THE MAINTENANCE OF UV CURING EQUIPMENT

REPRINTS FROM "RADIATION CURING" VOL. VII/NO. 1 FEBRUARY 1980 PUBLISH ED BY TECHNOLOGY MARKETING CORPORATION

17 PAR K STREET, NORWALK, CONNECTICUT 06851, U.S.A.

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.