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 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.
|
