In order to obtain good printing results, special attention needs to be paid to the most important parameters in rotary screen printing.The key factors affecting this printing method are the type of screen plate, the ink, the squeegee and the flow path. The focus is around the squeegee, as this is the parameter that the printing operator is most capable of influencing himself in screen printing.
The squeegee supplier suggests that the important requirement for squeegees is to ensure that the rubber is of the right hardness, to print with the best squeegee edge and to find the best squeegee position.
The key factors affecting this printing method are the type of screen plate, the ink, the screen printing squeegee and the flow path. The focus is around the squeegee, as this is the parameter that the printing operator is most capable of influencing himself in screen printing.
In most cases, absorbent squeegee rubber is made of polyurethane (PU, DIN abbreviation: PUR). Polyurethanes are plastics or synthetic resins which, depending on the manufacturing method, can be hard and brittle, but can also be soft and flexible. These different PU properties are reflected in the different hardness grades of the squeegee rubber, which are expressed using the Shore system. Squeegee rubbers with three Shore hardness grades (55-60, 65-70 and 70-75) are used for rotary screen printing. The lower the Shore index, the softer the absorbent pick-up rubber. Each hardness grade is also marked by a different colour. Shore 55-60 rubber is yellow or white, Shore 65-70 rubber is red and Shore 70-75 rubber is green.
In principle, more ink is transferred with soft absorbent steak rubber than with hard absorbent steak rubber, but the harder rubber is more suitable for writing. Screens have less abrasive effect on hard absorbent pick-up rubber and can therefore be used for longer than soft absorbent pick-up rubber before regrinding. Squeegee pressure should always be maintained at the highest and lowest values necessary.
A sharp blotter is an essential requirement for good printing results. With a clear and coplanar printing edge, the screen printing ink is pressed through the screen holes. If part of the squeegee edge is damaged or worn, it is no longer possible to obtain uniform pressure over the entire width of the squeegee. In other words, the ink is no longer pressed evenly through the screen aperture, which ultimately leads to imperfections in the printed image.
The hardness of the squeegee is influenced not only by the actual rubber, but also by the support length (= distance from the front of the squeegee to the support plate). The longer this length is, the softer the edge, which in turn affects the thickness of the applied ink layer. If the distance is too short, this can lead to printing gaps; if it is too long, this can lead to smudging. As a rule, the distance between the suction pick and the support should be between 4 and 6 mm, provided that the pick is prepared correctly.
The position of the suction pick also affects the printed image. On all Gallus presses, the standard position of the suction skate support is at an angle of 30° or 60°. The squeegee is always ground at an angle of 90°, so that the defined settings can be applied at any time. The squeegee should be positioned so that it runs centrally in relation to the impression cylinder, otherwise the squeegee will be too far back or too far away in the direction of web travel. However, to maximise print speed, depending on the machine system, the suction pick is always placed 1-3 mm relative to the direction of travel.