Abstract:
A sand-mold molding method for producing a molded article obtained by packing foamed sand may include stirring a binder with an aggregate to form foamed sand; forming a cavity via clamping a metallic mold closed; packing the foamed sand into the cavity of the metallic mold and then heating and solidifying the foamed sand; and opening the metallic mold partially to provide a gap in the metallic mold while maintaining the cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to Japanese Patent Application No. 2013-066335, filed Mar. 27, 2013, and International Patent Application No. PCT/JP2014/051273, filed Jan. 22, 2014, both of which are hereby incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a sand-mold molding method and a sand-mold molding apparatus in which a foamed sand is packed into a metallic mold and solidified to mold a sand mold such as a sand core. 
       BACKGROUND 
       [0003]    When casting a cylinder block, a cylinder head, or the like of an engine, a collapsible sand core (sand mold) is used for forming a hollow part such as a water jacket, an intake and exhaust port, or the like. The sand core is formed into a predetermined shape by, for example, blowing a shell sand coated with a thermoplastic resin into a metallic mold by compressed air from a nozzle of a sand blowing device to pack the shell sand into a cavity and then heating and solidifying it. 
         [0004]    In contrast, there is a technology for molding a sand mold such as a core using foamed sand that is foamed by stirring water and an water-soluble binder with an aggregate. In this way, when the foamed sand is molded by packing it into a cavity of a metallic mold, the internal pressure of the cavity increases due to evaporation of moisture and thermal expansion of air bubbles during the course of heating and hardening (baking) of the foamed sand. Thus, in the molded article, the water-soluble binder and the aggregate accumulate to form a dense hardened layer with high strength in a surface layer part, while a fragile brittle part with low density is formed in an inner part. Therefore, a core molded by foamed sand can ensure sufficient strength as a core during casting, and exhibits excellent collapsibility such that it can be easily removed from the casted article after casting. 
         [0005]    When heating and hardening foamed sand within a metallic mold, water vapor and gas are produced, and thus it is necessary to smoothly discharge such water vapor and gas to the outside of the cavity of the metallic mold. Thus, slits or passages for discharging water vapor and gas are provided in metallic molds used for molding foamed sand. Also, although not in relation to foamed sand, JP  2002 - 192305  A discloses providing gas venting holes to a metallic mold so as to discharge from the cavity of the metallic mold any gas that is produced when baking shell sand in the cavity of the metallic mold. 
       CITATION 
       [0006]    Literature 1: JP 2002-192305 A 
       SUMMARY 
       [0007]    The following problems may occur when molding a sand mold using foamed sand. 
         [0008]    In order to shorten the heating and hardening time (baking time) of foamed sand, how quickly water vapor can be discharged from the cavity of the metallic mold is important. However, since the water-soluble binder flows out together with water vapor and gas from the slits or passages for gas venting during the course of heating and hardening the foamed sand, solidified binder may accumulate on the slits or passages for gas venting due to repeated moldings, and this may obstruct the discharge of water vapor and gas. Thus, there has been a problem in that the baking of the foamed sand may require more time if the smooth discharge of water vapor and gas becomes obstructed. 
         [0009]    The present invention was created in consideration of the above-described problems, and an object thereof is to shorten a hardening time by smoothly discharging water vapor and gas from within the cavity when heating and hardening foamed sand within a cavity of a metallic mold. 
         [0010]    To solve the above-described problems, the present invention provides a sand-mold molding method in which a molded article is obtained by packing foamed sand, which has been foamed by stirring a binder with an aggregate, into a cavity of a metallic mold and then heating and solidifying the foamed sand, wherein, after forming the cavity by clamping the metallic mold and packing the foamed sand into the cavity, the metallic mold is opened slightly to provide a gap in the metallic mold while maintaining the cavity. 
         [0011]    Hereinafter, several examples of embodiments of the invention for which it is recognized that a patent claim is possible in the present invention (may also be referred to as “claimable inventions” below) are described below. The embodiments below are divided into aspects similar to the claims, and each aspect is assigned a number and described in a format where other aspect numbers may be cited as necessary. This division into aspects is for facilitating the understanding of the claimable inventions, and the combinations of components that constitute the claimable inventions are not limited to those described below in the following aspects. In other words, the claimable inventions should be interpreted upon referring to the descriptions in each aspect below and the descriptions of any examples and the like. As long as it is in line with such interpretations, the claimable inventions may include embodiments in which further components are added to the embodiments of each aspect or components are deleted from the embodiments of each aspect. The contents of (1) to (6) below correspond to various aspects of the disclosure. 
         [0012]    (1) A sand-mold molding method in which a molded article is obtained by packing foamed sand, which has been foamed by stirring a binder with an aggregate, into a cavity of a metallic mold and then heating and solidifying the foamed sand, wherein, after forming the cavity by clamping the metallic mold and packing the foamed sand into the cavity, the metallic mold is opened slightly to provide a gap in the metallic mold while maintaining the cavity. 
         [0013]    (2) The sand-mold molding method according to (1), wherein a mold clamping force of the metallic mold is reduced so that the gap is provided in the metallic mold by a counterforce from the metallic mold. 
         [0014]    (3) The sand-mold molding method according to (1) or (2), wherein the gap in the metallic mold ranges from 0.22 mm to 0.36 mm. 
         [0015]    (4) A sand-mold molding apparatus including: a metallic mold in which a cavity is formed by mold clamping; a mold clamping device that opens/closes the metallic mold; a packing device that packs a foamed sand, which has been foamed by stirring a binder with an aggregate, into the cavity; and a heating device that heats the foamed sand within the cavity, wherein the mold clamping device slightly opens the metallic mold after the foamed sand has been packed into the cavity so as to form a gap in the metallic mold while maintaining the cavity. 
         [0016]    (5) The sand-mold molding apparatus according to (4), wherein the mold clamping device reduces a mold clamping force so that the gap is provided in the metallic mold by a counterforce from the metallic mold. 
         [0017]    (6) The sand-mold molding apparatus according to (4) or (5), wherein the gap in the metallic mold ranges from 0.22 mm to 0.36 mm. 
         [0018]    With the structure of (1) and (4), by providing a fine gap in the metallic mold, water vapor and gas are smoothly discharged to the outside from the gap in the metallic mold during heating and hardening of the foamed sand within the cavity of the metallic mold, and thus the hardening time can be shortened. 
         [0019]    Even if solidified binder accumulates in a passage through which water vapor and gas are discharged from the cavity of the metallic mold, the water vapor and gas can be reliably discharged to the outside from the cavity by the gap in the metallic mold. 
         [0020]    At this time, since the gap in the metallic mold is sufficiently small and the shape and dimensions of the cavity are maintained, the dimensional precision of the molded article is not affected. 
         [0021]    With the structure of (2) and (5), the gap in the metallic mold can be adjusted according to a counterforce from the metallic mold. 
         [0022]    With the structure of (3) and (6), the gap can be optimized to carry out preferable molding. 
         [0023]    (7) The sand-mold molding apparatus of (5) or (6), in which the mold clamping device clamps the metallic mold by an air cylinder and reduces a pressure of compressed air supplied to the air cylinder so as to reduce a mold clamping force. 
         [0024]    According to the present invention, water vapor and gas can be smoothly discharged from within the cavity through a gap in the metallic mold during heating and hardening of foamed sand within a cavity of a metallic mold, and thereby the hardening time can be shortened. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1  is a schematic view illustrating a sand-mold molding apparatus according to an embodiment of the present invention; 
           [0026]      FIGS. 2A to 2C  illustrate the steps for molding foamed sand with the sand-mold molding apparatus shown in  FIG. 1 ; 
           [0027]      FIG. 3  is a table illustrating the relationship between a mold clamping set pressure and a gap in a metallic mold in the sand-mold molding apparatus shown in  FIG. 1 ; 
           [0028]      FIG. 4  is a graph illustrating the relationship between a mold clamping set pressure and a gap in a metallic mold in the sand-mold molding apparatus shown in  FIG. 1 ; 
           [0029]      FIG. 5  is a table illustrating the relationship between a mold clamping set pressure and the quality of a molded article in the sand-mold molding apparatus shown in  FIG. 1 ; 
           [0030]      FIGS. 6A and 6B  are image views illustrating a composition of foamed sand; and 
           [0031]      FIGS. 7A and 7B  are explanatory views illustrating the course of heating and hardening of foamed sand within a cavity of a metallic mold. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    An embodiment of the present invention will now be explained below in detail based on the drawings. 
         [0033]    As shown in  FIG. 1 , a sand-mold molding apparatus  1  according to the present embodiment solidifies foamed sand to mold a sand core (sand mold), and includes a metallic mold  2  that forms a cavity C, a packing device  3  for packing foamed sand S into the cavity C of the metallic mold  2 , and a mold clamping device  4  that opens and closes the metallic mold  2 . 
         [0034]    The foamed sand S used in the present embodiment is obtained by mixing and kneading sand, which serves as an aggregate, with an water-soluble inorganic binder including water glass (sodium silicate), water, and a surfactant to foam the sand. An image of the state of the particles that constitute the foamed sand S is shown in  FIGS. 6A and 6B .  FIG. 6A  illustrates a state in which bubbles  8  are adsorbed to the surface of a sand particle  7 , and  FIG. 6B  illustrates an enlarged portion of a bubble  8 . As shown in  FIG. 6B , the foamed sand S is foamed by coating the surface of a water glass aqueous solution ( 10  denotes water and  11  denotes water glass) with a surfactant  9  to form bubbles  8  and then adsorbing the bubbles  8  to the surface of a sand particle  7  via the surfactant  9 , and the foamed sand S has suitable viscosity. Herein, the foamed sand S having suitable viscosity can be obtained by setting the molar ratio and weight ratio of the water glass relative to the sand to approximately 1.0 to 3.0 and 0.4 to 3.0% respectively, setting the weight ratio of the water relative to the sand to approximately 1.5 to 5.0%, and setting the weight ratio of the surfactant relative to the sand to approximately 0.003 to 2.0%. 
         [0035]    Referring to  FIG. 1 , the metallic mold  2  includes a stationary mold  12  and a movable mold  13  that are divided by a parting line P. A cavity C is formed by clamping the stationary mold  12  and the movable mold  13 . The stationary mold  12  is fixed to a stationary base  14 . The movable mold  13  is fixed to a movable base  15  that is movable, and the movable mold  13  moves together with the movable base  15  to open and close the metallic mold  2 . The stationary mold  12  and the movable mold  13  are heated to approximately 150° C. to 300° C. by a heating device H such as a heater, and thereby moisture of the foamed sand S packed into the cavity C is evaporated to solidify the foamed sand S. In the metallic mold  2 , discharge passages such as slits  16  for discharging gas and water vapor to the outside during heating and hardening of the foamed sand S may be provided within the cavity C. 
         [0036]    A releasing device  17  is provided to each of the stationary mold  12  and the movable mold  13  of the metallic mold  2 . Each releasing device  17  includes a plurality of extruding pins  18  provided such that they can move towards/away from the inside of the cavity C, an extruding plate  19  connected to the bases of the plurality of extruding pins  18 , and an extruding spring  20  provided between the fixed base  14 /movable base  15  and the extruding plate  19 . The plurality of extruding pins  18  are compressed via the extruding plate  19  by the spring force of the extruding spring  20  to make the distal ends of the extruding pins  18  protrude into the cavity C. Refracting pins  21  whose distal ends oppose each other are attached to the extruding plates  19  on both sides. When the stationary mold  12  and the movable mold  13  are closed, the distal ends of the retracting pins  21  abut each other to move the extruding plates  19  counter to the spring force of the extruding springs  20 , which causes the extruding pins  18  to retract from the cavity C. Thereby, the extruding pins  18  move together with the opening/closing of the metallic mold  2 , such that they retract from within the cavity C when the metallic mold  2  is closed, and protrude into the cavity C when the metallic mold  2  is opened to release the molded core. 
         [0037]    The packing device  3  includes a sand tank  22  in which the foamed sand S is kneaded and stored, a pressurization mechanism  23  that pressurizes the foamed sand S within the sand tank  22 , and a packing port  24  that connects the sand tank  22  to the cavity C of the metallic mold  2 . The sand tank  22  is set onto the metallic mold  2  upon closing and clamping the stationary mold  12  and the movable mold  13 , and by pressurizing the foamed sand S within the sand tank  22  by the pressurization mechanism  23 , the foamed sand S is packed into the cavity C of the metallic mold  2  through the packing port  24 . 
         [0038]    The mold clamping device  4  includes a double-acting air cylinder  26  that drives the movable base  15 , an air source  27  that supplies compressed air to the air cylinder  26 , a switching valve  28  that switches the supply of compressed air from the air source  27  to the air cylinder  26  to make the movable base  15  advance forward or retract backward, a pressure adjusting valve  29  that adjusts the pressure of the compressed air to be supplied to the air cylinder, a pressure gauge  30  that detects the pressure of the compressed air supplied to the air cylinder  26 , and a control device  31  that controls the operation of the switching valve  28  and the pressure adjusting valve  29 . 
         [0039]    The control device  31  controls the switching valve  28  to switch the supply of compressed air so as to elongate/retract an activation rod  26 A of the double-acting air cylinder  26 . Thereby, the movable mold  13  is made to advance/retract together with the movable base  15  to open/close the metallic mold  2 . The pressure adjusting valve  29  is operated based on a detected pressure of the pressure gauge  30  during mold clamping to adjust the pressure of the compressed air to be supplied to the air cylinder  26 , and thereby the mold clamping force is adjusted. Thereby, when heating and hardening the foamed sand S within the cavity C of the metallic mold  2 , if the mold clamping force by the air cylinder  26  is decreased, the movable mold  13  will retract slightly due to the pressure within the cavity C, the spring force of the extruding springs  20  of the releasing devices  17 , and a counterforce by warping of the metallic mold  2 . Thus, a fine gap L (refer to  FIG. 2C ) is formed on the parting line P between the stationary mold  12  and the movable mold  13 , and this gap L can be adjusted. 
         [0040]    Next, the steps for molding a sand core with the sand-mold molding apparatus  1  will be explained. 
         [0041]    As shown in  FIG. 2A , the movable mold  13  is driven by the mold clamping device  4  to close and clamp the metallic mold  2  to form the cavity C, and then the packing device  3  is set onto the metallic mold  2 . At this time, the extruding pins  18  of the releasing devices  17  move together with the mold clamping so as to retract from the cavity C as explained above. In a state in which the metallic mold  2  has been preheated by the heating device H, the pressurization mechanism  23  of the packing device  3  is operated to pack the foamed sand S within the sand tank  22  into the cavity C of the heated metallic mold  2  through the packing port  24 .  FIG. 2B  illustrates a state in which the packing of the foamed sand S into the cavity C has been completed. The foamed sand S that has been packed into the cavity C of the metallic mold  2  is then hardened by evaporating the moisture therein via heating. 
         [0042]    Herein, the course of heating and hardening the foamed sand S packed into the cavity C will now be explained referring to  FIGS. 7A and 7B . As shown in  FIG. 7A , in the cavity C of the metallic mold  2 , the air bubbles  8  of the foamed sand S expand due to heating, leading to an increase in the internal pressure of the cavity C. As shown in  FIG. 7B , the air bubbles  8  move along the inner wall of the cavity C of the metallic mold  2  and are discharged to the outside via the passages formed by the slits  16  or the like that are provided on the parting line P. At this time, the water glass (binder) and aggregate that constitute the foamed sand S are pushed towards the inner wall side of the cavity C, and thus the density of water glass and aggregate near the inner wall of the cavity C increases. As a result, in a sand core W that has solidified within the cavity C, a dense hardened layer  33  in which the density of the water glass and the aggregate is high is formed in a surface layer part that is in contact with the inner wall of the cavity C, whereas the inner density is low such that an easily-collapsible brittle part  34  is formed in a center part. 
         [0043]    The water glass (binder) that is pushed toward the inner wall side of the cavity C penetrates into the passages consisting of the slits  16  or the like together with the water vapor and gas and is released to the outside. Binder that has penetrated into the passages may harden and adhere to the passages. If the passages become blocked due to accumulation of binder that adheres to the passages, the smooth discharge of water vapor and gas may become obstructed, leading to an increase in the baking time and poor molding. 
         [0044]    In the present embodiment, after completion of packing of the foamed sand S into the cavity C of the metallic mold  2 , the pressure adjusting valve  29  is operated by the control device  31  of the mold clamping device  4  based on the detected pressure of the pressure gauge  30  to reduce the pressure of the compressed air supplied to the air cylinder  26  by a predetermined pressure so as to decrease the mold clamping force. Thereby, as shown in  FIG. 2C , the movable mold  13  retracts slightly due to the pressure within the cavity C that has increased due to heating and hardening of the foamed sand S, the spring force of the extruding springs  20  of the releasing devices  17 , and a counterforce by warping of the metallic mold  2 . Thus, a fine gap L is formed on the parting line P between the stationary mold  12  and the movable mold  13 . Gas and water vapor that are generated during heating and hardening of the foamed sand S within the cavity C of the metallic mold  2  is smoothly discharged to the outside through the fine gap L. As a result, solidification of the foamed sand S can be accelerated, and the baking time can be shortened. 
         [0045]    At this time, the fine gap L is a slight gap of a size such that gas and water vapor can be smoothly discharged, and the amount of movement of the movable mold  13  is also small. Thus, the shape and dimensions of the cavity C are maintained, and the shape and dimensional precision of the core that is molded is not affected. Also, since the movable mold  13  is moved slightly by the pressure within the cavity C, the spring force of the extruding springs  20  of the releasing devices  17 , and a counterforce by warping of the metallic mold  2  to form the fine gap L between the stationary mold  12  and the movable mold  13 , a constant fine gap L can be secured regardless of the amount of accumulation of binder on the parting line P or the slits  16  in the metallic mold  2 . As a result, gas and water vapor that are generated during heating and hardening of the foamed sand S can be reliably discharged to the outside, and thus the baking time can be shortened and a molded article of good quality can be obtained. 
         [0046]    After the foamed sand S within the cavity C of the metallic mold  2  has solidified, the switching valve  28  is operated by the control device  31  to switch the supply of compressed air to the double-acting air cylinder  26  and make the movable mold  13  retract together with the movable base  15  so as to open the metallic mold  2  and remove the molded sand core. At this time, the distal ends of the retracting pins  21  of the releasing devices  17  separate from each other together with the mold opening, and thus the extruding plates  19  move by the spring force of the extruding springs  20  and the extruding pins  18  protrude into the cavity C to release the molded sand core. 
         [0047]    Next, in the above-described steps, the relationships between the pressure of the compressed air supplied to the air cylinder  26  when forming the gap L on the parting line P in the metallic mold  2  (refer to  FIG. 2C ) with the size of the gap L and with the quality of the sand core that is molded will be explained referring to  FIGS. 3 to 5 . 
         [0048]    As shown in  FIGS. 3 and 4 , by reducing the pressure of the compressed air supplied to the air cylinder  26  during mold clamping from a pressure of 0.35 MPa to a pressure of 0.15 MPa, 0.10 MPa, and 0.05 Mpa, the gap L became on average 0.22 mm, 0.25 mm, and 0.36 mm respectively. Thereby, as shown in  FIG. 5 , when the pressure of the compressed air that is supplied was kept unchanged at 0.35 MPa during mold clamping so that no gap L is provided, 60% of the molded articles were of good quality and 40% were of poor quality, whereas when the pressure of the compressed air that is supplied was reduced to 0.10 MPa so that the gap L is provided, 80% of the molded articles were of good quality and 20% were of poor quality. When the pressure of the compressed air that is supplied was further reduced to 0.05 MPa so as to increase the gap L, 100% good quality could be obtained. 
         [0049]    In the above-described embodiment, the pressure of compressed air supplied to the air cylinder  26  of the mold clamping device  4  (the mold clamping force) is reduced to form the gap L between the stationary mold  12  and the movable mold  13  by warping in the metallic mold  2 . However, a predetermined gap L can also be formed by directly controlling the amount of movement of the movable mold  13 . 
       Explanation for References 
       [0050]      1 : Sand-mold Molding Apparatus  2 : Metallic Mold C: Cavity H: Heating Device L: Gap S: Foamed Sand