The screen printing is over one thousand years old that started in China. Staring from the concept from serigraphy and the time of Rubylith technique of making screen, the modern time silkscreen printing and emulsion has come along to its newer, faster and highly sophisticated system and techniques, both science and art. The industry went through using dichromate (highly toxic acid that is still being used in the third world) to Diazo based emulsion, Dual cure Diazo emulsions and now to SBQ and hybrid emulsion technology.
Sceen Printing in Art
Though screen printing has some roots in Europe, the real story of the screen printing begins in early 1900 in USA-The first patent was submitted in 1907. Some artists in 1930 working on Serigraphy that is what they called the art created through screen printing. They used the photo imaging stencil technique to print the art work. They came pretty close to the modern silk screen printing. In 1960, Andy Whrol great art work made the technique more commercial rather than art only. By 1980, multi color design were developed and the printed shirts very popular. The liquid what we call Emulsion used to be pyre Polyvinyl Alcohol. They cooked the material on stove and open fire and used Chromic Acid to create an image on the screen. Advance was the first company that developed the concept of Dual Cure and refined the Emulsion to develop a screen as a sophisticated printing plate with half tones and strong stencil.
The Differences in Emulsion
The idea is that the liquid emulsion is being applied to the screen and then dried. The dried film is then exposed to UV lights with the positive, with the image to be printed, on the top of the screen and the image impression transfers to the screen with open lines showing the image – When you wash the screen, the image appears ready for the printing. The first step is to apply the wet emulsion on the screen by a scoop coater, starting from the print side to the squeegee side. You can apply once on the print side or one on the screen side and one to the squeegee side or two and one or two and two and more, depending on how thick stencil you may need for the print. Let the screen dry, putting the screen side down as you wish to have more emulsion on the screen side than the squeegee side. Once the screen is completely dry, it is ready for exposing. There are numbers of exposure systems and the light’s wavelength that we like to talk about. Let us look briefly how does the emulsion work? All the screen printing emulsion carries polyvinyl Alcohol (POVH) that contains a hydroxyl group (OH). When you add Diazo to the emulsion and apply the emulsion with Diazo on the screen, after drying, the light make the Diazo react. The light passes through the open areas of the image where the emulsion film gets cured and stays on the screen. The darker area of the film does not allow the light to go on to the emulsion film and that makes it to wash out as it was not cured. This interesting chemical reaction has contributed to what we call Silk Screen Printing. The next development was the emergence of Dual cure emulsions that added to water and solvent resistance and better image definition as the edges of the films becomes harder and well defined. The word Dual cures means that emulsion is curing in two different ways. The first was the diazo reaction; the second is the monomers are being polymerized by the photo-initiators activating the monomers. There is a lag behind the activations of these two systems. That makes the Post curing (second exposure after the screen was burnt and washed and dried for post curing) different than the simple diazo emulsion post curing. In the case of simple diazo emulsion post curing, the excess of Diazo is being utilized to its possible exposure period. With Dual cure the monomers have to be hardened to its maximum-We call this optimum exposure.
SBQ Changes the Screen Printing Emulsion Game
The SBQ/one part emulsions were introduced that made the emulsion four to five times faster exposing, depending on the exposure unit being used. The SBQ monomers for photo-polymer emulsion were invented in Japan and they held the patent to graft the SBQ monomer to the hydroxyl group. The Ion exchange procedure used by the Japanese and published the patent made it harder, unreliable, nasty and longer method of producing the SBQ grafted product that is being used worldwide (Only USA, Japan and Germany have this technology). We have improved the process and found a way to make it easier and more effective. Finally, the word Hybrid introduced, When they added monomers to the SBQ emulsion, they called a Hybrid emulsion. All our SBQ, one part emulsion are Hybrid emulsions. Again, by adding the monomer you get better resistance and better edge definition.
One of the important things to remember about the SBQ monomer that it does not have any water resistance – sometimes they add Diazo to the SBQ emulsion for water base inks. Let us look at briefly on the light system that helps in making of the image. As we know that the light travels in waves and the wavelength is very important. Every sensitizer in emulsions requires a specific wave length. The most applicable range varies from 360 to 390 nm and goes up to 420nm. So an exposing unit with certain output, the distance between the light source and the screen and the thickness of the emulsion film and the color of mesh contribute to the exposure time. If you are using a new emulsion, the best way to find the optimum time is to start what they call “ the wedge step test” that requires multiple exposure time by blocking at intervals the exposure side of the emulsion film. The demanding screen printers usually require the following properties out of an Emulsion
- Fast drying and exposure
- Easy stain-free reclaiming
- Wide exposure limits or wider latitude
- universal emulsion that may be used on different types of inks, such as water, solvent, Plastisol and UV inks
- Sensible pricing with great performance mentioned above
- Higher solids so that they may get heavier coating with fewer applications (XENON’S Nova is one of the best Emulsion in this regard)
- Higher EOMR (Emulsion over mesh ratio) EMO is a percentage of the mesh thickness on the substrate side of the mesh- So the Emulsion should be able to give a high EOMR with least applications of the Emulsion. The emulsion build up should be somewhere from 10 to 20 % of the mesh thickness. EMO has a great influence on the ink deposit on the fabric besides the viscosity and the flow of the ink, mesh count and the contact angle.
- An emulsion that would yield possible lowest RZ factor- RZ factor defines the smoothness of the dry film of the emulsion that is why the emulsion should have higher solids, the best possible flow and leveling properties.
- High acuity of edge definition. Dual cure emulsions have better edge definition; the reason is that when the monomers In a Dual Cure get fully polymerized they yield a stiffer and sharper edge. So there are Diazo based and SBQ Dual Cure available.
- Higher resolution- the longer curing of the emulsion yields a better resolution. There are lot of printers prefer the Diazo based emulsion for their half tones.
One of the most advance technology developed at Xenon Products is an emulsion line without Diazo what we call it ND FUSION technology. This line of emulsion products do not require Diazo and have the broadest exposure time than any other type of emulsion in the market place. With regular SBQ emulsion if you expose longer than the suggested exposure time that is usually in seconds, you over expose the emulsion and thereby may not open fine lines. By using ND FUSION emulsions, under Black light, 120 watts, you may expose the emulsion for 30 seconds or five minutes and you get perfect imaging stencil without the danger of under or over exposing the emulsion film. For higher resolution, you may expose longer and for faster jobs go for the shortest exposing time. The reclaiming is excellent.