The Screen Print Man

While many a firms have the capacity to embrace automation, there is a lag in their capacity to put to use the technology. Screen printing has applications in segments as varied as wedding cards to the printed circuit boards.

But what is of utmost importance is the technology know-how to operate and exploit the machine. To begin with, we will speak about the most important part of the screen printing machine - the Squeegee.

Some excerpts from Dawn Hohl-Nowlin's article published in the February 2014 issue of Grafica News India. 

Dawn is a Technical Screen Printing Consultant at Uncommon Enterprises LLC, USA and Member of the Academy of Screen and Digital Printing Technologies.

Squeegee selection
The screen is by far the most important component for quality screen printing. But the squeegee is the point of contact where the print is actually transferred to the substrate, making it a critical variable to understand and control.

Unfortunately, the squeegee is often one of the most neglected tools in the process.

The first step
Squeegees have been made out of three primary materials, rubber, neoprene and polyurethane. High density polyurethanesqueegees are the most widely used in screen printing today. Polyurethane squeegees are formulated to offer various levels of abrasion and solvent resistance to match different printing applications.

Durometer
Durometer describes the hardness/softness of a squeegee on the Shore 'A' scale. The higher the number the harder/stiffer the squeegee. Screen printing squeegees are typically available in 55A to 95A  hardness. Most manufactures color code their squeegees by durometer for easy identification on the production floor. However, that color coding is not universal and can be confusing if squeegees are purchased from different sources.

A gauge called a Durometer is used to measure Shore on the actual squeegee which is useful to monitor age/degradation and when hardness is unknown. In general, 55-65 durometer is soft, 70-75 is medium and is considered all purpose, and 80 and above is considered hard. Softer squeegees deposit more ink and conform better to irregular surfaces than higher durometer squeegees, but have resolution limitations. Medium to harder blades are more suited for finer detail, thinner ink deposits and may be required when printing highly viscous inks. Harder durometer blades will resist solvents and abrasion better.
 

Squeegees are available in solid durometer or multi-durometers sandwiched together. Dual and Triple durometer squeegeesuse a different durometer at the print edge and a harder durometer on the inside or main portion of the blade . The inside harder layer prevents excessive bending while the outside layer allows the edge to conform the substrate surfaces.

Profile and edge
Blade profile refers to the squeegee's printing edge shape and affects the squeegee's adaptability and ink delivery. There are many unique profiles available, such as round, double beveled, single bevel, diamond and blunt bevel, which are used for applications including bottle/container printing, high precision electronics printing, and irregular object printing. But rectangular blades are the most common for flat graphics printing.

The printing edge sharpness of a rectangular squeegee is the primary ink deposit and image definition control. A sharp edge offers controlled ink deposit and sharp detail during printing.

An edge with a small to large radius will give progressively greater ink deposit with less detail sharpness. An edge that is chamfered will give the greatest ink deposit for thick film applications. Squeegees will wear with use and the edge must be resharpened regularly to maintain desired results.

Effective printing angle and free height
Squeegee free height refers to the dimension of the blade extending out of the holder and determines how much a blade will bend under pressure. The effective printing angle is the actual angle that forms during printing between the squeegeeand the screen.

As the blade bends the actual printing angle changes and subsequently the printing force changes. The free height, squeegee angle, squeegee pressure, durometer, blade profile, screen tension, off contact, ink and substrate are all part of this equation.

As the free height is reduced from repeated squeegee sharpening and wear, it cannot flex as well which makes it stiffer during printing. A squeegee in this state will act like a higher durometer blade, changing ink deposit and print quality.

Triple and dual durometer blades were designed to minimise these changes as the very hard durometer in the center (or top) prevents the blade from bending, while the outside softer durometer allows adaption to the printing surface throughout the life of the squeegee.  

Another squeegee design called composite squeegees use fiberglass reinforcement to provide maximum stiffness, which keeps the effective angle the same regardless of free height or pressure. Another option is to use metal backing supports to standardise the free height of the blade, helping to maintain a consistent effective printing angle.

Length of squeegee
Using a squeegee extending 25 mm beyond each side of the image will maximise the free mesh area of a screen. Free mesh area is the distance between the ends of the squeegee and the inside of the frame. The larger the free mesh area the less off contact is needed improving registration, print clarity and ink deposit uniformity.

The practice of rounding off both ends of the squeegee helps minimise mesh stress at the ends of the squeegee. It can be accomplished either by clipping with a specially designed cutter, or rounding with abrasives.