How Materials React to a Laser

There are generally three ways that a material can react to a laser: Engrave, mark or cut.

Laser engraving is defined when material ablation (removal) occurs. In this case the laser engraving machine will vaporise some of the material to create some depth.

Some materials can only be laser engraved, such as wood for example. In other words, the material cannot be exposed to the laser without some ablation occurring, however, in this example ablation is desirable because the perceived quality and value of laser engraved wood is further enhanced when there is a significant degree of ablation.

Some materials work well with minimal ablation but react adversely when over-exposed to the laser. Cast acrylic is a good example in this case where some ablation by a 10.6µm laser creates a beautifully lustrous effect but when over-exposed the engraving can become very powdery and tainted by an adverse reaction to an excessive degree of thermal stress.

Laser marking constitutes the majority of laser applications and is where the material reacts without ablation to show a contrast between the area exposed to the laser and the area that has not been exposed to the laser. Laser marking is usually some form of colour change but can also include a reaction called ‘foaming’. Foaming is a material specific phenomenon that occurs when the laser marking machine heats the material to release gasses that raise and then solidify above the surface of the material.

Polycarbonate is a good example of a plastic that if exposed to a 1µm laser will colour change and foam with a highly contrasting and desirable effect.

Laser marking without ablation can provide for an incredibly detailed result, at super-high speeds and with zero to minimal adverse reactions from the base material.

Some materials can be laser marked or laser engraved according to the system configuration used and the laser parameters set. For example, a CO2 laser at the 10.6µm wavelength can only mark the surface of anodised aluminium. Increasing the intensity of the exposure can cause removal of the anodised surface but this is not very noticeable. Using a solid state laser, like a fiber laser at the 1µm wavelength, the same material can be surface marked or ablated to a considerable depth.

Laser cutting occurs when the laser vaporises the entire thickness of the material to create a void from upper to lower surface.

Solid, flat sheet materials are used for the vast majority of laser cutting where highly accurate and detailed cuts can be made by the laser that are impossible to produce by other forms of cutting technology.

For example, laser cut acrylic provides for an instantly polished edge and some textiles, such as silk for example, are heat-sealed by laser cutter where they may otherwise fray.

For some applications a finely controlled cut takes place by the laser. For example, ‘kiss-cutting’ of adhesive labels where the adhesive surface is laser cut but the backing material is not and also, the laser scoring of some materials to form a line that can form a break point, for example for snapping-off labels out of a sheet format.

At Lotus Laser Systems we manufacture a wide range laser marking and engraving solutions ideally configured for laser cutting, laser marking and laser engraving all types of materials. Our experts would be happy to recommend which configuration best suits your application.

Cooling of a DC Glass CO2 Laser

DC excited (glass) CO2 lasers at the 10.6µm wavelength are rapidly growing in popularity and fast overtaking the number of RF (metal) lasers sold, especially in the more price conscious markets such as laser cutting machines for schools and laser engravers for hobbyists.

Compared to an RF alternative, DC lasers have limited functionality, lower performance and a much shorter working life but they do offer the possibility of owning a laser cutter to some that would otherwise never be able to afford a laser cutting machine with an RF laser source.

All lasers require cooling and these days modern RF lasers are all cooled by air. DC lasers, however, are a less efficient technology and generate more heat as a by-product of the process so all DC laser need to be water-cooled.

Cooling the laser effectively and efficiently is a critical process. Failure to do so will cause massive fluctuations in the performance and reliability of the laser cutter, significantly shorten the working life of the laser itself and can in some cases lead to a premature, catastrophic laser failure.

Some very low cost machines ship with no more than what is an aquarium pump for the cooling device. Such devices are wholly inadequate as they do no more than to recirculate increasingly hotter water around the laser tube, some with variable rates of flow and pressure too.

For the laser to be cooled efficiently and effectively the coolant (water) must pass through a device specifically designed to control its temperature. In a laser cutting machine this device is called a chiller, although this can be a bit of a misleading description because the chiller will only actually reduce the coolant temperature after it reaches a set-point. It is therefore an ‘on-demand’ device, continually monitoring and keeping the coolant temperature constant.

Most chillers will use deionised water for the coolant, which helps to keep both the coolant and the internal workings of the laser clean. No matter what the coolant type the chiller must be regularly monitored and maintained to ensure that it is performing correctly.

Periodically, the chiller should be drained, the internal workings of the laser flushed and the chiller replaced with new coolant. Care should be taken to ensure that any air filters/vents on the chiller are also regularly cleaned/replaced. DC lasers are a consumable part. When replacing the laser the user should never use a chiller containing old coolant

The chiller should not be placed above the height of the laser. Ideally, it should be placed on a stable surface approximately 500mm from the floor and away from all other electrical devices. This minimises the likelihood of contamination of the cooling veins.

Pay careful attention not to allow the workplace to fall below 0c as this will cause the coolant to freeze and the chiller and the laser to become damaged.

Most DC lasers <100w work best when operated at a set temperature of 21c. If the laser cutting machine is within an environment that has a high temperature and/or relative humidity the operating temperature of the chiller should be set with 10c of the dew point, otherwise, condensation may form on the internal workings of the laser and the chiller causing a short. At maximum the chiller should not operate above 24c.

Maintaining an effective cooling device for your laser can save you significant time/money and prolong the life of your laser cutter.

At Lotus Laser Systems we manufacture a wide range laser marking and engraving solutions ideally configured for laser cutting and laser engraving all types of materials. Our experts would be happy to recommend which configuration best suits your application.

Laser Cutting Wood

Insofar as laser cutting is concerned, wood can be separated into two distinct groups:

1. Natural timber from various species of tree. For example, maple or cherry wood.
2. Man-made wood manufactured by bonding wood pulp, fibers or shavings. For example, MDF or plywood

All natural timbers can be cut by a CO2 laser at the 10.6µm wavelength. Limitations only apply with regard to thickness and the degree of charring that is acceptable to the customer.

Special versions of laser cutting machine are made for specific wood cutting applications such as, for example, laser cutting plywood for label dies. However, for the purpose of this article we are referring to a typical benchtop format laser cutter <100w.

For most industries laser cutting anything beyond 12mm natural timber and 9mm plywood produces an undesirable result because the feed rate (speed of cut) combined with optical limitations causes the wood to adversely burn.

When laser cutting wood the rate of laser fume extraction, particularly underside extraction and air assist delivered through the nosecone, can never be too much. When vaporised, wood produces a very dense fume that if not immediately evacuated from the cutting line will rise into the path of the beam and absorb laser power, significantly dropping the feed rate of the cut and in turn accelerating the degree of charring at the edge of the cut.

High amounts of debris from laser cutting wood can also contaminate laser optics causing them to fail prematurely and also cause HAZ (Heat Affected Zone), which is an undesirable staining to the edge and surfaces of the material.

In machines where the design of extraction is poor, surface HAZ can be minimised by first applying a low-tack, paper based tape to both surfaces of the wood. The tape acts as a masking barrier, attracting the contamination. After laser cutting the paper is removed and the wood surface underneath is clean.

Natural timber has a tendency to warp, which is challenging for laser cutting as this causes the beam to defocus, producing undesirable results. Therefore, if the application calls for laser cutting of natural timber it is best to process the material as soon as possible after receiving it from the mill.

Man-made or engineered woods can contain resins that are actually designed to be fire retardant. This is unhelpful when trying to burn through the wood with a laser cutter! Therefore, if the application calls for using a form of engineered wood it’s wise to be very selective about the exact type of material you use. For example, plywood is most commonly available in a WBP or exterior grade. The resins used to bond this type of plywood are among the least laser friendly and will resist thelaser cutting process, accelerating HAZ.

Interior grade plywood, sometimes called white glue plywood, is among the most laser friendly plywood’s. Not as commonly available as exterior grade, interior grade should be used as a preference for laser cutting if the job specification allows it as the resins in the material will vaporise much cleaner.

An often overlooked aspect is that of the surface finish of wood prior to laser cutting. If the part to be laser cut is the finished item then the surface should always be sealed/finished before it is laser cut. This reduces HAZ and any HAZ present can be wiped off with a slightly damp cloth. If however the material is to be post processed, such as sanding for example, then pre-sealing the wood is not absolutely necessary as the HAZ will be removed during post processing.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions ideally configured for laser cutting and laser engraving wood. Our experts would be happy to recommend which configuration best suits your application.

Cooling for a Low Power CO2 Laser

CO2 lasers generate heat as a by-product of the beam generation process. If this heat is not removed effectively it can cause the laser to overheat and at best perform unreliably and shorten the working life of the laser or in a worst scenario the laser can fail catastrophically.

An effective and efficient cooling mechanism is therefore a vital element of the laser cutting or laser engraving machine design and function.

For low power (<100w) laser cutters there are in simple term two types of laser source:

1)DC excited; generally made from glass

2) RF excited; generally made from metal with some now being made from a ceramic.

DC lasers are significantly less efficient than RF lasers and as a consequence they generate a higher heat load so they must be water-cooled. Most modern RF lasers are so efficient that air-cooling is perfectly adequate up to 100w.

Air-cooling is performed by positioning a number of fans around the laser cavity and by moving ambient temperature air over the surface of the laser the heat is removed. Air-cooling is a much tidier, lower cost method of cooling a laser, however, it can be somewhat noisy, especially for higher power lasers where the air flow must be increased to remain effective.

Water-cooling RF lasers is a more reliable and efficient cooling method, however, it adds significantly to the cost of the laser cutting machine so it’s not common to find RF lasers being water-cooled that are <100w. Higher power RF lasers generate significantly higher heat loads and so must be water-cooled.

All DC lasers require water-cooling. Some very low cost machines incorporate what is no more than an aquarium pump to recirculate a reservoir (a bucket) of water. This is a wholly inadequate and somewhat unsafe design for cooling a laser as the water temperature will consistently rise to a point where the coolant (water) is too hot to effectively remove heat from the laser. Also, it is wholly unsafe to have open containers of water close to any electrical device.

Professional, well designed laser cutting machines incorporating a DC laser will use a specifically configured recirculating water chiller. Here the coolant is pumped around the laser to pick-up the heat then returned through a cooling mechanism, which lowers the coolant to a set temperature. The function is similar to that of a radiator in a car. The coolant temperature is monitored by the unit, which self-adjusts to increase/decrease the rate of cooling and maintain the set-point.

It is important to note that devices like this require periodic maintenance, such as frequent replacement of the coolant for example. If the coolant is not changed frequently it can contaminate and in turn cause contamination of the laser itself resulting in a catastrophic failure.

Well designed water-cooled laser cutters will also incorporate a flow sensor so that the laser cannot be accidentally operated when the cooling unit is not powered-on or should the cooling unit fail.

I cannot emphasis enough just how important it is to effectively cool the laser source as significant variance in thermal load to the laser is the single biggest cause of premature laser failures.

At Lotus Laser Systems we manufacture a wide range laser, marking and engraving solutions and we have installed machines all over the World to the widest range of environments. Our experts would be happy to advise you on this or any other aspect of your laser system installation.