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UV Safety
Shielding is absolutely mandatory. Medium pressure UV lamps radiate harmful UV radiation that can cause serious burns to skin and eyes. While thermal burns are felt immediately, UV burns are not felt for several hours. Short exposure to lamp radiations can cause severe burning of skin and eyes. UV burn of the eyes affects the cornea and takes several days to heal. UV burn is identical to ‘Welder’s Burn’ and will feel like sand in the eyes that cannot be washed out. The discomfort is transitory. Extreme caution must be taken – high power UV radiation can cause blindness. Exposure to UV radiations, of only limited time, will evoke erythema (sunburn) on normal skin. Such erythema is transitory and will not produce blistering, nor tanning, as only a small amount of radiation penetrates the Malpighian layer. Extreme caution must be taken – high power UV radiation can cause severe burns to the skin. Shielding material can be of cloth, glass, plastic, wood or metal. As infrared energy is generated along with intense visible light, fireproof as well as opaque material that does not degenerate under UV radiation must be utilized. Direct light from the UV processor should not be visible to operator nor other personnel. Bounce (reflected) light should be minimized and avoided. Total shielding with openings minimized for product entrance and egress from the UV processor should be incorporated into processor design. Reflective surfaces coated with black UV absorbing paint reduce reflected UV radiations. Protective clothing and safety spectacles should be worn if optimum shielding cannot be attained.
Cooled heat skins should provide protection to the press, conveyor, and other process components in or near the UV processor. The cooling system should be carefully designed and properly maintained. In air-cooled systems, filters must be properly cleaned or replaced on a maintenance schedule related to powder, dust and dirt conditions where the UV processor is operating. Shielding design must allow for thermal expansion. Any exposed heated surfaces of the processor or related equipment should have guards to prevent contact by personnel. Time delay circuits are necessary to shut down lamps if the press stops with paper or other substrate under the UV processor: Low power switches are incorporated press systems so that energy is reduced automatically when the web is slowed or stopped. Regular inspection is required to ensure time limits and related circuits have not been altered or bypassed. Halon #1211 fire extinguishes are to be used in even to fire. Damage to press and lamps will not occur. CO2 fire extinguishers with dry chemical or water are NOT recommended. If a fire occurs, all residue of damaged substrate should be removed from curing area. Soot and ash must be cleaned from lamps and reflectors before re-start. Investigation to determine the malfunction causing the fire is most important. Correction must be made to eliminate re-occurrence. equipment in excess of ninety (90) inches arc length should have quartz supports incorporated into the irradiator design to relieve the strain on the l amps. The supports will also help prevent lamp bowing which can cause excessive heat to the substrate. High voltage and currents energize UV lamps in UV processors. Transformers and capacitors (stabilizers) designed to provide lamp starting voltages, and to limit current subsequently, provide a uniform output of energy. UV processor have stabilizers with starting voltages from 400V to 6000V, operating voltages from 50V to 2500V, and currents from 3.0 Amperes to 24.0 Amperes. Arc length of the lamp dictates the particular factors for its operational design. A short circuit in the stabilizer (secondary) lamp circuit will not reflect in the primary line circuit. No fuses will blow nor circuit breakers trip. Such a short circuit will continue to arc until an insulation space is burned in to the equipment at arc point. As the starting current of lamps equals the secondary short circuit current, no fusing of the secondary circuit can be made. High voltage/ high temperature wire is utilized in the UV processor circuits. Lamp wiring should be separately conduited – not with control nor power wiring. All wires must be kept from the lamp radiation area to negate infrared/UV degradation of wire insulation. Electrical interlocks should be provided on the processor and its control cabinets. Regular maintenance inspection should be made to ensure these are not defeated in use. Closed and locked junction boxes must be provided and should remain closed and locked when the UV processor is in operation. UV processor electrical systems should be serviced only by qualified electricians.
Ozone formation can be eliminated by using ozone-free quartz lamps. Certain dioxides are added to the quartz of these lamps which absorb the ozone producing wavelengths. Ozone free and pure fused quartz lamps are interchangeable. Ozone free lamp usage may affect cure speeds if ink or coating formulation is designed to utilize the absorbed wavelengths. A nitrogen atmosphere in a processor also eliminates ozone production by eliminating oxygen. Lower power lamps may be used in a nitrogen atmosphere however the cost of nitrogen will probably offset any operating cost savings attained. Ozone can be effectively eliminated in the processing area by exhausting air of the cooling system of the UV processor to outside the building. Such exhausting has no danger as the hot gas is very unstable and breaks down to oxygen rapidly in ducting. Neither a nitrogen atmosphere nor ozone-free lamps eliminate the cooling system required by lamps and related UV processor components.
Ink manufacturers should provide solutions to any ink misting problems. Misting ink can be drawn into the UV processor and deposited as dust on lamps. If UV lamps are contaminated, output will decrease, and overheating of equipment and electrical arcing at lamp fittings can occur. Slow curing speeds and higher operating costs will follow. Exterior contamination of lamp surface voids lamp warranty. Fused quartz (Silicon dioxide) with a high melting point and excellent UV transmissivity is used in fabrication of UV processor lamps. A 22 x 25 millimeter diameter tube will wall thickness of 1.0 to 1.5 nm is typically used with tungsten electrodes sealed into each end. equipment are manufactured from 1” to 180” arc length. Quartz is very fragile and special cushioned packaging is utilized for safe transportation. Upon receipt of lamp package, the carton should be examined for damaged incurred in shipping. Delivering carrier employee should sign off any apparent damages at time of receipt. The carton should be opened fully so that the lamp can be lifted out of packaging with no twisting or pulling. Unpacking should take place in an area large enough to remove the lamp safely. equipment must be wiped with alcohol before placing in service. Bare skin contact with the quartz envelope MUST be avoided. Compounds from the skin when heated on lamps operating at 600º to 850º C will form permanent etching (devitrification) on the quartz surface decreasing UV energy transmission. A contaminated lamp eventually will overheat, causing premature failure.
As the UV processor is an optical system, all types of dust, powder, grease, smoke and misting ink must be cleaned from lamp and reflectors especially if other operations are in the same area. Electrical fittings must also be cleaned to prevent arcing between fitting and lamp ends. Dirty reflectors will reduce cure rates and increase temperature. Approximately 50-60% of the lamp energy is returned by the reflectors. Overheating from a dirty condition can cause warping, possibly reducing electrical spacing, and cause a short of the arc to become possible. A mild detergent and distilled water mixed at a ratio of 1¼ ounces to 1 gallon makes a good cleaning solution. After cleaning rinse will clear distilled water and wipe or polish with a clean cloth.
Grease or ink on lamp or reflectors will require washing with a solvent, then cleaning with a detergent solution. Clean alcohol or ammonia and distilled water can also be used for cleaning. The use of steel wool, emergy paper, or abrasive powders is NOT recommended for cleaning lamps or reflectors. Technical
information was provided by the manual 'Superior Quartz Products,' |