Explanation of the printing process that prevents ink and imprints from rubbing off.
Printing onto plastic is a very complicated process, and knowledge of chemical reactions is a requirement. The operator must understand the chemical properties of various types of plastics as well as the properties of different kinds of inks. If the wrong ink is used to print on a plastic object, then the image may flake or rub off. It is imperative to integrate the ink into the very substance of the plastic in order to prevent rub-off.
Properties of the plastic object must be determined prior to printing. The surface tension (the energy of the plastic) is first determined by testing the “dyne” level. On a scale of 1 to 100, a dyne level of 38 – 50 is necessary to successfully print onto plastic. If the dyne level is lower than 38, then the ink will peel off because it won’t fully dry. Problems with static electricity will occur if the dyne level is above 50, which will then cause problems putting the product through a printing press. There are measures a printer can take to make sure the ink adheres to the plastic and to reduce static, but adding those measures increases cost in materials and man hours.
When the correct ink and/or catalyst is applied to plastic, a chemical reaction occurs that transforms the ink--which is normally considered a liquid--to a solid film integrated into the substance of the plastic. This action describes a chemical reaction because one substance turns into another. This change can be one of color, of shape, or of other physical properties. The resultant printing should be resistant to solvents, degreasers, alcohol, soap, water, and other chemicals.
Although there are many ways to imprint items like plastic promotional products, the most common techniques are foil stamping, pad printing, and silkscreen printing. These processes all incorporate the inks into the substrate of the product and make the imprint virtually impossible to remove.
Screen-printing is most common printing process used on plastic materials, and it's ideal on flat surfaces. Pad printing, a similar process, is used on irregular surfaces. Inks must have solvent bases that will bond chemically to the plastics, and the solvents used will vary according to the chemical make-up of the product material.
Most of the printing processes require a lengthy drying or curing time. Ultra-violet light is often used to cure or dry the ink. Some inks require a bath in a fountain solution after printing and others require dryer additives in the ink. Application of heat, as well as infrared drying, will initiate the chemical chain reaction that will accelerate the absorption of ink into the product.
Foil stamping is a process that uses a thin foil material on a ribbon that includes the desired color and a heat-activated solvent; when a hot die contacts the ribbon, the color fuses to the product. This process generally employs metallic ink after printing.
Dye sublimation is another popular technique, but it requires a water-resistant overcoating. After printing and overcoating, the imprint is heat-transferred to the product. The use of the Epoxy Dome technique permanently transfers an epoxy-covered image to paper, and an adhesive backing is applied that enables the dome to attach to a product.
Printing processes are diverse and complicated, and the image becomes a non-removable part of the product when the correct chemical reaction occurs between the ink and the product surface.
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