1. Smoke and dust hazards in laser cutting and smoke exhaust mechanism
In recent years, the laser cutting industry has developed rapidly, and the power of various lasers has become greater and greater.
With the continuous improvement of laser cutting thickness and speed, the amount of smoke and dust generated per unit of time is increasing.
If the original laser cutting machine dust removal system is still used, it will not be able to meet normal dust removal needs and will not be able to meet corresponding government and industry regulations.
In order to improve product quality and meet customer needs, this article focuses on the research and optimization of dust removal system for 6kW and above laser cutting machines.
1.1 Hazards of smoke and dust laser cutting
The smoke and dust released during laser cutting of metal sheets can be divided into dust and aerosols, of which 97% have a diameter of less than 5.7μm, which means that most of the smoke and dust can be inhaled by the human body.
Depending on the cutting material and the scene, other harmful substances may also be produced.
For example, if the leaf surface contains oil, it will produce gases made up of very complex hydrocarbons; If the surface of the leaf has a film, the cut will release lipid and olefinic compounds, accompanied by a noticeable odor.
If the smoke and dust produced during laser cutting are not captured and treated effectively, it will harm human health and the environment.
1.2 Laser cutting smoke exhaust mechanism
During laser cutting of metal sheets, a negative pressure is formed below the cutting surface by extracting air through a fan, and the smoke and dust produced by cutting are sucked out.
Currently, the industry generally uses a multiple partition method. The effective cutting area is divided into multiple zones, and the corresponding damper is opened according to the actual cutting position to achieve better dust removal effect. This is shown in Figure 1.
According to the formula, the volume of air required for the dust collector is:
I p =K×3600(5H 2 +F x )V x (m 3 /h) (1)
Where K is the selection margin coefficient, which is ≥1.2; H is the distance between the actual suction port and the cutting position, in meters; F x is the dust hopper partition area, in square meters; V x is the wind speed at the cutoff position, in meters per second.
From Equation (1), it can be seen that for laser cutting machines equipped with dust collectors of the same air volume, the smaller the hopper partition area and the closer the suction port is to the cutting, the greater the dust removal wind speed. on the surface of the work table and the better the dust removal effect will be.
Through various experiments on different metal sheets and cutting parameters, the ideal wind speed for removing dust from the surface of the cutting plate is between 0.8 ~ 1.2 m/s. In this case, the dust removal effect is good and the success rate of capturing smoke and dust is more than 95%. There is no obvious smoke seen with the naked eye or lungs. If the dust removal wind speed of the cutting plate surface is less than 0.5m/s, the dust removal effect will be poor and there will be obvious smoke when cutting carbon steel plates. If the dust removal wind speed of the cutting board surface is greater than 1.2 m/s, the dust removal effect is good, but it will also suck in more sparks or slag, causing damage to the dust removal equipment or even causing fires.
2. Structural optimization of dust removal system
2.1 Reducing the dust container partition area
From the above formula, it is obvious that one way to increase the dust removal wind speed at the cutting position without increasing the power of the dust collector is to reduce the partition area of the dust bowl.
From this perspective, we carry out practical comparative tests and experiments. We change the machine with the same cutting scope from 5 partitions to 6 partitions. After the improvement, the section length remained unchanged at 2.07m, while the section width was reduced from 0.85m to 0.69m, reducing the divider area by 19%, as shown in Figure 2.
(a) Before improvement (b) After improvement
According to the test, under the same conditions of air volume, temperature and measuring position, the average wind speed on the surface of the work table before the improvement was 0.63 m/s, and after the improvement, the speed Average wind speed increased to 0.75 m/s, which represents an increase of around 19%.
Therefore, reducing the partition area can directly increase the wind speed V x when the air volume L p remains the same.
However, increasing the dust removal partition will also have some disadvantages, such as increasing the number of air gates, which will cause more air leakage; an increase in the number of cylinders used to control the air gates can increase the cost and the possibility of failures.
Therefore, it is necessary to make choices based on the actual positioning of the product.
2.2 Reducing the distance between the suction port and the cutting surface
From the above formula, it is obvious that the second way to increase the dust removal wind speed at the cutting position without increasing the power of the dust collector is to reduce the distance between the suction port and the cutting surface.
Regarding laser cutting machines, increasing the height of the air duct means shortening the distance between the air gate suction port and the work table.
We also conduct comparative test experiments under the same conditions of dust removal partition and dust removal fan. The corresponding actual average velocity test data for three times the air duct height increase are shown in Table 1.
As can be seen from the table, when the H value decreases proportionally, the V x increases continuously. However, due to the structural limitations of the laser cutting machine, there are limitations to reducing the H value.
Furthermore, as the height of the air duct continues to increase, it is necessary to consider a protection scheme to prevent the laser from damaging the air duct. The air duct must be placed outside the cutting area, especially for high-power machines.
Table 1 – Wind speed test data record
| Foundations | Option 1 | option 2 | Option 3 | |
| Work table surface real average wind speed V x (EM -1 ) | 0.52 | 0.63 | 0.74 | 0.84 |
| Distance from work table surface to suction port H/m | 0.60 | 0.50 | 0.40 | 0:30 |
| Test the fan air volume L p (m 3 /h -3 ) | 6,000 | 6,000 | 6,000 | 6,000 |
2.3 Reducing the pressure loss of the dust removal system
According to the Moody chart, the damping coefficient λ along the path can be determined by the Reynolds number Re and the ratio ε/d (where ε is the absolute roughness of the air duct wall and d is the equivalent diameter of the pipe ) .
The greater λ, the greater the pressure loss along the path.
Combining with the actual air flow in the air duct, it can be seen that the larger the equivalent diameter d of the air duct, the smaller λ will be and the smaller the surface area s will be (with constant length and sectional circumference c), the smaller will be the friction loss.
Therefore, in terms of reducing pressure loss along the way, round tubes should be preferred first, followed by square tubes and then rectangular tubes.
As shown in Table 2, under the same cross-sectional area, the equivalent diameter of the round tube is the largest and the surface area inside the tube is the smallest.
Table 2 Equivalent diameter and sectional circumference of round tube, square tube and rectangular tube.
| Gas pipelines | Circular tube (Φ.114) |
Squared pipe (100×100） |
Rectangular tube (150×67） |
| Equivalent diameter d | 114 | 100 | 92 |
| Sectional circumference c | 354 | 400 | 434 |
Due to the restrictions on the structure of laser cutting machines, it is difficult to use circular tubes for air duct structures.
Generally, square tubes and rectangular tubes are used for the main air duct. For example, a rectangular tube with the size of 250×150 and a square tube with the size of 200×200 are used.
The equivalent diameter of the rectangular tube is 0.19m and that of the square tube is 0.2m. Tests have shown that at a constant air volume of 5000m3/h and the length of the air duct, the unit pressure loss of the rectangular tube is 34.86Pa/m and that of the square tube is 26.93Pa/m, with a reduction of 23%.
The recommended range for wind speed inside the air duct of the laser cutting machine is 15-18m/s.
According to the formula V=Q/S, whether the selection of the air duct section is reasonable or not can be checked according to the air volume of the dust collector and the recommended wind speed inside the duct.
If the wind speed inside the duct is too low, smoke and dust are likely to accumulate inside the duct; on the other hand, if the wind speed inside the duct is too high, the system pressure loss will increase and the dust removal efficiency will decrease.
Therefore, when selecting a dust collector, not only should a dust collector be chosen that matches the air volume, but the pressure loss of the laser cutting machine's dust removal system should also be considered. The dust collector inlet wind pressure should not be less than the pressure loss of the laser cutting machine's dust removal system.
It is necessary to select the corresponding fan performance curve provided by the manufacturer (see Figure 3) for selection, and not make generalizations based on fan power alone.
3. Summary
(1) The upgrade speed of the dust removal system of domestic laser cutting machines is far behind the development speed of laser power. Dust removal problems will be exposed in high-power machines.
(2) The dust removal effect of laser cutting machines is also related to factors such as hopper seals and the number of bends in the air ducts. Therefore, even with the same structured dust removal system, the dust removal effect of products produced by different laser cutting machine manufacturers may vary greatly.
(3) The amount of smoke and dust generated by laser cutting may be a problem that has been ignored. The amount of smoke and dust emitted by laser cutting depends on the material characteristics of the metal itself, as well as the cutting speed and pressure parameters during processing. Setting cutting parameters that minimize smoke and dust emissions for different materials is also an important way to improve the dust removal effect of laser cutting machines.
