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.