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UV in General – Choose or Ignore it – Some Theory
- We are going to study the UV technology. Much non-sense was (and very often still is!) uttered or written about the UV
technology: a «fantastic collection of howlers of the UV system». I have often been asked if it was a rule that would be almost universal as far as UV printing...
I have several years of experience in UV, (29 exactly... and 25 years without the solvent based systems and I can say :
- Everything, from stencilling to printing, must contribute to the reduction of the thickness of the deposibted coat of ink, and
- “OVERCURING” must be avoided.
I believe that this is the only good and important rules both for technical and commercial reasons.
The 18 problems below are still often mentioned :
| 1. |
The UV inks break under the blade of the guillotine cutter, the die-cutting and when folding. |
| 1.1. |
False. |
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| 2. |
The outdoor posters printed in UV end up on the ground. |
| 2.1. |
False. |
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| 3. |
The adhesives «curl» on the edges and/or end up on the ground. |
| 3.1. |
False |
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| 4. |
The «inter-coats» adhesion is poor. |
| 4.1. |
False |
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| 5. |
The «light resistance» of the UV ink is low. |
| 5.1. |
False |
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| 6. |
The chemical and physical resistance is low. |
| 6.1. |
False. |
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| 7. |
There is no «gold or silver» ink in UV. |
| 7.1. |
False. |
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| 8. |
The UV inks are not quite opaque. |
| 8.1. |
False and true. |
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| 9. |
The good mat UV inks are rare. |
| 9.1. |
Relatively true. |
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| 10. |
With the UV inks it is impossible to print «double-faced decalers (adhesive visible on both sides of a window). |
| 10.1. |
False. |
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| 11. |
In four-color process, it is difficult to print the third or fourth color. |
| 11.1. |
False. |
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| 12. |
The old generation of UV inks lacks some suppleness. |
| 12.1. |
Relatively true, in general but, the new generations are perfectly supple and support vacuum forming. |
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| 13. |
It is possible to print the UV inks with any type of screen fabric. |
| 13.1. |
Absolutely false, if you want succeed in it. One of the secrets in printing the UV inks is to obtain the thinnest possible
deposit of ink. |
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| 14. |
The “regular” capillary films are perfect for the UV printing. |
| 14.1. |
False if thick films are used, with a few and very rare exceptions. Only one capillary film - CP from Autotype – a 6 µ film
with an excellent Rz value – is quite valuable, despite its price, for UV screen printing. |
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| 15. |
The adhesion to certain substrates is not good. |
| 15.1. |
This is practically not true anymore today. |
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| 16. |
It is impossible to print the thermo-sensitive substrates in UV. |
| 16.1. |
False. To blow cold and humid air (5°C - 41°F - and 70% of hygrometry) is the solution. |
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| 17. |
It is impossible to print the «micro-encapsulated» inks with the UV inks systems. |
| 17.1. |
False with the water based UV inks. |
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| 18. |
The UV inks are expensive. |
| 18.1. |
False. If we follow the too classic reasoning of too many printers, thinking in term of «short term profit», it is true that
the direct cost price of one liter of UV ink is obviously more expensive than for a liter of a solvent based ink, OK?! Even
the direct cost is not that high if we take into account an ink mileage that can reach up to 70 m2/liter (for an ink and
140 m2/liter for a clear. In addition, we have the savings on the cleaning solvents, the polyvalence of these inks - just
one ink on many different substrates. Also, and because they do not contain any solvent, a «flash point» of 94°C
instead of 42°C with the solvent based inks – permits to significantly reduce the costs of the insurance policies covering
the company! And of course, the gains of productivity, space, energy, etc. already mentioned. And finally, a «plus»
directly related to the new laws in Europe and other parts of the world, as far as the health and protection of the
environment that forbid more and more the emission of VOCs inside and outside the printing plant. |
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Important fact: the dangers of “over-curing” (over-polymerization)
Most people have a poor knowledge of the existence of important factors like the «secondary polymerization and the postpolymerization».
| Rule number 1 : |
the deposit of ink should be thin in order to be polymerized at a minimal level of energy. |
| Rule number 2 : |
the polymerization should be light in order to make the ink hard on surface but soft in under-coat. |
| Rule number 3 : |
the factors determining the ink deposit are:
- for 50%, the fabric of the screen,
- for 20%, the stretching and stenciling techniques,
- for the remaining 30%, the rheological characteristics of the ink, the type of press, the hardness, the angle
of pressure of the squeegee blade, the fineness, the angle and the pressure of the flood bar. |
Today, and still for a few years, the UV or water based UV systems are the only practical and economic alternative being reliable
and ecological that is offered to screenprinting. |
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Physics and Economic Theory of UV Printing in Screenprting
The pressure from the measures of protection of the environment inside and outside the plant (health, safety and environment)
has become extremely strong. The solvent-based systems can be used of course but it is now, in many countries, absolutely
forbidden to reject (VOC emissions), any type of solvent inside or outside the plant...
Eliminating these emissions of solvent (by burning, catchment, filtering, recycling, etc.) is a very expensive procedure. Switching
to UV inks seems to be the best reasonable solution. True for digital solvent and so called eco-solvents ink-jet printing.
Is it Saft to Invest in UV or Not?
This choice implies different options :
- economic,
- technical,
- related to screen- and digital printing
The problems to examine are first, to know :
- what the UV are,
- why you should or not choose this technique,
- what their choice would imply in a screenprinting plant, as far as :
- the equipment
- the technology
- the training of the staff.
There is a need of knowledge of all practical problems, and also some relatively minimal modifications - but still fundamental - of
the basic technique of screenprinting. A certain theoretical knowledge of the UV is quite recommended.
UV Printing - AN Ultraviolet Polymerization, What is it?
In screenprinting - as in digital - this process implies the use of an ink of special formulation that polymerizes only in the presence
of a source of ultraviolet radiations.
The Relation Between the UV ink and the UV Curing System is Total and Absolutely Necessary.
The principle of “drying” is just and only “a reaction of polymerization”. But it is question of a very particular type of reaction: a
phenomenon of photo-catalysis or photo-polymerization. I will then talk about “reaction of polymerization or curing” and “curing
units”.
Some Theory on UV
So, What are these UV Radiations?
The UV radiations are, just like the visible light or infrared, one of the forms of energy of the electro-magnetic spectrum : exactly
the “ultraviolet” (from 0.002 to 0.04 microns.
The “useful” sector is in fact the one between 200 and 400 Nm, the “near UV”.
- from 105 to 200 Nm medium UV,
- from 200 to 400 Nm near UV,
- from 400 to 700 Nm visible light,
- from 700 to 5.000 Nm infrared.
The “useful wave lengths”, are situated between 200 and 270 Nm.
The near UV is the “useful” part. |
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