US3267527A

US3267527A - Method and apparatus for making ceramic molds

Info

Publication number: US3267527A
Application number: US235245A
Authority: US
Inventors: George M Brown
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-11-05
Filing date: 1962-11-05
Publication date: 1966-08-23
Anticipated expiration: 1983-08-23

Description

G. M. BROWN Aug. 23, 1966 2 Sheets-Sheet 1 Filed Nov. 5, 1962 F/G Z INVENTOR.

GEORGE M, BROWN G- M. BROWN Aug. 23, 1966 METHOD AND APPARATUS FOR MAKING CERAMIC MOLDS 2 Sheets-Sheet 2 Filed Nov. 5, 1962 M mi INVENTOR.

GEORGE M BROWN HA5 flTTO/ NEY United States Patent 3,267,527 METHOD AND APPARATUS FOR MAKING CERAMIC MOLDS George M. Brown, 7536 Jehnnycalre Ridge Road, Mentor Township, Lake County, Ohio Filed Nov. 5, 1962, Ser. No. 235,245 3 Claims. (Cl. 2214) This invention relates to investment molding and more particularly concerns the formation of ceramic molds defining impellers with blades having a spiral configuration.

Heretofore, precision casting of complex bladed impellers for pumps and related devices has conventionally been accomplished by the use of disposable patterns, such as wax, in the formation of ceramic molds defining the bladed casting cavity.

One of the objects of this invention is to provide a new and improved method and apparatus for forming ceramic molds for precision impellers having screw-type blades.

Another object of this invention is to provide a new and improved method and apparatus for forming ceramic molds for impellers of the type described by use of a permanent positive pattern.

A further object of this invention is to provide a new and improved method and apparatus for twisting a positive impeller pattern of the type described out of a ceramic mold.

An additional object of this invention is to provide a new and improved method and apparatus for forming ceramic molds for impellers of the type described which readily lends itself to the demands of economic manufacture.

Other objects and advantages will become apparent from consideration of the following specification taken in conjuction with the accompanying drawings in which like characters refer to the same of similar parts throughout, and in which:

FIG. 1 is a perspective view showing the pattern of an impeller according to the invention.

FIG. 2 is an elevational view of an assembled flask, partly in section to show an invested pattern according to the invention.

FIG. 3 is a top plan view of a flask according to the invention.

FIG. 4 is a front elevational view of a twist-off mechanism according to the invention.

FIG. 5 is a side elevational view of the twist-off mechanism of FIG. 4.

FIG. 6 is a fragmentary View, partly in section, showing the cross-slot rail securing arrangement according to the invention.

FIG. 7 is a fragmentary view, partly in section, showing operation of the flask assembly and twist-off mechanism according to the invention.

Referring now more particularly to the drawings, FIG. 1 shows a metal pattern 14 which is identical to the impeller to be cast except for the axial standard 15 at the hase thereof. The impeller pattern has a conical hub with one or more blades or fins 16 radiating therefrom in a helical or spiral configuration. Plural blades may be of varying length but each must terminate at the base end 17' of the hub and have the same constant lead angle to permit their simultaneous withdrawal from the ceramic mold. In addition to an identical constant lead angle for all blades, it is essential that the spacing between adjacent blades, or if one blade, between adjacent turns, be constant. A positive master pattern of the impeller is usually machined to shape and provided with a polished finish, usually on the order of 20 microinches R.M.S. of surface roughness. The pattern is usually cut with a A ice tapered end mill having a 1 degree included angle to compensate for flex of the radially outward portion of each blade so as to prevent radial backdraft which would preclude twisting of the pattern from the ceramic mold.

Referring now more particularly to FIGS. 2 and 3, the impeller 14 is shown secured to a cylindrical flask base 19, with the impellers standard 15 engaged in axial bore 20. Base 19 is further formed with an off-center bore 21 to receive bolt 22 therethrough for threaded engagement with :a suitably tapped hole in the base of impeller 14 to prevent rotation thereof on the base 19. Base 19 has four equally spaced feet 24, each projecting from the underside thereof so as to define opposed slots 25.

Adapted for engagement with flask base 19 is cylindrical flask 27 which has booking halves 2S and 29 releasably held by recessed bolts as at 31. is formed with a pair of opposed ears 32 projecting radially and extending along the side of flask 27 for slidable engagement in keyslot tracks of a twist-oif mechanism as will hereinafter be seen. To releasably secure flask 27 to base 19, flask 27 is formed with opposed bores, as at 37, through each of which a bolt, as at 338, is adapted to be inserted for threaded engagement with the tapped hole 40 in base 19. A circumferential step 41 is also provided between flask 27 and base 19 to maintain axial alignment of the flask assembly. Flask 27 is formed with a conical interior wall 33 terminating at the upper end thereof in a central sprue opening 34 through which a slurry of self-bonding refractory mix 42 is adapted to be disposed to invest pattern 14.

Conical wall 33 is shown spaced from the exterior envelop of pattern 14. The configuration of the interior of flask 27 will depend on the method and apparatus used for investing and casting. It has been found that a spacing of A1 to /2 inch provides suflicient strength to permit the ceramic mold to readily withstand the twist-01f operation, where conventional investments and investing techniques are utilized. Where the blade is to extend through to the conical wall 33, it is often advantageous. to loosely wind fiberglass or wire stringers about pattern 14 to be invested with the pattern to provide additional strength. And, of course, the pattern 14 and the interior of flask 27 is normally sprayed with a release agent, such as silicone, prior to investing. vDuring or after the setting of the investment, flask securing bolts 38 are removed and the flask assembly is placed inthe twist-off mechanism shown in FIGS. 4 and 5.

Referring now more particularly to FIGS. 4 and 5, numeral 44 designates a rectangular base to which spaced

parallel rails

45 and 46 are secured by :bolts 47.

Rails

45 and 46 are each formed along the top thereof with inwardly opening channels, as at 48, which are adapted to slidably engage theears 32 of flask 27.

Spaced from one end of the

rails

45 and 46 is a rotatably supported chuck 49 having four radial keyslot tracks 50 equally spaced thereabout. Tracks 50 are adapted to axially receive feet 24 of flask base 19 so that upon rotational movement of chuck 49, feet 24 engage with tracks 50 to prevent further axial movement of flask base 19.

Rails

45 and 46 are selectively positionable to accommodate different size flask assemblies by providing a cross-slot arrangement for each of the rail securing bolts 47. As shown in FIG. 6, elongate slot 53 in the footer 54 of rail 46, and perpendicularly disposed elongate slot 55 in the base 44, are in confronting registration so as to define a hole therethrough to receive securing bolt 47,

which is held by a clamping nut on the underside of base Flask half 29 Chuck 49 is supported for rotational movement by drive shaft 60 that is driven by reversible motor 61 through gear reducer 62 and pillow block 63. Direction of rotation of chuck 49 is determined by a suitable manually operated motor start switch 64. This arrangement provides for the chuck being driven in either a clockwise or counterclockwise direction depending on whether the pattern has a left-handed or right-handed spiral.

Operation of the twist-off mechanism and flask assembly may best be understood by reference to FIG. 6. The flask assembly, as shown in FIG. 2, is supported on

rails

45 and 46 with ears 32, slidably engaged in channels 48 so as to permit only axial movement of the flask assembly. The flask assembly is adapted to be slid axially into the chuck 49 so that feet 24 of base 19 are received freely within their corresponding tracks 50 in chuck 49, the arrangement being such that the circumferential length of each foot 24 is less than the opening to track 50 in the face of chuck 49. Upon actuation of switch 64, flask base 19 rotates with the chuck 49 at 2 to 3 rpm. forcing pattern 14 to rotate therewith. Initial rotation of chuck 49 in either direction causes tracks 50 to engage or lock-in feet 24 so as to preclude axial separation of the base 19 and chuck 49. Continued rotation of chuck 49 effects a like rotation of flask base 19, whereupon impeller pattern 14 unscrews or twists-off from the ceramic mold 42 forcing flask 27 along with mold 42 to ride axially away from flask base 19 in channels 48, the arrangement being such that flask 27 and flask base 19 separate on a plane at right angles to their axis. The interior wall of flask 27 may be formed with one or more inwardly projecting ears to assure that the mold 42 will not turn with the flask 27 during the twist-off operation.

The twist-off operation should be accomplished at the time of the initial set of the investment or before appreciable expansion thereof has taken place. Initial set of gypsum-type investments having a plaster of Paris binder occurs shortly after the material loses its water gloss. The investment will become warm when the set occurs. As soon as this heat can be felt, the twist-off operation may be accomplished. The length of time after the initial set which may expire before twist-off must be empirically determined for each type of investment. With conven tional gypsum-type investments, twist-off should be accomplished Within to minutes after set, since expansion after set continues for at least two hours. For investment materials which utilize other binders, such as colloidal silica sol, expansion after setting is much less pronounced. For results, twist-off should be accomplished after setting of the investment but before appreciable expansion has taken place, which for most impeller configurations is on the order of .001 inchper inch.

It is to be noted that in the production of molds for impellers having a plurality of blades, the blades of the positive pattern will have significant, although slight, variations in blade lead, thickness and spacing due to limitations in the conventional machining method described. These variations tend to develop stresses in the mold during the twist-off operation. The self-guiding twist-01f method herein described inherently provides for balancing or distribution of these stresses so as to minimize the incidence of mold fracture.

From the foregoing it should be apparent that I have provided a new and improved investment molding method and apparatus which is well adapted to fulfill the aforestated objects of the invention. Moreover, whereas the invention has been described in particularity with respect to a preferred embodiment which gives satisfactory results, it will be understood by those skilled in the art to which the invention most nearly appertains that other embodiments and modifications thereof may be resorted to without departing from the spirit or scope of the invention as defined by the appended claims.

I claim:

1. A method of forming a ceramic mold for an impeller having a conical hub with a plurality of equally spaced screw-type blades of constant lead radiating therefrom, including the steps of; forming a positive pattern of the impeller so that the spacing between adjacent turns is constant, coaxially securing the positive pattern to a cylindrical flask base for rotation therewith along its axis, securing a cylindrical flask to said flask base so as to define an investment cavity about said pattern, investing the pattern with a slurry of self-bonding ceramic refractory mix so as to form a ceramic mold within the flask, after setting of the slurry but before appreciable expansion thereof twisting the flask base so as to unscrew the impeller pattern from the ceramic mold which is held in the flask, with the thread formed by the pattern in the ceramic mold functioning as the sole pitch guide for movement of the pattern whereby the stresses on the mold developed during such twisting are equally distributed.

2. In combination, a twist-off mechanism and a flask assembly; said twist-off mechanism including a base, spaced parallel rails secured thereto having confronting channels, a rotatable chuck supported from said base spaced from one end of said rails, means for selectively driving saidchuck, said chuck being formed with oppositely extending radial tracks; said flask assembly including a flask and flask base, said flask base having an impeller pattern secured thereto, said flask assembly adapted to be filled with a self-bonding refractory ceramic mix so as to form a mold about said pattern, said flask having opposed ears adapted to be slidably received within said confronting channels, said flask base having feet adapted to be received within the radial tracks of said chuck so as to engage said flask base therewith, the arrangement being such that upon rotation of said chuck with the flask base engaged thereto, the mold thread formed by the pattern functions as a thread guide causing the flask and mold therewith to move in a direction away from said flask base as guided by said flask ears sliding in said confronting channels.

3. In combination, a cylindrical flask assembly adapted for separation into a flask and flask base at right angles to its axis, said flask base having secured thereto a pattern having a plurality of equally spaced screw-like blades, said flask assembly adapted to be filled with a self-bonding refractory ceramic mix so as to form a mold about said pattern, a first means for restraining said flask from rotational movement while permitting axial movement, a second means for restraining said flask base from axial movement while permitting rotational movement, and a third means for selectively rotating said second means so as to unscrew said pattern from said mold, the arrangement being such that the mold thread formed by the pattern functions as the sole pitch guide for separation of said flask assembly.

References Cited by the Examiner UNITED STATES PATENTS 133,592 12/1872 More 22-14 914,474 3/ 1909 Barnes 22-14 931,760 8/1909 Hattersley 22-14 1,544,060 6/1925 Dimick 22-14 1,856,166 5/1932 Nordfelt 22-14 2,509,692 5/1950 Miller 22-199 X 3,072,979 1/1963 Abbott 22-18 3,162,592 3/1964 Taccone 22-14 FOREIGN PATENTS 3,732 of 2/ 1869 Great Britain.

J. SPENCER OVERHOLSER, Primary Examiner.

MARCUS U. LYONS, MICHAEL V. BRINDISI,

Examiners.

Claims

1. A METHOD OF FORMING A CERAMIC MOLD FOR AN IMPELLER HAVING A CONICAL HUB WITH A PLURALITY OF EQUALITY SPACED SCREW-TYPE BLADES OF CONSTANT LEAD RADIATING THEREFROM, INCLUDING THE STEPS OF; FORMING A POSITIVE PATTERN OF THE IMPELLER SO THAT THE SPACING BETWEEN ADJACENT TURNS IS CONSTANT, COAXIALLY SECURING THE POSITIVE PATTERN TO A CYLINDRICAL FLASK BASE FOR ROTATION THEREWITH ALONG ITS AXIS, SECURING A CYLINDRICAL FLASK TO SAID FLASK BASE SO AS TO DEFINE AN INVESTMENT CAVITY ABOUT SAID PATTERN, INVESTING THE PATTERN WITH A SLURRY OF SELF-BONDING CERAMIC REFRACTORY MIX SO AS TO FORM A CERAMIC MOLD WITHIN THE FLASK, AFTER SETTING OF THE SLURRY BUT BEFORE APPRECIABLE EXPANSION THEREOF TWISTING THE FLASK BASE SO AS TO UNSCREW THE IMPELLER PATTERN FROM THE CERAMIC MOLD WHICH IS HELD IN THE FLASK, WITH THE THREAD FORMED BY THE PATTERN IN THE CERAMIC MOLD FUNCTIONING AS THE SOLE PITCH GUIDE FOR MOVEMENT OF THE PATTERN WHEREBY THE STRESSES ON THE MOLD DEVELOPED DURING SUCH TWISTING ARE EQUALLY DISTRIBUTED.