Influencing factors in rotary screen printing – the squeegee

Influencing factors in rotary screen printing – the squeegee

07.03.2016

To achieve a good printing result, it is important to pay particular attention to the most important factors influencing rotary screen printing. The decisive factors in this printing process are the type of screen plate, the ink, the squeegee, and the travel distance. Today's article will focus on the squeegee, as this is the variable that the printer can best influence in screen printing. When using a squeegee, it is important to ensure that the squeegee rubber is of the right hardness, to print with the best possible squeegee edge and to find the optimum squeegee position.

Choosing the right squeegee blade

The squeegee rubber is usually made of polyurethane (PU, DIN abbreviation: PUR). Polyurethanes are plastics or synthetic resins that can be hard and brittle or soft and elastic, depending on how they are manufactured. These different properties of PU are reflected in the various degrees of hardness of the squeegee rubber, which are specified in Shore. In practice, squeegee rubbers with 3 different Shore grades are used for rotary screen printing: 55-60, 65-70, and 70-75 Shore. The lower the Shore number, the softer the squeegee rubber. The individual hardness grades can also be distinguished visually: yellow or white (Shore 55-60), red (Shore 65-70) and green (Shore 70-75).

As a general rule, a soft squeegee blade achieves greater ink transfer than a hard one. A harder squeegee blade, on the other hand, is more suitable for lettering and text. The screen has less of an abrasive effect on a hard squeegee rubber, which means that it can be used for longer than a soft squeegee rubber without needing to be reground. The squeegee pressure should always be kept as high as necessary and as low as possible.

The different characteristics of the individual squeegee blades can be summarized as follows:

The sharp squeegee edge guarantees success

Sharp squeegee edges are essential for good printing results. With a defined and flat printing edge, the screen printing ink is pressed through the open screen mesh. If the squeegee edge is partially damaged or worn, it is no longer possible to produce a uniform print across the entire squeegee width. This is because the screen printing ink is no longer pressed evenly through the mesh, which ultimately leads to defects in the print image.

The printing edge can be reground manually or using a squeegee grinding device (Gallus ID no. 189300), whereby the latter ensures that the squeegee rubber is removed evenly across the entire squeegee width thanks to automated and standardized grinding. This ensures reproducible reworking of the squeegee edge. This means that the screen printing squeegee always has the same, predefined, sharp edge after grinding – in other words, it always has the same quality. This standardizes a potential factor influencing the print image, minimizes machine downtime due to improperly prepared printing accessories, and facilitates troubleshooting in the event of print image defects.

Image: Gallus blade grinding device (ID no. 189300)

The support length regulates the degree of rigidity of the squeegee.

However, the squeegee hardness is not only influenced by the squeegee rubber itself, but also by the length of the support (i.e., the distance between the front edge of the squeegee and the support plate). The longer the squeegee support, the softer the squeegee edge, which in turn influences the ink application thickness. If the support is too short, pressure losses occur; if it is too long, smearing occurs. As a rule, the distance between the squeegee and the support should be between 4 and 6 mm, provided that the squeegee is well prepared.

Correct squeegee position: clean print and high printing speed

The position of the doctor blade also influences the print image. The standard position for the doctor blade holder on all Gallus printing presses is 30° or 60°. The doctor blade is always ground at a 90° angle so that the defined settings can be maintained at all times. The doctor blade should be positioned so that it runs centrally to the impression cylinder, otherwise it will be too far back or too far forward in the web direction. However, to achieve maximum printing speeds, the doctor blade is always positioned 1-3 mm against the running direction, depending on the machine system.

Squeegee position too far back: smearing in the print image

If the doctor blade is positioned too far back in the direction of travel of the self-adhesive web, delayed pressure is exerted on the transferred print image. The result is smearing of the printing ink at the trailing edge of the print. The printing ink in the print image that has not yet dried is smeared by the delayed pressure of the doctor blade. If this printing problem occurs, the doctor blade must be moved further forward in the direction of web travel.

Squeegee position too far forward: missing ink at the trailing edge of the print

If the squeegee is positioned too far forward in the direction of travel of the paper web, premature pressure is exerted on the transferred print image. The result is a lack of ink at the trailing edge of the print. Because the squeegee pressure occurs further forward in the print image, the ink is not completely pressed through all the open mesh of the screen onto the substrate. To remedy this printing problem, the doctor blade must be moved further back in the direction of web travel.

Gallus is happy to support its customers with material recommendations, training, service, and technical assistance.