This is a divisional of application Ser. No. 11/667,958 filed Jul. 16, 2007, which is a National Stage Application filed under §371 of PCT Application No. PCT/FR2005/002826 filed Nov. 15, 2005. The entire disclosure of the prior application, application Ser. No. 11/667,958 is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates to the production of containers. 
     It relates more particularly to a method and an installation for producing containers—especially bottles—from thermoplastic parisons. 
     BACKGROUND ART 
     Such a method involves a first step during which the parisons are heated, within an appropriate heating unit, then a second step during which the parisons are introduced, hot, into a multiple-mold blow-molding or stretch-blow-molding unit where they are shaped into containers. 
     On leaving the blow-molding or stretch-blow-molding unit, the containers thus formed will be directed either toward a storage unit to await subsequent filling or directly toward a filling unit. 
     Let us remember that a container parison comprises a neck, intended to take the closure that seals the container that is to come and which is already at its final dimensions, extended by a body, the shaping of which will lead to the actual container proper. 
     The heating of the parisons is generally performed within an oven equipped with an array of tubular halogen lamps past which the parisons progress, while being rotated on themselves. More specifically, an oven contains several elementary modules, each containing several lamps, each of the lamps being controlled individually so that, ultimately, on leaving the oven, the temperature of the body of each of the parisons is above the glass transition temperature of their constituent material and a heating profile is obtained on each parison, which profile is predetermined such that the distribution of material is optimized in the container that is to be obtained. 
     This method of heating does have a certain number of disadvantages. 
     First, its energy efficiency (that is to say the ratio of the power absorbed by the parisons to the power consumed by the lamps) is extremely low, of the order of 11 to 15%. This is because of the spatial diffusion of the radiation emitted by the lamps, only a fraction of which reaches the body of the parisons. The low value displayed by this efficiency has a negative impact on production rates. 
     Next, the heating profile (that is to say the plot of temperatures measured along the length of the parison) cannot be obtained precisely; given the diffusion effect, the radiation from the lamps interferes with each other which means that seeking precisely to regulate the intensity of the combined radiation at a given distance from the lamps is an extremely fanciful notion. 
     In order to alleviate this disadvantage, there has already been the idea to make the parisons file past the lamps at the closest possible range. However, this then gives rise to an undesirable problem of overheating at the surface of the parisons, which phenomenon cannot be lessened unless an expensive ventilation system is fitted and operated. 
     Furthermore, there is also a significant phenomenon of thermal convection whereby the ascending air streams transfer some of the emitted radiation to the capital part of the parison. Now, the neck of this parison needs to be kept at a modest temperature so that it maintains its original dimensions. 
     Hence, in order to limit the incident heating of the neck by thermal convection, it has become judicious to orient the parisons neck down. As such a precaution proved to be insufficient in certain instances, it was combined with ventilation of the neck. Whatever the case, this orientation of the parisons entails, on entering the heating unit, an operation of inverting the preforms, because the preforms are generally introduced into the oven neck up, and also an operation of inverting either the preforms before they are introduced into the mold when the stretch-blow-molding step is performed neck up (which is the more common scenario), or of the containers as they leave the installation so that they can be stored or filled. These inverting operations entail installing and operating appropriate devices which make the installation more complicated and have a negative impact on cost. 
     SUMMARY OF THE INVENTION 
     In order in particular to alleviate the aforementioned disadvantages, the method according to the invention for producing a container from a thermoplastic parison involves:
         a step of heating the parison performed by means of at least one beam of coherent electromagnetic radiation, then   a step of forming the container from the parison thus heated.       

     The invention also proposes an installation for producing containers from thermoplastic parisons, which comprises a heating unit for heating the parisons with a view to forming the containers from the parisons thus heated. The installation defines a path that the parisons are intended to follow within the heating unit, which comprises at least one source of coherent electromagnetic radiation directed toward a region situated on the path of the parisons. 
     The radiation can thus be concentrated on to a localized part of the parison, making it possible to obtain a temperature profile close to a predetermined profile, the almost-total absence of diffusion and thermal convection allowing the parison to be heated while it is oriented neck up without this neck experiencing incident heating liable to alter its dimensions. 
     More specifically, the beam of electromagnetic radiation (such as a laser emitted for example by a laser diode) is preferably directed toward the body of the parison. The radiation is preferably emitted in the near infrared, in other words at a wavelength ranging between about 700 nm and 1600 nm. 
     The heating of the parison is preferably performed by means of a plurality of adjacent and/or superposed beams of electromagnetic radiation. In practice, heating may be performed by means of a plurality of juxtaposed and/or superposed laser diodes, for example, in the form of one or more arrays. 
     The or each beam may be linear or planar; it is, for example, directed in a predetermined overall direction, while the parison, at least locally is made to follow a path either substantially perpendicular or substantially parallel to the direction of the beam. 
     In the heating step, the parison is preferably rotated about a predetermined axis, for example, an axis that coincides with an axis of revolution of the parison, so as to obtain uniform heating around the circumference of this parison. 
     Furthermore, the neck of the parison may be ventilated in order to remove the overflow of hot air. 
     According to one embodiment, in the heating step, the beam is reflected at least once off a reflective surface. 
     The heating unit comprises, for example, a chamber comprising a first wall and a second wall facing one another and substantially parallel to the path of the parisons, these walls being positioned one on each side of this path and together delimiting an internal volume, the first wall being equipped with a plurality of superposed parallel slits facing each of which there is positioned, on the opposite side to the internal volume, a row of radiation sources. 
     According to one embodiment, the second wall at least has, on the same side as the internal volume, a reflective internal surface. 
     In order to ventilate the neck of the parison, the heating unit may comprise a ventilation system able to generate an air flow passing through a region situated vertically in line with said chamber. 
     According to an embodiment variant, the installation comprises two successive heating units of this type. 
     According to another embodiment, with the path of the parisons being substantially circular, the heating unit comprises a plurality of successive chambers positioned along the path, each chamber having two cylindrical walls facing each other and positioned one on each side of the path and together defining an internal cavity, each wall having several adjacent reflective facets facing toward the cavity, the source of electromagnetic radiation being directed toward one of these facets. 
     The heating unit, for example comprises an opaque screen adjacent to one of the facets, to absorb the beam after it has been reflected several times off the facets. 
     Whatever the embodiment adopted, the heating unit preferably comprises means for rotating the parisons about their axis of revolution. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will emerge from the description given hereinafter with reference to the attached drawings in which: 
         FIG. 1  is a schematic view of an installation for producing containers from thermoplastic parisons; 
         FIGS. 2 and 3  are perspective views of a block and of an array of laser diodes which may be chosen to equip an installation according to the invention; 
         FIG. 4  is a schematic perspective view showing the internal structure of an array of laser diodes; 
         FIG. 5  is a diagram illustrating the compared efficiency of three different laser sources for heating a PET; 
         FIG. 6  is a schematic perspective view illustrating a heating unit for an installation for producing containers according to a first embodiment; 
         FIG. 7  is an elevation in cross section illustrating the heating unit of  FIG. 6 ; 
         FIG. 8  is a schematic perspective view showing a container parison exposed to a laser beam in a heating unit as depicted in  FIG. 7 ; 
         FIG. 9  is a schematic perspective view illustrating a heating unit for an installation for producing containers according to a second embodiment; 
         FIG. 10  is a schematic perspective view similar to  FIG. 9  also illustrating a heating unit according to a variant of the second embodiment; 
         FIG. 11  is a schematic plan view, from above, illustrating the heating unit for an installation for producing containers according to a third embodiment; 
         FIG. 12  is a plan view, from above, on a larger scale, of a detail of the heating unit depicted in  FIG. 11 ; and 
         FIG. 13  is a view illustrating, in perspective, the detail depicted in  FIG. 12 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  depicts an installation  1  for producing containers, such as bottles, from parisons  2 , in this instance preforms, made of thermoplastic. It is recalled here that the term “parison” covers not only a preform, but also any intermediate part between the preform and the finished container. Some methods actually involve two successive shaping steps, namely a first step of forming an intermediate container from the preform then, after a certain time has elapsed, a second step of forming the finished container from the intermediate container. 
     A parison  2  in the form of a preform is depicted on a large scale in  FIG. 8 . It is a molded component in the form of a test specimen exhibiting symmetry of revolution about an axis A and having a neck  3  intended, as far as possible, not to undergo any deformation during the forming of the container, and a body  4  ending in a bottom  5  and intended to be heated and then shaped. Without implying any limitation to such an application, it is assumed in the remainder of the description that the containers are formed directly from preforms, which means that, for the sake of convenience, this term will be used arbitrarily to denote parisons or preforms. 
     The containers are, for example, made of polyethylene terephthalate (PET), of polyethylene naphthalate (PEN), or another appropriate thermoplastic. 
     As depicted in  FIG. 1 , the installation  1  comprises a feed unit  10  which supplies the preforms  2  to a forming unit  6 . The feed unit  10  comprises, for example, a hopper  11  into which the preforms  2 , produced beforehand by molding, are loaded loose, this hopper  11  being connected to an inlet  12  of the forming unit  6  by a sorting machine  13  which isolates and positions the preforms  2  (which are cold, that is to say at ambient temperature) on a slide  14 . 
     The preforms  2  are then mounted on a transfer line  15  then heated, as they pass through a heating unit  16 , before being introduced hot into a blow-molding unit  17  (or stretch-blow-molding unit) of the multiple-mold carousel type. 
     The containers are then transferred, by means of a conveyer  18 , such as a wheel with cavities, from the molds of the blow-molding unit  17  to an outlet of the forming unit  6 . 
     Within the heating unit  16 , the preforms  2  are heated by means of at least one beam  22  of coherent electromagnetic radiation. 
     For this, the installation  1  defines, within the heating unit  16 , a predetermined path  23  that the preforms  2  follow during the heating step. More specifically, this path  23  is defined by a conveyer (not depicted) equipped with links articulated to one another and from which the preforms  2  are suspended. This driving technique is well known to those skilled in the art and will not be described in detail; let us nonetheless specify that each link comprises attachment means in the form of a hanger, known as a “spinner” in the terms of the art, which fits into or on to the neck  3  of the preform  2 , this hanger having a pinion-shaped part which meshes with a fixed rack running alongside the line, so that as the line advances, the hangers, with their preforms are rotated. 
     The heating unit  16  comprises at least one source  24  of coherent electromagnetic radiation directed toward a target region  25  situated on the path  23  of the preforms  2 , and through which these pass, as we shall see later. 
     The description which follows first of all sets out the choice of the source  24  of electromagnetic radiation for heating the preforms (§1), and then, describes the heating unit  16  and the corresponding heating method, in three exemplary embodiments (§2). 
     1. Choice of the Source of Electromagnetic Radiation 
     Tests have shown that, across the light spectrum, the radiation that is of use for heating a thermoplastic such as a PET (the material from which container preforms for the most common applications are conventionally made) lies in the field of the near infrared, that is to say at wavelengths ranging between 700 nm and 1600 nm. 
     Several lasers available on the market have proved satisfactory in application to the heating of thermoplastics (the tests conducted by the inventors were conducted using a PET). 
     A PET preform generally has a wall thickness ranging between 1 mm and 3 mm, entirely dependent on the type of container that is to be obtained. 
     A first test was conducted by the inventors on PET test specimens 3 mm thick using three laser sources emitting in the near infrared, namely:
         1. first of all, a laser of the Nd:YAG type (this type of laser comprises a neodymium-doped yttrium aluminum garnet amplifier with a power of 4.4 kW, generating an infrared beam with a wavelength of 1064 nm,   2. secondly, a laser diode of the hybrid type, with a power of 3 kW, generating an infrared beam combining two wavelengths of 808 nm and 940 nm respectively, and   3. thirdly, a laser diode with a power of 500 W generating an infrared beam with a wavelength of 808 nm.       

     The diagram in  FIG. 5  shows, for each of these lasers, the plot of the time taken for the material to reach the core temperature of 130° C. (this is in fact the temperature to which PET preforms need to be heated), as a function of the transmitted power density. 
     It can be seen that, while the efficiency of the Nd:YAG laser seems to be superior to that of the diode lasers, the plots are, nonetheless, similar, which shows that the laser can be chosen on the basis of parameters other than efficiency alone, particularly on the basis of the shape of the beam, the size of the source and, of course, its cost. 
     Furthermore, it has been found that the choice of laser is also dependent on the need to safeguard the material from uncontrolled crystallization. A compromise is therefore needed. Although the Nd:YAG has proven its efficiency, the diode laser will take preference over it, being less expensive and less bulky, for an imperceptible difference in efficiency in the application to the heating of thermoplastic preforms. 
     While tests have shown that the domain adopted for the radiation is that of the near infrared, they have also shown that, before 1000 nm, the choice in wavelength has little impact on the heating quality (“heating quality” is to be understood as meaning heating which not only gives a lower exposure time, but also gives good accuracy and good diffusion of the radiation through the thickness of the material). 
     By contrast, for the same wavelength, the following parameters: beam shape, energy profile, power density, have an important effect on the heating quality. 
     As we shall see hereinafter, the first exemplary embodiment uses a planar beam  22 , generated by a laser diode  26  to which a spreading lens is added. Various manufacturers offer laser diodes which either come individually or assembled into arrays as depicted in  FIGS. 2 and 3 . 
       FIG. 2  depicts a block  27  of stacked diodes  26  with a total power of 1200 W, marketed by Thales, under the references TH-C17xx-M1 or TH-C55xx-M1. Each diode  26  generates a planar laser beam so that the block generates several superposed planar beams which may be parallel or divergent. 
       FIG. 3  depicts an array  28  of diodes  26  with a power of 40 W each, each diode  26  generating a planar beam. The array  28  thus generates a planar beam, formed by the juxtaposition of the beams generated by all the diodes. An array of this type is marketed by Thales, under the references TH-C1840-P or TH-C1841-R. 
     As can be seen in  FIGS. 2 and 3 , the block  27  and the array  28  are both equipped with an internal water-cooling circuit, the water inlet  29  and outlet  30  pipes of which can be seen in the figures. 
       FIG. 4  schematically depicts the structure of an array  28  of diodes  26 . The diodes  26  are jointly mounted and soldered onto a support  31  equipped with ducts  32  perpendicular to the beams  22  and through which the cooling fluid runs. 
     2. Producing the Heating Unit 
     The heating unit is now described in greater detail according to three distinct exemplary embodiments with reference to  FIGS. 6 to 11 . 
     2.1 Example 1 
     The first exemplary embodiment is described with reference to  FIGS. 6 to 8 . 
     As can be seen in  FIG. 6 , the path  23 , represented by a chain line, that the preforms  2  follow within the heating unit  16  is substantially rectilinear and defines a direction L termed the longitudinal direction. 
     In this example, the heating unit  16  comprises a chamber  33  comprising a first wall and a second wall  34 ,  35  which are vertical and face one another and run substantially parallel to the path  23 , being positioned one on each side thereof. 
     The walls  34 ,  35  together delimit an internal volume  36  through which the preforms  2  pass longitudinally. 
     As can be seen in  FIG. 7 , the walls  34 ,  35  extend over a height substantially equal to the length of the body  4  of the preform  2 . This preform is oriented neck up, the neck  3  protruding out of the chamber  33  above the walls  34 ,  35 . The chamber  33  is open at the bottom to allow an ascending air flow  37  to circulate to provide the chamber  33  with a certain degree of ventilation in order to remove the heat emitted by the body  4  of the heated parison  2 . 
     Each wall  34 ,  35  has a respective internal face  38 ,  39  facing toward the internal volume  36  and a respective opposite external face  40 ,  41 . 
     The first wall  34  is equipped with a plurality of superposed horizontal parallel slits  42  facing each of which there is positioned, on the external face  40  side, an array  28  of laser diodes, as described hereinabove. 
     As can be seen in  FIG. 6 , the heating unit thus comprises a matrix  43  of laser diodes formed by a plurality of superposed arrays  28 , which runs substantially facing the entire height of the body  4  of the preforms  2 . The arrays  28  may be cooled by means of their own circuits, which are connected to a common cooling liquid supply  29  and discharge  30  duct. 
     Each diode emits a beam  22  oriented in an overall direction T that is transverse to the path  23 , and runs in a horizontal mid-plane P parallel to this path  23 . 
     Each slit  42  subjects the beam  22  passing through it to a diffusion effect which means that the beam  22  has a tendency to diverge on each side of the horizontal midplane P. 
     Furthermore, the internal faces  38 ,  39  of the walls  34 ,  35  are reflective which means that the beam  22  undergoes several successive reflections and therefore crosses the preform  2  several times before it loses its energy. This results in an improvement in the energy efficiency and in a reduction in the time taken to heat the preforms  2 . 
     To produce the matrix  43  of diodes, it is possible to use several superposed arrays  28  of 40 W diodes of the type explained hereinabove (cf. §1) and illustrated in  FIG. 3 . 
     In  FIG. 7 , the angle of divergence of the beam  22  is exaggerated in order to demonstrate this dual phenomenon of divergence and reflection. 
     Rotating the preform  2  about its axis A makes it possible, on leaving the heating unit, to obtain a temperature profile that is substantially constant around the circumference of the body  4 . 
     Furthermore, it is possible to regulate the power of the diodes  26  in such a way as to obtain the desired temperature profile which is non-uniform over the length of the preform  2 , for example, with a view ultimately to obtaining a container of curved shape. In such an example, the middle arrays  28  will be set to a lower power than the lower and upper arrays  28  so as to keep the central part of the body  4  at a temperature that is lower (for example at around 115° C.) than the temperature of its end parts (which will be raised to around 130° C.). 
     Although the phenomenon of thermal convection in the chamber  33  is limited because of the use of coherent radiation, particularly so that the neck  3  does not experience any heating liable to soften it and cause an alteration to its dimensions during the blowing (which, as has been stated, allows the preforms  2  to be oriented neck up), it may prove preferable to ventilate at least the upper part of the chamber  33 , so as to create a cool air flow around the neck  3 . 
     Hence, as has been depicted in  FIG. 7 , the heating unit  16  is equipped with a ventilation system  44  generating an air flow  45  which, vertically in line with the chamber  33 , circulates transversely in order to remove the heat energy drained away by the upward air flow  37  due to natural thermal convection. This ventilation system  44  for example comprises a fan  46  arranged in a casing  47  positioned on the external face  41  side of the second wall  35  and having an opening  48  extending vertically in line with an upper edge  49  of the wall  35 , able to route the air flow  37  from the fan  46  transversely. 
     Each preform  2  is heated as follows. 
     The preform  2  originating from the feed unit  10  enters the heating unit  16  along the longitudinal path  23  locally defined by the conveyer. 
     The preform  2  is rotated about its axis A. The laser beams  22  emitted by the diodes  26  strike it along the entire path that it follows through the chamber  33 . Initially at ambient temperature, the body  4  of the preform  2  is quickly raised to a temperature of around 120° C., while its neck  3  is kept at ambient temperature. 
     On leaving the chamber  33 , the preform  2  is transferred to the stretch-blow-molding unit  18  to be shaped into a container. 
     2.2 Example 2 
     The second exemplary embodiment is now described with reference to  FIGS. 9 and 10 . This second example comprises a first embodiment illustrated in  FIG. 9 , whereby the installation  1  comprises a single heating unit  16 , and a second embodiment which, illustrated in  FIG. 10 , constitutes a variant of the first in that the installation  1  comprises two successive heating units  16 . 
     According to the first embodiment, the path  23  followed by the preforms  2  within the heating unit  16  is locally rectilinear, in a longitudinal direction L, between an upstream transfer region  50  where the cold preforms  2  are brought into the heating unit  16  by an upstream transfer wheel  51 , and a downstream transfer region  52 , where the hot preforms  2  are removed from the heating unit  16  by a downstream transfer wheel  53 . 
     The heating unit  16  comprises several superposed laser sources  24  positioned at a downstream end of the path  23 , along the axis thereof. The sources  24  here consist of collimating lenses  54  each connected by an optical fiber  55  to a diode laser generator  56  and together form a vertical block  57  of a height substantially equal to the bodies  4  of the preforms  2 . 
     As can be seen in  FIG. 9 , the lenses  54  are oriented in such a way as to generate longitudinal (linear or planar) beams  22  which strike the preforms  2  in succession before encountering an opaque screen  58  forming an energy sink, positioned transversely in the continuation of the path  23 , beyond the upstream transfer wheel  51 . 
     Thus, along the path  23 , each preform  2  is progressively heated by the laser beams  22  whose energy, transferred successively to the preforms  2  that they strike and pass through is, first of all, from the point of view of the preform, low at the exit of the upstream transfer wheel  51 , then increases as the preform  2  gradually nears the sources  24  before reaching a maximum in the vicinity of these sources before the preform  2  is taken up by the downstream transfer wheel  53 . 
     It is thus possible to heat the preforms  2  gradually using only a block of laser sources, rather than a matrix as explained in the first example described above. 
     However, in order to avoid excessively rapid dissipation of the energy of the laser beams, it is preferable to use laser diodes of a higher power. Thus, the laser adopted here is a diode laser of the type set out hereinabove (cf. §1), with an individual power of 500 W. 
     As illustrated in  FIG. 9 , the heating unit  16  comprises a confinement chamber  59  comprising two walls  60 ,  61  facing each other and positioned one on each side of the path  23 , between the upstream  51  and downstream  53  transfer wheels. 
     These walls  60 ,  61  have reflective internal faces which confine the laser beams  22  by reflecting their transverse components resulting from the diffraction through the preforms  2 . Thus energy losses are limited while at the same time improving the safety of the installation. 
     Although this is not shown in  FIG. 9 , the heating unit  16  may be equipped with a ventilation system similar to the one described hereinabove in the first exemplary embodiment. 
     According to the second embodiment, the installation  1  comprises two heating units  16 , similar to the heating unit  16  described hereinabove in the first embodiment and positioned in succession in the path of the preforms  2 , namely a first heating unit  16   a  designed to raise the preforms  2  to an intermediate temperature (that is to say to a temperature between ambient temperature, which corresponds to the initial temperature of the preforms, around 20° C., and the final temperature, prior to forming, of around 120° C.), and a second heating unit  16   b  designed to raise the preforms  2  to their final temperature (of around 120° C.). 
     The path  23   a  followed by the preforms  2  within the first heating unit  16   a  is locally rectilinear, in a longitudinal direction L between an upstream transfer region  51  where the cold preforms  2  are supplied to the first heating unit  16   a  by an upstream transfer wheel  51 , and an intermediate transfer region  62  where the warm preforms  2  are transferred from the first heating unit  16   a  to the second  16   b.    
     In the example depicted in  FIG. 10 , the heating units  16   a ,  16   b  are arranged parallel to one another, and the path  23   b  followed by the preforms in the intermediate transfer region  62  is curved. This arrangement makes it possible to avoid interference between the beams  22  of the first heating unit  16   a  and those of the second  16   b.    
     The path  23   c  followed by the preforms  2  within the second heating unit  16   b  is, also, locally rectilinear and longitudinal, between the intermediate transfer region  62  and a downstream transfer region  52  where the hot preforms  2  are taken up transversely by a downstream transfer wheel  53 . 
     Each heating unit  16   a ,  16   b  comprises a block  27  of superposed laser diodes of a height substantially equal to that of the bodies  4  of the preforms  2  and arranged at a downstream end of the corresponding path  23   a ,  23   c  along the axis thereof. 
     The blocks  27  of diodes are, for example, of the kind set out hereinabove (cf. §1) and illustrated in  FIG. 2 . 
     As can be seen in  FIG. 10 , the first heating unit  16   a  comprises an opaque screen  58  forming an energy sink, that the laser beams  22  strike once they have passed in succession through the preforms  2  present on the path  23   a , and which is positioned transversely in the continuation of the path  23   a  beyond the upstream transfer wheel  51 . 
     The second heating unit  16   b  also comprises such an opaque screen  58 , for its part positioned in the continuation of the path  23   c , on the same side as the intermediate transfer region  62 . 
     Furthermore, as can be seen in  FIG. 10 , each heating unit  16   a ,  16   b  comprises a confinement chamber  59  of which the reflective walls  60 ,  61 , positioned one on each side of the corresponding path  23   a ,  23   c , prevent the lateral dispersion of the laser beams  22 . 
     Thus, the preforms  2  are first of all raised to an intermediate temperature, for example of around 80° C., within the first heating unit  16   a , and then, from there, are raised to a final temperature of about 120° C. within the second heating unit  16   b  before being transferred to the stretch-blow-molding unit  18 . 
     It should be noted that for particular applications, more than two heating units could be envisioned. 
     2.3 Example 3 
     The third exemplary embodiment is now described with reference to  FIGS. 11 to 13 . 
     In this example, the path  23  of the parisons  2  within the heating unit  16  is substantially circular and, as can be seen in  FIG. 11 , the heating unit  16  comprises a plurality of adjacent chambers  63  arranged along the path  23  and through which the preforms  2  pass in succession. 
     The path  23  is defined between an upstream transfer wheel  51  which brings the preforms  2  from the feed unit  10 , and a downstream transfer wheel  53  carrying the stretch-blow-molding molds. 
     Each chamber  63  has two cylindrical walls facing each other, namely an internal wall  64  and an external wall  65 , positioned one on each side of the path  23 , and together defining an internal cavity  66  in which the preform  2  is positioned, its axis A therefore being temporarily coincident with an axis of symmetry of the chamber  63 . 
     Each wall  64 ,  65  has several adjacent reflective facets  64   a ,  64   b ,  64   c ,  65   a ,  65   b ,  65   c  facing toward the cavity  66 , each facet  64   a ,  64   b ,  64   c  of one wall  64  being positioned facing a corresponding facet  65   a ,  65   b ,  65   c  of the wall  65  opposite, these facets  64   a ,  64   b ,  64   c ,  65   a ,  65   b ,  65   c  not being exactly parallel with their pair but together defining an angle α of a few degrees, as can be seen in  FIG. 12 . 
     An upstream gap  67  and a downstream gap  68  are defined between the walls  64 ,  65 , through which gaps  67 ,  68  each preform  2  in turn enters and then leaves. 
     Furthermore, the heating unit  16  comprises, for each chamber  63 , an opaque screen  58  adjacent to one facet  64   c  of the internal wall  64 , on the same side as the downstream gap  68 . 
     For each chamber  63 , the heating unit  16  comprises a block  27  of stacked laser diodes positioned facing one  64   a  of the facets of the internal wall  64 , bordering the upstream gap  67 . The laser diodes, directed toward this facet  64   a  are designed each to generate a beam  22  that is either linear or contained in a vertical plane that is transverse with respect to the path  23  of the preforms  2 , the beam  22  making an acute angle with the normal to the facet  64   a  ( FIG. 12 ). 
     Thus, each beam  22  undergoes several successive reflections off the facets  64   a ,  65   a ,  64   b ,  65   b ,  64   c ,  65   c  before striking the screen  58  which, as it forms an energy sink, completely absorbs the beam  22  ( FIG. 12 ). 
     When a preform  2  is positioned at the center of the chamber  63 , neck up, each beam  22  thus strikes it several times in distinct regions distributed at its circumference, as can be seen in  FIG. 12 . 
     As is apparent from  FIG. 11 , each preform  2  passes in succession through all the chambers  63  and the diodes can be set in such a way that their power increases along the path  23 , the temperature of the preforms  2  therefore increasing as they gradually progress through the heating unit  16 . 
     As before, the preforms  2  may be rotated about their axis of revolution A, their progress within the heating unit  16  preferably being stepwise, each preform  2  for example remaining in each chamber  63  for a fraction of a second. 
     It is perfectly conceivable for the progress of the preforms through the heating unit  16  to be continuous, because of the good ability that the laser beams have to penetrate through the material of which the bodies of the preforms are made. 
     Of course, irrespective of the embodiment adopted, it is possible to regulate the speed at which the preforms  2  travel through the heating unit. 
     In fact, the various settings (rate of travel, power of diodes, length of chamber) will be chosen by the person skilled in the art according to the material to be used for the preforms, and the machine rates dictated by production. 
     As we have seen, the method and the installation described hereinabove allow parisons, such as preforms, to be heated both more quickly and more precisely than can be achieved by the known methods and installations. 
     This speed means that the size of the heating unit can be limited, while tests have shown it is possible, using coherent electromagnetic beams, to achieve energy efficiencies of 50%, something which seemed unthinkable with the known methods and installations. 
     Tests have in fact demonstrated a laser energy penetration into the materials commonly used in this application, that is superior to that of the radiation of the halogen lamps conventionally employed for heating, thus improving the uniformity of the temperature of the material through the thickness of the preform. 
     The precision of the heating makes it possible to obtain a vertical heating profile which more precisely matches the desired profile. More specifically, this precision makes it possible to achieve heating profiles which hitherto were impossible to obtain. That in particular means that the design of the preforms can be revised so that the weight (which in practice means the wall thickness) of the preforms can be distributed differently according to the desired temperature profile for a particular profile of the container. 
     Furthermore, the small amount of heating of the ambient air additionally means that the preforms can be kept in the neck up orientation throughout the container production process, thus avoiding inverting operations.
A method of producing a container from a thermoplastic blank, including a step in which the blank is heated using at least one beam of coherent electromagnetic radiation, and a step in which the container is formed from the blank thus heated. The invention also relates to an installation which is used to produce containers and which includes a unit for heating the blanks in order to form containers from the blanks thus heated. The inventive installation defines a path along which the blanks travel inside the heating unit. In addition, the heating unit includes at least one coherent electromagnetic radiation source which is directed toward a zone that is located on the aforementioned path.