UV丝网印刷油墨问题与解答（英文）（三）

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3、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 . This methd 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.

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14、What is corona treatment?
Corona discharge is another method for treating plastics to increase dyne level. Ionized air is created by running high voltage through a dielectric covered roll. The substrate is passed through the ionized space, which causes molecular bonds to break on the material’s surface. Treatment is affected by the air gap distance to the part and the speed of the material as it passes through the system. This method of treatment generally takes just fractions of a second. Corona treatment is commonly used on thin films for web printing applications.

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15、How do plasticizers affect ink adhesion on PVC?
Plasticizers are chemicals that are added to plastics during processing to make the finished material soft and flexible. Plasticizers are especially prevalent in polyvinyl chloride (PVC). The type and amount of plasticizer added to flexible PVC or any plastic material depend on the mechanical, thermal, and electrical properties expected from the plastic.
Plasticizers have the tendency to migrate to the surface and affect ink adhesion. Plasticizers on the surface are contaminants that have the ability to lower the surface energy of the surface. The more contamination, the lower the surface energy and greater the risk of poor ink adhesion. Substrates affected by this problem can be cleaned with a mild solvent before production to improve their printability.

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16、How many lamps do I need for curing?
Depending on the ink system and substrates to be printed, a one lamp curing system may be sufficient. However, if the budget allows, a two-lamp curing unit will provide the benefit of a faster cure, especially with slower-curing colors, and it will allow for faster overall production rates. Two lamps are always better than one because a two-lamp system would provide more millijoules or total energy at the same conveyor speed and lamp settings. The main factor to be taken into consideration is whether the curing unit will be efficient enough to cure the inks on any substrate at reasonable production speeds.

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17、How does the viscosity of the ink affect printability?
Most inks are thixotropic. This means that the viscosity changes with applied shear (stirring, floodbar and squeegee action), time, and temperature. Viscosity also decreases with increase in the rate of shear (e.g., speed of floodbar or squeegee). When the temperature gets warmer, the viscosity becomes lower as well. Screen printing inks are formulated to print well on press. However, depending on the press set up and prepress modifications to the ink, printability issues may occur. The viscosity of the ink on press will not be the same as it was in the container.
Ink manufacturers determine a specific range of acceptable viscosities for a particular product. Viscosity measurement is controlled within a specific range of shear force, time, and temperature. With thinner or low-viscosity inks, thickening agents may be added. For thicker or high-viscosity inks, thinners may be added. Contact your ink supplier for specific products and processing information.

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What affects the stability or shelf life of UV inks?
One factor that influences stability is ink storage. UV inks normally are packaged in plastic containers instead of metal because plastic containers are permeable to oxygen. Also, an air gap is desirable between the surface of the ink and the container lid. This air gap—specifically, the oxygen in the air—helps in minimizing any premature cross linking within the ink. In addition to the packaging, the temperature at which ink containers are stored plays a big part in maintaining their stability. High temperatures will cause the ink to react and cross link prematurely.
Modifications made to the original ink formula may also affect shelf stability if the remaining ink is stored. Additives, especially catalysts and photoinitiators, may lead to more limited shelf life. Finally, exposure to stray UV light from windows and lighting when a container is left open can affect stability.

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19、What is the difference between in-mold labeling (IML) and in-mold decorating (IMD)?
The basic principles behind IML and IMD are the same. A decorated film is placed into an injection mold and then melted plastic resin is injected to form a part with specific geometry such as an ice cream container or cellular phone cover (Figure 5). In IML, the labels may be printed using various printing technologies such as gravure, offset, flexography, or rotary or flat screen printing. These labels are generally printed on the top surface so the unprinted side would come in contact with the injected resin. The injected resin would normally be the same type of plastic as the label material or a compatible substitute.
In IMD, which is used to create more durable parts, the printing generally is done on the second surface of transparent films. Screen printing is the printing technology of choice for these applications. Compatibility of the film and a UV ink to the injection resin is a must. In this sector, IMD typically means in-mold or insert-mold decorating.

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20、What happens if I use a nitrogen curing unit to cure colored UV inks?
Curing systems that cure prints in a ni-trogen atmosphere have been around for decades. These systems are mainly used for texturing and curing clearcoats printed on the top surface of membrane-switch overlays. The main purpose of the nitrogen is to replace oxygen because oxygen is a cure inhibitor. However, due to the very low irradiance from the bulbs in the nitrogen chamber, pigmented or colored inks may not cure very well in these systems.
Most nitrogen systems still being used have a texturing or germicidal lamp installed before the nitrogen chamber. As its name implies, the main function of the texturing bulb is to create a texture. Clear UV inks used for this application are formulated with photoinitiators that absorb in the shorter wavelengths. When these clears are exposed to the texturing lamp, the surface cures faster than lower levels of the ink film, which causes the ink surface to wrinkle and become textured. Depending on the chemistry, colored UV inks may also texture when they’re exposed to a texturing lamp.