Abstract:
Method and apparatus for cooling a heated mold ( 12, 14, 16 ) of a device ( 10 ) for blow-molding thermoplastic containers, the device including:
       a mold ( 12, 14, 16 ) having an internal face which delimits a molding cavity ( 18 ) and which includes an external face ( 20, 28 ) which surrounds the molding cavity ( 18 );   heating elements for heating the mold to a determined temperature and which are deactivated when the cooling method is implemented;
 
characterized in that the mold ( 12, 14, 16 ) is cooled by circulating heat-transfer fluid directly in contact with the external face ( 20, 28 ) thereof.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for cooling a heated mould of a device for blow-moulding thermoplastic containers. 
     The invention relates more specifically to a method for cooling a heated mould of a device for blow-moulding thermoplastic containers, the device comprising:
         a mould having an internal face which delimits a moulding cavity and which comprises an external face which surrounds the moulding cavity;   heating means for heating the mould to a determined temperature and which are deactivated when the cooling method is implemented.       

     BACKGROUND OF THE INVENTION 
     In the known way, moulding devices of this type can be used to create containers, such as bottles, from thermoplastic preforms. The preforms are preheated to a glass transition temperature to make them sufficiently malleable. The preform thus heated is inserted into the moulding cavity, then a blow nozzle injects a pressurized blow-moulding fluid, generally air, into the preform so that the walls of the latter conform to the impression delimited by the moulding cavity. 
     When the plastic containers have to be filled with a hot liquid, a container moulded at an ambient temperature carries the risk of container shrinkage and deformation. To avoid this shrinkage phenomenon, the mould has to be heated to a determined temperature, for example between 130° C. and 180° C., during the moulding operation in order to make the plastic of which the container is made heat resistant. 
     Various ways of heating the mould are already known. Thus, it is known practice to heat the mould using a fluidic circuit created within the thickness of the mould. A hot heat-transfer fluid is fed into the circuit in order to heat the mould. 
     It is also known practice to arrange heating electrical resistances within the thickness of the mould in order to heat the mould electrically. 
     Certain operations require an operator to handle the mould. Such is the case for example when there is a change in format of container to be manufactured. However, the operator cannot handle the mould while it is still hot and it is not a viable option to wait for the mould to cool down passively, thus bringing the entire production line to a standstill. 
     In order to solve this problem, it is known practice to use mould cooling means. 
     Thus, when the mould is heated by a hot heat-transfer fluid circuit, it has already been proposed that the hot heat-transfer fluid be temporarily replaced with cold heat-transfer fluid. Such a method allows the mould to be cooled rapidly. 
     The manufacture of a mould equipped with a heat-transfer fluid circuit devoted to cooling is extremely expensive and complicated to achieve because of the presence of the heating resistances within the thickness of the mould. 
     SUMMARY OF THE INVENTION 
     The invention proposes a cooling method of the type described hereinabove, characterized in that the mould is cooled by circulating a cold heat-transfer fluid directly in contact with the external face thereof. 
     According to other features of the method:
         the heat-transfer fluid is formed of a pressurized gas which underdoes an expansion as it circulates against the external face of the mould;   the heating means are formed by electrical resistances which are interposed in the thickness of the mould between the moulding cavity and the external face.       

     The invention also proposes a moulding device for implementing the method according to the teachings of the invention, in which:
         the mould is formed by the union of two half-moulds each comprising a union face in which part of the moulding cavity is formed and an opposite external face that forms part of the external face of the mould;   each half-mould is fixed in a housing of an associated support with an empty space reserved between the bottom of the housing and the external face of the half-mould;       

     characterized in that the moulding device comprises at least one controlled heat-transfer fluid feed duct which opens into the empty space to allow the heat-transfer fluid to circulate between the external face of the half-mould and the bottom of the housing. 
     According to other features of the device:
         the empty space is open to the atmosphere to allow the heat-transfer fluid to be discharged;   the feed duct opens into the bottom of the housing;   the feed duct comprises several orifices opening into the bottom of the housing, the orifices being distributed over the said bottom;   the feed duct is produced in the thickness of the support.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages will become apparent during the reading of the detailed description which follows, for the understanding of which reference will be made to the attached drawings among which: 
         FIG. 1  is a perspective view depicting a mould of a moulding device created according to the teachings of the invention; 
         FIG. 2  is a perspective view depicting a moulding device created according to the teaching of the invention and comprising a half-mould mounted in an associated support; 
         FIG. 3  is an exploded perspective view depicting the half-mould of  FIG. 2  and its support; 
         FIG. 4  is a view in vertical section on the plane of section  4 - 4  of  FIG. 2 , depicting the space left between the mould and its housing in the support; 
         FIG. 5  is a view in horizontal section on the plane of section  5 - 5  of  FIG. 2 , depicting open-ended orifices of the supply duct; 
         FIG. 6  is a view in section on the plane of section  6 - 6  of  FIG. 5 , depicting the supply duct extending into the thickness of the support. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the remainder of the description, elements that have an identical structure or analogous functions will be denoted by the same reference numerals. 
     The following orientations will be adopted nonlimitingly in the remainder of the description:
         longitudinal indicated by the arrow “L” and directed from the rear forwards;   vertical indicated by the arrow “V” and directed from the bottom upwards;   transverse indicated by the arrow “T” and directed from left to right.       

     In addition, the terms “axial” and “radial” will be used with reference to the axis “A”. 
       FIG. 1  depicts a device  10  for manufacturing containers by blow-moulding or by stretch-blow-moulding. This device  10  comprises a mould  12  formed by the union of two substantially symmetric half-moulds  14 ,  16  made of a metallic material, for example of steel or of an aluminium alloy. 
     The mould  12  thus assembled has the overall shape of a hollow cylinder of revolution of vertical main axis “A”. As depicted in  FIG. 2 , the mould  12  comprises an internal face which delimits a moulding cavity  18  which opens axially towards the top so that the neck of a preform (not depicted) can pass through it. The mould  12  is radially delimited by an external face  20  which surrounds the moulding cavity  18 . 
     Each half-mould  14 ,  16  is carried by an associated support  22 ,  24 . 
     The half-moulds  14 ,  16  are articulated along a hinge (not depicted) of vertical axis so as to allow the mould  12  to be opened so that a preform can be introduced into it. A mould bottom  25 , depicted in  FIG. 1 , is interposed between the two half-moulds  14 ,  16  to form the bottom of the container. The overall structure of such a mould  12 , referred to as a hinged mould, is described in French Patent Application FR-A1-2.856.333 and in the corresponding International Application WO-A1-05/002.820, both in the name of the applicant company and to which a person skilled in the art may refer. 
     Since the two half-moulds  14 ,  16  and their support  22  are substantially identical, only the half-mould  14  and its support  22  will be described hereinafter with reference to  FIGS. 2 to 6 , the description being symmetrically applicable to the other half-mould  16  and to its support  24 . 
     The half-mould  14  has a semi-cylindrical shape. It thus has a planar front union face  26  in which half of the moulding cavity  18  is formed, and an opposite semi-cylindrical external face  28  that forms half of the external face  20  of the mould  12 , as illustrated in  FIG. 3 . 
     If reference is made once again to  FIG. 3 , the half-mould  14  also has an upper face  30  in the form of half a disc. The straight edge of the upper face  30  has a cut-out  32  intended to form an orifice through which the neck of the preform inside the cavity  18  can pass. 
     A mounting plate  34  of complementary shape is fixed to the upper face  30  of the half-mould  14 . The upper face of the mounting plate  34  forms a support face for a nozzle (not depicted) carrying pressurized air by virtue of which the container is blow-moulded. 
     The half-mould  14  comprises controlled means for heating the mould to a determined temperature. More specifically, the heating means here are formed by heating electric resistances  35  which are interposed within the thickness of the half-mould  14  between the moulding cavity  18  and the external face  28 . 
     For this, the half-mould  14  comprises a plurality of vertical orifices  36  which open at the top into the upper face  30  of the mould, as can be seen in  FIGS. 3 and 4 . The resistances are thus introduced into the orifices  36 . The mounting plate  34  covers the orifices  36  to enclose the resistances therein. The electrical resistances are supplied with electricity by electric wires (not depicted) which are sandwiched between the mounting plate  34  and the upper face  30  of the half-mould  14 . Each wire is connected to a connector  38  which in this instance is borne by the mounting plate  34  so that the heating resistances can receive a controlled supply of electricity. 
     The half-mould  14  is housed in a housing  40  which is made in a front face of the associated support  22 , as depicted in  FIG. 3 . The housing  40  has a bottom  42  which faces forwards and which has a shape that complements the shape of the external face  28  of the half-mould  14 . The half-mould  14  is intended to be fixed removably into the housing  40  of the support  22  so that changes of mould  12 , notably when the manufacturer wishes to change the format or shape of the end container, can be made. 
     The removable attachment is achieved using means that are already well known. One exemplary embodiment of such attachment means is described and depicted in document EP-B1-0.821.641. 
     The half-mould  14  needs to be kept hot. To avoid heat loss, as depicted in  FIG. 4 , it is known practice to leave an empty space  44  between the bottom  42  of the housing and the external face  28  of the half-mould  14  by interposing spacer pieces  46  between the half-mould  14  and the bottom  42  of the housing  40 . The spacer pieces  46  here are attached to the bottom  42  of the housing  40 . 
     This space  44  is usually filled with stationary air forming a thermally insulating layer. Advantageously, the spacer pieces  46  offer a more reliable possible area of contact with the external face  28  of the half-mould  14  for minimizing heat loss by conduction. For the same reasons, the spacer pieces  46  are advantageously made of a thermally insulating material. 
     The empty space  44  is made as one piece, i.e. the spacer pieces  46  do not divide it into several separate parts. 
     The empty space  44  is not fluidtight, thus it can communicate with the outside, notably via the front gaps  48  formed between the vertical edges of the union face  26  of the half-mould  14  and the vertical edges of the housing  40  as depicted in  FIG. 5 . 
     When the heating means are activated they heat the half-mould  14  to a temperature that is too high for an operator to be able to handle it bare handed. The temperature of the half-mould  14  is, for example, in excess of 100° C. 
     When the half-moulds  14 ,  16  need to be handled, it is therefore preferable to deactivate the heating means. Nonetheless, this operation is not enough because the passive cooling of the half-mould  14  is a very slow process. 
     The invention therefore proposes a method for actively cooling the half-mould  14 . This method is implemented after the heating means have been deactivated. 
     According to this method, the half-moulding  14  is cooled by circulating a cold heat-transfer fluid directly in contact with the external face  28  thereof. The heat-transfer fluid in this instance circulates in the empty space  44 . 
     To do this, at least one controlled heat-transfer fluid feed duct  50  opens directly into the empty space  44  to allow the heat-transfer fluid to be circulated between the external face  28  of the half-mould  14  and the bottom  42  of the housing  40 . In the example depicted in  FIG. 5 , the support  22  comprises two parallel feed ducts  50 . 
     Each feed duct  50  here extends vertically within the thickness of the support  22 , parallel to the bottom  42  of the housing  40 . Each feed duct  50  is positioned near to a vertical lateral edge of the housing  40 . 
     Each feed duct  50  comprises at least one orifice  52  opening into the bottom  42  of the housing  40 . Each feed duct  50  here comprises three orifices  52  opening out as depicted in  FIG. 6 . The orifices  52  are evenly distributed over the said bottom  42  along the vertical axis “A”. 
     Each orifice  52  is here arranged vertically between two spacer pieces  46  so as to allow an even distribution of the stream of heat-transfer fluid despite the presence of the spacer pieces  46 . 
     Each feed duct  50  is connected to a source or heat-transfer fluid via connecting means  54  which are arranged at a lower end of the support  22 . 
     The heat-transfer fluid here is formed by a pressurized gas which is discharged into the atmosphere by passing through the gaps  48 . The pressurized gas in this instance is compressed air, for example at between 8 bar and 40 bar. 
     The gaps  48  have a passage cross section that is large enough that the pressurized gas is made to expand as it circulates against the external face  28  of the half-mould  14 . This expansion causes a drop in the temperature of the gas thereby encouraging rapid cooling of the half-mould  14  by conducting heat from its external face  28  to the gas. 
     This allows the heat of the half-mould  14  to be removed into the atmosphere very rapidly. 
     The method is advantageously implemented with the two half-moulds  14 ,  16  parted from one another to allow the heat-transfer fluid to be discharged at maximum flow rate. 
     The device and the method for implementing it can thus effectively cool hot moulds  12 . For example, by using compressed air at 8.5 bar as the heat-transfer fluid, it is possible to cool several moulds  12  in under  15  minutes at a very low cost because of the relatively low pressure of the heat-transfer fluid. 
     By increasing the pressure of the heat-transfer fluid it is of course possible to shorten the time taken to cool the moulds  12  even further. This is because the flow rate of heat-transfer fluid can be increased and it is possible to obtain a heat-transfer fluid that is very cold because of the great amount of expansion it has undergone. 
     Furthermore, the external face  28  of the half-mould  14  has a larger surface area than the internal face of the cavity  18 . Thus, by circulating the heat-transfer fluid against the external face  28  of the half-mould  14 , it is possible to benefit from a larger area of contact between the heat-transfer fluid and the half-mould  14 . The removal of heat by conduction is thus greater than it would be if the heat-transfer fluid was circulated inside the moulding cavity  18 .