How surface tension and UV light intensity affects printing on packaging material with UV curable inks

How surface tension and UV light intensity affects printing on packaging material with UV curable inks?

Buskro Ltd

August 21st 2023?

UV Curable Inks are a liquid ink formulation consisting of monomers, oligomers and a small percentage of photo-initiators, which when exposed to high-intensity ultraviolet light, initiates a photochemical reaction that cures (harden) and causes the ink to adhere to most printing substrates.

Although UV-curable inks are versatile and are often touted as being compatible with virtually all printing substrates, it is important to note that the performance of such inks are dependent on both the i) the surface tension of the material and ii) intensity of the UV light source

A material’s surface tension is a function of its coating properties and the amount of time after producing it, which is to say that the material’s dyne level will often decrease over time. Material surface tension is measured and expressed in Dyne/cm. The general rule for the optimal application of UV inks onto materials is that the material’s dyne level exceeds the ink’s dyne level by at least 10 dyne/cm. With dyne level of a material decreasing over time, it is often the case that a material that exhibits good adhesion shortly after its manufacture may produce inadequate results a number of days afterwards as the material’s dyne level decreases.?

If there is an imbalance between the ink and the printed material’s surface tension, the adjacent ink drops deposited on the material may not flow freely and join giving the appearance of poor, “scratchy” print (lines in the printed image) in the best case or easily removable through abrasive contact in the worst case. Having poor print quality or ink removal after printing can lead to increased product waste, costly time spent reprinting and customer inconvenience & dissatisfaction.?

Glossy packaging materials including some plastic cards, UV coated, and aqueous-coated stocks are most susceptible to the aforementioned problems. Where a printing application requires a material with a challenging surface tension, a positive outcome may be achieved with the application of inline surface treatment prior to the printing process delivered by corona, plasma, or heat devices which act to increase surface tension.?

UV light intensity, or more importantly the amount of ink exposure to such light, determines the level of cure of the printed image. An insufficient amount of exposure results in a “wet” or partially cured image that may be susceptible to abrasion. It should be noted that the curing process occurs from the surface inwards and continues for some period (> 1hour) after exposure. Printed images which may appear to be insufficiently cured immediately after UV light exposure when exposed to aggressive scratching and/or abrasive mechanical elements, often are deemed to be acceptable by the time the printed material is handled for packaging. In cases where the printing application requires high-speed and/or high density (High DPI) images, it may be necessary to augment the exposure period by adding a second curing lamp to ensure full cure of the printed image with an acceptable scratch-resistance.

Things to Remember About Surface Tension and UV Light Intensity:

1.? ? Surface tension of glossy products may be insufficient, resulting in poor print and/or adhesion;

2.? ? Surface tension may adversely decrease over time;

3.? ? The curing lamp may not provide sufficient UV light exposure to fully cure images requiring high-speed and/or high DPI print;

4.? ? Printed images will continue to cure over time and such cure occurs from the surface inwards;

5.? ? ?Optimal surface tension range for the material is around 38-50 dynes/cm, but should not be higher than 54-60 dynes/cm;

6.? ? ?Surface treatment devices such as corona, plasma, or heat act to increase dyne levels and may be used to optimize surface tension prior to printing.