Patent Application
20210121971
The invention relates to a method and a device for plasma cutting of work pieces.
A plasma is a super-heated, electrically conductive fluid composed of positive and negative ions, electrons and excited and neutral atoms and molecules. Different gases/fluids can be used as the plasma gases/fluids. These gases/fluids dissociate and ionize by means of the electrical energy deposited into the plasma by the electric arc.
The plasma arc cutting process, also known as plasma cutting or arc cutting, a well-known manufacturing process, is commonly used for cutting, marking and gouging of conductive materials. Plasma cutting uses a highly constricted arc with a high energy density and high pressure to heat, melt and blow the resulting molten material off a workpiece to be cut. The process typically uses a plasma forming gas and a shielding fluid. The shielding fluid can be a gas or a liquid that is injected around a main plasma arc. In earlier developments, gas was commonly used as a shield, and subsequently liquid water was also used within the first few years of the invention of the process.
In plasma cutting, a cutting torch is utilized, which typically comprises an electrode, a nozzle, a plasma gas distributor, a shield gas/fluid distributor and a shield cap. In the following, such a shield cap is sometimes simply referred to as a shield. Herein, the nozzle coaxially surrounds the electrode, defining a passage for passing of a plasma forming gas therebetween, and the shield cap coaxially surrounds the nozzle, defining a passage for passing of a shielding gas or fluid therebetween. The nozzle is provided with passages for a plasma gas and the shield cap is provided with passages for a shielding fluid. Within the nozzle, a plasma arc is generated between the nozzle and the electrode during the piloting phase. As the arc exits the nozzle, under the action of the plasma forming gas flow, the power supply senses the extension of the arc towards the work piece and disconnects the nozzle from the circuit forcing the arc to fully transfer to the work piece. The parameters of a plasma arc can be influenced by the design of the nozzle, from which the plasma arc is ejected, and the electrode within the nozzle, shield nozzle, and both the gas/fluid distribution members for both the plasma and the shield lines.
It is the goal of plasma cutting system to provide a plasma cutting arc that achieves the highest cutting speed at the highest cutting quality and therefore at the best cost base. Quality is, in part, defined as the lack of bottom dross (solidified metal hanging on the lower edge of the cut face), angularity of the cut surface as well as its smoothness. To achieve such goals, the plasma cutting arc needs to provide high energy density at a very localized footprint during cutting. The stability of the arc must be maintained to avoid any striations and imperfections of the surface of the cut and provides controlled repeatability. Therefore, the plasma cutting arcs tend to be highly constricted. Traditionally, the constriction is achieved through a number of methods known in the prior art literature. Four main methods are typically used, (1) wall constriction and stabilization, (2) plasma gas swirl flow constriction and stabilization, (3) shield flow constriction and stabilization and (4) magnetic constriction and stabilization. There are limitations to all the above methods. For example, a small diameter nozzle that provides wall constriction and stabilization is limited by the high arc heat load (the smaller the diameter at a given current/power level the higher the heat load will be) and the nozzle material selection (typically copper or high thermal conductivity metals) that allows the removal of such heat. Similarly, a properly established plasma forming gas swirl enhances such constriction and stabilization when combined in a correct way with the selected plasma gas nozzle orifice design. Furthermore, properly injecting the correct amount of shield flow (gas or liquid, swirled or non-swirled) dramatically improves the constriction and stability of the arc. Magnetic constriction, while theoretically possible, tends to require a large magnetic field values, on the order of a Tesla rendering it industrially impractical for constriction purposes. The injection shield flow improves the arc constriction by properly shaping the boundary layer between the arc and the outside atmosphere. As the shield flow is injected around the arc, it cools the fringes of the arc effectively decreasing its foot print. The cooled arc fingers no longer possess the high temperature required to carry the electrical current. The effective decrease of the current carrying cross section, therefore, forces an increase in temperature of the plasma core, thereby increasing the electrical conductivity of the plasma to compensate for such cross section reduction and maintain the constant current provided by the power source. The shield flow also provides a buffer against the atmosphere and preserves the chemistry of the plasma flow, minimizes or enhances the flow velocity of the arc and the overall flow field.
It is the intent of this invention to improve on the use of shield flow gas and the shield flow liquid (typically water) by injecting CO2 snow. There is provided enhanced arc cooling and shaping of the arc. The CO2 snow sublimes and it leaves no liquid that flows over the surface of the metal to be cut. It also provides a means of preventing the atmospheric gases from contaminating the plasma arc and changing its desired chemistry.
Plasma cutting is an intense source of pollutants, these pollutants including metal particulates and gases (e.g. ozone, NO, NO2, . . . ), electromagnetic radiation (UV light) and sound emissions. In the prior art, a number of technologies have been introduced to reduce these sources of pollutants, such as a water muffler providing a curtain around the torch to capture fumes, UV light and to reduce sound levels. Further prior art methods include underwater cutting, where the torch and/or the work piece are fully or partially submerged in water during the cutting process. Furthermore, water tables, in which the water level is at or just below the bottom surface of a plate being cut, and down draft tables, in which the ambient air is sucked through the table and passed through filters, have been used. Furthermore, it is known to introduce water as the shielding fluid in the plasma cutting torch.
These technologies have drawbacks. For example, under water cutting or water tables produce waste water that has to be properly collected, stored and disposed of, which leads to higher operating costs. Underwater cutting and water table cutting produce lower quality cuts and reduce the consumable life of the plasma cutting torches, especially of torch components such as electrodes, nozzles and shield caps. In some applications, so called water mufflers are used. These are components that are used to introduce water around the plasma torches. Typically, these are provided as additional components that are not designed as part of the torch. Water mufflers also inject water that gets contaminated and requires collection and disposition. Also, water leaves marks on the work piece or plate, that are especially undesirable for aluminium and stainless steel, so that this leads to the requirement of further cleaning, and can also cause rusting on mild steel.
Down draft tables tend to be the most useful for dust collection, but in facilities where the plate being cut is very large, the design of the table becomes critical to be able to extract all of the fumes. Furthermore, water impacting the filters of down draft tables can cause damage to the filters, thereby reducing their effectiveness and life. Down draft tables are not able to reduce noise or block UV arc radiation.
A further solution that has been used in plasma cutting is the use of water as shielding fluid, which is injected around the main plasma arc as it exits the nozzle of the cutting torch. Water is introduced around the arc tangentially, radially or in at angular vector to further constrict the arc. The amount of water used varies with the particular design. Typically, substantially less water is used herein compared to water used as a curtain around the main torch in connection with water mufflers. Nevertheless, introducing water on top of stainless steel and aluminium plates are still not desirable. Also, cutting water caught in the filters of down draft cutting tables will reduce their usable life.
It is the object of the invention to provide a plasma cutting method and device, with which the disadvantages outlined above can be minimized or prevented.
This object is achieved by a method and a device comprising the features of the respective independent claims.
According to the invention, in a 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 plasma arc, the shielding flow comprises CO2-snow or a mixture containing CO2-snow. It is noted that the term “shielding flow” as used herein is meant to comprise any flow of material comprising solid and/or fluid, i.e. liquid and/or gaseous, components. The term “mixture containing CO2-snow” as used herein is to be understood as comprising mixture of CO2-snow with any expediently chosen gases and/or fluids and/or solids.
By injecting CO2 snow onto the work piece, i.e. by ejecting it from the cutting torch, there is provided enhanced arc cooling and shaping of the arc. The CO2 snow sublimes and it leaves no liquid that flows over the surface of the metal to be cut. It also provides a means of preventing the atmospheric gases from contaminating the plasma arc and changing its desired chemistry.
The CO2-snow thus ejected from the cutting torch around the plasma arc acts as a curtain to immediately cool, condense and nucleate any metallic fume generated on the work piece 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. CO2-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, electrode shield cup 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. According to a preferred embodiment, the shielding flow is provided together with or without a carrier gas or fluid. Using carrier gases or fluids provides an effective way of injecting a shielding flow in a desired amount and direction. On the other hand, providing a shielding flow without a carrier gas can be advantageous for certain applications.
According to a preferred embodiment, the shielding flow is provided in a flow path which is split into a first central flow component provided directly around the arc and at least one second coaxial flow component provided coaxially around the central flow component. Each flow component can provide an effective curtain around the plasma arc. The central flow component is especially provided to constrict the plasma arc and enhance the cutting process.
Advantageously, 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.
It is also possible to provide the first and/or the second flow components of the shielding flow directed in a direction forming a closing or an opening angle relative to the main extension direction of the plasma arc. For certain designs, an opening angle helps in protecting the torch during the piercing which causes metal blowback during the piercing process. Similarly, a closing angle can also help in this respect. The outer component of the flow, aside from protecting the torching during the piercing phase and the cutting phase of the process, also acts as a built-in “CO2 muffler” to reduce overall emissions, i.e. electromagnetic radiation including UV (causing ozone generation) NOx, particulate, noise, etc.
It is also possible to provide a shielding flow or at least one of the first and second flow components of a shielding flow directed towards the arc in a direction perpendicular or essentially perpendicular to the main extension direction of the arc.
It is also advantageous to provide the shielding flow with a rotational component defining a rotational movement about the main extension direction of the plasma arc. This further improves the constriction of the plasma's arc and therefore, improves the cuttings speed and quality.
According to a preferred embodiment of the device according to the invention adapted to implement the method according to the invention, it comprises a cutting torch provided with an electrode, which is coaxially surrounded by a nozzle, thereby defining a passage for passing of a plasma gas between electrode and nozzle, the nozzle being coaxially surrounded by a shielding cap, thereby defining a passage for passing of a shielding flow between nozzle and shielding cap, wherein the passage for a shielding flow is configured and adapted for use of CO2-snow or mixture containing CO2-snow as shielding flow.
Advantageously, the plasma cutting torch is provided with means to provide the at least one passage to supply a shield flow comprising CO2-snow such that the CO2 is injected around the main plasma arc.
Further, the device advantageously comprises a plasma cutting torch provided with means to provide at least two passages to supply a shield flow comprising CO2-snow and further another pathway to provide a carrier gas, the carrier gas especially being selected from a group comprising CO2 gas, N2 gas, air, oxygen, argon, argon-hydrogen mix, argon-hydrogen-nitrogen mix, or a combination of the above gases.
According to a further preferred embodiment, the device comprises a shield member such that the CO2-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 counterclockwise direction.
Advantageously, the device comprises a shield member such that the CO2-snow shield flow injected around the main arc is in an angular manner and/or swirling manner, especially either in a clockwise or counterclockwise direction.
Advantageously, the device comprises a shield member comprising multiple components to generate a swirling CO2-snow shield flow.
Expediently, the shield member is adapted to split the CO2 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.
According to preferred embodiments, 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. Preferred embodiments of the invention will now be described with reference to the figures.
In
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
In order to achieve a highly constricted plasma arc 160 for plasma cutting, it is proposed to use CO2-snow as a shielding flow passing through passage 114. This CO2-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 CO2 snow.
The CO2-snow may be injected without any further carrier gas through passage 114. In a preferred embodiment, however, CO2-snow is injected together with a carrier gas, such as nitrogen, oxygen, air, argon, etc. or a mixture thereof.
As can be seen from
The CO2-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. CO2-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.
Also, as CO2-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.
The main difference over the previously discussed embodiment is that the flow path through passage 114 for CO2-snow is split into a central section 114a, corresponding to the lower section 114a as described with reference to
In
According to a further embodiment shown in
Be it noted that the introduction of CO2-snow can be provided in such a way that CO2-snow and a carrier gas are introduced into the various passages 114. Alternatively, feed stock such as liquid CO2 or CO2-gas can be fed directly into passage 114, and the CO2-snow generation process effected within passage 114, as schematically indicated at reference numeral 170 in
A further carrier gas line may be provided to introduce a carrier gas for the CO2-snow into the cutting torch. This further gas line (not shown in
In a further implementation, not shown in the figures, a rotational or swirl flow could be imposed on the CO2-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.
All advantages discussed in connection with the embodiment of
Be it noted that a constant amount of CO2-snow can be used at all current levels and material thicknesses to be cut. However, in preferred implementations, the amount of CO2 used is an increasing function of the current of the plasma cutting arc. Specifically, it is advantageous to set the CO2-snow flow rate as a function of the current level of the plasma arc. Also, the CO2-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
The method as described, using CO2-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.
In a preferred embodiment, the following combinations of CO2-snow and carrier gas may be considered advantageous, for example for carbon steel cutting: oxygen plasma in combination with CO2-snow as shielding fluid and an oxygen gas as carrier gas, or oxygen plasma in combination with CO2-snow as shielding fluid and air as carrier gas.
For stainless steel or aluminum, and also for certain non-ferrous materials: nitrogen plasma can be used in combination with CO2-snow as shielding fluid and nitrogen gas as carrier gas, or Ar—H2 mixture (example: 35% H2 with the balance argon, often referred to as H35) plasma in combination with CO2-snow as shielding fluid and nitrogen gas as carrier gas, or Ar—H2 mixture (example H35) plasma in combination with CO2-snow and Ar—H2 gas mixture as carrier gas, or Ar—H2 and N2 plasma (with various mix ratios) in combination with CO2-snow as shielding fluid and nitrogen gas as carrier gas, or Ar—H2 and N2 plasma (with various mix ratios) in combination with CO2-snow and Ar-H2+N2 gas mixture as carrier gas, or an N2—H2 mixture (example: F5) as plasma in combination with CO2-snow as shielding fluid and nitrogen gas as carrier gas.
The ratios between CO2-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 CO2-snow flow rate, or is set to match the CO2-snow flow rate, or is set at twice the CO2-snow, or is set at 5 times the CO2-snow, or is set at ten or 15 times the CO2-snow flow rate. Intermediate or higher ratios are also possible.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1712301.9 | Jul 2017 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/025186 | 7/9/2018 | WO | 00 |
