Written by Master Screen Printer, Douglas Grigar
How to coat a screen may seem like a subject for complete newbies, but if you have been screen printing for a while, you may need to focus on the fact that we are reviewing how to coat a screen for best results in this article. Simple changes to commonly used methods for screen coating can produce greater consistency in your shop, improved product performance and money-saving efficiencies.
When did you last take a critical look at your procedures? They may work, but do they work as well as they can to make your business optimally efficient?
Avoid the “good-enough-is-good-enough” mindset. Aim higher and you will be rewarded.
In this article we will explore –
- Why you will obtain better performing screens with proper levels of EOM (Emulsion Over Mesh)
- The Shine Method for wet-on-wet screen coating to produce significant EOM
- Comparing the Shine Method of screen coating to traditional procedures
The Shine Method of Screen Coating – An Introduction
The primary goal with this article is to introduce you to a quick and production-friendly method of wet-on-wet screen coating that will produce a significant EOM layer thickness. Face-coating as an alternative technique will produce a flatter, smoother-surfaced stencil than the typical wet-on-wet method. However, the increased labour time and supplies required by the application of an additional face-coat of emulsion on a previously dried emulsion layer makes this technique unpopular.
Since wet-on-wet coating is most often the rule, we will not discuss face-coating here. Rather, we will focus on the facts and results of wet-on-wet screen coating procedures in an attempt to make the procedure as effective as possible. So let’s see how to coat a screen for best results…
The Basic Variables of Screen Coating
1. Travel speed of coater
2. Angle of the coating trough relative to the screen
3. Pressure of coating trough to screen
4. Fill level of coating trough
5. Coating trough edge profile
These variables are illustrated in figure 1.
1. Travel speed of the coater
The travel speed of the coater will change the amount of liquid emulsion transferred to and into the mesh. As the operator increases the coating speed, the amount of emulsion deposited will decrease and the amount of bubbles formed in the coating trough will increase.
2. Angle of the coating trough
Increasing or decreasing the angle at which the coater is held to the mesh will affect the amount of emulsion that is transferred to the screen.
3. Pressure of the coating trough to the screen
The pressure that is applied to the coating trough against the mesh will change the amount of emulsion transferred to the screen 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.
4. Fill level of the coating trough
The fill-level of emulsion in the coating trough 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 variances will have the same dramatic effect on EOM thickness whether you are using an expensive coating machine or coating manually.
5. Coating trough edge profile
The edge profile of the trough has an effect on the emulsion-transfer rate. On coating troughs with two sides, the rounded edge could transfer double or more the amount of emulsion into the mesh compared to the sharp edge (see Figure 2). A rounded edge will transfer emulsion faster in less time, resulting in a much higher amount of emulsion deposited.
The above factors give insight into why we would expect to see measurable differences, some of these quite pronounced, when comparing screens coated by different individuals.
It is important to keep all five these factors in mind when considering how to coat a screen for best results. Some time will be required to practice the technique, but the results will be well worth the effort.
Emulsion Over Mesh
Emulsion over mesh is simply the measure of how much the emulsion layer extends beyond the threads of the mesh, and is generally 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 definition and dot formation (see Figure 3).
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).
How To Coat A Screen For Best Results: The Shine Method
When learning how to coat a screen for best results, the desired outcome 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 35-40% or more 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 “shine” 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, de-hazed and degreased for this procedure. Heavy mesh fabric haze will cause the emulsion to flow differently and not shine on the squeegee side evenly; it will have the look of wet cloth but not shine 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 concluded on the 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.
Always, always coat the squeegee side last!
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.
Always dry your stencil squeegee side up.
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.
Fix Mistakes Easily
The shine or glisten procedure allows for an easy fix if the coating is botched: simply scrape off the emulsion from both sides and start over, coating for a shine as before. This is a useful fallback point if you need it.
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 fallback point described above will have less than a 4% change regardless of how much emulsion was coated on the substrate side.
Comparing Other Coating Methods
One very common technique is the 1+1 – one coat on each side of the screen, using the shard edge of the coating trough. This results in a very thin coating of EOM. Once dried, the EOM will be less than 2%. This falls woefully short of the recommended 5% for fine detail and small dots, and is hopelessly inadequate when compared to the suggested 20% EOM for general printing. (See Figure 5)
There is a common misconception among screen printers that a screen coated using the 1+1 technique shows fewer flaws and must therefore be better. The reality of the situation is that a thicker layer of emulsion shows flaws and contamination in the mesh from lack of complete or effective cleaning. Hiding such flaws with a thin layer of EOM does not eradicate them and believing that hiding these flaws is effective or helpful is illogical.
The typical spot colour job printed with a 60 – 63 TPC (150 – 160 TPI) mesh on light shirts may not suffer sufficiently from the lack of sharp stencil profile for the customer to notice, but the fact of the matter is that it will suffer. As tolerances get tighter and tighter, jobs will suffer increasingly until eventually your customer will notice. Do you really want to turn out work that is only just good enough?
A Side-by-Side Look
The micro-photographs shown in Figure 6 below clearly show the differences between 1+1 coating with a sharp edge, against the shine or glisten method of “coat-to-a-shine” and two strokes 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 Shine method coating on 60 TPC (150 TPI) mesh. The middle pair shows the same, on 60 TPC (150 TPI) yellow mesh, while the bottom shows the contrast in fine lines on the same mesh. The Shine method produces superior results every time.
The results are quite clear. The 1+1 method using a sharp edge only just covered the threads while the rounded edge with the shine method gained a considerable thickness of emulsion over the threads giving much sharper lines and dots.
How to coat a screen for best results – Use the shine method! Here is the proof…
All photos are of stencils that are fully exposed and developed with a 1,000-psi pressure washer at 75 – 90 cm.
Is Good Enough Really Good Enough?
Saddling your print shop with “good enough” procedures will only translate into problems when the need for precision is crucial in order to successfully finish a project. Getting your workforce trained to high standards and introducing production-friendly methods to increase quality will allow your business to take on more demanding jobs in future with less downtime. Learning how to coat a screen for best results is therefore imperative in assuring a quality and consistent end product.
Douglas Grigar now presents regular screen printing classes in the UK from our fully equipped workshop in Lancashire. The three days of in-depth, hands-on training will pay dividends on cost savings, time saving and overall improvements of procedures in your shop that will far exceed the outlay required. Can you afford to have staff trained to a “good enough” standard when they could be excellent? Learning how to coat a screen for best results is just one small part of this in-depth training class.