Electric heating – Metal heating – By arc
Reexamination Certificate
1999-04-09
2001-08-07
Walberg, Teresa (Department: 3742)
Electric heating
Metal heating
By arc
C219S137310
Reexamination Certificate
active
06271497
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to plasma torches, and more particularly to a sloughing and fracture resistant plasma torch head and a method for making the same.
BACKGROUND OF THE INVENTION
Plasma torches are frequently used for welding, hard facing, and cutting metals. These plasma torches have a torch head, to which a torch handle and a set of terminal components such as an electrode, a tip, and a shield cup are attached. The torch head has embedded therein various components for transferring fluids and electrical currents, which include a high frequency (HF) lead and a power lead/gas conduit. The power lead/gas conduit is separated from the HF lead by an insulating member. The torch head is fabricated from an electrically insulating and heat resistant material which is either a phenolic molding material or an elastomer such as a rubber compound. The torch head is typically formed by molding under pressure with the other components embedded therein.
For torch heads formed of heat resistant phenolic molding material, the life of the torch head is typically limited by fracturing of the torch head which is caused by abrasion of the surface in combination with thermal cycling of the torch head during use. To reduce such problems, multi-part torch heads have been fabricated having a core surrounded by a shell. The core segment is fabricated from a thermally stable epoxy or phenolic molding material which has a portion of the transfer components embedded therein. The core segment in turn is housed in a two-part shell cover, fabricated from a heat resistant phenolic molding material, which is fastened around the core segment. The use of a multiple part assembly allows safety switches to readily be incorporated into the torch head. The fabrication of the torch head from a multiple part assembly also reduces the mean thickness of the torch head elements and enhances the life of the torch head by reducing the thermal stresses. While the life of the multi-part torch head is greater than the life of the single part torch head, the resulting torch heads still suffer limited useful lives due to fracturing.
A discussion of the fabrication of torch heads is found in U.S. Pat. No. 5,519,185, which suggests that the use of elastomers for the housing is preferred to extend the usable life of the torch head. The '185 patent notes that the fabrication of housings from silicone rubber, which is one of the preferred elastomers for heat resistance, is particularly difficult since the flow characteristics of the silicone rubber are poor. The '185 patent teaches that torch heads formed from silicone rubber are usually formed by wrapping the components to be embedded in the housing with strips of silicone rubber and then pressing the resulting assembly in a mold to obtain the desired shape, with the excess material being squeezed out of the mold leaving a flash which is trimmed away.
It is noted in the '185 patent that these elastomer materials are subject to deterioration over time due to sloughing, and that this problem can be reduced by employing a three-layer structure having a first layer of a heat resistant elastomer, a second layer of a fibrous heat and abrasion resistant reinforcing material, and a third layer of a heat resistant elastomer. While the resulting structure provides a reduction in sloughing compared to single-layer elastomer torch heads, the improved life is obtained at the expense of complicating an already complex fabrication technique.
Thus, there is a need for a plasma torch head which is readily fabricated and which has a high resistance to damage from cyclic thermal stresses, mechanical wear, sloughing, and fracturing.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide a plasma torch head having improved resistance to sloughing and fracturing.
It is another object of the present invention to provide a plasma torch head having an elastic core surrounded by a rigid shell.
Still another object of the present invention is to provide a plasma torch head having inner transfer components embedded in an elastic material which does not require wrapping of the transfer components during fabrication.
Yet a further object of the present invention is to provide a plasma torch head having an elastic core for embedding the inner transfer components with enhanced resistance to sloughing.
It is an object of the present invention to provide a plasma torch head having a rigid exterior surface with extended surface life.
It is yet another object of the present invention to provide a durable plasma torch head which can be readily fabricated.
It is still another object of the present invention to provide a method for readily fabricating a torch head within improved durability.
It is still another object of the present invention to provide a torch head with improved durability which can incorporate safety switches.
SUMMARY OF THE INVENTION
The present invention is for a plasma torch head which has transfer components which are spaced apart and positioned to partially overlap, providing an overlapping region of the transfer components. The transfer components serve to transfer fluids and currents through the plasma torch head, and preferably include at least one insulating member to assure the electrical isolation of the current carrying transfer components.
A core fabricated from an elastomer is provided, into which a core-supported section of the overlapping region of the transfer components is embedded, while leaving an unsupported section of the overlapping region of the transfer components which extends beyond the core. The elastomer employed is preferably a moldable thermosetting rubber, and more preferably silicone rubber. To simplify fabrication, it is further preferred that a liquid moldable rubber be employed to form the core by injection of the liquid moldable rubber into the mold. Silicone rubbers in liquid form are commercially available from Wacker Silicones Corp. and GE Silicones.
The core of the plasma torch head is housed in a rigid shell. It is preferred that the rigid shell house not only the core, but also a portion of the unsupported section of the overlapping region of the transfer components. It is further preferred for the unsupported section of the overlapping region of the transfer components to be engaged by the rigid shell to provide support to and provide separation of the transfer components.
The shell can be fabricated as a two-piece shell which surrounds the core. The sections of the shell can be fabricated either by casting, in which case it is preferred for a two-part epoxy to be employed, or by molding, in which case it is preferred to employ a phenolic molding material such as Bakelite®.
Alternatively, the shell can be fabricated as a single piece by either injection molding, transfer molding, or by casting the shell around the elastomer core. More preferably, the shell should encase a portion of the unsupported section of the overlapping region of the transfer components. When a single piece shell is employed, it is preferred that the shell be cast from a two-part epoxy.
The plasma torch head of the present invention can be fabricated by various methods. Preferably, the plasma torch head is fabricated from injection molded elements, compression molded elements, cast elements, or a combination thereof. In one method, the core of the plasma torch head is fabricated by compression molding strips of an elastomer which are wrapped about the transfer components, as is discussed in the '185 patent.
The preferred method of fabricating the core is by injection molding. When the core is formed by injection molding, a core injection mold is employed, which is designed to position and hold the transfer components and any insulating members in the appropriate spacial relationship. The transfer components and any insulating members are set in the core injection mold and the mold closed to fixedly position the transfer components and insulating members in the core injection mold. An elastomer, which is prefer
Semprebon Jeffrey E.
Tatras, Inc.
Van Quang
Walberg Teresa
Weins Michael J.
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