To achieve a good print result, particular attention needs to be paid to the most important parameters in rotary screen printing. The key factors influencing this printing method are the type of screen plate, the ink, the squeegee and the flow path. The article in this edition looks at the squeegee, because this is the parameter that press operators are best able to influence themselves in screen printing. The important requirements for the squeegee are to ensure the rubber is of the appropriate hardness, to print with the best possible squeegee edge and to find the optimum squeegee position.
Selecting the correct squeegee rubber
In most cases, the squeegee rubber is made of polyurethane (PU, DIN abbreviation: PUR). Polyurethanes are plastics or synthetic resins that, depending on the manufacturing method, can be hard and brittle but also soft and elastic. These different PU properties are reflected in the different hardness grades of the squeegee rubber, which are indicated using the Shore system. Squeegee rubbers with three different Shore hardness grades – 55-60, 65-70 and 70-75 – are used for rotary screen printing. The lower the Shore figure, the softer the squeegee rubber. The individual hardness grades are also identified by different colours. Shore 55-60 rubbers are yellow or white, Shore 65-70 rubbers are red and Shore 70-75 rubbers are green.
In principle, more ink is transferred by a soft squeegee rubber than a hard one, but a harder rubber is more suitable for text. The screen acts less abrasively on a hard squeegee rubber and it is therefore possible to use it for longer than a soft squeegee rubber before regrinding. The squeegee pressure should always be kept as high as necessary and as low as possible.
The different characteristics of the individual squeegee rubbers are shown in the table below.
A sharp edge delivers successful results
Sharp-edged squeegees are a basic requirement for good print results. With a well-defined and coplanar print edge, the screen printing ink is pressed through the screen mesh openings. If part of the squeegee edge is damaged or worn, it is no longer possible to achieve a uniform pressure across the entire width of the squeegee. In other words, the ink is no longer pressed evenly through the mesh, which ultimately leads to flaws in the print image.
The printing edge can be reground manually or using a squeegee grinder (Gallus ID No. 189300). The advantage of the latter is that it removes the rubber evenly thanks to automated and standardised grinding across the entire squeegee width. This ensures reproducible finishing of the squeegee edge, which means that the screen printing squeegee always has the same predefined sharp edge after grinding – i.e. it is always of the same quality. This standardises a factor that can affect the print image, minimises press downtimes resulting from printing accessories not being prepared properly and makes it easier to establish the cause of flaws in the print image.
Figure: Gallus squeegee grinder (ID No. 189300)
Support length regulates the squeegee's stiffness
The squeegee's hardness is not only influenced by the actual rubber but also by the support length (= distance from front edge of squeegee to support plate). The longer this length, the softer the edge, which in turn influences the thickness of the layer of ink applied. If the distance is too short, this leads to printing gaps and if it is too long, this results in smearing. Normally, the distance between squeegee and support should lie between 4 and 6 mm as long as the squeegee is prepared properly.
Correct squeegee position for a clean print and high print speed
The squeegee's position also affects the print image. The standard position of squeegee holders on all Gallus presses 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 as to run centrally in relation to the impression cylinder, as otherwise it is too far back or too far forward in the direction of web travel. To maximise print speeds, however, the squeegee is always positioned 1-3 mm against the direction of travel, depending on the machine system.
Squeegee too far back: smearing in the print image
If the squeegee is positioned too far back in the direction of travel of the self-adhesive web, pressure is exerted on the transferred print image too late. This results in ink smearing at the rear edge of the print. The ink that has not yet dried in the print image is smeared due to the delayed pressure of the squeegee. If this printing error occurs, the squeegee must be moved further forwards in the direction of web travel.
Squeegee too far forward: insufficient ink at the rear edge of the print
If the squeegee is positioned too far forward in the direction of web travel, pressure is exerted on the transferred print image prematurely. The result is insufficient ink at the rear edge of the print. The fact that the squeegee pressure is exerted further towards the front of the print image means that the whole of the ink is not pressed through all screen mesh openings onto the substrate. If this printing error occurs, the squeegee must be moved further back in the direction of web travel.
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