Pad Printing Inks

Pad printing inks have very little in common with paint, screen printing inks or even other inks. The inks that we use today are designed specifically for the pad printing process. Even though a full spectrum of pad printing inks are available for printing on different surfaces, all pad printing inks share two common traits: they are solvent-based and they contain a special pigment density.

Pad Printing Ink Viscosity
Unlike other printing processes, ink viscosity plays a crucial role in the image transfer process. Most manufacturers recommend a ratio of between 15 and 20% of solvent to ink dependent on weight. It is the evaporation of the solvent that creates ink tackiness, this then allows the image transfer to occur. Without the creation of this tackiness, the transfer printing pad would be unable to function properly. The pad printing ink would then either be too wet or too dry, preventing proper image transfer onto the substrate.

Pad Printing Ink “Flashing” Process
The first stage of the pad printing cycle begins with the cliché immersed in the ink. As the cycle begins, the doctor blade or ink cup cleans the cliché surface and leaves the ink in the image exposed to the air. Upon exposure to the air, solvent in the top half of the image begins to “flash off” and becomes tacky. The transfer pad then compresses onto the cliché image and draws the ink from the etch. The portion of ink that was located at the bottom of the etch is now exposed to air for the first time. As this occurs, the solvent on the bottom half begins to evaporate and the tack process is complete. The image is then ready to transfer.

Pigment Opacity
The etch depth of a standard cliché is about 25 microns or the equivalent of .001 inches. This causes a layer of ink approximately .0008 of an inch thick to be transferred to the part to be pad printed on. To maintain good image opacity, the pigment in the ink must be small enough to avoid removal in the plate doctoring cycle.

Pad Printing Machine Considerations
Pad printing machines do not affect ink as much as they affect solvent evaporation. A typical open ink well pad printing machine will affect solvent evaporation differently than a machine that uses a sealed ink cup. The open inkwell machine will require more operator intervention to maintain ink viscosity because the solvent is continuously evaporating. In a closed cup system, the ink and solvent are encapsulated. This helps to slow down evaporation.

A pad printing machine with a faster cycle rate will not allow as much time for the solvent to evaporate as a machine with a slower cycle rate. This can result in incomplete image transfer due to the lack of time required to create the proper tack. To counter this phenomenon, solvents are available that provide different evaporation rates.

Single and Two Component Pad Printing Inks
Pad printing inks are available in two basic formats: single-component and two-component. When we speak of two-component inks we are always referring to hardener as the second component.

Single-component ink provides the benefit of extended pot life. Often, single-component ink is left in the machine overnight or in preparation for the next work shift. Although single-component inks are easier to use, they do not provide the same level of resistance to mechanical abrasion or solvents as their two-component counterparts.

Two-component ink is better suited to applications that require a specific performance or resistance. The addition of hardener allows the ink to resist mechanical abrasion and most solvents.

Pad Printing Ink Adhesion
Many inks are specially formulated to allow adhesion to different product surfaces. The ink used to print on polycarbonate may be different from one that is used to print on styrene.

The ability of the ink to adhere to the substrate is based on two factors: the ability of the ink to cross-link with the intended substrate and surface tension. When an ink is properly matched to a substrate, a chemical cross-link occurs that promotes adhesion.

Surface tension, measured in dynes per centimeter, can be described as the “wettability” of a substrate surface. Materials such as polyolefins that have low surface tension do not allow the chemical cross-link to occur. In these instances, pretreatment is required, thereby adding cost to the decorating operation in terms of equipment and labor.