US2119242A - Method and apparatus for casting - Google Patents

Description

May 31, 1938. J. FLAMMANG ET AL y y METHOD AND APPARATUS FOR CASTING Original Filed Aug. 12, 1931 2 Sheets-Sheet l imwbowi May 31, 1938. J. FLAMMANG ET AL METHOD AND APPARATUS FOR CASTING 2 Sheets-Sheet 2 Original Filed Aug. 12, 1931 a a 1 G A 7 H Hun a w x Patented May 31, 1938 UNITED STATES PATENT OFFICE Bowser and Herman Ernst, St. Louis Mo., as-

signors to The Sterling Corporation, St. Louis, Mo., a corporation of Delaware Application August 12, 1931, Serial No. 556,568

, Renewed July 8, 1936 8 Claims.

This invention pertains to a method and means for casting which is particularly adaptable to the casting of trunk pistons such as are used in automobile and aircraft engines. Such pistons are often cast of light metals, such as aluminum or its alloys and the casting is done in permanent metal molds.

A number of different methods of casting articles of this kind have been used heretofore 10 with varying degrees of success. Most of these methods have disadvantages of one form or another; In practically all previous methods the metal is delivered to the mold cavity or matrix thru a gate which is relatively narrow compared with the bulk of the casting. It has been desirable to make the gate narrow in order to reduce the amount of metal contained therein and also in order to leave a relatively small section of metal which must be cut in order to remove the g gate from the casting.

One serious drawback with the above method of casting is that the gate often sets'or solidifies before the interior metal of the casting has set. When this happens the flow of molten metal to 25 the casting must necessarily cease before the interior metal of the casting has become solid. Consequently the interior metal of this part of the casting solidifies after the exterior portion or outer skin has set. Slnce the setting or solidify- 30 ing takes place as a result of the cooling of the metal, there is still a contraction of the metal going on while the interior portions are still fluid. Those parts of the casting which have set continue to contract on account of their cool- 35 ing. The interior portion which is still liquid is also contracting at this time. Accordingly, the fluid metal which originally had suflicient volume to fill the entire interior portion of the casting will shrink so that its volume will be less than 40 it was originally. As a consequence some of this metal is withdrawn during cooling from the space it originally occupied so that as a result of this action empty spaces or blow-holes are left in the interior of the castings These blow-holes are 45 flaws which weaken the casting. and are otherwise objectionable.

Another drawback with former casting methods results from the fact that air is entrapped in the stream of molten metal as it passes into the 50 mold cavity. This is due in some cases to the high velocity at which the metal enters where the casting is carried out under a high pressure. The agitation of the metal entering at a high velocity entraps air with the metal. In other cases where 55 the metal is poured into the mold cavity from the top and flows in by gravity, air is entrapped during the pouring and is carried into the mold cavity where the coollng'of the metal entraps air bubbles which remain in the casting.

It is an object of this invention, therefore, to 5 overcome some of the above mentioned objections to the prior casting methods.

Another object is to provide a method in which the entrapping of air with the inflowing metal is avoided. l0

Another object is to provide a method in which the air in the mold cavity is permitted to escape therefrom before the cavity is entirely filled with molten metal.

Another object is to provide a method and 15 means whereby the supply of molten metal to the interior parts of a casting may be continued until those interior parts have set so as to avoid the formation of blow-holes and crystallization shrinkage by the contraction of the metal.

Further objects will appear from the following description taken in connection with the accompanying drawings, in which:

Figure 1 is a side view partially in section of a casting apparatus embodying this invention and adapted for carrying out the method thereof;

Figure 2 is a cross sectional view of a piston casting shown in its relation to the casting nozzle and representing the casting just after the metal of the casting itself has completely set. This figure is a section on line 2--2 of Fig. 3;

Figure 3 is a section on line 3-3 of Fig. 2.

In accordance with the method of this invention the metal is caused to flow into the mold cavity upwardly from the bottom. The mold cavity is vented in any suitable manner. In many types of castings the mold must be constructed so that it may be opened to free the casting. In

such cases the cracks between the adjoining faces of the mold sections usually provide suflicient 40 venting space for the escape of air from the mold cavity. The metal is flowed into the mold cavity under a moderate pressure so that it enters at a moderate velocity. The pressure is so adjusted that the metal will flow into the mold quietly without undue agitation so as to avoid entrapping air in the flowing metal. At the same time the mold cavity is filled slowly enough so that the air therein may escape thru the vent openings in advance of the flowing metal. Since the metal enters at the bottom and the air escapes at the top all danger of entrapping air is eliminated.

A particular feature of this invention is that arrangements are such that the flow. of molten metal into the mold is so regulated that the natural cooling of the casting metal in the mold causes the casting to set first in that portion which is most distant from the gate, and the setting 01 the metal progresses gradually toward the gate. As a consequence of this action those parts of the casting which are still fluid at any time are still in communication with the source of molten metal. Accordingly, the flow of molten metal is maintained to such portions 01' the casting as are still fluid.

In order to accomplish this result the manner and rate of cooling of different parts of the casting should be taken into account. Where the casting has certain portions which are more massive than other portions so that more metal is required to fill that part of the mold, such massive portions must be filled quickly so that ample time is allowed for cooling and setting before lighter portions nearer the gate become solid and cut ofl the supply of molten metal to such massive parts. A direct flow of metal to such massive portions is, therefore, desirable. In accordance with this invention the gate is so formed as to provide a direct path for the molten metal to such massive parts as the wrist pin bearings so that a rapid delivery of metal to those parts may be accomplished. This arrangement also provides a direct path for the flow of heat from the incoming molten metal to those parts in the path of such flow so that they are maintained fluid until the more distant portions have a chance to set.

In a trunk piston the head portion in which the packing ring grooves are subsequently formed is usually somewhat thicker and contains more metal than the skirt portions. In some types of pistons the head is also made heavy. Since these portions are somewhat massive and the cooling thereof takes place progressively from the outside toward the interior it is important to maintain a flow of molten metal and also a flow of heat to the interior portions of these massive parts until setting is complete. In other words the piston head should be the last part to become solid since the delivery of metal to the other parts in the method of this invention takes place thru the head.

In accordance with this invention, therefore, the gate in the mold is so formed as to facilitate maintaining the head portion of the casting in fluid condition until the remaining portions have set. This is accomplished by providing a gate having a contracted portion at a point therein spaced from the piston head. The gate is relatively short between the source of molten metal and the casting. Accordingly, the heat of the molten metal supply keeps the metal in the gate molten and also extends its influence to the adjacent portion of the piston head so as to maintain that portion fluid as long as possible. This contracted portion of the gate may be said to act as a sort of heat valve controlling the rate of flow of heat to the piston head. By adjusting the spacing of this contracted point from the piston head the rate of heat flow to the casting from the source of molten metal can be adjusted relatively to the rate of heat flow away from the casting thru the mold. Such adjustment is made so that the setting of the metal takes place progressively from the distant portions of the casting toward the gate and particularly from the outer circumference of the piston head toward its center where the gate enters the casting. When this adjustment is obtained the central portion of the piston head is the last to set. It may in fact be maintained fluid for a short time after its natural tendency to set by the flow of heat thereto thru the gate from the source of metal. During this final solidification, the contraction of the metal in that portion is supplied by a fiow of molten metal from the retort up to the final moment of setting. When the casting has completely set, the mold may be separated from the source of metal.

Referring to the drawings, l designates a furnace of any suitable type containing a retort 2, in which the casting metal is melted, and provided with any suitable means 3 for heating the retort. A nozzle or spigot 4 dips into the molten metal in the retort. The retort is equipped so that it may be closed air-tight and an air connection 5 is provided by means of which pressure may be applied to the interior of the retort in order to eject the molten metal.

A mold 6 which may be of any suitable type is mounted on a frame I hinged at 8 on the furnace structure so that it may be lowered upon the retort or raised thereabove. The mold 6 may be of any suitable type according to the shape of piston to be cast and is preferably suitably vented. In the illustration of the drawings a piston mold is shown having a removable core 9. This core may be of the collapsible type or of any other construction suitable for the purpose. In the apparatus illustrated in the drawings a hydraulic cyl- .3

inder I0 is arranged for withdrawing the core 9 and other suitable hydraulic or air-controlled devices I I may be arranged in any desired manner to open the mold 6. These devices for operating the mold form no part of the present invention. A description of suitable piston molding apparatus applicable to the present invention may be found in Patent No. 1,952,201 issued March 27, 1934. The weight of the mold and its connected parts may be counterbalanced by a weight l2 hung on a cable l3, passing over a pulley l4 on an upright support l5 and connected to the frame I in any suitable manner.

The top of the nozzle 4 has a face, the outer portion of which is plane to match the face of the bottom of the mold 6 with which it engages. The central portion of the nozzle 4 may be provided with an upstanding tip 30 arranged to project into the gate of the mold so as to assist in transferring heat to the metal therein. An insulating plate or washer 3| of asbestos or other suitable material may be interposed between the nozzle 4 and the mold 6 so as to avoid unduly heating the mold, and to prevent leakage of metal.

In the operation of casting the empty mold with its core in place is pressed down upon the nozzle 4 by swinging the frame I downwardly upon its hinge. The air pressure is then applied to the connection 5 so as to apply pressure to the surface of the liquid metal in the retort 2, thus forcing the metal out of the nozzle 4 and into the mold 6. A moderate pressure is applied for this purpose sumcient to lift the metal to the top of a mold cavity so as to completely fill the same. The pressure is not so great as to cause the metal to flow into the mold at a high velocity. Accordingly, the metal rises in the mold cavity at a relatively slow rate so as to permit the air in the mold to escape by the vent openings. The velocity of inflow is, however, suflicient to insure that the mold cavity is completely filled before the metal sets. It will be noted that this procedure differs from so-called pressure casting, in which the metal is forced into the mold under a high pressure, in that the moderate pressure here employed need not greatly exceed that required to lift the metal into the mold. The gentle flow obtained is, therefore similar to the flow obtained in gravity pouring, but in the present method the metal enters the mold at the bottom instead of at the top.

In such castings which have portions heavier (i. e., portions of greater volume) than other portions thereof, the mold bavity is preferably so arranged that these heavier portions will be lowermost and that the gate communicates as directly as possible with these heavier portions. In the illustrations of Figures 2 and 3 the piston casting has its heavier portion, representing that portion in which the packing ring grooves are cut and the head portion l1, placed at the bottom. In the piston illustrated, there is also a fairly heavy rib l8 extending across the head.

' In such castings when made by previous methods, it has been found diflicult to avoid blowholes or porosity due to crystallization shrinkage in the portions l6, I1, and i8. As is well understood the avoidance of such defects is particularly important in this type of piston and especially in the head portions thereof.

In accordance with this invention the gate I9 is placed so as to enter the head portion II of the casting at substantially the middle thereof. The gate I9 is provided with a contracted neck 20 spaced at appropriate distance from the head l1. Extending in opposite directions from the gate l9 and along the direction of the wrist pin axis are lateral wing portions 2i which may be tapered toward the outer circumference of the piston head. \These wing portions may be relatively narrow and provide a direct path for the flow of metal to those sides of the piston where the wrist pin bearings are located. Accordingly, a rapid flow of metal to these portions is possible and a direct path for fiow to massive portions of the casting is provided. This prevents the metal from setting before it arrives at its proper place in the mold, and maintains the same in a sufficiently fluid condition to work into all parts of the mold cavity so that a perfect casting may be formed. Also these wing portions very materially assist in taking care of crystallization shrinkage in portions l6, l1 and i8 by feeding metal directly to these portions.

The contracted neck 20 provides a point in the path of flow where that path is most restricted. This point is spaced appropriately both from the body of the casting and from the spigot 4. It will be noted that the flow of molten metal to the casting is most rapid at this point. There is also a flow of heat through the metal from the heated spigot to the casting and this fiow of heat is also most concentrated at the neck 20. Accordingly the best conditions for fluidity of the metal flowing to the casting are found at the neck 20. It will be seen that the entire organization is such that the metal flows to the mold cavity in such a manner as to rise in said cavity in a symmetrical manner. Once the cavity is filled with metal, solidification begins. The parts which first solidify are necessarily those most distant from the spigot, that is the upper rim of the skirt. As the heat flows away from the casting at all points within the mold but additional heat flows to the casting from the spigot through the gate, it will be clear that the solidification takes place progressively from the top downward along the piston. Under these conditions the last portion to solidify is the piston head and as the flow of heat from the spigot continues the central portion of the head which is next to this source of heat will remain liquid longest. During this solidification there is always molten metal capable of flowing situated between the solid parts and the source of new metal. Accordingly any contraction involved in the solidification is supplied by new metal from the source so that faults due to crystallization shrinkage are entirely avoided. The solidification of the casting, therefore, continues progressively until finally the gate portion becomes solid. As the neck portion 20 has the greatest concentration of heat it will still be liquid when the rest of the gate solidifies. Accordingly when the mold is removed from the spigot this liquid portion returns to the spigot and the gate is terminated practically at the neck 20.

In order to insure free flowing of the metal and that it will remain in a sufllciently fluid condition until it arrives even at distant parts of the mold cavity the nozzle 4 may be separately heated by any suitable means, such as jet 22 arranged to project a flameagainst the nozzle. In accordance with this invention the nozzle is heated to such a temperature as to maintain the liquid metal therein at a temperature substantially above the normal melting point of the metal. For instance, if the normal melting point of the casting metal is 1100 F. the nozzle might be maintained at such a temperature as to keep the metal therein at about 1300 F. It has been found that by thus heating the molten metal just as it enters the mold it is maintained in a highly fluid condition until it has had a chance to enter all crevices of the mold cavity so as to form a perfect casting, after which it cools gradually, setting first at the extreme ends furthest from the gate and progressively thereafter along the piston, the head portions setting last. The insulating pad 3| insures that the high temperature of the nozzle 4 shall. not unduly heat the mold itself. Accordingly the absorption of heat from the casting by the mold is not interfered with.

At the same-time the tip 3|] of the nozzle extending into the gate is adapted to deliver heat to the metal therein from the highly heated nozzle, and thereby prevent premature setting of the lower portion of the casting. By maintaining the molten metal as it passes to the casting at a superheat temperature, as specified above, and maintaining continual communication between the main body of metal and that which has already entered the mold, it will be noted that a definite temperature gradient is established from the point of superheat to the metal in the mold. By maintaining a superheat temperature, a gradient of considerable slope is established and, accordingly, there will be a flow of heat along the flowing column of metal to the metal in the.mold. Such a flow of heat does more than compensate for loss of heat by the metal en route to the mold. It supplies an extra quantity of heat to the metal already in the mold. As a'consequence the metal may be flowed into the mold at a low rate in a gentle manner and will retain suflicientfluidity to enter all of the minute interstices of the mold without the necessity of applying a high pressure thereto. By thus flowing the metal in a quiet and gentle manner into the mold, excessive agitation is avoided and the chances of entrapping air in the flowing metal are reduced to a minimum. By heating the spigot to a temperature substantially above the freezing point of the metal, the temperature gradient, which would be insignificant under the conditions of ordinary practice, is stepped up at the point where the metal passes the spigot and, accordingly, a more rapid flow of heat to the metal in the mold is promoted. By this method, therefore, the metal may be maintained in a fluid condition until it reaches the farthest points of the mold cavity and thereafter the cooling of the casting takes place in a symmetrical manner from the farthest points toward the gate. As the heat flow along the incoming metal is maintained, the portion near the gate is the last to become solid and, as it is in continual communication with fresh metal, shrinkage is compensated for continually as it takes place by a flow of new metal to the casting.

It will be seen, therefore, that this invention provides a simple and eifective method and apparatus by which castings may be formed free of blow-holes, crystallization shrinkage and other contraction flaws. The method is easily carried out and is inexpensive to apply to practically any type of piston casting.

While a certain theory of action has been used in describing this method, such theory is advanced only for the purpose of explanation. It is understood, therefore, that the invention is not limited to any particular theory of operation. It is also understood that while both method and {apparatus have been described as a. unitary whole,

#bertain individual features or sub-combinations thereof, may be useful and the employment of such individual features and sub-combinations is contemplated by this invention and is within the scope of the appended claims.

As stated previously, this invention is applicable to the casting of light metals of which aluminum and its alloys are examples, other examples being magnesium and its alloys. It is, therefore. to be understood that the word aluminum, appearing in the claims, is used descriptively and not limitatively.

It is further obvious that various changes may be made, within the scope of the appended claims, in the details of construction and operation without departing from the spirit of this invention; it is to be understood, therefore, that this invention is not limited to the specific details shown and/or described.

Having thus described the invention what is claimed is:

1. Apparatus for casting trunk pistons provided with wrist-pin bearings, comprising, a mold having a piston-forming matrix arranged with the piston head lowermost, and having a gate entering the matrix at the middle of the piston head and provided with lateral wing pora tions extending in opposite directions across the piston head along the wrist pin axis.

2. Apparatus for casting trunk pistons, comprising, a mold having a piston-forming matrix arranged with the piston head lowermost, and hating a gate entering the matrix at the piston head, and means for heating the metal flowing through said gate substantially above the melting point of the metal, said means extending into said gate.

3. Apparatus for casting trunk pistons, comprising, a mold having a pistonrming matrix arranged with the piston head lowermost, and

having a gate entering the matrix at the piston head, a spigot for the molten metal leading to said gate, and means for heating said spigot substantially above the melting point of the metal, saitd spigot having a part extending into said ga e.

4. Apparatus for casting trunk pistons, comprising, a mold having a piston-forming matrix arranged with the piston head lowermost, and having a gate entering the matrix at the piston head, a spigot for the molten metal leading to said gate, means for heating said spigot substantially above the melting point of the metal, and heat insulation between said spigot and said mo d.

5. Apparatus for casting trunk pistons, comprising, a mold having a piston-forming matrix arranged with the piston head lowermost, and having a gate entering the matrix at the piston head, a spigot for the molten metal leading to said gate, means for heating said spigot substantially above the melting point of the metal, and heat insulation between said spigot and said 'mold, said spigot having a part extending into said gate.

6. The process of casting trunk pistons in permanent molds, comprising, with the piston head lowermost, flowing the molten metal from a source upwardly into the mold in a path symmetrical with refercnce to the piston axis, and n heating the inflowing metal substantially above its fusion temperature so as to produce an inflow of heat to the metal in the mold along with the inflowing metal such as to cause solidification thereof progressively downwardly and symmetrically with respect to said axis.

7. In the art of casting aluminum, the process comprising, maintaining a large body of molten metal at a temperature sufiiciently above the freezing point thereof to render the metal fluid, lifting the molten metal in a gentle flow upwardly from said body into a mold, and adding heat to the flowing metal after it leaves said body and before it enters the mold so as to maintain a suiiicient temperature gradient from the point of heat application in the flowing metal stream to the remotest molten metal in the mold to cause a progressive setting of the casting from the remotest part toward said point.

8. In the art of casting aluminum, the process comprising, maintaining a large body of molten metal at a temperature sufilciently above the freezing point thereof to render the metal fluid, lifting the molten metal in a gentle flow upwardly from said body into a mold, applying to said body of metal a moderate pressure limited so as not greatly to exceed that required to lift the metal into the mold and maintaining such pressure so limited until the casting sets, and adding heat to the flowing metal after it leaves said body and before it enters the mold so as to maintain a sufllcient temperature gradient from the point of heat application in the flowing metal stream to the remotest molten metal in the mold to cause a progressive setting of the casting from the remotest part toward said point.

JOHN FLALJIVIANG. PERCY L. BOWSER. HERMAN ERNST.

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