METHOD AND DEVICE FOR PLASMA CUTTING OF WORK PIECES

Abstract

The present invention relates to a method and device for CO.sub.2 plasma cutting of a work piece, using a plasma cutting torch, wherein an arc is generated between the cutting head and the work piece, and a shielding gas is provided around the arc, characterized in that the shielding gas comprises CO.sub.2-snow or a mixture containing CO.sub.2-snow.

Claims

1. Method for plasma cutting of a work piece, using a plasma cutting torch, wherein a plasma arc is generated between the cutting torch and the work piece, and a shielding flow is provided around the arc, characterized in that the shielding flow comprises CO.sub.2-snow or a mixture containing CO.sub.2-snow.

2. Method according to claim 1, wherein the shielding flow is provided together with or without a carrier gas.

3. Method according to claim 1, wherein the shielding flow is provided in a flow path which is split into a first central flow component provided directly around the plasma arc and at least a second coaxial flow component provided coaxially around the central component.

4. Method according to claim 3, wherein the first flow component of the shielding flow and the second flow component of the shielding flow are directed essentially in a direction parallel to a main extension direction of the plasma arc between the cutting torch and the work piece.

5. Method according to claim 3, wherein the first and/or the second flow components of the shielding flow are directed in a direction forming a converging or a diverging angle relative to the main extension direction of the plasma arc between the cutting torch and the work piece.

6. Method according to claim 1, wherein the shielding flow or at least one of the first and second flow components of the shielding flow is directed towards the plasma arc in a direction perpendicular or essentially perpendicular to the main extension direction of the plasma arc between the cutting torch and the work piece.

7. Method according to claim 1, wherein the shielding flow is provided with a rotational component defining a rotational movement about the main extension direction of the plasma arc between the cutting torch and the work piece.

8. 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 at least one passage (114) for passing of a shielding flow between nozzle and shielding cap, wherein passage (114) for a shielding flow is configured and adapted for use of CO.sub.2-snow or a mixture containing CO.sub.2-snow as shielding flow.

9. Device according to claim 8, wherein the plasma cutting torch is provided with means to provide the at least one passage (114) to supply a shield flow comprising CO.sub.2 snow such that the CO.sub.2-snow is injected around a main plasma arc.

10. Device according to claim 8, comprising a plasma cutting torch provided with means to provide at least two passages to supply a shield flow comprising CO.sub.2-snow and further another pathway to provide a carrier gas, the carrier gas especially being selected from a group comprising CO.sub.2 gas, N.sub.2 gas, air, oxygen, argon, argon-hydrogen mix, argon-hydrogen-nitrogen mix, or a combination of the above gases

11. Device according to claim 8, comprising a shield member such that the CO.sub.2-snow shield flow is injected around a main arc in a coaxial manner or in a radial manner or in a radial and swirling manner, especially either in a clockwise or a counterclockwise direction.

12. Device according to claim 8, comprising a shield member such that the CO.sub.2-snow shield flow injected around the main arc is in an angular manner and/or swirling manner, especially either in a clockwise or a counterclockwise direction.

13. Device according to claim 8, comprising a shield member comprising multiple components to generate a swirling CO.sub.2 snow shield flow.

14. Device according to claim 8, comprising a shield member that splits the CO.sub.2 shield flow, one flow component directed around a main arc and a second flow component being provided around the shield member further away from the arc.

15. Device according to claim 14, wherein the second flow component exits the shield member in a direction parallel to a main arc, or in a direction pointing away from the main arc or in a direction pointing towards the main arc.

Description

[0029] FIG. 1 shows a schematic side sectional view of a plasma cutting torch adapted to implement a first preferred embodiment of the method according to the invention,

[0030] FIG. 2 a further schematic side sectional view of a plasma cutting torch adapted to implement a second preferred embodiment of the method according to the invention,

[0031] FIG. 3 shows a further schematic side sectional view of a plasma cutting torch adapted to implement a third preferred embodiment of the method according to the invention, and

[0032] FIG. 3 shows a further schematic side sectional view of a plasma cutting torch adapted to implement a fourth preferred embodiment of the method according to the invention.

[0033] In FIG. 1, a schematic side sectional view of a cutting torch for plasma cutting is shown. The torch is generally designated 100. 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 114a 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.

[0034] 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 a 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.

[0035] In order to achieve a highly constricted plasma arc 160 for plasma cutting, it is proposed to use CO.sub.2-snow as a shielding flow passing through passage 114. 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 drawbacks of liquid water injection due to the sublimation of the CO.sub.2 snow.

[0036] 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.

[0037] As can be seen from FIG. 1, the lower section 114a of passage 114 is arranged so that it is directed towards the plasma arc 160, i.e. defining a closing angle relative to plasma arc 160. This arrangement leads to an especially efficient constriction of the plasma arc. The direction of shielding flow ejected out of section 114a is indicated by arrow 124a.

[0038] 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 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.

[0039] Also, as CO.sub.2-snow sublimes, it leaves no residue on the work piece being cut, and it can cleanly and easily pass through filters for example of a down draft table.

[0040] FIG. 2 shows further preferred embodiments of a cutting torch adapted to implement two variations of an embodiment of the method of the invention. Similar components as already discussed referring to FIG. 1 are designated with the same reference numerals, and will not be described in detail again.

[0041] The main difference over the previously discussed embodiment is that the flow path through passage 114 for CO.sub.2-snow is split into a central section 114a, corresponding to the lower section 114a as described with reference to FIG. 1, and a further outwardly directed section 114b or, alternatively, 114c adapted to further protect the cutting zone and provide a second curtain surrounding the curtain provided by the first passage 114a through which shielding fluid is ejected.

[0042] In FIG. 2, two possible orientations of such a coaxial section are shown. In actual embodiments, only one of these two variants will be implemented within one nozzle. On the left hand side, a coaxial section 114b defining an opening angle relative to the main extension direction of plasma arc 160 is shown, i.e. an angle directed away from the main extension direction of plasma are 160 (as indicated by arrow 124b). On the right hand side of FIG. 2, a coaxial path 114c extending essentially parallel to the main extension direction of plasma arc 160 is shown, leading to an ejection of shielding flow parallel to the main extension direction of plasma arc 160, as indicated by arrow 124c. Be it noted that the coaxial path could also be directed in a closing angle, i.e. towards the main extension of plasma arc 160, although this variant is not shown in FIG. 2.

[0043] According to a further embodiment shown in FIG. 3, CO.sub.2-snow may also be directed in a radial direction directly at the plasma arc 160, i.e. in a direction essentially perpendicular to the main extension direction of the plasma arc (arrow 124e). To implement this variant, the lower section 114e of passage 114 is oriented in a direction perpendicular to the upper sections of passages 114 and to the main direction of plasma arc 160. Here again, passage 114 could be split, for example into one section as defined by lower sections 114e, and a further section in which CO.sub.2-snow is ejected in a coaxial manner with limited or no direct impact on the plasma arc 160, i.e. essentially parallel to the main direction of plasma arc 160.

[0044] Be it noted that the introduction of CO.sub.2-snow can be provided in such a way that CO.sub.2-snow and a carrier gas are introduced into the various passages 114. Alternatively, feed stock such as liquid CO.sub.2 or CO.sub.2-gas can be fed directly into passage 114, and the CO.sub.2-snow generation process effected within passage 114, as schematically indicated at reference numeral 170 in FIG. 3, which shows a snow generation zone.

[0045] A further carrier gas line may be provided to introduce a carrier gas for the CO.sub.2-snow into the cutting torch. This further gas line (not shown in FIG. 3) can be provided at the upper end of the torch, to integrally mix carrier gas with CO.sub.2-snow after its generation. Such a carrier gas line could also be provided externally and introduced into the torch after CO.sub.2-snow is generated.

[0046] FIG. 4 shows a further possible implementation of a nozzle 110 of a cutting torch for implementing a further embodiment of the method according to the invention. Here, the CO.sub.2-flow, as provided through passage 114, is split into multiple parts after CO.sub.2-snow generation at 170. As can be seen, passage 114 is provided with a first lower section 114e, corresponding to the lower section 114e of FIG. 3, which leads to a direction of CO.sub.2-snow radially or perpendicularly upon plasma arc 160. A further section 114f is provided, which leads to an injection of CO.sub.2-snow essentially parallel to the main extension direction of arc 160.

[0047] In a further implementation, not shown in the figures, a rotational or swirl flow could be imposed on the CO.sub.2-snow, providing it with a rotational component relative to the main direction of plasma are 160. This may be achieved by different methods such as having injection ports that are offset from the center of the cutting torch such that the flow is injected off center, thereby generating a swirling component.

[0048] All advantages discussed in connection with the embodiment of FIG. 1 are equally applicable to the variants shown in FIGS. 2 to 4.

[0049] Be it noted that a constant amount of CO.sub.2-snow can be used at all current levels and material thicknesses to be cut. However, in preferred implementations, the amount of CO.sub.2 used is an increasing function of the current of the plasma cutting arc. Specifically, it is advantageous to set the CO.sub.2-snow flow rate as a function of the current level of the plasma arc. Also, the CO.sub.2-snow flow rate can be set to match the plasma gas flow at a rate of 1:1, 0.5:1, 2:1, 5:1 and 15:1. Intermediate, lower or higher ratios are also possible. Also much higher flow rates are possible with the implementations shown in FIG. 2 through FIG. 4.

[0050] The method as described, using CO.sub.2-snow as a shielding fluid, may be used for cutting various materials, especially, but not limited to, mild steel or carbon steel, stainless steel, aluminum, copper, titanium, brass etc.

[0051] 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 shielding fluid and an oxygen gas as carrier gas, or oxygen plasma in combination with CO.sub.2-snow as shielding fluid and air as carrier gas.

[0052] 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 shielding 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 shielding 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.

[0053] 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 twice the CO.sub.2-snow, or is set at 5 times the CO.sub.2-snow, or is set at ten or 15 times the CO.sub.2-snow flow rate. Intermediate or higher ratios are also possible.