A printing problem characterized by the accumulation of paper fibers (called lint), bits of detached coating particles, or other debris (such as pickouts) on the printing plate or blanket, in either image or non-image areas, that affect print quality. Piling can result from a variety of causes in addition to basic surface debris, such as the use of tackier inks that overcome the paper's pick resistance, changes in the blanket's surface stickiness or increased blanket slippage, the chemical composition of the press's dampening system reacting with that of the paper, and various stresses imposed on a paper during feeding. Piling can be classified in a variety of ways, depending on the characteristics of the material. (See Whitening, Powdering, Milking, and Image Area Piling.) Piling not only affects print quality but can increase the rate of plate or blanket wear.
The term piling also refers to the accumulation on the plate or blanket of dry particles of ink. Ink piling commonly is caused by the inadequate viscosity of the ink vehicle, or an inadequate amount of vehicle, preventing the pigment from being transferred to the substrate with the vehicle and remaining behind on the plate or blanket. In some cases, an overly absorbent substrate will drain the vehicle away from the pigment while on the press, leaving the pigment to pile on the plate or blanket. In cases where the paper is to blame, the only solution is to use different paper. If the ink vehicle is inadequate, the addition of body gum may alleviate the problem, if there is no time for ink reformulation. Ink piling is also called caking.