The first main influencing factor you must consider before cutting any piece of acrylic is, what is the expectations out of the laser cut? Does the end product need to be perfectly flame polished with a optical finish, or is the part going to be used as a mechanical component and edge quality is not too important. This factor will determine what kind of of cut you should be looking to produce and ultimately could save you time if you establish that edge quality is not expected to be flame polished.
Acrylics are cut by vaporising the solid material. The laser is absorbed by the material causing it to change from a solid to a liquid then to a vapour. This is done with a very low level of chemical degradation. To help combat the copious amounts of vapour created by laser cutting, it is important to have a high quality, strong vacuum system to remove the vapour. The vapour that is emitted from the laser cutting process is highly flammable hence the laser system should never be left unattended while acrylics are being cut.
A gentle stream of air or nitrogen will blow the vapour away from the cutting area, to prevent the vapours igniting. This gentle stream of air also helps solidify the remaining liquid along the cut line; this will help create a glossy, smooth flame-polished finish on the cutting surface. But this is a double edged sword, if the stream of air is too powerful, the cutting edge will be disturbed while in the solidification phase and create a frosted or wavy cutting edge. So having a variable pressure regulator on the air assist pump is a must.
The basis of a good cut is good material. Sticking to name brand materials will give best results. These are the brands that Trotec uses and recommends.
|Plexiglas® / Acrylite®
Types Of Acrylic
There are two main types of acrylic that you will commonly encounter, cast and extruded acrylic. These two types of acrylic have slightly different chemistry and are manufactured differently, which results in vastly different cutting characteristics. For most applications, cast acrylic is probably the right material for you, but under specific circumstances extruded acrylic might have its advantages over cast acrylic. The main differences are listed in the table below.
- Many colours available
- More expensive
- More thickness’s available (2mm to 200mm)
- Greater thickness tolerance (>±5%)
- Less colours available
- Less thickness’s available (30μm to 25mm)
- Lower thickness tolerance (<±5%)
- Engraving has high contrast
- No bur on cut edge
- High molecular-weight
- Needs more power – higher melting point
- Parameters more critical for flame polishing
- Engraving has low contrast
- Sharp bur at laser cut line
- Low molecular-weight
- Needs less power – lower melting point
- Parameters not as critical for flame polishing
As a typical rule of thumb for every 10 Watts of power you will be able to cut 1mm / 0.04 inch of material. This will give you the ability to flame polish your cuts and give good production speed. That doesn’t mean you can’t cut 12mm / 0.5 inch with a 60 Watt laser, this is still possible, but the quality of the cut will not be nice and the process will be much slower. As a general rule, more power is always better; this gives you the versatility to cut extremely fast on thin materials and give you much better cut quality on thicker materials, peak power is the key!
Depending on the material and the type of cut you would like to achieve, the frequency value will change.
Flame Polished Cut
- Common to start with high frequencies (20 to 25 kHz) for cast acrylic.
- Common to start with low frequencies (2-5 kHz) on extruded acrylic.
- Common to start with frequencies (9 to 12 kHz) for cast acrylic.
- Common to start with frequencies (5 to 10 kHz) for extruded acrylic.
By decreasing the frequency of the laser you are essentially reducing the cutting energy of the laser, this is done by reducing the amount of laser pulses over a given time frame. This can however help reduce flaming, decrease boiling but does result in slower cutting speeds.
The cutting speed is defined by two factors, the quality requirements and the laser power. The typical types of cuts and their speed are shown in the image above. The light blue line represents a separation cut, which is a cut with no preference for edge quality. The second type of cut is represented by the dark blue line which is a quality cut.
As you can see there is a large discrepancy in speed of the cut depending on the quality level you would like to achieve with a given thickness of material. There is also a linear increase in speed as the power of the laser increases. For example a 400W laser cutting 10mm thick acrylic will be twice as fast as a 200W laser cutting the same material.
Lenses and Focus
Lenses are an important factor in the cutting process of acrylics, they have different focal lengths which affect their beam radius and waist length. Without getting too technical, the lenses are responsible for the shape and quality of the cutting edge.
For materials up to 20mm / 0.75 inch in thickness you should use a 2.5″ lens if the power of your laser permits this. For lasers with power levels <120 Watts you will notice a dramatic cut quality increase if you linearly increase the lens focal value as the material thickness increases. Trotec laser offers lens options from 1.5″ all the way up to 5″ versions. For materials of 3 to 6 mm (1/8 to 1/4 inch) in thickness you will be able to cut faster using a 2″ lens.
For material above 20mm / 0.75 inch in thickness or if your laser power exceeds 200 Watts, using a 5″ lens will give best results.
As for focus, for best results, follow these rules.
- Do not defocus the laser if the material you are cutting is below 10mm / 0.4 inch in thickness.
- When cutting material with thickness’s over 10mm defocusing the laser so the focal point sits 1/3rd into the material will give best results (for 12mm material use a z-offset of -4mm).
- Defocus the laser if cut edges are not straight or wavy at the bottom of the cutting edge.
Defocusing the laser results in better beam usage, you are taking advantage of the beam waist to get a straighter and higher quality cut.
When flame polishing acrylic it is essential to cut the acrylic using a cutting table. Trotec Laser offers an acrylic or metal cutting table option for your laser. The acrylic cutting table is essential to a high quality flame polish cut. When cutting acrylic on a metal cutting bed, the laser energy may hit the metal cutting bed and bounce back into the material causing divots or vertical marks along the cut line. By using the acrylic cutting bed, the bed will absorb the laser energy and not bounce it back towards your work piece. There are also acrylic lamellas available for purchase that will do the same job, but for larger objects.
As mentioned before suction from below is important to rid the laser cabin of flammable acrylic vapour. Trotec Laser offer a vacuum bed for their lasers which the cutting beds can sit on top of. This gives you the benefits of both the acrylic cutting bed with the essential use of the vacuum table. When the material is on the acrylic cutting bed, you can lay scrap material or thin sheets of aluminium on the laser bed to cover the exposed areas where the vacuum can escape from. By doing this, you are increasing the vacuum intensity under the acrylic, which will result in a better cut.
Without a quality exhaust, quality cuts are not possible. The exhaust system serves two purposes; firstly it avoids flaming of the material. A good acrylic cut is hot and produces a lot of gases, these gases are highly flammable. The exhaust system transports these gases away from the cutting area and thus avoids those gases from igniting.
Secondly the exhaust controls the solidification of the liquid acrylic. The airflow through the kerf of the cut allows the acrylic to melt from top to bottom, this gives the desired flame polished cut, without this you will get wavy horizontal lines through your cut edge.
One of the best all-in-one exhaust and filtration systems on the market is the Atmos duo plus.