01 Jan Improved Best Practice Screenmaking Techniques
For those who can never seem to get the very best detail out of their stencils, have to constantly block out pinholes, have stencils that breakdown on the press, and are thinking that there must be a better way. There is! This article describes a number of best practices to incorporate into your screenmaking skills.
Bill Hood, ASDPT Fellow
Updated March 1, 2020
If you learned your screenmaking skills from an old book, or by listening to other novices (and a few experienced screenmakers) on social media, you are probably having problems. And the bad news is that you may not even know you have problems, because it seems that everyone you know has the same outcome. This has led you to believe that the norm is to have the same experiences as others. But, it does not have to be that way. Some signs of improper screenmaking practices are…
- Failure to record the details in the art to the final stencil
- Blocking out pinholes in your stencil
- Experiencing jagged edges on your image rather than sharp edges
- Making multiple strokes to get the necessary opacity
- Having to resort to a print/flash/print technique
- Having your stencil breakdown prematurely on the press
Now the good news! You can improve your screenmaking skills and all of these problems can be eradicated from your life.
Before we get too deep into the screenmaking changes, we will need some tools to measure our results.
Mesh Tension Meter – The tension meter is an absolute necessity in screenmaking if you hope to have consistency in your screenmaking process. Those who do not use a tension meter risk sending a finished screen to the press, and then realizing that the tension is too low to hold proper registration. They end up fighting the ability to properly register the job and end up wasting far more than the one minute per color necessary to bring the colors into registration.
Mesh Tension meters can be analog or digital. The battery-powered digital types are no more accurate than the analog types, and when the battery power is reduced, these units can give false readings. There are two different probe types available, a round probe and a bar probe. The round probe will only provide an average between the warp and weft directions of the mesh and thus readings can be misleading. The bar probe gives a directional reading over 3/4-inch of threads and assures better tension levels.
Scoop Coater – For those who coat their emulsion manually, you will need a scoop coater that is approximately 6 inches narrower than the widest dimension of the frame to be coated. This leaves 3-inches on either side of the coated area, which is within the free-mesh area and does not need emulsion. The free mesh area can be filled with blockout or protected with a permanent or semi-permanent blockout.
Better scoop coaters have an angled end cap to assist you with the correct coater angle. They also have a dull (more radius) side and a sharp (less radius). Use the dull side for initial coatings and the sharp side for making face coats on all mesh counts.
Turning the screen on its widest side will decrease the coating distance and the time to coat screens. While this time may seem insignificant at first, those seconds saved can really add up at the end of the year.
Assuming that you are using an 18 x 20 inside dimension frame your maximum image size is 14.126 x 12.771 inches. You only want a coater that extends about 1/2-inch on either side of the maximum image resulting in a coater width of 15-inches. If you have multiple size frames, then you may need more than one scoop coater.
Moisture Meter – The moisture meter is an often overlooked tool. The meter is used to measure the moisture content of the dry emulsion to assure that the moisture is below 4 percent, which is necessary for proper exposure of the emulsion. Thus we will need a meter that has an accuracy of + / – 1%.
And, we need a contact-pad style meter versus the pin-type, which is used to measure the moisture content of wood, in which the pins have to be pushed into the heart of the wood to get a measurement.
When the contact pads of the moisture meter are pressed onto the surface of the dry emulsion, the meter sends out an electromagnetic (think radio) wave into the material to be measured. The meter then interprets the fluctuations in the wave to establish the moisture content of the emulsion being measured.
Moisture meters come in three basic types: inductive, capacitive and high-frequency galvanizing or dielectric constant. The inductive meters are some of the most accurate available. However, they are not inherently suitable for screen printing because they use two sharp probe pins to gather data. The repeatability of some very low-cost, pin-less moisture meters, is erratic and often incorrect because of the thin nature of the emulsion. Some contact-pad, capacitive models, but can give false readings due to the ambient moisture in the screenroom.
A relatively accurate moisture meter is the TQM Aqua-Check, sold by emulsion manufacturers through better distributors.
Thickness Gauge – The thickness gauge is a necessity for producing stencils that are consistent in quality. With the ability to measure the thickness of the manually coated dry stencil, you will get a consistent thickness of coatings, accurate exposures, control over the emulsion over mesh ratio, consistent ink deposits, and the ability to eliminate double strokes.
These are available for anywhere from $20 to $2,000 USD. I personally use a Positector 6000 FS by Defelsko with an FS Regular Probe ($725USD) because I want to connect to my iPhone or computer to record and analyze the readings. Earlier I used a HL II Thickness Gage from Highline ($149) that worked well but did not have compatibility I wanted. The thickness gauges are also known as mil gages or paint meters as they are more widely used by paint and body shops. They should come with a metal puck to place on the other side of the stencil for accurate readings.
Coating Stand – Emulsion flows into the mesh opening by gravity, thus the more angle on the screen, the more emulsion is deposited. This implies that you need to position the screen at a set angle to achieve consistency. While there are commercial holders for coating, they can be easily made in an hour from scrap wood. The Wall Mounted Screen Holder (shown below) has served thousands of screenprinters well for decades. The size of the holder will vary depending on your screen size. It is mounted so that the top of the unit is at elbow height. During the coating procedure, you want to end up with the scoop coater about the distance from your elbow to the ground. Any higher and it makes the coating process more difficult.
The screen is placed in the holder with the forward edge being held in place by the 2×2 inch woodblock on the front. Then the top of the screen is leaned against the backboard and the L-hook screw is rotated downward to hold the screen in place. This assures that the emulsion flows uniformly from screen to screen. The L-brackets are necessary for extending the life of the holder. Note, that while the illustration shows the L-brackets on the top, you may want to move them to the bottom for heavier or larger screen sizes. I know of several that have been in use for over 40 years and show no signs of wear.
Best Practice Coating Methods
Most of the best screenmakers that I know coat their mesh with the correct Emulsion Over Mesh Ratio (EMOR) in mind. They are more concerned with printed image quality and are coating for a specific stencil thickness. This also allows them to make a stencil that does not require multiple strokes to achieve opacity.
So, how do they coat? They make what is considered a base coat which completely fills the mesh openings, but without the necessary build-up. They allow this base coat to dry.
Once dry, they measure any EMOR that is present with an electronic thickness gauge. They then start building up the thickness on the bottom of the stencil (opposite the squeegee side) with thin coats that dry very quickly. This also promotes a better Rz factor (smoothness of the stencil) that increases the resolution of the image edge.
When they are close to the correct EOMR, they make a face coat on the squeegee side of the stencil to create what I coined as an Anti-Friction Coating in 1983, which allows for the squeegee to move fast across the surface as it shears the ink from the top of the mesh openings. As well, this Anti-Friction Coating decreases the “chattering” of the mesh caused by a squeegee that “drags” across the surface of the stencil. This creates two additional benefits. It improves the resolution of the ink flow at the image edge, and it decreases the opportunity for mesh movement which is reflected by a slight shadow in the printed image on the mesh that is not correctly tensioned.
Yes, it seemingly takes longer to prepare a stencil that works well, it is time well spent if one wants and needs to improve the quality of their printed images. When one benefits cost, the care that goes into the stencil not only creates benefits, it has a tremendous return on investment.
Choosing a Coating Method
There are many ways in which one can do things. And, for the sake of argument, there is no incorrect way if you the way you choose meets your goal.
For example, if you were in Los Angeles and wanted to go to New York, who is to say that going west would be a bad decision for a way to get to New York. It would probably be a more scenic route and you would have many adventures along the way. Or you could go north up into Canada and then head east. Or you could go south and turn around and head north when you arrived at Ushuaia. That would be a very scenic route as well. The problem with all of these and more routes that you might take is that they take up a lot of time and the cost is much higher than using a more direct route.
I have seen screenmakers take many different routes to accomplish the goal of producing a simple stencil. And, for them, their route produced no issues for them. They were happy with the route they chose and continue along their way over many years.
However, the most successful screenmakers take the shortest route to achieve the prescribed goal of producing an outstanding stencil – and taken together as a majority, they are doing what is known as the Best Practice way.
Drying the Emulsion
Continuing with our Best Practice method, let’s discuss the drying of the emulsion. Just as in the coating technique there are many ways to dry the emulsion.
If you do not allow the emulsion to dry thoroughly (with 2 to 4 percent moisture level) before making the exposure, the resulting exposure will take more time as the sensitizer cannot properly cross-link with the alcohol in the emulsion there is moisture within the emulsion. Your exposure, if set for a completely dry stencil, is 45 seconds, and you attempt to expose a stencil for 45 seconds that has an 8 percent moisture level, the stencil will not be fully exposed. The detail in the stencil will suffer and you risk having the stencil delaminate from the mesh during development or worse yet, on the press during the print run.
The solution is to invest in a moisture meter so that you can know the moisture level prior to exposure. This will assure that you will take stencils that are properly dried to the exposure unit and that your stencils will be properly exposed.
All emulsions have a sensitizer that is sensitive to UV energy, but also subject to cross hardening when exposed to temperatures above approximately 104˚F (40˚C). You should never allow the emulsion to be heated past the manufacturer’s suggested temperatures as this heat will cross-link the emulsion and cause the stencil to not expose or develop properly.
Screen Alignment During Drying
There are three ways to align the screen when drying the emulsion. Horizontally with the squeegee side of the mesh up, horizontally with the squeegee side of the mesh down, and or vertically. To dry the emulsion any alignment, if drying is your only goal, will allow the emulsion to dry.
However, when drying a stencil made with a direct, liquid emulsion the Best Practice way would be horizontal with the squeegee side of the mesh up. This allows the wet emulsion to move down to the bottom of the mesh and produce a better Emulsion Over Mesh Ratio (EOMR) on the substrate side of the mesh, which in turns provides for a shoulder to stop the flow of ink and form a higher resolution edge definition in the final print.
If you dry the direct, liquid emulsion in a vertical alignment, you risk having the emulsion progress down the mesh while it is drying. This can produce a slightly thicker thickness of emulsion at the bottom of the screen, which increases the EOMR and produces a different thickness of ink at the bottom of the screen. While not a great concern on spot colors with large areas, fine details and halftones would suffer in resolution.
When using a capillary stencil, the Best Practice method would be to dry the screen horizontally with the squeegee side of the mesh down as the emulsion on the capillary film is not a liquid and the slightly moist emulsion next to the mesh can move down and around the mesh threads to produce a better bond.
In your explanation of the coating method I gave, you will note that the base coat was dried before any additional coats were made. This changed the scenario of how the emulsion would flow in the drying process.
When you coat a mesh using direct, liquid emulsion the liquid is smooth on the surface. However, as it dries, the emulsion flows downward. As the water evaporates from the emulsion the surface is no longer smooth. If you were to dry a stencil coated with direct liquid emulsion and dry the emulsion with the squeegee side down, you would have a smooth surface on the squeegee side, but the emulsion on the print side, which was the top of the drying stencil you would note that the surface is rough. Without making a final face coat on the print side, your EOMR is mostly on the squeegee side and the resulting printed image would suffer in edge definition.
Once you have allowed the emulsion to dry thoroughly (with 2 to 4 percent moisture level), you are ready to head to the exposure unit. However, you need to know how to find the correct exposure for your unit.
Film Type Exposure Guides
The film type exposure guides are not what you might be led to believe. It is true that one can find the exposure for a single stencil, but you cannot use the information that the guide provides on the next stencil, because the exposure will be different. Exposure will vary depending on mesh color, weaving pattern, mesh count, thread diameter, mesh tension, emulsion type, emulsion moisture content, and the stencil thickness. If one of these changes, the exposure must be changed.
While it is possible to bring most of the above list of variables under control by recording the different base exposure times, the one thing that presents the biggest problem is the stencil thickness. This can be easily resolved by measuring the stencil thickness with the electronic thickness gauge.
The electronic thickness gauge is a great investment as it can be used to measure the thickness of any stencil prior to exposure and derive the exact exposure needed to cross-link the polymers without fail. This double checks of emulsion thickness before exposure will keep the screenmaker from having to remake screens due to incorrect coating methods. It can also be used to measure the thickness of the ink deposit and help decide on what mesh should be used to achieve a duplicate deposit.
Remember, the printed ink thickness on a substrate is directly related to the stencil thickness. A 300-micron thick stencil will lay down a 300-micron thickness of wet ink. Although the ink will be reduced in height slightly during subsequent colors and again in the dryer as it cures, the operator can be extremely close to duplicating the deposit.
The thickness of a stencil is directly related to the exposure time necessary to polymerize the entire stencil thickness. It should be obvious that a 230-48 mesh and a 110-80 mesh both coated with the same emulsion/mesh ratio would require different amounts of exposure to assure that the best results are obtained from each particular stencil.
Checking the thickness of the dry emulsion will give you a mathematical factor that can be used to assure proper exposure. That is if a stencil is 30% thicker than the control exposure of any given thickness, then the new exposure would be 30% more than that of the control exposure.
Using a pre-configured exposure based on the use of an exposure calculator for a given thickness, one can easily use the thickness of the measured screen to adjust the exposure based on the thickness of the emulsion. For example, if your benchmark exposure was 40 seconds for a stencil that was 50-microns and the current stencil was 75-microns, then the exposure would be 1.5 times 40 seconds or 60 seconds.
If your current thickness is more than the benchmark the formula is:
Current Thickness of 75 / Benchmark Thickness of 50 = 1.5, thus Benchmark Time of 40 Seconds x 1.5 = 60 Seconds.
75/50=1.5 and 40×1.5 = 60 seconds
If your current thickness is less than the Benchmark thickness the formula is:
Benchmark Thickness of 50 / Current Thickness of 45 = .9, thus Benchmark Time of 40 Seconds x .9 = 36 Seconds.
50/45=.9 and 40x.9= 36 seconds
There is a lot more to the coating technique and this article is only to provide a few thoughts on the subject. There are other articles on this site to help you better understand the coating process.