Heat Management Methodology for Successful UV Processing on Heat Sensitive Substrates

Transcription

1 Heat Management Methdlgy fr Successful UV Prcessing n Heat Sensitive Substrates Juliet Midlik Prime UV Systems Abstract: Nw in 2005, UV systems pssess heat management cntrls that fine tune the exthermic and delivered heat in the UV curing prcess. This paper will explre the varius methdlgies and cntrls that allw cnverters t cure UV adhesives, UV inks, and UV catings when applied t heat sensitive substrates. UV Fundamentals: The UV curing prcess generates tw types f heat, exthermic heat and delivered heat. Exthermic heat is created by a chemical reactin such as the plymerizatin f the UV chemistry (Figure 1). Delivered heat is generated frm the UV lamp. This delivered heat must be mnitred and cntrlled t avid damaging any f the cmpnents that cmprise the UV prcessing unit, as well as t maintain substrate integrity. UV lamps are made f quartz glass and can cme with electrdes internal t the lamp s ends r withut electrdes. Electrdeless UV lamps are generally limited in length up t ten inches and have narrw lamp diameters. The diameter f an electrde UV lamp can vary tremendusly. Typically, the higher the lamp wattage - the larger the lamp diameter needed t dissipate heat created frm the bulb. Als, the lnger the length f the UV lamp - the larger the diameter needed t maintain structural integrity. A narrwer UV lamp prvides less surface area frm which t dissipate the heat. Cnsequently, narrwer lamps typically emit a higher level f IR energy. All UV lamps require heat t generate UV energy. The heat prduced by the lamp is necessary t emit peak UV intensities alng the length f the lamp. Heat must be kept cnstant alng the length f a UV lamp. Ding s prvides cnsistent UV utput acrss the entire lamp length and greatly reduces the likelihd f either ht spts r cld spts develping at the surface f the lamp. Either ht r cld spts can rapidly destry the structural strength f the UV lamp and generate a defective prduct. The mst cmmn UV lamp used in the printing and cnverting industries tday is the standard mercury lamp. The standard mercury lamp generates peak intensity at 365nm (Figure 2a). Ttal UV utput ranges frm 200nm 440nm (Figure 3). Sme UV chemistries require a metal halide lamp fr curing. Metal halide r dped bulbs shift spectral emissins. Tw cmmn dped bulbs are Irn, cncentrated frm 330nm 390nm, and Gallium, cncentrated frm 400nm 440nm (Figures 2b & 2c). Metal Halide lamps require higher striking vltages and als have higher perating temperatures than standard mercury lamps. While energy distributin f UV lamps vary amng UV manufacturers. Typical UV lamp energy distributin is apprximately: 1/3 UV +1/3 light + 1/3 heat. Fr many UV curing applicatins heat generated frm the UV lamps is f minr cncern. Hwever, fr cnverters, mnitring and minimizing delivered heat t the substrate is f fundamental significance. The cnverter must limit the peak temperatures reaching their substrates. There are a wide variety f methds utilized t cntrl and limit heat t the substrate. Heat Management Methds fr UV Systems: All UV curing systems need t emply a cling prcess t effectively remve heat generated frm the UV lamp. The tw mst cmmn means f remving this heat is either air-cling r water-cling. Page 1 f 7

2 Air-cled UV prcessrs utilize a high vlume f air directed either perpendicularly r parallel t the UV lamp t remve a large prtin f the heat generated frm the UV lamp. The remainder f the heat transfers t the UV prcessing unit and t the substrate. Water-cled UV prcessrs circulate chilled water thrugh extruded aluminum reflectrs and husing cmpnents t remve the majrity f the heat generated frm the UV lamp. Water-cled UV prcessrs still need sme air-cling t scrub heat frm the surface f the UV lamp. The remaining heat typically gets remved via the substrate. As stated abve, bth air-cled and water-cled UV systems, in additin t UV energy, transfer IR energy and visible energy t the substrate. T further reduce and cntrl IR and visible energy transferred t the substrate, UV system manufacturers utilize a variety f heat management methds. Please nte that the effectiveness and csts assciated with any f the materials discussed belw may vary greatly between UV system manufacturers. UV prcessrs will have ne f the fllwing reflectr / shutter designs; parablic, elliptical r a cmbinatin f the tw. (Figure 4) Parablic systems are designed t fld the substrate and maximize the UV expsure time, in cnjunctin, the expsure perid f IR t the substrate. An elliptical design maximizes the UV peak irradiance and typically minimizes the time perid f expsure thus reducing the time permitted fr IR absrptin by the substrate. UV systems can be designed t autmatically vary pwer utput as a functin f prcess speed. By mnitring the UV energy needed fr prcess speed and autmatically matching the UV utput t this need, the cnverter can avid issues assciated with ver cure and reduce the heat transfer t the substrate. Dichric catings can als be used t reduce delivered heat. Cld mirrrs, which are dichric-cated liners / shutters, reflect UV twards the substrate and transmit IR away frm the substrate. Cld mirrrs typically prvide maximum UV reflectance between 220nm t 380nm and reduce heat via the absrptin r transmissin f visible and IR energy. These catings can be applied t aluminum, stainless steel, fused silica r ther glass substrates and exhibit frmability n metal reflectrs. Cld mirrrs can als be custmized with angles f incidence such as 0, 21, 45 degrees, r a range f ther degrees (Figure5). These catings are fairly hard and must be cleaned regularly t remain effective. Althugh effective, dichric cated materials, whether glass r metal, must be cleaned cnsistently t demnstrate the very prperties fr which they were purchased. Dichric-cated mirrrs and plates are expensive and are cstly t maintain. Quartz windws r fused silica barrier plates can be psitined between the UV lamp and the substrate. These barrier plates transmit UV. These barrier plates can als have dichric catings applied t them. Unlike cld mirrrs, these catings reflect IR energy and transmit UV energy. These barrier plates are called ht mirrrs. Ht mirrrs typically prvide maximum UV transmittance frm 220nm t 400nm and partially reduce heat t the substrate via the reflectance f bth visible and IR energy. If greater IR rejectin is necessary, it is pssible t air space mre than ne f these ht mirrr elements in a parallel array (Figure 6). Only ne heat management methd utilized actually wrks in a reverse prcess f remving bth delivered heat and exthermic as they are transferred t the substrate. Rather than reducing the IR energy and visible energy delivered t the substrate, a UV manufacturer can munt the UV prcessing unit directly ver a water-chilled rller. With the substrate running ver the chill rll, the heat created by the chemical reactin (exthermic) and delivered heat frm the UV prcessr is remved frm the substrate as they are being transferred t the substrate. Bth the temperature f the water and the vlume f water delivered t the chill rll can be easily changed thus effectively and efficiently prducing cnsistent and cntrlled substrate temperatures thrughut the prcess applicatin. Page 2 f 7

3 In additin, the chill rll methd eliminates the high csts assciated with maintaining many f the afrementined heat management methds. Objective: Illustrate the effects f the abve stated heat management methds n the UV dsage (J/cm 2 ), peak UV intensity (mw/cm 2 ), average and maximum temperature rise (ATR & MTR). Review the capital equipment, peratinal and maintenance csts related t each heat management methd. Tests: Tests were perfrmed n the fllwing: Prime UV Optimum Series Air-Cled UV prcessr, Prime UV Diamnd Series Water-cled UV prcessr, Prime UV Transprt Cnveyr and an 18 inch-lng, 22mm diameter, 600 watt mercury vapr UV lamp. UV dsage (J/cm 2 ), peak UV intensity (mw/cm 2 ) and temperature readings were measured and cllected with the UV PwerMAP frm EIT. UV spectral graphs were created using the Sla-Scpe and Sla-Check system frm Slatell Ltd. The fllwing heat management methds were tested: Reflectr / shutter design elliptical & parablic Pwer utput varies as a functin f prcess speed 50 fpm & 100 fpm Dichric cated reflectrs / liners i.e. cld mirrrs Quartz barrier plates Cmbinatin f quartz plates with dichric reflectrs Dichric-cated barrier plates i.e. ht mirrrs Cmbinatin dichric-cated barrier plates (ht mirrrs) with dichric reflectrs. Chill rll (Tests cnducted n-site at custmer utilizing high efficiency chill rll system belt driven by press drive shaft) Capital equipment csts are estimates as t industry average fr a 42-inch UV prcessing system perating at a maximum 600 watts per inch ffering 8 pwer levels. Maintenance csts assume perating time f 4000 hurs per year and d nt include csts assciated with time needed t replace r clean cmpnents, as this wuld vary widely amng UV manufacturers. Cnsumable csts d nt include cst f UV replacement lamps r the csts assciated with having plant persnnel replace r clean these cnsumable cmpnents. Results: Cnclusin: First determine the temperature limits f the prducts being prduced r may be prduced in the future. Only with this infrmatin in hand is it pssible t effectively evaluate which heat management methd(s) discussed wuld be best fr his r her peratin. As previusly shwn, the csts and effectiveness f the varius heat management methds can vary tremendusly. Certain methds are better suited fr specific applicatins and UV chemistries. Linking UV utput t prcess speed is ne f the least cstly yet mst effective ways t limit heat transfer t the substrate. Althugh the initial investment may be the highest, installing the UV prcessr ver a high efficiency chill rll is the mst effective way, if desired, t maintain a specific substrate temperature. An additinal benefit f this heat cntrl methd is that it necessitates the least amunt f maintenance time and dllars. Hwever, this methd isn t practical fr sme cnverting prcesses nr fr certain UV retrfit applicatins and linking UV utput t prcess speed may nt be enugh. In these cases emplying ne methd, r any cmbinatin f the ther, needs t be cnsidered. Page 3 f 7

4 When researching the ther methds, the cnverter shuld cllect the csts assciated with replacing all cnsumable cmpnents (i.e. reflectr liners, shutters, ht mirrrs, cld mirrrs, UV lamps, etc.) and maintaining these materials (i.e. cleaning and checking the effectiveness.) Be aware that these dichric-cated materials may lse much f their effectiveness if nt kept clean. Cnsequently, ne must cnsider the cleanliness f the prcess as well as the time restraints inherent in ne s prcess befre investing in any ne f these heat management methds. A harsh perating envirnment can significantly reduce the life expectancy f many f these materials. The heat management methds d reduce temperatures, but in sme cases, als affect the UV dsage and UV intensities being delivered t the substrate. Whether these effects are detrimental r beneficial t the prcess needs t be evaluated as well. Page 4 f 7

5 Figure 1. UV Curing Prcess Cmparisn Figure 2a. Standard Mercury Lamp Figure 2b. Metal Halide, Dped Bulb - Irn Figure 2c. Metal Halide, Dped Bulb - Gallium Page 5 f 7

6 Figure 3. UV Output Range Spectrum Figure 4. Parablic vs. Elliptical Reflectrs Page 6 f 7

7 Figure 5. Ht Mirrr Figure 6. Cld Mirrr Page 7 f 7

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