DEVICE AND METHOD FOR PLASMA CUTTING OF WORK PIECES

Abstract

The present invention relates to a device for plasma cutting, comprising a cutting torch (100) provided with an electrode (120), which is coaxially surrounded by a nozzle (110), thereby defining a passage (112) for passing of a plasma gas between electrode and nozzle, wherein the nozzle is coaxially surrounded by a shielding cap (122), thereby defining a passage (114) for passing of a shielding flow between nozzle and shielding cap, the device further comprising an annular member (200) coaxially surrounding the cutting torch (100) configured and adapted to provide a further curtain flow coaxially surrounding the shielding flow through passage (250a and 250b) wherein annular member (200) is configured and adapted for use of CO.sub.2-snow or a mixture containing CO.sub.2-snow as shielding flow.

Claims

1. Device for plasma cutting a work piece, comprising a cutting torch (100) configured and adapted to generate a plasma arc between itself and the workpiece, the cutting tool being provided with an electrode (120), which is coaxially surrounded by a nozzle (110), thereby defining a passage (112) for passing of a plasma gas between electrode and nozzle, wherein the nozzle is coaxially surrounded by a shielding cap (122), thereby defining a passage (114) for passing of a shielding flow between nozzle and shielding cap, the device further comprising an annular member (200) coaxially surrounding the cutting torch (100) configured and adapted to provide a further curtain flow coaxially surrounding the shielding flow through passage (114) between nozzle and shielding cap (122) wherein annular member (200) is configured and adapted for use of CO.sub.2-snow or a mixture containing CO.sub.2-snow as shielding flow.

2. Device according to claim 1, wherein passage (114) is configured and adapted for use of CO.sub.2-snow or a mixture containing CO.sub.2-snow as shielding flow.

3. Device according to claim 1, wherein the annular member (200) is configured and adapted to be provided with CO.sub.2-snow and to form the curtain flow using this provided CO.sub.2-snow.

4. Device according to claim 1, wherein the annular member is configured and adapted to be provided with liquid CO.sub.2, generate CO.sub.2-snow on the basis of this provided liquid CO.sub.2, and form the curtain flow using the generated CO.sub.2-snow.

5. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide a shielding flow comprising CO.sub.2-snow with or without a carrier gas.

6. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide a curtain flow which is directed in a direction forming a converging or a diverging angle relative to a main extension direction of the plasma arc generated between the cutting torch and the work piece.

7. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide a curtain flow which is directed in a direction parallel or essentially parallel to a main extension direction of the plasma arc generated between the cutting torch and the work piece.

8. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide a curtain flow in a direction perpendicular or essentially perpendicular to a main extension direction of the plasma arc generated between the cutting torch and the work piece.

9. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide a curtain flow with a rotational component defining a rotational movement about a main extension direction of the plasma arc generated between the cutting torch and the work piece.

10. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide the curtain flow as a continuous annular curtain.

11. Device according to claim 1, wherein the annular member (200) is configured and adapted to provide the curtain flow in form of a set of annularly arranged jets provided around the circumference of the annular member.

12. Method for plasma cutting a work piece, comprising the following steps: providing a cutting torch configured and adapted to generate a plasma arc between itself and the work piece, providing a shielding flow to coaxially surrounding the plasma arc, and providing a curtain flow coaxially surrounding the shielding flow, wherein the curtain flow comprises CO2-snow or a mixture containing CO2-snow.

13. Method according to claim 12, wherein an annular member (200) is provided to provide the curtain flow, the annular member being configured and adapted for use of CO2-snow or a mixture containing CO2-snow as shielding flow.

Description

[0037] Preferred embodiments of the invention will now be described with reference to the figures.

[0038] FIG. 1 shows a schematic side sectional view of a device for plasma cutting according to a first embodiment of the invention,

[0039] FIG. 2 a further schematic side sectional view of a device for plasma cutting according to a second embodiment of the invention, and

[0040] FIG. 3 shows schematic front views of an annular member as used in preferred embodiments of the device according to the invention.

[0041] In FIG. 1, a schematic side sectional view of a preferred embodiment of a device for plasma cutting a work piece is shown and generally designated by reference numeral 10. Device 10 comprises a cutting torch 100 and an annular member coaxially surrounding the cutting torch, also referred to as a muffler. The torch is generally designated 100, the annular member 200.

[0042] The cutting torch 100 comprises an electrode (cathode 120) coaxially surrounded by a nozzle 110. Electrode 120 and nozzle 110 defines a central passage 112 for passing of a plasma gas around electrode 120, i.e. between electrode 120 and nozzle 110. Coaxially surrounding nozzle 110 there is provided a shield 122, defining a passage 114 for a shielding flow between nozzle 110 and shield 122. The cutting torch 100 is arranged above a work piece 130 to be cut, the work piece acts as an anode during plasma cutting.

[0043] As is well-known in the art, an electrical cutting current flows from a schematically shown current source 140 to the plasma cutting torch 110 via electrode 120, a plasma arc 160 constricted by the nozzle 110 to the work piece 130 and back to the current source 140 (only shown in FIG. 1). As is also well-known in the art, this arrangement leads to formation of plasma cutting arc 160 between the electrode 120 and the work piece 130. The cutting arc 160 defines a main extension direction along the shortest line between the electrode 120 and the work piece 130.

[0044] Annular member 200 coaxially surrounds cutting torch 100. It can be provided as an add-on-extension for any plasma cutting torch, i.e. any kind of cutting torch can be retro-fitted with such an annular member. Annular member 200 comprises an inlet port 210 for an inlet path 212, via which CO.sub.2 feed in liquid form and, optionally, a suitable compressed carrier gas, such as nitrogen, oxygen, air, argon, etc. or a mixture thereof, is introduced into the annular member. Within the annular member 200, there is provided a (schematically shown) CO.sub.2-snow generator 220 for generating CO.sub.2-snow by expanding the CO.sub.2 feed. CO.sub.2 snow thus generated is ejected via an outlet port 230, i.e. injected towards the working piece 130, thus defining an outlet path a shielding flow 250a comprising i.e. injected generated CO.sub.2-snow. The compressed carrier gas acts to increase the momentum of the thus generated snow particles. This is for example especially advantageous in case of relatively high operating currents of the cutting torch 100, but also in case of low currents. By means of increasing the momentum or speed of the CO.sub.2-snow particles, the flow field around arc 160 and cutting torch 100 can be effectively improved. The type of carrier gas used may be a function of the material of the work piece being cut. The ejected CO.sub.2-snow together with the carrier gas thus forms curtain flow 250a around cutting torch 100 in an effective manner.

[0045] According to the embodiment as shown in FIG. 1, the curtain flow 250a is directed onto the work piece 130 in a direction parallel to the main extension direction of plasma arc 160. This is, for example, achievable by providing a nozzle or nozzles within outlet port 230, through which the curtain flow is ejected, with a corresponding orientation.

[0046] As schematically shown in FIG. 3, outlet port 230 can for example be provided with a continuous annular gap or nozzle 230a, or a multitude of discrete nozzles or jets 230b arranged in a circle, for example around an inner or outer circumference of annular member 230. To achieve the ejection direction for curtain flow 250a shown in FIG. 1, these nozzles are orientated such that, in the view of FIG. 3, the curtain flow is ejected perpendicularly to the plane of projection.

[0047] Alternatively, if the CO.sub.2 feed is introduced into annular member 220 without a carrier gas, the CO.sub.2-snow may be ejected without such a carrier gas through outlet port 230, such that the CO.sub.2-snow falls freely, for example due to a pressure within annular member 200 and gravitation, around the arc and cutting torch 100.

[0048] FIG. 2 shows a second preferred embodiment of the device according to the invention. All corresponding components are designated using the same reference numerals, so only the differences over the embodiment of FIG. 1 will be discussed in the following. The only difference is that according to the embodiment as shown in FIG. 2, the curtain flow (designated 250 b in FIG. 2) is ejected from outlet port 230 of annular member 200 in a direction forming a converging angle relative to the main extension direction of plasma arc 160. To achieve such a converging angle, it is, for example, possible to provide the nozzles 230a, 230b as shown in FIG. 3 with a corresponding orientation, i.e. in a converging manner relative to a central axis 230c extending perpendicularly to the plane of projection (not preferred as it will excessively disturb the main plasma arc flow and the shield gas flow ejected from paths 112 and path 114 respectively). It is also possible to provide the shielding flow in a direction forming a diverging angle relative to the main extension direction of arc 160, although this variant is not explicitly shown in the figures.

[0049] Providing the curtain flow 250b in a converging angle relative to the main extension direction of the plasma arc can improve the effectiveness of protection and cooling especially in cases of higher arc currents. At higher currents, the plasma arc is less susceptible to an impinging effect of the CO.sub.2-snow, so that a slightly converging angle can provide better protection and cooling without degrading the plasma arc.

[0050] Advantageous settings are for example that the curtain flow comprising CO.sub.2-snow is directed in a direction forming a converging or diverging angle relative to the main extension direction of the plasma arc of 5, 10, 15, 20, 25 or 30 degrees, or in a range from 5 to 30, 5 to 20, 5 to 15, 10 to 15, 10 to 20 or 20-30 degrees. Larger, smaller or any intermediate angles are also possible.

[0051] The method as described, using CO.sub.2-snow provided by an annular member of muffler surrounding a cutting torch, as a curtain fluid, may be used during cutting of various materials, especially, but not limited to, mild steel or carbon steel, stainless steel, aluminum, copper, titanium, brass etc.

[0052] At higher currents and in case of cutting thicker materials, more fumes, noise and UV light are typically generated. Thus, the amount of CO.sub.2-snow advantageously provided increases with the current of the plasma arc. Advantageously, the CO.sub.2-snow flow rate is set as a function of the plasma cutting torch current.

[0053] In a preferred embodiment, the following combinations of CO.sub.2-snow and carrier gas may be considered advantageous, for example for carbon steel cutting: oxygen plasma in combination with CO.sub.2-snow as curtain fluid and an oxygen gas as carrier gas, or oxygen plasma in combination with CO.sub.2-snow as curtain fluid and air as carrier gas.

[0054] For stainless steel or aluminum, and also for certain non-ferrous materials: nitrogen plasma can be used in combination with CO.sub.2-snow as curtain fluid and nitrogen gas as carrier gas, or Ar—H.sub.2 mixture (example: 35% H.sub.2 with the balance argon, often referred to as H35) plasma in combination with CO.sub.2-snow as curtain fluid and nitrogen gas as carrier gas, or Ar—H.sub.2 mixture (example H35) plasma in combination with CO.sub.2-snow and Ar—H.sub.2 gas mixture as carrier gas, or Ar—H.sub.2 and N.sub.2 plasma (with various mix ratios) in combination with CO.sub.2-snow as shielding fluid and nitrogen gas as carrier gas, or Ar—H.sub.2 and N.sub.2 plasma (with various mix ratios) in combination with CO.sub.2-snow and Ar—H.sub.2+N.sub.2 gas mixture as carrier gas, or an N.sub.2—H.sub.2 mixture (example: F5) as plasma in combination with CO.sub.2-snow as shielding fluid and nitrogen gas as carrier gas.

[0055] Especially, the following combinations of plasma gas/shield flow/curtain flow are advantageous in case of cutting mild steel: O2/O2/CO.sub.2-snow, O2/O2/CO2-snow+air, O2/O2/CO2-snow+any carrier gas, air/air/CO2-snow+any carrier gas.

[0056] It is also possible to use the following combination: O2/CO2-snow+O2/CO2-snow+any gas. Here, the shield flow (i. e. the flow surrounding the plasma arc) as well as the curtain flow (i. e. the flow surrounding the shield flow) both contain CO2-snow together with an appropriate carrier gas.

[0057] The ratios between CO.sub.2-snow and the carrier gas flow are advantageously related in such a way that the carrier gas flow rate is set at 0.5 of the CO.sub.2-snow flow rate, or is set to match the CO.sub.2-snow flow rate, or is set at 1.5 times or twice the CO.sub.2-snow flow rate, or is set at 5 times the CO.sub.2-snow flow rate, or is set at ten or 15 times the CO.sub.2-snow flow rate. Intermediate or higher ratios are also possible.

[0058] However, in a simple and thus easy to handle implementation, an expedient CO.sub.2 flow rate for annular member 200 is used for all current and cutting torch shield gas flow settings of cutting torch 100.

[0059] Be it noted that is also possible to provide annular member 200 with CO.sub.2-snow from an external source. In this case, for example, means for expanding CO.sub.2 feed within annular member 200 can be omitted. Such a simplified annular member or muffler can be provided with a carrier gas feed, if desired.

[0060] All embodiments as shown can help to minimize or eliminate drawbacks experienced in prior art plasma cutting. Once fume particles generated during plasma cutting are condensed from the gaseous phase due to the low temperatures of the CO.sub.2-snow applied and also providing nucleation sites, metal and oxide particles are carried towards the work piece and collected. As the CO.sub.2-snow warms up, it changes phase from solid to gas (i.e. it sublimes), so that no waste liquid remains, which would have to be collected and disposed of. The CO.sub.2-snow also acts as a barrier that can capture metal and metal oxide particles generated during cutting.

[0061] The added cooling provided by this CO.sub.2-snow ejected from annular member 200 improves survivability of the cutting torch as a whole. Using a device according to the invention, there is no need to use water tables and under water cutting, thus improving cutting quality and speed. Both noise levels and light emissions are reduced resulting in an improved working environment for an operator.

[0062] In order to further optimise a highly constricted plasma arc 160 for plasma cutting, it is additionally possible to use CO.sub.2-snow as a shielding flow passing through passage 114 of the actual cutting torch 100. This CO.sub.2-snow acts as a constricting flow (gas-solid mixture, i.e. a two phase flow) for cooling the fringes of arc 160. As the fringes of the plasma arc cool down, the arc diameter decreases, causing an increase in the core temperature of the plasma. This results in an increase in electrical conductivity of the plasma arc 160, thereby allowing conduction of the same current through a reduced cross sectional area of the plasma arc. This increase in arc constriction improves the piercing capacity, cutting speed and cutting quality achievable with plasma arc 160. This improves the constriction in traditional shield gas flows. It also eliminates the draw backs of liquid water injection due to the sublimation of the CO.sub.2-snow.

[0063] The CO.sub.2-snow may be injected without any further carrier gas through passage 114. In a preferred embodiment, however, CO.sub.2-snow is injected together with a carrier gas, such as nitrogen, oxygen, air, argon, etc. or a mixture thereof.

[0064] The CO.sub.2-snow thus ejected around the plasma arc 160 acts also as a curtain to immediately cool, condense and nucleate any metallic fume generated on the work piece 130 into particulates, preventing an uncollected escape. Furthermore, it effectively reduces noise levels generated by the process by acting as a damping barrier to the noise generated by the plasma arc. Also, it absorbs UV radiation generated in the process and prevents the formation of ozone further away from the arc zone along the radiation path. CO.sub.2-snow provided through passage 114 acting as a shielding flow also cools the outside of the torch during cutting or piercing of thick material work pieces and during higher current operation, whereby the life of a plasma cutting torch and its consumables, especially nozzle, and shield etc. can be increased. Also, it effectively cools thinner work pieces such as thin plates, thereby reducing warpage and thus eliminating complex procedures of nesting various cutting paths across the length and width of the work piece, which, in prior art applications, can increase cutting time and reduce the process throughput.