Key Components and Processes in Tinplate Printing

Key Components and Processes in Tinplate Printing

The printing plate serves as the carrier for transferring ink to tinplate. In modern offset printing, the prevalent choice is the presensitized plate (PS plate), which is a plate with a precoated photosensitive emulsion.

Composition:

The printing plate consists of a base, an oxide layer, a hydrophilic layer, and a photosensitive emulsion layer.

Characteristics:

1. The base has a finely textured surface with good moisture retention.

2. The photosensitive layer exhibits high resolution and excellent dot reproduction.

3. It demonstrates good ink transfer capabilities, accurately reproducing the tonal gradations and colors of the original.

4. Exhibits good printing adaptability.

Exposure Principle:

During the exposure process, ultraviolet light passes through the transparent areas of the negative, causing the photosensitive agent in the emulsion layer to decompose. This results in exposed areas on the plate, forming the image.

Rubber Blanket

In sheet-fed offset printing, the rubber blanket plays a crucial role as an intermediate transfer medium. It is used for both image transfer and cushioning the impression cylinder, ensuring a smoother printing surface.

Rubber blankets can be ordinary or air-cushioned. Air-cushioned blankets have an additional air layer, providing positive compression during the pressure process, preventing dot distortion and double imaging. Currently, tinplate printing predominantly employs air-cushioned rubber blankets.

Technical Requirements:

1. Transferability: Indicates the ability of the rubber blanket to transfer ink, expressed as ink transfer rate.

Ink Transfer Rate = Transferred Ink Volume / Adsorbed Ink Volume × 100%

Higher ink transfer rates denote better transferability.

2. Resistance to Oil, Acids, Alkalis, and Solvents: Reflects the surface's resistance to external substances.

3. Elongation Rate: The percentage increase in length under tension, with lower elongation rates being preferable.

Elongation Rate = (Length under Stress - Original Length) / Original Length × 100%

4. Hardness: The ability to resist external substances being pressed into its surface, with higher hardness providing clearer dots.

5. Flatness: Generally, the error value should not exceed 0.04mm to ensure clear and full dots without compromising ink absorption.

Fountain Solution

Fountain solution, typically diluted from a pre-prepared concentrate, aims to balance and consolidate the blank areas of the plate, ensuring cleanliness and normal ink transfer.

Control Parameters:

1. pH: An acidic pH enhances wetting efficiency, making blank areas cleaner and solid areas more defined. pH is generally maintained between 4.5 and 5.5.

2. Conductivity: Reflects the solution's conductivity and indicates the mixing ratio of concentrate to water. Typical conductivity is around 1200.

3. Temperature: Stability at around 10°C ensures consistent wetting properties.

4. Concentration: Generally around 2%.

5. Alcohol Content: Introducing alcohols or surfactants into the fountain solution aids in reducing surface tension and improving wetting. Alcohol content is typically around 10%.

Auxiliary Materials such as Protective Gel and Cleaning Solution

Protective gel, mainly composed of water-soluble resins, safeguards the printing plate, preventing oxidation. It should be uniformly applied.

Cleaning solution, containing citric acid and surfactants, maintains plate cleanliness by swiftly removing stains, foam, and scratches. It prevents oxidation, and usage requires careful shaking before application. The solution should be used sparingly to avoid excessive usage.

Correction paste can remove excess image or dirty spots. When using correction fluid, precautions should be taken to prevent seepage into surrounding images, as the gas produced can damage adjacent graphics.