Home > Drying & Curing, UV Inks > How to maintain consistency with curing UV Inks

How to maintain consistency with curing UV Inks

My company have sold hundreds of UV Curing system but when ever we get phone call or complaints about curing problems there are couple of thing lacking from the printers side to further invest and the main thing is UV INTEGRATOR to check UV LIGHT INTENSITY. Its like how can you drive a car judging the speed without a speedometer.

Printers buy machines worth millions but hesitate to invest on such petty tools or just neglect for no reason. But they forget that these meters are not luxury but essential tools for maintaining quality and consistency. UV machine only cures the ink and they are featured with an adjustable conveyor speed to set as per the desired value specified by the ink manufacturers in Joules or Milli Joules/cm2.

If you are a quality printer or looking towards upgrading your quality then you need to invest on a good UV INTEGRATOR if you are already using a UV Curing system.

Every ink on different substrate printed with different mesh counts needs different intensity to cure the UV Inks and don’t forget to involve the your ink supplier to give you proper specifications. They have it all ready but you need to ask for it. Even if you get those specifications you need a meter to check on regular basis.

What happens if you over cure the UV inks?
Kindly note that by over curing the surface becomes so hard that when another layer of ink is printed and cured over the first layers, the top layer would experience poor adhesion to the bottom layer. Also over cured ink becomes less flexible and makes the surface hard which is liable to break the ink film during punching or bending.

How to ensure whether my ink is properly cured?
Properly curing the ink film for the first time is very important. Proper cured ink minimizes substrate degradation, over curing, rewetting, and under curing thus optimizes proper ink-to-ink adhesion.

First before starting the production set your conveyor speed as per the meter reading specified by the ink supplier (ensure that the specification matches your lamp wattage i.e. 200/watts/inch or 300/watts/inch) or maintain data as per your last production parameters noted. If you do not have these data or if your ink supplier cannot help you on this then try the following:

First keep your conveyor speed on higher side then usual (kindly note that white and black colors cures slower than other colors and transparent varnishes) then test adhesion by using a cross-hatch tape test (use good quality tapes like 3M Scotch etc.). If you find improper adhesion after the test then reduce the conveyor speed and keep doing it until you find proper adhesion and make sure your substrate do not deform or change from its original shape or dimension. This will lead you cure your UV Inks at the optimum level without under over curing.

Thereafter when you begin the production decrease the speed 20% to 25% from the optimum to adhesion failures.

Also ensure that you clean the mirror polished skin of the reflector regularly. Dirty reflector decreases your curing speed plus over heats the substrates and affect your registration parameters due to the change in substrate dimensions.

So don’t hesitate to invest on a good UV METER. It is not an luxury but necessity to maintain your quality standards.

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  1. Kamlakar R. Wadekar
    February 19, 2010 at 2:33 pm

    Dear Bhargav,
    We generally only concentrate on the UV LIGHT INTENSITY and the Conveyor speed for proper UV Curing , but sometimes even after having your settings perfect we face a problems that the UV INK or UV Varnish comes off from the surface very easyly, my experience 1). When you do not have a proper Surface Tension ( DYNES LEVEL ) between 38 to 40 on your substract you can face problems like this, particularly on Matt Lamination Sheets as sometimes the film used is a old stock and the CORONA treatment becomes weak normally after three months from the date of manufacture. 2). Sometimes the inks printed by Offset has a high level of WAX contians, even in this condition we may face the same problem.

    Like

  2. Michel Caza
    February 19, 2010 at 10:35 pm

    A good integrator will only gives you the amount of UV light emitted (then if the amount of UV light is correct and to measure that it remains correct when the lamp get older, after 1,000 hours of use at maximum power. Unfortunately it does not tell you if your curing is correct. I made some researches with a Canadian University years ago to try to build a tool measuring the exact percentage of polymérisation of the UV ink. Unfortunately such a tool would be too expensive for most of companies and the research was stopped.

    This was not a problem in my company because, joined with my own researches in that field since 1976, we had an expérience of almost 30 years in UV printing on a lot of different substrates and we know exactly if the curing (polymerisation) was correct or not at a certain moment (just after curing and cooling and later)… But you need to know what is a « correct curing » of course !.

    Most of the inks manufacturers cannot give you precise data about what is the « correct polymérisation » of their own inks : too many variable involved in the printer’s plant.

    The worth ennemy is – as mentioned by Bhargav – overcuring.

    The human tendency – by fear of an unsufficient “drying” – is to overcure, a fact which create a lot of problem as Bhargav said.

    As matter of fact, most people, ink manufacturers included, forget what we name “post-curing”: a UV inks continue to polymerize between 24 to 36 hours after passed under the UV lamp. If you cure at the maximum (and control immediatly after curing with the cross-hatch test mentioned) the day after, your ink will be naturaly “overcured” and a lot of problems will happen.

    The secret is to cure only at 75% : sufficient for piling your prints without risk. The question is how to know if it is correct. The most simple – and cheapest – test is to scratch with the nail : the top of the ink must be hard but the underpart of it must remain soft (you will have some colour coming under your nail if you scratch hardly). As an example, do the same thing – scratching – the day after and you will find your colour “hard at heart”, the properly cured. OK, it does not look too “scientific”, but it is efficient and cheap !

    The “cross-hatch tape test”, then, will confirm that the curing is correct.

    To clean the lamp and the reflector is extremely important, as Bhargav wrote. It must be done each two weeks. Don’t touch the lamp with naken fingers (it would damage the quartz of the lamp), use cotton gloves to clean with isopropyle alcohol.

    Michel Caza

    Like

    • Sanjay Vora
      February 22, 2010 at 6:01 pm

      Hello Mr Caza

      Hope you are keeping good health now.

      There are two types of cures taking place – surface cure and deep cure. Longer UV wavelength is good for deep cure and shorter wavelength is good for surface cure. Is it possible to have good surface cure of all the inks as we normally do with the UV cure unit (cure at faster speed) and then pass the substrate under a low intensity longer wavelength UV light. With this we can avoid over curing of the ink.

      Like

      • Michel Caza
        February 22, 2010 at 8:34 pm

        Hi dear Sanjay,

        Where did you catch that idea of “two types of UV curing” ?
        Since I developed the UV technology from 1976, I never mentioned such a thing !
        And in practice, I never added a second curing to the first one for a “deep cure”. I simply cure at minimum and leave the “post-polymerisation” complete the curing in the depth of the ink.

        I generally had extremely thin coat of ink from 10 to 30 µ, cure with one lamp or sometimes 2 lamps emitting in a wavelength between 200 to 400 Nanometres (the most reactive point of the photo-initiators is situated between 258 to 264 Nm) that is to say most often at 80 W/cm (200 W/inch), UV light concentrated on a band of 2,5 cm (1 inch) on the substrate. Speed of the belt : between 25 and 50 metres/minute – mostly around 35 m/minute.

        I never disperse the IR radiations (UV lamps emit between 200 and 5,000 Nm – the infrareds) with dichroïc or quartz filters because some colours need a “cocktail” of radiations to cure properly. My solution is to add a 2 meters long “cooling unit” blowing cold air (5° to 10°C) with 80/85% of humidity. This way, paper recovers its size (with humidity) and plastics do not “extend”, even a 10 µ polyester sheet or adhesive decals : all remains flat and keep their size. (We had that behind each of our UV units on our 5 line of press, big size (220 x 140 cm) included, flat bed or cylinder press…

        I was using only two French UV inks, mat or satin finish on all substrates – paper, board, polyester, PVC rigid or supple, polycarbonate, PMMA, styrene, polypropylene (supple or rigid) and polyethylene – sometimes adding 5% of a catalyst (hardener).
        Only two types of fabrics : 180 and 140/cm T (threads 28 > 32 µ), yellow, PW, calendered (squeegee side), tension at 25 Newtons/cm.
        We use some others fabrics, but only for “special effects”.
        But for “normal” prints, only one coat of emulsion on substrate size of the screen… Squeegee 75° shore at 75° angle, very thin “kiss” flooding with a 500µ blue steel blade.
        All this help to reduce the ink thickness, then the problems of polymerization and dot build-up.

        To come back to your problem – of course, I don’t know which fabric, tension, ink etc. you use, and on which substrate you print,– but forget this “double polymerisation” : it cost you time and energy, different types of lamps or filters, etc..
        Follow simply the rules I gave about “minimum curing” and, with a good ink and thin coat, you will have a perfect and naturally cured coat “at heart”… The day after !

        In thirty years of UV, I never had an “over cured ink”. We had too much finishing in POP and too many superposed ink coats sometimes in fine art print to risk a bad inter-coat adhesion or breaking of ink when folding or cutting !

        Michel Caza

        Like

  3. Nikhil Ahuja
    February 20, 2010 at 11:13 am

    in my plant we often have jobs with more than 5 to 6 colors and effects to print,
    So this brings up a question in my mind when we talk about over curing.

    Even if we were to under cure the initial colors, would the multiple passes through the UV dryer keep escalating polymerization to an extent where some of the inks printed become over-cured and hard/brittle (causing chipping when cut or creased)?

    Nikhil Ahuja

    Like

  4. Michel Caza
    February 20, 2010 at 7:11 pm

    Hi Nikhil ? Barghav and all,

    Excellent question Nikhil.
    Most of time, of course, the jobs are in four or many more printed colours.
    YES. The curing of the following colours have an impact.
    It is very simple to catch : when your first printed colour goes under the UV lamp, it burns 75% of the “photo-initiators” (the molecules that initiate the reaction of polymerisation).
    75% of the remaining 25% of the first colour will be burn when you cure the second colour and some left – a very few of the first colour – will disappear when the third colour is cured…
    This is not related to the natural “post-curing” I mentioned yesterday but comes in addition if I can say so !
    Then this is a supplementary reason to stay to the minimum curing while printing the run, mostly for the first printed colours.
    To “feel” the correct curing is mostly a question of practice : the scrapping with the nail I mentioned is a simple manner to estimate.
    Then, trying with small runs – it depends also of the material you print on – is a good way : don’t take risks on big orders (!).
    Start with what you feel as the proper curing for your first colours and if necessary slow down your belt speed or increase the power of the lamp. It is always easier “to increase” the curing. If you start too high, it will be difficult “to reduce” : the evaluation is more difficult.
    To see the impact of the second and third curing on the first printed colour, pass immediately your first print a second time and then a third time. By scrapping, you will immediately know how those additional curing alter or affect the polymerisation.
    Remember : sufficient for “piling” your prints, no more.
    Let the second and third curing and MOSTLY THE NORMAL POST-CURING finish the proper curing job !

    Michel Caza

    Like

  5. Nikhil Ahuja
    February 22, 2010 at 11:55 am

    Hi Michel,

    I greatly value recieving advice from “The Screen Printing Guru”…
    Bhargav keeps talking about you, and your way of spreading knowledege among others is really inspiring.
    thanks for your help on my question, im sure to put it to practice.

    i have seen pictures of the beautiful and absolutely unique work that you do and believe it is unsurpassed rather unsurpassable.
    hope that you plan a trip to india in the near future,
    always very eager to meet you.

    regards,
    Nikhil Ahuja

    Like

  6. Sanjay Vora
    February 22, 2010 at 6:20 pm

    First of all all the ink users need to know the energy requirements (milijoles/cm.sq of the particular ink. If one knows that, one can make use of UV integrator to ascertain what should be the conveyor speed to cure the ink correctly. However if you do not have a UV integrator, you are playing blind. But at the same time, let us accept the fact that not many people can afford it, especially the new converts to the UV. So I can suggest some JUGAD.

    1.A new UV lamp emits about 30% useful UV, 15% visible and 55% IR light. As the lamp ages, UV light goes on decreasing and IR goes on increasing. Increased IR means more heat. So keeping all the conditions same, if you find that your printed substrate is getting heated more, it is an indication that the lamp has aged and thus you have to adjust the cure speed accordingly.

    2. When the lamp is new, check the current drawn by the lamp. As the lamp ages, it will draw less and less current. This also helps in determining the lamp life and adjust the cure machine speed as lamp ages.

    Like

    • Michel Caza
      February 22, 2010 at 8:45 pm

      Correct Sanjay !
      that can help – mostly N 1 – when you don’t have an integrator. Don’t follow too much the decreasing of power (N 2) in Amps… The power distribution in India is too erratic !
      But you must know that a good UV lamp (Philips or Hanovia) is warranted for 1,000 hours of use at full power… But as I work generally far from the 300 W/inch, but more at 120 W/inch, I can use the lamp around 3,000 hours.
      Don’t turn off and on the lamp too often : if you stop to print less than 20 minutes, you will save the lamp lenght of life – simply reduce the lamp to the minimum possible and… don’t stop the belt of course !

      Michel Caza

      Like

  7. Sanjay Vora
    March 4, 2010 at 7:24 pm

    Michel Caza :
    Hi dear Sanjay,
    Where did you catch that idea of “two types of UV curing” ?
    Since I developed the UV technology from 1976, I never mentioned such a thing !
    And in practice, I never added a second curing to the first one for a “deep cure”. I simply cure at minimum and leave the “post-polymerisation” complete the curing in the depth of the ink.
    I generally had extremely thin coat of ink from 10 to 30 µ, cure with one lamp or sometimes 2 lamps emitting in a wavelength between 200 to 400 Nanometres (the most reactive point of the photo-initiators is situated between 258 to 264 Nm) that is to say most often at 80 W/cm (200 W/inch), UV light concentrated on a band of 2,5 cm (1 inch) on the substrate. Speed of the belt : between 25 and 50 metres/minute – mostly around 35 m/minute.
    I never disperse the IR radiations (UV lamps emit between 200 and 5,000 Nm – the infrareds) with dichroïc or quartz filters because some colours need a “cocktail” of radiations to cure properly. My solution is to add a 2 meters long “cooling unit” blowing cold air (5° to 10°C) with 80/85% of humidity. This way, paper recovers its size (with humidity) and plastics do not “extend”, even a 10 µ polyester sheet or adhesive decals : all remains flat and keep their size. (We had that behind each of our UV units on our 5 line of press, big size (220 x 140 cm) included, flat bed or cylinder press…
    I was using only two French UV inks, mat or satin finish on all substrates – paper, board, polyester, PVC rigid or supple, polycarbonate, PMMA, styrene, polypropylene (supple or rigid) and polyethylene – sometimes adding 5% of a catalyst (hardener).
    Only two types of fabrics : 180 and 140/cm T (threads 28 > 32 µ), yellow, PW, calendered (squeegee side), tension at 25 Newtons/cm.
    We use some others fabrics, but only for “special effects”.
    But for “normal” prints, only one coat of emulsion on substrate size of the screen… Squeegee 75° shore at 75° angle, very thin “kiss” flooding with a 500µ blue steel blade.
    All this help to reduce the ink thickness, then the problems of polymerization and dot build-up.
    To come back to your problem – of course, I don’t know which fabric, tension, ink etc. you use, and on which substrate you print,– but forget this “double polymerisation” : it cost you time and energy, different types of lamps or filters, etc..
    Follow simply the rules I gave about “minimum curing” and, with a good ink and thin coat, you will have a perfect and naturally cured coat “at heart”… The day after !
    In thirty years of UV, I never had an “over cured ink”. We had too much finishing in POP and too many superposed ink coats sometimes in fine art print to risk a bad inter-coat adhesion or breaking of ink when folding or cutting !
    Michel Caza

    Like

  8. Sanjay Vora
    March 4, 2010 at 7:46 pm

    Dear Michel

    From practicality and from a printers perspective,I entirely agree with you about the procedure adopted by you to avoid over curing of UV inks for a multicolor job. However from ink chemistry point of view, most of the times, blends of photoinitiators have to be added in the ink wherein some do the job of surface cure and some of deep cure. It is possible to have inks where all the inks are surface cured with xenon flash lamps and after fully printed, the sheet can be passed under normal UV lamp for complete cure. Xenon lamps are the same as found in our camera flash. They do not need to be kept continuously on. Ignite them when the substrate is passing under it and then put it off till the next piece comes under it. In short, this is an available technology and one can use it if required.

    Like

  9. Michel Caza
    March 15, 2010 at 3:32 pm

    Hi dear Sanjay,

    Sorry for late answer, but I was not on Bhargav’s blog those recent days.
    Let us be clear : if photoinitiators have to be added in the ink, it simply means the ink is not good !
    To add photoinitiator is extremely délicate and you need a quite good knowledge of UV chemistry. This is not the case for 95% of screen printers. It exists an optimum concentration point for highest efficiency and if you add more photo-initiator, the efficiency falls and you obatin an opposite result : a loss of curing !

    On the other hand, I am against the pulsed xenon (ionised inside a quartz) and flash UV lamps. The xenon was popular in the US (a company from Chicago now disappeared) between 1978 and 1980 because efficient in offset… (remember, with an ink coat ten times thiner! than screen). I proved that time that it was unefficient, except for very transparent inks and the US company supplying those UV curing units was obliged to buy back from the users all the curing unit sold !

    Same thing in the beginning of 90ties where Svecia started to build multi-colour press with “UV flash” units. Even with transparent process colours, they need to add a “regular” curring unit at the end of the press to properly achieve curring. Something similar to what you mention about “surface curing with xenon flash”. Of course totally inefficient with relatively dark or opaque or metallised UV inks.

    Conclusion for me, a good ink + a good curing unit and if possible the addition of a cooling unit solve the problems of surface and deep cure in one single shot : never in my life I used different type of curing to face such a problem I never met, but don’t forget the “post-curing” !

    Michel Caza

    Like

  10. March 23, 2010 at 1:13 pm

    Dear Sir,

    We do jobs moslty with UV inks. Some time there are two or more than two impressions jobs. While in more than one impression i usualy face problem of registration. The registration does not match after the first imprtession and during the second impression on the same substrate. Please suggest how to reeduce this problem and what are the parameters that we must follow while multi impression jobs.

    Thanks

    Gitesh

    Like

    • March 23, 2010 at 6:42 pm

      Dear Gitesh,

      Are you using UV Meter? If yes than how many Milli Joules do you normally keep.

      What kind of substrate are you using? (Also specify the thickness)

      What mesh count do you use?

      From where do you buy your inks? (Also specify manufacturers name)

      Finally do you use Grafica’s machine? If yes, then please let me know the type or model no.

      Like

      • March 24, 2010 at 12:28 pm

        Dear Sir,

        Thanks for the reply..

        Kindly let me explain with the example. We are into printing and packaging industry. We were printing a tag on which the matter was white color with glossy effect. To Match the same quality we printed first white color in 10 UPS on the sheet of 800 GSM size 11.5 inch by 12.5 inch (with Matt Lamination both side) using 77 no. mesh polyester. When all the no. of sheets are printed with white color (Nylotex White Sericol Co.) Thereafter 24 hours of drying period the 2nd impression is Gloss UV (MICRO Inks/JN Arora) printing with 165 no. polyester on the same sheet. The white printing runs out. We have to print UV carefully to register in exactly which takes a lot of time than normally production time. Still 2 to 4 ups out of 10 ups are out of registration. The printing method is manually and we are not using graffica machine and UV meter.

        Please suggest.

        With regards

        Gitesh
        9718052157

        Like

      • March 24, 2010 at 3:58 pm

        First of all I do not feel any problem with your process what you are following currently and your registration problem is more before curing and not after curing so that UV is not distorting your sheet.

        In fact the sheet 800 gsm with lamination is quite thick and should not loose dimension stability so easily. I feel the white ink might be distorting the laminated sheet. So first check the white after drying whether it matches with your film properly after drying because that must be the main reason. Secondly also check whether you are maintaining the registration guides at the same place each color.

        If all the above parameters are OK then check whether you are using high tension screen (stretched at the optimum tension as per specified by the mesh manufacturer), use lower off-contact, maintain equal off-contact all four corners and use lower squeegee pressure, if you are using wooden frames then forget it you cannot get good registration.

        Also check are you maintaining 6″ (inches) clearance between the frame and image area (i.e. your frame ID should be 12″ bigger than your print area). Is your screen tension equal all over the print area.

        Ensure your squeegee should have rounded corner and do not use more than 1″ bigger each side (2 inch in total than your print area). Sharp edge corner generally distorts the image more often.

        Take care and have a nice day…

        Bhargav

        Like

      • March 24, 2010 at 4:04 pm

        If still you have problems than you are welcome at our institute DMI we can help you to resolve it easily with our expert team because some issues are difficult to answer without seeing it practically.

        Have a nice time…

        Bhargav

        Like

  11. Ram Kumar
    January 28, 2011 at 10:33 am

    Dear Sir,
    How we can standalised a Lab minicuresystem with the press curing ( Customers) syastem.

    Like

    • January 28, 2011 at 10:39 am

      Not understood your question kindly explain properly in details what you want to know.

      Like

  12. July 20, 2012 at 9:38 pm

    Hi, I have been following your blog because it provides great technical information. We are a large-format offset lithography printer in the USA. We run both conventional and UV inks in our presses, and I was wondering if all this information carries over to the offset world as well? I understand many of you are screen-printing experts. Thank you for your patience, I am only a little over one month in my experience with printing.

    Like

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