Metal cutting is a common economic and practical processing method.
In order to achieve the best cutting effect, it is necessary to select and apply the air compressor properly.
This article presents the method of selecting air compressor according to the nozzle theory, and introduces the problems needing attention when using compressed air as auxiliary gas in laser cutting machine.
Laser cutting is the most widely used laser processing technology in the world.
As early as the 1970s, the laser was first used in cutting.
Since the beginning of this century, with the rise and popularization of the third generation laser technology, laser cutting is more widely used in sheet metal, plastic, glass, ceramics, semiconductors, textiles, wood and paper materials processing.
Since the beginning of this century, many laser enterprises have vigorously developed high-power fiber laser cutting machine.
Due to the unique processing advantages of high-power fiber laser processing, the processing cost has been greatly reduced and a variety of flexible payment methods are combined.
At present, more and more sheet metal processing enterprises, kitchen and bathroom manufacturing enterprises and automobile parts processing enterprises are using laser cutting machine more and more, especially in the sheet metal processing industry, it has replaced the traditional processing method.
Laser cutting machine can meet the cutting requirements of various materials and complex shapes.
Besides the need for high energy laser, auxiliary gas is an indispensable material to complete the cutting process.
The auxiliary gases used for laser cutting are mainly oxygen (O2), nitrogen (N2) and compressed air.
Compressed air is easier to obtain than oxygen and nitrogen, and it is very cheap compared with oxygen and nitrogen.
It is very common to use compressed air as auxiliary gas cutting.
The quality of compressed air has a very direct impact on the quality of metal laser cutting, and the size and stability of gas pressure will affect the cutting effect.
The size of the air compressor used as auxiliary gas for laser cutting machine should be selected according to the design of laser cutting head, auxiliary gas pressure and nozzle size.
Thus, the best matching of air compressor and laser cutting machine can be obtained.
2. Principle of laser cutting
Laser cutting is to use the focused high power and high-density laser beam to irradiate the workpiece, so that the material at the irradiated place will melt, vaporize, ablate or reach the ignition point.
At the same time, with the help of the high-speed airflow coaxial with the beam, the molten material can be removed to cut the workpiece.
Laser cutting is one of the thermal cutting methods.
Fig. 1 Principle of laser cutting
2.1 Main methods of laser cutting
Laser cutting can be divided into laser gasification cutting, laser melting cutting, laser oxygen assisted melting cutting and controlled fracture cutting.
1) Laser gasification cutting
Using the laser beam with high power density to heat the workpiece, the temperature rises rapidly and reaches the boiling point of the material in a very short time, and the material begins to vaporize and form vapor.
These vapors are ejected at a very high speed, and at the same time, a notch is formed on the material.
The gasification heat of materials is usually very large, so the laser gasification cutting needs a lot of power and power density.
Laser gasification cutting is mostly used for cutting extremely thin metal and non-metallic materials (paper, cloth, wood, plastic and rubber, etc.).
Fig. 2 Laser gasification cutting
The cutting edge of laser gasification cutting is smooth without burr.
2) Laser melting cutting
Laser melting cutting is to melt metal materials by laser heating, and then inject high-pressure inert gas (N2, Ar and He etc.) through a nozzle coaxial with the beam.
Depending on the strong pressure of the gas, the liquid metal is eliminated and a slit is formed.
It is not necessary to vaporize gold completely in laser melting cutting, and the laser energy required is only about one-tenth of that of gasification cutting.
For sheet materials, if the cutting speed is too slow, most of the laser beams will lose energy directly through the incision.
With the increase of the speed, more beams irradiate the material, and the coupling power with the material is increased.
For thick plate materials, because laser evaporation or the speed of melting product removal is not fast enough, the beam is reflected on the material section in the slit many times.
As long as the melted product can be removed before it is condensed by cold airflow, the cutting process will continue.
Fig. 3 Laser melting cutting
The edge of the cutting edge of laser melting cutting is striped.
Laser melting cutting is mainly used for cutting some materials or active metals which are not easy to oxidize, such as stainless steel, stainless steel, stainless steel, titanium alloy, aluminum and its alloy.
The laser melting cutting uses nitrogen as the auxiliary gas for cutting quality, but the cost is higher than that of oxygen cutting.
3) Laser oxygen assisted melting cutting
The principle of laser oxygen-assisted melting cutting is similar to oxyacetylene cutting, which uses the laser as preheating heat source and oxygen as auxiliary cutting gas.
On the one hand, the oxygen emitted reacts with iron which is heated to about 1500 ℃ by laser.
Thus, a large amount of oxidation heat is continuously released to make the iron element heat up and burn;
On the other hand, molten liquid oxides and melts are blown out of the reaction zone to form a notch in the metal.
Due to the oxidation reaction in the cutting process, a large amount of heat is generated, so the energy required for laser oxygen cutting is only 1/2 of that of melting cutting.
Moreover, the cutting speed is far higher than that of laser gasification cutting and melting cutting.
The edge roughness of laser oxygen assisted melting cutting is between laser vaporization cutting and laser melting cutting.
Laser oxygen assisted melting cutting is mainly used with carbon steel and other easily oxidized metal materials.
It can also be used for stainless steel and other materials processing, but the section is black and rough, and the cost is lower than inert gas cutting.
4) Controlled fracture cutting
Through laser beam heating, brittle materials which are easy to be damaged can be cut off in a high speed and controllable way, which is called controlled fracture cutting.
The cutting principle is as follows: the laser beam heats the small area of brittle material, which leads to thermal gradient and serious mechanical deformation, which makes the material form cracks.
The cutting speed of fracture control is fast, only a small laser power is needed;
If the power is too high, the workpiece surface will melt and the cutting edge will be damaged.
The main controllable parameters are laser power and spot size.
2.2 Characteristics of laser cutting
Compared with other cutting methods, laser cutting is characterized by high cutting speed and high quality.
It can be summarized as follows:
1) The cutting quality is good.
Due to the small laser spot, high energy density and fast cutting speed, laser cutting can obtain better cutting quality.
2) The laser gas cutting incision is thin and narrow, the two sides of the cutting seam are parallel and perpendicular to the surface.
3) The cutting surface is smooth and beautiful, which can even be used as the last processing procedure.
The parts can be used directly without machining.
4) After laser cutting, the width of the heat-affected zone is very small, the properties of the material near the slit are almost unaffected.
The shape of the workpiece is small and the cutting accuracy is high.
The cutting speed comparison between laser cutting and other cutting methods is shown in Table 1, and the cutting material is low carbon steel plate.
Table 1 Comparison of cutting speed in several cutting methods
|Cutting method||Laser cutting||Plasma cutting||Oxyacetylene cutting||Hydraulic cutting|
|Cutting speed||Very fast||Fast||Slow||Very slow|
5) The cutting speed is fast, for example, when 2500 W laser cutting 1 mm thick cold-rolled carbon steel plate, the cutting speed can reach 16-19 m/min.
6) It is a non-contact cutting. There is no contact between the nozzle and the workpiece during laser cutting, and there is no tool wear.
3. Comparison of laser cutting with different auxiliary gases
When laser cutting, different cutting gas is selected according to the different material of cutting plate.
The choice of cutting gas and pressure has a great influence on the quality of laser cutting.
3.1. Types of auxiliary gas
The commonly used auxiliary gases for laser cutting are oxygen (O2), nitrogen (N2) and compressed air (compressed air), and sometimes argon (Ar) is used.
According to the gas pressure, it can be divided into high-pressure gas and low-pressure gas.
3.2. Selection of auxiliary gas types
The main functions of auxiliary gas in laser cutting are:
- combustion supporting and heat dissipation
- timely blowing off the molten stains produced by cutting
- preventing the cutting molten stains from rebounding upward into the nozzle, protecting the focus lens, etc.
According to the different cutting materials and combined with the power of laser cutting machine, when choosing different laser cutting technology, auxiliary gas selection is not the same.
The characteristics, uses and application scope of different auxiliary gases are as follows:
O2 is mainly used for cutting carbon steel materials.
The chemical reaction heat of oxygen and iron promotes the endothermic melting of metal, which can greatly improve the cutting efficiency.
It can also realize the cutting of thicker materials, and obviously improve the processing ability of the laser cutting machine.
However, due to the presence of oxygen, an obvious oxide film will be formed on the end face of the incision.
Moreover, it will produce a quenching effect on the materials around the cutting surface, improve the hardness of this part of materials, and have a certain impact on the subsequent processing.
The color of the oxygen cutting material incision end face is black or dark yellow.
General carbon steel plate uses oxygen cutting, low-pressure drilling and low-pressure cutting.
When N2 is used as auxiliary gas for cutting, nitrogen will form a protective atmosphere around the molten metal to prevent the material from being oxidized and the formation of oxide film, realizing non-oxidative cutting.
But at the same time, because nitrogen does not react with metals, the cutting ability is not as good as oxygen without reaction heat.
Moreover, the nitrogen consumption of nitrogen cutting is several times larger than that of oxygen cutting, and the cutting cost is higher than that of oxygen cutting.
The non-oxidation cutting surface has the characteristics of direct welding, coating and strong corrosion-resistance etc., and the end face of the incision is white.
Generally, nitrogen is used to cut stainless steel, galvanized sheet, aluminum and aluminum alloy plate, brass and other materials, with low-pressure perforation and high-pressure cutting.
When cutting with nitrogen, the change of gas flow rate has a great influence on cutting,
In the case of ensuring the cutting gas pressure, it is necessary to ensure sufficient gas flow.
3) Compressed air
Compressed air can be directly provided by air compressor, which is easy to obtain compared with oxygen and nitrogen, and the price is very cheap.
Although the air contains only about 20% oxygen, the cutting efficiency is far less than that of oxygen cutting,
However, the cutting capacity is close to that of nitrogen, and the air cutting efficiency is slightly higher than that of nitrogen cutting.
The end face of air cutting is yellow.
It is the most economical and practical choice to use compressed air instead of nitrogen when there is no strict requirement on the surface color of the material incision.
Economic comparison of compressed air laser cutting and nitrogen laser cutting
At present, the liquid nitrogen on the market is about 1400 yuan/ton,
The liquid nitrogen for laser cutting needs Dewar tank, which is generally 120 kg/can, and the price of 1kg is more than 3 yuan.
According to 1400 yuan/ton, the specific gravity of nitrogen in the standard state is 1.25 kg/m3.
Therefore, the maximum consumption of liquid nitrogen in a Dewar tank is about 120/1.25 = 96 Nm3.
The cost of nitrogen per Nm3 is 168/96 = 1.75 yuan/Nm3
If Baode PM15TK-16 air compressor is used to provide 16 bar compressed air, it can provide 1.27 m3 per minute.
The full load input power of this type of air compressor is 13.4 kW.
If the industrial electricity charge is calculated as 1.0 yuan / kWh, the air cost per m3 is 13.4 × 1.0 / (1.27×60) = 0.176 yuan/m3.
According to the actual consumption of 0.5 m3 gas per minute and the laser cutting machine working 8 hours a day, the cost saved by air cutting compared with nitrogen cutting is: (1.75 – 0.176) × 8 × 60 × 0.5 = 378 yuan
If the laser cutting machine works for 300 days a year, the cost of gas consumption can be saved: 378 x 300 = 113400 yuan,
Therefore, the use of compressed air instead of nitrogen cutting is very economical and practical.
The gas cost saved in one year is enough to buy three high-quality integrated air compressors.
Ar is an inert gas, which can prevent oxidation and nitriding in laser cutting, and can also be used in solution welding.
However, the price of argon is higher than that of nitrogen, so it is not cost-effective to use argon electrode in general laser cutting.
Argon cutting is mainly used for titanium and titanium alloy etc.
The end face of the incision is white after argon cutting.
4. Factors affecting the quality of laser cutting
4.1. Factors for evaluating laser cutting quality
There are many standards to judge the edge quality of laser cutting.
Standards such as burr form, depression and grain can be judged by naked eyes;
The perpendicularity, roughness and incision width need to be measured by special instruments.
Material deposition, corrosion, heat affected area and deformation are also important factors to measure the quality of laser cutting.
The most common poor cutting quality is over burning and slag hanging.
The power of laser cutting machine, laser pulse frequency, cutting speed, beam focus, beam polarization direction, nozzle size, auxiliary gas pressure and flow rate will affect the evaluation of laser cutting quality factors.
Fig. 4 Key factors of evaluating laser cutting quality
1) Overburning is caused by the fact that the heat from the melting of the workpiece cannot be carried away by the high-pressure airflow in time when the laser power is too high, cutting holes or cutting speed is too slow.
It can reduce the laser power and increase the gas flow rate to solve the cutting over burning.
Fig. 5 Overburning
2) Adhering slag is also called burr.
The laser power is insufficient, or the auxiliary gas pressure is insufficient, or the cutting speed is too fast, so that the auxiliary gas can not completely blow out the melted or gasified materials produced in the cutting process, resulting in slag adhering to the lower edge of the cutting surface.
Fig. 6 Adhering slag
4.2. Factors for evaluating laser cutting quality
The focus position of the laser beam has an important influence on cutting quality.
When cutting different materials, different focus positions need to be adjusted.
Sometimes, cutting burr is not due to insufficient auxiliary gas pressure, but the focus is too high.
The influence of beam focus position on cutting quality is shown in Fig. 7.
Fig. 7 Influence of beam focus on cutting effect (carbon steel: 15mm, stainless steel: 8mm)
4.3. Influence of cutting speed on cutting quality
1) The cutting speed is too fast
- It may cause unable to cut with splash sparks;
- Some areas can be cut off, some areas can not be cut off;
- The whole cutting section is rough;
- A diagonal cut pattern is formed and slag is produced in the lower part of the plate (Fig. 8).
Fig. 8 Effect of too fast cutting speed
2) Cutting speed is too slow
- The results show that the cutting plate is over melted and the cutting section is rough;
- When the cutting seam is widened, the whole area will melt at the small fillet or sharp corner, resulting in overburning;
- Cutting efficiency is reduced.
4.4. Influence of nozzle on laser cutting quality
When the center of the nozzle is different from the center of the laser beam, the high-speed airflow from the nozzle will blow the molten material, which makes it easier to have slag on one side and not on the other side.
This phenomenon is obvious when cutting 3 mm thick plate, and it can’t be cut in serious conditions.
Nozzle size should be appropriate.
Cutting thick plate requires high laser power, and the diameter of the spot projected on the plate by laser focusing is also large, which requires that the diameter of the nozzle passing through the beam is also large.
However, if the diameter of the nozzle is too large.
On the one hand, the velocity of the air jet from the nozzle will be slow, and the ability to blow out the molten material will be weakened, and the cutting section may form burr;
On the other hand, due to the large size of the nozzle and the slow air flow speed, the slag and smoke generated by laser cutting are easy to splash into the nozzle and damage the protective lens.
For high-power laser machine, the reflected laser beam may burn the laser head.
4.5. Influence of compressed air quality on laser cutting quality
The quality of compressed air has a very direct impact on the quality of laser cutting.
The compressed air contains water mist and oil.
If it is not cleaned, high-pressure jet to the protective mirror of the laser cutting head will seriously affect the transmission of the laser beam, disperse the focus, and cause the product to be cut incompletely and produce waste products.
If it is a super high power laser cutting machine, as long as the mirror or nozzle surface adheres with a little tiny oil film or water mist, it may also cause high-energy laser emission to burn the laser head.
As the laser head price of super high power laser cutting machine (above 12000 W) is more than 20000.
Most of the air compressor providers can not handle the oil and moisture in the compressed air well.
Most users of ultra-high power laser cutting machines are not willing to risk using compressed air as auxiliary gas.
Figure 9 and 10 shows uncontaminated lenses and contaminated lenses, respectively.
Fig. 9 Uncontaminated lenses
Fig. 10 Contaminated lenses
5. The factors affecting the thickness and efficiency of metal laser cutting
From a large number of practices, the main factors affecting the metal laser cutting thickness and cutting efficiency are the power of the laser cutting machine, the type of auxiliary gas, the pressure and the flow rate of auxiliary gas.
When choosing air compressor for laser cutting machine, these factors should be considered comprehensively.
According to experiment 4 of laser cutting research conducted by Tsinghua University, the influence of laser power and auxiliary gas pressure on laser cutting ability is shown in Fig. 9.
The cutting speed is 3 m/min, the spot diameter is 0.16 mm, the nozzle shape is conical, the diameter of the nozzle bottom is 1.5 mm, and the distance between the nozzle and workpiece is 1 mm.
Fig. 11 Influence of laser power and auxiliary gas pressure on laser cutting ability
As can be seen from Fig. 11 (a), if the laser power is lower than 860 W for 2 mm thick low carbon steel plate, no matter how much pressure of auxiliary gas nitrogen is increased, the workpiece can not be cut off because the energy provided by the laser is not enough to penetrate the workpiece.
If the laser power is high enough to penetrate the workpiece, the higher the laser power, the lower the auxiliary gas pressure required.
It can be seen from Fig. 11 (b) that if oxygen is used to cut low carbon steel plate with the same thickness, the influence of laser energy and auxiliary gas pressure on cutting ability is similar to that of nitrogen cutting.
However, the laser power and auxiliary gas pressure required for cutting are greatly reduced.
This is because oxygen reacts with iron during cutting, which releases about four times the laser power.
This also explains why the general medium power laser cutting machines prefer to use oxygen cutting when cutting low carbon steel plate of 2 mm or more.
When cutting carbon steel plate with compressed air, there is about 20% oxygen in the air.
This part of oxygen also reacts with iron element and releases heat during cutting.
Therefore, the efficiency of cutting carbon steel plate with compressed air is slightly higher than that with nitrogen.
In addition, different laser cutting machine manufacturers, due to the different laser and technology, cutting capacity is not the same.
This post extracts the public data of several representative manufacturers for reference.
Table 2 Maximum cutting thickness of Han’s laser cutting machine, mm
|Using IPG fiber laser|
|Low carbon steel||10||12||16||20||22||25|
Table 3 Maximum cutting thickness of HG laser cutting machine, mm
|Marvel series fiber laser|
|Low carbon steel||20||20||22||25||25||25|
Table 4 Maximum cutting thickness of Bystronic laser cutting machine, mm
|Low carbon steel||6||10||16||20|
Due to the decrease in laser cost, the overall cost of the laser cutting machine has been greatly reduced.
At present, the power of laser cutting machines purchased by general metal processing users is 3000-4000W.
A small number of metal processing users around the metal material market mainly buy 12000 -15000W ultra-high power laser cutting machine.
Therefore, in general, laser cutting can be used for low-carbon steel plate with thickness less than 22-25 mm, stainless steel plate, aluminum alloy plate and copper alloy plate less than 10-16 mm.
- Low carbon steel plate with a thickness less than 2-2.5mm should be cut with high-pressure compressed air;
- Low carbon steel plate with thickness of 2-2.5mm should be cut by oxygen;
- Stainless steel plate, galvanized sheet, aluminum alloy plate and copper alloy plate should be cut with nitrogen or high pressure compressed air.
6. Selection of air compressor
6.1. Selection of air compressor pressure and displacement
According to the previous discussion, the problem of how thick the plate can be cut by the air compressor equipped with the laser cutting machine mainly depends on the laser power, but has little to do with the pressure of the compressed air.
If the laser power is large enough, the pressure of compressed air can also be cut;
If the laser power is not big enough, the compressed air pressure can not be cut.
As long as the laser power is large enough, the higher the pressure of compressed air, the better the laser cutting quality and the higher the cutting efficiency.
With the development of laser cutting machine power from small power to medium power, then to high power and even super power.
The pressure requirement of the air compressor supporting the laser cutting machine has also increased from the initial 8 bar to 13 bar two years ago, and then to 16 bar now, and it is still developing to 20-30 bar.
For the laser cutting machine using compressed air as auxiliary gas, the compressed air is divided into three channels after drying and purification, which is respectively used as cutting gas, cylinder power source and optical path positive pressure dust removal gas to maintain the normal operation of the laser cutting machine.
The proportion of cutting gas is about 80-90% of the total consumption of compressed air.
For the laser cutting machine which does not use compressed air but uses nitrogen or oxygen as auxiliary gas, it still needs a small air compressor to provide cylinder power supply with a 6-7 bar atmospheric air source.
How much air compressor should be equipped for laser cutting machine with compressed air as auxiliary gas?
As auxiliary gas, compressed air is sprayed out of the nozzle coaxially with laser beam to assist laser cutting.
Calculating the compressed airflow through the nozzle can determine how much air compressor needs to be equipped.
The absolute pressure of air before and after the nozzle is set as P and P0 respectively.
According to the nozzle theory, when P0/P ≤ 0.528, the flow rate of compressed air passing through the nozzle in the standard state is as follows:
- Q – air flow, L/min
- S – effective area of nozzle, m2
- μ – flow coefficient, μ < 1, generally taken as 0.92-0.94
- A – nozzle area
- d – nozzle diameter, m
- p – absolute pressure in front of nozzle, Pa
- T – stagnation temperature of gas in front of nozzle, K
- p – absolute pressure in front of nozzle, Pa
- T – stagnation temperature of gas in front of nozzle, K
In the calculation of the air compressor discharge, it is necessary to consider the laser cutting machine free travel time, the actual working time is between 50-70%.
The actual required displacement QN is calculated as follows:
According to the power of the laser cutting machine, the common seven nozzle specifications are: 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 mm.
Fig. 12 shows the auxiliary gas flow rate of nozzles with different cutting pressure.
Fig. 12 Auxiliary gas flow rate of nozzles with different cutting pressure, m3/min
A user’s laser cutting machine uses a 2.0 mm nozzle, and the cutting pressure is expected to reach 1.6 MPa absolute pressure.
How should an air compressor be equipped?
As shown in Figure 12, a compressed air flow of 0.491 m3/min is required for 2.0 mm nozzle to maintain 1.6 Mpa absolute pressure;
If the actual outgassing time is 70% and 90% of the air displacement of the compressor is used for auxiliary cutting calculation, the required gas volume is: 0.491 * 0.7/0.9 = 0.382 m3/min, and that of two compressors is not less than 0.764 m3/min.
The nominal discharge capacity of a Baode11TK/16 compressor is 0.97 m3/min, which can be equipped with two laser cutting machines with 2.0 mm nozzle (one driven two), or a laser cutting machine with 2.5mm nozzle.
How can a Baode15TK/16 laser cutting integrated air compressor be equipped with a laser cutting machine?
The nominal discharge capacity of Baode15TK/16 compressor is 1.27 m3/min, which can be converted into: 1.27 * 0.9/0.7 = 1.633 m3/min
According to Fig. 12, one Baode15TK/16 integrated air compressor for laser cutting can be equipped with one laser cutting machine equipped with 3.5 mm nozzle or two laser cutting machines with 2.5mm nozzle (one driven two).
6.2. Selection of air compressor type and system
The quality of compressed air has a direct impact on the quality of laser cutting, and the laser cutting machine generally needs to work continuously, so the air compressor is generally placed near the laser cutting machine.
Therefore, the air compressor must be equipped with a good dryer to remove the moisture in the compressed air as much as possible, and the high precision filter group with enough precision must be configured after the dryer to fully filter oil and dust particles.
After the post-treatment, the pressure dew point of the compressed air reaches 2-5 ℃, the residual oil content is less than 0.001 ppm, and the particle content is less than 0.01 μM,
After the compressed air is dried, the temperature is returned to close to normal temperature, which can effectively avoid condensation again,
Such high-quality compressed air can meet the long-term stable operation of the laser cutting machine, without frequent wiping the protective lens.
The piston air compressor is easy to obtain high pressure, but the piston air compressor is noisy, has many vulnerable parts, and the exhaust oil is not handled well.
Therefore, it is not recommended to use piston air compressor below 1.6 Mpa.
It is generally recommended to select screw-type air compressor with suitable cold dryer and precision filter below 1.6 Mpa.
The compressor should adopt permanent magnet variable frequency motor, which can maintain the stability of air supply pressure and ensure the best cutting effect.
Because most users do not have the knowledge of air compressor and compressed air purification treatment, if the users purchase the air compressor, air storage tank, cold dryer and filter one by one, and then install them by themselves, it is not only troublesome, but also likely that the whole system can not meet the required compressed air quality requirements.
Integrated air compressor integrates compressor, air storage tank, water separator, cold dryer and filter into one, which saves the trouble of user’s self-provided gas tank, cold dryer and filter, the trouble of user’s on-site installation and connection, and the trouble of service provided by multiple suppliers.
It can provide continuous, stable and clean compressed air, which is an ideal choice for supporting high-power laser cutting machine.
Fig. 13 Integrated air compressor for laser cutting machine
At present, the laser cutting machine which needs more than 1.6Mpa compressed air mainly adopts piston compressor, but its development is relatively slow due to the restriction of reliability and quality of piston compressor.
With the popularization of high-power laser cutting machine, the demand for thick plate cutting is increasing.
It is necessary to develop a small flow high-pressure screw air compressor with rated discharge pressure ≥ 2.0 MPa.
7. Problems needing attention
7.1 Over–consideration of the purchase cost
When purchasing the air compressor used for laser cutting, if the purchase cost is excessively considered, it is likely to buy a pile of rotten goods in the end.
Because the air compressor supplier also wants to make money, if he sells the compressor system with high quality compressed air to you at the lowest price in the market, he will not be profitable.
For the sake of business and profit, he can only replace the inferior with the good, and replace the small with the big.
For example, for the sake of competition and profit, the cooling dryers with a nominal treatment flow of 1 m3/min are now on the market, none of them can reach the nominal pressure dew point temperature of 2-8 ℃, and the actual pressure dew point is above 12-16 ℃.
If the air compressor for laser cutting is equipped with such a cold drying machine, the protective lens must be wiped many times a day, and the quality of laser cutting cannot be guaranteed.
Similarly, the oil and gas separation core and its own value is not high precision filter is not cheap.
7.2 Greedy psychology
In the choice of air compressor, the user will generally choose the air compressor with larger displacement than the actual demand, and the sales personnel are also willing to sell the air compressor with large displacement.
However, if the air compressor displacement is too large, much larger than the actual demand.
Even if the air compressor has frequency conversion regulation, it may also cause the air compressor to run at the lowest frequency, the air supply is still excessive, and the machine has to enter the shutdown sleep state.
In order to maintain the stable gas supply pressure, once the gas continues, the air compressor has to start operation.
Such frequent “start-run-sleep-start-run-sleep” will not only shorten the service life of the air compressor, but also cause the lubricating oil in the oil-gas separator to bubble and increase the oil content in the exhaust.
When “the air compressor is too large” has become a reality, it is necessary to modify the controller parameters to try to reduce the number of automatic start and stop of the air compressor, such as changing the normal 10 s no-load delay to 180 s no-load delay.
7.3 Good operating habits
If the air compressor is started first and then the cold dryer is started, it is possible that the exhaust can not reach the required pressure dew point for a long time.
At this time, if the laser cutting machine is operated, it is possible that the water in the compressed air will be fogged on the protective lens of the laser cutting head.
The correct operation habit is to start the cold dryer first, and then start the air compressor when the refrigerant temperature drops to about 5 ℃.
7.4 Operating temperature setting
The exhaust temperature of the general air compressor with 7-8bar exhaust pressure is not more than 90 ℃ under normal use conditions.
The manufacturer generally sets the exhaust temperature at 78-85 ℃.
However, for air compressors with exhaust pressure of more than 1.6Mpa, if such a low exhaust temperature is required, it will be a disaster if it is used on laser cutting machine.
Because the high-pressure air compressor runs in summer and only about 80 ℃ exhaust temperature, the exhaust will be very easy to separate water, resulting in the emulsified foam of lubricating oil to cause the sharp increase of oil content in the exhaust.
As a result, it is impossible to cut or cut out waste products, and even damage the focusing lens.
The exhaust temperature should be set according to different places and seasons according to Figure 14.
Fig. 14 Saturated air pressure dew point
Dew point temperature at different inlet temperatures and exhaust pressures,
@Atmospheric saturation intake
Saturation dew point temperature, oC
Exhaust pressure, MPa (a)
7.5 pay attention to daily maintenance
1) Before the formal work every day, the condensate in the air tank should be drained and the oil stain of the condensate water should be observed.
2) It is necessary to observe the oil return condition of the oil return pipe from the oil-gas separator to the main engine of the compressor through the sight glass (Fig. 15) to judge whether the oil content in the exhaust gas is high or low, and whether the lubricating oil or oil separation core needs to be replaced.
a. High-speed airflow carrying a small amount of lubricating oil through the mirror – normal;
b. The sight glass is full of lubricating oil and moves slowly – the pipeline is blocked and needs to be cleaned up;
c. The high-speed airflow carries a large amount of lubricating oil through the sight glass- the oil separation effect is poor and the oil content in the exhaust gas is high, which needs to be solved immediately.
Fig. 15 Oil return sight glass
3) Replace the filter element of the precision filter within the time specified by the manufacturer
4) Replace the air filter element, oil separator, oil filter and lubricating oil within the time specified by the manufacturer.
Conclusion: compressed air is widely used as auxiliary gas in the laser cutting machine.
It can replace oxygen to cut carbon steel sheet and nitrogen to cut stainless steel, galvanized sheet, aluminum alloy and copper alloy.
The correct selection and use of air compressor can not only guarantee the quality of laser cutting, but also obtain good cutting efficiency and obvious cost-saving.