Screen printing and flexo printing machines for LCD manufacturing.
LCD Equipment
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In LCD manufacturing, several printing and dispensing technologies are used in order to dispense various layers and seals. Dispensing is mainly used for low throughput and variable design displays. Printing technologies are used for high precision and high volume production cycles.
Screen printing is used for seal printing and thick layer printing. Flexo-Printing is used for polyimide layer printing and printing of thin layers with a high precision.
The screen frame is the most fundamental element of the screen-printing process. The function of the screen frame is to providing a means for mounting the mesh, withstanding the force of the tensioned mesh, standing up to additional forces applied during printing, remaining flat and square and being light enough to handle easily.
Once tension is applied to the mesh and the mesh is affixed to the frame, the flatness of the frame becomes critical. The frame's job is to resist the forces of that tension. A suitable frame will remain flat and square for the time of use. Among other things, distortion of the frame causes mesh threads to follow the same, curved direction as the beams, resulting in mesh distortion at the edges of the screen. This drastically reduces the screen's "sweet spot" or maximum print area. It also leads to tension inconsistencies that further complicate your ability to achieve good registration from colour to colour.
The size of the screen frame is important for the max. resolution of the print. The image-to-frame ratio is critical for avoiding image distortion. This term refers to the size of the image area relative to the total screen area, which is usually represented by the frame's inside dimensions. The less a screen is deflected during the print stroke, the lower is the force against the mesh and the lower the possibility of distorting the printed image. Squeegee length and stroke length also figures into the relationship between image size and total mesh area.
Mesh selection is a series of compromises. A coarse mesh will allow the printing material to pass through the screen easily. This will result in thick layers, fast prints and low resolution. A fine mesh will create difficulties in printing a bright design on dark-coloured substrates and it is difficult to press viscose material through the screen. However you can achieve very good resolution using fine mesh screens. This is the most important point in LCD manufacturing, where silk screens are used for printing. High resolution silk screens have more than 130 meshes per cm, enabling them to print lines with a width down to 50µm. The layer thickness is in the range of 10µm normally, which is regarded as a "thick" layer in LCD manufacturing.
Squeegees for seal printing are made from a stiff material like hard polyurethane with a hardness of up to 80 durometer. The durometer of a squeegee is the measure of its hardness, and a guide to the blade's ability to resist bending during printing. Squeegee durometers are measured on the Shore A scale, an industrial standard of 1-100 used to indicate the hardness of rubbery materials. The higher the durometer, the greater the blade rigidity. The lower the durometer, the more the blade will flex during printing. Harder squeegees (for example, an 80 durometer) are much less forgiving and will not print an even layer of ink on an uneven surface. However, these higher durometers are required to stand up to the high pressure needed to print at high speed or with high-opacity, high-viscosity materials on glass plates.
Beside the hardness, the shape of the squeegee and the edge profile play an important role too. Various shapes are available. This shape also determines the squeegee's adaptability. In a sense, profile fine tunes this adaptability and limits the amount of force transmitted to the printing surface. The profile affects the relationship between the set squeegee angle and the effective squeegee angle relative to the screen in the direction of the print stroke. For seal printing squeegees with a square shape are used. The squeegee edge must be sharp, but also free of nicks and waves. Squeegee sharpeners can remove a thin layer from an 80- or 90-durometer squeegee blade and restore a perfect edge.
The angle of the squeegee to the mesh defines the amount of material printed and the forces necessary for the print. Pulling the squeegee at 15-20º off the vertical ensures an even deposit of material. The setting of the exact angle depends in part on the squeegee's profile and durometer.
a printing table, b LCD glass plate, c dispenser, d doctor blade, e anilox role, f resin letterpress, g printing role
