Patent Publication Number: US-2005121165-A1

Title: Method and apparatus for casting

Description:
The present invention relates to a method of casting and to casting apparatus for performing the method. The casting method has been developed principally for the casting of aluminium disc brake calipers and it will therefore be convenient to describe the method and apparatus of the invention as it applies to that application. However, it should be appreciated that the invention is not limited to that particular application and could be employed, for example, for other cast aluminium products, or for cast products of different metals, such as brass or magnesium.  
      The casting method of the invention is of the type in which the molten casting material is fed upwardly or in a counter-gravity direction. Counter gravity casting is disclosed in several patent specifications, including U.S. Pat. No. 4,733,714, U.S. Pat. No. 4,862,945 and U.S. Pat. No. 5,730,203. A particular benefit of such casting is that the injection pressure applied to the molten metal can be balanced against the downward gravity pressure so that the mould cavity can be filled in a non-turbulent manner, thereby minimising the formation of oxides through the cast product, which otherwise affect the strength of the casting formed. The minimisation or virtual complete elimination of oxides in the cast product is extremely advantageous in the production of disc brake calipers, because of the high loading such calipers are subject to in use.  
      One problem which has arisen with counter-gravity casting is that the feed pressure of the molten metal must be maintained while the metal solidifies, or the molten metal will flow back under gravity from the mould cavity, through the feed pipe to the supply of molten metal, when the feed pressure is released. While maintaining the feed pressure is not difficult, the necessity to do this severely limits casting productivity or cycle time.  
      As discussed in U.S. Pat. No. 4,733,714, counter-gravity casting can employ rotation or inversion of the mould after the molten metal has been pumped or injected into the mould cavity. This advantageously permits disconnection of the feed pipe from the mould, so that the pipe can be connected to a new mould to feed molten metal thereinto, while the molten metal in the mould from which the pipe has been disconnected, solidifies. This method accelerates the rate of production compared to other methods of counter-gravity casting in which the feed pipe remains connected to the mould until such time as the molten metal has solidified sufficiently for disconnection.  
      The above method employs a feed reservoir internally of the mould cavity, which is located below the mould cavity when molten metal is being fed into the mould, but which is located above the mould cavity when the mould is inverted. The feed reservoir feeds molten metal to the mould cavity when the mould is inverted and thereby provides a source of pressure by way of a metalostatic head.  
      The method is particularly suited for use with sand moulds, because such moulds can be produced continuously and then moved to the molten metal source for filling and then moved away for cooling. Also, in a sand mould, the runner which communicates with the mould cavity and the feed pipe can be closed simply by forcing the wall of the runner inwards, with a suitable plunger or the like. Therefore, it is a simple matter to close the runner to prevent reverse or backflow of molten metal to the metal source upon release of feed pressure.  
      Sand moulds are not however, very suitable for producing cast products requiring high tensile strength. This is partly because mould sands generally provide poor rates of heat transfer and consequently slow metal solidification rates, so that the cast product has a very coarse microstructure. For increased strength, a fine microstructure is desirable. Aluminium disc brake calipers are subject to very high loading and it is therefore extremely desirable that they be produced with a fine microstructure rather than a coarse microstructure. It is however the case that disc brake calipers have been cast previously from sand moulds, but this has been limited to cast iron calipers. However, the use of cast iron for disc brake calipers these days is undesirable, principally because of the significant weight increase compared to aluminium calipers.  
      According to the present invention there is provided a method of producing a cast product, said method employing a mould having a mould cavity and a runner which communicates with the mould cavity, said mould being connected to a feed pipe for feeding molten material to said mould cavity through said runner, said mould including a valve arrangement for closing said runner to material flow upon receipt within said mould cavity of sufficient molten material, said method including closing said runner by said valve arrangement upon receipt of said sufficient molten material within said cavity and thereafter disconnecting said feed pipe from said mould and inverting said mould, and further including relatively displacing said mould and said feed pipe for connection of said feed pipe to a further mould and allowing said molten material to solidify within said mould cavity prior to removal therefrom.  
      Further, the present invention provides a casting apparatus including a mould having a mould cavity, and a runner which communicates with said mould cavity, a feed pipe for connection to said mould for feeding molten material to said mould cavity through said runner, a valve arrangement for closing said runner to material flow upon receipt within said mould cavity of sufficient molten material, said apparatus being operable to disconnect said feed pipe from said mould following closure of said runner by said valve arrangement and said mould being mounted for inversion following said disconnection, said apparatus further being operable to relatively displace said mould and said feed pipe for connection of said feed pipe to a further mould of said apparatus.  
      The casting process and apparatus of the present invention is envisaged to be almost exclusively for use in the casting of metal and while it is possible that materials other than metals would be employed, hereinafter reference will be made to metals only.  
      In the method of the invention, the valve arrangement facilitates early disconnection of the feed pipe from the mould, to further accelerate this type of casting from prior art casting methods in which mould inversion does not take place. Accordingly, feed pipe disconnection can occur as soon as the valve arrangement has closed the mould runner and if desirable, even before the mould is inverted. The feed pipe can thus be connected to a further mould without having to wait for the first mould to rotate to the inverted position.  
      The above arrangement also simplifies the connection between the mould and the feed pipe because the connection is not required to permit relative rotation between the mould and the feed pipe. Thus, the connection may for example, be a simple abutting connection with engagement between the two components, which connection might otherwise be inappropriate in circumstances where relative rotation was required. The abutting connection would preferably seal against leakage through the connection.  
      The mould according to the present invention preferably is a permanent mould, in particular rather than a sand mould. In addition to the drawbacks associated with the use of sand moulds for disc brake caliper production discussed earlier herein, sand moulds typically are further inappropriate for disc brake caliper casting because sand granules can be dislodged from the mould during casting and become part of the cast product, affecting its strength characteristics. Castings produced in sand moulds also are typically dimensionally inaccurate and exhibit poor surface finish.  
      The mould may include a mould cavity of any suitable form and may include a feed reservoir to feed the cavity with molten metal when the feed pipe is disconnected and the mould has been inverted. Such a feed reservoir preferably is located above the mould cavity in the inverted condition of the mould, so as to feed the molten metal under gravity. Pumping or pressure may alternatively be adopted, in particular if the reservoir is located other than above the mould cavity in the inverted condition of the mould.  
      In the above arrangement, the runner can communicate directly with the feed reservoir so that feed of molten metal takes place through the runner and the feed reservoir prior to the mould cavity.  
      The feed pipe preferably feeds upwardly to the mould, so that when the runner is closed, molten metal within the feed pipe and possibly also within the runner, can flow back through the feed pipe to the molten metal source. The feed pipe may feed vertically into the mould, say at the centre-line of the mould, or at any angle thereto. When casting a disc brake caliper, an angle of attack of about 45° may be appropriate. Alternatively, for other cast products such as wheels or steering knuckles a vertical attack might be preferred.  
      The valve arrangement can take any form suitable to close flow of molten metal through the runner. A valve for use in the invention preferably is one which provides for automatic closure of the runner when pressure causing molten metal to flow counter to gravity is released. The valve may include a ball which floats in the molten metal and which is lifted away from a valve seat when molten metal is flowing under pressure in the direction towards the mould cavity, to permit that flow, and which closes the runner by cooperating with the valve seat when the pressure is released and the flow stops or reverses. The ball therefore can be made out of a material that has a relatively lower density than the molten metal. The valve seat may take any suitable form suitable to cooperate with the ball and to thereby block the runner against flow of molten metal in a direction away from the mould cavity.  
      In an alternative form, the valve arrangement includes a valve member in the form of a ball, which can be shifted by shifting means between an open position in which molten metal can flow past the ball, and a closed position in which flow of molten metal is blocked. The ball may be one which floats in the molten metal, so that the shifting means is operable to shift the ball only to the closed position from the open position, whereas the ball can shift by floating to the open position in circumstances when there is a flow of molten metal and the shifting means is not maintaining the ball in the closed position. The shifting means may include a rod or plunger which acts on the ball, to promote its movement between the open and closed positions. The runner may include a valve seat to cooperate with the ball and the valve seat may be formed by a conical portion of the runner into which the ball can enter and seat.  
      In one arrangement, the valve ball is biased away from the valve seat so that an actuating mechanism can be employed when necessary to shift the ball to the seated valve closed position. In an alternative arrangement, the ball may be allowed to float in the molten metal being pumped to the mould cavity and shifting means, such as described earlier, is arranged to cause the ball to seat within or against the valve seat. An external actuator can be provided to actuate the push rod or pin as necessary.  
      Preferably the ball is made from a sacrificial material which can be discarded at a suitable point in the casting process. For example, the ball may be formed of a material such as fly-ash, the binder of which can survive the temperature of aluminium casting (about 750° C.), but which will not survive at an elevated temperature, such as 1000° C., at which temperature the binder breaks down. The desirability for such a material is to simplify the recovery of waste cast product which is formed by molten metal which solidifies outside the mould cavity, such as within the runner. That waste material is removed from the cast product during finishing of the product and it is desirable to recycle that waste for subsequent casting, rather than to just discard it. However, it generally will be the case that the ball will form part of the waste cast product and because the ball will be of a different material to that of the product, the ball has to be removed from the waste cast product before the rest of that product can be recycled. A sacrificial material can be easily removed by heating the waste cast product to an elevated temperature to separate the material from the product by combustion. Fly-ash is a material that can be removed effectively by this process. Fly-ash additionally is advantageous because it will not deteriorate at the molten temperature of aluminium and therefore can be effectively used as a material for valve construction, but will decompose for removal by elevating the temperature well above the molten temperature.  
      Other methods can be used to separate the ball material from the waste cast product. For example, the ball material may be removed by sand blasting whereby the ball is broken down for removal. Alternatively the ball may be formed from a non-wetting material that can be separated easily from the cast material. Fly-ash can achieve this, as can certain ceramic composite materials.  
      The valve arrangement could employ a valve member of a form other than a ball and the invention includes within its scope a valve member of any suitable shape or construction that can close the runner as required. Other forms of valve member can include a rotational valve member that permits flow in one position of rotation and which closes against flow in a different position, or such valve members that are collapsible, or that chill to solidify the molten metal.  
      The method according to one embodiment of the invention involves the valve arrangement actuating to close the runner after the mould cavity and any feed reservoir is filled sufficiently, and thereafter disconnecting the feed pipe from the mould. Inversion of the mould can take place immediately, at any time following actuation of the valve arrangement. It may for example take place as soon as the feed pipe is disconnected from the mould or alternatively the mould may be shifted away from the feed pipe, or the feed pipe away from the mould before inversion takes place. The timing of the inversion may also be dependent on the type of material being cast and the type of casting being made.  
      In one preferred arrangement, an array of moulds is provided for successive feeding through a single feed pipe. Preferably the array is formed in a circular manner about a central axis, and the array is rotated incrementally, or is indexed about the axis as each mould is fed. A suitable turntable could be employed for this purpose. Conveniently, the array can include sufficient moulds such that the cast product of each mould solidifies as the array rotates and can be removed from the respective mould, prior to that mould returning for reconnection to the feed pipe. In this arrangement, inversion of the mould and indexing of the array can be immediate upon the valve arrangement closing the runner. Cooling of the molten metal in the mould can also commence immediately.  
      Other equally applicable arrangements can be employed, such as an array which is not circular, but which might for example move on a conveyor. Alternatively, the moulds may be generally stationary and the feed pipe may shift between them. Still alternatively, a combination of mould and feed pipe movement may be employed.  
      As discussed earlier, the rate of solidification of the cast product is important for achieving a desirable microstructure. Permanent steel moulds offer high heat transfer rates compared to sand moulds and those rates can be increased by forced heat extraction using air and/or water cooling. The rates can alternatively or additionally be accelerated by injecting suitable gas into the mould cavity, to fill the evolving shrinkage gap between the mould cavity wall and the solidifying casting. Shrinkage of the casting occurs during solidification and the casting tends to move away from the mould cavity wall, leaving a gap filled with air. The air gap actually insulates the casting from the mould cavity walls so that the rate of heat transfer from the casting slows and solidification takes longer, thereby increasing overall cycle time. A gas such as helium, has a high heat transfer coefficient and therefore it promotes heat transfer rather than insulating against it. Advantageously, the mould may include a suitable connecting part for connection of a gas hose thereto and the arrangement can be such as to connect a hose to the mould at a suitable time during metal solidification. That may be after mould inversion, but not necessarily. The connection therefore can be made at any suitable time, so that permanent connections are not required, thereby simplifying the construction of the mould and the casting apparatus.  
      The reduction in cycle time that can be made by cooling the actual mould, can alternatively, be made by cooling the plate or platen to which the mould is attached.  
      The present invention is very flexible in relation to cooling rates, because air and water cooling, and the use of high coefficient cooling gases can be varied as required to cause faster or slower solidification. Sand moulds typically do not have such flexibility.  
      The invention provides its principal advantages when the mould employed is a permanent mould, preferably a steel permanent mould. As discussed earlier, the use of a valve arrangement permits the use of permanent moulds rather than sand moulds and the use of permanent steel moulds, along with gases having high heat transfer coefficients and air and/or water cooling all contribute to faster casting solidification and therefore lower cycle times. That faster solidification also permits the production of aluminium disc brake calipers, with the necessary fine micro-structure. The valve arrangement therefore plays an important part in the invention, by facilitating the use of permanent moulds with the resultant advantages achieved thereby.  
      Also, the amount of cooling provided by gases having high heat transfer coefficients, and by air and/or water, can be adjusted by suitable control arrangements, so that in casting machinery that employs a plurality of moulds in a casting cycle, the moulds for a range of different products can be introduced into that cycle rather than being restricted to moulds for a single product. For example, in continuous casting machinery, in which moulds are presented to a feed pipe on a continuous basis for charging with molten metal and are then moved away for the molten metal to solidify, sand moulds are typically employed, with the moulds being produced in a different machine, or a separate part of the casting machine, on a continuous basis for supply to the casting machine. In that arrangement the nature of the sand mould machinery means that moulds suitable for a single product only are produced. Therefore, the sand mould process is suitable realistically for a single type of cast product only in any one production cycle and when a different type of product is to be cast, the operating parameters of the casting line must change, to suit the new manufacturing and casting requirements of the mould, such as the new mould shape and the change in volume of molten material. However, the present invention advantageously can adjust the rates of cooling applied to different moulds, so that the cycle time for the moulds for two different products can be identical. Accordingly, the casting machine can operate on a set cycle for a wide variety of different products by adjusting as necessary the cooling applied to different moulds to suit the cycle time of the machine. This adjustment is relatively simple and can be a programmed feature of the casting machinery. Also, it is a relatively simple matter when permanent moulds are employed, to introduce a different mould or moulds into the casting cycle and for the casting machinery to be adjusted to suit the introduced mould or moulds and this ideally can be an automatic adjustment that the casting machinery makes as it recognises a new mould entering the cycle. Thus, the present invention provides flexibility for short production runs, which are not suited to sand mould casting. 
    
    
      For a better understanding of the invention and to show how it may be performed, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawing.  
       FIG. 1  is a cross-sectional view of an apparatus according to one embodiment of the invention and shows a mould  10  in engagement with a feed pipe  11 . As shown, the feed pipe  11  is inclined upwardly and in practice would extend to and be immersed in a bath of molten metal. Molten metal is fed through the feed pipe by any one of a variety of mechanisms, such as by pumping or by pressurising the bath. The feed pipe  11  therefore feeds the mould  10  in a counter-gravitational manner. 
    
    
      The mould  10  includes a mould cavity  12 , which in the illustrated form shown in  FIG. 1 , is shaped to cast a disc brake caliper. The mould cavity  12  is in communication with a feed reservoir  13 , while the feed reservoir  13  is in communication with a runner  14 . It is clear from  FIG. 1 , that the mould cavity  12  is positioned above the feed reservoir  13  and this is the correct orientation for counter-gravity casting. Accordingly, molten metal is fed upwardly through the feed pipe  11 , through the runner  14  and the feed reservoir  13 , and into the mould cavity  12 . By this orientation, the advantages of counter-gravity casting can be obtained.  
      The feed pipe  11  is shown in abutting connection with a projecting portion  15  of the mould  10  and the nozzle  16  of the feed pipe  11  is aligned with an inlet portion  17  of the runner  14 . The nozzle  16  sealingly engages the projecting portion  15  to prevent leakage of molten metal past the connection. Any suitable sealing arrangement can be employed for that purpose.  
      The inlet portion  17  is formed to have a right angle curve. The upper end of the portion  17  is arranged to snugly accommodate the ball  19  of a ball valve  20 . Thus, the upper end  21  is of a diameter slightly greater than the diameter of the ball  19 .  
      The ball  19  is engaged by a pin  22  which is accommodated in a bore formed in the projecting portion  15  and biasing means acts between the pin head  23  and the upper surface  24  of the projecting portion  15  to bias the ball  19  towards or into the upper end  21  of the inlet portion  17 . The pin engages the surface of the ball  19 , rather than being connected to the ball, so that the ball  19  can be removed with the cast product when casting is completed without requiring disconnection from the pin. When valve actuation is required, an external actuator is operable to engage the pin head  23  and to push against the pin head to lower the ball  19  for valve actuation.  
      The inlet portion  17  is arranged to cooperate with the ball  19  to close the runner  14  and to achieve this, the inlet portion  17  may include a valve seat formed by a conical portion into which the ball  19  may be lowered into or against. Accordingly, while it is not apparent from the cross-sectional view of  FIG. 1 , the neck area  25  may converge in a conical manner so that the ball  19  seats within the neck area  25  upon its downward movement.  
      The biasing means may provide only a light bias, sufficient to maintain the ball  18  away from the valve seat in the absence of a valve actuating force being applied to the pin head  23 . During feed of the molten metal into the mould  10 , the ball  19  may float within the inlet portion  17 , but allowing the passage of molten metal therepast.  
      The feed pipe  11  is movable forward and back by a pneumatic cylinder  26  and that movement facilitates connection and disconnection between the feed pipe and the mould  10  at the projecting portion  15 .  
      The apparatus of  FIG. 1  is operable as follows. The mould  10  is brought into a position for connection to the feed pipe  11 . The feed pipe  11 , through the pneumatic cylinder  26 , is moved to connect to the mould  10  at the projecting portion  15 . Feeding means of whatever kind is provided to supply molten metal through the feed pipe  11  and into the inlet portion  17  of the runner  14 . As discussed, the ball  19  will either float in the molten metal or be raised into the upper end  21 , depending on the arrangement of the biasing means provided. Molten metal will then flow through main portion  18  of the runner  14  into the feed reservoir  13  and then into the mould cavity  12 .  
      When sufficient flow of molten metal has taken place, the ball valve  20  will be actuated to seat the ball  19  in the neck area  25 , to close the runner  14 . Thereafter, the feeding means terminates feed of molten metal, and such material that remains in the feed pipe  11  and the inlet portion  17  on the feed pipe side of the ball  19  can flow back under gravity to the bath of molten metal. The pneumatic cylinder  26  withdraws the feed pipe  11  to disconnect it from the mould  10  and the mould  10 , in any sequence or simultaneously, is shifted away from the feed pipe  10  and is rotated to be inverted. As the material in the mould cavity solidifies, molten metal in the feed reservoir feeds into the mould cavity.  
      Upon solidification of the molten metal, the mould  10  may be disassembled to remove the cast product. That product will also include any solidified material that remains in the feed reservoir  13  and the main runner portion  18 , as well as the ball  19 . This is the waste cast product. When the cast product is finished by other required machining or processing stages, the waste cast product can be removed and returned to the molten bath for reuse.  
      Advantageously, the ball can be made from materials of the kind discussed earlier, which are easily removable by suitable procedures.  
       FIG. 2  shows an alternative arrangement to  FIG. 1 , but in respect only of the valve arrangement. Accordingly, where the same features of  FIG. 1  are shown in  FIG. 2 , the same reference numerals, plus  100 , are employed.  
      In  FIG. 2 , the runner  114  includes an inlet portion  117  and an outlet portion  101  that communicates with the feed reservoir  113 . The runner  114  has a open seat  102  and a closed seat  103 . The open seat  102  is so named because the ball  119  can engage the seat  102  when molten metal is fed under pressure into the runner  114  from the feed pipe  111  and in that position a flow path  104  beneath the ball  119  permits flow of molten metal past the ball  119 . However, when the pressure is released and the flow of molten metal attempts to reverse due to gravity, the ball  119  will move with the molten metal in the direction toward the feed pipe  111  and will quickly engage the closed seat  103 , closing flow path  104 . As is clear from  FIG. 2 , the runner  114  is constricted at the closed seat  103  to have a smaller diameter than that of the ball  119 .  
      The ball valve  119  necessarily must float in the molten metal to seat in the manner required. Many suitable ball materials can be employed that will provide this characteristic.  
      The arrangement of  FIG. 2  advantageously provides for automatic valve operation and therefore does not require the actuating arrangement of the ball valve of  FIG. 1 . Also, because the ball movement between the open and closed seats is not great, valve operation is extremely quick and efficient.  
      The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.