Aluminium lithographic offset printing plates are a widely used type of printing plate in offset lithography. Using an aluminum-based plate as the core, they achieve selective ink adsorption and efficient printing through surface treatment and photosensitive technology.

Structure and materials of lithographic offset printing plates:

The substrate of aluminum offset lithographic printing plates is typically an electrolytic aluminum oxide plate with a thickness of 0.15-0.3 mm. The aluminum plate surface is anodized to form a microporous sand-like structure with a roughness Ra value of 0.5-1.5 μm. This sand-like structure helps retain water and control ink, giving non-image areas hydrophilic and oleophobic properties. A diazo resin or photopolymer photosensitive layer is also coated on the sand-like surface, with a coating amount of approximately 1-3 g/㎡.

Working principle of aluminum lithographic offset printing plates:

Aluminum offset lithographic printing plates belong to the category of offset printing, where the image and non-image areas are almost on the same plane. During printing, the principle of oil-water repulsion is utilized:

Image areas: After surface treatment, they become oleophilic, absorbing ink.

Non-image areas: Through hydrophilic treatment (such as an alumina layer or coating with a hydrophilic colloid), they absorb dampening solution and repel ink.

The ink is indirectly transferred to the substrate (such as paper) via a blanket, achieving high-precision printing.

Core characteristics of lithographic offset printing plates:

1. Material advantages:

Lightweight and durable: Aluminum plates have low density and high mechanical strength, suitable for long-term operation on high-speed printing presses.

Surface stability: After anodizing or electrolytic sandblasting, the aluminum substrate forms a uniform microporous structure (sandblasting), enhancing adsorption capacity and ensuring stable performance of both image and non-image areas.

Corrosion resistance: The aluminum oxide layer (Al₂O₃) has stable chemical properties, reducing erosion from dampening solution or ink components and extending plate life.

2. Printing performance:

High resolution: The sandblasting structure ensures clear image edges, suitable for fine printing (such as color images and text).

High print durability: Aluminum plates exhibit superior abrasion resistance compared to traditional lithography or zinc plates, allowing for single-plate printing yields ranging from tens of thousands to hundreds of thousands of impressions.

High adaptability: Compatible with both sheet-fed and web-based offset printing presses, meeting diverse printing scale requirements.

Plate making process flow of offset lithographic printing plates:

1. Plate pretreatment:

Grinding: Microporous structures are formed on the aluminum plate surface through mechanical grinding, electrolytic sandblasting, or sandblasting, increasing the specific surface area and enhancing adsorption capacity.

Anodizing: An aluminum oxide layer (Al₂O₃) is formed on the aluminum plate surface, improving hardness and hydrophilicity, providing a stable foundation for subsequent photosensitive layer coating.

2. Photosensitive layer coating:

A photosensitive emulsion (such as diazonaphthoquinone sulfonate) is applied to form a uniform photosensitive film for recording image and text information.

3. Exposure and development:

Exposure: The printing plate and film are brought into close contact, and ultraviolet light is used to harden (positive plate) or dissolve (negative plate).

Developing: Unexposed areas are removed using a chemical solution, exposing the hydrophilic aluminum base or image substrate layer.

4. Post-processing:

Coating with a hydrophilic colloid: A substance such as gum arabic is coated onto the blank areas to enhance hydrophilicity and prevent ink adhesion.

Baking: Some printing plates are baked at high temperatures (e.g., 230℃, 5-8 minutes) to improve the hardness and abrasion resistance of the photosensitive layer.

Applications of lithographic offset printing plates:

Commercial printing:

High-volume printing of newspapers, magazines, books, etc., relying on their high durability and low cost.

Packaging printing:

Printing of packaging boxes for food, pharmaceuticals, cosmetics, etc., requiring high resolution and color reproduction.

Advertising printing:

Posters, flyers, etc., utilizing aluminum plates to adapt to different paper types and printing speeds.

Aluminium lithographic offset printing plates have a wide range of applications. Traditional PS plates are relatively inexpensive and suitable for medium to long print runs. With the development of computer-to-plate (CTP) technology, aluminum-based CTP plates have become the mainstream in the market. For example, thermal CTP plates are suitable for long-run printing, while violet laser CTP plates are suitable for commercial quick printing and newspaper printing.