GB2135157A - Plasma torch - Google Patents

Abstract

A plasma torch comprises an anode body into which a plasma working fluid is introduced. A nozzle 2 of the body defines an aperture from which a plasma jet issues. A cylinder 4 is fixed to the outside of the body 1 to hold a mask retaining ring 5 into the centre of which a mask 6 is located. When the mask is located in the ring it encircles the plasma jet 3. Consequently when the plasma jet is used to cut a material, the mask 6 is heated by the plasma jet to a working temperature exceeding the freezing point of the material to be cut. In this way any material vapour or molten spray occurring during the cutting process does not accumulate on the nozzle or the mask. Preferably the mask comprises an annular disc secured to the nozzle either as a friction fit or by means of a retaining ring provided on the torch. <IMAGE>

Description

SPECIFICATION Plasma torch This invention relates to plasma torches used particularly for cutting materials.

A plasma torch typically comprises a cathode, and an anode having a main body into which a gas for the plasma is fed. The main body generally has a water cooled nozzle from which the plasma jet issues. During the process of cutting with the plasma jet material from the region being cut can accumulate on the nozzle either through condensation of hot vapour on the relatively cool nozzle surface or through molten material splashes. Accumulation of frozen material on the nozzle is detrimental to the cutting process, in particular to the cut quality and cut speed.

It is an object of this invention to provide a plasma torch where accumulation of material on the nozzle during cutting of the material is restricted.

According to the invention there is provided a plasma torch having a plasma nozzle including a substantially planar mask, interposed between the nozzle and the material to be cut, which mask is arranged to encircle a plasma jet issuing from the nozzle and also to be heated by said jet in use to a working temperature exceeding the freezing point of said material.

Preferably the mask comprises an annular disc secured to the nozzle either as a friction fit or by means of a retaining ring provided on the torch.

The nozzle is therefore separated from the material to be cut by the mask which itself is arranged to be heated by the plasma jet. Since the mask is heated to a working temperature exceeding the freezing point of the material to be cut any material vapour or molten spray occurring during the cutting process will not accumulate on the nozzle or the mask, that is to say the accumulation of frozen material on the torch in general is restricted.

Consequently the above-mentined detrimental effects on the quality of the cut or the speed of the cut are reduced. The area of the mask encircling and exposed to contact with the plasma jet can be altered to thereby select the working temperature of the mask required for the different freezing points of different materials being cut and in addition the material comprising the mask can be changed.

A projection can also be arranged to extend from the mask away from the nozzle to depend into a cut produced in said materiai to be cut. The use of this projection serves to separate flexible materials after the cutting operation, thus preventing resealing of the cut by molten edges of the material.

The mask can also act as a spacer between the nozzle and the material to be cut. In this case the mask contacts the top surface of the material to be cut and material produced during cutting is either blown through the cut or is drawn away on the cut edge to thereby give a well sealed edge in the case of non-metals.

To illustrate how the invention can be put into effect reference is now made by way of example only to the accompanying drawings, in which: Figure 1 illustrates a section through a mask for use with a plasma torch in accordance with the invention; Figure2 illustrates a section through a plasma torch with the mask illustrated in Figure 1; Figure 3 illustrates a section through a plasma torch with an alternative mask; Figure 4 illustrates a section through a plasma torch shown in Figure 3 whilst cutting a material.

The features which are common to the figures bear the same reference numerals.

Referring to Figure 2, a plasma torch comprises an anode body 1 into which a plasma working fluid such as argon gas is introduced. A nozzle 2 of the body, illustrated in outline, defines an aperture from which a plasma jet 3 issues. The cathode of the torch is not shown for reasons of clarity. A cylinder 4, fixed to the outside of the body 1, holds a mask retaining ring 5 into the centre of which a mask 6 is located.

When the mask is located in the ring it encircles the plasma jet 5.

Referring to Figure 1 the mask comprises an annular disc having a central aperture 7 of diameter D through which the jet passes. The aperture opens out with an angle alpha on the side of the mask remote from the nozzle. By variation of both D and alpha the area of the mask which encircles and is exposed to contact with the plasma jet can be selected to thereby set the amount of heat transferred from the jet to the mask. Typically the angle alpha has a value of 60". In addition, such heat transfer can be changed by either selection of different materials to manufacture the mask or by changing the contact area between the mask and the nozzle of the body, which is generally water cooled.

An alternative form of mask is illustrated in Figure 3, the primary difference from the mask in Figure 1 being that the annular disc 6 includes a circular recess 10 which can slot on to the end of the torch nozzle to give a friction fit. In this way the mask is easily replaceable and a retaining ring is not required.

Figure 4 illustrates the plasma torch shown in Figure 3 producing a cut 11 in a material 8. The mask 6 includes a projection 9 extending therefrom in a direction away from the nozzle. The projection is arranged such that in the cutting process the projection depends into the cut produced in the material and therefore keeps the cut material separated.

In operation the plasma torch in accordance with the invention cuts material 8. Any molten material spray or material vapour can spray on to or condense on the mask. However material does not accumulate there since the plasma jet is arranged to heat the mask, as described hereinabove, to a working temperature exceeding the freezing point of the material. Consequently liquid material either drips on to the material to be cut or is drawn downstream of the cutting position to give a good sealed cut edge. It will be apparent that the thickness of the mask can be selected so that it acts as a spacer between the nozzle and material to be cut. Although the invention is described for a non-transferred plasma torch it can also be employed for the transferred type. In addition, although the mask is illustrated as an annular disc other planar shapes can be employed as can other methods of fixing the mask in position between the plasma torch and the material to be cut.

Claims (10)

1. A plasma torch having a plasma nozzle including a substantially planar mask, interposed between the nozzle and the material to be cut, which mask is arranged to encircle a plasma jet issuing from the nozzle and also to be heated by said jet in use to a working temperature exceeding the freezing point of said material.

2. A plasma torch as claimed in claim 1 wherein said mask is arranged to contact the plasma jet over a contact area selected to provide a desired said working temperature.

3. A torch as claimed in claim 1 or 2 wherein the nozzle is cooled and the mask is adapted to have a selected heat transfer relationship with the nozzle to provide a desired said working temperature.

4. A torch as claimed in claim 3 wherein the mask is arranged to have a predetermined area of contact with the nozzle to provide said selected heat transfer relationship.

5. Atorch as claimed in any preceding claim wherein said mask comprises an annular disc.

6. A torch as claimed in claim 5 wherein said disc is adapted to be frictionally secured to the nozzle.

7. A torch as claimed in claim 5 including retaining ring adapted to secure said disc to the nozzle.

8. Atorch as claimed in any preceding claim wherein the thickness of the mask is selected to provide a predetermined distance between the nozzle and the material to be cut when the mask is in contact therewith.

9. A torch as claimed in any preceding claim wherein the mask includes a projection extending therefrom away from the nozzle to depend between the edges of a cut produced in said material by the jet.

10. A plasma torch substantially as herein described with reference to the accompanying drawings.