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What should be paid attention to in laser processing?

  26 Oct , 2020         Chris         8476

Although the same offset compensation value can ensure the accuracy of the same machining under the same conditions, the setting of the focus position depends on the perception of the processor operator, and the thermal lens will also cause the change

1. Change the processing size

This is due to the burning area created by focusing the laser on the diameter and width of the incision.

   Although under the same conditions, the same offset compensation value can be used to ensure the accuracy of the same machining material, the setting of the focus position should be made by the processor's perception operator and the action of the thermal lens will also cause a change in the focus position, so offset compensation values should be checked periodically.

2. Dimensional error of machining direction

   The upper dimension precision of the board is different from that of the board.There are two reasons to consider this phenomenon.First of all, the roundness and intensity of the beam are not evenly distributed, resulting in different incision widths along the processing direction.The solution is to adjust the optical axis or clean the optical elements.Secondly, the thermal expansion of the processed object will shorten the shape, length and size of the processed object.

3. Changes caused by distortion

   Although the dimensional accuracy is within the required range, warping may occur due to thermal deformation and other causes.Processing aluminum, copper, stainless steel is very important, it is affected by linear expansion coefficient, thermal capacity and other physical properties.As far as machining shape is concerned, the greater the aspect ratio is, the greater the amount of warping will be.Lower heat treatment conditions and processing lines are used in the processing scheme, but this problem has not been completely solved.The residual stress of the processed plate also affects the warping and size error, so we need to always maintain a certain configuration direction in the processing program.

4. Variation of spacing accuracy

   When the machining pore is large, the accuracy of hole spacing will be deviated.Since the hole is opened under the condition of thermal expansion, the space becomes smaller after cooling and contraction.We can supplement the precision of the positive part in the program, and we can also use the shape zooming function flexibly.No matter under what circumstances, should be in the initial processing, measurement of its processing size, filling error.When the clearance precision does not change with the machining position, but deteriorates in the whole machining area, the reason is that the mechanical precision deteriorates. 

5. Roundness changes

   In laser machining, the inclination of the cutting surface of the hole will inevitably occur.The lower diameter is larger than the back diameter, so the roundness of the smaller side of the back is generally evaluated.



Difficulty of piercing

   At the beginning of the cutting the hole needed to start the cutting is called a hole.The thicker the plate, the more unstable the perforation.It can be said that 70% of the bad machining in plate cutting with plate thickness greater than 12.0mm is due to poor perforation.In order to implement stable perforation, the machining characteristics of the perforation are described here.

 

Principle of perforation

   In the process of perforation, the molten metal produced during the processing prior to transfixion accumulates around the hole in the surface of the processed material.From the heating process of the surface of the processed object after luminescence to the perforating process of slow heating until the final transfixion is continuous.In this method, if the thickness of the plate is greater than 9.0mm, the perforation time will increase sharply, but the aperture is about 0.5mm, which is narrower than the incision and has less thermal impact.Therefore, if the processing capacity is increased and the output energy is increased, it is difficult for the molten metal to be discharged completely from the upper part of the aperture, resulting in excessive combustion.The CW condition is a method of setting the focus position slightly above the surface of the processed object, increasing the machining aperture, and heating rapidly.Although a large amount of molten metal appears and flies to the surface of the processed material, the processing time is greatly shortened.The absorption of laser energy also occurs in perforated hole walls.During the perforation process, the irradiated laser is reflected multiple times in the perforation, being absorbed while propagating underneath.In order to shorten the perforation time, it is necessary to supplement the reduced energy absorbed by the hole wall, that is, it is necessary to increase the output power during the perforation.In addition, in order to reduce the thermal impact around the hole wall, it is necessary to shorten the perforation time as much as possible while increasing the output power to reduce the laser irradiation around the hole wall.

 

Problems with perforations

(1) The moment when the defect occurs

   Identify defects that occur during the perforation or when cutting begins after the perforation.If it occurs during the piercing process, the output power and air pressure conditions are corrected according to the specific conditions at the beginning of the piercing or when the conditions are switched during the piercing process.If the defect occurs before the end of the punch, it is necessary to extend the punch time by switching to the cutting condition before transfixion.  

   If machining defects occur at the beginning of the cutting, it is because the molten metal that has accumulated around the surface of the hole is difficult to pass through, so it is necessary to set pulse conditions or low speed conditions at the starting position.

 

(2) The location of defects

   If there is a concentrated perforation defect at a specific location on the machining platform, it is due to the laser optical axis deviating from the nozzle center.This requires adjustment for optical path deviations.

   If the perforation location is too concentrated or near the cutting line, the perforation defect will also be caused due to the high temperature of the machining location.

   The SS400 plate with a thickness of 12.OMM was taken as the processed material, and the material temperature changed from normal temperature to 200℃. The results of the investigation and the relationship between the machining defects were obtained.The data represent 50 perforations at each temperature, the proportion of perforation defects and cutting defects.The higher the temperature, the greater the incidence of defects.Therefore, it is necessary to study the processing sequence and improve the procedure to perforate and cut along the line that has not been overheated.

 

(3) The time of bad perforation

   As the processing time goes by, when the number of machining defects increases but does not decrease, the reason may be the output power change caused by the vibration generator fault.If the cooling time can be increased, the reason may be caused by the thermal lens of the optical component.In this case, optical components need to be repaired and suppliers contacted.

 

(4) Materials with poor perforation

   For materials with poor perforation, it is important to confirm that they have been properly processed in the past.If there is a record of past processing, there is no need to adjust the processing conditions, can be identified as machining and optical parts of the defect, check to find out the cause.

 

Appropriate perforation conditions

   The thicker the processed material is, the higher the proportion of the perforation time in the overall processing time will be, and the higher the requirement for shortening the perforation time will be.The effective processing parameters for shortening punch time are pulse peak output power, pulse waveform and average output power.

 

Prevent whiskers from appearing when perforating stainless steel

   When cutting stainless steel, there will be scattered whiskers around the surface of the hole metal slag, in the mirror and striped surface material will appear scratches.Moreover, when the whisker metal slag is in contact with the nozzle of electrostatic induction machining head, there will be an alarm of abnormal focusing.According to the thickness of the plate, the corresponding treatment methods are different. 

 

Perforation of highly reflective material

   When cutting copper, pure aluminum and other highly reflective materials, it is necessary to smear beam absorbent on the surface of the processed material.Beam absorbers can not only improve the processing capacity, but also resist reflection from the safety point of view.The processing condition needs to reduce the pulse frequency and increase the energy of each pulse peak.And by increasing the gas pressure, the molten metal is squeezed into the plate to improve the effect of processing capacity.


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