Screen Printing: Production

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January 21, 2014

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Every day, thousands of printers across the country tighten, coat and pass their squeegees over thousands of screens.

As the very name implies, screen printing depends utterly on these mesh marvels. Proper preparation of screens is, therefore, crucial to creating quality prints.

No other printing medium has a primary printing carrier (the screen in this case) that moves during the process. Eventually, the stress applied to a screen through repeated use begins to elongate its mesh filaments. Screen tension will drop, in turn affecting ink film thickness, color control, registration and printing speed.

Getting Framed

The frame is key to controlling screen tension. The more stable the frame, the more control you’ll have over screen tension. The primary objective is to make sure the screen maintains its tension for as long as possible during a production run. Regardless of frame type, once the screen is stretched, tension can only move in one direction: down. Here’s a look at the pros and cons of each frame type.

Wooden Frames. Wooden frames are the most popular with novice screen printers for several reasons. First, they’re relatively inexpensive. Second, they’re lightweight and easy to work with. Wooden frames perform well for one- or two-color prints requiring minimal or no critical registration.

The greatest drawback to wooden frames is this: As the stress of the stretched mesh starts to pull on the frame, the frame will bow inward, and screen mesh tension will drop. Once this occurs, the initial screen quality is lost. In time, mesh tension will drop to the point where it will have to be removed and re-stretched.

The best defense against this scenario is to pre-deflect the frame during the stretching process so the frame pushes out against the stress while the mesh pulls inward. Of the screen stretching devices available, almost all will pre-deflect the frame in this manner. The downside is that once the mesh adheres to the frame, screen tension can only drop and the screen will inevitably require replacing. This constant need to replace mesh is the most expensive aspect of using wooden frames.

Wooden frames are also absorbent. Their tendency to soak up solvents and water over time leads to further loss of frame stability. Coat or dip your frames in a solvent-resistant coating such as polyurethane to protect against absorption.

Tubular Aluminum. Aluminum frames are the typical next step in the evolution of a screen printing facility. Although more expensive than wood, they are impervious to chemicals and water, lightweight and considerably more stable.

As with wooden frames, you can achieve suitable initial screen tension with an aluminum frame. Once the mesh adheres, tension will drop and in time the mesh must be replaced. However, tubular aluminum frames do allow for a higher initial screen tension and will sustain that tension longer than wooden frames.

Retensionable Frames. Although they are the most expensive of the three main printing frame choices, retensionable frames allow for the greatest degree of control, tension and registration. They also offer a screen life far beyond that of other choices.

The primary benefit to retensionable frames is that they allow you to tension the mesh to required levels, relax the mesh and then retension it back to the desired tautness. In addition, the mesh can be retensioned following every production cycle so you can use a new screen for every press run.

The additional benefits of retensionable frames are numerous: minimal off-contact distance; minimal squeegee pressure; maximum color control in process or flat printing; minimal ink film thickness for a softer hand; minimal screen exposure time; minimal squeegee drag; decreased emulsion consumption; and faster printing speed.

Making It Mesh

Mesh preparation plays another crucial role in screen production. If the screen mesh and stencil aren’t properly prepared, the stencil’s screen life will be greatly diminished. Stencil breakdowns are among the primary causes of lost production time on a press, whether you’re printing manually or automatically.

The steps required for screen preparation are: reclaiming (if the screen is being reused); dehazing; and degreasing (this is the only step required for a new screen).

In all three steps, you will need a high-power pressure washer. Units that go as high as 3,000 PSI and above can tear the mesh if not properly applied. I use 2,000 to 2,500 PSI to ensure removal of as much residual chemicals and greases as possible.

Many new printers opt not to degrease new screens, assuming that a new screen is clean. But there are numerous contaminants and greases on the fabric that must be removed prior to coating the screen. Failure to clean the screen will result in a poor quality stencil with a short life. Plus, you’ll spend as much time repairing screen breakdowns as you do printing.

Reclaim screens to strip and recycle them once the print run is complete. Many printers who use wooden frames store screens for future reorders. This can work, but keep in mind that screen tension will drop as the mesh and frame age. Retensionable frames, however, are specifically designed to be reclaimed, retensioned and reshot for every production run.

Dehazing removes contaminant particles caught between the knuckles of the mesh that the pressure washer may have missed. The screen must be completely rinsed of all chemicals and contaminants with the pressure washer to produce a quality stencil.

Screen Coating. Although no two printers use exactly the same coating technique, they all seek a common goal: placing the stencil on the print side of the screen rather than the ink side.

The number of coats applied to a screen also depends on the desired ink film thickness. The standard coating process for everyday wet-on-wet printing is two coats on the print side of the screen and one coat on the ink side.

Once the screen is coated, it should be dried in a low humidity environment of about 80ºF with the print side facing down. The coating will settle to the ink side of the screen and allow for a clean cut-off when printing.

Elements of Exposure

Proper exposure depends on the stencil receiving the correct amount of light during the exposure process. Virtually every emulsion manufacturer offers an easy-to-use exposure calculator to help you determine how much light you need.

Simply place the film test sheet on a blank, unexposed screen and expose that screen for its standard time. Once the screen is developed, you can easily see the calculator’s results, and adjust the exposure time if needed.

This review covers the basics of screen making. However, the true key to success from one print run to the next is to establish and standardize proper screen making procedures.

Rick Davis is the southeastern regional sales manager for Triangle Ink Co. He is a 35-year veteran of the textile screen printing and apparel manufacturing industries. His background includes plant design, management, troubleshooting, apparel manufacturing and consulting. He also is a member of the Academy of Screen Printing Technology (ASPT). For more information or to comment on this article, email Rick at rdavis@triangleink.com.