US2221302A - Investment ring heater - Google Patents

Description

1940- v D. D. MlZZY ET AL 2,221,302

INVESTMENT RING HEATER Filed Feb. 13, 1940 A TTORNEY Patented Nov. 12, 1940 UNITED STATES INVESTMENT RING HEATER Dan D. Mizzy and David L. Durst, Sacramento,

alif.

Application February 13, 1940, Serial No. 318,684

3 Claims.

This invention relates to a dental casting device, and especially to an electrically operated heater designed for the reception of investment rings and for heating the same to a predetermined temperature, the present application being a continuation in part of our pending application entitled Inlay heater and ring, filed May 26, 1937, Serial No. 144,878, now issued as Patent No. 2,188,915, Feb. 6, 1940.

It is common practice in making dental castings or inlays to form a wax impression or pattern conforming to the shape of the tooth cavity to be filled. After the pattern is removed from the cavity it is placed on a sprue pin and then embedded in an investment material. When the material is hardened, the wax is melted out by subjecting the investment material to a sufficient temperature to melt and permit complete removal of the wax. The result of this operation is that a cavity corresponding to the tooth cavity is formed in the investment material, and as such is ready for the gold or other material from which the inlay is to be cast. The sprue hole leading to the cavity in the investment material is generally so small that molten metal will not flow freely into it. For this reason pressure or centrifugal force is required to force the metal into the cavity.

Various devices have been provided for this purpose, but with varying degrees of success, as it is usually necessary to transfer the investment material from a furnace or other heating device to the casting machine before the molten metal can be poured. This transfer takes time, and very often causes a considerable temperature drop in the investment material, which is undesirable due to the comparatively high coefficient of expansion or contraction of the molten metal. Plainly speaking, the investment material should be heated to a point of, for instance, 125i) R, where it is expanded an amount equal to the shrinkage of the metal when it changes from a molten to a solid state. This obviously will not take place unless the investment material is first heated to bring about the proper expansion and then maintained at that temperature until the metal is actually poured.

The object of the present invention is generally to improve and simplify the construction and operation of dental casting devices of the character described; to provide a heater and investment ring which is adapted to be inserted in a centrifugal, pressure, or like casting machine; to provide a heater and investment ring which is adapted to be electrically heated directly in the casting machine, and maintained at the required temperature until the very moment the molten metal is to be cast; to provide a heater in which the investment ring is removable so that a number of investment rings may be prepared and cast one after another in the same heater Without cooling the heater; to provide a heater and investment ring which will require a minimum of investment material and from which the investment material and the completed casting may be quickly and readily removed; to provide an electrically actuated heater of the character described which is brought up to a predetermined temperature in a comparatively short time, and in which electric current consumption is maintained at an absolute minimum; to provide a heater which is so constructed and designed that it may be received and secured in practically any type of casting machine, whether centrifugal or pressure actuated; and further, to provide an electrically operated heater which is compact and rugged in construction to withstand abuse of every kind.

The heater is shown by way of illustration in the accompanying drawing, in which-- Fig. l is a perspective View of the support on which the electric resistance heating element of the heater is wound and secured;

Fig. 2 is a similar View showing the resistance heating element covered with insulating material, and the clamping rings and terminal posts or prongs which are connected with the opposite ends of the resistance heating element;

Fig. 3 is a central vertical section of the heater;

Fig. 4 is a perspective view of the same;

Fig. 5 is a side elevation of the sprue former, showing the sprue pin and wax pattern mounted thereon;

Fig. 6 is a vertical section of the investment ring, showing it in position on the sprue former, and also showing the investment material after it has been poured into the investment ring;

Fig. '7 is a View similar to Fig. 6, showing the sprue former and pin removed and the investment ring ready for insertion in the heater; and

Fig. 8 is a perspective view of the investment ring.

When a dentist is working on a cavity in a tooth, and has clean d it out and prepared it for the insertion of a filling, if a gold inlay filling is to be employed an impression is taken of the cavity with a small piece of wax. After the impression or pattern has been made, the pattern is removed and may have the appearance of that indicated at 2 in Fig. 5. It is placed on a pin 3 which is known as a sprue pin, and this pin is in turn placed in a rubber plate known as a sprue former, which is indicated at 4. A tubular sleeve 5, known as an investment ring, terminates in a rounded head section 6, on which is formed an annular flange 6a. A thin composition of inlay investment material is poured into the inlay ring from the upper end, until it is full, as shown in Fig. 6, and the investment-painted pattern on the sprue pin and former is gently worked down into the investment ring and seated. The investment ring and sprue former are left until the investment material has substantially set, and the sprue former, together with the pin, is then removed. This leaves the wax pattern within the investment material, as shown at 9 in Fig. '7, and it leaves a passage 8 where the sprue pin has been removed.

The next operation in general practice is to place the investment in a furnace and there to heat it until the wax pattern melts and the wax flows out of the hole 8. A portion of the wax usually remains and carbonizes on the wall of the mold, leaving a residue. The gold or other molten metal to be employed is then poured in to fill the cavity left by the wax pattern, and after the metal has hardened the investment material is broken out and the casting recovered and prepared for final insertion in the cavity of the tooth. A great deal of skill, experience and practice is required to operate in this manner, as the casting will be either too small or too large, depending upon the temperature to which the investment material was heated. If it was heated to the exact or proper temperature, the casting would be of correct size, but even so, its exterior surface has a tendency to be porous or rough, due to the carbonized wax left within the mold cavity.

The heater forming the subject matter of the present invention is designed to utilize a known principle, to wit, that molten wax flows towards a cold surface; hence if the investment ring and investment material can be gradually heated from the exterior, and not at the sprue opening end 8, the wax as it melts will flow outwardly through the comparatively cool sprue pin opening, and will be completely dissipated, thereby preventing carbonization or the forming of carbon particles on the inner face of the mold cavity.

The heater is best illustrated in Figs. 1 to 4 inclusive. It consists of an inner metallic cylinder 9 about which is placed a layer IQ of any suitable form of insulating material, such as asbestos, mica or the like. Placed on top thereof is a thin strip of mica II, which extends from end to end of the insulated cylinder 9. Perforations are formed at opposite ends of the mica strip. One end of an electric resistance wire, constructed of nichrome or the like, is passed through the upper perforations as shown at l2, and is then wound about the cylinder to form a heating coil or element l3. The lower end of the wire is passed out through the perforation M, and two free ends I2 and I4 are thus provided. The heating element I3 is now covered with a layer of asbestos l5, or similar insulating material, and two rings l6 and H are slipped over and clamped around the insulating material. The free end of the wire I! is bent down through a notch in the insulating material and soldered or welded to a lug l8 on one ring, and the other free end is similarly bent over and secured to a lug IS on ring ll. Both rings are reinforced with plates such as shown at 20, and these are drilled and tapped to receive the threaded end of the pair of pins or contact prongs 2| which are applied later. The rings l5 and H are made of any suitable material, and form conductors between the prongs 2| and the respective ends of the heating element l3. Also, they form a sturdy support for the prongs, this being exceedingly essential as they are subject to considerable wrenching and bending action when an electric connector or socket is applied, as will hereinafter be described.

' After the heating element has been prepared as shown in Fig. 2, it is again covered with insulating material, as shown at 23, and it is inserted within an outer cylinder 24 (see Figs. 3 and 4) An upper head member in the form of a flaring ring 25 is then introduced, and flanges 26 and 21 formed thereon are welded or otherwise secured in place to permanently seal and enclose one end of the electric heating element. A ringshaped bottom plate 28 is inserted in the lower end, and flanges 29 and 30 formed thereon are welded or otherwise secured to the respective cylinders 9 and 24, thereby permanently enclosing and sealing the heating element. One side of the exterior cylinder has a rectangularlyshaped opening formed therein which is bridged by a clamp generally indicated at 3|. A rectangular block of insulating material is slipped between the clamp and the cylinder, and its face when inserted is exposed by forming a rectangularly-shaped opening in the clamp. The insulating block has two holes formed therein which register with the threaded openings formed in the reinforcing plates 20 on the rings l6 and H. The prongs or pins 2| are inserted through the openings and screwed into the tapped reinforcing plates, and the heater is thus completed. The heater when completed is exceedingly compact and rugged in construction, its actual size being surprisingly small, for instance it is only 1% inches in diameter and has an overall length of 1 inches. This shape, size and length makes it suitable for insertion in practically any type of casting machine, whether centrifugally or pressure actuated.

The heater is in actuality a small furnace, and does away with the larger furnaces in general use at present. The current consumption is so small as to be almost negligible, and at that it assumes the required temperature, which in actual practice is about 1250 F., in fifteen minutes or less. The heater has the advantage that it may be placed directly in the casting machine with the investment ring mounted therein. Current is applied to the heater by merely placing a socket or connector, such as shown in dotted lines in Fig. 4, over the prongs. As the heater heats up, the heat is applied to the exterior of the investment ring and around the cylindrical portion 5 thereof. The investment material is thus heated from the exterior inwardly and comparatively little if any heat is applied at the ends; as the natural tendency of melting wax is to flow toward a cold surface, it will flow freely out of the sprue opening 8 as. the temperature increases, and will be completely dissipated before carbonization can take place.

The number of turns and the size of wire used in the heating element I3 is such that a temperature of 1250 F. will be reached quickly, and the investment material will be maintained at that temperature, conduction and convection losses being such that that temperature will not be exceeded. Thus when the molten gold is ready to be poured or cast, it is only necessary to pull or remove the socket and to start the casting machine in operation. The moment the gold has been poured, whether centrifugally or otherwise,

it hardens almost immediately. The heater and investment ring may thus be removed, and a second investment ring inserted, this being accomplished without the cooling of the heater. The cast investment ring may be plunged into cool water, and thus the investment material may be immediately broken out and the casting recovered. When it is recovered it will be of exact size, as the investment material has been heated to the exact temperature required; and the casting will be smooth in finish, as no carbon particles were formed during the heating of the investment material.

The several parts forming the heater are made of metal, and these are welded with relation to each other, thus making a strong, rigid housing or body, and as the prongs or terminal ends of the heating element are supported at their inner ends by the threaded connection formed between the rings l 6 and I1, and are supported also by the insulating block inserted within the clamp 3|, they are so rigidly supported that any wrenching or bending to which they may be subjected when applying or removing the socket member will neither tend to bend or break the electric connections. In plain words, the heater may be subjected to unusual abuse without detrimental effect.

While certain features of our invention have been more or less specifically described for purposes of illustration, we wish it understood that changes may be resorted to within the scope of the appended claims and that the materials and finish may be such as the experience of the manufacturer may dictate and other conditions demand.

Having thus described our invention, what we claim and desire to secure by Letters Patent is:

1. An electric heater of the character described comprising an inner cylindrical tube, a heating element in the form of a helical wire coil sup ported on the tube and insulated therefrom by insulating material exterior of the coil, a pair of clamping and current-conducting rings surrounding and securing the last named insulating material, a connection between each end of the wire coil and the respective clamping rings, an exterior cylindrical tube, a head member at each end secured to the tubes and closing the space between the tubes, and a pair of spaced contact pins extending through the exterior tube and insulated therefrom, said pins being secured at their inner ends to the clamping and conducting rings to supply electric current to the helical heating coil.

2. An electric heater of the character described comprising an inner cylindrical tube, a heating element in the form or" a helical wire coil supported on the tube and insulated therefrom by insulating material exterior of the coil, a pair of clamping and current-conducting rings surrounding securing the last named insulating material, a connection between each end of the wire coil and the respective clamping rings, an exterior cylindrical tube, a head member at each end secured to the tubes and closing the space between the tubes, a pair of spaced contact pins extending through the exterior tube and insulated therefrom, said pins being secured at their inner ends to the clamping and conducting rings to supply electric current to the helical heating coil, a clamp secured on the exterior face of the exterior tube, a block of insulating material secured thereby, said block having a pair of holes formed therein through which the contact pins extend, said insulating block forming a guide and support for the contact pins to increase their rigidity and to prevent bending or displacement of the same.

3. An electric heater of the character described comprising a pair of inner and outer concentrically positioned metal tubes having an annular space formed between them, a ring-shaped head secured to the respective ends of the tubes and forming a closure for each end of the annular space, an electric resistance wire in the form of a helical coil disposed in the annular space, a pair of combination clamping and conductor rings surrounding the helical coil, said coil and rings being electrically insulated from each other and the tubes, an electric connection between each end of the helical coil and the conductor rings, a pair of contact pins secured at their innor ends one to each conductor ring, the outer ends of said pins projecting through the outer tube, and a block of hard material secured on the exterior tube and having openings formed therein through which the contact pins extend, said block insulating the pins from the exterior tube, and forming a support for the outer ends of the pins to prevent displacement or bending of the same.

DAN D. MIZZY. DAVID L. DURST.

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