US3685569A

US3685569A - Method for gravity pressure permanent molding

Info

Publication number: US3685569A
Application number: US59473A
Authority: US
Inventors: Bryce A Denton
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1970-07-30
Filing date: 1970-07-30
Publication date: 1972-08-22
Anticipated expiration: 1989-08-22

Abstract

The invention is a method to reduce the porosity in gravity pressure permanent molding of metal objects. It involves gravity filling a mold cavity through an opening with a charge of liquid metal that has high solidification shrinkage characteristics. The metal is allowed to slush and then, after a predetermined time, a momentary pressure is applied to the slush by means of pistons operating into the mold while the fill opening is still mechanically unclosed causing the metal to fill voids caused by shrinking. The pistons are then removed preferably before opening the mold. Apparatus for timing the cutoff and piston movement is provided in a structure that is used for making flatiron soleplates.

Description

United States Patent [151 3,685,569 Denton [451 Aug. 22, 1972 [54] METHOD FOR GRAVITY PRESSURE PERMANENT MOLDING Primary Examiner-J. Spencer Overholser Assistant Examiner-John E. Roethel [72] Inventor Bryce Denmnomanocahf' Attorney-Lawrence R. Kempton, Leonard J. Platt, [73] Asslgnee: General Electric Company John F. Cullen, Frank L. Neuhauser and Joseph B. 22 Filed: July 30, 1910 [2|] Appl. No.: 59,473 [57] ABSTRACT The invention is a method to reduce the porosity in 52 us. (:1 ..l64/l20, 164/319 g y pressure permanent molding of metal j 511 Int. Cl. .3220 27/10 11 y q gravity filling m cavity through an 581 Field of Search ..l64/75, 100, 120, 319, 320, 1 3 2 Y a q s of q me that has g 164/321 sol1d1ficat1on shrinkage charactensucs. The metal 1s allowed to slush and then, after a predetermined time, [56] References Cited a momentary pressure is applied to the slush by means of pistons operating into the mold while the till open- UNITED STATES PATENTS ing is still mechanically unclosed causing the metal to fill voids caused by shrinking. The pistons are then g gi removed preferably before opening the mold. Ap- 3387646 6,1968 EE; 62/120 paratus for timing the cutoff and piston movement is 1 l t l t d t t th I d f ak' H t 3,344,848 10/1967 Hall et al. ..l64/3l2 55:23:2 a S we a use or m mg a 3,270,383 9/1966 Hall et al. ..l64/l20 3,106,002 10/1963 Bauer 164/120 2 Clains, 3 Drawing Figures LIQUID METAL Patented Aug. 22, 1972 uaum METAL Eyre 4 007;?

METHOD FOR GRAVITY PRESSURE PERMANENT MOLDING BACKGROUND OF THE INVENTION 1 Field of the Invention The invention pertains to a method for gravity pressure permanent molding of soleplates for electric flatirons to reduce the porosity that normally occurs in the heavier sections of the cast soleplate. This is achieved by using piston-applied pressure to the cast liquid metal in a particular timed sequence while the fill opening remains mechanically unclosed thus resulting in dense and uniform castings that are capable of a high lustre smooth finish.

2. Description of the Prior Art In electric flatirons, it is known to produce the soleplate by pouring or forcing a molten material such as iron or aluminum alloys into a mold or die cavity in which the temperature of the enclosure is below the freezing point of the liquid. Such cast objects are frequently found to have holes in the parent material and these are usually caused by entrained bubbles of air or gas picked up by the molten material during the pouring process. Additionally, gas dissolved in the mo]- ten material is precipitated as bubbles when the parent material freezes and gas is also found to emanate from mold coatings or foreign material on the mold surface or from inserts in the casting. Finally, in the case of metal having a relatively high solidification shrinkage characteristic such as aluminum, porosity occurs from the natural shrinkage of the material being cast as it changes from the liquid to the solid state. Such porous castings are referred to as unsound castings.

A well-known property of aluminum alloys is that upon passing from the molten to the solid state there is about a 7 percent reduction of volume. To compensate for this shrinkage, sand molds and permanent molds are usually provided with risers which are connected to the heavier sections of the casting in such a manner as to be filled with molten metal when the mold is filled. The object of the risers is to provide a supply of molten metal which will feed the casting to make up for shrinking as the metal freezes. The risers are seldom fully effective and they cause the mold to be complicated in design and are costly because they must be cut off or removed and the riser metal must be remelted. In addition, after the riser is cut from the casting the connecting area is often quite porous. By comparison, in the die casting process the outer skin of the part is usually sound because it is rapidly chilled by being in contact with the relatively cool die surface but the metal midway between the die surfaces is often porous, especially in the thick sections. This occurs in spite of the high pressures used to force the metal into the cavity because the pressure is no longer available to move metal into the cavity as soon as the thinnest part of the gate freezes. When the gate freezes it acts as a plug to prevent the flow of additional matal required to compensate for the shrinkage caused by freezing of the metal.

It is known to use the concept of plungers wherein the plungers are designed to eliminate the shrinkage by packing" the metal at the proper time. Generally this has involved the use of pistons either directly through the gate or, if not through the gate, then additional structure must be supplied to mechanically close the gate so that it is possible to apply relatively high pressure to the molten or partially solidified metal. This has required the use of rugged and expensive machinery because of the pressures involved in handling solidified or partly solidified metal with the result that the methods and apparatus available do not lend themselves to low cost gravity pressure permanent molding of cast objects which require a high lustre finish such as that in electric flatiron soleplates.

SUMMARY OF THE INVENTION Briefly described, the invention is directed to a method for making a dense casting of different thickness sections from a metal having relatively high solidification shrinkage which comprises the steps of gravity filling a mold cavity through an opening with a charge of liquid metal. The metal is allowed to slush with the fill opening mechanically unclosed except by the slush. While the metal is slushy, a movable piston means, separate from the filling means, applies momentary pressure to the slush by moving a predetermined distance into the mold at the thick sections of the casting while the fill opening remains mechanically unclosed to cause the metal to fill the voids caused by shrinking. Timed means are provided to actuate the movable means assoon as the metal becomes slushy which, in a typical iron soleplate casting, occurs between 10 to 40 seconds after the filling is cut off. The timed means operates the movable means into the mold for several seconds to exert momentary pressure and then withdraw so that no pressure is exerted after the metal solidifies. Thus, the main object is to provide a method that introduces the best features of gravity permanent molding with the best die casting techniques by using movable pistons in portions of the casting wherein pressure is exerted on that metal last to become slushy thereby densifying the metal to produce sound castings that are capable of being finished to a high lustre.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional view showing the general concept of the gravity permanent molding of the invention;

FIG. 2 is a schematic view of the apparatus used by the invention; and

FIG. 3 is a perspective view of a typical iron soleplate in its mold showing the location of the pistons and part of the mold in phantom.

DESCRIPTION OF THE PREFERRED EMBODIMENT While the invention is applicable to any cast metal, it is particularly applicable to aluminum alloys which have a high shrinkage solidification characteristic. Consequently, the invention will be described as applied to such alloy as used in electric flatiron soleplates but the invention is not so limited.

In order to visualize the shrinkage that occurs when aluminum alloys freeze, consider completely filling a mold cavity that is one-half inch in diameter with molten aluminum. If all of the shrinkage were confined along the center line when the mold solidified the shrinkage would form a hole more than one-eighth inch in diameter. Another characteristic of many aluminum alloys is that they do not suddenly pass from the liquid to the solid state. Many of the alloys, particularly those used in permanent molding, remain in a slush state for several seconds during which time the metal is sufficiently pliable that a hard object may be pushed into the casting by applying pressure.

Referring to FIG. 1, the invention overcomes the shrinkage problem in a mold by providing a movable means such as piston 12 fitted into a suitable hole 14 in the mold and fitted tightly enough to prevent leakage of molten metal when the mold is filled through opening 16. At a given time after the mold is filled the metal becomes slushy and the thinner sections freeze and become relatively hard while the thicker sections are still pliable. At this stage, pressure is applied to the outer end of piston 12 causing the piston to move inward displacing sufiicient material to compensate for the reduction in volume of the freezing metal. This described concept is generally known.

When the timing of the application of pressure by the piston is correct the metal adjacent to hole 14 is slushy and is shrinking so that when the piston starts to move inward a high force is required to pierce the shell of hardened metal which surrounds the slushy core. As soon as the shell is pierced very little additional force is required to move the piston because the piston is merely filling a space that otherwise would be spongy and largely made up of voids. Thus, with proper timing, this technique dispenses with any need for complex mechanical gate closures and related structure. Consequently, there is no need to close ofi the fill opening at gate 16 because, with this proper timing, the cast metal in the opening has solidified sufficiently to efi'ectively close it and the piston will not force material up into the gate. Areas of porosity are quite consistent in castings which are produced to identical configuration in high volume such as iron soleplates. Thus, the application of pressure at the same thick areas at the proper moment produces uniformly sound castings.

Referring next to FIG. 2, a schematic apparatus is illustrated which comprises a mold in two halves l8 and 20 between which is formed an internal cavity, as shown in FIG. 3, for producing an aluminum fiatiron soleplate 22. The soleplate is cast around heating element 24 of a known construction and being of the sheathed type. An internal electrical resistance element extends through the sheath with the heating element separated from the sheath by an insulating compound resistant to heat such as a granulated and compressed magnesium oxide. The heating element 24 generally extends in a loop beginning at the rear of the iron as shown and along one side to the forward or nose end and rearwardly along the other side. Thus, uniform heat distribution is provided when the iron is plugged in and activated. The soleplate has relatively thick and thin sections so that porosity is found in the thick sections. In order to produce such a soleplate, a supply container 26 is provided to hold the liquid metal and this is connected to the mold for gravity filling by means 28. The metal supply is terminated by a cut-off means 30, which may be in the form of a shutter, and which is suitably actuated to block filling means 28 at the proper time. In accordance with the invention, it is necessary to provide a movable means equivalent to piston 12 in FIG. 1 to enter the mold at the thick sections and this may take the form of piston means 32 which, in the case of a soleplate, comprises at least a pair of

separate pistons

34 and 36 operable on thick sections of the soleplate as shown in FIG. 3. Piston means 32 is connected to a suitable transmission 38 to be moved into the mold at the thick sections whereby

pistons

34 and 36 are moved a predetermined distance into and then out of the mold in a timed relation with respect to the cutoff of the metal from filling means 28. To this end, an interconnection 40 including a clock or triggering mechanism 42 connects the transmission with the cut-off means 30.

As previously noted, aluminum alloys of the type used in a soleplate in permanent molding remain in a slush state for several seconds during which time the metal is pliable.

l have found that by permitting the gate or fill opening 16 to remain open it will tend to freeze before the center sections or thick sections are out of the slushy state. This occurs rather rapidly when the cut-off means is operated to end the gravity supply of metal. This is, in effect, low pressure permanent molding. By tinting the pistons to apply pressure to the plastic metal and then removing the pressure after the metal solidifies and doing this after the open or unclosed gate solidifies, it is possible to compact the thick sections and eliminate the porosity normally inherent in freezing of the metal. It is important that the timing sequence be triggered so that the steps occur in the proper order if satisfactory castings are to be obtained. To this end, the cut-off means 30 is connected to operate trigger means 42 which, in turn, actuates the movable piston means 32 between 10 to 40 seconds after cut off occurs. The piston means are timed to move into the mold for several seconds and exert a pressure less than 2,000 psi and then withdraw from the mold. Thus, momentary pressure is applied to the slush at the thick sections while the gate opening is mechanically unclosed whereby the metal fills the porous voids and the pressure is then removed. No problems are encountered in closing off the mold because of the timed sequence whereby the open pressureless gate permits any expansion necessary. Thus, the molten metal is poured into the mold, the metal is allowed to become slush and the open gate to partially freeze whereupon the metal starts to shrink. At this point, makeup metal may feed in the gate to take up shrinkage in the interior. When the metal has partly frozen in the gate then the pistons are moved into the thick slushy sections and momentary pressure is applied. The pistons are preferably withdrawn to exert no more pressure alter the metal completely solidifies and before the mold is opened. It has been found unnecessary to exert over 2,000 psi for longer then 5 seconds to pierce the shell and then to achieve the dense and solid castings when this timed sequence is followed. The pressure applying piston movement may also be selected so that the final inward location of the piston faces before again moving outward will be such that the desired thickness of the molded part can be achieved by machining away the excess metal.

In a soleplate it is desirable to use a pair of separate pistons as shown in FIG. 3 directly on selected and different thick sections of the casting. it has also been found that it is desirable to operate the pistons on the casting side of the soleplate, i.e., the inner side as opposed to the finished bottom surface when the soleplate is polished to reduce any tendency to show by a different grain structure on the finished soleplate, where the pressure has been applied.

While there has been described a preferred form of the invention, obvious equivalent variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practised, otherwise than as specifically described, and the claims are intended cover such equivalent variations.

lclaim:

l. The method of making a dense casting of different thickness sections from a metal having a relatively high solidification shrinkage characteristic comprising the steps of;

gravity filling a mold cavity through an opening with a charge of liquid metal, allowing said metal to slush with said opening unclosed, applying momentary pressure to said slush at the thick sections of said casting while said opening is unclosed to cause said metal to fill voids caused by shrinking in said sections, said pressure being applied for up to 5 seconds between 10 to 40 seconds after said metal is poured, and then removing said pressure before said mold is opened. 2. The method described in claim 1 wherein the metal is a flatiron aluminum alloy soleplate, and

said pressure is applied at plural locations on the casting side of said soleplate.

I I I l

Claims

1. The method of making a dense casting of different thickness sections from a metal having a relatively high solidification shrinkage characteristic comprising the steps of; gravity filling a mold cavity through an opening with a charge of liquid metal, allowing said metal to slush with said opening unclosed, applying momentary pressure to said slush at the thick sections of said casting while said opening is unclosed to cause said metal to fill voids caused by shrinking in said sections, said pressure being applied for up to 5 seconds between 10 to 40 seconds after said metal is poured, and then removing said Pressure before said mold is opened.

2. The method described in claim 1 wherein the metal is a flatiron aluminum alloy soleplate, and said pressure is applied at plural locations on the casting side of said soleplate.