Laser Cutting Thermoplastics

Lasers have been utilized for pointing, engraving, welding and cutting applications for decades. The process for Laser Light generation, originally known as “Maser” was developed in 1953 to amplify microwave signals detected in distant stars. The first evidence of Laser Light generation was in 1960 by Theodore Maiman who successfully oscillated a ruby with solid state control. Over the next few years, development was rapid with the introduction of He Ne gas (Helium Neon) laser. In 1964 Kumar Patel invented the CO₂(Carbon Dioxide) laser while another team, J.E. Geusic, H.M. Markos and L.G. Van Uiteit invented the Nd:YAG, Neodymium:Yttrium Aluminum Garnet Laser. (Figure 1)

CO₂ and N₂ (Nitrogen) are natural found gases that exist in our atmosphere: when combined with He (Helium) they form the Lasing gas in CO₂ lasers. The CO₂ molecule is called the active medium and is the source for the laser emission. Solid state lasers utilize atoms combined with crystals such as ruby or YAG. When doped with a medium Cr (Chromium Ions) or Nd, they constitute the active lasing medium.

The gas laser resonator consists of a glass vacuum tube that has a rear mirror (Figure 1) and an output coupler at the other end; the medium is placed inside the tube. In the case of CO₂ lasers, the medium (He, N₂, CO₂) is sealed within the glass tube and has a limited life span, once discharged has to be replaced or recharged. With unsealed CO₂ lasers, gas is pumped into the vacuum tube from gas bottles. The bottles of gas are readily available and can be purchased ready mixed or separate per the laser needs. There are also gas mix stations available that will mix the gas into the appropriate proportions. A typical mix would be 82% He, 13.5% N₂, 4.5% CO₂.

The wave length of laser light produced between Nd:YAG and CO₂ is used for specific material types and applications. CO₂ (typical 10.6 m wave length) is mainly used for cutting applications in metal, plastic, wood and with certain controls is used to engrave non-metallic materials at high speed. CO₂ Lasers have working powers up to 10,000 watts and beyond.

Lasercutting Table Types

Cutting Head

The cutting head consists of a nozzle, lens, gas assist and may include Z axis control. The Lens is an important feature associated with cutting. The characteristics are usually meniscus shape and made of Zn Se (Zinc Selenium) with a focal length (F.L.) of 5”. As the beam passes through the lens a hot spot is focused, (focal point, F. P.,in this case at 5”) from the lens and is assisted with clean dry air (or nitrogen for its cooling non oxidant properties) to carry the intensity through the cutting nozzle orifice vaporizing the material at the point of contact. A typical width of cut (known as Kerf Width) would be around 0.013”. The position of the F.P. is important with relationship to the material thickness and type. A normal start point would be to set the F.P. to just sit on top of the material surface. This is where a “Z” axis control is very desirable. This is a motorized axis that allows the cutting head to move up or down with control from a potentiometer, software or a surface follower. It also allows the cutting head to fully retract on job completion for ease of loading and unloading. “Z” Axis control is important to ensure that the cutting nozzle and lens are adjustable and allow for easy beam alignment. A typical cutting nozzle would have a 0.060 orifice. The focused laser beam, when passed through the cutting lens has to be central and perpendicular to the lens, and in turn central to the nozzle orifice. Any deviation from this would drastically affect the quality of cut. If the beam clips the nozzle orifice, by even 0.001”, a poor quality material surface cut would be produced that would also be out of perpendicularity.

Fixed Beam

The laser power source remains static and is mounted onto a solid support stand. The beam delivery and nozzle are held in a fixed position over the work area. The Laser table moves in X and Y directions to replicate the programmed design. The laser beam has a fixed beam length ensuring uniform quality cuts across the working area. This process is excellent for manufacturing die boards and provides excellent kerf. However, the large foot print takes up floor space. In some instances floor space may be at a premium. For instance, a system having a work area of 50”x 60” would require in excess 130” x 150”, more than double the work area.

HyBrid Laser System

The Hybrid is similar to the Fixed Beam but has one moving axis and one flying optic. With flying optic, the laser source is usually positioned to the side or rear of the table. The beam is guided from the power source through a series of elbows, housing mirrors and steered to the cutting lens position. The farther the cutting lens distance from the power source, the greater the beam diameter. Divergence relates to the laser beam increasing in diameter as the distance increases between Laser source and cutting lens. With a flying optic this becomes critical as one mirror (axis) is continually moving back and forth in order to cut the part. The diameter of the beam continually increases and decreases as it passes through the cutting lens which in turn affects the quality of the Kerf width. Also any slight misalignment of the beam as it passes through the 90ฐ elbows, or mechanical looseness in the system will cause the beam to change position from the center of the cutting lens and in turn move the F. P. position, again degrading the quality of cut. A normal control employed with a flying optic is to move the F.P. position with the Z axis for compensation. The amount of displacement would depend on the divergence and laser power. The flying optic portion of the system should work under pressurized conditions to prevent dust particles settling on to the 90ฐ mirrors and cutting lens. The Hybrid has a small foot print as the work table only moves in one axis. However, if the beam requires regular alignment to maintain a consistent Kerf, it is usually difficult for the novice.

Flying Optics

Flying Optics has the same criteria as the Hybrid, but both X and Y have optics that move and the cutting area is static. It has a very small foot print, almost equal to the work area, but is not suitable for Die making, However it is excellent for plastics and metal cutting.

Articulated Arm

This is the ultimate setup for quality control issues because the beam length is always consistent. The bed is static and renders a footprint equal to the flying optics system. As the beam exits the laser power source it is guided into a closed pressurized hollow arm that has flexible joints. The arm is allowed to swivel at each joint; each joint has a beam bender mirror that reflects the laser beam from joint to joint reaching the cutting lens with a constant beam diameter. This method of control is ideal for all applications including plastic, die board and metal cutting.

Galvo Controlled Marking And Cutting Systems

The Galvanometer is an instrument for measuring a small electrical current or a function of the current by deflection of a moving coil. The deflection is a mechanical rotation derived from forces resulting from the current. This rapid deflection is used to direct the laser beam at high speed across a material surface to engrave and or cut material. The Galvo head consists of 2 mirrors that are attached to coils. An electrical current excites the coils and causes them to orient in the magnetic field generated by high power magnets. This rotation is controlled from the programmed image via software. The laser beam passes through a series of lenses, (dynamic focusing system) is bounced between mirrors by the Galvanometric action and then projected down onto the work surface. A typical engraving speed of 7 ft per second is attainable with working areas of 64” x 64”. This principle is used with CO₂ and Nd:YAG laser sources. CO₂ is primarily considered for engraving/cutting non-metal material.

Cutting Speeds

Material thickness and cutting speed has a direct relationship to power. Increased power allows for cutting thicker materials and for faster speeds. A typical cutting speed chart is shown for Acrylic cutting (Figure 8). It should be noted this relates to 250 and 500 watt power sources having a polished finish and non-polished finish speeds. Powers can be well in excess of 500 watts if required. For 1/8” polished finish at 250 watt, and 1/8” polished finish at 500 watt, the cutting speed is the same. This demonstrates that the process with polished finish is speed-critical. For “finished polish” the speed is always slower than “non-polished” finish. However, with correct power and speed the acrylic surface temperature melts to attain a highly polished edge. Critical to this process is the need for a quality beam mode and a well tuned laser. This ensures that the edges remain cool enough and not to fuse back together. This helps in product removal and, in the case of adhered polished parts, reduces the risk of crazing. The advantages of using 500 watt and up is increased cutting speed and ability to cut and polish 1” thick acrylic. Another advantage is to have the ability to add a second head. In the example for 1/8” acrylic, double the production throughput by splitting 500 watts into two 250 watts per head. (Figure 9) Gas consumption and running cost remains unchanged with double the throughput. This is a low cost solution for increased productivity.

To Laser Cut Or Not Laser Cut

Lasers offer many advantages when compared to conventional cutting techniques; a quality edge finish is achievable in a single pass, limited waste due to a thin kerf and increased productivity due to eliminated steps in fabrication.

Most materials can be laser cut, but it is always recommended to obtain Material Safety Data Sheets (MSDS) to ensure that the material for process does not contain harmful chemicals that may be emitted when laser processed (as any other thermal process). The general guide is that PVC-type materials should not be laser processed because of the harmful emission of Hydrochloric Acid, the poor cut quality, having darkened to black edges, possible damage to the machine and optics and it is environmentally unsafe. Types of material that do not cut well are thermosets and high temperature materials. Epoxies, Phenolic resins and most natural rubber products are included in this category.

Materials that cut well (thermoplastics) with laser processing are: Acrylics, Polystyrene, Polyethylene, Polyamide (nylon) and Polypropylene. With correct power range, speed and good quality mode (profile of the beam), materials can be processed in excess of 1” thick with clean polished edges.

Cutting Speeds for Acrylic Type Material

	Nominal Laser Power		Nominal Laser Power
	250W		250W
	Polished Finish		Non-Polished Finish
Thickness	Cutting Speed		Cutting Speed
1/8	80”/min		160”/min
3/16	40”/min		120”/min
3/8	20”/min		60”/min
9/16	15”/min		31”/min
3/4	120”/min		24”/min
	Nominal Laser Power		Nominal Laser Power
	250W		250W
	Polished Finish		Non-Polished Finish
Thickness	Cutting Speed		Cutting Speed
1/8	80”/min		315”/min
3/16	60”/min		157”/min
3/8	40”/min		118”/min
9/16	31”/min		80”/min
3/4	24”/min		47”/min
1”	16”/min		31”/min

Figure 9

Written by Terry Grainger, Lasercut, Inc., manufacturers of systems for laser cutting and high speed galvanometric marking for the plastics industry.

For more information, contact Lasercut, Inc., 69 North Branford Road, Branford, CT 06405, 203-488-0031, Fax: 203-483-0463, E-mail: lasercut@lasercutinc.com, Web: www.lasercutinc.com.