Abstract:
In a method for the internal cooling of a rotating object ( 4 ), liquid gas from at least one inlet channel ( 61 ) in a fixed object ( 5 ) is pressed into a ring-shaped groove ( 62 ) located between the fixed object ( 5 ) and the rotating object ( 4 ). From the ring-shaped groove ( 62 ) the liquid gas is pressed into at least one channel ( 63, 64.1, 64.2 ) in the rotating object ( 4 ) and brought to a part to be cooled ( 15 ). The liquid gas, upon contact with the part, evaporates and expands while absorbing heat, thereby cooling the part. The area surrounding the part to be cooled ( 15 ) may be designed as an expansion chamber ( 65 ). The method can be used to cool molded parts in injection molding machines with rotating molds ( 1 ), thereby achieving shorter cycle times.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a method for the internal cooling of a rotating object with liquid gas and to a device for implementation of the cooling method. The method is suitable in particular for the cooling of molded parts in injection molding machines with rotating molds. 
     In many fields of technology there is a requirement for an internal cooling of a rotating object. Such an internal cooling is achieved by introducing a cooling liquid, for example water, from a non-rotating, fixed object into the at least partially adjoining rotating object. In the rotating object, the cooling liquid is then conveyed to the part to be cooled. The liquid absorbs the thermal energy from the part and takes heat away, thereby producing a cooling effect. In doing so, the rotating object, for example, can be designed as a shaft, the fixed object, for example, as a bearing for supporting the shaft. 
     The introduction of the cooling liquid from the fixed object into the rotating object usually takes place axially. The transfer point between the fixed and rotating objects is situated on the axis of rotation. Such an arrangement is desirable, because with it the transfer point is not moving relative to the fixed object. There are, however, instances, where the rotating object is not axially accessible. 
     In some applications it is desirable to cool with a liquid gas instead of with a conventional liquid. Cooling processes with liquid gas are known as such. In them, a liquid gas is conveyed to the part to be cooled, whereby it usually is compressed all the more, the closer it is to the part to be cooled. At the part to be cooled an evaporation and an expansion of the initially liquid, compressed gas is permitted. During evaporation and expansion, thermal energy is withdrawn from the part to be cooled, as a result of which a cooling effect is produced. The gas is then removed in a gaseous physical condition (condition of aggregation). 
     If such a liquid gas is to be introduced into a rotating object from a fixed object, particular problems occur. The liquid gas possibly-depending on its chemical composition, temperature and pressure-is in a special physical condition (condition of aggregation), which, while advantageous for the cooling, is exceedingly delicate with respect to the handling. In any case, an evaporation and/or expansion of the liquid gas has to be avoided, because this would lead to freezing of the transfer point. Because of the special physical condition (condition of aggregation) of the liquid gas, in particular sealing problems at the transfer point have to be solved. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for the internal cooling of a rotating object with liquid gas, which solves or minimizes the problems set forth hereinbefore. It is furthermore an object of the invention to create a device for the implementation of this method. 
     In case of the method in accordance with the invention for the internal cooling of a rotating object, liquid gas is pressed from at least one inlet channel in a fixed object into a ring-shaped groove disposed at a contact surface between the fixed object and the rotating object. From the ring-shaped groove, the liquid gas is pressed into at least one channel in the rotating object and fed to at least one part to be cooled at which point the liquid gas evaporates, absorbs vaporization heat, and is removed as gaseous gas. 
     In preference, in an area surrounding the part to be cooled a greater cross-sectional surface area is made available to the liquid gas for flowing through, such as the sum of the cross-sectional surface areas of the at least one channel. As a result of this greater cross-sectional surface area in the area surrounding the part, the liquid gas evaporates and expands while absorbing thermal energy. The volume of the gas following the expansion, for example, can amount to 600 times its volume prior to the expansion. 
     The total cross-sectional surface area, which is made available to the liquid gas on the way from the fixed object to the object to be cooled, is preferably maintained constant or reduced, so that the liquid gas does not expand on the way to the part to be cooled and so that an optimum cooling effect is obtained at the part to be cooled. It is particularly advantageous to reduce the total cross-sectional surface area at least once, which is made available to the liquid gas on the way to the part to be cooled, so that the liquid gas is compressed. This can be achieved, for example, by contractions of the channels. While the inlet channel in the fixed object may have a diameter of several millimetres, the last section of the channel in the rotating object may have a diameter of 0.5 mm or less; even capillary dimensions can be utilized. In this regard, “total cross-sectional surface area” is: the cross-sectional surface area of the one channel, if only one channel is present, resp., the sum of the cross-sectional surface areas of all channels, if several channel are present; in this, the cross-sectional surface areas are always measured vertical or perpendicular to the direction of flow of the gas. 
     The device in accordance with the invention for the implementation of the method has a fixed object, in which an object rotating around a rotation axis is rotatably fixed. The fixed object has at least one inlet channel for liquid gas. The rotating object is surrounded by a ring-shaped groove, into which the at least one inlet channel leads and the center of which is situated on the rotation axis of the rotating object. The ringshaped groove can be machined into the rotating object and/or into the fixed object. The rotating object has at least one channel for liquid gas that leads out from the ringshaped groove into the area surrounding a pan to be cooled. 
     The area surrounding the part to be cooled preferably has a greater cross-sectional surface area than the sum of the cross-sectional surface areas of the at least one channel in the rotating object, whereby these cross-sectional surface areas are measured in essence vertically or perpendicular to the rotation axis. As a result of this, the gas is provided with sufficient volume for an expansion. The area surrounding the part to be cooled can, for example, be at least one expansion chamber. 
     The cross-sectional surface area of the inlet channel or, if several inlet channels are present, the cross-sectional surface area of the inlet channels, is preferably greater than double the cross-sectional surface area of the ring-shaped groove. If this requirement is fulfilled, then the liquid gas does not evaporate and/or expand in the vicinity of the ring-shaped groove. Evaporation and/or expansion could have the consequence that too much heat would be removed from the surroundings of the ringshaped shaped groove and that this surrounding area would freeze, which is undesirable. 
     The double cross-sectional surface area of the ring-shaped groove is preferably greater than the sum of the cross-sectional surface areas of the at least one channel. The sum of the cross-sectional surface areas of the at least one inlet channel to the ring-shaped groove preferably remains constant or is reduced. The sum of the cross-sectional surface areas of the at least one channel leading to the cooling place preferably remains constant or is reduced. By means of such measures, however, the liquid gas is compressed on its path to the place to be cooled, so that an optimum cooling effect is obtained. 
     In order to prevent the occurrence of accumulations of heat, advantageously at those points where an accumulation of heat could occur, porous steel is utilized. This material stores the cold and, if so required, absorbs thermal energy. 
     The method in accordance with the invention can advantageously be used for the cooling of molded parts in injection molding machines with rotating molds. Rotating molds like this provide many advantages. There is, e.g., the possibility to inject the molten mass (for example, molten plastic mass) into the mold from several injection stations. With this, first of all molded parts with different geometrical shapes, different colors, or made of different materials can be manufactured. Secondly, with this it is also possible to make molded parts out of several components (multi-component process). Therefore, molded parts that have several colors or consist of several materials (assembly injection molding) can be made. Rotating molds apart from this also make possible the utilization of intermediate stations for different operations and shorter cycle times. The liquid gas can be fed in continuously or else also in batches, e.g., only then, when cooling within an injection molding cycle is necessary. 
     With the method in accordance with the invention, and with the device in accordance with the invention, cycle times can be even more massively reduced thanks to the exceedingly efficient cooling of just-injected molded parts. The invention presented here solves the problem of the introduction of liquid gas into the shaft rotating from time to time, on which the injection mold is suspended. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and further features of the invention will be apparent with reference to the following description and drawings, wherein: 
     FIG. 1 is a (p,T) phase diagram for CO 2 , 
     FIG. 2 is a longitudinal section through an exemplary embodiment of a device in accordance with the invention, used in an injection-molding machine. 
     FIGS. 3-5 is a detail of different embodiments of the device in accordance with the invention in longitudinal section and 
     FIG. 6 is a longitudinal section through a further exemplary embodiment of a device in accordance with the invention, used in an injection-molding machine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Used as a cooling medium with the method in accordance with the invention, and in the device in accordance with the invention, in preference is CO 2 . FIG. 1 shows a (p,T) phase diagram for CO 2 , the numerical values of which have been taken from Landolt-Börnstein, Numerical Values and Functions, volume IV, 4th part, Springer-Verlag, 6th printing, 1967, pages 178-179 and 296. In this diagram, on the horizontal axis the temperature T in ° C. is marked linearly and on the vertical axis logarithmically the pressure P in 10 5  Pa (which approximately corresponds to one atmosphere). A region of the solid phase  91 , a region of the liquid phase  92  and a region of the gaseous phase  93  can be differentiated between. These regions  91 - 93  are separated from one another by the melting pressure curve  94 , a vapour pressure curve  95 , resp., a sublimation pressure curve  96 . The curves  94 - 96  meet at a triple point P T . Further characteristic points in the (p,T) diagram are a critical point P K  and a sublimation point P s . 
     The cooling medium CO 2  is preferably brought to the part to be cooled in a compressed liquid condition at temperatures of −50 to −20° C. (223 to 253 K). This temperature range  97  in FIG. 1 is indicated with broken lines. The course of the vapour pressure curve  95  in this temperature range  97  shows that high pressures between approx. 7*10 5  and 20*10 5  are necessary in order to keep the CO 2  liquid. If this requirement is to be fulfilled, then the cross section of inlet channels on the way to the part to be cooled must not significantly increase. Also no tight places must occur; this makes the transfer of the cooling medium from the fixed object to the rotating object particularly difficult. 
     A further possible cooling medium is nitrogen (N 2 ). 
     FIG. 2 illustrates a schematic longitudinal section through an exemplary embodiment of a device in accordance with the invention. The device is built into an injection molding machine with a mold  1  rotating around a rotation axis a. Shown of it schematically is only the mold  1 , composed of first half mold  11  and a second half mold  12 . The first mold half  11  has a cutting  13 , through which molten plastic mass can be injected into a forming hollow space  14  between the first mold half  11  and the second mold half  12 . Of an injection nozzle  2  only a part, for example, a heat-conducting torpedo  21  is illustrated. The second mold half  12  is mounted on a shaft  3 , for example, a hollow shaft and together with it forms a rotating object  4 . The shaft  3 , for example, can be rotatably supported on ball bearings  31 . 1 ,  31 . 2  in a fixed object  5 . Such a rotating injection mold  1 , as mentioned at the beginning, has many advantages. 
     The object now is to efficiently cool with liquid gas a part  15  located close to the forming hollow space  14  of the second mold half  12  and/or the molten plastic mass, or a molded part created from it by solidifying and situated in the forming hollow space  14 . For this purpose, the part  15  to be cooled of the second mold half  12  is equipped with and, in this example of an embodiment ring-shaped expansion chamber  65 , in which pressed in liquid gas vaporizes and expands. For the purpose of pressing in the liquid gas, the fixed object  5  has an inlet channel  61  for liquid gas. The rotating object  4  is surrounded by a ring-shaped groove  62 , into which the inlet channel  61  merges. The ring-shaped groove is located on a contact surface  45  between the fixed object  5  and the rotating object  4 ; this contact surface  45  corresponds to the cylindrical external surface of the shaft  3 . The transfer point between the inlet channel  61  and the ring-shaped groove  62  can be sealed with a seal implemented as a cutting (not illustrated). The rotating object  4  has a channel  63  consisting of, for example, two channel parts  63 . 1 ,  63 . 2  for liquid gas, which leads from the ring-shaped groove  62  into the expansion chamber  65 . In the example of FIG. 2, the channel  63  splits-up into two or also several channel branches  64 . 1 ,  64 . 2 . The latter channels  64 . 1 ,  64 . 2  in the rotating object  4  typically have very small diameters of 0.5 mm or less. 
     In order to enable an expansion of the gas at the desired place  15 , the expansion chamber  65  has a much greater total cross-sectional surface area  2 A E  than the sum A K641 +AK 642  of the cross-sectional surface areas of the channel branches  64 . 1 ,  64 . 2 ; the total cross-sectional surface area 2A E  of the expansion chamber  65  is in preference some hundred times, for example, 600 times greater than the sum A K641 +AK 642  of the cross-sectional surface areas of the channel branches  64 . 1 ,  64 . 2 . In doing so, these cross-sectional surface areas A E , A K641 , A K642  are in essence measured in a plane vertical to the respective direction of flow of the gas. If the gas has two paths at its disposal, such as, e.g., in the ring-shaped expansion chamber  65 , then for the calculation of the total cross-sectional surface area 2A E  the corresponding cross-sectional surface area AE has to be counted double. 
     In order to, on the contrary, prevent a vaporisation and/or an expansion of the gas on the way to the expansion chamber  65  and to compress the liquid gas even more for the purpose of achieving an optimum cooling effect, the inlet channel  61 , ring-shaped groove  62 , and channels  63 . 1 ,  63 . 2 ,  64 . 1 ,  64 . 2  are dimensioned as follows. The cross-sectional sectional surface area A z  of the inlet channel  61  (measured in a plane vertical or perpendicular to the direction of flow of the liquid gas) is greater than the twice 2A N  the cross-sectional surface area A N  of the ring-shaped groove  62  (also measured in a plane vertical or perpendicular to the direction of flow of the liquid gas, i.e., in a plane that contains the rotation axis a). Twice 2A N  the cross-sectional surface area A N  of the ring-shaped groove  62  is greater than the cross-sectional surface area A K631 , A K632  of the channel  63 , resp., the sum A K641 +AK 642  of the cross-sectional surface areas of the channel branches  64 . 1 ,  64 . 2  reduces towards the expansion chamber  65 , for example, at one or more contractions  66 . 1 ,  66 . 2 , in preference each time by 5 to 10%. 
     In summary, therefore for the cross-sectional surface areas A z , A N , AK 631 , A K632 , AK 641 , A K642 , A E  therefore the inequality 2A E&gt;A   z &gt;2A N ≧A K631 ≧A K641 +A 642  is applicable, whereby the factor ahead of A E  for the example of a ring-shaped expansion chamber amounts to 2, for other geometries, however, can also assume another value, for example, 1. 
     After the expansion of the gas in the expansion chamber  65 , the gas is taken away in a gaseous condition. For this purpose, e.g., the second mold half  12  in the region of the expansion chamber  65  can at least partially be made of porous steel, and the gas can be removed through the pores. Alternatively, the second mold half  12  can be equipped with evacuation bores  67 , through which the gas is brought to the outside or into the forming hollow space  14 . Such evacuation bores  67  can also be implemented as expansion bores with great cross-sectional surface area; their total cross-sectional surface area can preferably be some hundred times, for example 600 times, greater than the sum A K641 +AK 642  of the cross-sectional surface areas of the channel branches  64 . 1 ,  64 . 2 . The evacuation bores  67  can, for example, be produced by galvanizing. On the surface of the second mold half  12 , the gas removed can either simply escape to the ambient atmosphere. It can, however, also be collected, liquefied, brought back to a tank and used again for cooling; with a recycling like this, approx. 70-95% of the gas can be re-used after a cooling process, which is very efficient. 
     In the FIGS. 3-5, a detail of different embodiments of the device in accordance with the invention is depicted in longitudinal section, namely the ring-shaped groove  62 , a part of the inlet channel  61  in the fixed object  5  and a part of the channel  63  in the rotating object  4 . For the sake of simplicity, in FIGS. 3 and 4 only one half of the longitudinal section of the rotating object  4  is illustrated. In the example of an embodiment of FIG. 3, the ring-shape groove  62  is also, as in FIG. 2, machined into the rotating object  4 . FIG. 3 shows a variant with several, for example, two inlet channels  61 . 1 ,  61 . 2 . FIG. 4 depicts an example of an embodiment, in which the ring-shaped groove  62  is machined into the fixed object  5 . FIG. 5 concerns a combination of the FIGS. 3 and 4 to the extent that the ring-shaped groove  62  is located both in the rotating object  4  as well as in the fixed object  5 . 
     In the examples of embodiments of the FIGS. 2-5, the liquid gas was respectively pressed radially inwards, vertical or perpendicular to the axis of rotation a, into the ring-shaped groove  63 . This, in accordance with the invention, does not necessarily have to be the case. FIG. 6 shows (an otherwise analogous to FIG. 2) an embodiment of the device in accordance with the invention, in which the liquid gas is pressed into a ring-shaped groove  62  parallel to the axis of rotation a. The ring-shaped groove  62  in this example is machined into the second half mold  12 . The contact surface  45  in which the ring-shaped groove  62  is located in this example of an embodiment is vertical to the axis of rotation a. Other embodiments are conceivable, in which the liquid gas is even pressed into the ring-shaped groove  62  radially outwards or in another direction. It goes without saying, that combinations of the embodiments illustrated in the FIGS. 2-6 belong to the invention.