Cleaning of metal wires by using rotating thermal plasma.

Kettler, Roman ; Cesarec, Paulina ; Katalinic, Branko 等

1. INTRODUCTION

The ambition of continuous development of methods and products is one of the main features of the science and technology. The metalworking industry is very interested to increase the quality of their products. One very important role has the cleanliness of the metallic raw materials.

The company wants to develop a new surface cleaning device for metallic wires, based on thermal plasma. The first step was the investigation of the suitability of thermal plasma as a cleaning medium for metal wires (0.8 to 1.5 mm). The second step was development of several prototypes of cleaning device. The third step was improvements of device based on tests and relevant patents. The investigation, analysis and optimisation of magnetic behaviour was made with the help of simulation software (ANSYS 11). A scanning electron micrograph was used to prove the effect of the cleaning method.

2. TECHNICAL BASICS

2.1 Principle of cleaning with thermal plasma

In the physic plasma is a (partially described) ionized gas. Ionization is the process by which one or more electrons are removed from an atom or molecule through external power influence. So a positively charged ion and negatively charged free electrons will be created. The plasma state is also called the fourth state. (Falbe & Regitz, 1997 & 1999)

In the welding technology Argon is often used as an inert gas to protect the welding surface against oxidations. The Argon atoms have a complete filled valence bowl, which makes them chemical inert. Plasma means that at least one electron is missing in the atom. That can be realised by using high voltage. Electrons will flow and collide with electrons from the argon atom and blast them out. So we get argon plasma and the electric arc can burn. (Beuth, 1997)

[FIGURE 1 OMITTED]

In the picture (Fig. 1) a dirty metallic surface is shown. If the power source will be turned on, an electric arc will start to burn and the electrons will flow into the metallic surface. But on the other hand the much heavier ions (displayed as the yellow arrow in the figure) will flow out of the surface and because of their kinetic energy they will put away the dirt.

2.2 Principle of cleaning a wire

To clean a wire the electric arc has to burn around the wire. Therefore a ring electrode (Fig. 2) can be used where the wire goes through. The electric power, which causes the arc, has to flow through the wire, so the current is limited by the diameter of the wire. That's the reason why the electric injection will come from 2 sides as shown in the figure below.

[FIGURE 2 OMITTED]

To assure an effective cleaning effect, the arc should start to rotate around the wire fast and controlled. That can be made by using the Lorentz force. Each electric conductor (including arcs) causes a magnetic field around it. If this conductor will be placed in the homogenous magnetic field, the two fields will influence each other. This effect is called the Lorentz force. (Bauckholt, 1992)

[FIGURE 3 OMITTED]

Rectified magnetic fields will pull and oppositely orientated magnetic fields will push each other. And so the conductor in the picture (Fig. 3) will start to move up. If now a ring electrode (Fig. 2) will be placed in the homogenous field, the burning arc between this ring and the wire cannot move up and will start to rotate. The rotating speed depends on the current and the length of the arc and the power of the homogenous magnetic field.

3. THE PROTOTYPE

3.1 Configuration and function of the prototype

The whole device consists of two standard wire memories, two standard wire-deliver-units for moving the wire and injecting the electric power, a cooling pipe and the cleaning chamber. The cooling pipe is just a normal pipe which is flooded with Argon. It avoid that the cleaned hot wire comes in contact with the air and oxidize on it. The cleaning process itself will be done in the cleaning camber.

[FIGURE 4 OMITTED]

The cleaning camber (Fig. 4) can be closed and flooded with Argon. The cleaning process can be observed through a glass window. The camber consists of the following main parts:

(1) Entry injector. The wire enters the cleaning camber through this cone. It makes sure that the Argon will not leave the camber and it also isolates the wire from the camber.

(2) Ring electrode. It is water cooled and isolated from the rest of the camber.

(3) Magnetic coil. Is also water cooled and has 2000 windings.

(4) Leaving injector. The wire leaves the camber through it.

(5) Cooling Pipe. Its length is about 500mm.

The whole camber is made by iron, so that the homogenous magnetic field can be flow through it. The cleaning arc is powered by two standard welding current sources from "solution by innovation" SBI. During the cleaning process the wire is pulled through the armature and an arc burns between the wire and the ring electrode. The coil produces a homogenous magnetic field, which is closed through the camber and turns on the arc.

3.2 Problems during the prototype development

During the development of the prototype several problems had to be found out and solved by adapting the prototype. Finally the adjusted prototype number 16 was able to clean wires in an acceptable quality level. In the following text just two main problems are elaborated.

[FIGURE 5 OMITTED]

The problem of the pulling effect (Fig. 5) was that the electric arc quivered forward and backward till it braked off. The solution was quite easy. By using a conic ring electrode it was possible to stabilise the position of the arc by regulate the inert gas speed.

[FIGURE 6 OMITTED]

Another problem was the cooling pipe. The cooling effect was just too weak. For solving of this problem a cooling camber with water cooled rolls was designed (Fig. 6). The distances between the rolls must be different otherwise the wire starts to turn like a bolt in a nut. The whole cooling camber has to be flooded with Argon.

4. ANSYS SIMULATION AND SEM ANALYSIS

4.1 Simulation and improvement by using ANSYS 11

The rotation speed of the electric arc is decisive for the cleaning effect. The higher speed means better cleaning. On the other hand the speed depends mainly on the power of the homogenous magnetic field. To optimise this field, the simulation software ANSYS 11 was used (Fig. 7).

[FIGURE 7 OMITTED]

Different dimensions, distances and adjustments have been simulated. The result shows that the coin should be as big as possible, the diameter of the entry and leaving injector (Fig. 4) has to be the same like the hole in the coil and the distances between the entry injector, the ring electrode and the coil should converge zero.

ANSYS was also used to calculate the value of the magnetic flux density in the arc area. With the realised prototype it was possible to generate 0.0415 T. A complex calculation via hand results almost the same size.

4.2 Analyse of the cleaning effect by using SEM

A scanning electron microscope (SEM) was used to check cleaning effect of prototype devices. Therefore an unpolluted, a less oily and a less rusted wire were cleaned with the electric arc and afterwards the treated and untreated wire was inspected with the SEM. The result of the rusted wire is shown below (Fig. 8).

[FIGURE 8 OMITTED]

Comparing the two SEM pictures (Fig. 8) it can be shown that the oxygen pollution (left side) of the untreated wire disappeared after the cleaning process (right side). Also the surface structure seems to become more harmonic.

5. RESULTS OF THE PROJECT

The project has shown that it is possible to clean wires by using thermal plasma as a cleaning medium. Finally the prototype was able to examine clean wires and wires with very less oil or grid pollution. The relevant parameters of the final prototype are shown in the following table (Tab. 1):

6. CONCLUSION

The project has shown that the investigated cleaning principle is functioning. This study is made in the lab conditions, and the way to develop and produce a market-ready cleaning device can be produced needs additional steps. Knowhow generated during this feasibility study is sufficient for the development of new improved prototypes which can reach the level of market products.

7. REFERENCES

Bauckholt, H. J. (1992), Grundlagen und Bauelemente der Elektronik, Carl Hanser Verlag, Munchen

Beuth, K. (1997), Bauelemente--Elektronik 2, Vogel Verlag, Wurzburg

Bohler, E. (1994), Elemente der angewandten Elektronik, Vieweg Verlag, Wiesenbach

Falbe, J. & Regitz, M. (1997 & 1999), Rompp Lexikon Chemie, Georg Thieme Verlag, Stuttgart

Kettler, R. (2007), Reinigung metallischer Drahte mittels thermischen Plasmen, Diplomarbeit, FH Technikum Wien

Lenk, R. & Gellert, W. (1973), Brockhaus abc, Brockhaus Verlag, Leibzig

KETTLER, R[oman]; CESAREC, P[aulina] & KATALINIC, B[ranko] *

Tab. 1. Final Parameters of the prototype

Parameter Value

Current of the two welding units apiece 10 A
Current for the magnetic coil 3 A
Pulling speed of the wire 5 m/min
Material / Diameter of the wire C01 Si0.9 /
 [phi] 1 mm