This paper is a review of how the past several years of technological and chemical changes in screen printing have affected the squeegee — the one tool which has remained relatively unchanged through the years.
We have succeeded in fine tuning the stencil materials to where we can now reproduce an image, which more closely resembles the original positive. With the fabric technology, precise stretching techniques and the precision designed printing presses offered in today’s market, we are able to consistently reproduce an image print to print.
Once the proper screen mesh for a particular ink system, or end use product has been determined, there are variables controlling ink deposit and consistent print definition or quality — the ink and the squeegee.
Throughout this paper the emphasis on all variables will be primarily related to the conventional right-angled cut, rectangular blade ranging from 60-90 durometer. This type of blade is used most often in sheet or web printing on various substrates. Although there are exceptions to these, such as some cylindrical screen printing, printed circuits, and textile, most principles and procedures herein will apply to all screen printing.
The screen printing ink will most often require some modification with viscosity modifiers or flow controls to assure proper printing characteristics. The squeegee blade, on the other hand, may be more responsible for printing characteristic variables than all the modifications in the ink.
To assure optimum printing quality, the ink and squeegee must function in harmony. The proper viscosity of ink must be taken into consideration with relation to squeegee, composition, durometer, and proper press setting. Proper selection of the squeegee blade, the durometer, angle setting, and pressure are all critical factors in printing efficiency.
The first task in selecting the proper squeegee is to determine what style, composition, and durometer is best suited for your particular application.
The durometer will usually be determined by the substrate and screen mesh. If the material to be printed has a non-smooth or irregular surface and a rather coarse mesh is required, the squeegee selected will usually range from 50-60 durometer. If the printed surface is smooth, utilizing a mesh count between 104 and 158 threads/cm (265 and 400 threads/inch), a harder durometer would be required.
The next area of concern will be the specific screen ink. Compare various blade compositions, looking for the best resistance to the ink chemistry as possible.
Some blades, as studies have shown, have better solvent or chemical resistance than others. For example, a blade which works well in poster ink or lacquer may fall apart or swell severely in an acrylic polyester, or ultraviolet formulation.
Between the ink and press manufacturers, instructions should be given as to style and durometer for your specific needs.
Squeegees have changed little in composition through recent years, however, ink chemistry, screen mesh, and other applications have undergone considerable change, affecting the over-all performance of the blade.
Within the last few years screen mesh counts have averaged 69 to 95 threads/cm (175 to 240 threads/inch), with screen inks averaging 25 to 40 percent solids. With these solids levels, we were putting 60-70 percent solvent through these coarse meshes. More solvent with minimal mesh interference has allowed for an increased amount of variables in the print stroke and squeegee to achieve uniform build and flowout characteristics. Furthermore, these variables have reduced the amount of waste normally due to poor fabric weave, nicks in the squeegee blade, dirt, etc.
Today we are working with screen meshes ranging from 104 to 165 threads/cm (265 to 420 threads/inch) with screen inks between 40 and 100 percent solids.
Imperfections in mesh, improper ink grinds and flowout, now show up more readily. If we have slight imperfections in the squeegee, the print quality will definitely show. Very close attention to the squeegee, and to its proper settings and maintenance, are a must.
A squeegee must be sharp. You cannot properly transfer ink uniformly or consistently through a mesh with a dull or nicked squeegee. The first indication of poor printing characteristics should be a visual inspection of the inside of the screen. If the screen is not reasonable dry by appearance or touch (leaving a slight residue of ink on the inside of the screen), it is obvious the squeegee did not properly transfer the ink. By leaving a slight residue of ink in the screen, the print will not be as sharp, and every subsequent print under those conditions will be progressively worse. (See Figure 1)
This condition may be due to a too high viscosity of the ink to flow out properly, however, in many cases the squeegee is responsible for creating the problem. This condition may stem from the squeegee not being sharp enough, insufficient pressure, or too much, resulting in an improper flexing of the blade.
Most problems are created by applying too much squeegee pressure. Any time the squeegee has enough pressure applied to bend the blade, or to severely change the angle, you will immediately lose the proper “cutting edge” of the blade resulting in poor print quality. (See Figure 2)
Prior to adjusting squeegee pressure, check screen tension, size of print area, inside dimensions of the screen, and off-contact setting.
Excessive off-contact with a tight screen will distort the image. A greater problem occurs when excessive squeegee pressure is applied in an attempt to achieve a good ink deposit on the substrate. Too much pressure will bend the blade beyond the proper angle, resulting in poor print quality. When attempting to print too close to the edge of the screen frame, with a reasonable amount of off-contact, the amount of pressure required to achieve good ink deposit will result in poor print quality. (See Figure 3)
Many problems with excessive flexing of the blade or loss of proper angle and pressure, are usually caused by too much blade below the squeegee holder or support frame. The lower the durometer, the more prevalent this condition of blade distortion will become.
For optimum squeegee function there should be a minimum amount of blade below the squeegee holder or support frame. When looking at many squeegee assemblies it seems apparent the only solution to this problem is cutting the blade down to approximately a quarter to half an inch from the squeegee holder.
Another solution is to adapt a support or shim behind the squeegee blade. (See Figure 4) This will not only straighten the blade but will allow more pressure while maintaining the straight and proper angle.
When situations arise where all indications for solving a printing problem lead to a higher durometer, you may often gain the effect of a durometer increase by back supporting the blade. (See Figure 4)
Remember that most squeegees will be affected by excessive use, exposure to various inks and solvents, and have a rather limited life span.
There are a few basic procedures and practices which will prolong the effective life of the blade, and most importantly, decrease down time and improve the quality of printing.
- Begin printing with a sharpened squeegee.
- Do not allow the blade to be submerged or exposed to the ink longer than necessary. When stopping for breaks, lunch, or other long periods of time, remove the squeegee from the ink and wipe dry.
- Do not print with the same squeegee more than one shift or one day at a time. Some squeegees may require changing more often.
- Allow a minimum of 24 hours after printing prior to resharpening. I usually suggest having three squeegees per press. One you are using today, the second for tomorrow, and the third for day three. With this rotation you allow two days between printing for the squeegee to properly “recover”. Just prior to using the dried squeegee again, properly sharpen and reset.
- The sharpening process requires more than a block and sandpaper. With the changes in today’s inks and screen meshes, the squeegee will not tolerate imperfections without obvious penalties in the print. One of the most important pieces of equipment you may purchase today is a good squeegee sharpener. Very precise sharpeners are readily available and although more expensive than some belt sanders, they are by far more efficient in consistently sharpening the squeegee.
- Prior to printing, check for correct off-contact, assuring proper pressure and that the squeegee blade is not bent or flexed.
All the preceding steps and procedures may be of little effect with a squeegee that is not properly tuned. Conversely, the squeegee can optimize all the previous steps and assure the quality print you desire.