Printable Forms: Types, Manufacturing, and Preparation
Printable Forms and Offset Printing Processes
A printable form is an element placed on a printing machine that makes copies of an original graphic area. The image area retains ink, while the non-image area (contragrafismo) retains water and repels ink.
Planographic Printing Methods
Planographic printing is a method where the image and non-image areas are on the same level. Common examples include:
- Lithography: Uses oil-based ink directly. Historically, lithographic forms were made of stone, but now zinc and aluminum are also used.
- Offset Lithography: Uses methods adaptable to planographic printing. The printing is indirect. Disadvantages include color variation along the roll, paper distortion due to moisture (causing registration problems), paper draft issues, and difficulty drying.
Manufacturing of Printing Plates
The plate preparation process involves graining, anodizing, and applying an emulsion.
Graining
Since aluminum has poor adhesion, the surface is treated to improve it:
- Dry-brushing: Steel brushes rotate on the aluminum layer, creating a coarse grain.
- Wet-sanding: The aluminum surface is treated with chemicals and a rotating brush.
- Electro-graining: The plate is introduced into an acid mixture, and an electric current is passed through it. This produces high print quality.
Anodizing
Anodizing improves water retention in non-image areas. Controlled oxidation is performed to adjust the dot. This provides increased resistance to rubbing and chemicals, making it suitable for high-volume runs.
Emulsion
A layer of radiation-sensitive emulsion is applied to the anodized layer. These emulsions are typically composed of diazo compounds (for positive plates) or polymers (for negative plates).
Types of Printing Plates
- Polyester Plates: Use a polyester base. They are exposed using films produced in a film recorder and are suitable for short runs and low resolution.
- Photopolymer Plates: The emulsion is made of polymers that change properties depending on light exposure. A drawback is the need for precise light control to prevent fogging.
- Silver Base Plates: Use silver halide emulsions as photosensitive materials. Suitable for medium and high-quality runs. A drawback is the cost of materials.
- Thermal Imaging Plates: Provide high quality (up to 200 lines per inch) and are suitable for very long runs. An advantage is that they can be handled in normal light.
Preparing the Plates (Positive Plates)
- Exposure: Through positive mounting, image areas receiving light become soluble, while non-image areas do not.
- Developing: The plate is placed in a developer bath to reveal the latent image formed during exposure.
- Fixing (Optional): Removes any residual ink receptivity in non-image areas. The diazo compound, sensitive to light, was protected by dark areas.
- Retouching: Unnecessary elements are eliminated from the plate surface.
- Gumming: Protects the hydrophilic areas during waiting periods.
- Baking: Increases the lifespan of the plates.
Features of Fixing
- Creates a protective layer.
- Strengthens olefin action.
- Protects the emulsion.
- Enhances contrast between image and non-image.
Waterless Offset Plates
- Exposure: Uses ultraviolet light with positive mounting. Areas exposed to light harden and stick to the silicone layer.
- Developing: Dissolves the silicone layer in unexposed areas.
- Washing: Removes residual chemicals.
- Touch-up: Uses a special concealer and solvent to remove unwanted elements.
Advantages of Waterless Offset: Less dot gain, no ink-water emulsion, greater color intensity.
Disadvantages of Waterless Offset: Requires special inks, temperature increase due to lack of water cooling, sensitive non-image area.