US2887745A - Casting mold and method and apparatus for making the same - Google Patents

Description

May 26,1959 T. J. R. BRIGHT 2,887,745

CASTING MOLD AND METHOD AND APPARATUS FOR MAKING THE SAME Filed Dec, 6, 1954 5 Sheets'-Sheet 1 Q I 1% 20 20 4a. 48' v 4.9 49 4 49 Q I I: a} I 1 l 1/ i i I I I i 44- l "*1? i I [K 5 i E 22 29 q "27 v I HUM." H1 41 l 1 Cd 0 -z7 I 3a ET A as 14 INVENTOR HamwsJKfiry/zl' mama-MM ATTORNEYS May 26, 1959 T. J. R. BRIGHT CASTING MOLD AND METHOD AND APPARATUS FOR MAKING THE SAME Filed Dec. 6, 1954 3 Sheets-Sheet 2 g. 5. as 25 2 6 ,34

INVENTOR 1710mm Jiiibnjbl i Z Q? J J ATTORNEYS May 26, 1959 T. J. R. BRIGHT 2,887,745

' CASTING MOLD AND METHOD AND APPARATUS FOR MAKING THE SAME Filed. Dec. 6, 1954 s sheets-Sheet s MIIIIIIIIIII gala ATTORNEY 5 United States Patent CASTING MOLD AND METHOD AND APPARATUS FOR MAKING THE SAME Thomas John R obert Bright, Warwick, England Application December 6, 1954, Serial No. 473,354

Claims priority, application Great Britain December 7, 1953 8 Claims. (Cl. 22-496) This invention relates to molds for precision casting and to methods and apparatus for making the same.

Itis well knoWnin the production of cast metal articles orc'omponents of intricate shape or critical tolerances to make use of molds, a pattern of the article which. it is proposed to cast being produced by pouring mercury in a die, freezing the mercury, removing the frozen mercury pattern and dipping the pattern whilst in its solid frozen state into a ceramic slurry, the dipping process being repeated until a mold of the requisite thickness has been built up. The mercury is then melted out and the ceramic shell mold put through a firing operation at a high temperature. The resultant ceramic shell has a fine cavity surface which is capable of producing a fine surface finish on a steel or other component subsequently cast in the mold, such molds having the additional advantage that they are capable of resisting, the thermal shock of the hot metal being poured into a comparatively cold mold, although in actual practice his more usual to heat the molds to a high temperature before pouring in the molten metal. Furthermore, by

virtue of their thin walled construction and the nature of the material used the hot gases find'an escape route actually through the walls of the mold, thereby insuring the production of castings free or substantially free from blow-holes or surface markings. Once the casting: has beenmade and cooled the ceramic is broken away from the cast article.

While articles produced by the frozen mercury process or. by other processes involving the temporary SOIldlfiCEl tion of a patternrnaterial are characterized by very high.

quality, the: process has important economic disadvantages. The chief one of these is that a. new pattern and anew mold has to be-made for every article cast. This makes for high labor cost and precludes mass production; Furthermore, for high production, even on a pieceworkbasis, a large investment in mercury is necessary. :It-is" therefore an object of the present invention to produce high precision ceramic molds that can be reused many times in the mass production. of cast articles but which are nevertheless fully possessed of the advantages of: single use ceramic molds of the type known to the art before the present invention.

It is' a further object of this invention to provide a high precision, high strength, fully permeable partible ceramic mold which can be cleaned, inspected and repaired internally.

Itis contemplated'according to the present'inve'ntion to orientthe components of the ceramic mold astlie ceramic is applied to the pattern thereby to insure perfect orientati'ion' of the mold parts after the mercury or other pattern has been melted away.

Other objects and advantages of this: invention willrbe apparent upon consideration of the followingv detailed description of several embodiments thereof in conjunctionwiththe annexed drawings wherein:

Figure 1 is an exploded perspective view of a die .may be fastened by screws as shown.

for receiving a low melting pattern forming material such as mercury;

Figure 2 is a vertical sectional view through the die of Figure 1 as it appears when assembled to receive the pattern forming material;

Figure 3 is a view in section taken on the line 3-3 of Figure 2;

Figure 4 is a view in section similar to Figure 3 but showing the pattern in position with most of the die components removed;

Figure 5 is a view similar to Figure 4 but showing the ceramic surrounding the pattern;

Figure 6 is a view similar to Figure 5 but showing the mold after firing of the ceramic and after removal of the parting strip;

Figures 7, 8 and 9 are fragmentary views in section showing successive steps in preparing orienting projections and sockets in the parting strip of the present invention;

Figure 10 is a view in horizontal section of a die for producing a mold with specific external contouring;

Figure 11 is a view in horizontal section of the pattern produced from the die of Figure 10;

. Figure 12 is a view in horizontal section of the pattern of Figure 11 after coating with ceramic;

Figure 13 is a View in horizontal section of the ceramic mold produced from the pattern of Figure 11; and

Figure 14 is a view of a ceramic mold top with locating rib for orienting the mold parts of a mold having the configuration depicted in Figure 6.

While the present invention is suitable for the pro duction of molds for making high precision solid and hollow articles of most intricate shapes, the invention will be described with respect to the production of a. simple hollow tube of rectangular cross section. A die for producing a pattern for such a hollow, rectangular section tube is shown in Figures 1 and 2. The die is comprised of an outer surface defining portions or outer components 19, an inner surface defining portion or inner component 11, a parting strip 12 and orienting base blocks 13. The outer die portions 10 are comprised of side walls 14 and 15 which define internal cavities at 16 and 17. These cavities define and hence conform to the external shape of the rectangular section pattern which is to be produced. The side walls 14 and 15 are provided at the top'with cap plates 18 and 19 respectively which The cap plate 13 has therein a sector shaped gate 18a and two risers of generally cylindrical form indicated at 18b. The cap plate 19has a sector shaped gate at 1% and risers at 1%. In filling the die the mercury is poured into the gates and the gases escape through the risers. The side plates 14 and 15 are provided with dowel apertures Ztl through which dowels are passed in order to fasten the outer component10 of the die to the inner component 11 thereof. The inner component 11 includes two rectangularv blocks 21 and 22 on opposite sides of parting strip 12. Block's 21 and 22 are adapted to register in spaced relationship with cavities 16 and 17 and to define with the recessed portions of the outer component 10, a space of uniform thickness. This space registers with the gates or pour apertures 18a and 1% as well as with the risers 18b and 19b and is the space in which the pattern is formed. The parting strip 12 is provided with dowel apertures at 23 for cooperative registry with the apertures 20 in the outer die portion 10. The parting strip is also provided with dowel apertures near its base through which removable dowels 24 extend for the purpose of temporarily connecting the base blocks 13 to the parting strip 12 on opposite sides thereof. also: provided with a number of orienting cavities 25. Projecting from the partingstrip on the side opposite to The parting strip 12 is the cavities 25 are plugs 33, the structure and function of which will be hereinafter more fully described. The side plates 14 and 21 have longitudinal grooves 33a therein in order to clear the plugs 33. The side plates 14, 15, 21 and 22 also have cavities in their bases which register with plugs 26 which project up from the blocks 13 in order to orient the removable blocks 21 and 22 in correct spaced relation within the cavity defined by the outer die structure 10.

The parting plate 12 is provided with a series of holes or cavities 27 which are spaced apart and lie in registry with the pattern cavity within the die.

When the outer components 10 of the die of Figure l are placed around the inner components 11, the assembled die as viewed in vertical section has the appearance depicted in Figure 2 and, as viewed in horizontal section, has the appearance depicted in Figure 3. Dowels 28 are passed through the apertures 20 and 23 to hold the die parts together.

With the die assembled as in Figures 2 and 3, the cavities 29 and 30 which are defined by the die body at opposite sides of the parting strip 12 are filled with mercury, wax or other low melting pattern forming material. This material completely fills the cavities and, in addition to filling them, fills the gates 18a and 19a and the risers 18b and 19!), passes through parting strip 12 at the location of the apertures 27 therethrough in a manner which can be best appreciated by reference to Figure 3. Once the cavities 29 and 30 are filled, the die is chilled to freeze the mercury or other material in position. While maintaining the mercury at a desired low temperature the die is disassembled. This involves removal of the dowel pins 28 and lifting up and ofi the outer die components 10 as complete subassemblies as it appears in the upper portion of Figure 1. At this stage, dowel pins 24 are removed. Orienting blocks 13 are lifted away to permit the blocks 21 and 22 to be knocked out through the bottom of the pattern. The base blocks 13 are then restored to position and the dowels 24 reconnect them to the parting strip 12. When this operation has been completed, the top plan view appearance of the remaining structure is as shown in Figure 4.

The frozen mercury pattern includes a part 31 which is the complement of cavity 29 and a part 32 which is the complement of cavity 30. Although these parts are neither internally nor externally supported they stay in perfect registry in relation to one another because they are tied together by the frozen connections which pass through the parting strip 12 at apertures 27. The dowels 24 are not removed at this stage and the base blocks 13 are kept connected to the parting strip 12.

Having achieved the pattern shown in Figure 4, the next step is to coat the pattern with ceramic. The entire assembly shown in Figure 4 is subjected to repeated dipping in a slurry of ceramic material. For the first and possibly for a small number of successive dips a comparatively fine slurry will be used. A coarser slurry is thereafter used until the required thickness of ceramic material has been built up, it being preferred that the thickness of ceramic material shall be at least one sixteenth of an inch for good mechanical strength and not substantially in excess of one eighth of an inch in order to insure the maintenance of adequate porosity for the escape of gases evolved when metal is cast within the ceramic mold.

After an adequate amount of dipping, the pattern coated with ceramic will correspond in appearance to Figure 5. In Figure the slurry does not completely cover the base blocks 13 as one might expect after repeated dippings. It will be understood that after each dipping it is possible to wipe any parts of the die. This accounts for the clear ends of the parting strip 12 and clean portions of the blocks 13 as they are shown in Figure 5. It also accounts for the clean tops of the sector shaped bases of mercury which filled the gate and the clean tops of the mercury complements to the risers. Once an adequate thickness of ceramic has been built up the material is allowed to set until it is sufiiciently hard to permit melting out of the mercury or other low melting pattern material. After the pattern material has been melted out the remaining structure still keeps the position shown in Figure 5. At this stage, the ceramic mold is comprised of outer portions 34 and 35 and inner portions 36 and 37. The portions 34-36 and 3537 may be pulled away from the opposite sides of the parting strip 12, or, if desired, the parting strip may be made in sections or laminae so that it can be collapsed to be withdrawn in pieces. In either case the portions 3436 and 35-37 may be reassembled without parting strip to define a partible mold as shown in Figure 6. For the particular type of construction shown, the blocks 13 are retained since the projections 26 extending upwardly therefrom serve to orient the internal mold portions 36 and 37 in relation to the external portions 34 and 35. While the blocks 13 shown are a part of the die they may be made of ceramic also if desired. The projections 33 and cavities 25 result in the production of projections on the ceramic mold parts 35 and 37 and registering complementary cavities on the parts 34 and 36. This being the case, the mold of Figure 6, although partible, is completely accurate and can produce repeated castings of superior accuracy without being required to be broken after each casting operation as was prior art practice.

It will be understood that much of the success of'the processheretofore described depends upon the accuracy of the registry of the orienting plugs and sockets. To this end, the elements which produce the orienting parts of the mold are made with great accuracy by a procedure which may be best described with reference to Figures 7, 8 and 9. The first step is to locate on and drill through the partition plate a number of holes indicated in Figure 7 at 38. Using these holes as centers, the next step is to machine out the frusto-conical cavities 25. Thereafter, the frusto-conical projection 33 is made and this is mounted on a spindle 39. This projection 33 is made to approximate size and while still on this spindle 39 is ground into the cavity 25 by procedures such as are used in the grinding of the valves of internal combustion engines. Once the fit between projection 33 and its corresponding socket 25 is absolutely perfect, the projection is located on the opposite side of the plate 12 from the socket 25, see Figure 8. At this point, the projecting end of the spindle 39, which was used during the grinding-in operation, is removed and the final structure is as shown in Figure 9.

. In discussing the steps of preparing a mold according to the present invention, it was pointed out that the projections 33 may be applied to the parting strip 12 incident to its manufacture or applied after the formation of the pattern but before the immersion of the pattern in the slurry of ceramic material. It is understood that the latter case presupposes a pattern of such size that the projections can be installed without interference with the pattern. It also is to be understood that in either case the steps described in conjunction with Figures 7, 8 and 9 are precedent to the manufacture of a parting strip so that after the pattern has been formed all that is necessary is to press the projection in position. In the case of small patterns it is usually necessary to install the projections before the pattern is poured. In this case the die will require machined out clearance grooves to accommodate the projections, see Figures 1, 2 and 3.

One of the principal advantages of the present invention is that the mold, such as is shown in Figure 6, can be internally inspected, repaired and cleaned. Another advantage is that it can be repeatedly used with large savings in labor and savings in capital investment in mercury. In order that the mold may be conveniently repetitively used it is proposed as a part of this invention to contour the mold externally as well as internally in order to adapt it to use in presses or other automatic machinery. If now reference is made to Figures to 13, inclusive, it is seen how this may be accomplished. For convenience of illustration, Figures 10 to 13, inclusive, do not illustrate any sort of base structure corresponding in function to the plates 13 of Figures 1 to 6, inclusive. Since these figures are horizontal sections rather than end views the pattern for the gate and risers does not show in them nor do they show in the mold. These parts, however, correspond to the showing inFigures 1 to 6, inclusive. It is to be understood, however, that some equivalent to these plates is actually present. As in the case of the process described with Figures 1 to 6, inclusive, the shape selected for purpose .of illustration is a hollow structure of rectangular cross section. Upon reference to Figure 10 it can be seen that the die is comprised of six parts, three on each side of the parting strip 40. The innermost portions 41 and 42 correspond in structure and in function to the parts 21 and 22 of the die of Figure 1. The intermediate parts 43 and 44 have no counterpart in Figures 1 to 6 but they define the outside of the inner pattern cavity. and the inside of the outer pattern; cavity. The outer components 45 and 46 correspond generally in structure and in function to the parts 14 and 15 of the die of Figure 1. The parting strip 40 has sockets 47 therein corresponding in structure and in function to the sockets 25. The parting strip is also provided with apertures therethrough at 48 in the regions where the pattern will abut the surface of the strip. These apertures correspond in, structure and in function to the apertures at 27 in the strip 12. r

When the die has been assembled, as shown in Figure 10, the four cavities 49, 50, 51 and 52 are filled with a low melting point liquid such as mercury. This mercury is then frozen in position and the die parts are removed leaving a pattern in mercury which is fastened to the parting strip 40 by the bits of frozen mercury which lie Within and extend through the apertures at 48. The pattern as viewed in plan is shown in Figure 11. In Figure 12 the pattern is shown at the completion of its investiture with ceramic. Also in Figure 12 the projections 53 which correspond to the projections 33 of Figures 1 to 6, inclusive, are shown. These are put in position as previously described after removal of the die and before the pattern is invested with ceramic. After the pattern is melted away from the hardened ceramic of Figure 12 and when the ceramic has been fired, it can be assembled to form a mold of the type shown in Figure 13. In view of the fact that the ceramic which surrounds the portion of the pattern within cavities 49 and 52 is not needed as part of the mold, it may either be wiped away during the investiture or else, as shown, discarded after the pattern has been melted away. The portion of the pattern within cavities 49 and 52 of course functions to contour the outer surfaces of the ceramic mold. Thus, the mold of Figure 13 is not different from the one shown in Figure 6 insofar as the castings made therein are concerned, but externally it is provided with a series. of sockets 54 which may be used as orienting sockets to locate the mold in a molding press or other automatic machinery. The deep cavities 54 also perform an important heat exchange function by increasing the overall surface of the mold without increasing its thickness. The irregular apertures at 55 give the mold a honeycomb construction which is strong and yet favorable to good heat exchange. The orientation of the parts of the partible mold shown in Figure 13 is accomplished in the same manner as has been more fully described in connection with Figure 6, see the projections 56.

While the description of the accompanying drawings is concerned with a specific parting strip structure, it is to be understood that this invention is concerned with the production of molds by the use of parting strips in sufficient number and of such configuration .as to divide parting the mold into a plurality of parts which are separable after the ceramic is set. The parting strip may be of any suitable thickness or configuration so long as it forms on each mold part a full face of a width sufficient to produce a good closure of the mold when the edges of 'the mold are brought into an abutting relationship after the pattern has been removed.

' It will be appreciated that if the pattern is produced from mercury or other material remaining in a liquid state at normal temperature, it will be necessary to freeze the mercury or like material after it is poured into the die. Frozen mercury is more readily removed from a die if the latter is first coated with a suitable antifreeze liquidsuch as ethylene glycol or polymers thereof.

Although mercury is prominently mentioned as a material from which the pattern may be made, a bismuth-tin alloy which is 58% bismuth and 42% tin is very satisfactory. It is likewise satisfactory in some instances to use some'mercury in a pattern alloy which is predominantly composed of some other metal.

The ceramic mold parts when at room temperature will be in a green state and consequently the procedure adopted in the removal of the low temperature melting alloy, mercury, wax or other material will to a certain extent depend upon the material used for the pattern. For example, when using frozen mercury, it is preferred to pour liquid mercury at normal temperature onto the ceramic mold parts or to immerse the mold parts in liquid mercury so as to melt out the pattern without risk of distortion of the ceramic parts. If a low temperature melting alloy is used the mold parts can be subjected to a degree of heat such that the alloy will be melted out leaving the ceramic substantially in its original green state. It will be appreciated, therefore, that it is advisable to use an alloy which will have a co-efficient expansion comparable with that of the ceramic material so as to avoid distortion of the ceramic during the melting out process. Alternatively the mold parts may be removed from the pattern and the pattern used over and over again.

To enable the ceramic mold parts to be successfully removed from the parting strip it may be desirable partially or completely to fire the mold parts, whilst still in position on the strip.

While it is apparent that the ceramic which bridges over the pattern parts 31 and 32, see Figures 4, 5 and 6, serves to orient the inner and outer portions of the mold in relation to each other, it will be appreciated that there are cases where it may be desirable to wipe the top of the pattern during investiture with ceramic and to reorient the parts with a separate cap. In this case a cap such as that shown in Figure 14 may be used. This may be prefabricated from ceramic or made of metal, if desired. The cap shown is indicated as being of ceramic and having a main body portion 57 with an integral locating rib 58 corresponding to the top contour of the mold cavity.

What is claimed is:

1. A method of producing a reusable multiple part ceramic or other high temperature resisting mold, the mold parts incorporating means for ensuring accurate relative location but being capable of separation for the removal of a finished casting, which comprises applying high temperature resisting material to form mold parts to a pattern formed from a material of low melting point, the pattern including two or more parts on an intervening parting plate which serves to separate the mold parts during their formation, the parting plate on its opposite faces having locating means for producing identical registering recesses and projections in and on the mold parts which when interengaged, when the mold parts are separated from the parting plate and assembled together to form the finished mold, will ensure accurate relative location of the mold parts during use.

2. A method as claimed in claim 1 wherein the high temperature resisting mold material is applied to the pat-, tern parts and their intervening parting plate in liquid form by a clipping operation.

3. A method as claimed in claim 1 wherein the high temperature resisting mold material is applied to the pat tern parts and their intervening parting plate in liquid form by a spraying operation.

4. A method as claimed in claiml wherein the high temperature resisting material is substantially uniformly applied so as to produce a mold having a substantially uniform wall thickness.

5. In a die for use in the production of a heat fusible pattern for use in the production of ceramic mold parts therefrom which comprises an assembly of pattern forming parts in combination with a parting plate for receiving the parts of the pattern, the improvement for insuring precise alignment between completed ceramic mold parts which comprise a parting plate having on its opposite faces oppositely positioned identically shaped projections and recesses an integral orienting portion of each projection extending through the parting plate to the bottom of the opposite recess, said projections and recesses forming corresponding recesses and projections in said completed ceramic mold parts produced therefrom, which when the ceramic mold parts are finally placed together will ensure accurate relative location.

6. A reusable multiple part high temperature resisting mold having two or more parts on an intervening parting plate which separates the mold parts during formation, the parting plate on its opposite faces having 10- cating means for producing identical registering recesses and projectionsin and on the mold parts to ensure accurate relative location of the mold parts during use,

the mold parts being of cellular formation, the walls of the mold and the external cellular parts being of substantially uniform thickness.

7. A reusable mold as claimed in claim 6 wherein the external shape of the cellular part of the mold is such that it can be used in a press of corresponding internal shape, the mold being accurately formed externally.

8. A method for insuring accurate registry of mold parts on opposite sides of a parting plate comprising drilling through a parting plate a hole having a coaxial frusto conical cavity at one end, orienting a complimentary frusto-conical body in said cavity with one projecting portion of a coaxial spindle projecting from both ends of the body, grinding in the mutually engaging faces of the body and cavity while the body is so oriented, removing the body and spindle from the hole, inserting the other projecting portion of said spindle into the hole from the opposite side of the plate and cutting off said one projecting portion of the spindle leaving an identically dimensioned body and recess on opposite sides of the plate.

References Cited in the file of this patent UNITED STATES PATENTS 703,169 Baldt June 24, 1902 1,369,683 McTighe Feb. 22, 1921 1,912,889 Couse June 6, 1933 2,160,645 Cooper May 30, 1939 2,206,034 Hagemeyer July 2, 1940 2,448,827 Reder Sept. 7, 1948 2,460,213 Capps Jan. 25, 1949 2,632,216 McQuaid Mar. 24, 1953 2,652,609 Sudia Sept. 22, 1953 2,682,692 Kohl July 6, 1954

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