PROPER SCREEN COATING: MAKE WHITES WHITER AND PRINTS CLEARER

Once your dark room is set up with essentials and you’ve chosen an emulsion, we come to the art of screen coating.  Yes, we describe it as an art because it takes an experienced touch gained through lots of practice to achieve a consistent coating every time.  Properly coated screens provide an even depth ink channel across the entire print surface.  An even ink channel results in a superior final print, meaning whiter whites and clearer prints.  Read on to learn more about proper coating techniques and how to troubleshoot issues you may be experiencing.

Things to Consider: Mesh Count and Substrate

            When coating a screen, you should always begin with the end in mind.  What artwork are you printing, and what will you be printing it on?  Emulsion creates a stencil “well” or “channel” that gets loaded with ink before the ink passes to the substrate itself.   Your coating decisions will depend on the stencil thickness required by the level of design in the artwork and the printing surface at hand. 

Your first important consideration will be mesh count.  Higher mesh counts (200 and above) will allow less ink to pass through the screen and can hold highly detailed designs.  You’ll generally need high mesh counts for printing hard and smooth surfaces like posters, stickers, and signs, or textiles printed with CMYK process or high detail designs.  Mid-range mesh counts (155 to 180) will allow a moderate amount of ink to pass through, and are primarily used for textiles.  Lower mesh counts (140 and below) will allow more ink to pass through with limited detail.  Low mesh counts are mainly used for printing textiles with block or athletic style designs.

When we talk about coating screens we use a coating ratio defined by “substrate side to squeegee side” of the screen (eg – 1:1 refers to one coat on the substrate side of the screen, and one coat on the squeegee side of the screen).  If you’re printing on a hard and smooth surface like a poster or sign and using a higher mesh screen, you will likely need less ink deposited and will probably only need a shallow ink channel achieved with a 1:1 coating ratio.  If you’re printing on textiles, the mesh count and ink well depth needed for any particular job can vary.  For example, a CMYK print on a t-shirt will probably need a higher mesh count with a shallow channel achieved with a 1:1 or 1:2 coating.  An athletic block print will probably need a lower mesh count and deeper channel achieved with a 2:2 coating. 

The solids content of your emulsion will also need to be taken into account (read “Choosing the Right Emulsion” here).  These suggestions are not set-in-stone, but use them as a starting point until you gain experience that will allow you to make informed choices based on the job at hand. 

Preparing to Coat: Getting Set Up

The Screen - Your screen needs to be fully reclaimed.  Don’t skip the degreasing step!  Also make sure your screen is fully dried with no wet spots.  Be careful to not touch the mesh with your fingers, as this transfers oils to the mesh that can result in emulsion irregularities.

The Scoop Coater – Scoop coaters come in two different styles.  Single edged scoop coaters (like our Pro Angle scoop coater) have a handle on one side and a moderately sharp edge on the other. The handle is a nice feature to make coating a little easier, but it’s not necessary. 

Dual edged scoop coaters (like this basic Scoop Coater with removable end caps, the Pro Angle 2, or the Monster Max) feature one sharp edge for a thinner channel and one round edge for a thicker channel.  Make sure you understand the scoop coater you’re working with to achieve the coating you desire.

Your scoop coater should also start out clean, with no dried chunks of emulsion inside.  Check the coater’s edge to make sure there are no burrs or nicks that might damage your screen mesh.  Fill the scoop coater generously with emulsion – don’t skimp!  The weight of the emulsion pressing against the screen will aid in pushing the emulsion fully into the openings of the mesh.

Coating Location – The main factor in achieving a good coating is that you are comfortable and able to smoothly coat from the bottom to the top of your screen without having to raise your hands or arms above shoulder height.  The specifics of your set up will depend on your height, the size of the screen being coated, and the equipment you have available to you. 

Screen stands (like the Vastex C-100 Screen Coater Rack) will hold the screen in a stable 90º vertical position so you can hold the scoop coater with both hands, and are adjustable in height to give you the best position no matter what size screen you are coating. 

If additional equipment isn’t in the budget, you can set up to coat screens on the floor or a bucket up to about 18” off the floor.  This lower position will help when it comes time to press/angle the screen towards your coater as you move from the bottom of the screen towards the top.  Coating on a standard tabletop will likely be too awkward to achieve a smooth coat.

Properly Coating a Screen: The Technique

1. Start coating on the substrate side of your screen.
2. Stabilize the bottom of the frame on the floor or raised surface, and grab the top of your screen on the frame.
3. Place the coater’s edge at the bottom of the screen about 1” up the mesh from the frame.
4. Pressing the edge FIRMLY against the mesh, tilt the coater towards the screen until the end caps are flush against the screen.

1. Tilt the screen down to about a 45º angle from the floor.
2. KEEP FIRM PRESSURE AGAINST THE SCREEN. Start drawing the coater up the mesh.
   1. Speed – maintain a slow, consistent pace from beginning to end in order to eliminate pinholes. 20x24” screens should take approximately 8 seconds per pass.  23x31” screens should take approximately 12 seconds per pass.
3. Once coater is within a few inches of the top of the frame, return screen to the 90º vertical position.
4. While maintaining firm pressure of the coater’s edge against the mesh, tilt the coater down to allow the excess emulsion to refill the trough.
5. Once emulsion is away from the screen, gently “shimmy” the coater side to side while pulling away from the mesh.

1. Repeat the process on the squeegee side of the mesh while emulsion is still wet. ALWAYS FINISH ON THE SQUEEGEE SIDE OF THE SCREEN.
2. Once all coats are complete, the screen should be dried as it will be on the press – squeegee side up, substrate side down – to allow the emulsion to work with gravity and form the channel on the substrate/bottom side of the screen.

Troubleshooting Coating Issues

Heavy Deposits of Emulsion – A “tiger striping” effect or large areas of obviously heavy emulsion are caused by inconsistent pressure and speed “shaky”.  Pressing too hard will create thin spots, while not pressing hard enough will result in too much emulsion remaining on the mesh. 

Pinholes – Pinholes can be caused by coating too fast. A slower stroke will allow the emulsion to really get into all the nooks and crannies in the mesh.

Fish Eyes – Look like white dots in your emulsion.  Typically from backsplash of reclaiming chemicals left in the screen.  A flood rinse after degreasing should leave you with a clean screen to resolve this issue.

Specialty Screen Coating Options

            Face Coating is a technique that adds layers of emulsion to an already coated and dried screen on the substrate side of the mesh.  Face coating is effective in creating very deep channels appropriate for specialty inks like glitter flakes.

 Practice Makes Perfect!

            Proficiency in coating screens takes practice!  Don’t give up on yourself if you’re having trouble.  Be patient, take your time, follow the tips above and in due time you’ll find yourself producing consistent coatings that result in less issues further in the screen printing production process!