Abstract:
A method for placing an oven ( 12 ) for heat-treating preforms ( 13 ) into standby, including: a device ( 16 ) for conveying the preforms ( 13 ); at least one heating lamp ( 22 ); at least one heat-accumulation element ( 24, 26 ); and an electronic unit for controlling the heating lamps ( 22 ), characterized in that the method includes, in series: a first switching-off step (E 1 ) which is triggered when a stop in production is determined, and in which the heating lamps ( 22 ) are completely switched off; and then a second temperature-maintaining step (E 2 ) which is triggered after the first step (E 1 ), and in which the heating lamps ( 22 ) are activated at a predetermined intensity in order to reheat the accumulation elements ( 24, 26 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a method for placing on standby an oven for heat-treating thermoplastic material preforms. 
     The invention more particularly concerns a method for placing an oven for heat-treating thermoplastic material preforms on standby during a temporary cessation of production, the oven including:
         a device for conveying preforms that moves the preforms through the oven;   at least one heating lamp that is adapted to emit infrared radiation when it is switched on, each storage element being disposed along the route of the preforms;   at least one heat-storage element that is made from a material having a high thermal capacity, the storage element being adapted to be heated by the heating lamp;   an electronic unit for controlling the heating lamps.       

     2. Description of the Related Art 
     The manufacture of thermoplastic material containers, such as large or small bottles, etc., is generally effected on the basis of preforms, sometimes called blanks, which are introduced into a molding device with which are associated blowing or drawing-blowing means. 
     Before they are molded by blowing or drawing-blowing, the preforms are heated in a heat-treatment oven so as to confer on them a structure sufficiently malleable for the blowing operation. 
     Installations for mass production of such containers are equipped with treatment ovens including a heating tunnel provided with means for heating the preforms. The preforms are generally transported through the heating tunnel at high speed without stopping. The tunnel has a length sufficient to enable heating of the preforms as they pass through. 
     The preform heating means are generally formed by heating lamps that emit infrared radiation. These lamps traditionally take the form of horizontal tubes that are disposed on a wall of the heating tunnel. Each tunnel section is equipped with a plurality of lamps, which are stacked in order to effect appropriate heating of the whole of the body of the preform. 
     The wall facing the heating lamps is equipped with ventilation means for agitating the air in order to prevent some areas of the preforms from being overheated by the infrared radiation. 
     In order for all of the infrared radiation to be used for heating the preform, reflectors are disposed behind the lamps in order to reflect the infrared rays in the direction of the preforms. 
     Some operating incidents make it obligatory to interrupt production by the installation temporarily. In this case, the oven is no longer fed with preforms. The preforms present in the oven at the time feeding is interrupted finish their travel normally and then exit the oven in the direction of the blowing device. When the last preform is evacuated, the oven no longer contains any preforms. 
     Some vulnerable parts of the oven are liable to be damaged by a high temperature. These vulnerable parts risk being damaged if the heating lamps remain switched on with no preforms passing through. 
     Furthermore, it is also preferable for the interior of the oven to be maintained at a temperature sufficient for production to be restarted rapidly, avoiding a long step of restoring the oven temperature. 
     The oven emptied in this way is then placed on standby. It is known to employ a method of placing the oven on standby during which the intensity of the heating lamps is lowered in order to prevent the vulnerable parts of the oven being overheated and therefore damaged. The ventilation means are furthermore activated in order to evacuate the air heated by the lamps and to cool the vulnerable parts of the oven more effectively. 
     Such a method consumes a great deal of energy, however. The invention therefore proposes a putting on standby method enabling the consumption of energy to be reduced at the same time as ensuring optimum operation of the treatment oven. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention therefore relates to a method of the type described above characterized in that it includes in series:
         a first or switching-off step, which is triggered at the time of a cessation of production and during which the heating lamps are switched off completely;   and then a second or temperature-maintaining step, which is triggered after the first step and during which the heating lamps are switched on at a intensity determined so as to heat the storage elements.       

     According to other features of the method:
         the method is repeated from the first step until the oven is returned to production;   the first step ends at the end of a predetermined first or switching-off duration;   the second step ends at the end of a predetermined second or switching-on duration;   the first step ends when the temperature of the storage element is less than or equal to a first or lower threshold temperature;   the second step continues until the temperature of the storage element is greater than or equal to a second or upper threshold temperature;   the temperature of the storage element is measured by a probe which is disposed near or in the storage element;   the oven includes ventilation means that are adapted to be controlled by the electronic control unit, the ventilation means being deactivated during the first step and activated during the second step;   if the cessation of production exceeds a limit duration, the method is stopped and the heating lamps are switched off completely.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the method will become apparent on reading the following detailed description, to understand which reference should be made to the appended drawing, in which: 
         FIG. 1  is a plan view that represents diagrammatically a heat-treatment oven that is to be equipped with an installation for mass production of thermoplastic material containers; 
         FIG. 2  is a view in section taken along the line  2 - 2  in  FIG. 1  which represents a preform travelling in the tunnel of the treatment oven, the tunnel being equipped with heating lamps and reflectors; 
         FIG. 3  is a block schematic that represents the method for placing on standby the oven produced in accordance with the teachings of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the remainder of the description there will be adopted in a non-limiting manner a longitudinal orientation “L” oriented from back to front, a vertical orientation “V” oriented from top to bottom, and a transverse orientation “T” oriented from left to right. These orientations are indicated by the trihedron “L, V, T” in the figures. 
     In the remainder of the description, elements having analogous, identical or similar functions will be indicated by the same reference numbers. 
     There is represented in  FIG. 1  an installation  10  for mass production of thermoplastic material containers. The installation  10  mainly includes an oven  12  for heat-treatment of preforms  13  and a station  14  for blow-molding the preforms  13  treated in this way, i.e. heated, to form the thermoplastic material containers. 
     In a known manner, the heat-treatment oven  12  takes the form of a tunnel  15  through which the preforms  13  pass. The preforms  13  are heated as they pass through. 
     The installation  10  includes a device  16  for transporting preforms  13  through the heat-treatment oven  12  that is adapted to move the preforms  13  one behind the other inside the oven  12 . The device  16  for transporting the preforms  13  thus moves the preforms  13  through the oven  12 . 
     The oven  12  includes a plurality of sections. At least one of the sections is provided with heating means. One of these sections equipped with heating means is represented in more detail in  FIG. 2 . 
     As represented in  FIG. 2 , the tunnel  15  is delimited transversely by a vertical interior first wall  18  and a vertical exterior second wall  20 . Each preform  13  moves so that its axis is vertical. 
     In the example represented in  FIG. 2 , the preform  13  moves in the oven  12  with its neck directed downward. This configuration enables the body of the preform  13  to be heated effectively whilst keeping the neck of the preform relatively cool. To enable homogeneous heating of the body of the preform  13 , the latter is mounted on the transport device  16  to rotate about its vertical axis. 
     The second wall  20  is equipped with means for heating the body of the preforms  13 . The heating means are formed by at least one heating lamp  22  that is adapted to emit infrared radiation when it is switched on. 
     Here the heating means include a plurality of heating lamps  22  that take the form of longitudinal tubes that are carried by the second wall  20 . Thus each heating lamp  22  is disposed along the route of the preforms  13 . The heating lamps  22  disposed in this way emit infrared radiation in all directions and notably in the direction of the body of the preform  13  to be heated. 
     The heating lamps  22  are disposed vertically above each other so as to heat all of the body of the preform  13  globally homogeneously. 
     To make it possible to improve the efficacy of the heating lamps  22 , it is known to dispose reflectors  24  between the second wall  20  and each heating lamp  22 . The reflectors  24  are conformed so that infrared radiation that is emitted by the heating lamps  22  in the direction of the second wall  20  is reflected in the direction of the body of the preform  13 . 
     Here the reflectors  24  are formed in one piece by a vertical plate pierced with horizontal vents  25 . The vents  25  enable the passage of a flow of air as explained hereinafter. 
     In a nonlimiting manner, to improve further the efficacy of the heating lamps  22 , reflectors  26  are also disposed on the first wall  18  so that the infrared rays that cross the tunnel  15  transversely are reflected in the direction of the preform  13 . The reflectors  26  on the first wall  18  are produced in the form of a single plate pierced by vents  25 . This plate is identical to the plate incorporating the reflectors  24  on the second wall  20 . 
     The oven  12  also includes at least one heat-storage element that is made from a material having a high thermal capacity, the storage element being adapted to be heated by the heating lamps  22 . 
     Here, in the example represented in  FIG. 2 , the heat-storage elements are formed by the reflectors  24 ,  26 . This advantageously enables two functions to be implemented with the same component in order not to clutter the oven  12 . 
     According to a variant of the invention that is not represented, at least some heat-storage elements do not have a reflector function. They may then be blocks that are disposed vertically between two heating lamps. 
     The reflectors  24 ,  26  are made from a material having a high thermal capacity and a low thermal conductivity. Here this means reflectors  24 ,  26  made from a ceramic material. Accordingly, the reflectors  24 ,  26  are able to store a large quantity of heat. 
     By way of nonlimiting example, the ceramic employed to make the reflectors  24 ,  26  has a thermal conductivity less than 30 W.m −1 .K −1 . 
     Thus the reflectors  24 ,  26  form the heat-storage elements. As will be explained in more detail hereinafter, the invention proposes to use this stored heat. 
     The oven  12  further includes ventilation means (not represented) that enable air to be circulated in the tunnel  15  so as to prevent the preform  13  from being heated non-homogeneously by the infrared radiation emitted by the heating lamps  22 . 
     The ventilation means include for example a fan which blows air transversely through the vents  25  that are produced in the plates incorporating the reflectors  24 ,  26 . 
     The heating lamps  22  and the ventilation means are controlled by an electronic control unit (not represented). This electronic control unit notably enables switching on or complete switching off of the heating lamps  22  to be commanded. The electronic control unit also enables the intensity of the heating lamps  22  when they are switched on to be controlled. 
     It happens that operating incidents make it obligatory to interrupt the production of containers. The supply of preforms  13  to the oven is then stopped and the preforms  13  already present in the oven finish their heating cycle before being evacuated from the oven. If these operating incidents can be resolved quickly, for example in less than 30 minutes, it is advantageous not to cool the oven  12  in order to be able to restart production rapidly, without it being necessary to warm up the oven  12 . 
     The invention proposes a method for placing the heat-treatment oven  12  on standby during a temporary cessation of production. Such a method is notably represented in  FIG. 3 . 
     When cessation of production is signaled to the electronic control unit, the placing on standby method is triggered. This placing on standby method notably includes:
     a first or switching-off step “E 1 ” that is triggered when production is to be halted and during which the heating lamps are switched off completely by the electronic control unit once the oven has been emptied of preforms;   followed by a second or temperature-maintaining step “E 2 ” that is triggered after the first step “E 1 ” and during which the heating lamps are switched on at an intensity determined by the electronic control unit.   

     During the first step “E 1 ”, the presence of the storage elements formed by the reflectors  24 ,  26  with high thermal capacity makes it possible to switch off the heating lamps  22  completely without the internal temperature of the oven  12  suddenly falling. The heat stored by the reflectors  24 ,  26  is returned, notably by convection, to the ambient air contained inside the oven  12 . Thus the interior of the oven is kept hot to enable faster restarting of production. 
     During the first step “E 1 ”, the electronic control unit also deactivates the ventilation means so as not to cool too quickly the storage elements formed by the reflectors  24 ,  26 . Thus the reflectors  24 ,  26  are cooled only by natural convection. 
     By way of nonlimiting example, when the ventilation means are activated, a cooling rate of 100° C. per minute is observed, whereas when the ventilation means are deactivated, the rate of cooling is only 10° C. per minute. During production, the reflectors  24 ,  26  reach a temperature between 300 and 500° C., for example. Accordingly, when the ventilation means are deactivated, the reflectors  24 ,  26  remain hot for a long time in relation to a cessation of production of the order of 10 to 20 minutes. 
     The object of the second step “E 2 ” is to prevent the internal temperature of the oven  12  falling too far after a long interruption of production. Too great a temperature drop would lead to the obligation to warm up the oven  12  on restarting production. The electronic control unit switching on the heating lamps  22  then enables the reflectors  24 ,  26  to be heated to enable them to store heat again. The heating lamps  22  may be switched on at a moderate intensity, for example 50% of their maximum intensity. 
     During this second step “E 2 ”, the ventilation means are activated by the electronic control unit. The heating lamps  22  then heat the interior of the heating tunnel  15  and there would be the risk of damaging some vulnerable parts, such as metal parts or the means  16  for transporting the preforms  13 . 
     In a first embodiment of the invention, the first step “E 1 ” and the second step “E 2 ” are programmed to last a predetermined duration, for example 5 minutes each. Accordingly, the first step “E 1 ” ends after a predetermined first or switching-off duration “D 1 ” and the second step “E 2 ” ends after a predetermined second or switching-on duration “D 2 ”. 
     The duration “D 1 ”, “D 2 ” of each of these two steps “E 1 ” and “E 2 ” is defined by trial and error, for example. 
     If the interruption of production lasts beyond the planned duration “D 2 ” for the second step “E 2 ”, the method is repeated from the end of the second step “E 2 ”. Thus the method is repeated until production in the installation  10  is restarted. 
     In a second embodiment of the invention, the oven  12  is equipped with at least one temperature probe  28  that is adapted to measure the temperature near or in a reflector  24 ,  26 . The temperature measured in this way is transmitted to the electronic control unit. 
     The second step “E 2 ” is triggered if the measured temperature is less than a first or lower threshold temperature “T 1 ”. 
     Then, if the interruption of production continues, the method is repeated if the measured temperature has become greater than or equal to a second or upper threshold temperature “T 2 ”. The second temperature “T 2 ” is higher than the first temperature “T 1 ”. 
     If the duration of the interruption of production exceeds a predetermined duration, for example 30 minutes, the standby method may be terminated and the heating lamps  22  switched off and the ventilation means deactivated until production is restarted. The oven  12  will then have to be warmed up before production can be restarted.