Laser cutting process for acrylic ( Lucite ) sheets
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 vaporizing the solid material. The laser is absorbed by the material causing it to change from a solid to a liquid then to a vapor. This is done with a very low level of chemical degradation. To help combat the copious amounts of vapor created by laser cutting, it is important to have a high quality, strong vacuum system to remove the vapor. The vapor 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 vapor away from the cutting area, to prevent the vapors 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. Brand Manufacturer Plexiglas® / Acrylite® Evonik Perspex® Lucite Altuglas® Akemar Plazit® Plazit Quinn® Quinn Oroglas® Oroglas 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. Cast Extruded General Many colours available More expensive More thickness’s available (2mm to 200mm) Greater thickness tolerance (>±5%) Less colours available Cheaper Less thickness’s available (30μm to 25mm) Lower thickness tolerance (<±5%) Laser 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 Laser Power 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! Frequency 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. Separation Cut 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
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