Types of emulsions in screen printing
Screen printing emulsion, are applied directly to the cleaned and degreased screen, through a scup coater. After drying, it is light sensitive and can be exposed to ultraviolet light. In this way, you will transfer your design (film) onto the screen.
The area where the light hits the emulsion becomes waterproof. The area where no light hits the emulsion, by covering it with the film, remains water-soluble and can then be washed out. Now, you have transferred your positive film to the screen.
Today’s Screen Printing direct emulsions can be divided into three types:
- SBQ Photopolymer
Diaso emulsions consist of a two-component system containing a sensitizer mixed with a non-sensitizer. They are inexpensive, lightweight diazo emulsion getting nowadays replaced by Dual cure and SBQ emulsions. And have a shelf life of 30-45 days after mixing.
Dual cure emulsions
Dual cure emulsions are also a two-part system but differ in that the sensitizer is mixed into a pre-sensitized base, which allows for fast exposure, durability, sharp edge definition, and long shelf life. Dual cure offers a wider exposure latitude to capture more of the image more easily.
Washing out and developing a Dual Cure emulsion is somewhat easier than with a pure photopolymer emulsion. Dual Cure emulsions retain the properties of the emulsion even when slightly underexposed or overexposed. It is more expensive than Diaso emulsion but less expensive than SBQ emulsions.
SBQ emulsions for Screen Printing
SBQ emulsions are referred to as pure photopolymer emulsions and have the fastest exposure times. They do not require mixing and have a shelf life of one year.
This premium product is more expensive than diazo and dual-cure emulsions. Excellent mesh bridging. Stencil surface (Rz), is reduced compared to the diazo emulsion, as it has less water inside.
Tips for screen printing emulsion coating
- Make sure that the screens are degreased and thoroughly dried before coating.
- Screen tension should be a min. of 20 N/cn for best results.
- Coat the screen on the contact side first. The contact side is the side that touches the garment during printing. The last coat should be applied on the inside (squeegee side) of the screen.
- Screens should be dried horizontally with the contact side down.
- Store the screens in a light-protected room or cabinet after coating and drying.
- Use the blunt edge of the coater for maximum EOM, or select the sharp edge if a thin screen is desired.
- Do not use excessive heat when drying the coated stencils 30° to 40° C (86° to 104° degrees F), as this will cause premature activation of the sensitizer.
- After drying, do one face coat again, to reduce the RZ.
- Store your coated and dry screen at 30% to max. 50% relative humidity.
- Your coating room should be dust-free!
- Don’t blow direct air on the screen with a ventilator on the coated screen to dry, you will just push the dust on it, which can lead to pinholes later.
Important to know, what ink you are gone be printing
You need to see, what ink you are printing, as mostly, for Plastisol inks, or water-based printing inks the emulsion is different. Like most water-based inks, you would need a hardener. But please check this out with your supplier.
„Emulsion over mesh (EOM) = total stencil thickness minus (-)
the mesh thickness“
All of these emulsions are applied to the screen with a scoop or automatic screen coater. The sharp edge applies a thin layer of emulsion that is ideal for making stencils with fine detail, halftone printing, and other applications that require a thin layer of ink on the substrate. The blunt edge applies a thicker layer and provides optimal EOM results.
Sometimes, you will use a capillary film. The advantages of capillary films include superior mesh bridging, no pinholes, excellent Rz value. Thick capillary films are the secret to success when executing high-density (HD), tall puff, and other types of special effects (SFX) prints. Capillary films range in thickness from 10-50 microns for standard printing and 100-400 microns for HD and SFX printing.
What Emulsion do we have in Screen Printing