US 3136831 A
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R. D. ZINN CASTING METHOD June 9, 1964 2 Sheets-Sheet 1 Filed July 14, 1961 Fly. 5
R. D. ZINN CASTING METHOD June 9, 1964 2 Sheets-Sheet 2 Filed July 14, 1961 INVENTORZ 2A man/0 0. Z//v/v BY I A T 7' Ol/VE X5 United States Patent 015 ice 3,136,831 Patented June 9, 1964 This invention relates to a novel casting technique and more particularly relates to a core structure whereby castings with complicated coring can be made with cores which are easily removable from the casting after the latter has hardened. The invention is particularly applicable to the casting of propellants in solid propellant rocket enginesand will be described in terms of casting such propellants. However, it will be apparent from the specification which follows that the invention is been used wherein the coreis made of a plurality of keyed segments of a solid material which can be removed in pieces. Suchmethods havenot been fully satisfactory from several standpoints. For, one thing, in the removal of such cores, there is always the possibility of damaging the casting since many castings are made of materials which can be easily damaged. Another disadvantage is that the castings are frequently made of a highly combustible material and there is always the possibilty of fire or explosion when heat or mechanical force is applied to the casting to remove the core. Further, many of the prior art methods do not leave a completely smooth inner surface but the surface frequently has small ridges or other irregularities therein. The core material of many prior art methods contaminates the propellant grain, causing ignition difliculties.
In accordance with the present invention, a method and apparatus are provided for the manufacture of castings wherein the core comprises a flexible member which is made temporarily rigid by filling it with a soluble or meltable filler compound so that the rigidized core is held to a desired shape during casting but can be collapsed and removed through a small opening after the casting has hardened.
A primary object of this invention is to provide a casting method wherein a core is provided which can be easily removed after the casting is completed.
Another object of the present invention is to provide a casting method which has a high safety factor in core removal.
Still another object of this invention is to provide a casting method wherein a core is used which leaves a completely smooth surface on the casting.
Still another object of this invention is to provide an economical core which does not utilize expensive materials and which can be fabricated and used with a minimum of labor.
A still further object of this invention is to provide a core structure which does not contaminate the propellant surface.
Another object of this invention is to provide a core structure which can be installed and withdrawn through an opening smaller than the core.
Other objects and features of advantage of the present invention will be apparent from the specification which follows.
In the drawings forming part of this application:
FIGURE 1 is a perspective view of a rigidized cor showing the mold in which the core is made.
FIGURE 2 is an enlarged sectional view of a core made in accordance with the present invention.
FIGURE 3 is a sectional view showing the method of making a casting employing the present invention.
FIGURE 4 is a sectional view similar to FIGURE 3 showing one method of de-rigidizing the core.
FIGURE 5 is a similar sectional view showing the removal of the flexible member from the otherwise finished casting.
FIGURE 6 is a sectional view showing a casting which has been manufactured in accordance with the present invention.
FIGURE 7 is a sectional view showing the method of utilizing the core of the present invention wherein the core is withdrawn through a small opening in a motor.
FIGURE 8 is a view similar to FIGURE 7 showing a completed rocket motor made in accordance with the present invention.
Referring now to the drawings by reference characters, there is shown in FIGURE 1 a mold 7 made of segments 8 and 10 which mold is used for'making the core of the present invention. In the present illustration the core is relatively simple and the mold merely has a first cylindrical section 12 and a second cylindrical section 14 connected by a disc-like section 16. It will be understood, of course, that the simple symmetrical shape shown is only for purposes of illustration and that the mold for the core can be made in substantially any desired shape. The mold 7 can be made of any suitable material such asmetal, wood, plastic, ceramic, plaster or the like. In making the core, a flexible member 18 is provided which generally conforms to the desired shape of the finished core or which can be stretched to the desired shape. The member 18 can be of any flexible material which is inert to the material being cast, which has sufiicient flexibility to conform to the desired shape and suficient mechanical and thermal stability to stand the ultimate casting and/ or curing operation. Preferably the flexible member is a rubber impregnated cloth preformed to the desired shape. The rubber impregnated cloth forms an excellent release agent for castable solid propellants. The member 18 is placed in the core mold 7 and the mold sections are closed around the member 18. A mandrel 20 is then placed through the core and a filler compound 22 is then run into the flexible member 18, preferably under pressure so that the member 18 is made to conform to the inner configuration of the mold. The filler material 22 can be any desired soluble or meltable material but is preferably a Water soluble plaster. Although the flexible member 18 can be rigidized by filling it in its entirety with the filler compound in some instances, portions of the rigid filler can be precast. Thus, to make the core shown in FIGURE 2, one might first place a cylindrical pre-cast filler element 22a on the mandrel 20, enclose the same in the mold and then cast in place the center disc-like section 22b. A third pre-cast cylindrical section 22c can then be inserted to complete the core.
After the core has been completed, the core, now generally designated 24, is placed in a mold 26 and the casting compound 28 poured through the opening 30. In the instance illustrated, the casting compound 28 might well be a rubber-like material with an oxidizer salt therein, as is well known in the solid propellant art. After the mold is filled, it is heated to polymerize the propellant, whereupon it will form the solid polymerized material 32. At this point, the mandrel 20 is withdrawn leaving a central opening through which Water can be passed as at 34. This dissolves out the filler compound leaving only the flexible member 18 in place. Although for purposes of illustration water has been shown as being passed through the center,
an organic solvent might be used in the case of some 1 filler materials or in the case of other filler materials the filler might be merely heated and allowed to drip out. If only one end of the core is accessible, the solvent must be introduced and withdrawn from the s'arne'end; After substantially all of the filler material has been withdrawn as' is shown in FIGURE 4, the flexible 'm'ember 18 can now be withdrawn from the casting as is shown in FIG- URE 5. The mold 26 is then disassembled leaving the' casting with its irregular opening 38 as is shown in FIGURE 6. In FIGURES 7 and 8 the invention is illustrated in connection with a rocket motor leavingianopening smaller designated'40 is made of fiberglass 42. The core includes a small neck portion 44 which is smaller than the desired core. Here the rubber impregnated-cloth mandrel 46 is inserted through the neck opening 44 and filled with .a water soluble plaster 48. The propellant 50 is then than the desired core. Here a single'piece case generally I poured in place and cured. The plaster48 is then dissolved out and the cloth 46* can then be pulled out. This (q) placing in a die a continuous open-ended-webbing I member capable of being filled to form a solid, openended body, said webbing member having suflicient flexibility to conform to the desired shape of ahollow portion of said die, said die having" a passage extending through at least one end thereof to said hollow portion; a
' (b) inserting a mandrel through said passageiinto said webbing material in said hollow portion whereby 'to envelope the sides of said mandrel, with said webbing material; 7
'(c) injecting into the space between said' mandrel and the adjacent webbing a flowable' filler material capable of being treated to form a rigid, soluble filler, 'the position of said mandrel beingradjusted such that s after said filler is injected the'ends ofjsaid' mandrel remain uncovered;
(d) treating said filler material to cause to harden l I to forms a rigid'core; f
('e) molding casting' mateiial about'said core while 7 maintaining the ends of said mandrel in an exposed condition and withdrawing said mandrel to provide a passage entirelythrough said core;
(f)- flowinga-solvent for said filler material thronghr I the Pa SOformed wherebyto dissolveandre- 'moveisaidfiller; l, r r I casting so formed;
2. The method of claim wherein the filler material a I I water-solubleplasten' i v V 3'. The method of claim 1 wherein thewebbing member is a rubber impregnated cloth. V
7 References Cited; in the'fil'e of :this patent V UNiTED STATES PATENTS (g) and removing the said fleidble webbing'jfromth'ev