1. Special Equipment
To reduce the change of focus spot size caused by the change of beam size before focusing, the manufacturers of laser cutting system at home and abroad provide some special devices for users to choose from:
(1) Parallel light tube. This is a commonly used method, that is, in the output end of the CO2 laser with a parallel light tube to expand the beam processing, the beam diameter widened, the divergence angle smaller, so that in the cutting work range of near-end and distal focus beam size near the same.
(2) The lower axis of an independent moving lens is added to the cutting head, and the z-axis of the distance (standoff) from the control nozzle to the material surface is two separate parts. When the machine tool worktable moves or the optical axis moves, the beam from the proximal to the distal f axis also moves simultaneously, causes the light spot diameter in the entire processing area to be consistent in the beam focusing.
(3) control the pressure of the focusing mirror (usually the metal reflective focusing system). If the beam size becomes smaller and the focal spot diameter becomes larger, the automatic water pressure changes the focusing curvature to make the focus spot diameter smaller.
(4) To increase the x and Y direction of the Compensation optical path system on the Flying optical Road cutting machine. This means that the compensating optical path is shortened when the cutting distance is increased, and conversely, when the cutting near-end optical path decreases, the compensating optical paths are increased to keep the length of the light path consistent.
2. Cutting perforation Technology
Any kind of thermal cutting technology, except a few cases can start from the edge of the plate, generally must wear a small hole on the board. Earlier in the laser stamping composite machine is a punch first punch out a hole, and then the laser from the small hole to start cutting. There are two basic methods for punching a laser cutter without a stamping device:
(1) Blasting perforation: (blastdrilling), the material after irradiation by continuous laser in the center to form a concave pit, and then by the coaxial with the laser beam oxygen flow will quickly remove the molten material to form a hole. The size of general hole is related to the thickness of the plate, the average diameter of the blasting perforation is half of the thickness of the plate, so the perforation aperture of the thicker plate is larger and not round, so it is unsuitable to use (such as oil sieve pipe) on the parts with higher requirements. In addition, due to perforation of the oxygen pressure is the same as the cutting, splash larger.
(2) Pulse perforation: (pulsedrilling) The use of High peak power pulse laser to make a small amount of material melting or vaporization, commonly used air or nitrogen as an auxiliary gas, in order to reduce the exothermic oxidation of the hole expansion, gas pressure is less than the oxygen pressure when cutting. Each pulsed laser produces only a small particle jet that is gradually deepened, so that the perforated time of the thick plate takes a few seconds. Once the perforation is complete, the auxiliary gas is immediately replaced with oxygen for cutting. The perforation is better than blasting perforation because of its small diameter. The lasers used in this field should not only have high output power, but also the time and space characteristics of more important time bundles, so the general Cross-flow CO2 lasers cannot adapt to the requirements of laser cutting.
3. Nozzle design and airflow control technology
When laser cutting steel, oxygen and focused laser beams are shot through the nozzle to the cut material, thus forming a gas beam. The basic requirement for airflow is that the airflow into the incision is large and the speed is high, so that enough oxidation can make the incision material fully exothermic, and there is enough momentum to blow the molten material out. Therefore, in addition to the quality of the beam and its control directly affect the cutting quality, the nozzle design and airflow control (such as nozzle pressure, workpiece position in the airflow, etc.) is also a very important factor. The nozzle for laser cutting uses a simple structure, i.e. a tapered hole with a small round hole in the end.
Because the nozzle is generally made of copper, small size, is easy to damage parts, need to be replaced frequently, so do not carry out fluid mechanics calculation and analysis. In use, from the nozzle side to a certain pressure pn (table pressure for PG) of the gas, said nozzle pressure, from the nozzle exit, through a certain distance to the workpiece surface, the pressure is said to cut the pressure pc, the last gas expansion to the atmospheric pressure Pa. The research shows that with the increase of PN, the airflow velocity increases and the PC increases.
please calculate by following formula : V=8.2D2 (pg+1)
V-Gas Velocity l
D-Nozzle diameter mm
pg-Nozzle pressure (gauge pressure) bar
For different gases there are different pressure thresholds, when the nozzle pressure exceeds this value, the airflow is the normal oblique shock wave, the flow velocity from subsonic to supersonic transition. This threshold is related to the PN, PA ratio and the degree of Freedom (n) of the gas molecule two factors: such as oxygen, air n=5, so its threshold value Pn=1barx (1.2) 3.5=1.89bar. When the nozzle pressure is higher pn/pa= (1+1/n) 1+N/2 (Pn;4bar), the airflow normal oblique shock seal becomes the positive shock wave, the cutting pressure pc drops, the airflow velocity is reduced, and the surface of the workpiece is formed Eddy, which weakens the effect of the airflow to remove the molten material and affects the cutting speed. Therefore, the nozzle pressure of the nozzle with a small round hole with the end of the cone hole is usually below 3bar.