US3627019A

US3627019A - Method of casting a continuous series of slugs

Info

Publication number: US3627019A
Application number: US764905A
Authority: US
Inventors: Robert Edward Hitchcock; Alan Younger
Original assignee: R T Z METALS Ltd
Current assignee: R T Z METALS Ltd
Priority date: 1967-10-06
Filing date: 1968-10-03
Publication date: 1971-12-14
Anticipated expiration: 1988-12-14
Also published as: NL6814287A; FR1589521A; BE721878A; DE1801659A1

Abstract

Making castings, such as slugs, in low and medium melting point metals and alloys, by vacuum casting, wherein the metal is introduced through an entry port in a bottom plate into a mold cavity defined by a mold plate, the bottom plate and a top plate, the top plate being so constructed as to allow evacuation of the mold cavity, the metal being caused to solidify progressively towards the entry port in the bottom plate, characterized in that after solidification the mold plate is moved transversely to shear the casting from any solidified metal in the entry port without any movement of the bottom plate.

Description

Inventors Appl. No.

Filed Patented Assignee Priorities Robert Edward Hitchcock Harttordshire;

Alan Younger, Buckinghamshire, both of England Oct. 3, 1968 Dec. 14, 1971 R. T. Z. Metals Limited London, England Oct. 6, 1967 Great Britain Oct. 6, 1967, Great Britain, No. 45,908/67;

Oct, 6, 1967, Great Britain, No. 45,909/67; Oct. 6, 1967, Great Britain, No. 45,910/67 METHOD OF CASTING A CONTINUOUS SERIES [56] References Cited UNITED STATES PATENTS 2,708,298 5/1955 Beckes 164/323 X 2,863,188 12/1958 Harrison 164/257 2,923,040 2/1960 Goodwin et all 164/154 X FOREIGN PATENTS 512,254 7/1952 Belgium 164/127 615,484 1/1949 Great Britain 164/63 Primary Exuminer-R. Spencer Annear A!t0rney Holman & Stern ABSTRACT: Making castings, such as slugs, in low and medium melting point metals and alloys, by vacuum casting, wherein the metal is introduced through an entry port in a bottom plate into a mold cavity defined! by a mold plate, the bottom plate and a top plate, the top plate being so constructed as to allow evacuation of the mold cavity, the metal being caused to solidify progressively towards the entry port in the bottom plate, characterized in that after solidification the mold plate is moved transversely to shear the casting from any solidified metal in the entry port without any movement of the bottom plate.

Patented Dec. 14, 1911 3,627,019

FIG]. I

Patented Dec. 14, 1971 3,627,019

7 Sheets-Shoot 2 FIG 2 Patentea Dec. 14, 1971 7 Sheets-Shoot 3 L an f H Wm M a V0 7 Mu .7 /W A w y m a Patented Dec. 14, 1971 7 Sheets-Sheet 4 Patented Dec. 14, 1971 3,627,019

'7 Sheets-Sheet 5 Patented Dec. 14, 1971 3,627,019

7 Sheets-Sheet 6 33 674 bl /1 //////,v r///////4/// FIG.9.

73 76 b4 bl Halo.

Patented Dec. 14, 1971 7 Sheets-Sheet 7 MlE'llllllUD 01F (IASTHNG A CONTIINIUQlUS SERIES OF SLUGS This invention relates to a process and apparatus for the production of castings in low and medium melting point metals and alloys such as aluminum, magnesium, zinc, lead, copper and their alloys. The castings may be utilized in the ascast condition or they are suitable for further fabrication by wrought processes, especially by extrusion, coining, forging and particularly impact extrusion for the production of containers.

it is to be understood that the term vacuum casting is intended to comprehend the equivalent differential pressure technique.

The castings are typically of such shape and dimensions as to be aptly termed slugs.

The term "slugs" is used in this specification to denote a cylindrical or polygonal shape wherein the diameter of the cylinder, or the diameter connecting the apexes of the polygon, is at least equal to the thickness of the said slugs. This however is not to be construed as a limitation on the size and shapeof casting produced within the scope of the present invention. A preferred size range is comprehended as approximately to 4 inches in diameter.

An important aspect of the manufacture of metal slugs which are characteristically relatively small is that they should be produced very rapidly. The conventional process for the production of metal slugs has been to punch the slugs out of rolled sheet or strip. In that process a principal disadvantage was that in the blanking operation, even when blanking hexagonal rather than circular section slugs, a residual web of metal was created, the slugs having been punched from the holes therein. The web typically constituted from 30 percent to 55 percent of the original rolled sheet or strip and the process was thus very wasteful. Further, at least two disadvantages existed in the product manufactured by the conventional means.

The first of these is that in the rolling operation of the sheet or strip a preferred grain orientation was created in the rolling direction; this in turn produced earing" on a container impact extruded from a slug blanked out of such a sheet or strip. The term earing" is well known to those familiar with the art and is understood to refer to unequal height of the wall of the container, the variation being ofa symmetrical form and being due to preferential metal flow during impact extrusion. A similar phenomenon is found in cupping rolled products. The resultant earing has to be trimmed from the top of the container thus increasing the amount of trim material; this again is wasteful.

The second ofthe principal disadvantages has been found in slugs blanked from rolled sheet or strip in that the blanking operation leaves a burr on one side of the slug. This burr has to be substantially removed prior to impact extrusion if a good surface finish is to be achieved. Further, owing to the burr it is highly desirable to orientate the slug for impact extrusion such that the buried edge is towards the punch.

Other developments have occurred in the past wherein a machine casts the slug directly, no rolling or blanking being undertaken. To gain high production rates in the manufacture of these relatively small castings by such a technique, for instance pressure die casting, it has been conventional to cast simultaneously a large number of units in a single mold and subsequently to sever eachcasting from the common runner or gate and to trim off any risers. Even where single castings have been made it has still been necessary to sever the casting from the gate and riser. Further, die castings are prone to the inclusion of minute voids which are highly undesirable where such castings are to be subsequently worked by impact extrusion or other similar precision processes.

Castings made by pressure die casting techniques have not found wide acceptance as impact extrusion slugs due to the aforementioned problems. Where the height or the thickness of the slug is substantial, past practice has been to cut the slug from rolled, continuously cast or extruded rod.

Various proposals have been. made for the production of such slugs by vacuum casting methods, but hitherto, none has been commercially successful.

One machine embodying vacuum filling which has found limited commercial use for production of castings of substantial height was found to be uneconomic when modified to produce slugs due to the very substantially reduced rate of output in terms of metal throughput. In this machine the casting head operated within, but separate from a pan unit. The casting head and pan, in close contact, were immersed into the liquid metal causing the molten metal to flow upwards into the mold cavity due to differential pressure. This upward movement was assisted by the application of a partial vacuum, through narrow passageways, to the mold cavity. After solidification of the metal in the mold cavity the two members were withdrawn and the gate metal was sheared by a relative rotary movement between the members, the cast slugs finally being ejected.

in another machine embodying the same general feature of vacuum filling of a mold cavity situated above the liquid metal, the mold was not immersed relative to the surface of the liquid metal but relied wholly on the application of vacuum to fill the mold cavity. After casting the head moved vertically from a fixed base which was permanently immersed in the liquid metal. Such vertical movement ensured that the solidified metal in the gate remained attached to the underside of the slug and thus left the gate clear from the next cycle. This solidified gate metal had to be removed from the underside of the slug as a secondary operation. It was suggested however that the process could be controlled in such a manner as to operate the vertical movement of casting head at the precise time that the solidification front was flush with the gate orifice. Such precision in timing is unlikely to be achieved in industrial practice.

The present invention consists in a method of making castings in low and medium melting point metals or alloys, by vacuum casting, wherein the metal is introduced through an entry port in a bottom plate into a mold cavity defined by a movable mold plate, the bottom plate and a top plate, the top plate being so constructed as to allow evacuation of the mold cavity, the metal being caused to solidify progressively towards the entry port in the bottom plate and after solidification the mold plate being moved transversely to shear the casting from any solidified metal in the entry port without any movement of the bottom plate.

Preferably, the mold plate is moved by the introduction of a further mold plate into the position between the top plate and the bottom plate.

The invention further consists in apparatus for vacuum casting low and medium melting point metals and alloys comprising means for holding a body of molten metal, a top plate ineluding cooling means, a bottom plate with an entry port, a movable mold plate adapted to cooperate with the bottom plate and the top plate to form a mold cavity, and means for moving the mold plate from between the bottom plate and top plate to shear the solidified casting from the solidified metal in the entry port without any movement of the bottom plate.

Preferably the apparatus includes a plurality of mold plates which are movable in sequence between the casting head and the bottom plate to displace a preceding mold plate.

The mold plates may be guided in tracks to pass along a closed path.

Where the output of one casting head is inadequate, one or more extra casting heads and their bottom plates may be fitted at other locations in the track followed by the mold plates, thereby achieving an improved utilization of mold plates.

The casting molds are easily and rapidly changed. This is of importance if the process is to be commercially acceptable. The mold plates may be of simple and inexpensive form and may have one or more cavities.

During casting, the casting head, mold plate and bottom plate are held in contact but at completion of solidification of the casting a small separation may be achieved between the casting head and mold plate thus relieving the contact pressure between mold plate and the bottom plate. The mold plate is moved substantially transversely thus shearing off the solidified metal in the entry port within the bottom plate. No transverse movement is necessary in either the casting head or the bottom plate. The casting, retained within the mold plate is ejected when the mold plate is clear of the casting head. After clearing of the solidified metal in the entry port, the casting cycle may be recommenced.

It will be appreciated that the use of a multiplicity of small mold plates means that each mold plate itself is not subject to undue warping by uneven heating.

One feature common to prior machines was that the bottom plate through which molten metal was fed to the mold cavity was heated by contact with the liquid metal, this contact being continuous in some designs and intermittent in others. This construction necessitated the use of a heat-conducting material not corroded by contact with theliquid metal. A suitable material of construction was said to be cast iron. When casting aluminum however, the temperature level and distribution in the baseplate due to heat conduction was inadequate unless the plate was thin, in which case it die not have a long life. Further, in the shearing process to pan the casting from solidified metal in the gate of the baseplate, wearing of the gate orifice was a significant problem.

According to a preferred form of the invention, therefore, the lower surface of the bottom plate is adapted to be spaced from the body of molten metal at all stages of the casting cycle and means are provided for heating the bottom plate in addition to any heating effect arising from approximity of the body of molten metal.

The bottom plate may be supported above a metal feedpipe and has a ceramic or other substantially chemically inert material conduit to provide communication between the body of molten metal and the entry port in the bottom plate.

Alternatively, the bottom plate may be situated above a feedpipe for the liquid metal, but may be supported by independent means and the space between the lower surface of the said plate and the upper surface of the said feedpipe is sealed with a flexible sealing material which will withstand elevated temperature. A suitable material is asbestos cord.

The distance between the level of liquid metal in the feedpipe and the upper surface of the bottom plate is kept to a minimum.

The bottom plate may be formed as a hollow element and hot fluids are circulated within it to provide heating. Alternatively, an electrical heating element may be incorporated.

In either case the heating effect may be closely controlled and is virtually independent of the level of the molten metal.

The bottom plate is so situated that it forms a lower boundary to the casting mold cavity. The bottom plate is used as has been previously proposed, to restrain solidified metal in the entry port while the casting is sheared from it. It is to be appreciated that this arrangement allows the bottom plate to have a temperature very closely approaching, equal to, or even in excess of the melting temperature of the metal being cast. Further, this may be achieved without substantial removal of heat from the liquid metal. The liquid metal is in direct contact only with the entry port in the bottom plate and then only during casting thus reducing susceptibility to contamination. When not casting, the height of liquid metal is so adjusted to ensure that it is retained below the lower surface of the bottom plate.

Earlier work has demonstrated the desirability of having a hot bottom plate in order to ensure satisfactory metal flow within the mold cavity and further, cooling of the mold has to be incorporated in such a manner as to ensure that solidification commences at the side and outer upper surface of the mold and progresses towards the entry port this being the last zone to solidify. This particular arrangement fulfills these requirements and further allows better control of temperature distribution on the lower surface of the mold cavity. In particular, it permits the introduction of greater heat at the entry port. This in turn means that optionally, the apparatus may be operated in such a manner as to effect remelting of the solidified metal left in the gate after the shearing of the slug or casting.

A further advantage of the arrangement is that the assembly including the bottom plate is easily replaced in the event of failure. This, together with its general construction permits a significant improvement in machine utilization and a signifi' cant reduction in the total cycle time for casting as compared with existing apparatuses, while retaining entry port clearing as a desirable part of the total cycle.

In the direct casting of slugs it is important that the temperature of the molten metal and the rate at which it is supplied should be accurately controlled, and also that contamination of the molten metal from contact with the feed system and casting apparatus should be minimized.

Prior art feed systems rely on an open bath of metal which exposed supported members to heat and corrosion.

Accordingly, the apparatus may include an arrangement for feeding molten metal comprising a pipe or tube mounted essentially horizontally and connectable with a source of molten 'metal, such as a melting furnace or a holding reservoir, the said pipe or tube having outlet openings in its upper surface and at least one substantially vertical feed pipe dipping into the metal in the said horizontal tube through these outlet openings, the level of molten metal being controlled so as not to overflow from the horizontal pipe and the vertical feed pipes not being in physical contact with the horizontal pipe.

When there is no applied pressure differential the level within the pipe or tube is controlled to a position essentially below the upper outside surface and preferably above the upper inside surface of the horizontal pipe or tube. Further, in order to facilitate control of the metal level, the upper surface of the horizontal pipe or tube may be increased in thickness at the outlet openings. During casting, which is achieved by withdrawal of metal through outlet openings, it will be necessary to add further metal to the furnace in order to maintain the level of molten metal in the horizontal tube.

The horizontal pipe, its attachment and joints therein must be constructed of a material which is substantially inert under conditions of constant contact with the molten metals at their respective casting temperature. The material of pipe work construction differs with different liquid metals, but for magnesium, zinc and aluminum and most of their alloys a suitable material is silicon carbide or more preferably silicon nitride. This last material is attacked to a negligible extent and shows other desirable characteristics being highly resistant to thermal shock, suitable for fabrication and joining into the shapes envisaged and having the desirable thermal insulation characteristics.

Preferably, heating means are incorporated around the horizontal pipe or tube for the two reasons that liquid metal within the pipe must be maintained at or near to that optimum temperature required for casting and that for recommencement of casting operations after a prolonged stop it is necessary to melt the metal which has been permitted to freeze in situ. lt is recommended that oxide build up will be minimized by allowing metal to solidify in the pipework when the pipework is taken out of use for long periods.

Alternatively, means may be included for draining the system in which case the external heating is required only for temperature control of the liquid metal. At commencement of operations on a new unit, it is envisaged as desirable to sweep the unit with a nonoxidizing gas which does not readily combine with the liquid metal to be introduced, and subsequently to retain the said atmosphere until the unit is full of liquid metal. Where the metal is aluminum a suitable gas is nitrogen or preferably argon.

The external heating means may be of any form suitable for such heating. For instance, the heating may be by immersion in a liquid, by combustion of gas or oil or by electricity and in the last case by resistance, induction or other means.

In the last two cases, a suitable heat-insulating material will be incorporated around the horizontal pipe to utilize the heating element efficiently and to reduce heat loss from the pipework and particularly from the liquid metal. The external heating means might, where suitable, be the sole source of heat and then be used to melt metal as and when required during casting operations.

The advantages of the feeding arrangement are principally as follows:

I. The use of a horizontal feedpipe as opposed to an open reservoir allows the lower surface of the bottom plate of the vacuum mold cavity to be easily and readily supported without the supporting members being subjected to heat or corrosion clue to the proximity of the molten metal.

2. A minimum physical separation may be readily achieved between the lower surface of the bottom plate and the surface of the liquid metal by incorporating external support members as in l above.

3. By having only a small annular area of molten metal in the outlet opening of the horizontal feedpipe exposed to the atmosphere, oxidation of the molten metal in the zone from which it is drawn for casting is reduced to a minimum.

4. As the volume of metal in the horizontal feedpipe is relatively small as compared with that in a holding furnace, precise temperature control of the contained metal by external heating is readily achieved.

In vacuum casting processes, it is necessary for the mold cavity to be open to the source of the lower pressure through a gap which is sufficiently narrow for flow of molten metal therethrough to be prevented. When casting aluminum, a gap of about 0.004 inch maximum width has been found suitable.

In order to achieve close control of the gap, the top plates may have a lower face forming part of the mold cavity and having a peripheral zone surrounding he cavity, adapted to be slightly spaced from the upper surface of the movable mold plate to form a gap and an adjustable annular portion surrounding the top plate and spaced therefrom to form a vacuum chamber when the mold plate is in position, the said annular portion having a lower face in a plane slightly below the plane of the lower face of the top plate to bear on the upper surface of the mold plate to seal the vacuum chamber while accurately controlling the spacing of the lower surface of the top plate and the upper surface of the movable mold plate.

Preferably, the annular portion is secured to a casting head allowing the interposition of one or more shims so that the spacing of the top plate from the mold plate may be controlled by suitable choice of shim thickness.

The annular portion may be initially secured to the casting ahead without the interposition of shims and the lower surface of the annular portion and the outer edge of the peripheral zone of the top plate ground to coplanarity, dismantled and reassembled with the shims to achieve the required spacing.

The invention will be further described with reference to the accompanying drawings, wherein a preferred form of ap paratus according to the invention, given by way of example only, and not by way of limitation, is shown.

In the drawings:

FIG. I is an isometric schematic view of the apparatus;

FIG. 2 is a plan view of the general layout of machine and mold track assembly;

FIG. 3 is a section on the line A-A ofFIG. 2;

FIG. 4 is a partial cross section front elevation of FIG. 2 showing the machine and mold track with the metal feed system and casting head support in cross section;

FIG. 5 is an end elevation view of FIG. 2;

FIG. 6 is a schematic plan view of the activating cylinders;

FIG. 7 is a sectional view ofa modified form of mold plate;

FIGS. 8, 9 and 10 are each partial sectional views of different forms of embodiment of bottom plate assembly for use according to the invention;

FIG. II shows a sectional elevation ofone preferred form of feedpipe for use with the apparatus according to the present invention;

FIG. 12 is a side elevation of the pipe shown in FIG. 111;

FIG. 13 is a plan view of a casting head assembly according to a preferred form of the invention; and

FIG. M is a section on the line A-A also a mold plate.

The mold plates 1, are shown retained in a rectangular track which is created by mold track guides 2: these are attached to a track support plate 45, as shown in FIG. d. In the operating condition two mold plates positions 3 are vacant in order to permit movement of the mold plates ll around the track. The mold plates l are pushed around the track by four pusher plates 4 (omitted from FIG. I The casting head 5 is mounted above the mold plate track and moves in a vertical direction only. The casting head 5 is restrained in the transverse direction during shearing by a head steady 5a (FIG. 2). The casting head 5 is raised and lowered by a rocker assembly, driven by cylinder A, see FIG. 6, comprising a rocker arm 6 mounted at a fulcrum 7 and received in fork ends 5, one at each extremity, one for attachment to the casting head assembly and the other to the casting head cylinder A. A slug ejector rod 9 (see FIG. l) is mounted above one mold plate, preferably in a transfer track between the mold plate line under the casting head 5 and the line parallel to it. FIG. d shows the casting head 5 and the line parallel to it. FIG. 4 shows the casting head 5 mounted on the stripper bolts 10 (see also FIGS. 13 and M) which are countersunk into a stripper bolt plate 111, the bolts being retained by a stripper bolt retaining plate H2. The plate 112 is also the sprue clearing cylinder back plate and is bolted to the stripper bolt plate llll by bolts 43. The stripper bolts 10 run in bearings 113, mounted in a head support bracket M. The head support bracket M is attached to a vertical channel support 115. The sprue clearing cylinder 16 passes through the head support bracket M and the drive rod to the cylinder 16, is attached to the sprue clearing pin 17 by a sprue drive connector ill. The casting head 5 abuts onto the mold plate 1 below it which in turn is positioned over a bottom plate assembly 115 (see FIG; 5). The bottom plate assembly 118 is retained in a bottom plate retaining ring Ml (see FIG. 4), attached to the underside of the track support plate 45. The bottom plate assembly 13 is thermally insulated with mica 419, from both the bottom plate retaining ring 441 and the track support plate t5 through which it passes. The dimensions of these parts are such that the upper surface of the bottom plate stands slightly proud of the upper surface of the track support plate 45, the edge of the bottom plate being chamfered.

The bottom plate assembly will be further described in detail hereinafter with reference to FIGS. 8 to 10, but FIG. 4 shows diagrammatically that it includes an upper 19 and a lower 20 bottom plate formed to create an annular cavity 211, which carries entry and exit ports 22 for passage of the heated fluids. The third part of the bottom plate assembly is a bottom plate pipe 23 of which the lower end protrudes below the surface ofliquid aluminum 24} contained in a horizontal feed pipe 25. The upper end of the pipe 23 together with parts of upper bottom plate 19, create the metal entry port A7. A gap 26 exists between the bottom plate pipe 23 and horizontal feedpipe 25 in order to reduce the susceptibility to breakage of the vertical feedpipe 23, this preferably being manufactured in ceramic, for instance silicon nitride.

The mold plate thickness governs the thickness of the finished casting. However to avoid a multiplicity of mold track guides 2 the edges of all mold plates l are manufactured to a standard thickness thus allowing rapid. change of slug dimensions, both diameter and thickness simply by changing the suite of mold plates. Indeed it is possible to have a variety of mold plates operating at any one time.

It is to be appreciated that by creating a slight curvature onto the upper surface of the baseplate l8 and the lower surface of the top or central anvil 311, domed" slugs may be cast directly. Further, almost any shape of casting which may be ejected without essentially using a split die, may be cast on the machine as described, it being necessary only to create a cavity of the desired shape in the areas defining the mold cavity.

of FIG. 13 showing ln operation the sequence of activities is interlinked. Seven separate moving items are involvedsee FlG. 6and are as follows:

Cylinder A to lift and lower casting head Cylinder B, to move mold plate line under casting head and shear casting Cylinder B, to move mold plute line parallel to those of B,

Cylinder C transfer of mold plates from track 8, to track B Cylinder C, transfer of mold plates from B, to track B,

Cylinder D sprue clearing pin drive and return slug ejector.

Cylinder E It will be appreciated that only a very small movement is I required by cylinder A to relieve the sealing" pressure between casting head, mold plate and bottom plate.

To commence a casting cycle, the casting head is lowered and the vacuum applied: after a set time the following actions occur simultaneously.

Head raised (A) Vacuum off (optionally may be left on) 8, forward immediate return ti, forward immediate return E forward immediate return.

On completion of movements 8,, B and E, the second group of movements is triggered preferably by an interlink and thus automatically.

C forward immediate return C, forward immediate return D forward immediate return A lowered this trips vacuum on Cylinder B, is of greater diameter than B C, or C as the movement of B severs the casting from the solidified metal in the metal entry port. This solidified metal is then returned into the liquid metal by the action of the sprue clearing pin. The machine is now ready to recycle.

FIG. 7 shows a form of mold plate In which has a vacuum groove 51 in the lower surface connected by holes 52 to the porated in the base plate: these are not located centrally as in the solid slug casting machine but feed metal outside a central core directly into the annular mold cavity. The central core replaces the sprue clearing pin the pin being located over the eccentrically placed metal entry ports. In operation conventional flat mold plates are used. After casting the central core is retracted before operation of cylinder 8,. An accurate locating mechanism is incorporated between the mold plates and casting machine in order to ensure concentricity of hole within the casting.

In the second embodiment ofa machine for direct casting of hollow slugs, a core is attached to each mold plate by a bridge of metal above the plate. The bridge of metal conforms closely in shape with a groove cut into the under side of the central anvil-the close conformity essentially existing but not necessarily elsewhere in the groove. This approach ensures concentricity of hole within the slag.

FIG. 8 shows an upper bottom plate portion 61 which is attached to a lower bottom plate portion 62 in the region of a gate 63 which includes an entry port 64. A further contact between upper and lower

bottom plate portions

61 and 62 is at the peripheral ring 65 of the unit. Such a construction creates a chamber 66 through which it is possible to pass a heated fluid via suitable entry and exit connections. This fluid may be passed through the chamber under a pressure or suction. A suitable fluid is any fluid either liquid or gaseous which will not corrode the material of construction of the bottom plate assembly this material being selected by consideration of the metal to be case rather than the heating fluid. For example the heating fluid could itself be a liquid metal, for instance molten lead, or a preheated gas. A preferred fluid is hot combustion gases. The said gases may be only partially burnt, before admission to the chamber 66 allowing further combustion to take place within the said chamber. For this purpose, air ports 67 may be provided into the chamber 66 or to the fluid entry and exit ports. The chamber exhaust gases may optionally be used to preheat incoming gases.

The combustion gases may be drawn through the chamber 66 by means of a passage of fluid through a high-velocity nozzle on the exit side of the chamber 66.

The lower bottom plate portion 62 is supported on an independent support means 68, but it is sealed to a ceramic feed pipe 69, of the metal by a flexible sealing ring 70, e.g. of asbestos cord. It is to be appreciated that the degree of vacuum to be applied to the mold cavity above the bottom plate unit is not high, being typically 1-4 inches Hg when casting aluminum. Thus the requirement for sealing by the sealing ring 70 is not very arduous. The independent support means 68 is thermally insulated from the bottom plate assembly by an insulating lining 680, a preferred material for such a lining being mica.

The design of the chamber 66 and the flow of fluid through it, is such as to create a preferred temperature distribution in that the zone around point 71 is equal in temperature, or hotter than that around point 72. During casting, liquid metal is drawn into the mold cavity and subsequently solidified. The solidification front will progress down the mold cavity of which the upper bottom plate 61 forms the lower boundary and will ultimately pass into the entry port 64 and when it has done so, a shear movement between mold cavity and the oriflce will cause a substantially clean parting between the solidified metal in the orifice and the casting. The temperature of the base plate at 71 may optionally be then raised so as to cause remelting of the solidified metal in the orifice 64, there being then no necessity for further clearing of the gate 63 by other external means such as a sprue clearing pin.

FIG. 9 shows a form of bottom plate which is heated electrically, and which consists of essentially the same construction as that in FIG. 8. In this embodiment there is an upper bottom plate portion 61 which is attached directly to the lower bottom plate portion 62. An electrical heating element 73 is sandwiched between

plate portions

61 and 62 from which it is suitably electrically insulated. The lower plate portion 62 may optionally be omitted in which case the heating element 73 is situated between the upper surface of the metal feed pipe 69 and the lower surface of the upper bottom plate portion 61. The flexible sealing ring 70 is incorporated in the same manner as described above. The upper plate portion 61 incorpoi-ates an orifice 64. The heating element 73 is so constructed as to give the desired temperature distribution within the upper plate portion 61. The preferred temperature distribution has a temperature gradient from the center to the periphery of the plate, the center being equal to or hotter than the periphery and being optionally capable of attaining a temperature in excess of the melting temperature of the metal being cast for sprue clearing purposes.

FIG. 9 shows a form of baseplate similar to that of FIG. 8, but with the sealing by ring 70 replaced by a ceramic or other inert material vertical feed pipe 74 extending from the lower bottom plate portion 62 and below the surface 75 of the body of molten metal. The pipe 74 is threaded into the lower base plate portion 62 by threads coarse enough to accommodate thermal movements, and the end of the pipe 74 is sealed against the base plate portion 62 by a ring 76, e.g. of asbestos.

The pipe "M should be of such a diameter that there is no physical contact between it and the orifice in the pipe 69 in which it is placed. This is not essential but desirable in order to avoid fracture of pipe '74 due to vibration and physical movement of the bottom plate during casting and particularly during the shearing operation.

It will be understood that this form of inert insulating, preferably ceramic, vertical feed pipe may be also used in conjunction with the arrangement of FIG. 8.

It is further to be understood that the embodiments herein described are specific examples of the invention which shall not be construed as limiting the scope of the invention, such modifications being envisaged as including a nonplanar configuration to the lower end of the gate 63. This configuration may be reentrant or protruding, in either case being so constructed as to desirably ensure that the gate 63 is not in permanent contact with the liquid metal, to reduce risk of liquid metal contamination and heat extraction from the melt. In no embodiment is permanent contact between the gate 63 and liquid metal a limiting factor.

FIGS. 11 and 12 show a preferred form of metal feeding arrangement, FIG. 11 shows a horizontally disposed pipe 81 into the upper surface of which has been cut an exit orifice 82, which is positioned in a thickened part 83 of the pipe.

Induction or resistance heating coils 84, substantially surround the pipe along its entire length except at the exit orifices. No extra heating is deemed necessary at the exit orifices in this embodiment of the invention. Insulating material 35 surrounds the heating coils.

In FIG. 12, a side elevation view of the embodiment in FIG. 11 shows the positioning of a number of exit orifices 82. Further, it can be seen that the pipe 31 does not have a thickened upper surface along its entire length, but only around the orifices 82, thus allowing the heating coils 84 to encircle the pipe 81 along a cylindrical zone 89. The pipe 81 is attached to the liquid metal feed system by a compression clamp.

FIG. 13 is a plan view of the casting head assembly 3 and stripper bolts previously described. The casting head is cooled by the passage of a cooling fluid via a coolant entry part 27 and a coolant exit port 28. A blockage 29 in the cooling ring 30 ensures efficient flow of the coolant. Vacuum ports 42, see FIG. Ml, are positioned along line XY.

FIG. M shows a detailed cross section view of the casting head assembly. The casting head 5 is made up of three parts, a top plate or central anvil 31, a main body 32, which is shrunk and brazed onto the central anvil 31, and an outer ring 33 which is bolted to the main body at four points by bolts 34. The construction of the central anvil 31 and main body 32 is such that the cooling ring 30 is created between them. A sprue clearing pin 17 slides in a vented cylinder 35 but has a close fitting tolerance where the lower portion 36 ofthe pin slides in the central anvil 31. The sprue clearing pin 17 is retained in the central anvil by a sprue pin adjuster 37. The sprue pin adjuster 37 is threaded internally onto the central anvil 31 and butts against the upper surface of the main body 32 but is clear of the central anvil 31 at point 48. Sprue shims 33 are inserted such that, at equilibrium operating temperatures, the lower end of the sprue clearing pin 17 is flush with the lower surface of the central anvil 31. The lower surface of the outer ring 33 is constructed flush with the lower surface of the central anvil 31. Differential height between the lower surfaces of the central anvil 31 and the outer ring 33 is achieved by inserting vacuum shims 39 between the upper surface of the outer ring 33 and the adjacent surface of the main body 32.

Owing to the insertion of the vacuum shims 39, a vacuum gap 40 is created around the upper periphery of the casting mold cavity 46 when the casting head 5 is lowered onto the mold plate 1. An annular vacuum chamber 41 is created between the outer ring 33 and central anvil 31 and one or more vacuum ports 42 run directly into the vacuum chamber 41 through the wall of the outer ring 33.

SIIL! The use of a sprue clearing pin 1 which also defines part of the lower surface of the top plate leads in some circumstances to difficultles in matching the varied heat conduction requirements of the pin. During casting, to initiate solidification, it must be able to cool the metal in the mold cavity, whereas during sprue clearing it should not cool the only partially solidified metal in the sprue or there will be a tendency for sprue metal to adhere to the pin. This has led to a modification of the arrangement in which the top plate assembly does not incorporate the sprue pin 17, but is moved away during the shearing action to allow a sprue pin located above the top plate assembly to be advanced to clear the sprue. After clearance, the pin is retracted and the top plate assembly returned to its casting position.

It is to be appreciated that by creating a slight curvature onto the lower surface defining the mold cavity as and the lower surface of the central anvil 31, "domed" slugs may be cast directly. Further, almost any shape of casting which may be ejected without essentially using a split die, may be cast on the machine as described, it being necessary only to create a cavity of the desired shape in the areas defining the mold cavity.

It will be appreciated that the metal of the mold plate will normally be hard, and that the wear involved in the continual moving of the mold plate relative to the casting head can be taken on the ring 33, which is comparatively easily replaced, and also adjusted to compensation for wear by replacing the shims 39 with thicker ones as necessary.

In casting aluminum slugs, it has been found that a casting rate of per minute for slugs of l-inch diameter and 0.156- inch thickness can be achieved using a vacuum gap of 0.004 inch, a melt temperature of the aluminum of 710 C. and a vacuum level of between 24 and 38 inches water gauge. The casting time in respect of each cycle is between 0.3 and 0.4 seconds, which leaves a dead time in each cycle of about 0.35 seconds. It is believed that this dead cycle time could be reduced from this comparatively high figure in relation to the casting time, and this would lead to an output level of perhaps I20 or I50 per minute, from a single molding cavity. It was found when operating under the conditions mentioned that the castings were free of porous zones, and had controlled corner radii suitable for feeding directly to an impact extrusion machine, subsequent to barrel lubricating.

Deviation from the melt temperature of 710 C. resulted in flash metal being pulled into the vacuum gap, or in inadequate mold filling. The inclusion of dross was virtually presented by the use of the submerged ceramic nozzle, or vertical feed pipe.

While the figures given above are basically for aluminum, the concept of the invention and the general ideas of the apparatus are applicable to the vacuum casting of zinc and other low and medium melting metals and alloys.

Various other modifications may be made within the scope of the invention. Thus although the axis of the casting head, mold plate and bottom plate is shown as vertical in the drawings this axis may be inclined at an acute angle to the vertical or may be horizontal.

We claim:

1. A method of casting a continuous series of slugs of low and medium melting point metals and alloys comprising laterally moving a sequence of mold center plates along a progressive path between top and bottom plates, allowing each said mold center plate to remain periodically between said top and bottom plates in a slug casting station, said plates together defining a mold in said casting station, casting a slug in said mold in said casting station, displacing each said mold center plate laterally from said casting station by means of the next succeeding mold center plate after the cast slug has been formed to shear the cast slug, said top and bottom plates being fixed against lateral movement during said shearing step, and passing the said mold center plate containing the cast slug to a slug discharge station.

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