Chapter 7
Coating The Screen Mesh With Direct Liquid Emulsion
Now the screen is ready to coat with emulsion. Emulsion is a light sensitive liquid chemical that is applied to the screen; it becomes most light sensitive when dry. For this step the best tool is a scoop coater even if you are the home printer. This is a tool devised to be used by hand that resembles a trough the width of the screen with a rounded smooth edge or "blade" on one side. Typically, a scoop coater for a 20 X 24 screen is about 16 inches wide so that you may coat a screen length wise with one pass from bottom to top. It will give you the most consistent, smooth coat and provide even thickness in the emulsion as well. The thickness of your stencil is directly related to the amount of ink that will be deposited onto the shirt. The thicker the emulsion is, the deeper the stencil is and more room the ink will have to be deposited onto the shirt. This gives better "opacity" or "coverage". However, do not waste time trying to build up this thick emulsion over mesh, or EOM, because you can negate it very easily on press. Your off contact and press settings along with your flood and stroke will be more critical in getting decent coverage and opacity.
VIDEO: Emulsion Scoop Coater Tips
Be aware that when printing basic lower detail art, imperfections in the coating of emulsion are less likely to cause a problem with print results. On the other hand, with higher mesh counts and finer detail, especially halftones, you will need to be sure the emulsion is coated evenly and as consistently as possible.
The side of the screen that comes in contact with the tee shirt is known as the substrate side. The side where the ink is placed and pushed through the screen with the squeegee is known as the ink well side. You will want the thickness of the stencil to be apparent on the substrate side. This is accomplished by how you coat and dry the emulsion on the screen. The result of this will be more prominent on lower mesh counts where you will be able to feel the EOM.
Coating your screens with emulsion should be done in subdued yellow or red light. This part of the process is often done in a "dark room" or an "exposing room" where the exposure table is also located. This is very handy as when the screens are dry, they will need to be handled in the same lighting conditions to place artwork on prior to exposure.
In order to deposit a thicker layer of emulsion on the substrate side, you will need to coat the substrate side first. Always make coating passes length wise starting at the bottom. Then follow with a second coat from the ink well side of the screen. This will push much of the emulsion to the substrate side and it should look very glossy with a wet sheen. Now we dry the screen in a level rack with the substrate side facing down. This will allow gravity to keep the emulsion thickest on the substrate side. Make sure the drying rack is either light safe or in a light safe room, level, and has sufficient air circulation. Poor air circulation will result in dimpled emulsion and an unusable screen.
VIDEO: Coating & Storing Screens Without A Darkroom
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Depending on how good your air circulation is and how many screens you coat at one time, your drying time can be anywhere from an hour for one screen to overnight for a rack full. The drying time will also depend on the relative humidity in your geographic region. Remember that the area where the screens are drying with emulsion should be a clean room with minimal dust being airborne. Dust that falls onto the surface of the wet emulsion on the screen is most likely to cause pinholes or blockage within your stencil area. Play it safe and allow ample time for the emulsion to dry. It is a big waste of time and chemicals if you rush this part because emulsion will not expose properly if it has too much moisture in it. That means a screen may appear to be dry to the touch but in fact it is still very moist. I usually let my screens dry overnight for good measure.
Once the screen is dry it is very light sensitive and all steps hereafter up until the screen is taken out of the exposure unit should be done in subdued yellow or red light.
Some screen coating tips to remember:
It is always best to start with the highest quality direct liquid emulsion for the best performance. Make sure to choose your emulsion properly for your printing application. Understanding emulsion characteristics will help you do that with beneficial results.
When coating screens manually it is often difficult to achieve a smooth uniform surface with consistent thickness. To ensure complete coverage, filling in all the mesh openings, you will want to use a multiple pass method with 2 passes being the minimum. That means once on the substrate side and once on the inkwell side. Scoop coaters come with a thin, sharp edge and a duller, round edge. The thin, sharp edge will leave less emulsion on the mesh where the round edge will leave more emulsion on the mesh.
- Only coat your screens under red or yellow safe light conditions.
- Mix your emulsion well before dispensing it into your scoop coater.
- A scoop coater should be clean and its edges free of dents and nicks. A smooth edge is best.
- Make sure the scoop coater is well filled. You do not want to run out of emulsion halfway up the screen.
- Once the scoop coater is filled, the emulsion should be applied immediately. Emulsion left sitting in the open is a target for dust and debris.
- In low humidity environments, an open container of emulsion can begin to form a skin quickly, so keep your emulsion in an air tight container until you need it and after you apply it wash out your scoop coater right away.
- Once the emulsion contacts the mesh, proceed slowly. Emulsion is viscous, and it needs time to fill the mesh openings. Use a slow smooth passing action to move the scoop coater up the screen. Moving too fast can trap air bubbles in the emulsion and when they pop, the mesh can remain open creating pinholes.
- Coat the substrate side of the screen first. Begin your coating pass at the bottom of the screen. Firmly support your screen as you press the edge of the scoop coater against the mesh. Move slowly up the screen and slow down to a stop near the top. Twist the scoop coater upwards and off the mesh. Flip the screen quickly and repeat on the ink well side.
- If you want a thicker stencil, use more passes for each side always ending with the ink well side. If a thinner stencil is desired, then use less passes, again always ending with the ink well side. This forces the emulsion through to the substrate side. This is where the stencil must extend above the threads of the mesh to achieve a good emulsion over mesh rating.
- When drying your screens, make sure to place them substrate side down and in a rack that is parallel to the ground. This allows gravity to pull the emulsion toward the substrate side resulting in better stencil thickness. The screens should be allowed to dry in the dark and worked with in safe light conditions only.
- Direct emulsions must be dried completely before being exposed. Any dampness in the emulsion during exposure will interfere with the chemical reaction that takes place to cure the emulsion. This could also create a poor stencil and result in a screen that is difficult or perhaps impossible to reclaim. Allow your screens to dry slowly and make sure there is ample air circulation to allow moisture to dissipate away from the drying screens.
- For extra thick stencils it is possible to coat a screen and allow it to dry. Then the substrate side may be coated again to fill in imperfections in the emulsion surface and gain some minimal thickness. The thinner, sharp edge of your scoop coater will be ideal for this technique.
- Always be sure to use fresh emulsion and work in a clean, dirt-free environment. Proper mesh preparations are also necessary to ensure excellent emulsion performance. Try to learn about your emulsion’s characteristics. Knowing this information will help you achieve the results you desire.
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VIDEO How Long Does The Stencil Last On Screen?
VIDEO: Cleaning The Scoop Coater Experimental Hack
Understanding Direct Liquid Emulsions:
A direct emulsion is one that is applied to the screen in liquid form with a scoop coater. This type of emulsion has specific qualities that differ from capillary film stencils. The first and foremost important thing to remember with emulsions is that they react to ultraviolet light. This reaction causes it to "cure" or become hard. Normal ambient white light has enough UV to expose emulsions over longer periods of time. This is why emulsions are worked with under yellow or red safe light conditions.
That means your exposure source must output a good amount of UV light to expose your screens within a reasonable time. The less UV your light source outputs, the longer your exposure times will be.
Because emulsions can also react to excessive heat, prolonged exposure times to high intensity light sources with low UV output can be problematic. An example of such a light source would be the sun. Light sources like the sun can literally cook the emulsion with heat alone causing it to solidify.
Different emulsions vary in their photo sensitivity. Some will expose faster than others depending on light sources. This is why it is necessary to consider your light source when choosing an emulsion.
Today's emulsions use only two types of photo sensitizers. Diazo, which has been around for some time and Styryl Basolium Quaternary photopolymers called SBQ photopolymers which are relatively newer. The sensitizers are added to a resin base of polyvinyl alcohol or polyvinyl acetate to make the emulsion.
This means that emulsions can be categorized into three groups based on the two photo sensitizers; diazo, photopolymer, and dual cure emulsions. Dual cure emulsions combine the best features of the other two. At the same time, they compensate for some of their individual flaws.
The basic three types of direct emulsions: Diazo, Photopolymer, and Dual Cure.
Diazo emulsions are the least expensive and have the lowest light sensitivity of the three. If your light source is weak, a diazo emulsion can cause you difficulty. However, they also have the widest exposure latitude and can be forgiving. Diazo emulsions are not well suited for fine detail because they tend to make a thicker stencil. You will want a thinner stencil for the reproduction of fine details and halftones.
Another thing to note about diazo emulsions is that they come in either solvent-resistant or water-resistant types. That means if you use both solvent and water-based inks, you would need to stock two types of diazo emulsions. They also must be mixed with the sensitizer correctly to achieve optimum results. Once mixed, they have a shelf life of about two to three months depending on conditions.
Photopolymers or SBQ photopolymers are also called one-pot or one-part emulsions. These are the most light sensitive and fastest emulsions. They also have excellent shelf lives more than one year but they tend to cost a lot more too. Because the sensitizer is mixed with the polyvinyl base by the manufacturer, SBQ photopolymer emulsions are excellent at the reproduction of fine detail.
Although SBQ photopolymer emulsions are faster, they tend to perform best with good light sources and superior film positives because their exposure latitude is small. If you work with water-based inks, you can find some SBQ emulsions formulated to be specifically water resistant. They are also less affected by ambient humidity most of the time.
Dual cure emulsions have a combination of the qualities of both diazo and SBQ emulsions. As a result, their exposure times fall in the middle range of diazo and SBQ emulsions. The exposure latitude is wider than diazo and they can reproduce fine detail better as well. A dual cure direct emulsion of high quality will have a wider exposure latitude meaning there is a wider range of exposure times that will produce a working stencil. These are more forgiving with poor film positives. Dual cure emulsions are usually less expensive than SBQ photopolymers and work well in high humidity. The shelf life of a dual cure is often much less than that of a SBQ photopolymer emulsion.
The dual cure emulsions are also a two-part mixture. They must be mixed well to achieve their best performance much like the diazo emulsions. You can find many water-resistant varieties of dual cure emulsions.
VIDEO: Photopolymer vs Dual Cure Emulsions
Viscosity and solids content of emulsions can also affect the performance in your application. The higher viscosity, the thicker the liquid is. Thicker emulsions do a better job of clinging to lower mesh counts. Thinner emulsions, on the other hand, easily penetrate higher mesh counts. Higher viscosity emulsions will have a higher percentage of solids. Emulsions with higher solids content will reproduce fine details and halftones better. Solids can also help fill gaps in course mesh.
Tips:
- Emulsions do not need to be stored in a refrigerator to prolong shelf life.
- Make sure not to let the emulsion freeze.
- Do not expose your emulsion to excessive heat. Above 100 degrees Fahrenheit, heat may cure emulsion.
- Always work in darkroom conditions with your emulsion to avoid
problems.
- Mix your emulsions well before each use.
- Make sure the emulsion container lid is secure and tight during storage.
- Do not use a wooden stirring stick for emulsions to eliminate pinholes.
Capillary film is the type that is applied as a film in roll form with a smooth water squeegee. There are advantages and disadvantages to using either direct emulsions or capillary films.
Because they are more than 50% water, direct emulsions can dry unevenly as the moisture evaporates. As the water evaporates out of the emulsion, it contracts and shrinks slightly. As it does, it conforms easily to the surface of the mesh and imitates its surface characteristics. These two factors combined can make it difficult but not impossible to obtain a smooth emulsion surface with direct emulsions.
Believe it or not, the smoothness of the emulsion surface on the substrate side will affect your print results. The smoother, more consistent the surface, the tighter and more complete the stencil contacts the substrate providing a good "seal". This ultimately restricts ink flow and will increase sharpness and resolution.
Direct emulsions are more durable. This is one of the reasons they are chosen most often in textile print shops. A direct emulsion is better suited for long duration print runs for excellent stencil life. Because direct emulsions encapsulate the mesh from both sides, it has better adhesion qualities and better durability. Direct emulsions also have the benefit of being able to be layered or coated to different thicknesses or emulsion over mesh values, (EOM).
VIDEO: What Is The Substrate Side Or Ink Well Side Of The Screen?
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Capillary films have a predetermined thickness, (EOM), and surface qualities because they are manufactured to factory specifications. It is almost like a fruit roll up in that it is a film of light sensitive emulsion that is adhered to the screen. But these roll ups are made in varying widths and are perfectly smooth. Obviously, this makes getting a smooth surface very easy. As stated above, this can affect print results.
Capillary film also has the disadvantage of being applied from only the substrate side. This means it does not have the same adhesion qualities as direct emulsions. Therefore stencil wear and durability can be an issue with capillary films on high volume print runs.
All direct emulsions are evaluated and/or rated by the following standards which will affect stencil performance:
Resolution - the ability to reproduce fine detail.
Definition - the ability to accurately reproduce the edges of the artwork as it appears in the film positive.
Bridging Characteristics - the ability to span or "bridge" the open spaces between mesh threads.
Viscosity - refers to a liquid's ability to flow. This is important during coating. Low viscosity emulsions flow easily, which can help fill in the tiny mesh openings in higher mesh counts.
Solids Content - refers to the solid particulate count in the emulsion. Emulsions that have higher solids content generally do better at bridging mesh openings.
Rz Value - the surface difference between the hills and valleys of the woven mesh caused by contraction and shrinking of the emulsion during drying is expressed in a numerical measurement.
Bridging characteristics describes the emulsion's ability to span or "bridge" the open spaces between mesh openings smoothly. Stencils that tend to take a longer zigzagging route around the mesh opening will create a saw-toothed edge. That is known commonly as saw-tooth-ing. This results in poor stencil resolution and definition. An emulsion with higher solids content will perform better in this respect. Higher solids content emulsions will also experience less contracting and shrinking during drying.
VIDEO: What is “Sawtoothing” And How To Prevent It
When coating your own screens with direct liquid emulsions, it is necessary to easily identify the cause of common problems. And of course, it would then be important to understand how to solve them as well. Some of the most common direct emulsion problems can be directly related to screen making. Some problems are as easy to identify. Check out this briefing on the most common issues with direct emulsions.
Formation of fish-eyes, crescents, half-moons - This could be the result of inefficient mesh prep procedures. It is very important to remove all the grease and dust. Any dirt or debris will cause emulsion problems. These problems may be easily avoided with proper mesh prep. Poorly mixed sensitizers and photopolymer emulsions that have been sitting for long periods of time and not mixed before use may also cause these symptoms.
Pinholes and/or air bubbles - Pinholes are often caused by dust or debris on the screen or in the emulsion during coating. But pinholes may also be produced by air bubbles being left in the screen due to rapid coating passes. It is best to use a slow pass when coating the mesh with emulsion. Also, after mixing the emulsion, it is recommended to allow it to sit for some time so that all the air bubbles introduced during mixing may settle.
Poor emulsion adhesion - This too can be caused by improper mesh preparation. It is usually caused by not abrading new mesh before first use and/or poor de-greasing. But it is a problem that may occur if the screen is exposed before it is completely dry. That is to say that if there are still high amounts of humidity in the emulsion, it can affect adhesion. This should be kept in mind if you work quickly and especially in high humidity regions. Another cause of this may be incompletely dissolved sensitizers. Make sure your emulsion is well mixed when using two part diazo or dual cure emulsions.
Undercutting or loss of detail - White mesh may produce this problem by halation. This is light scatter caused by light passing through the clear film base on its way to the emulsion. This can be minimized by making sure your film positive comes in contact with the emulsion on the same side that the image was applied to the clear film base. Over exposure can also cause undercutting. Be careful to expose the emulsion just enough to thoroughly cure it.
Saw tooth effect - Poorly coated screens can be the direct cause of this all too common result. Saw tooth effect happens when the emulsion cannot bridge the gap between mesh openings especially in diagonal directions from corner to corner. It may also occur if the emulsion coating is too thin and forms to the geometry of the mesh surface. Using High quality emulsions with good bridging characteristics and creating a smooth, thick emulsion surface on the substrate side will help reduce or eliminate this.
Emulsion is difficult to reclaim - Leaving ink or tape adhesives on emulsion after using a screen for long periods of time may cause the emulsion to become permanently hardened. Especially if the emulsion was under exposed. This may also happen with other chemicals or ink removers. Leaving excessive ink wash on a screen directly prior to reclaiming may also cause difficulty. And never spray any stencil remover onto emulsion and let the screen dry. That will harden the emulsion beyond reclaiming especially if it is allowed to sit a long time. Usually, most emulsion removers will work with most emulsions but you may want to make sure your stencil remover is compatible with your brand of emulsion.
VIDEO: Preventing & Fixing Pinholes
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VIDEO: Why Won’t My Emulsion Stencil Reclaim?