Scientists Discover New Possibilities for Screen Printing
By Sean Milmo, European Editor
Printing is being put under the microscope of scientists in an effort to improve the efficiency of technologies at a time when the graphics industry is under intense competitive pressure, particularly from the Internet.
As they investigate unexplored parts of printing processes, some scientists are arriving at controversial conclusions, particularly for makers of printing equipment and consumables like ink.
Screen printing is probably among the oldest of the modern printing processes, having its roots in early medieval China and Japan. Yet it is perhaps the least understood.
“In the early 20th century, screen printing was reinvented, but since it was mainly used then for artistic and craft purposes, no one until relatively recently cared about the technology,” said Alex Zucherman, a member of the Academy of Screen Printing Technology (ASPT) and chairman of Fimor S.A., Le Mans, France, a maker of screen printing blades or squeegees.
The basic screen printing technology has not changed much over the last few decades. As a result, screen printing has been condemned in some quarters to a gloomy future because it is considered to be low tech and becoming rapidly outdated.
Ink jet is already making inroads into large format sectors like posters and point-of-sale displays, which have been dominated by screen printing. Some experts are predicting that ink jet will soon be taking over big sections of the screen printing market.
However, recent research and new applications, particularly in Asia, have shown that screen printing has a much brighter future than predicted.
The process is turning out to be far more intricate than commonly thought. Michel Caza, one of Europe’s leading experts on the technology, calculates that as much as 125 different parameters influence the screen printing process.
“It is so complex that there is a lot more to be discovered about it,” said Mr. Zucherman. “The analysis of each of the parameters and their interaction with each other would be an enormous task.”
For makers of screen printing inks, there could be long-term opportunities for the development of a more versatile range of inks. In existing segments such as CDs and labeling, they will be able to do more to provide higher quality and more precise color printing.
Japanese and Taiwanese companies are demonstrating that in areas like large volume electronics, screen printing is more efficient and cost effective than ink jet. Screen printing can do higher resolution work than its rival digital process and at a much lower cost.
Increasingly, scientists are targeting their investigations on the point in the process when the mesh separates from the substrate. This stage seems to be crucial in determining the quality of the print, especially definition and resolution.
Researchers at the school of mechanical engineering of Leeds University, northeast England, have been conducting experiments which highlight the gaps in knowledge about the way the ink is deposited on the substrate.
A team at the university, including Professor Philip Gaskell, a specialist in engineering fluid mechanics, carried out a systematic and scientific analysis of screen printing by investigating the basic physics of the technology.
“We believe that by gaining a close understanding of the underlying mechanisms of the process it should be possible to widen the use of screen printing into many new areas,” said Nikil Kapur, a scientist on the research team.
“We looked very closely at the process, applying our knowledge of fluid mechanics, and for the first time we understand the fundamentals of screen printing,” he added. “What we discovered is that generally held ideas about the process are in fact wrong.”
Plans for the Leeds research project stemmed from Professor Steven Abbott, research and technical director of Autotype International, Wantage, England, and a visiting professor at the Leeds mechanical engineering school.
“I suspected that the traditional view of the way screen printing works was not right,” said Professor Abbott, whose company is a leading manufacturer of stencils for screen printing. “I talked to Professor Gaskell about it over a drink and he immediately drew a few diagrams to show that indeed the physics was wrong. In fact, the usual explanations goes against the laws of physics.”
The Leeds researchers carried out a series of tests to assess factors in screen printing such as the speed of the squeegee on the print stroke, how much force it applied, the distance between the mesh and substrate and the viscosity of the ink.
Among the techniques used was white light interferometry to investigate the exact way in which the ink is transferred from the mesh to the substrate.
The scientists examined in particular the importance in the process of the squeegee’s printing stroke which is applied after the blade loads the surface of the mesh with ink.
The general belief is that the pressure of the squeegee’s second stroke pushes the ink through the open holes of the mesh as defined by the pattern of the stencil and then forces it onto the substrate. The Leeds team are skeptical about this, suggesting that the key moment is when the transfer of the ink occurs during the next step, when the mesh is separated from the substrate.
“The mesh comes out of the ink rather the ink comes out of the mesh,” said Professor Abbott. “It is like honey being deposited from a spoon onto a surface. At a crucial point a liquid bridge or viscous drag forms between the mesh and the substrate. When the bridge breaks, it creates ink splatters, a phenomenon in screen printing for which no one has been able to provide a convincing explanation before.”
When carrying out microscopic examinations of the ink on the screen printing substrates, the researchers found that it had peaks and troughs. As the mesh moved away from the surface, the drag from its threads would create the peaks while the holes between the threads would form the troughs.
Possibilities for High-Tech Applications
This discovery has enabled the Leeds team to carry out work with a much higher definition and registration than in normal screen printing because it has provided closer control of the positioning of the ink on the substrate. The researchers have achieved resolutions in lines of less than 50 microns across.
“I passionately believe that screen printing can be a high-tech process,” said Professor Abbott. “This research shows that it can achieve high resolution graphics in areas like CD printing. In color printing it can position dots very precisely on top of each other so that dot spread can be considerably reduced.
“More importantly in the longer term, it is capable of providing the high resolution required for the next generation of electronics with conductive polymers, which is something that the Asians are already grasping,” he continues. “Screen printing is capable of higher resolutions than ink jet while at the same time it has a greater degree of flexibility with types of inks and substrates.”
More understanding of the screen printing process is helping to clarify what are the most important properties in inks for the process.
“Often screen printing inks are expected to do things which are now shown to be unnecessary,” said Professor Abbott. “There are interesting opportunities for ink designers if they concentrate on the two main requirements of ink – color and, if necessary, conductivity. The science shows that to obtain precision the viscosity of the ink needs to be right.”
Professor Abbott said that the critical elements in gaining the best performance from the ink is the thickness and flatness of the stencil.
“A thin and flat stencil will provide better registration and little dot-on-dot gain,” he said. “If the stencil is too thick and rough, the registration is less exact and there is dot gain.”
Professor Abbott’s view is an example of how interpretations of the results of new research can be highly contentious. He believes that once the squeegee fills the mesh with ink and removes any excess, its job has been completed.
“It’s a heretical view,” he said. “Tests show that after the squeegee has done the filling and scraping of the mesh, it has nothing to do with the printing process because it is the mesh which controls the ink deposit.”
Not surprisingly, this analysis has little support among screen printing specialists, particularly among those involved in making squeegees.
“No one in the screen printing community shares (his opinion on squeegees),” said Mr. Zucherman. “The slightest variation in squeegee compound, hardness, sharpness, thickness, free height or swelling after solvent attack can induce a measurable change in the quantity of ink sheared through the screen and transferred onto the substrate.”
In fact, a small change in the hardness of a rubber squeegee as measured on the shore durometer scale of 0 to 100 can affect print quality. “A two-point change on the shore scale can influence the precision of the print on the substrate,” said Mr. Zucherman.
Disputes about the relative contributions which squeegees and stencils make to the screen printing process underline the need for more research into how the technology operates.
“There is a lot of research work to be done before we fully understand how the ink is released from the mesh,” said Tim Claypole, director of the Welsh Centre for Printing and Coating at the engineering school of the University of Wales, Swansea, considered to be Europe’s leading research institute in screen printing. “We cannot say yet who has the right ideas and who the wrong ones. No one can yet say they’ve found the solution.”
Some have doubts, however, about what difference the new scientific research will make to the long-term fortunes of screen printing.
“I fear that screen printing will continue to be at a disadvantage because more and more people want push-button systems which are not dependent on the skills of the operator,” said Mr. Zucherman. “This research may have been done a little too late. It might have been different if it had been conducted 10 years earlier.”