| UV curing technology is now being used in many different types of Industrial
applications and the maintenance of this new equipment is some-thing that must be
established to ensure optimum running. Preventative maintenance Is something that must be
considered. Perhaps more Important than the repair of broken equipment is the work of
keeping this equipment in proper operating condition. The effort to create a preventative
maintenance program is one which evaluates the entire manufacturing process In which the
UV curing equipment Is being used. This evaluation is necessary as the use of UV curing
equipment Is unique to the process in which It is being used, just as is the type of
coating or ink used for each particular product. This approach is also necessary because
UV technology is often new to the employees who are expected to run the equipment as well
as those who are to maintain it. Without specific knowledge of preventative servicing of
UV equipment, the result is lost production time due to equipment that is inoperative.
The best way to avoid learning by one's mistakes and
losing valuable production is to establish a program to periodically check the features of
the equipment which, if not maintained, would result in the shutdown of a production line.
Most of today's UV curing equipment has been designed with "safety Interlock"
systems, each of which is designed to atop the entire curing unit as soon as a specific
problem develops. Depending on the specific design, the machine may or may not tell you
why it shut itself down, and you may still be faced with a list of variables which will
have to be checked to reveal the problem. in any case, the ability to service the machine
quickly and efficiently is important. More important than the ability to get the machine
running again is to maintain the equipment so that it does not shut down in the first
place.
The areas of UV curing equipment maintenance attention include:
1. Bulbs
2. Reflectors
3. Ballasts, condensors and other electrical components
4. Conveyor belts and drive assemblies
5. Shutter assemblies
6. Cooling systems
a. Air filters
b. Air blowers
c. Water hoses
d. Water jackets
e. Water pumps, etc.
These items will not apply to all equipment and processes 55 each
has its own unique features. The frequency of use of the equipment will also determine the
type of maintenance program established. A machine that is running part of one shift will
naturally require different attention than one running 24-hours a day or three shifts.
Bulb maintenance must consider several variables. These variables
will determine how frequently the lamps should be changed or inspected. The production
usage or time on will have an effect on the bulb life. Most UV curing units have meters
for each bulb which measure the length of time which has accumulated. This time does not
reflect high power versus low power but only the time which current has passed through the
lamp. This means a maintenance log must be kept to ensure proper information about the use
and performance of a particular lamp. Bulb suppliers usually have a pro-rated guarantee
which is valuable if the lamp goes soon after purchase. Proper record keeping will verify
the fact that a new bulb was defective. The fact that bulbs are pro-rated is of little
value after the lamp has lasted close to the end of the guarantee period. But the lamp
meter and the guarantee do not consider several important points. The number of times the
lamp is turned on will have a bearing on the bulb life. This information is difficult to
maintain but the understanding that bulb life is associated with starts will help define
the operating procedures as well as maintenance programs.
Bulb maintenance must also consider the fact that
the spectral output may change over a p5riod of time. This means it is possible for the
lamp to be on without producing the proper energy to effectively cure 5 reactive ink or
coating. The change in spectral output is a variable that must be controlled, since the
effect is to produce products with uncured or partially cured inks or coatings. This may
exhibit itself in an obvious way where you can see the difference and you can quickly
associate the incomplete cure to the spectral range of the ink or coating with the actual
spectral out-put of the lamps being used. This may not be as easy as it sounds since
spectral output may not be the first place that you think to look. This problem is more
important when the spectral change produces only a slight change in the cure. A small
change may only effect the coating's physical properties, such as adhesion, intercoat
adhesion, or long-term effects associated with outdoor weatherability or embrittling These
slight changes could produce enormous problems that don't make themselves known until
after the end product is in service. A preventative maintenance program will catch changes
in spectral output before they become a problem.
A proper maintenance program can avoid these
problems by identifying where the problem exists before a production run. Bulb maintenance
must go beyond "is it on?" The periodic use of a UV intensify analyzer will
establish the level of energy the bulbs should produce. This can then be used to check and
monitor the established information. There are several types of UV intensify analyzers
available, each of which will produce information which can be interpreted for this
evaluation. The frequency of this test would depend on each shop's requirements. At the
start of a maintenance program, very frequent tests will allow the operator to document
the performance of a unit, as well as the output of each of the lamps. It would also be
practical to test the spectral output periodically throughout a production to ensure
consistency. It is possible to equip a UV unit with intensity sensors which will indicate
the potential of a problem automatically. There are UV unit manufacturers who offer this
as a part of their safety interlock systems. It is important that the user recognizes this
factor and addresses his maintenance program accordingly.
There are other problems associated with the bulb
which will have an effect on the processing but are not as frequent or as critical a worry
as the ones already discussed. During the installation and the handling of the bulb or
cleaning of the reflectors, it is possible to get foreign matter on the bulb. This will
have an effect on the transmission of light. A fingerprint will etch itself right into the
quartz, presenting two problems. The etched area will transmit light as a clear section of
the bulb. This may not appear to be significant on a large lamp, but a one-inch
fingerprint is a big percentage of a 12-inch lamp. The reduction of curing efficiency is
equal in both lamps since exposure is made perpendicular to the lamp. In addition to a
reduction of light transmission, the area etched will weaken the quartz itself and
increase the possibility of breakage. This is especially of concern for lamps which
operate at high pressures, such as capillary lamps. This breakage may occur during the
machine's operation or even while handling.
The collection of dirt on the lamp from other
sources is possible and it is recommended that bulbs be cleaned before installation to
ensure there is nothing on the quartz. A clean rag and pure cleaning solution must be
used. If the rag is not clean or an improper cleaning solution is used, a thin film of
foreign matter may be applied to the bulb. When the lamp is later used, the heat generated
by the arc will slowly darken the thin film on the surface. This darkening will, of
course, reduce the amount of UV energy that will be transmitted and effect the cure of the
ink or coating. This problem would be detected by a UV intensity analyzer, as well as by
periodic inspections.
Very large UV bulbs will sometimes sag as a result
of the heat generated by the lamp during operation. The extra large size of the bulb will
cause the lamp to be out of focus as the distance relationship of the lamp to the
reflectors will have changed This will show up only in the middle of the exposure area and
should be checked with a UV intensity analyzer
A good positive maintenance program will includes
replacement bulb plan. Since you will never know a bulb will burn out or exhibit standard
levels of UV energy. It is necessary to keep an inventory of replacement bulbs. The best
system will maintain one bulb for each bulb in service. This will protect against any
problem that may arise, as well as have on hand the bulbs required for a scheduled change.
Maintaining one bulb in reserve for every one in may be too costly and a small number of
bulbs may be desired. The minimum number of bulbs would then be determined by several
variables. The type of bulb or its popularity will indicate whether there are a variety of
replacement bulbs which can be used while a very specific bulb may be available only from
one source or may have to be made to order. A general rule that will provide good support
protection and minimum inventory is one bulb for each bulb in service on the most active
machine and 50% of the bulb requirements for the set of lamps in the next three machines.
This quantity of bulbs in inventory is substantially cheaper than the dollars lost per
hour in a down production line, not to mention the cost of long distance phone calls to
track down a replacement and airfreight expenses to fly a bulb to your location.
When changing bulbs, the connector terminals should
be inspected and replaced on a regular basis. High volt age will have effects on terminals
over a period of time. Each equipment manufacturer has his own design for bulb connections
and some will require more frequent attention than others. Connector terminals should be
inspected often and replaced before they show signs of deterioration.
The regular maintenance inspections should also
include reflector evaluation. How clean are the reflectors? Dirty reflectors will cut down
cure efficiency in a very dramatic way. With some ink or coatings a 10% reduction of UV
energy will result in a much greater reduction of cure speeds. Depending on the cooling
design and airflow within the particular machine, varying amounts of foreign materials may
collect on the reflectors. These materials may result from volatiles releasing from the
substrate, from the ink or coating, or even be the ink or coating itself (depending on the
individual formulation).
A substrate which is being printed or coated may
react with the heat generated by the UV lamps so that it releases small amounts of
material from within itself. The result may be a very minute release that eventually
deposits itself on the reflectors. The smallest amount of this deposit should be of
concern, yet it would never be noticed except over a long period of time. To guard against
this accumulation, cleaning of the reflectors should be done on a regular basis with a
soft, lintless wipe, wetted with pure acetone. It is important to leave nothing on the
lamps or reflectors which may darken with heat and reduce the transmitted energy. The
reflectors themselves need only be changed when they are damaged or soiled beyond
cleaning. This should be done when the slightest amount of interference is noticed. Use of
a UV intensity analyzer will also indicate when a reflector is in need of cleaning or
replacement.
When the UV intensity analyzer indicates a problem,
it can be in the bulb or in the reflectors. Checking the reflectors can be done by a
simple visual inspection, but preventive maintenance would render even this inspection
unnecessary. This is desirable because performing the inspection would require them to
cool down for several hours before it could be inspected and restarted. This could mean
more than an hour of production time lost for a very minor problem.
Ballasts, condensors and other electrical components
will possibly malfunction at any time. If is difficult to prepare for such circumstances.
The availability of replacement parts should be researched and established if a
replacement inventory is not maintained.
Other maintenance procedures would be established
depending on specific types of equipment. Some equipment is air cooled and requires air
filters which need to be replaced. Others are water cooled and are usually tied into a
safety interlock system so that if the wafer flow is below a specific level, the entire
machine will shut down. If a heat exchanger is used to conserve water, there would be
several maintenance items within the system that, if not followed, could shutdown an
entire line. These other support areas should not be forgotten or treated as any less
important than any other maintenance procedure. Water jacketed bulbs require some special
attention as the cleanliness of water jackets is important as well as the clarity of the
water. This type of cooling system requires some additional attention which can also be
indicated by a UV intensify analyzer. (Note that the analyzer will only indicate reduction
of energy and will not pinpoint the exact cause.) Within the heat exchanger of this type
system there would be water deionizer which would have to be maintained. If not, materials
in the water would collect on the interior of the quartz water jackets.
One must also be aware that foreign materials can
collect on the outside of the jacket. If an ink or coating contains volatiles, when it
passes under the exposing area the volatiles will collect on the cooler surfaces of the
water jackets. This material will build up over time and reduce the energy output.
Some equipment may have shutters whose mechanisms
should be checked as often as possible. These devices fend to operate many times per shift
and are mechanical. Replacement for each of the wearing parts should be on hand stall
limes.
Conveyorized Systems require similar maintenance
attention as in conventional dryers. The belt must track properly, must be set at the
right tension, and be checked for tears, rips and frays. Repairs should be made with
materials which will not be affected by UV light or by continuous exposure to ozone. This
limits what can be used. Therefore, it is recommended that a belt be replaced rather than
repaired when problems arise.
Periodically, all electrical connections, wires and
water hoses should be checked. UV units tend to have extreme conditions of heat, UV light,
and ozone. Each of these conditions will deteriorate the integrity of the internal
components. Any replacements or repairs to these parts should be made with materials whose
specifications will withstand the UV unit environment.
In summary, the essential component of a successful
maintenance program is a preventative maintenance routine that is adhered to faithfully
and includes attention to all of the variables. In addition, care should be taken to
maintain an adequate inventory of bulbs and other parts that are not readily available. |
