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Twenty Questions and Answers about UV Curing and Related Concerns

(February 2009) posted on Wed Feb 18, 2009

Trying to adjust to the realities of working with UV screen-printing inks? Use this Q&A discussion to clear up any misunderstanding about the inks, the curing process, and other aspects of UV technology.

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By Bea Purcell

In screen printing, variables that can greatly affect density values are ink-film thickness (determined by mesh, tension, off-contact, and floodbar and squeegee settings), color, size and number of pigment particles in the ink, and the color of the substrate. Optical density combines the opacity and thickness of the ink film. Opacity is affected by the number and size of the pigment particles and their light-absorbing and light-scattering characteristics.

What is dyne level?

Dyne per centimeter is a unit of mea-surement used to quantify the amount of force created on a surface. The force is caused by the intermolecular attraction of a particular liquid (surface tension) or solid (surface energy). For the sake of practicality, this measurement is referred to as the dyne level.

The dyne level or surface energy of a particular substrate is an indication of the material’s wettability and ink adhesion (Figure 3). Surface energy is a physical property of a substance. Many films and other substrates used in the printing industry have very low dyne levels, such as polyethylene with 31 dynes/cm and polypropylene with 29 dynes/cm, and need to be treated.

Treatment raises the dyne level of a particular film, but only temporarily. So factors such as the amount and time since initial treatment, handling, storage, humidity, dust, and contamination will affect the dyne level when you’re ready to print. Since dyne levels change with time, most printers find it necessary to treat or retreat these films immediately before printing.

How does flame treatment work?

Plastics, by their very nature, are non-polar and have essentially inert surfaces (low surface energy). Flame treatment is one of several methods of pre-treating plastics in order to raise the dyne level of the surface (Figure 4). This method is commonly used in the bottle-printing industry but is also used in the automotive and film-converting industries. In addition to raising the surface energy, flame treatment also helps eliminate surface contamination.

In flame treatment, the high temperature effects changes on the surface of the substrate by oxidation and breaking down hydrocarbons (from propane or natural gas) to free radicals, ions, and other charged particles. This changes the polarity of the plastic’s surface, which enhances ink adhesion and wettability.

What is corona treatment?


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