Abstract:
A furnace ( 22 ) for heating a preform ( 10 ), includes an upper longitudinal heating tunnel ( 24 ) bounded transversely by the longitudinal vertical internal faces ( 36, 38 ) of two walls ( 40, 42 ), one wall ( 40 ), being equipped with a heating system ( 46 ), and the other wall ( 42 ), having aeration orifices ( 50 ) intended to let the air blown by a blower ( 52 ) pass transversely through them, and in which one portion ( 16 ) of the preform ( 10 ) is held outside the heating tunnel ( 24 ), a blower ( 52 ) blowing air transversely towards the portion ( 16 ) of the preform ( 10 ) so as to keep the constituent material of this portion ( 16 ) of the preform ( 10 ) at a temperature below its softening point, characterized in that the blower ( 52 ) includes two independent fans ( 54   a,    54   b ) that deliver air to the aerated wall ( 42 ) and to the portion ( 16 ) of the preform ( 10 ), respectively.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a furnace for heating a preform, especially a plastic container preform. 
   The invention relates more particularly to a furnace for heating a preform, especially a plastic container preform, which comprises a longitudinal heating tunnel bounded transversely by the mutually parallel longitudinal vertical internal faces of two walls, one a heating wall, being equipped with a heating system, and the other an aerated wall, having aeration orifices that are intended to let the air blown by a blower pass through them transversely from the upstream, outside the heating tunnel, to the downstream, inside the latter, and in which furnace a first portion of the preform is heated in the heating tunnel, while the second portion of the preform is held outside the heating tunnel through a longitudinal opening made between the longitudinal edges of the walls of the heating tunnel, the blower blowing air transversely to the second portion of the preform so as to keep the constituent material of this second portion of the preform at a temperature below its softening point. 
   2. Description of the Related Art 
   Plastic containers, and especially bottles, for example made of polyethylene terephthalate (PET), are generally produced from preforms that are shaped into bottles by an operation of blow or stretch-blow moulding of their body and their end wall. The preforms generally have the form of a test tube, the neck of which already has the final shape of the throat of the container to be produced. The neck has for example a thread intended to receive a screw stopper. 
   Before carrying out the blow moulding operation, it is necessary to heat the preforms in a heating furnace so as to soften the plastic. For this purpose, the constituent plastic of the preforms is heated to a temperature above its glass transition temperature. 
   Preform heating furnaces of the type having a longitudinal heating tunnel are already known. The cold preform is transported by a transport device from a first end of the tunnel and then travels along the tunnel, in which it is heated, before emerging via the second end of the tunnel. The preform thus heated is ready for the blow moulding operation. 
   To ensure depthwise heating of the end wall and the cylindrical wall of the body of the preform over its entire periphery, the preforms are generally rotated about themselves as they travel through the furnace. Moreover, one wall of the tunnel is equipped with radiation heating means, while the other wall is provided with aeration orifices for blown air to flow through so as to promote uniform heating throughout the thickness of the cylindrical wall of the body of the preform without overheating the layer of surface material forming the external face of the cylindrical wall of the preform. 
   In fact, the blown air removes the convective heat caused by the heating means, in order to promote penetration of the radiation that it produces in the thickness of the constituent material of the body. 
   However, the neck of the preform, which has its final shape and its final dimensions, must not be deformed during the heating and/or blow moulding operations. It is therefore essential to keep the neck at a temperature below the glass transition temperature. For this purpose, the neck of the preform is held outside the tunnel through an opening in the tunnel that forms a longitudinal slot. 
   To prevent the heat from the heating tunnel from being communicated to the neck of the preform, said neck is cooled by air blown by a blower. 
   It is known to equip furnaces with a single blower, which makes it possible, on the one hand, to distribute the heat uniformly in the heating tunnel and, on the other hand, to cool the neck of the preform. The blower is then equipped with a deflector, which is located upstream of the tunnel and of the preform relative to the flow direction of the air. The deflector thus deflects a portion of the stream of blown air on to the neck of the preform. 
   BRIEF SUMMARY OF THE INVENTION 
   The aim of the present invention is to improve the effectiveness with which the neck of the preform is cooled during the heating operation, while optimally keeping the surface layer of the body of the preform at a temperature close to the glass transition temperature of the plastic. For this purpose, the invention proposes an furnace of the type described above, characterized in that the blower comprises two independent fans that deliver air to the aerated wall and to the second portion of the preform held outside the heating tunnel, respectively. 
   According to other features of the invention:
         the delivery ducts are formed by the upper and lower compartments of a common box having a dividing partition;   the blower includes two separate air inlet ducts that are each associated with a fan;   the blower includes two coaxial vertical shafts, each driving an associated fan;   the speed of the air stream expelled by the fan associated with the aerated wall is controlled by a control device; and   the fans are controlled by two separate control units.       

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Other features and advantages of the invention will become apparent on reading the following detailed description, for the understanding of which the reader should refer to the appended drawings, in which: 
       FIG. 1  is a side view showing a preform intended to be heated in a furnace before being converted into a container after a blow moulding operation; 
       FIG. 2  is a side view showing an furnace for heating the preform of  FIG. 1  and produced according to the teachings of the invention; and 
       FIG. 3  is a cross-sectional view in the plane  3 - 3  of  FIG. 2 , showing the furnace equipped with a blower comprising two fans. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the rest of the description, a longitudinal, vertical and transverse orientation, indicated by the (L,V,T) coordinate system shown in  FIG. 2 , will be adopted, but without being limited thereby. 
   In the rest of the description, an upstream-to-downstream orientation will be used to describe the flow of air. 
   The same references will also be used to denote identical, similar or analogous elements. 
     FIG. 1  shows a preform  10  made of a plastic, such as polyethylene terephthalate (PET), which is intended to be converted into a plastic container after a first operation, in which it is heated to soften the plastic, and then a second operation, in which it is blow moulded in order to shape the preform  10  into a container. 
   The preform  10  has a test tube shape of vertical axis A. The tubular body  12  of the preform  10  is closed at its upper end by a hemispherical end wall  14 , while its lower end includes a neck  16  that forms a throat. 
   The neck  16  already has the final shape of the throat of the finished container. Thus, to obtain a finished container, only the tubular body  12  and its end wall  14  have to be deformed by blow moulding. During the blow moulding operation, it is therefore important to heat only a first portion of the preform  10 , comprising the tubular body  12  and the end wall  14 , above the glass transition temperature of the plastic, while keeping the temperature of the neck  16 , forming a second portion of the preform  10 , below the glass transition temperature. 
   The neck  16  is bounded at the top by a flange  18 . The external cylindrical face  20  of the neck here has a thread that is intended to receive a threaded stopper (not shown) so as to seal the container. 
     FIG. 2  shows an furnace  22  for heating the preform  10  during the heating operation. The furnace  22  comprises here a longitudinal heating tunnel  24  through which the preform  10  is intended to move along the direction indicated by the arrow F. The tunnel  24  has a first end  26  where the preform  10  enters, on the right in  FIG. 2 , and a second end  28  where the preform  10  leaves, on the left in  FIG. 2 . 
   As illustrated in  FIG. 2 , the furnace  22  includes a device  30  for transporting the preform  10  from the inlet  26  to the outlet  28  of the furnace  24 . The transport device  30  comprises a train of mandrels  32  for gripping the preforms  10 , these mandrels extending vertically upwards from a lower rail  34  which runs beneath the tunnel  24  so as to be parallel therewith. The mandrels  32  are intended to keep the preforms  10  vertical during their travel along the tunnel  24 . 
   The rectilinear longitudinal shape of the tunnel  24  as shown in  FIG. 2  is given by way of non-limiting example. 
     FIG. 3  shows a section of the tunnel  24 . It is bounded transversely by two mutually parallel vertical longitudinal internal faces  36  and  38  belonging to a first wall  40 , located on the right in  FIG. 3 , and to a second wall  42 , located on the left, respectively. 
   The tunnel  24  here is closed neither at the top nor at the bottom. Thus, the lower longitudinal edges of the internal faces  36  and  38  define a lower opening  41  in the form of a longitudinal slot. 
   The first wall  40  is equipped with a heater  46  for heating the tunnel  24 . In the example shown in  FIG. 3 , the heater  46  comprises eight infrared lamps, which here are referenced IR 1  to IR 8  and have the form of longitudinal tubes. The tubes IR 1  to IR 8  are held by each of their ends close to the internal face  36  of the first wall  40  using means, here vertically distributed longitudinal channels  48 , each intended to house one end of the lamps IR 1  to IR 8 . 
   The second wall  42  here includes orifices  50  that pass transversely through its thickness so as to allow a stream of air produced by a blower  52  to penetrate into the tunnel  24 . As explained in the preamble, the stream of air thus cools the cylindrical wall of the preform  10 . 
   As shown in  FIG. 3 , the tubular body  12  and the end wall  14  of the preform  10  are held inside the heating tunnel  24 , while the neck  16  and the flange  18  are held outside the tunnel  24 , passing through the lower opening  41 . 
   To keep the neck  16  at a temperature below its glass transition temperature, a stream of fresh air is blown transversely towards the neck  16  by the blower  52 . 
   The blower  52  is located transversely to the left of the second wall  42  in  FIG. 3 . 
   According to the teachings of the invention, the blower comprises two separate blowers, namely an upper blower  54   a  and a lower blower  54   b . In the rest of the description, the references for the components of the upper fan  54   a  will be followed by the letter &lt;&lt;a&gt;&gt;, while the references for the components of the lower fan  54   b  will be followed by the letter &lt;&lt;b&gt;&gt;. 
   Each fan  54   a  and  54   b  mainly comprises a motor-driven bladed wheel  56   a  and  56   b , also called a turbine, which is mounted so as to rotate about a vertical axis B, a vertical axial air inlet duct  58   a ,  58   b  and a transverse tangential air outlet or delivery duct  60   a ,  60   b . The fans  54   a  and  54   b  are overall symmetrical with respect to a horizontal mid-plane, that is to say the bladed wheels  56   a  and  56   b  and the delivery ducts  60   a  and  60   b  are adjacent, while the inlet ducts  58   a  and  58   b  are oriented in the opposite direction. 
   The bladed wheels  56   a  and  56   b  are mounted so as to rotate about the same vertical axis B. They are each rotated by an associated motor  62   a  and  62   b  via a shaft  64   a ,  64   b . The motors  62   a  and  62   b  here are placed vertically between the two bladed wheels  56   a  and  56   b.    
   In an alternative embodiment (not shown) of the invention, the motors  62   a  and  62   b  are located some distance from the fans  54   a  and  54   b  and they drive the shafts  64   a  and  64   b  via movement transmission mechanisms, such as belts. The two bladed wheels  56   a  and  56   b  may thus be brought close to each other vertically so that the blower  52  is more compact heightwise. 
   The air inlet ducts  58   a  and  58   b  are placed vertically along the axis B of the bladed wheels  56   a  and  56   b , one opposite the other. 
   Thus, the air inlet duct  58   a  of the upper fan  54   a  extends vertically above the bladed wheel  56   a  so as to take in air from above the blower  52  downwards into the bladed wheel  56   a.    
   The air inlet duct  58   b  of the fan  54   b  extends vertically below the bladed wheel  56   b  so as to take in air from below the blower  52  upwards into the bladed wheel  56   b.    
   The delivery ducts  60   a  and  60   b  are oriented transversely to the tunnel  24  and are adjacent to each other. 
   In the example shown in  FIG. 3 , they are formed in a common output box  66 , which is divided into two compartments, an upper compartment  60   a  and a lower compartment  60   b , by a horizontal partition  68  being substantially in the plane of symmetry of the two fans  54   a  and  54   b . The first, upstream transverse end  70  of the box  66 , located on the left and at the outlet of the bladed wheels  56   a  and  56   b  with regard to  FIG. 3 , and the second, downstream transverse end  72  include openings so as to let a stream of air flow transversely from the bladed wheels  56   a  and  56   b  into the tunnel  24  and the neck  16  respectively. 
   The upper compartment  60   a  thus forms a delivery duct that extends transversely from the bladed wheel  56   a  to the second wall  42  of the heating tunnel  24 . 
   The lower compartment  60   b  thus forms a delivery duct that extends transversely from the lower bladed wheel  56   b  into the region of the neck  16  of the preform  10 . 
   Advantageously, the motors  62   a  and  62   b  are controlled by two separate control units. It is thus possible to regulate the stream of air passing through the second wall  42  by acting on the upper fan  54   a , without modifying the stream of air needed to cool the neck  16 . 
   During operation of the blower  62 , the motors  62   a  and  62   b  rotate the bladed wheels  56   a  and  56   b  which induct air via the axial inlet ducts  58   a  and  58   b  as shown by the arrows in  FIG. 3 . 
   The upper bladed wheel  56   a  blows the air, thus inducted, into the upper delivery duct  60   a . The delivered air passes through the orifices  50  of the second wall  42  so as to penetrate into the tunnel  24 . 
   The lower bladed wheel  56   b  blows the air, thus inducted, into the lower delivery duct  60   b . The delivered air is directed directly on to the neck  16  so as to cool the latter. 
   According to an alternative embodiment (not shown) of the invention, the inlet duct  58   b  of the lower fan  54   b  is contained coaxially inside the inlet duct  58   a  of the upper fan  54   a . The downstream end section of the inlet duct is extended downwards through a vertical recess reserved in the axis of the hub of the upper bladed wheel  56   a , and then the air thus flowing through the hub is inducted by the lower bladed wheel  56   b  before being delivered into the lower delivery duct  60   b . Advantageously, this configuration prevents the lower fan  54   b  from sucking up elements likely to be beneath the furnace  22 , for example dust on the floor beneath the furnace  22 .