Lithographic Ink Problems
Lithographic printing problems are frequently interaction problems, stemming from deficiencies among paper, ink, press, and operational factors. A press adjustment might offer a temporary solution, only for the issue to recur later or manifest as a seemingly different problem.
Even when ink is not the primary cause, modifying it is often more practical than changing the paper or the form to be printed. For instance, a picking problem on marginally suitable paper, caused by incorrect ink film thickness, is often addressed by softening the ink rather than replacing the stock or reducing print solids.
The majority of ink-related issues typically result from two factors: (1) ink that is incompatible with the paper, and (2) excessive acidity or dampening. These conditions destroy or reduce the ink's drier, and excessive acidity can lead to tinting and plate blinding.
Ink appearing in the nonimage area encompasses a variety of issues, some easily confused (see Figure 5-15).
Dot growth, slur, and doubling increase the printed area on paper and alter color in process printing. Except for dot growth from excessive ink or overly soft ink, these are not typically classified as ink problems.
Catch-up, tinting, toning, and scumming are ink/plate/dampening problems often correctable on press. Catch-up, or dry-up, describes ink in the nonimage area due to either insufficient plate dampening or an excessively thick ink film relative to dampening levels. Tinting or toning results from ink emulsified in the fountain solution, while scumming is a permanent image—usually spots—in the nonimage area. The wet-thumb test helps distinguish these: the operator stops the press, applies a dampened finger to the plate's nonimage area, and observes the ink's behavior.
Scumming can originate from multiple sources simultaneously, including ink. Potential causes include film and film processing, plates and plate processing, dampening solution, lighting, press adjustment, paper, or ink.
Ink can induce scumming for several reasons. Adding oleic acid to ink can create a scummy plate. Uncontrolled changes in pigment or resin treatment may also cause issues. Improper grinding that leaves grit in the ink accelerates plate wear, leading to scumming and/or plate blinding.
Piling in image areas refers to the accumulation of ink pigment or paper coating on the blanket or plate. The primary cause is ink shortness resulting from emulsification with water and dilution with paper dust, pigment, or other debris.
An emulsified ink becomes short. Poorly ground or gritty ink also tends to be short. The addition of pigment or dust from the paper exacerbates the issue. Water mixes into the ink at the form rollers, while paper dust and debris are added at the blanket, where piling becomes most severe.
The blanket plays a critical role in piling alongside ink and paper. A very smooth blanket surface forms a tight bond with the paper, pulling dust and pigment from its surface. Switching to a rougher blanket may help, but adding a nonpiling additive (a glycol or wax emulsion) to the dampening solution can also improve blanket release and resolve piling. These additives should be sourced from printers' suppliers.
Piling in nonimage areas, known as dusting or milking, appears as white specks accumulating on the blanket's nonimage areas. It occurs when the paper surface or coating dissolves in the dampening solution and worsens as the sheet travels through the press (see Figure 5-16).
Linting, the removal of lint from uncoated paper surfaces, is generally a paper problem but can often be mitigated by ink manipulation. Reducing ink tack and increasing dampening solution flow are two effective methods to decrease linting.
Trapping variations cause color inconsistency in lithography. The causes differ significantly between trapping on wet ink versus dry ink. Wet trapping is a wet ink film's ability to accept another wet film printed over it, while dry trapping is a dry film's ability to accept a wet film.
Automation and data-driven press controls help mitigate traditional ink-paper interactions. Furthermore, a sustained focus on sustainable practices influences the development of inks and substrates, aiming to reduce environmental impact while maintaining print quality.