyethylene and on the other hand from polyvinyl chloride; in this case the bristles of the brushes are made of polyamide, the articles of polyvinyl chloride having a higher speed of movement than the articles of polyethylene.

 As regards in particular the selective sorting of bottles, and in particular of plastic bottles with a capacity of 1.5 liters which are articles which have a length of the order of 300mm, it is important that the contact surface between the bristles brushes and the bottle is generally sufficient to cause the desired selective training. Preferably, each rotary brush consists of an endless band, stretched and driven in rotation between two cylinders, the centers of which have a spacing of between 400 and 800 mm.

 It is another object of the invention to provide an installation for processing plastic articles in which a selective drive device having the above characteristics is used and which is furthermore equipped with means for detecting the speed of movement of the article inside the guide channel. Typically, this installation further comprises a processing unit disposed at the outlet from the guide channel, provided with control means capable of controlling the operation of said unit, for each plastic article introduced, as a function of the speed which is detected, so as to adapt the treatment according to the type of plastic material.

 Thus in this installation, use is made of the selection which has been made in the selective drive device due to the difference in speed of movement as a function of the material of the article. The actual treatment can be of any type, it can for example be simply a separation treatment, in which the articles having substantially the same speed of movement in the guide channel are grouped together in homogeneous batches.

 Preferably it is a treatment aimed at compacting the plastic article optimally taking into account the material of which it is made. In this case, advantageously, the processing unit comprises a) a preheating duct, capable of preheating one plastic article at a time, said duct being inclined downwards from the outlet of the guide channel and being provided with heating means and possibly means for closing its outlet orifice, b) and a device for flattening plastic articles, located at the outlet of the preheating duct; moreover the control means of the processing unit are capable of controlling the heating means and possibly the means for closing the preheating duct so as to adapt the interior temperature of the duct and possibly the heating time of the plastic article staying in the duct, depending on the type of plastic material.

 The purpose of preheating the plastic article is to bring it to a temperature close to its softening temperature so that the action of the flattening device can effectively compact said article, without it being able to resume its initial shape by elastic deformation. This heat treatment is of course conditioned by the power of the heating means and also the residence time of the plastic article in the preheating duct. In particular, the applicant has noticed that the obturation of the preheating duct allows, by simple insulation vis-à-vis the outside, substantially increase the temperature prevailing inside the duct; moreover, this closure makes it possible to block in position inside the duct the article which has a natural tendency to descend towards the flattening device, due to the inclination of the preheating duct.

 In the case of an installation intended for the flattening of several types of plastic articles which are distinguished by different softening temperatures, according to a preferred embodiment, the heating means of the preheating duct or possibly the heating means of the flattener have a first heating level capable of bringing the article to be heated to a first temperature level which is necessary for flattening the articles having the lowest softening temperature; moreover, the means for heating the preheating duct have one or more other heating levels, actuated by the control means, so as to bring the article to be heated to one or more higher temperature levels corresponding to the softening temperature of the other types of articles, finally the actuation of the control means is a function of the speed of movement of the article in the guide channel, characteristic of the article of a given type.

 In particular this is so when it comes to the flattening of polyethylene bottles and polyvinyl chloride bottles. For example, the means for heating the preheating duct have two heating levels. The lowest corresponds to the softening temperature of a polyvinyl chloride bottle and the second corresponds to the softening temperature of a polyethylene bottle.

 In a simplified version of the preheating duct for flattening installation of plastic bottles, the part of the duct over which each bottle is made to slide, is a slide having a V-shaped profile whose walls are made of a material with low power for hanging hot plastic, in particular polyglass stainless steel.

 This particular arrangement is intended to prevent blockage of a bottle in the preheating duct. The V profile minimizes the contact surface and therefore friction between the bottle and the walls of the guide duct. It is this small contact surface combined with the choice of the material constituting said walls which makes it possible to obtain perfect reproducibility of the sliding of the bottles in the preheating duct to the flattening device.

 Another difficulty may arise in the case where the flattening device consists of a pair of compression rollers, mounted to rotate at the outlet of the preheating duct. Indeed even when said rollers are filled on their periphery with teeth intended to come to nest into each other, there are however malfunctions in the passage of the article between the compression rollers, malfunctions which result in a certain skating of the rollers on the heated material which prevents entrainment of the article between the rollers.

 To overcome this drawback, it is provided that in the above installation the heating means of the preheating duct are arranged so that the surface of the preheating duct which is intended to be in contact with the plastic article is not directly heated by said means; moreover the center distance of the compression rollers is determined so that the plastic article, coming from the preheating duct, first comes into contact with the lower compression roller, which is positively driven in rotation while the other is mounted free to rotate.

 The fact that the surface which is in contact with the plastic article is not directly heated by the heating means creates during preheating, a certain heterogeneity in the softening of the article; the lower part of the article is relatively less heated, which results in higher hardness and therefore better adhesion during the action of the compression rollers.

The present invention will be better understood on reading the description which will be given of a preferred embodiment of a selective drive device integrated in an installation for flattening polyethylene and polyvinyl chloride plastic bottles, illustrated by the attached drawing in which
FIG. 1 is a schematic representation in section of the installation,
FIG. 2 is a diagrammatic plan view of the same installation,
Figure 3 is a schematic sectional view of the preheating duct.

 The selective drive device for articles which is the subject of the present invention preferably relates to, but is not limited to, the selective collection of household waste, more precisely the collection relating to the recovery of bottles. It is in particular intended to equip a container for the selective collection of plastic bottles. In Figure 1 is shown, equipping such a container 1 not only the selective drive device 2 but also a device 3 for flattening plastic bottles.

 The selective drive device 2 proper consists of a guide channel 4 and a set of two rotary brushes 5,6.

 The guide channel 4 has its inlet orifice 4a which opens into the upper part of the container 1. This orifice 4a is dimensioned so as to allow the user to introduce a bottle 7 of large dimensions, in particular having a capacity of 1.5 liters and making about 80mm in diameter by 300mm in length.

The guide channel 4 is inclined upward by a determined angle, between 15 and 45 ", preferably of the order of 30
Each rotary brush 5, 6 is arranged, facing one another, on either side of the path of the bottle 7 in the guide channel 4. More precisely the bristles 8 of each brush are caused to penetrate in the guide channel 4 and somehow form the side walls of said channel. The spacing d between the ends 8a of the bristles 8 of the brushes 5, 6, forming the two lateral walls of the guide channel is less than the diameter D of the bottle 7 whose selection is to be made using the drive device 2.L 'difference e which corresponds to the difference between the diameter D and the distance d between the ends 8a of the bristles 8 of the two brushes 5 and 6 facing each other must be sufficient so that when the bottle 7 passes through the guide duct 4 there is penetration of said bottle 7 into the bristles 8, as will emerge from the explanation which will be given below.

 Preferably each rotary brush 5, 6 consists of an endless strip 9 on one face of which tufts 10 of bristles 8 are implanted and which is stretched between two rollers 11,12 one of which 12 is driven in rotation.

 In a specific embodiment, each endless band had a width! about 80mm; it was furnished with tufts of hair each having a diameter of the order of 0.45 mm and a length of 50 mm.

The center distance L between the cylinders 11,12 was of the order of 500mm.

The spacing d was 78mm, knowing that a plastic bottle with a container of 1.51 has a diameter D which is 84mm. Thus the gap e was 6mm, which corresponded to a penetration of the bottle 7 into the bristles 8 of each of the brushes of the order of 3mm.

The operation of the device 2 is as follows. The user introduces through the inlet orifice 4a the bottle 7 into the container 1. This introduction is necessarily in the longitudinal direction of the bottle 7 due to the size of the inlet 4a. It can either be the base of the bottle forward as shown in Figure 1 or possibly the bottom. Once the user has made the bottle 7 penetrate into the guide channel 4 to a certain depth, the latter penetrates into the bristles 8 of the brushes 5,6 which, due to the rotation of said brushes in the direction of the arrow F exerts a brushing action on the external surface of the bottle 7, in a continuous and balanced manner. This brushing action causes the bottle 7 to move in the guide channel 4 all along the side walls formed by the penetrating bristles 8 in the guide channel 4 to the outlet 4b of said channel.

 Such a drive can effectively only occur if the diameter of the bottle is sufficient for penetration into the bristles 8 of the brushes and if the weight of said bottle 7 is low enough for the frictional forces of the bristles 8 are greater than those generated by the weight of the bottle 7 and the possible friction forces between said bottle 7 and the surface with which it is in contact on the guide channel 4.

 Thus if the user presents in the device 2 a glass bottle, having the adequate dimension, the action of the rotary brushes 5,6 will be insufficient not only to cause the displacement of this bottle in the guide channel but also to maintain that -this in the channel. As a result, the glass bottle, as soon as it is released by the user, will fall towards the latter, being in a way rejected by the selective drive device.

 This device 2 is advantageously used in a complete installation for compacting, by flattening, plastic bottles. The advantage of this installation is to reduce in a very large proportion the volume occupied by plastic bottles and therefore to increase the collection capacity of the same container 1 in terms of quantity of bottles.

 This installation comprises, at the outlet of the selective drive device 2 which has just been described, a flattening device 3 which comprises, on the path of the bottle, a preheating duct 13 then two compression rollers 14,15.

The preheating duct 13 is generally inclined downward so as to allow displacement by simple gravity of the bottle inside it as far as between the compression rollers 14,15. This angle inclination ss is preferably between 15 and 45C
Its upstream end 13a is connected to the outlet 4b of the guide channel 4 so that the bottle 7 can pivot from the position inclined towards the front by an angle a to a position inclined towards the rear by an angle p , and this without said bottle getting stuck during this maneuver. For this, the front end 13a of the preheating duct has a height H greater than that h of said duct in the upstream part which includes the heating means 16.

 In addition, the respective bottoms 17, 18 of the guide channel 4 and of the preheating duct 13 are connected near the center distance between the two outlet cylinders 12 of the rotary brushes 5,6 so that the pivoting of the bottle occurs as soon as the said brushes exit.

 The heating means 16 consist of two sets 19, 20 of three infrared emitters 21, 22, 23 for the first set and 21 ', 22', 23 'for the second set.

 In FIG. 3, the arrangement of the three infrared emitters of a given clearance has been shown: the first emitter 21 is arranged in the upper part of the duct 13, while the other two emitters 22, 23 are arranged laterally, being made face, in the lower part of the conduit 13. The overall action of the infrared emitters is to bring the temperature inside the conduit 13 to a value sufficient to obtain a softening of the bottle 7 when it passes between the rollers compression 14,15, so as to obtain a flattening of the bottle 7, without there being any volume recovery by elastic deformation. The presence of the two sets 19,20 aims to allow the regulation of this temperature at two levels different, corresponding to two softening temperatures, on the one hand that of polyethylene and on the other hand that of polyvinyl chloride.

 In a specific embodiment, the two lateral transmitters 22,23 of each 19,20 had a power of 400W, while the two upper transmitters 21 of each set 19,20 had a power of 250W.

 As can be clearly seen in FIG. 3, the surface 24 which is in contact with the bottle 7 in the bottom 18 of the preheating duct 13 has a V-shaped profile, with the two wings of the V which are inclined towards the center line 18a of the bottom 18 of a determined angle y.

The object of this particular V shape of the surface 24 is to minimize the contact zone between the bottle 7 and the preheating duct 13.

In this case, these are two contact lines 25, 26, which generate minimal friction forces between the bottle 7 which is already in a softening state and the surface 24. This makes it possible, without fail, to obtain the sliding of the bottle 7 in the preheating channel 13 as far as between the compression rollers 14,15, even if this bottle 7 remains in said conduit 13 for a determined time as will be explained below.

 The installation comprises an electronic control circuit 27 which makes it possible to control the operation of the first set 19 of infrared emitters or of the two sets 19 and 20, as a function of the temperature T internal to the preheating duct 13 which is desired to obtain the softening of the plastic bottle 7. This temperature T will differ depending on the material of which the bottle is made 7. The first set 19 of infrared emitters makes it possible to obtain the lowest temperature T while the two sets 19 and 20 actuated simultaneously achieve the highest temperature.

 The choice between these two heating levels can be controlled by detecting the speed of movement of the bottle 7 in the guide channel 4.

 In fact, by using brushes 5,6 for the bristles 5,6 a material capable of being charged with electrostatic energy during friction by contact on the surface of plastic bottles, it has been found that for a bottle 7 of the same weight and the same dimensions , its speed of movement in the guide channel 4 differed from one material to another depending on the electrostatic charge used. In particular with regard to polyethylene and polyvinyl chloride, this difference in speed is significant enough to make it possible to differentiate these two materials from one another.

 In a specific embodiment, the bristles 8 of the brushes 5,6 were made of polyamide, in particular of the brand NYLON; the speed of rotation of the driving brushes 5,6 was adjusted so that the passage time in the guide channel of a polyvinyl chloride bottle was approximately 5 seconds, while that of a bottle in polyethylene was around 7 seconds.

 This speed of movement of the bottles is measured by means of presence detection sensors 28,29, either mechanical or optical, connected to the electronic control circuit 27. Said circuit 27 can easily transform the difference between the detection pulses given by the sensors 28 and 29 in order to detect the time necessary for the bottle 7 to cross the guide channel 4 and to control the actuation of one or two sets of infrared emitters according to the detection which has been made, and thereby the type of plastic material.

 In order to optimize the operation of the heating means 16, the outlet of the duct 13 from the preheating duct is provided with a shutter 30, the opening and closing of which are controlled by the electronic control circuit 27. In addition, a temperature 31 penetrates inside the preheating duct 13 towards this outlet end. This probe 31 is also connected to the electronic control circuit 27.

 The bottle 7 which comes from the guide channel 4 and which rocks in the preheating duct 13 is stopped by the shutter 30 and remains in the duct 13 for a determined time t, which depends on the material of the bottle and therefore on the temperature T to which the bottle should be brought. It is the electronic control circuit 27 which automatically controls all of these operations in the following manner. The speed of movement of the bottle 7 in the guide channel 4 is detected thanks to the time which elapses between the detection obtained by the sensors 28 and 29. Depending on the value of this time, the electronic control circuit 27 will control the actuation of one or two sets of infrared emitters: one set if it is polyvinyl chloride and two sets if it is polyethylene. The opening of the shutter 30 will be automatically controlled when the temperature measured by the probe 31 has reached the predetermined value corresponding to one or other of the materials, polyethylene or PVC.

 It should be noted that the presence of the shutter, in addition to making it possible to stop the bottle 7 and therefore its stay in the conduit 13, improves the thermal insulation of said conduit and therefore the energy consumption of the installation.

Once the bottle 7 has reached the required temperature,
The order of 70 "in the case of PVC, of the order of 1200 in the case of polyethylene, the opening of the shutter 30 is controlled and the bottle 7 in a softened state leaves the preheating duct 13 to reach to the compression rollers 14 and 15 between which it is flattened to obtain a reduced volume.

 In order to obtain, for sure, the sliding of the bottle 7 in the softened state which remains in the preheating duct 13, in a specific embodiment, the angle y of inclination of the wings of the surface V 24 was of the order of 17 and the material chosen to produce this surface 24 was polyglass stainless steel, that is to say a material with low adhesion power of the plastic in the softened state.

 The compression rollers 14 and 15 are preferably furnished on their periphery with teeth 32 which mesh with one another.

 Ced makes it possible not only to flatten but also to drive the plastic bottle in the softened state due to the penetration of the teeth 32 into said bottle.

 However, it has been noted that there may be, in certain cases, a slip of the plastic bottle which is bothersome or even preventing its penetration between the two rollers 14,15.

 To overcome this drawback, it was chosen to position the rollers 14 and 15, the axes of rotation of which are horizontal, so that, leaving the preheating duct 13, the bottle 7 first touches the teeth 32 of the roller lower 15. It is this lower roller which is driven in rotation, while the upper roller 14 is free in rotation, being driven itself by the gear of the teeth 32. Because in the example illustrated, with three infrared emitters, two arranged laterally and one at the top, the portion of bottles placed facing the contact surface 24 is not heated directly by said emitters, it would therefore be relatively less softened than the other portions, exposed directly infrared transmitters. This could explain why the direct contact of this relatively harder portion with the teeth 32 of the roller 15 allows better bonding with said teeth better than if this surface were in a more advanced softening state.

 The present invention is not limited to the embodiment which has just been described both with regard to the selective drive device 2 proper and with regard to the flattening device 3.

 In particular in the case of an installation which would not aim to compact plastic bottles in a container but simply to carry out a sorting in batches between polyethylene bottles and PVC bottles, device 2 could be followed a referral system which would be controlled by the speed of movement of the bottles in the guide channel. 0n could therefore, thanks to the speed detected at each bottle, direct it to the corresponding collector.

 Of course, the present invention is not limited to the mode of presentation of the articles in the form of a bottle. It also relates to any other form of presentation, in particular of bottles, capable of being driven, selectively by driving brushes on an inclined wall, characteristics of the invention.

Claims (11)

1. Device for selectively driving articles, in particular bottles or bottles, characterized in that it comprises a guide channel (4) for the articles, inclined upwards, and at least one set of two rotary brushes d drive (5,6), which are placed opposite one another, on either side of the path of an article in the guide channel (4) and in that the angle of inclination (a) of the guide channel (4), the spacing (d) and the speed of rotation of the two brushes, as well as the length and flexibility of the bristles of said brushes (5,6) are determined by so that only the articles having given external dimensions (D) and a weight lower than a given weight value are entrained, in the guide channel (4), by the action of the bristles of the rotary brushes. 2. Device according to claim 1 characterized in that the bristles (8) of the rotary brushes (5,6) are in a material capable of being charged with electrostatic energy during friction on the surface of plastic articles (7), such so that the speed of movement of a given article under the action of said brushes, inside the guide channel (4) depends on the type of plastic material constituting the article. 3. Device according to claim 2 characterized in that the plastic articles being on the one hand in polyethylene and on the other hand in polyvinyl chloride, the bristles of the brushes are in polyamide, the articles in polyvinyl chloride having a speed of greater displacement than polyethylene items. 4. Device according to one of claims 1 to 3 for the selective sorting of plastic bottles with a capacity of the order of 1, Slitre, characterized in that each rotary brush (5,6) consists of a strip endless, taut and driven in rotation between two cylinders (11,12) whose spacings have a spacing between 400 and 800mm. 5. Installation for processing plastic articles in which is implemented the selective drive device of one of claims 1 to 4 which is equipped with means for detecting (28,29) the speed of movement of the article (7) inside the guide channel (4), said installation further comprising a processing unit disposed at the outlet (4b) of the guide channel (4), provided with control means able to control the operation of said unit, for each plastic article introduced, as a function of the speed which is detected, so as to adapt the treatment as a function of the type of plastic material. 6. Installation according to claim 5 characterized in that the processing unit comprises switching means controlled as a function of the speed detected so as to group together in homogeneous batches the articles having substantially the same speed of movement in the guide channel . 7. Installation according to claim 5 characterized in that the processing unit (3) comprises a) a preheating duct (13), capable of preheating a plastic article (7) at a time, said duct being inclined downwards from the outlet (4b) of the guide channel (4) and being provided with heating means (16) and possibly with means for closing (30) its outlet orifice, b) and a device for flattening the articles in plastic, located at the outlet of the preheating duct, and in that the control means (27) of the processing unit (3) are capable of controlling the heating means (16) and possibly the sealing means (30 ) of the preheating duct (13) so as to adapt the internal temperature (T) of the duct and possibly the heating time of the plastic article remaining in the duct, depending on the type of plastic material. 8. Installation according to claim 7 characterized in that the heating means (16) of the preheating duct (13) or possibly the heating means of the flattening device have a first heating level capable of carrying the article to be heated (7) at a first temperature level which is necessary for flattening the articles having the lowest softening temperature, in that the heating means (16) of the preheating duct (13) have one or more other levels heating, actuated by the control means (27), so as to bring the article to be heated (7) to one or more higher temperature levels corresponding to the softening temperature of the other types of articles; and in that the actuation of the control means (27) is a function of the speed of movement of the article in the guide channel, characteristic of the article of a given type. 9. Installation according to claim 8 characterized in that, as regards the flattening of polyethylene bottles and polyvinyl chloride bottles, the heating means (16) of the preheating duct have two heating levels, the most bottom corresponds to the softening temperature of a polyvinyl chloride bottle and the second corresponds to the softening temperature of a polyethylene bottle. 10. Installation according to one of claims 7 to 9 for the flattening of plastic bottles characterized in that the part of the preheating duct (13) on which is caused to slide each bottle, is a slide having a profile in the form of V which is made of a material with a low grip power on hot plastic, in particular polyglass stainless steel. 11. Installation according to claim 7 characterized in that the heating means (16) of the preheating duct (13) are arranged so that the surface (24) of the preheating duct (13) which is intended to be in contact with the plastic article (7) is not directly heated by said means (16), in that the flattening device consists of two compression rollers (14,15) whose positioning is determined so that the The plastic article, coming from the preheating duct, first comes into contact with the lower compression roller (15), which is positively driven in rotation.