Abstract:
A method of forming a hollow sand core involves placing a preform into a cavity defined in a mold, where the preform has a predetermined configuration. A granular material is then introduced into the mold cavity and around the preform. The introduced granular material is established around the preform to form the hollow sand core. The preform is deformed in a manner sufficient to enable removal of the preform from inside the hollow sand core, and then is removed from the sand core. The removal of the preform exposes a hollow portion of the sand core.

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to methods of forming sand cores and, more particularly, to a method of forming a hollow sand core. 
     BACKGROUND 
     Sand cores are often used to manufacture parts via casting processes. The sand core serves as a mold of the desired part shape. Sand cores may be made, for example, via cold box or no bake technologies. Such processes utilize organic and/or inorganic binders which adhere to the sand, thereby strengthening the resulting core. During both the cold box and no bake processes, a catalyst is used to harden the binders. 
     SUMMARY 
     A method of forming a hollow sand core involves placing a preform into a cavity defined in a mold, where the preform has a predetermined configuration. A granular material is then introduced into the mold cavity and around the preform. The introduced granular material is established around the preform to form the hollow sand core. The preform is deformed in a manner sufficient to enable removal of the preform from inside the hollow sand core, and then is removed from the sand core. The removal of the preform exposes a hollow portion of the sand core. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features and advantages the present disclosure will become apparent by reference to the following detailed description and drawings, in which like reference numerals correspond to similar, though perhaps not identical, components. For the sake of brevity, reference numerals or features having a previously described function may or may not be described in connection with other drawings in which they appear. 
         FIG. 1A  is a semi-schematic top view of an embodiment of a preform prior to deformation; 
         FIG. 1B  is a semi-schematic top view of an embodiment of a sand core having the non-deformed preform therein; 
         FIG. 1C  is a semi-schematic top view of an embodiment of a preform both after partial deformation and after full deformation, and also a cross-sectional view of the sand core of  FIG. 1B  taken along the  1 C- 1 C line; 
         FIGS. 2A and 2B  illustrate semi-schematic perspective views of a core box having the preform therein both before ( FIG. 2A ) and after ( FIG. 2B ) introduction of granular material and binder; 
         FIG. 3  is a schematic and partially cross-sectional view of a core box having the preform therein; and 
         FIGS. 4A and 4B  illustrate semi-schematic top views of another embodiment of a preform in its permanent shape ( FIG. 4A ) and its expanded temporary shape ( FIG. 4B ). 
     
    
    
     DETAILED DESCRIPTION 
     Examples of the method disclosed herein utilize a removable preform to form and shape the interior surface of a hollow sand core. This deformable preform advantageously enables the sand core to remain intact after formation and during preform removal. Furthermore, the hollow sand core formed using the preform may be desirable, as the amount of sand needed to form the core is reduced. It is further believed that the hollow portion of the sand core also enables gases generated during the casting process to be readily removed. The process disclosed herein is particularly advantageous in that typical processes, such as cold box and no bake technologies may be used to form the hollow sand core. 
     Referring now to  FIGS. 1A through 1C , depicted are embodiments of a preform  10  prior to sand core  12  formation ( FIG. 1A ), the preform  10  after sand core  12  formation and prior to removal ( FIG. 1B ), and both the fully deformed preform  10 ′ and the partially deformed preform  10 ″ after removal from the sand core  12  ( FIG. 1C ). It is to be understood that two preforms  10  are generally not used in formation of the sand core  12 , but rather  FIG. 1C  is merely illustrating the types of deformation of the preform  10 . 
     The preform  10 ,  10 ′ is generally formed of a material that is capable of deforming from its temporary shape T (such as that shown in  FIG. 1A ) to a permanent shape P (e.g., the shape shown in  FIG. 1C ) that is generally smaller than the temporary shape T. By “generally smaller”, it is meant that the preform  10 ′ (shown in  FIG. 1C ) is removable from the sand core  12  via the hollow portion  14  at least one of the two ends E 1 , E 2 . As such, in the embodiments disclosed herein, the temporary shape T is the desirable shape of the inner core, and the shrunken, deformed shape is the permanent shape P. In one embodiment, the permanent shape P has the same overall shape as the temporary shape T, but has a smaller diameter than the temporary shape T. In another embodiment, the permanent shape P is an entirely different shape than the temporary shape T, and has a smaller diameter D than the temporary shape T. 
     It is to be understood that in some instances, the permanent shape P of the preform  10 ′ is not completely obtained. This may be due to the fact that the entire preform  10  is not heated above the switching or glass transition temperature, or the non-deformed portion is placed onto a mandrel for introducing pressure inside the preform  10 . A non-limiting example of this embodiment is shown as reference numeral  10 ″ in  FIG. 1C . It is to be understood that the permanent shape P is not completely obtained, and thus the diameter D is not consistent along the entire length L of the partially deformed preform  10 ″. Partial deformation may be suitable as long as at least a portion of the diameter D is small enough along a portion of the length L such that the preform  10 ″ is removable from the sand core  12 . For example, the partially deformed preform  10 ″ shown in  FIG. 1C  has multiple diameters d 1 , d 2 , d 3  While diameter d 3  is not smaller than that corresponding portion of the temporary shape T, the diameters d 2 , d 3  enable the preform  10 ″ to be removed from the sand core  12  by being pulled through the hollow end portion  14  at end E 2 . 
     While expansion and contraction of the preform  10  is shown in two directions (e.g., the diameter expands/contracts), it is to be understood that expansion/contraction may cause the preform  10  to change shape in three dimensions, similar to a balloon. 
     Non-limiting examples of suitable materials for the preform  10  include shape memory polymers (e.g., thermoplastics such as polyolefins, polyurethanes, polyacrylates, or thermosets, such as polyolefins that have been covalently cross-linked), or elastomeric materials (e.g., natural rubber, synthetic polyisoprene, butyl rubber, halogenated butyl rubbers (e.g., chloro butyl rubber, bromo butyl rubber, etc.), polybutadiene, styrene-butadiene rubber, nitrile rubber, hydrogenated nitrile rubber, chloroprene rubber, ethylene propylene rubber, epichlorohydrin rubber, polyacrylic rubber, silicone rubber, fluorosilicone rubber, fluoroelastomers, perfluoroelastomers, polyether block amides, chlorosulfonated polyethylene, ethylene-vinyl acetate, or thermoplastic elastomers). Some elastomeric materials are also shape memory materials. 
     Prior to being used to form the sand core  12 , the preform  10  is shaped. The shaping process used will depend, at least in part, upon the material used. Very generally, the shaping technique is selected from blow molding, injection molding, compression molding, rotational molding, extrusion, stretching, or any combination of heating and force. 
     In one embodiment, the materials may be initially in the permanent shape P (e.g., via extrusion). The material may then be crosslinked using irradiation or a combination of heat and chemical means (depending upon the polymer used), blow molded above the glass transition temperature of the polymer, and then cooled to below the glass transition temperature to achieve the desirable temporary shape T. 
     In another embodiment, the materials may be initially in an expanded form that is even larger than the desirable temporary shape T. The material may be shrunk, via heating, to reduce the size of the material to a desirable temporary shape T. 
     When a shape memory polymer is used, the permanent shape P (i.e., the shrunken shape) may be set by bringing the material to a temperature that is at or above its melting temperature, forming it into the desirable shape P, and then cooling it below the glass transition temperature to set the shape P. If a thermoplastic shape memory polymer (with physical crosslinks) is used, then the permanent shape P may be reshaped by bringing the material again to a temperature that is at or above the melting temperature, reforming the shape, and cooling below the glass transition temperature. However, if the material used is a thermoset shape memory polymer (with covalent crosslinks), the permanent shape P may not be reprogrammed. Rather, this embodiment of the shape memory polymer preform  10 ,  10 ′,  10 ″ may be reused with the set permanent shape P. 
     In either case, to make the temporary shape T, the shape memory polymer is deformed above the glass transition temperature, molded into the desirable shape T, and cooled below the glass transition temperature. Heating the shape memory polymer above its glass transition/switching temperature causes the polymer to become pliable. Once pliable, a force (e.g., pressure, stretching, mechanical force, etc.) may, in some instances, be applied to expand the shape memory polymer into the desirable temporary shape T. An exterior mold may be used to achieve the desirable temporary shape T when the shape memory polymer is heated and becomes deformable. As mentioned above, once in the desirable shape, the polymer is cooled to set the temporary shape T. 
     Once the temporary shape T is set, if the shape memory polymer is again heated to above the glass transition temperature, it will revert back to the permanent shape P. As such, once the sand core  12  is formed (discussed further hereinbelow), the shape memory polymer is heated above its glass transition temperature again to recover the permanent deformed shape P. When the shape memory polymer is heated to a temperature above its glass transition temperature, the presence of physical or covalent crosslinks allows for the reversion of the shape memory polymer from one shape (e.g., the temporary shape T) to another shape (e.g., the permanent shape P) by releasing energy i) previously imparted to the system by the deformation of the polymer, and ii) stored in the system by subsequent cooling processes. 
     Referring now to  FIG. 2A , when the desirable temporary shape T of the preform  10  is achieved, the preform  10  is positioned within a cavity  16  of a mold  18  (e.g., a core box). The preform  10  may be anchored within the cavity  16  on its own, or via mechanical means or via the application of pressure. If the preform  10  has sufficient rigidity to stand on its own in the cavity  16 , no pressure would be required. The mold  18  may include one or more locating tabs  22  (shown in phantom) which protrude into the cavity  16  from a bottom surface of the mold  18 . The locating tab(s)  22  are configured to support the preform  10 . It is to be understood that both ends of the core box  18  may include locating tabs  22  to secure the preform  10  in the cavity  16 . In such instances, the cavity  16  would be enclosed and the core box  18  would be opened/closed lengthwise (in the embodiment of  FIGS. 2A and 2B , vertically) along a parting line. In instances in which the core box  18  has a vertical parting line, the locating tabs(s)  22  would be pulled out of, or otherwise removed from, the core box  18  before sand core  12  ejection/removal. 
     In other embodiments, a low amount of pressure (e.g., 1-5 psi) may be used to maintain the rigidity of the preform  10  during the core  12  generation process. In some embodiments, the preform  10  may be pressurized and sealed prior to the core  12  generation process. In other embodiments, the preform  10  may be pressurized while in the cavity  16 . One end of the preform  10  may be configured to receive such pressure (e.g., via a port formed in the core box  18 ), and the pressure may be constantly supplied such it is maintained throughout core  12  formation or the preform  10  may be sealed once pressurized. In some cases when pressure is constantly supplied or the preform  10  is sealed to maintain rigidity, the core forming process may be repeated using the same preform  10  multiple times without its removal from the cavity  16 . This may be accomplished because either the releasing of pressure and/or heating shrinks the preform  10  to its partially or fully deformed shape  10 ′,  10 ″ within the cavity  16 , and the sand core  12  may be removed therefrom. 
     In still other embodiments (see  FIG. 3 ), the mold  18  may have one or more holes  24  formed therein which receives the preform  10 . The holes  24  are formed through a portion of the thickness T of the core box  18  walls such that each hole  24  respectively receives an opposed end of the preform  10 . In such instances, the preform  10  is supported by the thickness T of the core box  18  at opposed ends. A plug or locating tab  22  (not shown in  FIG. 3 ) may be inserted into the preform  10 , thereby squeezing the preform  10  against the portion of the mold  18  which defines the hole  24  and providing rigidity to the preform  10 . Such a plug or locating tab  22  would have a diameter just less than the diameter of the corresponding hole  24 . In one embodiment, the plug or locating tab  22  may also have an aperture defined therein, which enables pressure to be applied to the preform  10  during core formation (e.g., if a suitable pressure port (not shown) is formed in the core box  18 ). In such instances, it may also be desirable to seal the other end of the preform  10  via another plug or locating tab  22  that does not include an aperture therein. 
       FIG. 3  also illustrates one blow tube  26  for the introduction of the sand  20  into the cavity  16 , and vents for the release of air and/or other gas from the cavity  16 .  FIG. 3  also illustrates a horizontal parting line  30  for opening/closing the core box  18 . 
     Referring back to  FIG. 2B , a granular material  20  is introduced, under pressure or via gravity, into the mold cavity  16  and around the preform  10 . In one embodiment, the granular material  20  is sand mixed with resin. This process is generally referred to as a cold box process. In this cold box process, the granular material  20  and resin is blown into the cavity  16  such that any space between the cavity  16  wall(s) and the exterior of the preform  10  is filled. A gaseous catalyst (e.g., triethylamine (also known as TEA gas) is used to initiate bonding of the sand and resin. In this embodiment, the catalyst is passed through the mold  18  such that it initiates curing of the resin and hardening of the materials to form the sand core  12 . In another embodiment, the granular material  20  is sand mixed with resin and the catalyst. This process is generally referred to as a no bake process. In this no bake process, the sand/resin/catalyst mixture is rained into the cavity  16  such that any space between the cavity  16  wall(s) and the exterior of the preform  10  is filled. Ultimately, the catalyst initiates the bonding of the sand to the resin. In this embodiment, curing is accomplished within a specific time period. The resin ultimately cures and the bonded mixture hardens, thereby forming the sand core  12 . 
     It is to be further understood that when pressure is utilized to support the preform  10  during core  12  formation, the pressure is released prior to any casting processes. 
     The formed sand core  12  still has the preform  10  therein, as shown in  FIG. 1B . The sand core  12  may be used in subsequent casting processes to form parts. In some instances, it may be desirable to remove the preform  10  prior to the casting process, and in other instances, it may be desirable to remove the preform  10  after the casting process is complete. Generally, removing the preform  10  prior to casting is desirable. If the shape of the cast part and the preform  10  render the preform  10  readily removable after the part is formed, then preform  10  removal may be accomplished after part formation. When removed after casting in complete, such removal is often accomplished during the shake-out process. 
     Regardless of when preform  10  removal is desirable, such removal may be accomplished by deforming the preform  10  to its permanent shape P (i.e., deformed preform  10 ′, shown in  FIG. 1C ) or its partially deformed shape  10 ″ (also shown in  FIG. 1C ). Deformation may be accomplished by a variety of different methods. The method selected may depend, at least in part, upon the material used. In some instances, the casting process could heat the preform  10  sufficiently that it shrinks during such process. It is to be understood, however, that if the preform  10  removal is accomplished after casting, it may be removed without any shrinking, since the core  12  would be broken during the shakeout process. 
     In one embodiment, depressurization may be used to obtain the deformed (i.e., permanent shape P) preform  10 ′ or partially deformed preform  10 ″. This is generally used when pressure is used to maintain the temporary shape T during sand core  12  formation. The removal of pressure will cause the temporary shape T of the preform  10  to shrink to the permanent shape P. Once in the shrunken permanent shape P (or at least partially shrunken shape), the preform  10 ′ (or preform  10 ″) may be readily removed from one of the two ends E 1 , E 2  through the hollow portion  14 . This form of deformation is particularly suitable for the preform  10  formed of elastomeric materials. 
     In another embodiment, the preform  10  may be heated in order to initiate deformation. This technique may be used when a shape memory polymer preform  10  is utilized. Heating may be accomplished by introducing a fluid (e.g., gas (e.g., air, nitrogen, or any other gas that does not react with the sand core  12 ), liquid, etc.) having a temperature sufficient to deform or otherwise at least partially switch the state of the preform  10  into the smaller shaped preform  10 ′ or preform  10 ″. The fluid may be heated prior to being introduced or after being introduced into the preform. 
     It is to be understood that removal of the preform  10 ,  10 ′,  10 ″ will not deleteriously affect the shape of the sand core  12 , at least in part because the core  12  has been cured and hardened prior to preform  10 ,  10 ′,  10 ″ removal. 
     Referring now to  FIG. 1C , a cross-section of the sand core  12  taken along the  1 C- 1 C line of  FIG. 1B  is depicted. The removed shrunken preform  10 ′ and the partially shrunken preform  10 ″ are also depicted. As shown, the interior of the sand core  12  includes the hollow portion  14  which has conformed to the temporary shape T of the preform  10 . Since the preform  10  is shrunken to preform  10 ′ or preform  10 ″ prior to its removal, the sand core  12 , and thus the hollow portion  14 , remain set in the desirable shape. 
     In another embodiment, the permanent shape P of the preform  10 ′ is a smaller version of the desirable part shape, and the temporary shape T is an expanded version of the permanent shape P and is the desirable part shape. This is shown in  FIGS. 4A and 4B . The application of temperature enables the preform  10 ′ to become pliable, and the application of pressure causes the pliable preform to expand to the desired temporary shape T,  10 . In this embodiment, the temperature is above the glass transition temperature of the material used for the preform  10 , and the pressure is sufficient to expand the preform  10 ′ to the desired temporary shape T. Heated gas may be used to raise the temperature and apply the pressure. Generally, the preform  10 ′ expands proportionally to the pressure applied and the initial shape P. 
     This embodiment may be particularly suitable when the permanent shape P has different section thicknesses along the length (not shown). When pressure is applied above the glass transition temperature of the preform  10 ′, the final temporary shape T will depend on, at least in part, the initial permanent shape P, the local material thickness, and the pressure applied. 
     The transition of the preform  10 ′ to its temporary shape T may also be achieved by localized crosslinking. For example, in a material where the covalent cross linking is achieved by irradiation, the irradiation may be locally applied rather than to the entire preform  10 ′. For another example, where the cross linking is initiated by heat, heat may be selectively applied to local areas. Once cross linked, applying pressure above the glass transition temperature will result in different rates of expansion between the cross linked locations and the under cross linked locations. 
     It is believed that the embodiment shown in  FIGS. 4A and 4B  may be suitable for an automated process in which the preform  10  may be reused. 
     After the pressure is applied to achieve the desired temporary shape T, the pressure may be maintained, but the temperature changed such that it is decreased to below the glass transition temperature. This causes the temporary shape T to set so that the preform  10  becomes rigid in the core box cavity  16 . The pressure may then be maintained or removed since the temporary shape  10 , T is set to the desired core  12  inner shape. 
     In the embodiment shown in  FIGS. 4A and 4B , the application of pressure may be accomplished by flowing a gas from one end of the preform  10 ,  10 ′ to the other. If the preform  10 ,  10 ′ were sealed at one end, two tubes may be used, one to introduce the gas therein and the other to remove the gas therefrom. In the latter embodiment, the difference in flow enables the pressure in the preform  10 ,  10 ′ to be regulated. 
     While several embodiments have been described in detail, it will be apparent to those skilled in the art that the disclosed embodiments may be modified. Therefore, the foregoing description is to be considered exemplary rather than limiting.