Exposure unit structure and type of light source in screen printing

In screen printing, the printing quality is mainly determined by the quality of the positive film and the quality of the template image obtained therefrom. One of the most important factors in order to obtain the best template image through a positive film is the light source, especially the type of light source and the quality of the emitted light. Although many different light sources can image the stencil, mastering the effects of high-intensity ultraviolet light (UV light) will help you make a reliable stencil and get a fine print.

The quality of the template is mainly based on four aspects: image resolution, image clarity, solvent resistance (water) resistance and abrasion resistance. Image resolution refers to the fineness of the image. At present, resolutions of 4-7 microns can be achieved on the template, much finer than the resolution we can print.

Image sharpness is closely related to resolution. It emphasizes the sharpness of edges that can distinguish details, so the definition is sometimes called sharpness. The longitudinal section of the template we want to obtain is sharp, like a steep peak, not a gentle valley. The template with poor resolution looks like it has been eroded, and it is softer. It is impossible to print sharp details with a soft-edged template.

Anti-solvent (water) properties are particularly stringent in the printing shop and recycling workshops. If the template does not have anti-solvent (water) properties, it will become soft and sticky during printing. Poor solvent resistance (water) stencils can lead to "sealing" and non-renewable. The anti-solvent (water) property is related to the type of resin in the solvent, but it is mainly related to the drying of the stencil and exposure conditions, and the quality of the UV light is a direct influencing factor.

Finally, the stencil must have abrasion resistance. If it is a short-lived version, there is nothing wrong with wear resistance. But if your screen is scrapped after printing 1200-5000 copies, its importance is shown. A properly processed direct-resin photomask shall have a print resistance of 75,000 impressions, with a fine mesh print, and an imprint resistance of more than 100,000 impressions.

In order to meet the requirements of the four factors above and obtain good template quality, we must have a better understanding and understanding of the light source. Here we will analyze the light source in detail.

1. Photographic Sensing The electromagnetic spectrum contains all wavelengths of light from gamma rays to radio waves. Between x-rays and infrared rays is ultraviolet and visible light. Their frequency is in nm, and 1 nm is one-billionth of a meter. The frequency represents the wavelength of light at this frequency.

The visible spectrum refers to the light from violet to red light, with a wavelength of 400-700 nm. The light outside the violet light in the visible spectrum is UV light, which is invisible.

The spectral band that causes a chemical reaction of the photosensitive material is called actinic light, for example, 365-440 nm light required for the diazo used in the emulsion and the photosensitive resin sensitizer. The most important actinic frequencies are 365, 380 and 420 nm.

The diazo sensitizer is most sensitive to 365 nm light. Sensitizers in liquid emulsions are sensitive to light in the 400-420 nm range, which is already part of the visible light range. Double hardened emulsions rely mainly on diazo sensitizers and also contain acrylics sensitive to light at 400-420 nm. The closer the light output frequency of the light source is to the photosensitive frequency of the emulsion, the shorter and more effective the exposure time. However, it should be remembered that only a small portion of visible light contributes to emulsion exposure.

Output UV light should pay attention to two important requirements: output consistency and output intensity. Output consistency is related to lamp life and voltage. Since the actinic light is essentially invisible, we cannot visually judge when the lamp is declining. Most light bulbs can be used continuously for 300-1000 hours, ie 2-6 months. For simple lack of detail work, the light bulb may have to last longer, but for the fine work, the shortest and most effective exposure must be used, and the lamp replacement frequency should be more frequent. If you often do random halftone images, change the bulbs every six months. The number of light sources turned on will slow down and reduce the service life. According to a lighting engineer, each time you turn on the power, it will take 4 hours of lamp life. The general principle of use is to open it when it is used, and to avoid frequent switching.

2. Output monitoring The life of the exposure lamp indicates the end of UV output validity. Therefore, the measurement and monitoring of the UV output is necessary for screen exposure. The intensity of the lamp is obtained by measuring the light on the surface of the screen, and during the screen exposure process, the energy concentration point per square centimeter on the screen is mainly measured. The higher the energy at this point, the shorter the exposure time during the printing and the more effective it is. The usual measurement unit is watts/cm2.

The closer the light source is to the surface of the emulsion, the higher the energy of the surface (not the closer the distance the better, too close will cause exposure "hot spots"). The most accurate energy measurement instrument is a UV radiometer. The use of such tools in the image field is very common, but screen printers rarely use radiometers. In the image area, the tool can also be used to measure lamp life in UV ink curing reactors. Without a radiometer, the best measurement instrument is a light integrator. The instrument functions to measure the predetermined unit light from the exposure light source and control the corresponding exposure time. Use must be calibrated so that one light unit is roughly equal to 1 second of exposure. This unit-second correspondence is very important to the lamp life, because the system will automatically increase the exposure time according to the corresponding relationship to compensate for the lack of bulb UV light output.

A sophisticated light integrator traces the illuminance of the lamp at each energy stage, including low energy, medium energy, and high energy until the lamp is rejected. You can preset so that the UV output will give an alarm below a certain value. Generally less than 50% of the initial value should be replaced with a new bulb. For example: Assuming that the new light bulb is 60 units per second, the bulb should be replaced when it takes 90 seconds to reach 60 units of exposure.

There are two situations to be aware of when using a light integrator. First, not all integrators are the same. The light sensor in the integrator must be sensitive to light at the wavelength you are measuring. This light refers to the actinic light used to expose the emulsion and stencil film. For example, if you use diazo emulsions, and you use an integrator that is sensitive to wavelengths of 420 nm, there is no use because diazo emulsions are most sensitive to 365 nm. You can check with the manufacturer to confirm that you have chosen the right sensor.

Another situation is the position of the integrator sensor. The best position is on the surface of the vacuum frame, which is where the light actually reaches the stencil. If the sensor is mounted on the head of the exposure unit, the distance between the light and the frame changes, affecting the amount of light reaching the emulsion.

The reduction in light energy is calculated as the inverse of the square. That is, intensity = 1/x2. X represents the multiple of the new distance to the original distance. For example, if the surface is 30 inches away from the exposure light source and 60 inches after the change, the power will be 1/4 of the original power if the magnification is 2. 20 inches will become 60 inches and the energy will be 1/9 of the original. This is very important, especially in halftone printing, because a 10% exposure deviation will greatly affect the restoration of the high-profile point.

The minimum distance of the vacuum frame is the diagonal length of the exposed area. If the vacuum frame is 4060 inches, its diagonal is 72 inches. But in more work, the diagonal method can not get satisfactory exposure time, it often extends the exposure time. To reduce the exposure time, the diagonal of the exposure image can be used as the distance. For example, the image itself is 3040 inches, and the diagonal and minimum distance is 50 inches. Reducing from 72 inches to 50 inches will reduce the exposure time to the original 52%.

If the sensor of the light integrator is installed on the head of the lamp, make sure that the exposure unit should be accurately fixed on the floor. It helps to fix the position and adjust the light to apply it at the same point. You can use chains, gauges or lines as the method of determining the correct point. The ultimate goal is to ensure that the distance is fixed and constant.

3. Output Light Source Type In order for the edge of the template to cure well, the light from the exposure unit should be emitted from a relatively small point. Therefore, the light source is often referred to as a point light source. Mercury lamps and metal halide lamps are the most typical point sources. They are filled with metallic mercury, which can be turned into a conductive gas by turning on the power. This gas creates an arc that emits the high-intensity UV light we need.

The output frequency control of the lamp is achieved by "passivating" the lamp. Mercury vapor has a very specific output spectrum, contains more actinic UV light, and also emits a significant fraction of the frequency of emulsion cross-linking. In order to optimize light output, metal halides or halogens, such as iron iodide and cesium iodide, have been added to mercury lamps. Metal halides cause output spectral shifts that match the emulsion photosensitivity, increase exposure efficiency, and reduce the number of light units used.

4. The consistency and strength of the correct light source UV light source is the guarantee of obtaining a high quality stencil. Regardless of your screen tension or the quality of the emulsion, it is not possible to get a good template without the right UV light source. From this it can be seen that controlling the exposure is critical to obtaining better print quality.

Reviewing the content of this article, and following the analysis of this article, I think your printing quality will miraculously improve.

Source: Bisheng Printing Professional Network