Patent Publication Number: US-2010122788-A1

Title: Temperature controlled mold

Description:
TECHNICAL FIELD 
     Embodiments of the subject matter disclosed herein generally relate to methods and molds and, more particularly, to mechanisms and techniques for controlling heat diffusion in a mold. 
     DISCUSSION OF THE BACKGROUND 
     Certain industrial machines have to be fabricated such that their cast parts are substantially free of imperfections to prevent the development of defects or friction between various moving parts. Defects might develop when the parts of the machine are formed in one or more molds and the cooling during the casting process is not appropriate. Various fractures or other structural defects may then develop in the cast part. 
     One such device that has at least a part formed with a mold is a compressor. The compressor is a machine capable of raising the pressure of a compressible fluid (e.g., a gas) through the use of mechanical energy. A compressor uses the mechanical energy, for example, to rotate two moving parts against each other. By rotating the moving parts relative to each other, the compressor takes in a certain amount of fluid in a given time and increases the pressure of the fluid, thus compressing the fluid. 
     Compressors may have different sizes. For a compressor having a body or moving parts as long as a few meters, the amount of heat stored by the material that is cast in the mold is large and the cooling process associated with such large parts is not simple. The parts need to be cooled in a controlled way such that structural formations inside the parts are not impaired while cracks and other defects should be prevented. 
     Among the various types of compressors used in industrial processing plants there are screw compressors, in which two screws or rotors with reverse pitch and with different diameters mesh with each other, so as to create a cavity that progressively moves from the intake area to the delivery area of the compressor, thus compressing the fluid. 
     For screw compressors, the compression ratio (the ratio of (i) a given volume of fluid that is input to the compressor to (ii) the compressed fluid at the delivery area) depends upon the length and profile of the screws, as well as upon the shape of the discharge hole at the delivery area. Such compressors are not equipped with valves and thus, mechanical forces that may create imbalances are not normally generated in this type of compressors. The shaft of this type of compressor can reach high speeds while combining high flow rates with a low size. 
     A cross section of such a compressor is illustrated in  FIG. 1 . The compressor is provided with a casing or shell  10 , inside which two screws or rotors  12  and  14  are housed and supported parallel to each other, with reverse pitch and different diameters. The screws  12  and  14  meshes with each other. The screws  12  and  14  move against one another to reduce the volume of the gas processed by the compressor and expel the compressed gas at suitable discharge ports (not shown). 
       FIG. 2  shows the casing  10  of the compressor exposing an inner cavity  16  in which the screws  12  and  14  are housed. The casing  10  may be formed in one or more parts. If the casing is formed in more than one part, those parts have to be assembled thereafter. 
     Screw compressors have the advantage of having simple mechanics, since the motion of the screws is continuous, and consequently undergo low mechanical stresses. Compared to other types of compressors, the screw compressors are capable of obtaining lower, but still high, compression ratios (3:1 to 4:1). In addition, the screw compressors may be disposed in series so to achieve a higher compression ratio. The mechanical efficiency of screw compressors is greater than that of alternative compressors, and therefore they are preferable for medium to large applications. 
     For the treatment of highly-corrosive process gases and, in general, in all applications where the compressed gas does not have to be contaminated, so-called oil-free screw compressors are normally used. In such compressors, special sealing systems allow the treated gas to be insulated from the lubrication system of the machine through various combinations of gaskets. Because there is no lubrication between the surfaces of the rotors, these surfaces should be synchronized to avoid undesired friction. 
     For the reason outlined above, the cast casings and screws of oil-free screw compressors should also have smooth surfaces with a minimum number of irregularities or defects of any type. In the same way, any hole or cavity of the casing that houses the gaskets and the support bearings should have minimum defects. 
     The casings of the compressors, including the oil-free screw compressors, once formed in the suitable sand molds, may develop cracks, microporosities and/or gas inclusions. To correct these defects, the compressors have to undergo repairs, which may require time-intensive and expensive welding operations that have to be inspected in order to assure that the affected area was repaired. Further machining steps may also be required to ensure proper repair. However, each additional machining step involves set up time, inspection and re-machine to the finished size. The additional machining steps are added to ensure that the cast casing is free of defects. Defects found in the pre-machined condition are weld repaired and locally heat treated to relieve stresses associated with welding. 
     The cast casings of oil-free screw compressors are manufactured by known sand casting processes. The sand casting process uses sand based molds for pouring the molten metal to form the desired parts. Depending on the desired machine, the mold may include the so-called forming cores, which are designed to reproduce the hollow parts of the machine (e.g., the casing of the compressor has a hollow portion or cavity in which the screws are placed). Each mold is obtained by packing sand into a suitable metal or wood container in which empty spaces are provided for the aforementioned hollow parts, so as to obtain the desired shape. The inner cores are also made from sand. The molds and the cores are assembled with the aid of glues, bolts to hold the cores from floating due to the buoyancy generated by the molten metal during the pouring step, or other fastening means. The cope part of the mold may be held by stacking weights on the cope so that the drag and cope do not separate during the pouring operation. After the castings cools in the sand molds, the sand molds are shaken and the castings are extracted from the molds. These sand molds may only be used one time. 
     A type of mold used for producing the casings of screw compressors may be made with a no-bake process, which uses a ready mixture including grains of sand precoated with thermosetting resins. In more details, the grains are of various sizes and may include chromite, zircon or silica sand. The silica sand has lower heat conductivity than chromite or zircon sands. However, these sands alone cannot make the molds to generate casings without defects after the casting process, since the cooling of the cast piece does not occur uniformly and cannot be controlled. 
     Therefore, cooling elements may be used in an attempt to somehow control the cooling process. One way to achieve this limited control of the cooling process is to use chills, which are metal plates that may be molded into the molds. The chills may be used, for example, molded inside the forming cores, and may be made in the form of steel or iron plates or nails, to promote uniform solidification of the part to be produced. However, since the chills act locally on the cores/molds, it is necessary to use a considerable amount of such chills to obtain improved results. Moreover, the presence of the chills tends to structurally weaken the forming cores. Thus, a mold made from sand and including chills has a reduced structural strength, which could lead to weak cores. 
     In addition, once inserted, the chills cannot be controlled or modified to adjust the cooling process as desired. In other words, no control may be exerted on the solidification process of the casting after the chills have been molded. The chills also have the disadvantages that they have to be held in place during the core making process, they may move during the pouring step, they may adhere to the cast part, thus generating further defects, etc. 
     Accordingly, it would be desirable to provide molds and methods that avoid the afore-described problems and drawbacks. 
     SUMMARY 
     According to one exemplary embodiment, there is a sand based mold for casting a device having a cavity. The sand based mold includes a core part including grains of sand bonded with a resin, where a shape of the core part fits a shape of the cavity of the device; and a first pipe embedded into the core part and configured to allow a compressed fluid to flow through the embedded pipe during a solidification process. The first pipe has an inlet and an outlet that exit the core. 
     According to another exemplary embodiment, there is a method for controlling a cooling process in a part of a device that includes a cavity, the device being cast into a sand based mold. The method includes embedding a first pipe into a core part of the sand based mold, where a shape of the core part fits a shape of the cavity of the device; attaching the core part to an external part, where the external part is part of the sand based mold; pouring liquid material into the mold such that the cavity is formed by the core part; and controlling the cooling process by circulating a coolant fluid through the embedded first pipe for a predetermined period of time. 
     According to still another exemplary embodiment, there is a sand based mold for casting a casing of a compressor having a cavity. The sand based mold includes a core part including grains of sand bonded with a resin, where a shape of the core part fits a shape of the cavity of the compressor to be formed; a first pipe embedded into the core part and configured to hold a fluid, the first pipe having an inlet and an outlet that exit the mold; an external part including the grains of sand bonded with the resin and configured around the core part such that molten material to be poured into the mold cannot escape; and a second pipe embedded into the external part and configured to hold the fluid, where the first and second pipes are completely embedded into the core part and the external part, respectively. 
     According to another exemplary embodiment, there is a method for forming a sand based mold. The method includes shaping an external surface of a box to match an external surface of the mold; suspending a first pipe above the external surface of the box; and pouring a mixture of sand and at least a binding resin into the box such that the first pipe is completely embedded into the mold except an inlet and an outlet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings: 
         FIG. 1  is a schematic view, in longitudinal section, of a screw compressor; 
         FIG. 2  is an schematic view of a casing of the screw compressor of  FIG. 1 ; 
         FIG. 3  is a schematic view, in cross section, of a mold used for casting the screw compressor; 
         FIG. 4  is a schematic view of a core part of the mold according to an exemplary embodiment; 
         FIG. 5  is a schematic view of the core part having two parts according to an exemplary embodiment; 
         FIG. 6  is a schematic view of a mold for forming a core part of the mold according to an exemplary embodiment; 
         FIG. 7  is a schematic view of a core box in which the core part is formed according to an exemplary embodiment; 
         FIG. 8  is a schematic view of an external part of the mold according to an exemplary embodiment; 
         FIG. 9  is a flow chart illustrating steps for casting a part of a device according to an exemplary embodiment; 
         FIG. 10  is a flow chart illustrating steps for casting a device with a cavity according to an exemplary embodiment; and 
         FIG. 11  is a flow chart illustrating steps for forming a sand based mold according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the subject matter disclosed. Instead, the scope of the exemplary embodiments is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a screw compressor. However, the embodiments to be discussed next are not limited to this type of compressor but may be applied to other compressors or other parts of machines that are made with a mold. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the present subject matter. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily all referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to an exemplary embodiment illustrated in  FIG. 3 , a mold  18  for forming the casing  10  shown in  FIGS. 1 and 2  may have two parts, i.e., an external part  20  and a core part  24 . Each of the external part  20  and the core part  24  may include one or more parts.  FIG. 3  shows an exemplary arrangement in which the external part  20  includes two parts  20   a  and  20   b  and the core part  24  includes a single part. After the parts of the machine are cast in the mold  18 , the parts corresponding to the external parts  20   a  and  20   b  of the mold have to be fasten to each other through known means, e.g., welding, screws, glue, etc. 
     The external part  20  may define the exterior shape of the machine to be cast while the core part  24  may define the cavity of the machine. For example, in  FIG. 2 , the core part  24  forms the cavity  16  while the external part  20  forms the casing  10 . As will be appreciated by those skilled in the art, the shapes, sizes and profiles of the external part  20  and the core part  24  may vary from machine to machine depending on the desired machine to be cast. According to an exemplary embodiment, the mold  18  may include only the external part  20 , only the core part  24 , or a combination thereof. 
     According to an exemplary embodiment illustrated in  FIG. 4 , the core part  24 , which is part of the mold  18 , may have a cylindrical shape. This shape is exemplary and not intended to limit the scope of the exemplary embodiments. The core part  24  may be used when the machine intended to be made has inner cavities, like for example the cavity  16  in  FIG. 2 , or when the machine has profiles that cannot be made with external molds. The core part  24  may have a shape that reproduces the cavity  16  of the compressor  10 . One or more core parts  24  may be used for carrying out the casting.  FIG. 5  shows such an example in which the cavity  16  of the compressor  10  is formed with a core part  24  having two members, i.e.,  24   a  and  24   b.    
     There are various production processes for obtaining the core part  24 . Such processes, as already discussed, are using sand casting methods. One of these processes, provided merely as an example, is a no-bake process. The no-bake process uses sand, binders and a catalyst mixed in a sand mixer. This mixture may be compacted/blown in a core box/pattern. The catalyst hardens the sand mold and the core/mold can be removed from the core box or the patterns. In more detail, the sand grains may be of various sizes and may include various types of sand such as silica, chromite, zircon, olivine, phosphatite, alumina, etc. The organic binder may, for example, be a dry phenolic resin (2 to 4% by weight), a thermosetting resin obtained by reacting phenol and formaldehyde, etc. Hexamethylenetetramine may be used as a catalyst and zinc or calcium stearate may be used as lubricants. 
     According to this exemplary embodiment, the mixture  26  may be inserted inside a suitable core box  28 , as shown in  FIG. 6 . The mixture  26  is then shaped to obtain the desired shape of the core part  24 . The core box  28  may be heated to a temperature around 250° C. This temperature may be achieved with an electrical resistance  30  or else through combustible gas burners  32  disposed around the core box  28 . After the mixture  26  is shaped inside the core box  28  to the desired profile, the high temperature melts the precoating on the grains of sand (the sand is formed of grains), causing the grains to stick to each other and thus hardening of the core part  24 . Once hardened, the core part  24  is extracted from the core box  28  through appropriate extractors (not shown). Prior to increasing the high temperature inside the core box  28 , chills  34  may be inserted into the mixture  26 , as shown in  FIG. 6 . Chills  34  may be placed in the core box  28  such that one surface of the chills is exposed (not shown) to the molten material to be poured to control the solidification of the casting. Because the bulk of the chill is embedded in the core, the structure of the core is weakened. 
     Instead of providing the chills  34  inside the core part  24 , according to the exemplary embodiment shown in  FIG. 4 , a first pipe  36  may be provided embedded in the core part  24 . The first pipe  36  may be fully embedded in the core part  24 . The first pipe  36  may be a one piece of seamless pipe with inlet and outlet ports  38  and  40  and configured to be connected to, for example, a pump (not shown) that is configured to circulate a compressed fluid through the first pipe  36 . In this exemplary embodiment, no chills are embedded into the core part  24 . However, according to another exemplary embodiment, both the chills and the pipe may be embedded into the core part  24 . 
     The first pipe  36  may be a steel duct manufactured without welding. Other materials may also be used to produce the first pipe. According to an exemplary embodiment, the first pipe  36  may be positioned inside the core part  24  at a distance from the surface of the core part  24 . A distance from the first pipe to the surface of the core part may be between about 2.5 cm to about 5 cm. The inlet and outlet  38  and  40  may be made to pass through a joining line between two external half-shells that may make up the mold  18 .  FIG. 5  shows the inlet and outlet  38  and  40  exiting at the top of the core parts  24   a  and  24   b.    
     According to an exemplary embodiment, the first pipe  36  may be distributed across a predetermined surface, inside the core part  24 , that follows an external surface of the core part. In other words, if the shape of the core part  24  is cylindrical as shown in the exemplary embodiment illustrated in  FIG. 4 , the first pipe  36  may be distributed across a cylindrical surface that mirrors the surface of the core part  24 . A thickness of the first pipe  36  may be in the range from about 0.2 cm to about 0.3 cm. 
     According to another exemplary embodiment, a second pipe  42  may be embedded into the core part  24 . The second pipe  42 , as shown in  FIG. 4 , may be identical or different from the first pipe  36 . One or more of the pipes embedded into the core part  24  may be distributed uniformly such that the cooling process is uniform. A coolant liquid or gas that flows through the first and/or second pipe may be air, compressed air, liquefied gas, or any appropriate fluid as will be recognized by those skilled in the art. 
     The core box  28  for forming the core part  24  is illustrated in  FIG. 7 , in which the first pipe  36  is exposed as the mixture  26  has not been added. The inlet and outlet  38  and  40  of the first pipe  36  are shown connected to a pump  50 . The shape of a surface  52  of the core box  28  indicates the corresponding places of the screws  12  and  14  of the compressor  10  shown in  FIG. 1 . 
     According to another exemplary embodiment, the mold  18  may include, in addition to the core part  24 , the external part  20 , which may be formed in one or more rigid outer containers  44  as shown in  FIG. 8 . The outer containers  44  may be manufactured in metal or wood. Two half-shells  20  may be provided and the two half-shells may be coupled together to make the outside part of the mold  18 .  FIG. 8  shows only one of the two half-shells  20 . Inside the container  44  or the half-shells that form the mold  18 , a shaped surface  46  is made, which defines the outline of the component to be manufactured through casting, for example, the casing  10  of the compressor. The shaped surface  46  of the mold  18  may be made up of a sand of suitable material, mixed with binders and inhibitors (similar or different than the core part  24 ) that allow the sand to harden and that prevent undesired reactions from occurring when such sand comes into contact with the molten metal that will constitute the component  10 . 
     According to an exemplary embodiment, at least one cooling pipe  48  is provided, as shown in  FIG. 8 , inside each external part  20  used in the mold  18 . A coolant fluid may circulate inside the cooling pipe  48 , similar to the first pipe  36  of the core part  24 . The structure, composition and distribution of the cooling pipe  48  of the external mold  20  may be similar to those of the first pipe  36 . 
     By distributing the first pipe  36  and/or the cooling pipe  48  uniformly inside the mold  18 , according to an exemplary embodiment, it is possible to obtain the forced but controlled cooling of the sand core  24  and the external mold  20  so as to cause a controlled solidification direction of the component  10  during the casting process. Directional solidification is a process in which the castings&#39; geometry does allow the solidification (of the molten material) to move towards the insulated risers that feed the shrink that occurs as the casting experiences a phase transformation. The embedded cooling pipe promotes directional solidification by accelerating the freezing process, thereby creating a surface skin that is free of defects. Without the cooling pipe, the surface skin free of defects might not be obtained. This controlled cooling process produces less cracks and defects in the cast parts and also less imperfections. According to an exemplary embodiment, at least one pipe is provided in the mold  18 , either in the core part  24  or the external part  20 . Various coolant temperatures and speeds may be selected for cooling the mold as desired during a given time interval. 
     A method for forming at least a part of a machine by using the mold  18  discussed above is now described with reference to  FIG. 9 . The mold  18  is formed in step  900  and may include at least one of the core part  24  and the external part  20 . The first pipe and/or the cooling pipe are disposed away from the surface of the mold  18 . In step  902 , the mold system  18  is assembled such that molten metal may be poured into the mold. If the mold system  18  includes two external parts and one core part, the core part is secured to at least one of the external parts and the external parts are connected to each other such that the molten metal cannot escape the mold system  18 . The inlet and outlet of the first pipe and the cooling pipe may be, in an optional step  904 , connected to a corresponding pump for circulating a corresponding cooling fluid. Different cooling fluids may be circulated in the first pipe and the cooling pipe by different pumps. However, according to an exemplary embodiment, the first pipe and the cooling pipe may be connected to each other such that a single cooling fluid is circulated by a single pump. With the mold system in place, the molten metal (or other fluid material) may be poured in step  906  to cast the desired machine part. The cooling of the poured, molten metal is controlled in step  908  by controlling various parameters of the cooling fluid, e.g., flow rate, pressure, temperature, etc. 
     According to an exemplary embodiment illustrated in  FIG. 10 , there is a method for controlling a cooling process in a part of a device that includes a cavity, the device being cast into a sand based mold, a shape of the sand based mold fitting a shape of the cavity of the device. The method includes a step  1000  of embedding a first pipe into a core part of the sand based mold, a step  1002  of attaching the core part to an external part, where the external part is part of the sand based mold, a step  1004  of pouring liquid material into the mold such that the cavity is formed by the core part, and a step  1006  of controlling the cooling process by circulating a coolant fluid through the first pipe during a solidification process of the device. 
     A method for forming a mold having the above discussed properties is now described with reference to  FIG. 11 . The method for forming the sand based mold includes a step  1100  of shaping an external surface of a box to match an external surface of the mold, a step  1102  of suspending a first pipe above the external surface of the box, and a step  1104  of pouring a mixture of sand and at least a binding resin into the box such that the first pipe is completely embedded into the mixture except an inlet and an outlet. 
     Such methods advantageously allow the cast components to have a surface with a low degree of roughness and also with a reduced amount of gas bubbles. This is true because it is known that the residual gases that accumulate inside the molten steel have a decreasing solubility as the temperature decreases. Thus, gases inside the molten steel would move towards the inside of the cast piece, e.g., far from the surface skin. 
     Therefore, the use of the first pipe  36  and/or the cooling pipe  48  may allow a controlled cooling of the core part  24  and/or the external part  20 , which cannot be achieved with metallic chills, consequently eliminating most of the problems associated with the presence of chills in the mold. 
     The controlled cooling of the mold also promotes the directional solidification of the molten metal, so as to obtain a final product that is as uniform as possible in terms of its mechanical and structural characteristics. 
     The disclosed exemplary embodiments provide a mold and method for casting parts of a machine in which a cooling process of the mold is controlled. It should be understood that this description is not intended to limit the disclosed subject matter. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other example are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements within the literal languages of the claims.