US3259949A - Casting method - Google Patents

Description

July 12, 1966 w. H. MOORE 3,259,949

CASTING METHOD Filed Jan. 16, 1964 l/V VE/VTOR.

W/L LIAM H. MOORE BY Una-Ln CUI J.

United States Patent Filed Jan. 16, 1964, Ser. No. 338,044 2 Claims. (Cl. 22--196) This invention relates to foundry molds for making castings and, more particularly, to foundry molds using consumable patterns, instead of the more conventional wooden, or metal patterns, which are removed from the molds prior to casting and which may be used over and over again.

Cavityless molds, or molds made from consumable patterns have several distinct advantages, the chief of which are that the shape to be cast may take any configuration, and molding itself comprises ramming the sand, or other suitable molding material, around the pattern, without regard to a parting line or pattern draft. This allows for a considerable decrease in cost of molding, particularly on parts where a single casting is required from a given pattern.

Conveniently, the expendable pattern may be made from an expanded plastic material such as expanded polystyrene or polyethylene which, as is well known, is porous because it contains air voids in this expanded condition and which is lightweight, cheap, and easy to form into any desired shape. This pattern material is capable of being volatilized or burned particularly in the absence of oxygen without appreciable residue remaining. The method of casting conventionally employed by those skilled in the art, is to utilize the molten charge for combusting and destroying the pattern. Molten metal poured into a mold containing a polystyrene pattern consumes the pattern and fills the resultant cavity with molten metal, which later solidifies, leaving a casting having the exact contour of the original pattern.

This method has several drawbacks, the chief of which are a rapid evolution of combustible vapor, gas and smoke during the casting and the production of dirt and dross on the castings, resulting from residue from incompletely combusted patterns. This residue tends to float to the upper surface of the casting, resulting in defects of sufficient magnitude to cause rejection of the casting.

It is the primary object of this invention to provide a means of removing an expanded plastic pattern such as polystyrene fiom a mold prior to the casting operation. Another object is to provide a means of eliminating casting defects resulting from pattern residue.

A further object is to provide a means of reducing explosion hazards from the rapid evolution of vapor and gas during the casting operation.

A further object is to allow the use of conventional gating and risering means in producing castings from expendable expanded plastic patterns.

Other objects and advantages will appear to those skilled in the art from the following-considered in conjunction with the accompanying drawing which is identified as:

FIGURE 1 which is a diagrammatic representation of a preferred method of this invention.

Reference numeral 1 identifies a polystyrene pattern complete with riser 2, ingate 3, and vent 10, rammed in a flask 5 in such a way that it is surrounded by molding sand 4. Reference numeral 6 is a carbon doxide gas supply passing through a spiral conduit 7 contained in a heating chamber 8, which is heated by means of a gas torch 9. The heated carbon dioxide gas passes through an adapter pipe 11 and into the mold cavity through the gating inlet 3.

Referring to the drawings, the pattern 1, preferably shaped fiom polystyrene, is coated with beeswax and moldwash, or other suitable coatings known to those skilled in the art. The pattern is placed in the flask S and is surrounded by the molding sand 4 packed in position by any desired ramming method.

At the same time, the risers 2 and ingate 3 and Vent 10 are placed in position in the mold. These items may be formed from polystyrene, or may consist of conventional wooden forms which are withdrawn from the mold immediately after ramming, thereby leaving a cavity connecting to the pattern. When made from polystyrene, they may be left in the sand, to be subsequently consumed along with the polystyrene pattern.

The chamber 8 is then placed in position, relative to the mold, so that the adapter pipe 11 enters the mold at the gate 3, the riser 2, or any other convenient cavity connected to the polystyrene pattern 1. The chamber 8 is heated by igniting the gas torch 9 to a temperature suflicient to cause the carbon dioxide gas, which passes through the spiral 7 to be heated to a temperature in excess of 500 F. and preferably, in excess of 800 F.

The valve on the carbon dioxide gas cylinder 6 is opened, so that carbon dioxide flows through the spiral 7 contained in the heated chamber 8 and thence through the adapter pipe 11, into the mold cavity.

Passage of the heated carbon dioxide gas into the mold causes volatilization and complete removal of the polystyrene pattern, leaving a clean, well hardened cavity in place of the pattern. The heating chamber and gas cylinder are then removed from the mold and the molten metal is introduced by conventional means through the ingate 3.

In the case of polystyrene, while there is some vaporization or volatilization at 500 F. and above, it is found that it may be completely consumed in an oxygen free atmosphere at temperatures in excess of 800 F.

As noted it is advisable to exceed 800 F. and for rapid vaporization, a higher temperature, namely, about 1,000 or 1,200 F. may also be used. I have found that the range of from 800 F. to 900 F. is the most advantageous. This temperature may be obtained quite readily, Without any special heating means other than a heated chamber, as shown, but at the same time, it is sufliciently high to cause fairly rapid vaporization of the pattern from the mold. I do not wish to be strictly held to the exact mechanism by which the plastic pattern is consumed, that is whether by volatilization or vaporization or by some other mechanism so long as most of the pattern passes from the mold cavity. It will be appreciated that these tempera tures will vary for different materials and the gas which is used herein for causing consumation of the pattern should be at or above the volatilization temperature of the material to be consumed.

It is, of course, quite possible to remove the polystyrene pattern from the mold by passing heated air, or oxygen through the mold. In fact, I have also consumed the pattern by first heating a portion of it with a gas torch and then passing oxygen onto the heated portion. I have found, however, that when oxygen is employed, a partial combustion and a partial vaporization occurs; very high temperatures are produced in the mold cavity, which tends to destroy this cavity unless it is made from a refractory other than conventional molding sand and, further than this, tars and liquid polystyrene and other residue tend to form in the lower portions of the mold cavity and these residues resist further removal by combustion. On subsequent casting these residues will be entrapped in the casting itself, thereby causing a defective casting.

By using an inert gas, such as carbon dioxide, nitrogen, or argon and by heating this gas to a temperature which will cause it to vaporize the polystyrene without substantial burning, I am able to'completely prevent the formation of unwanted residues in the mold cavity. I am also able to control the rate of removal of the polystyrene to a point where explosions from ignition of volatile vapors are not a serious factor.

While any inert gas may be used for this purpose, for obvious reasons, carbon dioxide and nitrogen are preferable becauseof their lower initial cost. Carbon dioxide is particularly preferred, because its presence may also cause suitable hardening when the molding material is bonded with sodium silicate, as is well known to those skilledin the art.

I have, found that when a polystyrene pattern is consumed by. the process of this invention, the polystyrene vapor tends to leave the mold cavity through the pores of the, sand, thereby hardening this sand and binding it to a high degree of'efiiciency, also providinga suitably neutral atmosphere during the subsequent casting operation. I prefer to provide a vent. at some upper portion of the mold, to remove excess polystyrene vapor and to speed up the complete-removal of the polystyrene pattern. The vent in FIGURE 1 may represent such a vent andothers may be provided;

I find thatwith average patterns, ranging in size up to about 12" or 15 in thickness, I am able to completely remove a pattern in from 10 to minutes, although time is no particular object and may be accelerated by increasing the temperature of the inert gas, or may be slowed down by decreasing the number of vents in the mold. This may be conveniently adjusted according to the particular conditions that pertain in each foundry.

Removal of the polystyrene by the process of this invention results in a moldcavity which can be cast by conventional means. Where the pattern is not removed prior to casting, as in other methods, a tremendous back pres-.

sure is exerted on the molten metal entering the cavity and polystyrene vapor is,evolved at extremely rapid rates, causing. a danger of explosion and the entry of the metal is delayed by approximately for any given gating area.

In the. process. ofv this invention, conventionally sized gatesand, conventional pouring speeds most, suited to any particular casting may be used withoutdifficulty, as the pattern is removed from the mold prior to the pouring operation, This overcomes some of the serious objectionsfound when patterns are not previously removed from the mold, as by other methods.

By usingthe process of this invention, I have been able to successfully, cast extremely small castings, which would be a matter, of difliculty, where the pattern is notrernoved from the mold prior to pouring, because of the long time taken to fill the mold cavity. Where the time is long on small castings and the back pressure is high, due to evolution of polystyrene vapor, the tendency is to get mis-run castings. This objection is completely overcome by the process of this invention.

At the same time, I have been able to cast extremely large castings weighing up to 10 tons or more, without any undue danger resulting because of the rapid evolution of polystyrene gas during the, casting operation as in the prior art methods.

In the process of this invention, also, the rate of removal of thepolystyrene pattern from the mold by means of the hot inert gascan be controlled in such a manner. that the baking, or hardening of the mold is relatively complete. Where this is not. capable of. control, as in other methods where the polystyrene pattern is left in the mold, to be consumed by the molten metal, the mold is not always hardened to a sufficient degree and the danger of a misshapen casting exists. In the process of my invention I am able to produce accuratecastings which are not misshapen and which can be gated, risered, and poured according to the procedures normally used in any foundry. This means that it is unnecessary for anyone using the process of this inevntion to evolve special methods of gatingand risering andspecial methods of. pouring.

In filling a mold with-molten metal, where the polystyrene pattern is left in, as in other methods, it is necessary to apply special gating techniques to exclude as much air as possible because the presence of air will, lead to the production of unwantedresidues in themold cavity. For this reason, under these methods it is normal to, gate at the bottom of a moldandfor the advancing metal to consume the pattern in such a. wayv that air. is. excluded. Ingthe process of my invention, none of these special gating techniques are necessary, becausethe pattern; isfirstremoved.

from the moldby meanspt the hot, inert. gases.

While I am able tosecure the, advantages of. conyen.-.

tional casting means by. first. destroying the. polystyrene pattern, according to the process of this.invention, I. am also able to capturethe advantages of; the cavityless HIOld': ing method, whichare chiefly; no partinglinegnoneed. for suitable draft on the pattern; andnoneedfor careful mold ramming and mold finishing, closing and pouring, to pre vent leakage at the mold joint. 7

While I. have described thisv invention withyav certain degree of particularity, itv is understood.that'disclosure, of.

of expanded polystyrene and. polyethylene resins, rama.

ming sand around saidpattern, introducing an inert gas selected from the groupconsisting of carbon dioxide; n i-.

.trogen, and argon at a temperature in excess of. 5009 1 said gasimpinging directly on said pattern, thereby substantially. volatilizing said material of construction in. the

substantial absence of oxygen with subsequent passing of same from its position in thesand, and thereafter pouring molten metal into the-resultant; cavity,

2. The method of making a metal shape including the steps of selecting. a consumable plasticpattern havingv a material of construction comprising an. expanded plastic r esin,.placing sand around said pattern, introducing an inert gasat a temperaturein excess of 5009 F. directly on said pattern, thereby substantially volatilizingsaidjmaserial of construction in the substantial absence. ofjoxrygen with subsequent passing of the same from its. position in the sand, and thereafter pouring molten metal into.

the resultant cavity.

References Cited by the Examiner.

UNITED STATES PATENTS 2,362,507 11/1944 Steinbacket'al. 2 2.1 -96 2,518,040. 8/1950 Mann 22?19 6y 2,830,343 4/1958 Shroyer 227-196. 3,132,388 4/1964 Grant 22]96 MARCUS U. LYONS, Primary Examiner.

Claims (1)

1. THE METHOD OF MAKING A METAL SHAPE COMPRISING THE STEPS OF SELECTING A CONSUMABLE PLASTIC PATTERN HAVING A MATERIAL OF CONSTRUCTION SELECTED FROM THE GROUP CONSISTING OF EXPANDED POLYSTYRENE AND POLYETHYLENE RESINS, RAMMING SAND AROUND SAID PATTERN, INTRODUCING AN INERT GAS SELECTED FROM THE GROUP CONSISTING OF CARBON DIOXIDE, NITROGEN, AND ARGON AT A TEMPERATURE IN EXCESS OF 500*F., SAID GAS IMPINGING DIRECTLY ON SAID PATTERN, THEREBY SUBSTANTIALLY VOLATILIZING SAID MATERIAL OF CONSTRUCTION IN THE SUBSTANTIAL ABSENCE OF OXYGEN WITH SUBSEQUENT PASSING OF SAME FROM ITS POSITION IN THE SAND AND THEREAFTER POURING MOLTEN METAL INTO THE RESULTANT CAVITY.

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