Most Papersmith and Son products will accept a UV varnish, but to obtain a good hold out and a gloss effect, the following recommendations are given:
Recommendations
To provide a good gloss image with UV varnishing it is recommended that the varnish is applied with a Screen Print application rather than a machine (litho) varnish. To counteract the varnish soaking into the uncoated stock, a two pass screen print application should be carried out, with the first pass using a viscous matt varnish and the second pass printing the gloss varnish on top of the cured matt (the varnish provides a good surface seal to hold the gloss onto the surface). Failure to carry out this procedure might result in the varnish travelling into the body of the sheet, resulting in a lack of gloss. Litho varnish sealing does not produce the same surface sealing effect.
Care should be taken with tight register work due to the heat emitted from the UV lamps and also when trying to print fine lines, as the first matt varnish can spread laterally more than the second gloss so there is a risk of a matt halo effect.
UV Inks for thermal Papers
Thermal papers have now become well established in the reel-fed label printing industry. At present, three grades are commercially available:
- Uncoated
- Top coated
- Semi-resistant
The uncoated grade of paper is widely used as fax paper and for supermarket pricing labels. This grade is not suitable for printing with UV inks.
The top-coated grade of thermal paper is the type used in letterpress of flexo-graphic label printing. These papers have special secondary top PVA coating applied, giving them product resistance to water, ice, fats and grease. These papers are also resistant to many solvents and plasticizers, but prolonged exposure to solvents like alcohol will adversely affect the coating and lead to the activation of the colour-reaction. Acid and alkaline exposure can also lead to adverse reactions. When using UV inks on these top coated grades, it is important to use an ink system, such as Intercolor’s Opticure, that has been formulated in recognition of these potential difficulties. The use of a specially designed UV varnish is also recommended.
As the name suggests, the semi-resistant grade is a ‘compromise material’ between the two other grades. It offers a definite price advantage, but the appearance of print and the general texture is poorer than obtained from top-coated thermal paper.
Greying or Flecking of thermal papers
Generally, thermal papers can be printed and converted with minimum problems. One of the most common problems that can occur is the greying or flecking of the printed area, when UV ink has also been UV varnished. Immediately after printing, the results are excellent, but after about 24 hours, a marked discolouration or patches can be seen. This problem only occurs when UV varnish is used in conjunction with UV ink, although the ink is deemed necessary. Too much varnish is thought to cause this. Although the top coating is resistant, the acrylic monomers used in UV inks and varnishes will dissolve the binder used in the initial dye-containing coating and activate localized colouration development. If the film of UV varnish is too thick, the curing may be incomplete and partial penetration of very small quantities of untreated monometer can cause very weak colour activation. The same can occur if the varnish film is not too thick, but inadequately cured.
The printer must ensure that the applied varnish film thickness is correct and that complete and thorough curing has been activated. There are occasions where it is desirable to protect the area to be imaged with a coating of varnish. Unlike many UV curing varnishes, it is possible to complete the thermal imaging through films of Intercolor’s Optiflex Thermal Varnish DL 0500.
Thermal imaging head life
The thermal imaging ‘head’ consists of a row of small pins with ceramic-coated heads. The pinheads can be individually super heated or cooled using microchip technology and the thermal paper is subjected to micro second pulses of heat (200C) at 2psi to activate colour development. During the imaging process, the pins can coat previously printed ink and varnish. If the ink and varnish films are not completely cured, minute transference to the pinhead can occur. The transference will gradually build up giving an insulating layer that increases the temperature to the point where the protective ceramic layer on the thermal printing head will eventually crack or splinter.
Poor ink adhesion will also cause transference to the thermal head. To improve ink adhesion to top coated thermal papers, pre-treatment by Corona Discharge is recommended. This process renders the PVA binder amorphous, thereby increasing its receptivity to coating. This in turn, leads to stronger interfacial bonding and less transfer to the thermal head.
Where no varnishing is taking place, an addition of 1 to 2% of slip compound added to the ink will optimize the co-efficient of friction and help to maintain head life.
A partially related matter is the use of thermal wax transfer imaging onto UV inks on conventional label papers. Many UV inks cannot readily accept overprinting with variable data by thermal wax transfer. Such information would include weights, prices, and sell-by data on food packaging. The latest Optiflex formulation has overcome this problem by providing a fully receptive surface, even when fully cured, to most wax ribbons. There is now no need to leave ink free areas of the design to ensure trouble-free overprinting.
The Optiflex system also benefits from high pigmentation, low viscosity and excellent flow, combining to give sharp dot reproduction and allow for the highest print quality to be achieved. Instant cure at high speeds (such as 150m/min) gives a film with good adhesion to a wide range of synthetic substrates whilst remaining highly receptive to overprinting. As with all aspects of label printing inks, Intercolour endeavours to confront technical problems that exist to provide solutions for their customers’.