Based on the principle of how the image areas and non-image areas on a printing plate produce printed matter during the printing process, printing can be categorized into two types: **Physical Printing** and **Chemical Printing**.
**1. Physical Printing**
In physical printing, the ink on the image areas is purely an accumulation or carriage. The non-image areas are either recessed or raised; because their height differs from the image areas, they cannot hold ink, leaving them blank. Consequently, the ink on the image areas is transferred to the substrate solely through physical mechanical action. Generally, **Relief Printing (Letterpress)**, **Intaglio Printing (Gravure)**, **Porous Printing (Screen Printing)**, and **Dry Planographic Printing** all belong to physical printing (where the printing surface is either higher or lower than the non-printing surface).
**2. Chemical Printing**
In chemical printing, the fact that the non-image areas (non-printing surfaces) of the plate do not hold ink is not because these areas are recessed, raised, or blocked. Rather, it is due to chemical action that creates a film which is water-receptive and ink-repellent (hydrophilic and oleophobic).
Admittedly, the image areas (printing surfaces) are ink-receptive and water-repellent (oleophilic and hydrophobic), while the non-image areas are water-receptive and ink-repellent. The mutual repulsion between water and grease is technically a physical phenomenon. However, during the printing process, the water-receptive and ink-repellent film on the non-image areas must be constantly replenished; therefore, it is classified as chemical printing.
**Offset Lithographic Printing** belongs to this category. This is because gum and colloidal substances must be added to the fountain solution to continuously supply a mucous layer of carboxyl groups, ensuring that the non-printing surfaces of the plate are not contaminated by grease
