Dealing with LGACs
Laser Generated Airborne Contaminants (LGACs) is a term used for dust and smoke, which is generated when a material is processed with a laser. Jon Young - Marketing Manager for Purex International, manufacturers of purification equipment explains why and how LGACS should be dealt withÖ
Laser Generated Airborne Contaminants (LGACs) is a term used for dust and smoke, which is generated when a material is processed with a laser. Jon Young - Marketing Manager for Purex International, manufacturers of purification equipment explains why and how LGACS should be dealt with…
Lasers are increasingly used to mark, etch, cut and weld a wide variety of materials. They offer an alternative to traditional machining methods and coupled with the right fume extraction system, deliver excellent results in terms of precision, machining time, flexibility and performance. To comply with COSHH regulations, fume extraction is required for most laser operations in order to protect personnel and to enhance the performance of the laser system itself. It is therefore of paramount importance that the correct fume extraction system is specified.
What happens when laser processing?
When a laser beam is applied to the surface of a material, several conditions can occur:
1. High temperatures that are generated cause the air near to the contact point to expand, generally back in the direction of the lens.
2. The laser beam causes incineration, vaporisation, melting and softening of the material depending upon the distance from the contact point. Rapidly expanding gases pick up and carry the removable particles and droplets at relatively high velocities away from the product material. (fig.1)
3. The contaminants released consist of a wide variety of gases, in some cases noxious i.e. benzene, phosgene. They also consist of the products of complete and partial combustion, including possible oxides of the base material. Stainless steel, for example, releases Chromium and Nickel, which can cause chronic toxicological effects such as liver/kidney disease and cancer.
Production problems caused by LGACs
During laser processing, gases, vapours and particulate matter are released as airborne contaminants, which in most cases, can be classified as harmful if inhaled. Airborne particles also cause problems for the laser if they are allowed to be deposited on the lens, or remain in the processing area, causing beam attenuation and therefore reject rates to increase (fig.2)
Deposition of particulates within the laser enclosure can result in contamination of the product and also the need for regular cleaning of associated operating equipment. The only practical method of overcoming the above problems is to remove these airborne contaminants as quickly and completely as possible by the provision of a fume extraction and purification system.
The range of materials that are processed with lasers is wide and varied so any purification system must be flexible enough to cope with different volumes and types of contaminants.
Purification system design
Airflow rates of between 80 and 5000 m3/hr will ensure the correct level of extraction from most laser processes. The correct rate required depends on many interacting factors and should therefore only be specified by experienced professionals to prevent the provision of an inadequate extraction system. The position and design of the extraction nozzle or enclosure is also a prime consideration if effective extraction is to be achieved, oval or round nozzles should generally be used in preference to thin, slot varieties.
One of the major cost benefits of using lasers instead of conventional methods to process materials is that there are few or no consumables such as drills, milling cutters, saws, etc. Therefore it is important that any purification system does not nullify this benefit by incurring excessive filter costs upon the user.
One method of prolonging the life of a main filter is to use a pre-filter to remove larger particles (=1µm) from the air stream before they enter the main filter. These are supplied in different forms, ranging from pads to bags to a patented concertina design that, as filter life is proportional to the area of media employed, offers around 10 times the life of a normal pre-filter. Pre-filters are made from a variety of filter media and it is vitally important that the correct type is specified by the supplier, otherwise the life of the main filter will be significantly reduced. This is especially true if the material that is processed releases oily or sticky particles.
Many purification systems work on the outdated principle of "top down" filtration, which has been proven not to offer the best filter life and may cause filters to split. A filter for laser processes generally contains HEPA (High Efficiency Particle Arrestor) media to filter out harmful particles (99.997% at =0.3µm and 95% at =0.01µm) plus a chemical layer to filter gases. In low pressure, "top down" purification systems, the contaminated air enters the machine at the top and takes the least line of resistance, at a high velocity through the filter (fig.3). This means that only part of the HEPA and chemical filter media is used and that the air is allowed to pass through the media at much more than the most effective dwell time. Both of these factors reduce filter life and therefore increase consumable costs for the user. Particles can also collect in the pleats of the HEPA media due to gravity and can cause it to split (especially when the particles are moist), releasing hazardous material into the workplace, without the operator's knowledge.
The solution to the aforementioned problems is found in the "Reverse Airflow Principle". This causes the contaminated air to slow and turn through 90 degrees when entering the purification machine. This action causes larger particles to drop out of the air-stream, thus preventing premature filter blockage (fig.4) and particles can no longer collect in and cause the pleats of HEPA media to split because the air is travelling upwards. Air equalisation plates containing holes with a cross sectional area which, correspond mathematically with that of the pump air inlet, should also be used. These, in conjunction with the reverse airflow system, cause the air to slow to the speed at which the filter media is most effective and ensure that the full area of filter media is used. They also increase the rigidity and security of the filters themselves. Some materials release corrosive vapour when processed with a laser, for example, PVC produces Hydrochloric Acid vapour, which coalesces inside the purification machine. In "top down" systems, the liquid HCl can travel through the filters and collect on the electrical wiring and the motor below, causing corrosion and a possible fire risk. Reverse airflow systems on the other hand, allow the acid to drip onto an absorbent pad where it can be safely dealt with (fig.4)
Another method of greatly increasing filter life is to utilise high-pressure pumps that are able to overcome the resistance in a filter as it becomes blocked, for far longer than a low-pressure system is able. The high pressures that are generated necessitate the use of reinforcing that should be employed between the pleats of HEPA media to prevent them from splitting.
Health and Safety
A pressure monitor should be employed to alert the user when a filter is nearly full, this allows them to schedule the purchase of filters, ensuring that extraction from the process is continuous. However, the pressure sensor does not alert the user if:
1. A filter is ruptured
2. There is no filter fitted
3. A seal is broken
4. A filter is damaged
5. The chemical layer that filters out noxious gases is exhausted
Although the use of reinforcing guarantees that filters do not split and a pressure sensor indicates when a filter is full, it is vitally important that the exhaust of any purification system that re-circulates air to the workplace is constantly monitored. It is very concerning that many commercially available laser fume purification systems rely on the operators nose to tell if a filter is blocked or there is a problem with the system. Automatic gas sensors should be used to identify if a chemical filter is exhausted and particle sensors should be employed to ensure that the operator is alerted, if for example a filter has not been fitted or it is damaged and is allowing hazardous particles to pass through it. If these monitoring systems are not in place, conformity with the COSHH regulations will be prejudiced and the users are at risk of potentially developing serious medical complaints through exposure to harmful substances. Such purification systems should also be annually certified by the installer and be well maintained to ensure compliance with the COSHH regulations.
Helping the environment
Perhaps the first thought of many is to vent contaminated air from a laser process out to atmosphere. This method of extraction is not environmentally friendly and there are stringent regulations which govern what and how much can be released into the atmosphere. These regulations can carry large financial penalties should the limits be exceeded.
Apart from regulations, it costs money to either heat or cool a factory. Therefore the venting of air that has in effect been paid for, into the atmosphere is not economical.
External venting does not usually offer precise control of airflow rates. Two common problems with this method are that if the airflow rate is two high, then small components can be sucked into the pipe-work and if the airflow rate is too low, then particles and fume will escape into the workplace.
In a modern production environment, it is necessary to have flexible production lines that can be moved as the factory grows or the production emphasis is altered. Vent to atmosphere extraction requires extensive, fixed pipe-work that cannot easily be moved. If moved, this pipe-work can harbour contaminants that must be cleaned up and made safe. Also, planning permission is often required to cut holes in the factory roof or walls and these holes must be sealed again if the pipe-work moves! This lengthy, expensive process interrupts valuable production time.
A re-circulating system avoids any regulations that govern external emissions. Purified air is returned to the workplace in virtually the same conditioned state/temperature, as when the air entered the machine, therefore air conditioning costs do not rise. Vacuum levels can be set and altered easily for each individual process and most systems can be easily moved, should the process move. Altogether a re-circulating system is much more cost effective, flexible and environmentally friendly than an external system.
Closed loop flow control
A good purification system should employ a closed loop flow control system to automatically adjust motor speed to provide several desirable outcomes. When a new filter is fitted into a purification machine there is little resistance to the air flowing through it, therefore a lower motor speed is required. As a filter becomes blocked, a higher speed is required to pull adequate air through the filter until the filter becomes "blinded" or full.
A fixed motor speed system fails on several counts because the motor will always run at 100%, so the extraction rate cannot be adjusted to the optimum level. If the motor speed is fixed then the extraction would either:
1. Be correct when the filter is new, but as the filter becomes blocked it would fall uncontrollably.
2. Or be far to high while the filter is new (which leads to reduced filter life), then slow as the filter becomes full.
A closed loop flow control system automatically adjusts the motor speed as a filter becomes full, so that the extraction rate remains constant with a variance of less than 1% throughout the life of the filter. The climate change levy introduced recently has seen energy costs rise for manufacturers, so another benefit of flow control is that energy usage is kept to a minimum, which is a most welcome feature.