Coating Screens for Maximum Effect by Douglas Grigar
Submitted By: blue moon Date: June 13, 2011, 07:29:30 PM Views: 4134
Summary: The facts to be explored in this article revolve around three parts: first, the reasons behind better performance given proper levels of emulsion over mesh (EOM); second, a method for coating wet-on-wet to attempt to produce a significant EOM; and third, comparing this method to standard procedures.
Coating Screens for Maximum Effect
by Douglas Grigar, Master Screen Printer

http://www.thegrendel.com


Special-Effect Garment Printing
(A version of this article originally appeared in the June 2005 issue of PRINTWEAR.)

 

While any stencil that can hang onto its mesh and make a successful print could qualify as a “money maker,” does this mean it is “good enough”? In many ways, the screen-printing industry is saddled with a “good-enough-is-good-enough” mind set. Many of us seem to suffer from a resistance to take a logical and critical look at long-standing methods, asking whether, while they may work, they work well enough? It is important to understand that, while common methods may produce useable results, it is often a simple change that can produce greater consistency, improved product performance and money-saving efficiencies.

The facts to be explored in this article revolve around three parts: first, the reasons behind better performance given proper levels of emulsion over mesh (EOM); second, a method for coating wet-on-wet to attempt to produce a significant EOM; and third, comparing this method to standard procedures.

Introducing a method of coating screens using the “wet-on-wet” procedure, while additionally increasing the emulsion-layer thickness (in a quick and friendly production-friendly manner) is my primary goal. Clearly, face-coating will produce a flatter smoother-surfaced stencil than the typical “Wet-on-wet” method. It is the extra time, labor, and supplies, though, that make the application of an additional face-coat of emulsion on a previously dried emulsion layer unpopular; so it typically doesn’t get done. Because wet-on-wet coating is most often the rule – and we will not be discussion face-coating here – this article will focus on the facts and results of wet-on-wet coating procedures, in an attempt to render such procedures as effective as possible.

Inescapable Basic Variables

Speed, angle, pressure, coating-trough fullness, and coating-trough edge are the basic variables we have to deal with when coating a screen with a coating-trough (see Figure 1). We have to take into consideration that from each person coating screens to the next we should expect to see measurable changes, some quite pronounced.

[Figure 1: Speed, angle, pressure, and coaster fill-level all effect the flow and amount of emulsion transferred to the mesh of a screen.]



The travel speed of the coaster will change the amount of liquid emulsion transferred to and into the mesh. As a coater increases the speed, the amount of emulsion deposited will decrease and, unfortunately, the amount of bubbles formed in the coating trough will increase.

The angle of the coating trough will also vary the amount of liquid emulsion: increasing or decreasing the angle at which the coater is held to the mesh will affect the transfer of emulsion.

The pressure that is applied to the coating trough against the mesh will also change the emulsion transferred along with the consistency of horizontal distribution of the liquid. Too little pressure can cause heavy uneven streaks of emulsion to form in the center, while too much pressure can result in damage to the screen and mesh.

The fill-level of emulsion in the coater also will affect with the amount transferred to the mesh, resulting in a measurable change in transfer of emulsion from 5% to 25% depending on the fill-level difference. Fill-level changes will have the same drastic effect on layer thickness if you are using an expensive coating machine as it will when coating manually.

The coating-trough edge profile will also change the emulsion-transfer amount with the rounded edge transferring double or more the flow of emulsion into the mesh than the sharp edge (see Figure 2). A rounded edge will transfer emulsion faster, translating to less time for higher value of emulsion deposited.

[Figure 2: The coating-trough edge changes the amount of emulsion transfer. With a rounded edge often doubling (or more) the flow of emulsion into the mesh fabric, the rounded edge attains the “glisten” goal in fewer strokes.]



While discussing these coating variables along would fill an article, my objective is to point out why even small changes in coating procedure will affect the final product, and why drastic changes in product, from screen maker to screen maker, is common.

Emulsion Over Mesh

Emulsion over mesh is simply the measure of how much the emulsion layer extends beyond the threads of the mesh, and represented as a percentage of the total thickness, threads and all. Too little EOM and the threads and their profile have a detrimental effect on edge definitional and dot formation (see Figure 3).

 
[Figure 3: EOM is a percentage of the total thickness of the stencil. Low EOM will follow the thread profile while proper EOM levels help left the threads from the stencil edge, forming sharper prints.]

As ink travels through the mesh, it will often take a path under the divots formed in the face of the emulsion by the thread profile. If there is not a “wall” of emulsion layer, past the threads, the ink literally squirts out from the edges. Too little EOM will also allow the threads of the mesh to come into direct contact with the substrate and form an unwanted temporary “stencil,” blocking ink from forming a full, even dot or sharp line (see Figure 4).


[Figure 4: The three panels show how dots will print with less interference from the mesh as the EOM layer increases. Note how the low EOM on the left lets the mesh sit directly on the substrate and you can see where the mesh thread knuckles form almost stencil-like blockages inside the dots.]

 
Suggested Procedure: The “Glisten” Method

As the desired result is to increase the deposit of emulsion to raise the EOM of the stencil, we want to make sure we include the procedures and supplies that will help attain this goal. A good quality emulsion with a solids content of over 35-40% is the best choice: any less and the shrinkage when dry will be excessive. Increasing the emulsion layer will also increase the exposure time; depending on your prior coating method, this could double or more the exposure time needed.

Using the rounded edge of the coating-trough will be necessary to cut down on the number of strokes needed to gain the volume of emulsion desired. Switching from the sharp to the rounded edge can be a shock to the user as the emulsion deposit increases drastically. Practice will be required.

Coating screens with this procedure depends on a particular order of application, and the user will always start the application on the substrate or shirt-side and always end on the squeegee-side. The “glisten” method starts with coating strokes onto the print-(shirt) side until the opposite side (squeegee) shines or glistens, from where the name for the procedure originates.

Your screen must be clean, dehazed and degreased for this procedure. Heavy mesh fabric haze will cause the emulsion to flow differently and not glisten on the squeegee side evenly; it will have the look of wet cloth but not glisten in all areas. Look for signs of shininess outside of areas where haze has altered the ways the mesh changes the emulsion travel.

Coating is now concluded on the screen’s squeegee side. Regardless of the number of strokes on the substrate side, the amount of emulsion deposited is always controlled by the final strokes on the squeegee side pushing emulsion to the print side while metering the total emulsion by returning excess back into the coater.

Squeegee side last, always. Coating the squeegee side last pushes the emulsion from the squeegee side to the print side where the EOM will have the most positive effect.

Dry your stencil squeegee side up, always. Gravity will pull the emulsion down and, when dried with the squeegee side up, the emulsion will dry with the thickest layer pulled to the substrate side.

Interesting Side Notes

The glisten procedure allows for an easy fix if the coating is botched: simply scrape off the emulsion from both sides and start at the beginning, coating for a shine; a fall-back point every time.

Higher mesh counts begin to “self-meter” emulsion levels as the constriction of flow by the smaller openings helps even out flow and make the coating more consistent; in other words, the higher the mesh where detail demands are higher, coating with this method become even more consistent. Fixes using the fall-back point described above will have less than a 4% change regardless of how much emulsion was coated on the substrate side.

The Comparisons

The most common screen-coating procedure is to use a coating trough with a sharp edge to apply a particular number of application strokes on one or both sides of a screen. While the typical spot-color “Joe’s Bar & Grill,” printed with a 150 – 160 mesh on light shirts may not necessarily suffer enough from the lack of sharp stencil profile (or at least most customers may never notice it), it will suffer. And, as the degree of job tolerance gets closer and closer, it will suffer more until, eventually, the customer will notice.

The common 1/1 – one coat on each side – with sharp edge will guarantee the coating of dried emulsion over the mesh threads will be less than 2%; that is less than half of the recommended 5% for small-dot resolution and one-tenth the recommended 20% for general printing (see Figure 5).

[Figure 5: 1/1 sharp-edge coating will produce less than 2% EOM, one-tenth the recommended level of 20% for general printing.]

 
   
Often, printers will point out that, when they coat sharp-edge 1/1, the screens show fewer flaws…which could only mean such screens are “better.” What this is really pointing to is that a thicker layer of emulsion is darker and will show flaws and contamination in the mesh from lack of complete and effective cleaning. Hiding flaws does not eradicate them, and thinking that hiding them is effective and helpful is not logical.

A side-by-side look: The micro-photographs shown in Figure 6 represent 1/1 with a sharp edge, against the glisten method of stroke-to-a-shine and one stroke on the squeegee side with a rounded edge.

      

      

   


[Figure 6: The top pair of micro-photographs shows the difference in dots between sharp-edge 1/1 coating and round-edge “glisten/1” coating on 150-tpi mesh. The middle pair shows the same, on 150-tpi mesh, while the bottom shows the contrast with fine lines on 150-tpi mesh. Clearly, the “glisten/1” method produces superior results.]

 
   

The results are quite clear where the 1/1 sharp just covered the threads, the rounded edge with the glisten method gained a considerable thickness of emulsion over the threads giving us much sharper lines and dots. All photos are of stencils that are fully exposed and developed with a 1,000-psi pressure washer at 30-36 inches.

Never “Good Enough”

While no one can argue that any job that is “good enough” to get paid for may not be considered a business success (because the order made money). But saddling the print shop with “good enough” procedures will only translate into problems when the need for precision is necessary in order to successfully finish a project. The introduction of a production-friendly method to increase quality will help any shop tackle more demanding jobs in the future with less downtime attempting to make changes specific to a new task.

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