US3446265A - Process for making permanently backed shell molds - Google Patents

Description

United States Patent 3,446,265 PROCESS FOR MAKING PERMANENTLY BACKED SHELL MOLDS Ronald H. Buck, Jr., Farmington, Mich., assignor to Eaton Yale & Towne Inc., Cleveland, Ohio, a corporation of Ohio N0 Drawing. Filed May 17, 1966, Ser. No. 550,632 Int. Cl. B22c 13/08 U.S. Cl. 16421 8 Claims ABSTRACT OF THE DISCLOSURE A process for making permanently backed shell molds by which a particulated refractory material containing a resin binding agent is introduced under controlled pressure into a cavity defined by a heated pattern and metal backing member which are vibrated at high frequency during the injection of the refractory material and continuously vibrated thereafter until a partial curing of the binding agent occurs.

This invention relates to the art of shell-lined, permanently backed molds and to a method of casting articles with such molds.

In recent years, shell molding techniques have substantially advanced the art of metal founding. In this process a sand and thermosetting resin mixture is placed on a pattern which is heated to cure and harden the resin. Thus, a shell mold of resin-bonded sand is produced over the pattern. Final baking produces the finished shell mold which is suitably backed up by shot or by solid members prior to the castingoperation.

More recently, shell molds have been formed by injecting a suitable mixture of sand and resin into a cavity defined by a pattern positioned in closely spaced relation to a backing member. After final baking and removal of the pattern, the shell mold with the backing member is used for casting.

The advantages of the shell-molding process are that the shells produce very little gas during the casting operation and, in addition, mold draft angles can be reduced so that articles can be cast in such molds with greater precision and very good surface smoothness as compared with earlier methods.

A problem encountered with shell molding processes heretofore used is that the quality of the casting surface of the shell has not been controllable with the highest degree of accuracy so that casting finish has not been consistently uniform, especially for articles of thin crosssection and intricate configuration. Furthermore, when using permanent backing members in such processes, difiiculties are encountered in properly effecting uniformity of density in the shell casting surface and in effecting proper flow during injection of the sand-resin mixture into intricate shaped cavities.

The present invention provides a method of making permanently backed casting shells which provide a superior casting surface finish by accurately controlling the uniformity of density in the casting surface of the shell mold.

The present invention provides a method for making shell molds by using an effective combination of sand blowing and vibration for tamping the sand-resin mixture within the shell-forming cavity so that the density of 3,446,265 Patented May 27, 1969 the shell is uniform throughout the shell-casting surface while using a minimum amount of resin.

The present invention further provides for the making of thin shell molds having intricate and complex shellcasting surfaces of uniform density.

In a further aspect of the invention, permanently backed shell molds produced by this invention may be filled with molten metal having a higher effective metal head producing castings of thin cross-section and intricate configurations. Also, increasing the effective metal head, either by vacuum, positive gas pressure, or by a mechanical means, provides for better mold fill without increasing the static metal head so that the length of sprue necessary to fill a given mold cavity is reduced. Also, a higher metl velocity is obtained, thereby making abetter utilization of fluid life without resorting to excessive superh'eat.

In accordance with this invention, permanently backed, thin shell-lined molds are formed by preparing a charge of dry, free-flowing sand which is coated'with a thermosetting resin and injecting such material under pressure into and while vibrating a shell-forming cavity so that upon subsequent curing of the thermosetting resin, the shell liner density is uniform throughout the casting surface thereof and such surface is exceptionally smooth and has high porosity.

In the present invention the shell-forming material consists of sand of a uniform fineness which is dry, freeflowing and the particles of which are coated with a thermosetting resin. The grain size usable in this invention may be in the range of -180 AFS but preferably within the range of l20150 AFS grain fineness. The sand particles are uniformly coated with a thermosetting resin such as the phenolics or the urea-formaldehydes, although other such resins may be used.

In the present invention, the amount of resin is carefully limited to 0.73.0% by weight of the sand. When utilized with a specific sand grain size described above, improved shells are provided giving superior casting surface finish.

In general, as the grain fineness number increases, resin content is also increased because of the greater surface area of the finer grains. Total surface area is therefore correlated to grain fineness number. Resin is primarily necessary for mold liner strength. Therefore, a 150 AFS sand would require approximately 3% resin in order to be as strong as 75 AFS with 1.5% resin.

Resin content is also related to sand density. Zircon, a silicate of zirconium, ZrSiO is approximately 2 times as dense as silica sand. Therefore, only /2 as much resin is required by comparison to silica sand in order to obtain a resin coating of equivalent thickness on the refractory grains. For example, 150 AFS zircon is used in' accordance with the present invention with 0.7l% resin; while with AFS silica sand, 1.5% resin is required for similar strength properties.

In general, as the sand particle size decreases, e.g., as grain fineness increases, mold permeability decreases. Also, excess resin content will further decrease mold permeability. Therefore, with very fine sand, e.g., AFS grain size, the resin content is kept as low as possible. Good permeability is essential for proper flow of molten metal into thin sections. Air or entrapped gas in the mold cavity reduces the effective metal head. Metal flow velocity in the mold is a function of the metal head. Therefore, a reduction in metal head reduces the velocity of metal flow and, thus, for a given fluid life, the metal will not fiow as far. The result is improperly filled mold cavities. In other words, fluidity would be decreased in a mold of low permeability compared with that in a mold of high permeability.

In accordance with the present invention, the resincoated sand is blown into the liner cavity with a pressure suflicient to completely fill the liner cavity, generally at pressures of approximately 50 to 80 p.s.i. Simultaneously with the blowing operation, the back-up member and pattern forming the shell cavity are vibrated to assist the resin-coated sand to completely fill the cavity, especially for intricate configurations, and to aid in properly tamping the sand to effect a uniformity in density of the shell. Vibration of the pattern and backup member may be accomplished by mechanical vibration of the piston type or rotating ball type having frequencies within the range of 6,000 to 7,000 cycles per minute when the sand-blowing pressure is within the range of 50 to 80 p.s.i. Sonic and electronic vibrators of equivalent energies may also be used.

Applicants have discovered that the duration of the vibration cycle is important and must be shorter than the time necessary to cure the thermosetting resin coating the sand. Otherwise, rough surface on the shell will result. Generally, the vibration cycle begins with the start of the blow and is continued after the termination of the blow until the resin is only partially cured. This vibration cycle is longer than the blowing cycle but shorter than the curing time of the resin, preferably within the range of 1 to 30 seconds.

Prior to the blowing and vibration step, the patterns are preheated in the range from about 350 to 550 F. Since phenolic resins will melt quickly prior to cure, excess heat is avoided. If the resin melts too quickly, this minimizes the distance the sand can freely travel. In order to blow and fill complex thin liner cavities with the sand and resin mixture, it is often necessary to lower the preheat temperature to 350 to 400 F. By so operating, baking times are approximately 1 minute. The longest baking time has been found to be about 2 minutes. At high temperatures, baking times are so low at seconds.

Bake or cure time is dependent upon a particular phenolic resin. The higher melting point resins provide increased flow time for the mold mixture before melting and curing starts. However, the total curing time is longer for the higher melting resins. Therefore, a compromise is usually, made between the melting point of the resin, and liner mixture flow-ability in order to optimize the shortness of the cure cycle. In accordance with the present invention, it preferable to use resins having a melting point of about 220 F. When so operating, cure times of to 30 seconds are obtainable, using preheat temperatures of about 400 to 500 F.

Ceramic additives can be used to conduct heat away from and out of the cast molten metal at varying rates. Zircon sand is an example of a high conductivity ceramic that can be used. In an extension of this aspect of the invention, metal fibers can be used. In this extension, excess thermal conductivity is to be avoided because fluidity of the metal will be unduly reduced. Therefore, a balance is desired between fluidity and high conductivity to give an increase solidification rate to give good surface finish and casting soundness.

However, the shell-lined molds of the present invention invention do provide a fast cooling rate by conduction through the liner, by radiation because of the high permeability of the shells and by conduction through the backing members. This results in good casting soundness with the casting having a fine grain size. Finer grain size generally results in increased strength.

In accordance with the present invention, vacuum assist is employed during metal pouring to remove air and gas from the mold cavity and thus effectively increase metal head. Vacuum in the range of up to 21.75 inches of water has been applied in accordance with the practice of this invention. Commonly a vacuum equivalent .4 to about 8 inches of water is employed. This magnitude generally produces a significant increase in fluidity of the molten metal. Vacuum has been applied in two ways:

(1) Continuous aspiration.-An evacuated tank is attached directly to the mold. In pouring, the mold is sealed and evacuated ahead of the metal stream.

(2) Vacuum is applied after start of metal pour. An evacuated tank is attached to the mold. When metal is poured, a switch is either mechanically or electrically actuated by the metal stream, which opens a valve and applies vacuum to the mold.

Generally a static metal head of 4 inches is employed. Added to this is the effective metal head from vacuum application. For example, if 2 inches of water vacuum is used, this corresponds to 0.95 p.s.i. For a high temperature alloy with a density of 0.3 pound/inch 0.95 p.s.i. is equivalent to a metal head of 3.18 inches. Accordingly, the total head would be 4+3.18=7.18. As mentioned above, vacuum on the order of 8 inches of Water has been used. For the above example, this is equivalent to a metal head of 25.6 inches and a total head of 29.6 inches.

Metal pouring temperatures are generally the same as used for other types of molds. For example, nickel base alloys are usually poured between 2850 and 3150 F. Ductile iron is poured from 2450 to 2650" F.

In accordance with the present invention and using standard C-lO silica sand AFS grain fineness number), surface roughness (smoothness) is in the range of 70-400 microinches throughout the area cast against the shell-lined mold. Normal sand casting produces a surface roughness of up to 1,000 microinches.

An important advantage of the use of rigid molds is that dimensional control is greatly improved. Dimensional repeatability, by using the present invention, has been held within 0.002" on jet engine nozzle vanes.

A characteristic of resin-bonded shell molds is expansion on heating. This expansion has been measured to be up to 0.006/ inch at 450 F. In accordance with the present invention, a solid metal backing minimizes this thermal expansion and contributes substantially to better dimensional control.

The backing members also allow very close alignment between mold halves and significantly increase dimensional control because of the repeatability of accurate V alignment.

By the present invention, rapid pouring is utilized in order to minimize temperature loss. Pour times for small castings generally range from 0.5 to 2 seconds.

It is to be understood that this invention is subject to logical extension as will be evident to those skilled in the art. Also, the terminology employed is for the purpose of description and not of limitation.

I now claim:

1. A method of making permanently backed, shelllined molds comprising;

preparing a charge of free-flowing, dry, homogenous,

shell-forming material comprising sand coated with a thermosetting resin,

positioning a pattern in close proximity to a backing member to from a cavity therebetween,

heating the pattern and backing member to a temperature above the melting point of the thermosetting resin,

pressure injecting said charge into said cavity for a time sufiicient to fill the cavity,

vibrating said pattern and backing member at high frequency during said injecting step and continuing said vibration until such time that the resin ispartially cured,

and completing the cure of said resin after termination of said vibration to form a shell linear against said backing member of uniform density, high surface smoothness and high porosity.

2. A method as claimed in claim 1 in which said charge consists of essentially sand having a grain fineness numher in the range of 90 to 180 AFS and a resin binder in the range of about 0.7 to 3 percent by weight.

3. A method as claimed in claim 1 wherein the sand has a grain fineness of 130 AFS and the resin content is less than 1.5 percent.

4. A method as claimed in claim 1 wherein the sand is Zircon having a grain fineness in the range of 105 to 150 AFS and the resin content is present in the range from 0.7 to 1 percent by weight.

5. A method as claimed in claim 1 in which the pattern is heated to a temperature Within the range of 350 to 550 F.

6. A method as claimed in claim 1 wherein the charge is injected into the said cavity at a pressure in the range of 30 to 120 p.s.i.

7. A method as claimed in claim 1 in which the mold is vibrated for a period of from 1 to 30 seconds beginning with the sart of the injecting step.

8. A method as claimed in claim 1 in which the thermosetting resin is completely cured by baking the shelllined member at a temperature of 350 to 550 F. for a period of 15 to seconds.

References Cited UNITED STATES PATENTS I SPENCER OVERHOLSER, Primary Examiner.

EUGENE MAR, Assistant Examiner.

US. Cl. X.R.