In the screen printing process, the emulsion is a key material for making screens. Based on their form and method of use, they are primarily divided into two categories: photoresist and photosensitive film, from which various screen-making methods have evolved.

I. Photopolymer: Direct Coating to Form a Film

Photopolymer, also known as photosensitive emulsion, is a liquid photosensitive material commonly used in direct screen-making. The film-forming process involves directly coating the photopolymer onto the mesh, which then dries to form a photosensitive film layer. The main components of photopolymer include film formers, photosensitizers, and additives. Early photopolymer emulsions used gelatin as a base, but later evolved to utilize synthetic polymers such as polyvinyl alcohol, resulting in improved performance. The advantages of photopolymer emulsions include a simple process, lower cost, and the ability for the operator to control the film thickness according to specific requirements.

II. Photoresist Film: Pre-formed for Higher Precision

Photoresist film, also known as film or photoresist paper, is another widely used plate-making material. It consists of a transparent plastic film substrate pre-coated with a photosensitive emulsion of a specific thickness through a precision coating process, resulting in a uniform photosensitive layer and a smooth surface. Based on their applications and usage methods, photosensitive films are primarily classified into the following types:

Water-based film (capillary photosensitive film): A pre-sensitized photosensitive material with hydrophilic properties, hence commonly known as “water-based film.” It relies on capillary action to adhere the photosensitive film to a damp screen; after drying, exposure, and development, a screen is formed.

Indirect film: Exposure and development are first performed while the film is separated from the screen. After the image is formed, the film is transferred to the screen, and the backing is removed once the screen is complete. This method produces sharp image edges and offers high precision.

Hybrid Film: This method combines elements of both the direct and indirect methods. The photosensitive film is applied directly to the mesh, dried, and then the backing is peeled off before exposure and development. This approach balances operational convenience with high-quality film formation.

III. Direct and Indirect Methods: Fundamental Differences in Plate-Making

The use of plate-making films is closely linked to plate-making methods, which are primarily divided into two categories: the direct method and the indirect method.

Direct Method: A photosensitive emulsion is applied directly to the screen to form the plate. This method is simple to perform, and the plate adheres firmly to the screen, offering good print durability. However, the thickness of the emulsion layer is influenced by the operator’s technique, making it difficult to ensure uniformity and surface flatness.

Indirect Method: First, an image is exposed and developed on a photosensitive film, which is then transferred to the screen to create the printing plate. This method allows for the pre-production of highly precise images with sharp edges, unaffected by the screen’s color or weave pattern. Its drawbacks include lower adhesion between the plate and the screen, relatively limited print durability, and higher costs.

Photosensitive resins and photosensitive films each have their own strengths, and the direct and indirect methods each have their own advantages. In actual production, the appropriate plate-making materials and process routes must be selected based on factors such as graphic precision, print volume, and cost budget.