Burn marks / heat marks and how we prevent them
C02 laser cutting is essentially finely burning through a material and engraving is delicately burning into it. Markings are an inevitable part of the process but they can be minimised. Burn marks and heat marks occur in following ways:
- On the material edge as laser cuts through
- Top surface marks from fumes and debris
- Laser tails from the laser piercing material
- Marks on the material reverse
- Ways in which we minimize burn / heat marks
On the material edge as laser cuts through
This differs from material to material. On plastic for instance the edges appear melted / slightly glossy. On more combustible materials such as woods, papers and card edges look more brown / black.
Top surface marks from fumes and debris
Fumes and debris are emitted when cutting / engraving. These fumes are extracted from the rear or base of the machines. The fumes are dragged over the material surface causing marks. Marks also appear from hot debris which glazes or scorches the exhaust-facing edges of the cut line. (most noticeable on thick materials and more combustible materials.)
Laser tails from the laser piercing material
During vector cutting, the beam follows a linear path. At the beginning of a vector shape the laser needs to begin firing to pierce though the material. This entry point is most susceptible to damage since it receives the most sustained blast of heat. Due to this, small flare marks can appear on material surface. These are known as laser tails.
Marks on the material reverse
When the laser comes into contact with the cutting bed as it slices through the material, debris is deposited onto the reverse face.
There are 3 types of laser beds we use depending on the job type and material. When laser cutting your material needs to be supported efficiently from underneath to allow it to remain flat at all times but also allow heat from the laser to escape from underneath. Unfortunately the more contact the reverse of the material has with the laser bed the higher the risk of marks on the reverse. The ideal scenario is for us to completely float the material so no heat builds up underneath. The ability to do this depends on your artwork and the type of material being cut. Paper for instance needs quite a lot of support as it is not a rigid material.
When your artwork is very dense or we are cutting a less rigid material such as paper or fabric we need to use the honeycomb bed to keep it evenly flat as the pieces are cut. The aluminium honeycomb cells support the material whilst allowing heat and debris to dissipate from beneath.
Due to the nature of repeat cutting, residues from materials tend to build up on the honeycomb beds. Reverse marks occur when the laser runs over the honeycomb cells. The heat from the laser causes these residues to transfer back onto the material being cut. For this reason we have alternative honeycomb beds for different classes of material: Plastics, Woods, Papers & card to minimise contamination from material to material. On thicker materials flashback. can also sometimes occur.
Aluminium lamella slats are our priority laser bed option. The material can be supported with a small number of slats reducing the contact with the reverse of the material. In some cases the material can be “floated” completely minimising marks on the reverse. We usually need to vector order your artwork (select the order in which pieces are cut) to achieve this. Residues from repeat cutting can also transfer back onto the reverse of a material from lamella but unlike honeycomb, the marks are a lot less.
Aluminium Engraving table:
The engraving table is a essentially a flat metal surface used more to support the material for engraving only. Very thin fabrics and some papers can also be cut on the engraving table because they require a very low laser power.
Ways in which we minimize burn / heat marks
All our material settings are optimised to maintain the cleanest cutting and engraving conditions and to keep these burn and heat marks to a minimum. Methods used vary from material from material but generally involve the combination of the appropriate cutting bed selection, protective backing and compressed air.
Large format protective paper backing
Large format protective backing is a sticky back masking tape that can be applied to a material surface prior to any cutting or engraving is carried out. It comes in rolls up to 1220mm wide. Applying the tape helps to limit surface burn marks particularly on the reverse surface. All the markings will transfer onto the protective tape and not the material. You will then need to peel off the backing to reveal the clean material surface. Plastic materials such as acrylic usually have the a thin plastic film applied as standard to help prevent scratches. Protective backing can be used on most materials but not all. Paper, card and some leathers / fabrics pose the biggest problems because the surface can get damaged / ripped when peeling off.
The tackiness of large format protective backing is sometimes insufficient for some materials particularly those with a rough surface. Birch plywood for instance has such a high moisture content and uneven grain structure that the backing peels off after a few minutes, and sometimes during the cutting process which can make it appear even more burnt than if it wasn’t used at all. In this case thinner strips of high tack masking tape are layered in strips to cover the reverse of the material.
Compressed air (up to 4 bar / 58.02 psi) is used in most laser cutting applications to remove heat and combustible gases from the top material surface. By directing a constant stream of compressed air at the point where the laser meets the material, flaming, scorching and charring is reduced on materials such as wood, acrylic and rubber. It also helps to limit smoke deposits on the material reverse. Smaller Kerf widths are also maintained with compressed air by minimizing the heat of the laser at point of contact with the material surface.
Not all materials react to the laser in the same way. Browse the materials gallery to see more on how certain classes of materials react to the laser and how we go about minimizing them.