Expert Tips

1.1 Laser cut edge on thick and thin materials

Thin materials

The edge of a laser cut material will never be entirely 90 degrees to the material surface. It’s very nearly there in most cases you don’t really notice and difference.

On thinner materials the edge can sometimes be very slightly angled. During vector cutting, the beam follows a linear path. A lot of material is being removed in the wake of the path, and this material vents up and out from the cut line in the direction of the exhaust. Even though the laser beam is perfectly straight sided, the walls of the cut do tend to angle up slightly in a v-shape, because of the heat is more concentrated at the top of the material and less on the reverse. This subtle angle is less apparent the thinner the material being cut. In most cases on materials less than 3mm it is unnoticeable.

 

Thick materials

 

The edge of thicker materials has a slightly different appearance caused by the focal distance of the lens and the shape the laser beam forms as it exits the lens. The laser cutting process involves a optimum focussed laser beam:

 

The laser beam that comes from the laser source (laser tube) is typically around 6.5mm (240 thousandths) in diameter. This is too large for practical cutting applications. In order to achieve high quality engraving and precise cutting, the laser beam is directed through a series of mirrors and a focus lens. The focus lens narrows or “focuses” the laser beam to a very small, precise spot. As the beam leaves the focus lens, it takes on a slight “hourglass” shape, with the ideal focal distance within the center of the “hourglass.”

 

The following images are accentuated to demonstrate the concept of the focal point of the lens. In reality the width of the beam and the “hourglass” focal shape is much smaller.

 

We use different lens for laser cutting depending on the thickness of the material being processed. This works in the same way as a camera that uses different lenses for close-up and wide angle photographs. The various lenses differ by their respective focal distances. A 2.5 inch lens is used for cutting materials between 0.5mm to 8mm. When cutting materials between 10mm – 25mm we use either a 3.5 inch or 5 inch lens. A longer focal length helps to de-accentuate the slight variance in the edge but does not prevent it entirely. Bare in mind that some areas of a laser cut edge will appear slightly more angled than others, especially if cutting over a large portion of the laser bed.

 

Pulsing of the laser.

When cutting thicker plastic materials (particularly acrylic) the edge appears to be made from lots of vertical lines. This is caused by the pulsing of the laser. While it appears that the laser is firing constantly when processing a job, it is actually quickly turning on and off which is known as pulsing. We cut acrylic at a high frequency and at a very fast pulse rate. This helps to maintain a more polished edge.

 

 

 

Examples of laser cut edge on thick materials

22mm thick clear cast acrylic 

12mm thick birch plywood 
12mm mdf 12mm zotefoam

 



 


 


1.2.    
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

 

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.

Honeycomb cells:


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:
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.

Masking tape

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

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.

 


 

1.3.Understanding the "kerf" of the laser

  • What does kerf mean?
  • Accounting for the kerf in a drawing

What does kerf mean?

The laser burns away a portion of material when it cuts through. This is known as the laser kerf and ranges from 0.08mm – 0.45mm depending on the material type and other conditional factors. Any areas in your design where cut lines come closer than 0.5mm together could burn away entirely. Any details narrower than 1mm are likely to be very fragile and in some cases can cause the material to warp whilst cutting. As a benchmark, we recommend that minimum cut widths be no smaller than the corresponding thickness of the material. For example, if cutting from 3mm acrylic, it’s best not to allow any widths less than 3mm. We can go smaller (see the cut width image on the material pages) but this can make your pieces very fragile which might not be suitable for your application. We will advise if your drawing has cutting tolerances that are too small, but we can’t be held responsible if your components do not hold together sufficiently.

Kerf is determined by material properties and thickness. But other factors also have an impact on how much the laser takes away. The focal length of the lens, pressure of compressed air both have an impact. Kerf widths can vary even on the same material sheet, whether cutting a straight line or a curve line or from laser cutting in the x or Y dimension. The manufacturing tolerance of the material can also impact the kerf.


Accounting for the kerf in a drawing

For some applications (for example a slot together product), you will need to account for the kerf within your drawing by adding or subtracting the kerf width from your component dimensions. The following table gives an overview of the average measurement the laser will take away when cutting on the most commonly cut materials and thicknesses. Please bear in mind that these measurements are to be treated as guidelines only, use them as a starting point. We always recommend prototyping a portion of your design taking into account the kerf and tweaking if necessary.

MaterialsThicknessAverage Kerf
Acrylic 1mm-3mm 0.18mm
Acrylic 5mm-8mm 0.21mm
Acrylic 10mm-15mm 0.3mm
Acrylic 20mm 0.32mm
HIPS, PETG, Styrene, 1mm-3mm 0.45mm
Birch plywood 0.8mm 0.08mm
Birch plywood 1.5mm 0.16mm
Birch plywood 3mm 0.2mm
Birch plywood 6mm 0.22mm
Birch plywood 12mm 0.3mm
MDF 3mm 0.16mm
MDF 6mm 0.2mm
MDF 12mm 0.28mm
Veneered MDF 6mm 0.17mm
Greyboard 1200microns 0.08mm
Greyboard 2400microns 0.12mm
Paper 90 – 350 gsm 0.08mm

 


 
1.4. Understanding power distribution and raster engraving
  • What is laser power distribution?
  • How it impacts raster engraving results
  • Arranging components economically on a sheet

What is laser power distribution?

 

When raster engraving small surface areas and larger surface areas the engraving result can differ slightly even when the same laser settings are used. Minor fluctuations can occur in depth and in some cases tone of engraving. This is based on the power distribution principal.

Typical if we are engraving a small surface area, the laser power is more concentrated because the strokes of the laser head ( the movement from left to right ) are in a smaller more focused area. As soon as the size of the engraving increases the stroke size i.e the distance the laser head travels increases, meaning the power distribution of the laser becomes slightly less. All engraving has a variance tolerance for this reason. The variance is very minor, in terms of depth we are talking in microns, but it can affect the appearance of the engraving.

How it impacts raster engraving results

 

When your artwork is staggered between differing sizes, the laser completes portions of the artwork a stage at a time. To demonstrate what we mean have a look at the following example.

The laser will begin to work it’s way down the artwork. To begin with, the stroke size is fairly thin in relation to the artwork meaning the laser power will be more concentrated. As soon as the laser reaches lower down, the stroke size will increase dramatically corresponding to the larger width. This can have a impact on laser power distribution causing a minor variance in the engraving depth and strength between the horizontal and vertical strip:

 


 1.5. What is flashback?

Because the laser beds are manufactured from aluminium they are by nature heat conductors. Flashback is caused when the laser meets any metal element of the cutting bed on the reverse of the material as it slices through. This can result in very small nicks from the underside laser cut edge. Flashback is most common on materials thicker than 5mm particularly acrylic as it is a brittle.

In all cases we try to keep flashback to a minimum. All our laser settings are configured to an optimum power to minimize damage. . We try to avoid using honeycomb where possible and use lamella slats but in some cases if your artwork is particularly dense, we need to use it to maintain support which can result in flashback.

 


 1.6.Laser engraving acrylic techiques

Laser engraving cast acrylic

Acrylic is manufactured in two different ways: Cast and Extrudedand reacts differently during laser engraving.

Cast acrylic is used for almost all engraving purposes as when engraved, it produces a white, frosty look that in contrast to the clear material.

 

Extruded acrylic

When laser engraving extruded acrylic it remains clear and does not produce much of a contrast. Extruded acrylic, on the other hand is ideal if you are only going to laser cut. The material has a lower melting point which produces an almost flame finished edge from the laser.

The quality of engraving is also affected by the purity of the acrylic. Cheaper acrylic doesn’t usually cut very cleaning due to inconsistent melting points over the sheet material. At Cut laser Cut we only use Perspex™ branded acrylic as it is manufactured to a benchmark of quality.

Mirrored reverse side engraving

A common laser engraving technique on clear (cast) acrylic is to mirror the engraving on the back side of the perspex. This produces a ‘look through effect’ from the front of the surface. We will reverse your artwork prior to starting an an engraving job like this.

Paint veneered engraving

Another popular technique is to paint one side of a clear acrylic piece. We then engrave directly through the paint to reveal the acrylic below. Great for definition and contrast detail.

 


 1.7. Engraving bare metals

Usually when engraving metals the best results can be achieved using a pre-coated metal that has been surface treated; anodised, lacquered, powder coated or spray veneered. The laser will then remove this surface to reveal the metallic surface below.

To mark a bare metal surface we have to run the laser very slowly at a high power. When doing this, the laser is not actually removing any of the material, it is altering the surface structure of the metal. This results in different colour variations ranging from gold to purple (depending on the metal used.). Different powers / speeds produce different results.


1.8. Photo laser engraving guidelines

There isn’t one correct method of engraving photos. The same photograph will engrave differently from one material to the next using the same image. Its worth baring in mind that different materials will produce different engraved look. We often advise to run a series of tests to begin so we can establish the correct settings for the sort of thing you would like to achieve.

  • Best types of images
  • Resolution
  • Digital Internet Images
  • Image size

Best types of images

Not all photos are fit for engraving. Photographs that produce the best results usually contain a wide depth of shading that range from light to dark.

Photographs containing large areas of a single color typically do not engrave well – especially when working with wood. We recommend avoiding pictures with expansive backgrounds containing only one or two small subjects.

The photo which is a good choice for engraving should have the majority is a gradient pattern of light to dark. There is not an abundance of large blocks of a single color and it contains a good amount of detail.

Resolution

The resolution at which you engrave does not have to be massively high. In most cases a resolution of 300 or 400 DPIwould be ok. You might need a high resolution, such as 600 or 1200 DPI for photo engraving for some applications but again this depends on what you want to achieve and the size of your material.

Digital cameras over 2.0 mega pixels produce great high-resolution images that engrave nicely . Additionally, photos snapped with a iphone or other camera phone often do not produce high resolution photos.

If scanning in a physical photo, Scan your photos at a low resolution such as 300 DPI. If we are going to engrave your photos at 300 DPI, there is no reason to scan it any higher. This just makes your image ridiculously large and difficult to work with in our graphics software. Always scan your image in colour and then convert to grey scale afterwards.

Digital Internet Images
Be sure to obtain the highest resolution digital image you can from the the web. Low resolution photos and graphics copied and pasted from the Internet are never any good! I repeat, NEVER any good! They might look ok on screen, but in reality they are usually not above 72dpi and very pixelated which would show on your engraving. If you look hard you can occasionally find high resolution graphics online.
Image size

Get the largest size image available, especially if you are scanning it. Stretching a photo in your graphics software will reduce the resolution of your image. Additionally, if your photo isstretched too much, it will become too pixelated to engrave.