Home > DMI Workshop, screen printing, UV Inks > Understand UV curing process in simple way

Understand UV curing process in simple way

UV curable inks and coatings render a durable, high-gloss finish with excellent mar and abrasion resistance. Because UV curable are nearly 100 per cent solids, they will not evaporate solvents into the air. In addition, UV inks and coatings cure only by exposure to UV light, so they will not dry out in the screen.

Finer mesh screens can be used with UV curable inks, which result in greater detail printing. Also UV saves significant floor space compared to wicket dryers or conventional hot air dryers. The nearly instant drying of UV results in more output from existing presses and fast completion of a job order. Also, UV consume less energy than gas or electric conventional dryers.

In general, UV curing or “drying” is achieved through photo-polymerization. Ultraviolet energy is generated by the mercury vapor lamps, which have a strong spectral peak of 3,650 angstroms. This energy is absorbed by the photo-initiator within the liquid formulation of the ink or coating. Free radicals are then produced, which attack the double bonds of the resin (monomeric) molecules. The molecular weight increases as the particles add to them-selves and form cross-linked chains, resulting in a solid polymer film. The whole process of converting a mobile liquid to an immobile solid coating takes place in a fraction of a second. It is accomplished without solvent waste while usually consuming less energy than conventional methods.

There are several factors governing the cure rate of which the most important ones are:

1. The photo-initiator to resin ratio in the liquid or ink.

2. The ability of the pigment and the concentration of the pigment to transmit UV energy, in the case of a screening ink.

3. The intensity of UV energy in watt-seconds/ CM2 produced from the mercury vapor lamps.

4. The thickness of the ink or coating to be cured.

5. The substrate on which the ink and coating are to be cured.

The photo-initiator/resin ratio is determined by the material supplier and is premixed prior to shipment. In almost all cases, the UV inks and coatings are ready to use in their packaged form.

The material supplier also chooses those pigments at an optimum concentration level, which are capable of transmitting UV light. However, some pigments transmit UV energy more readily than others: black and white are perhaps the most difficult inks to cure because black inhibits UV energy penetration and white reflects UV energy. They both can be cured, however, with increased exposure. The spectral hues cure faster than black and white and exhibit little difference in cure rate between themselves.

The intensity of the UV energy is, for the most part, determined by the type of lamp and reflector used. The current standard for the industry is 200 to 300 watts/inch mercury vapor lamp housed in an aluminum reflector of which the inside surface is plated and polished for maximum UV reflectance. At Grafica we have even successfully sold UV Curing Machine up to 600/watts/inch. Higher wattage leads to faster production.

The thickness of the ink and coating is a concern to the screen printer because if too much ink is deposited, the UV will be unable to penetrate, therefore an incomplete cure results. Because the UV curable inks are 100 per cent solids, mesh counts per inch ranging from 140 to 180 threads/cm (355 to 457 threads/inch) can be used to keep thickness in the curable range.

The thickness of the clear coatings is not as critical because they do not contain pigments which reduce cure rates. The substrates on which the ink and coating are to be cured often effect the cure rate. For instance, a surface that reflects UV energy would cure faster than a dark surface that would absorb the UV energy.

When purchasing a UV curing machine for screen printing, two very important features to look for are: how well the manufacturer handles the heat generated by the UV lamps, and how well the conveying system transports the light stock through the reactor.

UV lamps produce three types of energy: ultra-violet energy (which is absorbed by the coating or ink and produces cure), infrared, and visible light. Visible light is contained by the sheet metal enclosure of the curing tunnel, which therefore protects the operating personnel. Infrared or heat produced by the lamps must be controlled to keep the stock dimensionally stable. This is done by either water-cooling the lamps and reflectors or air-cooling the curing tunnel or both.

A separate cooling module can also also be added that allow the screen printer to process the most heat-sensitive substrates, such as vinyls and polyester films after curing.

For conveying light stock, such as pressure sensitizes, a vacuum holdout conveyor should be provided to insure a smooth transport through the curing tunnel. The vacuum holdout should begin one foot before the stock enters the reactor to prevent the sheet from fluttering as it enters the curing chamber. Also, the conveyor belt should consist of a material which rapidly dissipates heat such as a Teflon-coated, open-mesh, fiberglass belt.

Other considerations for purchasing UV curing machine are the number of lamps required to achieve proper cure at desired speed. This should be coordinated with the ink supplier who would recommend the number of lamps needed at a given production speed. Conveyor speed control should be precise and repeatable to prevent over- or under-curing, and drives should be variable from 30 to 150 FPM.

When converting a conventional line to UV, a few problems must be dealt with. Screen fabrics and mesh counts will have to be changed. Again, UV curable are 100 per cent solids and will not discharge solvents; so the amount of ink initially printed is the amount of ink left after cure. Therefore, monofilament screens with 140 to 180 threads/cm (355 to 457 threads/inch) should be used for screen printing UV curable materials.

Opacity in UV inks is somewhat of a problem. If the pigment level in a UV ink is too great, the ink will not cure; so, the screen printer requiring true opacity for a job cannot use UV inks at this time. Anything less than true opacity can be achieved by UV. Adhesion of the UV curable to some substrates has been a problem, but material suppliers are over-coming this obstacle by altering the formulation of the coatings for a specific substrate.

In some UV curable, the monomers in the formulation are cause for concern regarding toxicity. Proper handling should be exercised by the user. The coating supplier will inform the user on the degree of toxicity of his material and will instruct on proper handling.

Incidentally, UV energy is now used in the field of dentistry for curing photo-reactive material in filling cavities. UV curing has a bright future in the screen printing industry. The process is widely accepted and will continue to grow enabling the industry to provide new and better products for its customers. Again, UV curable are 100 per cent solids and will not discharge solvents; so the amount of ink initially printed is the amount of ink left after cure.

Avoid SOLVENT INK SYSTEM where ever you can.

Go for UV CURING SYSTEM and make your business profitable.

Keep growing…

Bhargav

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  1. Nikhil Ahuja
    March 30, 2010 at 12:16 am

    Hi Bhargav Sir,
    u wrote about white and black inks requiring more energy to cure…
    i have noticed one more phenomenon in my unit when printing on colored substrates,
    when we print on a more yellowish or even red substrate (metallic papers for wedding cards) we have to run the dryer very slow, as compared printing the same job on white paper.
    can you explain why this happens and if there is any way we can work around this?

    regards,
    Nikhil

    Like

  2. Kamlakar R. Wadekar
    March 30, 2010 at 11:38 am

    Keepup the good work.
    Kamlakar

    Like

  3. Kamlakar R. Wadekar
    March 30, 2010 at 12:33 pm

    Nikhil Ahuja :Hi Bhargav Sir,u wrote about white and black inks requiring more energy to cure…i have noticed one more phenomenon in my unit when printing on colored substrates,when we print on a more yellowish or even red substrate (metallic papers for wedding cards) we have to run the dryer very slow, as compared printing the same job on white paper.can you explain why this happens and if there is any way we can work around this?
    regards,Nikhil

    Well it’s surprising, but that’s what keeps us going, check the lamp intensity and the reflectors if they are good enough, normally you will have to adjust the speed on different subtracts e.g.: on metallic foil boards because of the reflection of the subtracts over curing will be noticed & with darker colours undercuring is noticed with your normal speed used for white paper.
    Best Regards
    Kamlakar R. Wadekar
    Ace Printers
    Mumbai

    Like

  4. murtaza kapadia
    March 30, 2010 at 12:54 pm

    fantastic information on curing which will open more avenues
    goodluc
    murtaza kapadia

    Like

    • March 31, 2010 at 8:09 am

      Thanks Mustafa, I am glad to know that you are regularly taking advantage of the information from my blog and always sharing your views

      Friends like you encourages me to write and share my knowledge

      Yours Friendly

      Bhargav

      Like

  5. Michel Caza
    March 30, 2010 at 6:32 pm

    Hi Nikhil and Kamlakar,
    What Kamlakar suggests is quite right : check the cleanliness of your lamp and reflector. They must be cleaned each two weeks (with isopropyl alcohol – use cotton gloves to not touch the quartz of the lamp) and don’t forget to check the power emitted : if your lamp is too old the power emitted will be rich in infrared but poor in UV.

    I hope you don’t have a UV unit with dichroic filters because the efficiency is reduced, especially when printing with opaque dark inks and on dark substrates.

    : 1, motion is very important

    : 2, 3, 4 added reflexions

    : 5, and 6, more complex problems

    The figures I join show how the UV rays work in the ink coat, according to different configurations. If you print on metallic paper, it will work as shown on fig.5, using the reflexion of the metal to help curing. But this depends upon the transparency of your ink. In general and “normally”, with non opaque inks, the speed of reaction increases and you have the risk of over-curing (as said Kamlakar).
    If the colour of the substrate is dark you loose a part of the incident reflexion and the reaction slows down (part of the UV rays are absorbed by the substrate and not reflected). But this has also to do with the colour of the ink (because of an eventual competition between pigments –and photo-initiators to catch the UV rays) and relative transparency.
    Two theoretical possibilities then: increase the power of the lamp or slow down the belt.

    This said, once more, I printed UV for years on many types of substrates, white, transparent, metalized or dark papers, plastics or metal and I never as to make big adjustments. Only to go from 35/40 meters/minute to 30 or 25/m or to increase the power of the lamp (or lamps when 2) from 200 watts/inch to 250 watts/inch. Even with cylinder press at a speed of 2,000 sheets 70 x 100 cm/hour.

    But, once more, you need a good curing unit (with clean lamp and reflector of course) and good inks… Ask Bhargav ! It’s not my role to give you brands !

    Michel Caza

    Like

  6. Sanjay
    August 27, 2010 at 12:46 pm

    Dear Mr.Bhargav Mistry,

    Its a very usefull information for the beginers.
    We are into Pigment manufacturing and want to intoduce pigment range for UV offset and UV Flexo inks.
    I want to know about
    1. which properties a pigment should have
    2. Which type of UV-Resin should we use for testing purpose so that the results
    will correlate to other UV-resin systmes also.
    3. What is the role of monomers in the UV resin (Oligomers )

    Regards,
    Sanjay

    Like

    • September 2, 2010 at 5:53 am

      Please note that generally I am not obliged to answer this questions because it’s not related to screen printing process and secondly your query is basically related to the technology of ink making and chemistry but still I like to help the industry and my people so in that case my answer to your questions point wise is mentioned below.

      This would be my final reply in this regards.

      1. The pigments are classical (BASF for example) for offset, flexo or screen. They simply need to be very stable. They are generaly coated with a resinate to trap the free radicals. The black must be mono-component.

      2. The UV-resin to use is an oligomer. It can be an epoxy acrylate, an polyurethane acrylate or, best for offset and flexo, a polyester acrylate.

      3. The monomer act as a « diluter » (to change the rheology of ink in a more liquid component), but of course it does not evaporate – it will remain a part of the finalpolymer after curing. For offset and at a higher degree in flexo where the ink must be liquid or very liquid (if compared with screen printing inks), the best monomers are di-acrylates or tri-acrylates, modified with fat acid, mostly because of offset. The « emulsion ability » must be carefully controled because of the mixing principle « water/oil » of offset.

      Regards,

      Bhargav

      Like

  7. September 23, 2010 at 10:13 am

    Great info. Got a question in regards to curing an adhesive applied to aluminum. Which would be a more cost effective, b) provide a better overall curing: Infrared or UV?

    Like

    • September 23, 2010 at 9:17 pm

      I am sure your adhesive must be SOLVENT based so in that case you will require Hot Air dryer if your substrate and drying requirement is around 80 to 90 dg celsius generally IR is used when the temp requirement is 120 to 180 dg celsius.

      Like

  8. October 30, 2010 at 4:40 pm

    Hello Bhargava

    Congratulations for the developements you have done in printing Industry.

    I operate a Label Printing company in Nairobi based on Flexography printing.
    It is our aim and ambition to further this industry in Africa and will be happy to join hands with you in future whereevr we can colloborate.

    Meantime, I have a technical query. We are having a issue of slow curing of Black ink. We have checked the Lamps, Shutters etc. Any other suggestions.
    We use Gallus Narrow web machines , do you have any suggestions how we can determine which would be the best UV lamps tuse??

    Thanks

    Samri Gandhi
    Adpak International

    Like

    • October 30, 2010 at 11:01 pm

      Hi Samir,

      Since it was a good question I thought instead of replying I should put your reply on my blog so check my blog now. I have highlighted some important points which effect the curing.

      Regards,

      Bhargav

      Like

  9. Umesh
    July 12, 2011 at 6:52 pm

    Dear Bhargav,

    We are into Watch Dials Manufacturer.
    I Need an UV ink for watch dials & how to process it & availablity of the UV inks at your end.
    Pl help me out

    Regards,
    Umesh

    Like

    • July 12, 2011 at 8:27 pm

      Kindly contact any ink manufacturers who can help you better…

      Like

  10. Nurul Maisyarah
    August 15, 2011 at 8:28 am

    hye.. Thanks for the great info. I try to study how to apply UV polymerization technique to my Molecular Imprinted Polymer (MIP) project. Most of the journal just state polymerization via UV at certain time. But I’ve no idea how & what is the procedure of UV polymerization.

    Like

    • August 16, 2011 at 7:56 am

      I need your introduction in order to reply your question on UV Polymerization

      Like

  11. Nurul Maisyarah
    August 16, 2011 at 8:20 am

    I’ve made the polymer solution then the solution i’ll coat on PET plastic followed by curing/polymerize at certain condition to obtain the polymer matrix on PET surface. But i dont know how carry on the UV polymerization method(in term of equipment needed)

    Like

    • Sanjay Vora
      September 3, 2012 at 5:57 pm

      If you are applying polymer solution, make sure to evaporate the solvent before curing the dry film of the polymer. I am assuming that your polymer is UV crosslinkable and if yes, depending upon the sensitivity to the spectrum range, cure it with a UV lamp emitting the same wavelength to which your coating is responsive.

      Like

  12. Ani
    March 7, 2012 at 12:53 pm

    dear sir,
    we are manufacturer of ice cream paper cone sleeves from aluminium foil paper.
    currently we are doing gravure printing on that. but problem is its doing lots of
    westage during printing registration.

    we are planning to do UV offset printing on it. complete thickness of aluminium paper
    foil 100gsm. but i wantto ask lots of company telling me during IR dryer your paper
    will be roll or band. kindly suggest us solution.

    thanks
    Anil

    Like

    • March 11, 2012 at 12:35 pm

      Generally in offset and in gravure printing the speed is so fast that according to me a 100 gsm foil paper should not have much problem but why don’t you try or the manufacturers who gives you the UV curing system or the ink manufacturer should be aware of such issues. Anyways if your substrate is heat sensitive you may try to go for COLD UV techniques where underneath the UV lamps there are special quartz filters used to reduce the heat and a good cooling system with in the UV Lamp chamber could solve this problem.

      We have cured much thinner plastics our UV Curing system and there were no issues even the pre gummed vinyl stickers are now a days screen printed with UV Inks without any shrinkage etc.

      Like

  13. Sanjay Oak.
    August 29, 2012 at 4:12 pm

    Sir, Is it possible that while printing by offset uv on metpet, ink curing is okay but ink adhesion
    not satisfactory? why? Also pl.suggest where can we purchase protective hand gloves & eye glasses while working with uv process. What is the shelf life of the Solution that is used for checking the dyne value of metpet.
    Thanks & Regards
    Sanjay Oak.

    Like

  14. mustafa
    August 30, 2012 at 3:19 pm

    dear sanjay,
    i have expirienced this problem many times that ink is cured but the adhesion is poor. the reason being the dyne value of metpet detoriates over period of time, hence an overcoat of uv varnish or lamination is highly recommended. in my lab we change the dyne solution every 3 months (this actually varies with the usage). The gloves are available at medical equipment supplies stores.

    Like

  15. Sam Purnomo
    September 5, 2012 at 9:19 pm

    Dear Mr Bhargav,

    I am very impressed with the information you provide for the screen print industry in India. Wish we had the same institution doing the same. Bravo !

    Whilst am here, I’d like to ask you about UV Effect specifically UV Refraction. I noticed your Valentine’s card turn out very nicely. I have tried to produce the same effect and till date am still not successful despite changing mesh count, mesh quality, UV ink thickness, squeege pressure etc. Any tips would certainly be appreciated.

    By the way, can I join your classes for the Screen Printing ? Thank you

    B.Regards
    Sam

    Like

    • September 9, 2012 at 11:58 am

      Reply your questions separately check your email… Thanks,
      Bhargav

      Like

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