01 Mar PREVENTING INK TRANSFER THROUGH ABSORBENT MATERIALS
It is quite common for an ink to penetrate absorbent substrates leading to a waste of ink and time needed to clean the ink from the platen or press bed. The cost of excessive ink transferred through the absorbent material can effectively double (or more) the cost of ink. Even in a small shop with a $65 USD per hour cost of operation, every minute saved results in a savings of $1.08 USD. Fortunately, this excessive waste can easily be reversed.
This problem is easily resolved by printing only on the surface of the substrate. You need only to apply enough pressure to bring the bottom of the mesh into contact with the substrate, which allows the ink filled mesh opening to properly adhere to the substrate. Any additional pressure will drive the ink through the substrate.
If you are printing with a low viscosity (thinner) ink, such as water-based ink, you will want the ink to completely penetrate into the substrate, however, you do not want the penetration to continue past the bottom of the substrate. Printing with low viscosity inks will need more care to transfer than that of a high viscosity (thicker) ink, which requires additional attention to the squeegee angle, pressure and speed.
The purpose of the fill stroke is to completely fill the mesh opening with ink to require minimal pressure during the squeegee stroke. Excessive ink transferred by the fill stroke will only increase the amount of ink transferred onto and into the absorbent material. This may also result in a loss of resolution and waste of ink.
There is no hard and fast rule as to what is the best squeegee angle, squeegee pressure, or squeegee speed. The optimum angle between the squeegee blade and the screen will depend upon the blade free height, printing pressure, mesh opening, durometer, blade profile, screen tension, off contact, type and viscosity of the ink, and characteristics of the substrate. The setting of the proper angle will be determined by all of these different properties. The sharp edge of the squeegee blade is the only portion of the squeegee that should come into contact with the mesh. If you reduce the angle too much, you will push too much ink through the screen. The general rule to remember to obtain the optimum squeegee pressure is “less is better.”
If all the variables described above are carefully considered and carefully set, the amount of printing pressure necessary to obtain the best results should be minimal. Aside from unwanted printing characteristics, excess squeegee pressure will cause excessive wear on your screens, rapid deterioration of your squeegee, and it will waste ink. The manual screen printer should also have a strong incentive to keep squeegee pressure at a minimum as studies have indicated that excess manual squeegee pressure is a factor in the development of carpal tunnel syndrome.
The rate of ink flow into the mesh is generally a function of the fill stroke combined with squeegee speed. Most inks tend to print better with a proper fill stroke followed by higher squeegee speeds. Too high a speed, however, may result in poor coverage. The optimum squeegee speed varies with the viscosity of the ink and the mesh being used and both factors must be considered to obtain the proper squeegee speed for the proper ink coverage. As the ink’s viscosity is altered during production (due to manual thinning or heat generation) the squeegee speed may need to be adjusted.
- Cease waste of ink that is thrown away.
- Eliminate time consumed in correcting errors in ink transfer.
- Assure complete filling of the mesh opening with the fill stroke.
- Use only the amount of squeegee angle to bring the sharp edge of the squeegee into contact with the mesh.
- Use minimal squeegee pressure to achieve ink transfer.
- Most inks tend to print better with a proper fill stroke followed by higher squeegee speeds.
- Too high a speed, however, may result in poor coverage, especially with high viscosity inks.
- As the ink’s viscosity is altered during production (due to manual thinning or heat generation) the squeegee speed may need to be adjusted.