Patent Publication Number: US-2020276738-A1

Title: Method and apparatus for producing objects made of polymeric material

Description:
The invention relates to a method and an apparatus for producing objects made of polymeric material, particularly by means of compression moulding. 
     The objects that can be produced by means of the method and the apparatus according to the invention can for example comprise caps for containers, preforms for obtaining containers by means of blow moulding or stretch blow moulding, or containers. 
     The polymeric material that can be processed by means of the method and the apparatus according to the invention can be any material usable in compression moulding, in particular a semi-crystalline material such as polypropylene (PP), high-density polyethylene (HDPE) or polyethylene terephthalate (PET). More generally, the method and the apparatus according to the invention can be used for processing any polymeric material that has a melting temperature higher than its crystallization temperature and/or glass transition temperature. 
     Conventionally, the objects obtained by compression moulding semi-crystalline polymeric materials are produced by inserting, into a mould, a dose of polymeric material having a temperature higher than the respective melting temperature. The dose is shaped between a male element and a female element of the mould so as to obtain the desired object, which is then cooled inside the mould and finally extracted from the latter. 
     EP 1265736 discloses a method for producing an object by compression moulding a semi-crystalline polymeric material, in which the polymeric material is heated, inside an extruder, until the polymeric material reaches a temperature higher than the respective melting temperature. Subsequently, the polymeric material is cooled to a working temperature lower than the melting temperature, but higher than a crystallization starting temperature, at which crystallization starts during cooling. From the polymeric material thus cooled, doses are obtained having a pre-established mass. These doses are introduced into respective moulds in which they are shaped between a male element and a female element. While the polymeric material is being shaped in the mould, its temperature is maintained at a value close to the crystallization starting temperature. Subsequently, the object obtained by shaping the polymeric material is cooled and then extracted from the mould. 
     The method disclosed in EP 1265736 allows the cycle time to be reduced if compared with the conventional methods in which the dose was introduced inside the mould at a temperature higher than the melting temperature. By introducing the dose of polymeric material in the mould at a working temperature that is lower than the melting temperature, but a little higher than the crystallization starting temperature, there is a reduction in the time required for cooling the moulded object from the working temperature to a temperature at which the moulded object can be extracted from the mould without being damaged. 
     Nevertheless, the method disclosed in EP 1265736 can be further improved, above all with regard to the energy efficiency of the apparatus used for carrying out such method. An object of the invention is to improve the methods and the apparatuses for producing objects, in particular by compression moulding doses of polymeric material, especially semi-crystalline thermoplastic material. 
     A further object is to increase the energy efficiency of the methods and of the apparatuses for producing objects, in particular by compression moulding doses of polymeric material, especially semi-crystalline thermoplastic material. 
     Another object is to provide a method and an apparatus that allow speeding up the production of objects made of polymeric material particularly by means of compression moulding, while simultaneously ensuring an energy savings with respect to the state of the art. 
     In a first aspect of the invention, there is provided an apparatus for producing an object from a polymeric material having a melting temperature, the apparatus comprising:
         a melting device for melting the polymeric material;   a heat exchanger for cooling the polymeric material melted by the melting device below the melting temperature;   a mould for forming the object from the polymeric material while the latter has a temperature lower than said melting temperature,
 
wherein the apparatus further comprises a heat recovery device associated with the heat exchanger, the heat recovery device being configured to recover at least part of the heat released by the polymeric material to the heat exchanger.
       

     The heat recovery device allows at least part of the heat released by the melted polymeric material to the heat exchanger to be made available for further use, thus preventing this part of heat from being dispersed in the environment. Thus, it is possible to obtain an apparatus having high energy efficiency. 
     In other words, advantage is taken of the fact that the polymeric material is cooled between the melting device and the mould, in order to at least partly recover the heat released by the polymeric material, so that the recovered heat can be addressed to other uses. 
     In addition, the apparatus according to the first aspect of the invention allows productivity to be considerably increased with respect to the known apparatuses. In particular, the time necessary for producing an object made of polymeric material by shaping the polymeric material inside the mould, is drastically reduced with respect to the state of the art. 
     Indeed, by inserting the polymeric material in the mould at a temperature lower than the melting temperature, there is a decrease in the time required for cooling the formed object to a temperature at which it can be handled and then extracted from the mould without undergoing damage. Hence, there is a reduction of the cycle time. 
     In an embodiment, the heat recovery device is configured to make the recovered heat available for further use in said apparatus. 
     In this manner, it is possible to decrease the amount of energy that must be supplied to the apparatus from the outside for allowing the apparatus to work properly. 
     In an embodiment, the heat recovery device is connected with a preheating device for preheating granules of said polymeric material upstream of the melting device. 
     In this manner, there is a reduction in the energy that must be supplied to the melting device in order that the melting device can melt the polymeric material. Indeed, the melting device receives the granules of polymeric material at a temperature higher than the ambient temperature, which implies a reduction in the heat that must be supplied to the granules in order to bring them to the melting point. 
     In an embodiment, the heat recovery device is connected to the melting device so as to release heat directly to the melting device. 
     Also in this case, the energy that needs to be supplied from outside the apparatus to the melting device is decreased, since a part of the heat required for melting the polymeric material in the melting device is recovered from the heat that the melted polymeric material releases to the heat exchanger. 
     In an embodiment, the mould is configured to compression mould the object from a dose of the polymeric material. 
     In particular, the apparatus can comprise a severing element for severing a dose of polymeric material from a flow of polymeric material coming from the heat exchanger. 
     In a second aspect of the invention, there is provided a method for producing an object from a polymeric material having a melting temperature, the method comprising the steps of:
         melting the polymeric material,   cooling the melted polymeric material below the melting temperature,   forming the object from the polymeric material while the latter has a temperature lower than said melting temperature,
 
wherein at least part of the heat released by the polymeric material during said step of cooling the melted polymeric material is recovered and made available for further use.
       

     The method provided by the second aspect of the invention allows to obtain the advantages, in terms of energy efficiency and decrease of cycle time, that have been described above with reference to the apparatus according to the first aspect of the invention. 
    
    
     
       The invention can be better understood and carried out with reference to the enclosed drawings, which illustrate an exemplifying and non-limiting embodiment thereof, in which: 
         FIG. 1  is a schematic view of an apparatus for producing objects by compression moulding; 
         FIG. 2  is a graph that shows how crystallization of a particular type of polypropylene varies as a function of time; 
         FIG. 3  is a graph that shows, for the polypropylene of  FIG. 2 , how the percentage of crystallized mass varies as a function of time; 
         FIG. 4  is a graph that shows, for the polypropylene of  FIG. 2 , how the time necessary for obtaining crystallization of 50% of the mass of material varies as a function of temperature; 
         FIG. 5  is a graph that schematically shows how the temperature of a polymeric material varies, in the apparatus of  FIG. 1 , as a function of time. 
     
    
    
       FIG. 1  shows an apparatus  1  for producing an object by compression moulding a dose of polymeric material. 
     The object produced by means of the apparatus  1  can be a cap for a container, or a container, or a preform for obtaining a container by means of blow moulding or stretch blow moulding, or more generally any object with a concave or flat shape. 
     The polymeric material used by the apparatus  1  can be any polymeric material that can be compression moulded, in particular a semi-crystalline material such as polypropylene (PP), high-density polyethylene (HDPE) or polyethylene terephthalate (PET). 
     Semi-crystalline materials are materials that exhibit, in their solid state, a fraction of crystalline mass and a fraction of amorphous mass. 
     For semi-crystalline polymeric materials, a melting temperature T F  and a crystallization temperature T C  may be defined. 
     In particular, the melting temperature T F  is the temperature at which a polymeric material which is heated, passes from solid state to melted state. 
     The crystallization temperature T C  is the temperature at which a fraction of material crystallizes during cooling. The crystallization temperature T C  is lower than the melting temperature T F . 
     To be more precise, the crystallization process does not occur at a specific temperature, but within a temperature range which is defined between a crystallization starting temperature T IC  and a crystallization ending temperature T FC . 
     Furthermore, the crystallization temperature T C , as well as the difference existing between the crystallization starting temperature T IC  and the crystallization ending temperature T FC , are not constant for a given material, but depend on the conditions according to which the material is cooled. In particular, the lower is the temperature at which the melted polymeric material is maintained, the faster crystallization thereof takes place. Moreover, the more quickly the melted polymeric material is handled, the more the temperature range at which crystallization occurs lowers. 
     This is confirmed by  FIG. 2 , which shows the results of an analysis carried out by differential scanning calorimetry (DSC) on polypropylene samples. The material samples analyzed were brought to a temperature higher than the melting temperature, at which temperature the samples were kept for a few minutes so as to melt all the crystals present therein. The samples were then cooled down to a pre-determined temperature and maintained at such temperature for the time necessary for each sample to be crystallized. Thus, crystallization times and modes were tested for each sample. 
       FIG. 2  shows the energy released by the samples analyzed as a function of time, during the crystallization step. 
     In particular, the curve indicated by A refers to the sample which was cooled to the lowest temperature, namely 108° C. In this sample, the crystallization has occurred in a shorter time and within a lower temperature range than the other samples analyzed. Curve A exhibits an exothermic crystallization peak which is the narrowest one among all the samples analyzed. This means that the difference between the crystallization starting temperature T IC  and the crystallization ending temperature T FC  for that sample, is minimum with respect to all other samples analyzed. 
     The curve referred to with B is instead relative to the sample which was cooled to the highest temperature, i.e. to a temperature of 115° C. In this sample the crystallization process did not occur, because the high temperature at which the sample was maintained did not allow crystals to be formed during the period of time in which the sample was observed. 
     This proves that the polymeric material crystallizes faster if a lower temperature is maintained. 
     A similar reasoning applies to the melting process and related melting temperature. 
       FIG. 3 , based on data obtained from  FIG. 2 , shows how the crystallized mass percentage varies in a sample as a function of time. Each curve refers to a different temperature down to which the sample was cooled, after which the sample temperature was kept constant. In particular, the temperature of each sample increases by moving from left to right in the graph. It is noted that, the lower is the temperature at which the sample is cooled, the more is reduced the time required for a 100% crystallization of the sample mass to occur. 
     A half-crystallization time t 1/2  can be defined, which is the time needed by a sample to have half of the mass thereof crystallized.  FIG. 4 , based on data from  FIGS. 2 and 3 , shows the half-crystallization time t 112  as a function of temperature at which the sample was maintained. It is noted that, upon increasing of the temperature at which the sample was maintained, the half-crystallization time t 1/2  increases. 
     In summary, the behaviour of a semi-crystalline polymer during melting and crystallization thereof cannot be univocally determined, but is affected by the cooling conditions, following which the polymer is cooled. In particular, the lower is the temperature at which the molten polymer material is kept, the faster crystallization takes place. 
     The above considerations derive from studies concerning behavior of semi-crystalline polymeric materials which were carried out under static conditions, that is to say, while the sample analyzed was not undergoing any deformation. Crystallization occurring under these conditions is called quiescent crystallization. 
     Nevertheless, when a semi-crystalline polymeric material is subject to deformation, as it happens when the polymeric material is handled in a machine, for example for being subjected to compression moulding, a phenomenon occurs called Flow Induced Crystallization. While the material flows, anisotropic crystallites are formed which are oriented in the flow direction, and this modifies the crystallization kinetics of the material with respect to the condition under which the only quiescent crystallization occurs. 
     When a polymeric material is cooled below the melting temperature T F  and at same time it is deformed, the quiescent crystallization and the flow induced crystallization combine, thus causing a globally faster crystallization of the material. 
     It was noted that, by displacing fast a molten polymeric material, the crystallization temperature thereof decreases and the temperature range within which crystallization takes place narrows. This is due to the fact that, by keeping the molten polymeric material in an agitated state, the polymeric chains of the polymeric material have less capability of organizing and solidifying in an ordered configuration. 
     The phenomena described hereinabove can be used to improve compression moulding of a semi-crystalline polymer, particularly in an apparatus  1  of the type shown in  FIG. 1 . 
     The apparatus  1  comprises a melting device  2 , which can comprise an extruder device  3 , suitable for melting and extruding the polymeric material. 
     Upstream of the melting device  2 , a feeding container  4  is provided, the feeding container  4  being suitable for containing a quantity of granules of the polymeric material to be processed in order to continuously feed the melting device  2 . In particular, the feeding container  4  can be connected to the melting device  2  by means of a hopper  5 . 
     In this case the granules of polymeric material descend from the feeding container  4  into the hopper  5  and from here into the melting device  2 . 
     In the melting device  2 , the polymeric material is heated until reaching a temperature T M  higher than or equal to the melting temperature T F . The polymeric material in this manner passes from the granular solid state to the melted state. 
     Downstream of the melting device  2 , a cooling zone is provided which, in the depicted example, is defined inside a heat exchanger  6 . The cooling zone is configured to cool the flow of polymeric material coming from the melting device  2  to an intermediate temperature T INT  lower than the melting temperature T F , but higher than the crystallization temperature T C . 
     The heat exchanger  6  can comprise a static mixer. The latter can comprise a conduit through which the polymeric material passes, in which a mixing element is arranged. 
     The mixing element comprises a plurality of diverting bars arranged in a stationary position for homogenizing the flow of polymeric material, both from the thermal viewpoint and, where appropriate, from the composition viewpoint. In particular, the diverting bars may divide the main flow of polymeric material into a plurality of secondary flows which mix with one another along their path inside the static mixer. 
     The heat exchanger  6  can be associated with a circuit  17 , in which a conditioning fluid circulates, e.g. diathermic oil, water, steam or other, in order to control temperature of the flow of polymeric material downstream of the melting device  2 . 
     More in detail, the circuit  17  in which the conditioning fluid circulates can comprise a coil  7 , which surrounds the conduit in which the polymeric material passes when it flows through the heat exchanger  6 . 
     In the depicted example, the exchanger  6  is a counter-current heat exchanger. In other words, the conditioning fluid enters into the coil  7  at a point arranged downstream of a further point from which the conditioning fluid exits from the coil  7 , with respect to an advancement direction F according to which the polymeric material advances inside the heat exchanger  6 . 
     Along the circuit  17  in which the conditioning fluid of the heat exchanger  6  circulates, a pump  15  can be provided for moving the conditioning fluid in the desired direction. 
     The apparatus  1  further comprises a heat recovery device  16  configured to cooperate with the heat exchanger  6 . The heat recovery device  16  is configured to recover at least part of the heat released by the melted polymeric material to the heat exchanger  6 , so that the recovered heat can be made available for further use. 
     In particular, the heat recovered by the heat recovery device  16  can be reused in the same apparatus  1  in which the heat recovery device  16  is inserted, so as to decrease the amount of energy that must be provided from the outside in order to enable operation of the apparatus  1 . 
     The heat recovery device  16  can be conformed as a recovery heat exchanger for removing heat from the conditioning fluid circulating in the circuit  17  associated with the heat exchanger  6 . Such conditioning fluid, which has cooled the polymeric material coming from the melting device  2 , is at a relatively high temperature and can heat a heat recuperator fluid circulating in the heat recovery device  16 . 
     The heat recovery device  16  can for example comprise a casing  18  which houses a recovery circuit  19 . Within the casing  18 , a portion of the circuit  17  associated with the heat exchanger  6  is also arranged. The circuit  17  and the recovery circuit  19  can face each other, e.g. near respective coils, or in any case be arranged close to each other, so as to allow passage of heat from the conditioning fluid circulating in the circuit  17  to the heat recuperator fluid circulating in the heat recovery device  16 . 
     In this manner, the heat recovery device  16  allows to recover, from the heat exchanger  6 , heat which would otherwise be lost, consequently increasing the energy efficiency of the apparatus  1 . 
     In the example shown in  FIG. 1 , the heat recovery device  16  is connected with a preheating device  20  for preheating the polymeric material intended to enter into the melting device  2 , e.g. for preheating the granules that are present in the feeding container  4 . To this end, the preheating device  20  can be conformed as a winding that encloses a lateral wall of the feeding container  4 . Such winding is connected to the recovery circuit  19 , such that the heat recovered in the heat recovery device  16  can be transferred to the preheating device  20 . 
     A further pump  21  can be provided for moving, towards the feeding container  4 , the heat recovery fluid that was heated by the conditioning fluid associated with the heat exchanger  6 . 
     The apparatus  1  further comprises a severing device  8  for severing dosed quantities or doses of polymeric material from the flow of polymeric material coming from the heat exchanger  6 . 
     In particular, the severing device  8  allows the doses to be severed from the flow of polymeric material which exits from a nozzle  9  arranged downstream of the heat exchanger  6 . In the depicted example, the nozzle  9  is directed upward. In particular, the nozzle  9  can be configured in a manner such that the flow of polymeric material that exits from the nozzle  9  is directed along a vertical direction. 
     Other arrangements of the nozzle  9  are also possible, however. 
     The severing device  8  can comprise at least one collecting element  10 , e.g. shaped as a concave element extending around a vertical axis, suitable for passing near the nozzle  9 , particularly above the latter, in order to sever a dose of polymeric material therefrom. The collecting element  10  therefore acts as a severing element for severing the dose from the flow of polymeric material exiting from the nozzle  9 . The dose remains attached to the collecting element  10 , which carries the dose towards a mould in which the dose can be shaped so to obtain the desired object. 
     The collecting element  10  is rotatable around a rotation axis Y, particularly vertical. To this end, a plurality of collecting elements  10  can be provided, supported by a carousel structure  11 . 
     The mould is arranged downstream of the heat exchanger  6 . 
     The mould can comprise a female element  13  and a male element  14 . The male element  14  can be arranged above the female element  13 , and be aligned with the latter along a vertical axis. Nevertheless, other mutual arrangements are possible for the female element  13  and the male element  14 . 
     The apparatus  1  can comprise a plurality of moulds arranged in a peripheral region of a moulding carousel  12 . The moulding carousel  12  can be rotatable around a vertical axis Z. 
     The apparatus  1  comprises a movement device for moving the female element  13  and the male element  14  towards each other and alternatively moving the female element  13  and the male element  14  away from each other. In particular, the movement device is configured to move the female element  13  and the male element  14  with respect to each other between an open configuration, in which the female element  13  and the male element  14  are spaced apart from each other, and a forming configuration, in which a forming chamber is defined between the female element  13  and the male element  14 , the forming chamber having a shape corresponding to the object that it is desired to obtain. 
     The movement device can be associated only with the female element  13 , so as to move the female element  13  with respect to the male element  14  that is instead maintained in a stationary position. It is also possible to associate the movement device with the male element  14 , which is thus moved with respect to the female element  13 , which is instead maintained stationary. 
     Alternatively, the movement device can simultaneously act on the male element  14  and on the female element  13 , which are both moved. 
     The movement device can for example be of mechanical or hydraulic type. An example of a mechanical movement device is a cam device, while an example of a hydraulic movement device is a hydraulic actuator. 
     In any case, the movement device is configured to move the male element  14  and the female element  13  with respect to each other along a moulding direction which, in the depicted example, is vertical. 
     The collecting element  10  is, as stated above, movable around the rotation axis Y. In particular, the collecting element  10  can be in a collecting position, in which the collecting element  10  is located above the nozzle  9  to remove a dose of polymeric material therefrom. The dose, due to its highly viscous fluid state, remains adherent to the collecting element  10  which, while rotating around the rotation axis Y, carries the dose towards the mould. The collecting element  10  can also be n a delivery position in which it is arranged above the female element  13 , so as to release—e.g. with the aid of pneumatic or mechanical devices—the dose of polymeric material inside a cavity of the female element  13 . 
     The portion of the apparatus  1  interposed between the heat exchanger  6  and an outlet mouth of the nozzle  9  is thermally conditioned, such that the temperature of the polymeric material that flows through it remains, at its interior, at a controlled value below the melting temperature T F , for example at an intermediate value T INT  between the melting temperature T F  and the crystallization starting temperature T IC . 
     The apparatus  1  can comprise an accelerator device for accelerating the flow of polymeric material coming from the melting device  2 . In the depicted example, the nozzle  9  acts as an accelerator device, since it is provided with passage sections for the polymeric material which progressively decrease in the advancement direction F, so as to accelerate the flow of polymeric material that passes inside. 
     Alternatively, or in combination with was stated above, a conduit within which the polymeric material passes before reaching the nozzle  9  can also act as accelerator device, if its internal sections are suitably sized for accelerating the flow of polymeric material. 
     In a non-depicted version, a different accelerator device may be provided, which can be arranged in any point between the heat exchanger  6  and the nozzle  9 . 
     During operation, as shown in  FIG. 5 , the granules of polymeric material are heated at a preheating temperature T 1  by the preheating device  20 , while the granules are located inside the feeding container  4 . The preheating temperature T 1  is higher than the ambient temperature. 
     From the feeding container  4 , passing through the hopper  5  (if present), the preheated granules enter into the melting device  2 , which heats the granules until they reach temperature T M , higher than or equal to the melting temperature T F . The granules are maintained at such temperature for a time sufficient to cause melting thereof, after which the melted polymeric material is extruded and sent into the heat exchanger  6 . Here, the melted polymeric material is cooled to the temperature T INT , which as stated above is comprised between the melting temperature T F  and the crystallization temperature T C . More specifically, the temperature T INT  is higher than the crystallization starting temperature T IC . 
     Subsequently, the polymeric material coming from the heat exchanger  6  reaches the nozzle  9 , from which a flow of polymeric material exits; the severing device  8  severs a dose from the flow of polymeric material. 
     In the nozzle  9 , and possibly also in a portion of the apparatus  1  comprised between the heat exchanger  6  and the nozzle  9 , the flow of polymeric material is accelerated, so that the flow reaches an average speed higher than the average speed that it had in the heat exchanger  6 . 
     The dose that the severing device  8  has severed is carried by a corresponding collecting element  10 , until the dose arrives above a female element  13  of a mould. 
     In this moment, the mould is in the open configuration, in which the female element  13  is spaced apart from the male element  14 . 
     The collecting element  10  can therefore be interposed between the female element  13  and the male element  14  and release the dose into the female element  13 . 
     When the collecting element  10  is above the female element  13 , the dose is detached from the collecting element  10  and deposited in the cavity of the female element  13 . The female element  13  and the male element  14  move towards each other due to the movement device, until a forming configuration is reached, in which the forming chamber defined between the male element  14  and the female element  13  has a shape corresponding to the shape of the object to be obtained. At this point, the female element  13  and the male element  14  are maintained in the reached mutual position in order to allow the object to be crystallized, so that the object may reach a sufficient strength which allows the object to be extracted from the mould without undergoing damage. 
     At the end of the forming step, the mould is opened, by moving the male element  14  and the female element  13  away from each other. The formed object is removed from the mould and a new forming cycle can start. 
     The dose is introduced in the mould at a working temperature T LAV  lower than the melting temperature T F  of the constituent polymeric material, but higher than the crystallization starting temperature T IC  at which, in static conditions, crystals would start to form. The working temperature T LAV  can be equal to the intermediate temperature T INT  or lower than the latter. 
     While the polymeric material that constitutes the dose is shaped between the female element  13  and the male element  14  of the mould, its temperature is maintained higher than the crystallization starting temperature T IC . 
     This does not mean that the temperature of the female element  13  and of the male element  14  of the mould is also higher than the crystallization starting temperature T IC . The female element  13  and the male element  14  can be provided with respective cooling circuits, a cooling fluid circulating within each of these circuits. Even if the temperature of the polymeric material that is shaped is higher than the crystallization starting temperature T IC , the temperature of the cooling fluid, like that of the respective mould elements, can be lower than the crystallization starting temperature T IC , even significantly lower. 
     When the compression-moulded object has reached its substantially definitive form, the object is cooled below its crystallization temperature T C . The object can be cooled with a cooling speed greater than 3.5° C./s, so that solidification occurs as quickly as possible. Inserting the dose in the mould at a temperature lower than the melting temperature T F  allows a reduction in the time necessary for cooling the compression-moulded object to a temperature at which the moulded object can be extracted from the mould and handled without being significantly deformed. 
     In addition, subjecting the flow of polymeric material to high speeds, upstream of the mould and/or inside the mould, allows increasing the deformation speed of the polymeric material and hence accelerating the crystallization kinetics, since in addition to the quiescent crystallization that would occur in static conditions there is also the flow induced crystallization. 
     Inside the melting device  2 , the polymeric material is supplied with an amount of heat sufficient for heating the polymeric material from the temperature T 1  to the temperature T M . Indeed, the polymeric material is brought from the ambient temperature to the temperature T 1  by the preheating device  20 , which exploits the heat that the heat recovery device  16  has recovered from the heat exchanger  6 . 
     If the heat recovery device  16  were not present, the other conditions being the same, the polymeric material would enter into the melting device at a temperature T 2  lower than the temperature T 1 . For example, temperature T 2  could be equal to the ambient temperature. The melting device  2  would therefore heat the polymeric material from the temperature T 2  to the temperature T M , with a consequent greater energy consumption. 
     Assuming—for the sake of simplicity—that the heat losses inside the apparatus  1  are negligible, the difference ΔT between the temperature T M  and the intermediate temperature T INT  is about equal to the difference ΔT between the temperature T 1  and the temperature T 2 . In other words, still assuming that the heat losses can be ignored, the heat released by the polymeric material to the heat recovery device  16  in the heat exchanger  6  corresponds to the heat savings can be obtained in the melting device  2 , by preheating the polymeric material by means of the preheating device  20  connected to the heat recovery device  16 . 
     This allows to obtain a reduction in the amount of energy that must be supplied, from outside, to the apparatus  1 . 
     The preheating device  20  can use, for preheating the granules of polymeric material, a preheating fluid in the liquid state, or a preheating fluid in the gaseous state, e.g. in the form of heated air. The preheating fluid used by the preheating device  20  in any case receives heat from the heat recovery device  16 . 
     In a non-depicted version, the heat recovery device  16  can be configured to release the recovered heat directly to the melting device  2 , instead of releasing the recovered heat to the preheating device  20 . In this case, the heat recovered owing to the heat recovery device  16  is used for assisting heating of the polymeric material in the melting device  2 . This still allows to save the quantity of energy that, from the outside, must be supplied to the apparatus  1 . 
     It is also possible to optimize the energy efficiency of the apparatus  1  by improving the thermal insulation thereof and optimizing the insulation. For example, the melting device  2  could be inserted inside a thermally insulated containment casing in order to reduce heat dissipation. Also the heat exchanger  6 , and/or the heat recovery device  16 , and/or the preheating device  20  could be inserted inside the abovementioned containment casing. 
     In an alternative embodiment, the heat recovery device  16  can be associated with a cogeneration device. The cogeneration device can for example be of the type operating with methane gas. 
     In a further embodiment, the heat recovery device  16  can be connected to, inserted in, or in any case associated with a heat pump. 
     In the preceding description, reference was made to a heat exchanger  6  comprising a static mixer. 
     Nevertheless, this condition is not necessary. 
     Indeed, the heat exchanger  6  can be defined not by a static mixer but by a dynamic mixer, i.e. a mixer provided with mixing elements that are moved during operation. 
     In addition, the heat exchanger  6  can be defined inside a cascade extruder or a planetary extruder, in particular arranged immediately downstream of the extruder device that melts and extrudes the polymeric material. 
     The heat exchanger  6  could also be defined inside a suitably conditioned twin-screw extruder. 
     In theory, the heat exchanger  6  could be defined within the same melting device  2  that melts the polymeric material, which could be provided with an end part configured to cool the melted polymeric material. 
     In general, the entire section of the apparatus  1  interposed between the melting device  2  and the severing device  8  can be thermally conditioned so as to cool the polymeric material. In this case, the cooling zone starts immediately downstream of the point at which the polymeric material is melted and continues up to the nozzle  9 . 
     Alternatively, the cooling zone can affect only a portion of the apparatus  1  arranged downstream of the point in which the polymeric material is melted. In this case, the cooling zone ends upstream of the nozzle  9 , and between the cooling zone and the nozzle  9  a maintenance zone is interposed in which temperature of the polymeric material is maintained at desired values. 
     In this case, temperature of the polymeric material in the maintenance zone arranged downstream of the cooling zone can be comprised, or substantially comprised, between the crystallization temperature T C  and the melting temperature T F . With the term “substantially comprised” it is meant that at least 90% of the polymeric material has a temperature in the range comprised between the crystallization temperature T C  and the melting temperature T F . Small portions of polymeric material can however have a temperature higher than the melting temperature T F , particularly near the surface of the polymeric material that flows in contact with the walls of the apparatus  1 . 
     In the depicted example, reference was made to a situation in which the dose is shaped between a female forming element and a male forming element belonging to the mould, i.e. the female element  13  and the male element  14 . 
     It can further happen that the dose is shaped in contact with an object which is not integrated in the mould, although it behaves like a moulding element while the dose is being formed. This is what happens, for example, in the case of the so called lining, in which the dose is shaped so as to form a liner inside a previously formed cap. More generally, the dose may be moulded inside the cavity of an object, so as to form a component anchored to the object. 
     In this case, the cap or, more broadly, the object provided with a cavity inside which the dose is shaped, acts as a female forming element, whereas the male forming element is integrated in the mould. In addition to the male forming element, the mould also comprises in this example a support element facing the male forming element and suitable for supporting the object inside which the dose has to be shaped during moulding. 
     In broad terms, it can therefore be stated that the mould comprises a male forming element and an opposite element facing the male forming element. The opposite element may be a female forming element or, alternatively, a support element for supporting an object inside which the dose is shaped. 
     What was previously described with reference to the embodiment in which the female forming element is a part of the mould, is to be understood also as referring to an embodiment in which the dose is shaped inside of an object which is not integrated in the apparatus  1  and which acts as a female forming element. 
     In the preceding description, reference was made to an apparatus and to a method for producing an object by compression moulding. The apparatus and the method according to the invention can indeed also be used for producing an object by means of injection moulding or injection-compression moulding. 
     In addition, instead of using an apparatus comprising a plurality of moulds mounted on the moulding carousel  12 , it is possible to use a plurality of moulds mounted according to an arrangement different from a carousel, or an apparatus provided with a single mould, as occurs in the so-called “single-impression” machines.