GB2111359A

GB2111359A - Microwave heating

Info

Publication number: GB2111359A
Application number: GB08224405A
Authority: GB
Inventors: Gregory Allen Benedict; Albert Miller Rockwood
Original assignee: Howmet Turbine Components Corp
Current assignee: Howmet Turbine Components Corp
Priority date: 1981-10-23
Filing date: 1982-08-25
Publication date: 1983-06-29
Also published as: JPS5884638A; DE3234658A1; FR2515096A1

Abstract

A crack-free seal is formed between adjacent, complementary, thermoplastic solid bodies 4, 5 by placing an electro-magnetically absorptive wafer 3 between the thermoplastic bodies, bringing the bodies into close proximity with the wafer therebetween, and irradiating the wafer and bodies with electro- magnetic radiation at a frequency that only the wafer substantially absorbs. The process is especially useful for sealing wax patterns and gating as part of the lost wax process for producing metal alloy castings. <IMAGE>

Description

SPECIFICATION Sealing thermoplastic bodies BACKGROUND OF THE INVENTION The present invention relates to an improved process for forming a crack-free seal between thermoplastic bodies. The invention has particular applicability to sealing wax patterns and gating in the "lost wax" process.

The "lost wax" process is one of the oldest techniques for producing metal alloy castings. To make castings in the process, a wax pattern identical in shape to the desired final metal casting is formed. A ceramic mold is then formed around the pattern and fired into a negative of the desired metal casting. During the firing process, the original wax pattern is melted or burned away.

Molten metal is introduced into the ceramic mold, and after the metal has cooled and hardened, the ceramic mold is chipped away, leaving the desired final metal casting.

To introduce molten metal into the ceramic mold, a conduit must be provided. To form such conduits or channels in the ceramic molds, wax gating must be attached to the wax patterns.

Since a ceramic mold is only used once before it is chipped away and destroyed, many wax patterns are often linked to a series of wax gating bars all linked to a pouring cup making a single wax cluster of many patterns.

One of the most labor intensive operations in the lost wax process has been the linking of the wax patterns and gating to form a completed wax cluster. The seal between a wax pattern and gating must be free of cracks lest the liquid ceramic slurry solidify in those cracks, and, after dewaxing, intrude into the route to be followed by the molten metal. Ceramic particles broken off by the metal flow would contaminate and destroy the utility of the final casting. The wax itself must also be fairly pure, so that when it is burned off, no residues remain that might contaminate the metal casting. Any process for sealing these wax clusters must, therefore, provide a liquid-tight seal without either contaminating the final casting or deforming the wax patterns and gating.

The current process for sealing the junctures in wax clusters involves highly skilled labor.

Workmen carefully seal each individual juncture by applying a heated implement, such as a hot knife, to the juncture. This process is expensive and time consuming. A need has long existed for a way to automate this step, but previous attempts have been unsuccessful.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a simple, reliable, and relatively inexpensive process for forming a crack-free seal between adjacent, complementary, thermoplastic solid bodies.

Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, the process of this invention for forming a crack-free seal between complementary, thermoplastic solid bodies comprises placing an electromagnetically absorptive wafer between the thermoplastic bodies; bringing the bodies into close proximity with the wafer therebetween to form an assembly; and irradiating the assembly with electromagnetic radiation at a frequency that the wafer substantially absorbs, to melt adjacent thermoplastic surfaces and form the seal between the bodies. The wafer preferably projects beyond the outer perimeter of at least one of the bodies.

Furthermore, the wafer is preferably substantially ashless upon combustion.

In a preferred embodiment according to the invention, the wafer comprises crystalline graphite in a polystyrene binder. It is also preferred that the electromagnetic radiation be in the microwave region so that an assembly occupying a substantial volume can be sealed simultaneously.

In an especially preferred embodiment, the thermoplastic bodies are wax patterns and gating that are sealed in the lost wax process. The wax pattern and gating preferably have complementary male and female portions, and the wafer preferably has an opening that fits around the male portion, so that the wafer may be oriented accurately in the assembly. The fit between the male portion and the female portion should be snug enough to hold the pattern to the gating without stressing the contact area during the melting and solidification that occur during the sealing process.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one embodiment of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a sectional view of thermoplastic bodies to be united and sealed in a preferred embodiment of the invention; FIGURE 2 is a plan view of a gating bar employed in the embodiment of Figure 1; and FIGURE 3 is a plan view of a wafer employed in the embodiment of Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to the presently preferred embodiments of the invention, an example of which is illustrated in the accompanying drawings.

In accordance with the invention, a crack-free seal is formed between adjacent, complementary, thermoplastic solid bodies. A wide range of thermoplastic bodies may be sealed with the process of the invention, as will be appreciated by those having ordinary skill in the art. The thermoplastic bodies preferably should be substantially unaffected by the incident electromagnetic radiation used according to the invention, so that the thermoplastic bodies will not be directly heated by the radiation. As embodied herein, the thermoplastic solid bodies are wax patterns and gating used in the lost wax process. For example, in Figure 1 are illustrated wax patterns 1 and gating bar 2. Any wax and wax fillers conventionally employed in the lost wax process may be used in this embodiment.

Particularly preferred wax compositions that provide consistently good sealing are paraffinbased waxes.

In accordance with the invention, an electromagnetically absorptive wafer is placed between the thermoplastic bodies, as illustrated in Figure 1 , and the thermoplastic bodies are then brought into close proximity with the wafer therebetween. Those having ordinary skill in the art will be able to select electromagnetically absorptive materials from which the wafer can be made for particular embodiments. The material should be rapidly heated by the electromagnetic radiation to melt and seal the abutting surfaces of the adjacent thermoplastic bodies. The wafer preferably comprises a carbonaceous material dispersed in a substantially electromagnetically non-absorptive material. An especially preferred material is crystalline graphite in a polystyrene binder.The composition of the wafer is preferably about 20% to 30% by weight crystalline graphite dispersed in 70% to 80% by weight polystyrene.

The grain size of the carbonaceous material is preferably between about 0.5 and 0.6 microns.

The thickness of the wafer is preferably about 0.005 inches. The wafer preferably projects about 1/16 of an inch beyond the perimeter of at least one of the thermoplastic bodies that are to be sealed to obtain optimum results. If the wafer does not project beyond the perimeter of at least one of the thermoplastic bodies, undercutting and uneven results may occur.

In the preferred embodiment in which the thermoplastic bodies are wax patterns and gating employed in the lost wax process, the wafer is preferably ashless upon combustion and certainly non-metallic. Although the material that the wafer is made of may float away in the dewax process or burn off in the preheat process, to avoid contamination of the alloy castings, the electromagnetically absorptive material in the wafer should be substantially ashless upon combustion. For example, crystalline graphite is commerically available that has an ash content of only 0.2% by weight.

The binder for the preferably carbonaceous electromagnetically absorptive material is preferably a substantially electromagnetically nonabsorptive material. Additional factors in selecting the binder are low melting point and low ash content, but with adequate strength for convenient handling. The binder should also be readily mixed with the carbonaceous electromagnetically absorptive material, and when mixed with the carbonaceous material, easily formed into a thin film. Preferred binders are polymeric materials, such as polystyrene and polyethylene. Those of ordinary skill in the art will be able to determine other binders having such properties, but the binder presently most preferred by the applicant is polystyrene.

The thickness of the wafer is not important, as long as there is sufficient electromagnetically absorptive material to cause melting of the thermoplastic bodies and the wafer is thick enough to provide satisfactory handling characteristics. As already indicated, a thickness of about 0.005 inches is preferred.

In the preferred embodiment relating to the lost wax process, the wax patterns and gating should be held together without stressing the area of contact during the melting and solidification phases to develop a smooth, crackless fillet. A loose fit may allow parts to sag or to separate from each other, and the force of gravity may cause molten wax to bulge or run.

One advantageous method, in accordance with the invention, for achieving a desirable physical fit between the thermoplastic bodies during formation of the seal is to provide complementary male and female portions in the thermoplastic bodies. As illustrated in Figure 1 , pin 4 is a noncircular, tapered male member and mating cavity 5 is a female member. Wafer 3, shown in Figure 3, may be provided with opening 6, which fits around pin 4. Wafer 3 should extend slightly beyond the perimeter of at least one of the thermoplastic bodies that are to be sealed, such as pattern 1 as illustrated in the drawing. Other expediencies for maintaining the position of the thermoplastic bodies and the wafer during the formation of the seal without applying stress will occur to those having ordinary skill in the art.

After the thermoplastic bodies and the wafer have been brought into close proximity in accordance with the present invention, the wafer and bodies are irradiated with electromagnetic radiation at a frequency that the wafer only substantially absorbs. The radiation causes the molecular structure of the absorptive material to align with the alternating electrical field, thereby causing the absorptive material to heat rapidly due to the heat generated by the friction of its molecules. The thermoplastic bodies, on the other hand, are unaffected by the radiation and therefore maintain their integrity. The complementary contacting surfaces of the thermoplastic bodies, however, are heated by conduction from the water to above their melting point. After the radiation is discontinued, the melted portions of the thermoplastic bodies blend and solidfy to form a crack-free seal.

Electromagnetic radiation over a wide range of frequencies can be used for heating, especially radiation having a frequency of between 103 and 1014 hertz. The preferred electromagnetic radiation is that in the microwave region. The use of microwave energy is advantageous, because of its selective effect on many materials. Materials that are non-polar, such as wax, aluminium oxide, and most ceramics, are unaffected by microwave radiation. Materials that are polar, such as water and graphite, are rapidly heated in the presence of microwave energy.

Standard microwave equipment operating at a frequency between 915 M Hz and 2450 M Hz can be used to supply microwave energy. The sealing time may vary widely; successful sealing has been accomplished at times ranging from 2 seconds to 2 minutes. The greater the power level of the microwave source is, the shorter the sealing time will be. With a specific test pattern, with all other variables being held constant, the time for sealing varied from about 45 seconds at about 100 watts to about 4 seconds at about 700 watts of power.

For most practical applications, a microwave unit capable of supplying power up to 2.5 KW should be adequate. Other variables that affect sealing time are the proportion of graphite in the lossy film, the type and size of graphite particle, the binder used, the thickness of the wafer, the degree of protrusion of the wafer from the pattern periphery, the size of the sealed sprue, the number of patterns being sealed, and the design of the microwave resonant cavity.

In the preferred embodiment relating to the lost wax process, sealing and bonding wax patterns to wax gating can be quickly accomplished by first joining them mechanically with a tapered pin connection, so that a thin polar wafer is trapped between the mating surfaces as shown in Figure 1. When an assembly is placed in a microwave energy field, bonding and sealing preferably will occur in less than 15 seconds. The wax of the mating surfaces of the patterns and gating melts and flows around and embeds the projecting edges of the wafer, which remains solid and intact, to form a smooth, crackless fillet. After about 30 seconds the wax liquified by the hot wafer will resolidify, leaving a sealed bond between pattern and gating that reaches full strength in several minutes, depending upon the configuration of the pattern.

Successful joining of patterns to both sides of a gating bar has been achieved as shown in the drawings. The process of the present invention thus can advantageously join a plurality of thermoplastic bodies simultaneously. It can also be used to join multiple- as well as single-gated parts. It is preferred that the planes of the pairs of surfaces to be sealed be substantially parallel and horizontal, to prevent running of liquified thermoplastic material. This problem can be minimized, however, by proper selection of the thermoplastic material, since some are more resistant to running when liquified than others. In any event, after the entire assembly of thermoplastic bodies has been appropriately mechanically linked, all of the junctures may be sealed in a single operation with a single exposure to the electromagnetic radiation.Since the thermoplastic bodies are unaffected by the radiation, the source of the electromagnetic radiation does not have to be focused on the individual junctures, but rather may safely be directed at the entire assembly.

There are achieved with the present invention, as broadly described and embodied herein, compared with prior techniques, a greater consistency in achieving satisfactory seals at far greater speeds with significant labor savings. As is apparent, the process can be easily automated.

For example, because of the speed with which seals can be formed using electromagnetic radiation, a single source of electromagnetic radiation can handle an entire plant's production.

A conveyor system can deliver assemblies from a plurality of work stations. A clicker press, with dies, can be used to cut appropriately-shaped wafers from lossy film rolls at the same rate that assemblies enter the electromagnetic radiation field. A control system can be used to automatically set cycle time, field intensity, and pulse shape to ensure optimum performance for each assembly. Workers alongside the conveyor can assemble the thermoplastic bodies and wafers and place them in position for sealing on a tray that is substantially unaffected by the electromagnetic radiation. Information encoded on the tray or otherwise provided can be automatically fed into the control system to set the electromagnetic radiation source conditions for each assembly as it passes into the electromagnetic field.

It will be apparent to those skilled in the art that various modifications and variations could be made in the process of the invention without departing from the scope or spirit of the invention.

Claims

1. A process for forming a crack-free seal between adjacent, complementary, thermoplastic solid bodies comprising: a) placing an electromagnetically absorptive wafer between said thermoplastic bodies; b) bringing said bodies into close proximity with said wafer therebetween to form an assembly; and c) irradiating said assembly with electromagnetic radiation at a frequency that said wafer substantially absorbs, to melt adjacent thermoplastic surfaces and form said seal between said bodies.

2. The process of Claim 1 wherein said thermoplastic solid bodies each are substantially unaffected by the incident electromagnetic radiation.

3. The process of Claim 1 wherein said wafer projects beyond the outer perimeter of at least one of said bodies when said bodies are in close proximity with said wafer therebetween.

4. The process of Claim 2 or 3 wherein said wafer is combustible.

5. The process of Claim 4 wherein said wafer is substantially ashless upon combustion.

6. The process of Claim 5 wherein said wafer comprises a carbonaceous material dispersed in a substantially electromagnetically non-absorptive material.

7. The process of Claim 1 wherein the electromagnetic radiation is in the microwave region.

8. The process of Claim 1 or 7 wherein at least one of said bodies comprises wax.

9. The process of Claim 1 or 7 wherein both of said bodies comprise wax.

10. The process of Claim 9 wherein said bodies are a wax pattern and a wax gating.

1 1. A process for forming a crack-free seal between adjacent, complementary wax parts comprising: a) placing a wafer of substantially electromagnetically absorptive material between said parts; b) bringing said parts into close proximity with said wafer therebetween; and c) irradiating said wafer and said parts with electromagnetic radiation in the microwave region of the spectrum to form said seal between said parts.

12. The process of Claim 1 1 wherein said wafer perimeter projects beyond the perimeter of at least one of said parts.

13. The process of Claim 1 1 wherein said wafer is combustible.

14. The process of Claim 13 wherein said wafer is substantially ashless upon combustion.

15. The process of Claim 14 wherein said wafer is approximately 0.005 inches thick.

16. The process of Claim 1 or 1 1 wherein said wafer comprises about 20 to 30% by weight of crystalline graphite.

17. The process of Claim 16 wherein said wafer comprises about 70~80% by weight of polystyrene.

18. In the lost wax process for making castings of metal alloys by preparing a wax pattern and gating, forming a seal between said wax pattern and gating, forming a ceramic mold about said wax pattern and gating, and removing said wax pattern and gating by melting or burning out said wax pattern and gating, the improvement comprising: a) placing a wafer that is substantially ashless upon combustion and comprises about 20 to 30% by weight of crystalline graphite and about 70 to 80% by weight of polystyrene between said wax pattern and gating; b) bringing said wax pattern and gating into close proximity with said wafer therebetween; and c) irradiating said wafer and wax pattern and gating with electromagnetic radiation in the microwave region to form a crack-free seal.

19. The improvement of Claim 18 wherein the edge of said wafer projects beyond the outer perimeter of at least one of the wax pattern and gating.

20. The improvement of Claim 19 wherein said wax pattern and gating have complementary male and female portions and said wafer has an opening that fits around said male portion, and wherein steps (a) and (b) comprise placing said wafer around said male portion and then inserting said male portion into said female portion.

21. The improvement of Claim 20 wherein said wafer is approximately 0.005 inches thick.

22. A process for forming a crack-free seal between adjacent, complementary, thermoplastic solid bodies substantially as herein described.

23. A process for forming a crack-free seal between adjacent, complementary, thermoplastic solid bodies substantially as herein described with reference to any one of the figures of the accompanying drawings.