01 Jul Ink Transfer and Squeegee-Less Printing
I am often asked about the ink transfer process. The question usually comes from a textile printer who is trying to print thick white plastisol ink. Of course, without knowledge of the ink transfer process, he is quite simply scooping out a large amount of white ink into the screen and attempting to print it by pushing it through the mesh, much like Tim “The Toolman” Taylor might have done in the sitcom Home Improvement if he were screenprinting. Of course, no one wants to read a book on the subject, so I will provide the short answer here.
by Bill Hood, ASDPT Fellow
This article is for those who are trying to push the ink through the mesh openings and using excessive pressure (over 30 pounds of pressure) to do so. With a little thought and a better understanding of the ink transfer process, the printing task can be much easier, increasing productivity and eliminating waste.
Think of screenprinting as an extrusion molding process, with the mesh being the mold, the ink being the material, and the substrate being the item to which the molded ink will be placed.
You begin by bringing your material (the ink) to the correct temperature and rheology (flow rate) so that you can extrude it easily into the mold (mesh). This is the job of the fill blade or if you do not have one, you could use an 80 Shore A squeegee.
After the material (ink) has completely filled the mold, you quite simply bring the bottom of the mold (mesh) into contact with the dry substrate and there is a capillary action that takes place to transfer the material (ink) to the substrate. No pressure is needed if everything is done correctly.
The squeegee has only two real purposes;
1) to shear the ink from the top of the mesh, and
2) to bring the bottom of the mesh into contact with the top of the substrate, which takes no more than perhaps 25 to 30 psi.
For those that do not understand how capillary action takes place, consider what happens when you bring the corner of a dry paper towel into contact with a puddle of water. No pressure is needed for a capillary action to take place. Now, think in reverse, with the dry paper towel on the bottom and the drop of water above. The same thing happens, i.e. the dry surface will absorb the water.
Actually, other than the fill stroke, the squeegee is not even needed for printing a variety of substrates. Watch the video below that I filmed at a shop in Puebla, Mexico of a young man printing 211 pieces an hour, without using a squeegee to make an actual print stroke. Yes, he does use a squeegee for a fill stroke, but not for printing. I have seen this process being used around the world, including during my trips to India, China, the Philippines, the UK, and yes, in the USA.