Method of forming a hollow sand core

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.

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.

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 toFIGS. 1A through 1C, depicted are embodiments of a preform10prior to sand core12formation (FIG. 1A), the preform10after sand core12formation and prior to removal (FIG. 1B), and both the fully deformed preform10′ and the partially deformed preform10″ after removal from the sand core12(FIG. 1C). It is to be understood that two preforms10are generally not used in formation of the sand core12, but ratherFIG. 1Cis merely illustrating the types of deformation of the preform10.

The preform10,10′ is generally formed of a material that is capable of deforming from its temporary shape T (such as that shown inFIG. 1A) to a permanent shape P (e.g., the shape shown inFIG. 1C) that is generally smaller than the temporary shape T. By “generally smaller”, it is meant that the preform10′ (shown inFIG. 1C) is removable from the sand core12via the hollow portion14at least one of the two ends E1, E2. 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 preform10′ is not completely obtained. This may be due to the fact that the entire preform10is not heated above the switching or glass transition temperature, or the non-deformed portion is placed onto a mandrel for introducing pressure inside the preform10. A non-limiting example of this embodiment is shown as reference numeral10″ inFIG. 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 preform10″. 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 preform10″ is removable from the sand core12. For example, the partially deformed preform10″ shown inFIG. 1Chas multiple diameters d1, d2, d3While diameter d3is not smaller than that corresponding portion of the temporary shape T, the diameters d2, d3enable the preform10″ to be removed from the sand core12by being pulled through the hollow end portion14at end E2.

While expansion and contraction of the preform10is shown in two directions (e.g., the diameter expands/contracts), it is to be understood that expansion/contraction may cause the preform10to change shape in three dimensions, similar to a balloon.

Prior to being used to form the sand core12, the preform10is 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 preform10,10′,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 core12is 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 toFIG. 2A, when the desirable temporary shape T of the preform10is achieved, the preform10is positioned within a cavity16of a mold18(e.g., a core box). The preform10may be anchored within the cavity16on its own, or via mechanical means or via the application of pressure. If the preform10has sufficient rigidity to stand on its own in the cavity16, no pressure would be required. The mold18may include one or more locating tabs22(shown in phantom) which protrude into the cavity16from a bottom surface of the mold18. The locating tab(s)22are configured to support the preform10. It is to be understood that both ends of the core box18may include locating tabs22to secure the preform10in the cavity16. In such instances, the cavity16would be enclosed and the core box18would be opened/closed lengthwise (in the embodiment ofFIGS. 2A and 2B, vertically) along a parting line. In instances in which the core box18has a vertical parting line, the locating tabs(s)22would be pulled out of, or otherwise removed from, the core box18before sand core12ejection/removal.

In other embodiments, a low amount of pressure (e.g., 1-5 psi) may be used to maintain the rigidity of the preform10during the core12generation process. In some embodiments, the preform10may be pressurized and sealed prior to the core12generation process. In other embodiments, the preform10may be pressurized while in the cavity16. One end of the preform10may be configured to receive such pressure (e.g., via a port formed in the core box18), and the pressure may be constantly supplied such it is maintained throughout core12formation or the preform10may be sealed once pressurized. In some cases when pressure is constantly supplied or the preform10is sealed to maintain rigidity, the core forming process may be repeated using the same preform10multiple times without its removal from the cavity16. This may be accomplished because either the releasing of pressure and/or heating shrinks the preform10to its partially or fully deformed shape10′,10″ within the cavity16, and the sand core12may be removed therefrom.

In still other embodiments (seeFIG. 3), the mold18may have one or more holes24formed therein which receives the preform10. The holes24are formed through a portion of the thickness T of the core box18walls such that each hole24respectively receives an opposed end of the preform10. In such instances, the preform10is supported by the thickness T of the core box18at opposed ends. A plug or locating tab22(not shown inFIG. 3) may be inserted into the preform10, thereby squeezing the preform10against the portion of the mold18which defines the hole24and providing rigidity to the preform10. Such a plug or locating tab22would have a diameter just less than the diameter of the corresponding hole24. In one embodiment, the plug or locating tab22may also have an aperture defined therein, which enables pressure to be applied to the preform10during core formation (e.g., if a suitable pressure port (not shown) is formed in the core box18). In such instances, it may also be desirable to seal the other end of the preform10via another plug or locating tab22that does not include an aperture therein.

FIG. 3also illustrates one blow tube26for the introduction of the sand20into the cavity16, and vents for the release of air and/or other gas from the cavity16.FIG. 3also illustrates a horizontal parting line30for opening/closing the core box18.

Referring back toFIG. 2B, a granular material20is introduced, under pressure or via gravity, into the mold cavity16and around the preform10. In one embodiment, the granular material20is sand mixed with resin. This process is generally referred to as a cold box process. In this cold box process, the granular material20and resin is blown into the cavity16such that any space between the cavity16wall(s) and the exterior of the preform10is 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 mold18such that it initiates curing of the resin and hardening of the materials to form the sand core12. In another embodiment, the granular material20is 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 cavity16such that any space between the cavity16wall(s) and the exterior of the preform10is 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 core12.

It is to be further understood that when pressure is utilized to support the preform10during core12formation, the pressure is released prior to any casting processes.

The formed sand core12still has the preform10therein, as shown inFIG. 1B. The sand core12may be used in subsequent casting processes to form parts. In some instances, it may be desirable to remove the preform10prior to the casting process, and in other instances, it may be desirable to remove the preform10after the casting process is complete. Generally, removing the preform10prior to casting is desirable. If the shape of the cast part and the preform10render the preform10readily removable after the part is formed, then preform10removal 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 preform10removal is desirable, such removal may be accomplished by deforming the preform10to its permanent shape P (i.e., deformed preform10′, shown inFIG. 1C) or its partially deformed shape10″ (also shown inFIG. 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 preform10sufficiently that it shrinks during such process. It is to be understood, however, that if the preform10removal is accomplished after casting, it may be removed without any shrinking, since the core12would be broken during the shakeout process.

In one embodiment, depressurization may be used to obtain the deformed (i.e., permanent shape P) preform10′ or partially deformed preform10″. This is generally used when pressure is used to maintain the temporary shape T during sand core12formation. The removal of pressure will cause the temporary shape T of the preform10to shrink to the permanent shape P. Once in the shrunken permanent shape P (or at least partially shrunken shape), the preform10′ (or preform10″) may be readily removed from one of the two ends E1, E2through the hollow portion14. This form of deformation is particularly suitable for the preform10formed of elastomeric materials.

In another embodiment, the preform10may be heated in order to initiate deformation. This technique may be used when a shape memory polymer preform10is 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 core12), liquid, etc.) having a temperature sufficient to deform or otherwise at least partially switch the state of the preform10into the smaller shaped preform10′ or preform10″. 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 preform10,10′,10″ will not deleteriously affect the shape of the sand core12, at least in part because the core12has been cured and hardened prior to preform10,10′,10″ removal.

Referring now toFIG. 1C, a cross-section of the sand core12taken along the1C-1C line ofFIG. 1Bis depicted. The removed shrunken preform10′ and the partially shrunken preform10″ are also depicted. As shown, the interior of the sand core12includes the hollow portion14which has conformed to the temporary shape T of the preform10. Since the preform10is shrunken to preform10′ or preform10″ prior to its removal, the sand core12, and thus the hollow portion14, remain set in the desirable shape.

In another embodiment, the permanent shape P of the preform10′ 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 inFIGS. 4A and 4B. The application of temperature enables the preform10′ 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 preform10, and the pressure is sufficient to expand the preform10′ to the desired temporary shape T. Heated gas may be used to raise the temperature and apply the pressure. Generally, the preform10′ 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 preform10′, 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 preform10′ 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 preform10′. 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 inFIGS. 4A and 4Bmay be suitable for an automated process in which the preform10may 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 preform10becomes rigid in the core box cavity16. The pressure may then be maintained or removed since the temporary shape10, T is set to the desired core12inner shape.

In the embodiment shown inFIGS. 4A and 4B, the application of pressure may be accomplished by flowing a gas from one end of the preform10,10′ to the other. If the preform10,10′ 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 preform10,10′ to be regulated.