Screen exposure control

For those who work in the screen production workshop all day, the operation of screen exposure is familiar. All of them are experienced and able to handle most practical problems and are qualified plate makers. However, it is still necessary to do a cliche about the screen exposure, which may help the novice.

For any plate maker, the right exposure is the top priority in optimizing the performance of the template. The only way to ensure correct exposure is to master some key parameters in the platemaking process and the factors that affect them in the exposure process. The following describes the parameters and their influencing factors in detail.

Whether it is a direct emulsion or a photo-sensitive dry film, exposure first considers the most basic parameters, and then considers how best to perform exposure control for the chemical formulation of the stencil material. Finally, formulate some practical plans that are suitable for daily operations and ensure the quality of production.

The main source of exposure is the light source. Exposure light sources used in plate making can emit a wide spectral range, output a variety of different light intensities, and have different light transmission geometries. Although these parameters are invisible to the naked eye, they have an important influence on the performance of the template, such as resolution and print durability. Each stencil material, whether it is a direct emulsion or a finished photographic dry film, essentially contains a photosensitizer sensitive to a specific spectrum. The correct exposure requires the correct spectrum, which is the correct light source. So the first thing to discuss is the light source. Discussing the light source is to discuss the effective spectral range emitted by the light source.

Only a small fraction of the rated output energy of the light source is converted to the correct wavelength for hardening the stencil. The useful portion of the output energy is called actinic light, including the wavelengths of blue, violet, and ultraviolet light. Commonly used metal halide, multi-spectrum and a certain fluorescent tubes produce spectra that are rich in actinic light. Other types of light sources cannot be used to make high-quality stencils.

There are two types of photosensitizing chemicals contained in direct photographic templates, one being diazo and the other being double nitrogen. These two types of substances can be used together because both of these chemicals contain the basic substance diazo sensitizer that determines the exposure time. In addition, there is a substance called single-component photopolymerization emulsion. The so-called single-component photopolymerization emulsion and photo-sensitive film use "SBQ (dimethyl benzene-enyl vinyl methoxide)" photosensitizer, which has a specific gravity of nitrogen. Photosensitizers react fast. The "SBQ" photosensitizer is not described in too much detail here.

Figure 2 shows the spectral output of a metal halide exposure light source with two curves superimposed, one representing the diazo absorption curve and the other representing the diazo sensitivity curve. These light-sensitive curves refer to the wavelength of light that causes the photosensitizer to react. The peak of the absorption curve is located at 373 nm, in the range of UV light, indicating the diazonium salt template-insensitive region emitted by a particular light source.




Note that the light absorption intensity of the absorption peak at the tail of the absorption curve is small, but there is more penetration and the light sensitivity curve reaches the peak. It is here that the metal halide lamp with a wavelength of 390-420 nanometers reaches the peak position. Obviously, the metal metal halogen lamp is the best choice for the diazonium salt or dual nitrogen salt template.




Figure 3 shows a comparison when the photopolymerizable emulsion is exposed to a multispectral light source. Here, the absorption peak is located at 342 nm. The light-sensitive peaks have moved to the 360-390 nm range, which is a region where the multi-spectral light source has a strong light output. This makes multispectral light sources the optimal exposure option for photographic emulsion emulsion stencils.

Due to the different photosensitivity wavelengths of the diazonium salt and the photopolymerization stencil material, their relative exposure speeds differ. For example, assuming that the photopolymerizable emulsion is 20% of the exposure time required for a diazonium salt or dual nitrogen salt product, the exposure time is diazonium salt or double nitrogen due to poor sensitivity of the photopolymerized emulsion to the metal halide lamp. Salt products require 50% of the exposure time. Exposure to blue fluorescent tubes at 420 nm, due to poor light absorption, most light passes directly, so it takes longer, which is 75% of the exposure time required for diazonium salts or dual nitrogen salt products.

In judging whether the exposure is correct or not, most people use the resolution as the standard. In fact, the resolution cannot be used as the criterion for judging whether the exposure is correct or not. "Detailed" doesn't necessarily mean overexposed. Not enough resolution does not necessarily mean that the template is completely hardened. Image resolution is affected by many factors, not just one factor. For example, a gap or poor vacuum on the exposed cloth may affect the resolution. Similarly, mismatched bonds may also have an impact on exposure. For example, if the photopolymer emulsion is applied to a white screen, fluorescent tube exposure should not be used.

In the process of exposure, choose the most effective exposure light source and control the correct exposure time. If the exposure equipment is intact, it will ensure the correctness of the exposure.