Technical Tips on Screen Printing by Bhargav Mistry, DMI – #5
Q – Why is my direct emulsion stencil breaking down during printing?
Underexposure (the most common cause of stencil failure)
Use an exposure calculator to determine proper exposure time.
Screen not fully dry before exposing
Use proper screen dryer after coating emulsion and give enough time to dry before exposing.
Screen not fully dry before printing
After developing let the water to completely evaporate before sending the screen for production. If possible keep screen in dryer for min 20 mins @40℃ after developing
Too many coats (wet-on-wet) will increase the emulsion build up drastically and slow down emulsion dry. Reduce number of coats (wet-on-wet).
Use yellow safelights. Insufficient degreasing or degreased screen must have been touch by naked hands/fingers.
Screen Tension too low
Use the screen tension recommended by the mesh manufacturer.
Emulsion thickness too low
Ensure that the emulsion build up is proper apply multiple coats or emulsion solid content too low.
Emulsion not compatible with Inks
Use water resistant emulsion for Water based Inks and use Solvent Resistant Emulsion for Solvent Based Inks.
Too much squeegee pressure
Reduce squeegee pressure and reduce off-contact distance (distance between screen mesh and substrate). To high off-contact also increases squeegee pressure.
Screen is clean to often
Do not let the ink dry in the screen. Flood/Fill the mesh after each print stroke. Do not use fans on the top of the print area. Use less aggressive solvents to clean the screen. If the ink is becoming too tacky this can also lead to emulsion break down. Use screen hardeners to increase the resistance and durability of the emulsion.
Emulsion not stored properly or outdated
Always store the liquid emulsion in a refrigerator temperature between 20℃ to 23℃. Write the date on the can after sensitised. Dual Cure Emulsion has to be consumed within 1 to 3 months depending on storage and manufacturer. Single Component ready to use emulsions generally last for 6 months to 1 year depending on storage and manufacturer.
Ensure that diazo is thoroughly mixed into the added water, and then into the emulsion. Use only recommended amount of diazo. Always stir diazo in the emulsion very slowly and use only after one over night or 12 hours after mixing.