The critical technologies of CO2 laser cutting machine

The critical technologies of the CO2 laser cutting machine are Light, Machine, Electric integrative Synthesis Technique.

For the CO2 laser cutting machine he Paramater of the laser bean, machine and the performance of the numerical controls will direct influence the efficiency and the quality of the laser cutting. Especially for cutting high precision and thick components.

It needs to grasp and solve the following critical technology:

1. The control technique of the focal position: One advantage of the laser cutting is the high density laser bean, it is about 10W/cm

2.because the energy density is in direct proportion to 4/πd2, so the dia of the focus flare can be as small as they can, in order to produce a thin lancing; at the same time, the dia of the focus flare is in direct proportion to focus depth of lens. The smaller the focus depth of lens is, the smaller the dia of the focus flare is. But it will engender splash when cutting, it may cause the damage to lens if the lens is close to the work  piece, so some of the big power CO2 cutting machine in industry use 5"~7.5" (127~190mm)focus.The actual focal spot facular dia is between 0.1 and 0.4mm. Refer to the high quality cutting, the effective depth of focus is also related to the dia of lens and the material. For example, to cut the carbon steel by 5" lens. The depth of focus is in the scope of +2%(about 5mm) of focus. Concidering the cutting quality and the cutting speed, in theory, for a 6mm metal material, the focus will be on the surface. For the 6mm carbon steel, the focus is on the surface. For the 6mm stainless steel, the focus is under the surface.The concrete size will according to the experiment.

In industrial production, there are three simple methods for confirm the location of the focus:

(1) Stamp pad method: Let the cutting head move from up to down direction, then adopt the laser bean print on the plastic board, the smallest print dia. Is the focus.

(2) Tilting plate method: Use a inclined plastic board which is angled with vertical direction, then find the smallest point of the laser bean.

(3) The blue spark method: Take out the muzzle, blow air, let the pulsed laser beat on the stainless steel board, let the cutting head move from up to down, the focus in the biggest blue spark location. Refer to the flying optics cutting machine, because of the divergent angle of the laser, so the cutting proximal and distant ends’ optical path length is different, there is some different for the laser bean before the focucing. The larger the entrance beam’s dia is, the smaller the flare is. In order to reduce the change of the focus flare by the change of the laser beam’s dia. before focusing, the manufacturer of laser cutting system at home and abroad provide some equipment for customers to choose:

(1) Parellel light pipe. This is a usual method. It is to add a parellel ligh pipe at the output pin to dispose the laser beam, the dia. of the laser beam will grow bigger after expand. The divergent angle goes smaller, it makes the laser beam proximal and the distant ends before focucing nearly the same in the cutting scope.

(2) Add a separate moving lens lower shaft on the cutting head, the stand off(Z axes) from the surface and the controlled muzzle and the lens are two saperate parts. When the machine table moves or the light axes moves, the light beam from proximal and distant end (F axes) will move together, so that will makes the flare’s dia. of the laser beam after focucing the same in the whole process scope.(Picture 2)

(3) Control the water pressure of the focusing mirror (generally, it is the metallic reflection focusing system). If the size of the laser beam changed and the dia. of the focused flare goes bigger, the auto controled water pressure will change the curvature of the wavefront in order to minify the flare.

(4) Add the X and Y direction compensate optical path on the flying optical path cutting machine. When the cutting distant end optical path increased, it will shortened the compensate optical path. Conversely, when the cutting proximal end optical path reduced, it will increase the compensate optical path.

2. Cutting perforation technique: Every thermal cutting techniques, with several exceptions(Cutting from the edge of the board), most of them need to bore a small hole on the board. Previous laser pressing complex machine use punch to crash a hole on it, then use laser to cut from the hole.

For the laser cutting machine without punch system, there are two mathods for them to crash a hole on the material:

(1) Blast drilling, through the continuous laser irradiation, it will engender a pit on the material, then the Oxygen flow which is in the same axes with the laser beam will soon fused the material to form a hole. Generally speaking, the size of the hole is relate to the thickness of the board, the average dia. of the hole by blast drilling is about half of the thickness of the board, so the hole will bigger if the boards thickness thicker, but the hole is out of round, this method should not be used in processing demanding components(eg. petrolic sieve slit-tube), just can be used on flotsam. Furthermore, because of the oxygen pressure when drilling is the same with the cutting process, so the splash is big.

(2) Pulse drilling:The high-peak power pulse laser will vaporize and melt a few material. In common use the air and nitrogen gasas the auxiliary gas, in order to reduce the expansion of the hole by exothermic reaction, the air pressure is smaller than the oxygen pressure when cutting. Every pulse laser only produce small micelles spray, step-down, so it will take a few seconds when drilling the thick board. Once finish the drilling, change the auxiliary gas to the oxygen to do the drilling at once. The dia. of the hole will be small, the quality of the hole will higher than the one by blast drilling. For this reason, the laser should not only have the high output power, but also should have the time and space characteristic, so the general crosscurrent CO2 laser machine is not suitable for the requirements of the laser cutting.

In addition, the pulse drilling needs reliable air channel controlled system, in order to achieve the control of the drilling time, the changing of the gas type and gas pressure. When using the pulse drilling, in order to obtain the high quality incision, the transition technology should be regarded from the static pulse drilling to continuous cutting. In theory, the cutting condition of the changeable accelerating section: For example the focus, the muzzle’s location,gas pressure etc. But actually, for the time is short, it is impossible to change the above conditions. In industrial production, we often use the changing of the laser’s average power to achieve the above method.

There are three methods:

(1). Change the width of the pulse;
(2)change the Pulsar Frequency
(3) concurrent change of the width and the frequency of the pulse.

The nozzle design and the air current control technology:

When the laser cutting the steel, Oxygen and focused laser beam through the nozzle to reach the cutted material, thereby to inform a air beam. The basic requirements for the air beam is the quantity should large when enter the cut, the high speed, so it can let the cut has a sufficient hot reaction. At the same time, it has enough momentum to blow out the fused refractory. So except the quality of the laser beam and control the direct effect about the cutting quality, the nozzle design and the air control(eg. Nozzle pressure, the location of the work piece in the air beam) are two important factors. Currently, the nozzles in the laser cutting field is simple configuration ones, viz. a bell mouth with small hole ends. Generally use experiment and errors method to design. Usually, the nozzles are made of red copper, small volume, it is quick-wear part, it needs to change frequently, so do not need to carry through the computation and anylasis about the fluid mechanics. Input some air which has the pressure Pn(gage pressure Pg) from the side of the muzzle., named nozzle pressure, eject from the muzzle, it through a little distance to reach the workpiece’s surface, the pressure called cutting presure Pc, the air will expend to the atmospheric pressure Pa. Studies show that along with the the increase of Pn, the air speed will increase, Pc will also increase.

The following formula can be used: V=8.2d2(Pg+1) 
　　V- gas flow rate L/min 
　　d-dia of muzzle mm 
Pg-muzzle pressure(gage pressure)bar

For the different gas has different threshold pressure, when the muzzle resure goes bigger than this calue, the air flow will be the normal obligue shock, the gas speed will increase from the subsonic speed to supersonic. The his Threshold to Pn and Pa ratio and the degrees of Freedom of molecule are related to two factors: e.g. Oxygen’s and air’s n=5, so the threshold Pn=1bar×(1.2)3.5=1.89bar. When the mozzle pressure more higher Pn/Pa=(1+1/n)1+n/2 (Pn; 4bar), the normal air obligue shock change into the bow wave, cutting pressure Pc goes down, the speed goes slow and form a backset on the surface of the workpiece, it reduce the the action of the airflow without the fused material, effect the cutting speed. That is the reason for use the muzzle with counterbore with small hole, to let the Oxegen’s muzzle pressure under 3bar.

In order to increase the cutting speed of the laser cutting, it can on the basis of aerodynamic principle, do not form the bow wave under the premise of raise the muzzle pressure, design a scalable muzzle, named Laval muzzle. In order to easy the manufacture, please according to picture four. Germany Hannover U Laser Centre use 500WCO2 laser, Lens’s focus is 2.5", tested with small hole muzzle and Laval muzzle(Picture 4). The result is in picture 5: It shows NO2,NO4,NO5 muzzle under the different pressure, the cutting coarse surface Rz and the cutting Vc function relationship. From the picture, it shows No2 small hole muzzle’s cutting speed under the Pn 400Kpa only can be 2.75m/min(the thickness of the carbon steel board is 2mm).NO4 and NO5 Laval muzzle’s speed can be 3.5m/min and 5.5m/min under the Pn from 500Kpa to 600Kpa.　Because of the obligue shock's multihop　at the boundary of the air blow, let the cutting pressure be the periodicity change. The first high pressure section is near, the surface of the workpiece is about 0.5~1.5mm near the exit, cutting pressure Pc is high and static,it is the usual process parameters in current production. The second high cutting pressure section is 3~3.5mm near the exit.The cutting pressure Pc is big, it also make the good effect, it also can protect the lens and save its life.The other cutting point on the curve are far from the exit of the muzzle, they are not suitable with the focus light beam, so abandond.

In summary, CO2 laser cutting technology is more and more used in the industrial production, the researching of the high cutting speed and the cutting technique of the thicker material is going on in other countries. In order to satisfy the requirements by high quality and production efficiency, we must pay attention to solve all kinds of technical questions and quality standard, so this new technology can be used more extensive in our country.

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