Preventing and Removing Haze
Ghost haze is a problem you can conquer when you arm yourself with the proper chemicals and use sound techniques.
As the number of products referred to as haze removers continues to grow, so does the confusion over
what classifies as a haze remover. Some degreasers also dehaze, and some dehazers also degrease. There are abraders that dehaze and dehazers that abrade. To simplify matters, let’s organize and define common screen-prep and cleaning products.
Degreasers consist of one or more of the following ingredients: detergents, emulsifiers, surfactants, and wetting agents. They clean the mesh prior to application of emulsion or film and remove most forms of contamination and foreign matter from the screens so a smooth, uniform, blemish-free coating can be achieved. Screens should be degreased just before the mesh is coated. Degreasers are not intended to dehaze.
Abraders scratch the knuckles of the mesh at a microscopic level, which increases the surface area and helps improve stencil adhesion. The active ingredient is silicon carbide, which is a dark, insoluble, crystalline compound used to abrade the mesh. These products are most frequently used to improve the durability of capillary and indirect films when longer production runs are required. Abraders are not intended
Ink removers consist of various blends of solvents used to dissolve inks for cleanup after printing, or for use while printing. If used after printing, they are classified as ink washes or ink degradants. These ink removers evaporate slowly, contain emulsifiers, and are used during the reclaiming process. Ink removers used while printing are classified as screen openers, press washes, or on-press cleaners. These ink removers are used to open up a clogged screen, to clean excess ink from the substrate side of the screen, and for making color changes at press. They usually evaporate faster than ink washes and preferably leave little to no oily residue so tapes can be used promptly thereafter. Ink removers are not intended to dehaze, but some show promise when used only for removing ink haze.
Haze removers traditionally contain some form of caustic ingredient, such as sodium or potassium hydroxide and/or sodium hypochlorite, and may include a solvent or solvent blend, as in the case of most single-component haze removers. They remove ink and/or emulsion ghost-haze images (stains) that remain in the mesh after removing the ink and emulsion from the screen during the reclaiming process.
Classifying ghost images
The two basic types of ghost haze are stains left by the printing ink and those left by the emulsion. The severity of ghosting often depends on the type of ink used. For example, catalyzed inks contain solvents that react with low-surface-energy substrates to improve ink adhesion. These types of inks can create ghost haze that is difficult to remove. Inks also contain pigments that get trapped in between the knuckles of the mesh, bond to the mesh, or become partially dissolved by hot solvents and slightly penetrate the mesh.
Another area we think about is the negative image created by the stenciled parts of the screen. Here, the emulsion or film used to define the customer’s artwork most often causes the ghost haze; however, the ghosting can be attributed to a combination of the emulsion and the ink staining the mesh in these areas.
Underexposure prevents the emulsion that encapsulates the mesh from the squeegee side from absorbing enough UV light to become anchored firmly to the screen. When developed, most of this emulsion rinses away along with the unexposed emulsion from the image area. The only firmly anchored emulsion resides on the side closest to the light source—the substrate side. This leaves mesh threads on the squeegee side exposed and susceptible to staining from ink and emulsion. Additionally, a reaction may occur between partially hardened emulsion, which may remain on the squeegee side of the screen, and the ink. This reaction makes reclaiming difficult and often results in excessive ghosting.
When deciding which haze remover will best meet your needs, you want to consider which type of haze is most prevalent in your shop. If you determine that ink is the only cause of your haze, you may be able to remove it by using a highly effective ink remover, sparing your screens the damage caused by unnecessarily using aggressive and caustic haze removers, thus extending the screens’ useful life. If, on the other hand, you’ve determined that emulsion is the only cause of your haze, you may not need to use a solvent activator with your haze remover in the case where two-part haze removers are used, thereby saving you time and money.
In most cases, however, printers experience haze caused by both the ink and the emulsion. Two-component haze removers usually work best in such situations, followed by haze removers that contain a caustic ingredient and some form of solvent. Typical two-component haze removers work most effectively by applying the first component to a dry or nearly dry screen. These systems are less harmful to the mesh, but they work slower than caustic systems. For maximum effectiveness, they should be allowed to dry naturally on the screen. Once dry, they are activated by applying a solvent ink wash onto the dried haze remover. This ensures that ink and emulsion stains are treated. If you find that time is a factor, or you do not want to use two products for removing haze and are not as concerned about potentially harming the mesh, you should select a caustic haze remover that contains solvent.
Ink and emulsion are not always to blame for haze. Other contributing factors, such as process variables, are often overlooked. Process-related variables contribute to the severity of ghost images as much, if not more, than your inks and stencil systems. Since process-related variables are often more controllable than the type of ink or emulsion you use, let’s take some time discussing how these variables contribute to ghosting.
Low and unstable screen tension Low and unstable screen tension can contribute to high degrees of ghost haze. Low-tension screens require excessive off-contact on press in order to achieve proper peel/snap-off during printing. This causes inordinate mesh elongation as the squeegee stretches the screen during the print stroke. At this point, the ink transfers through the mesh, and pigment particles get trapped in the knuckles of the mesh when the screen relaxes to its resting position. High, stable tension reduces ghost haze.
Unnecessary use of roughening agents Roughening agents, or abraders, improve stencil bonding and durability, but they should not be used unless absolutely necessary. An alternative is to use a degreaser/wetting agent that treats and conditions the mesh for improved stencil durability.
Improper drying Improperly dried and/or exposed emulsions are the leading cause of emulsion ghosting. Emulsions with residual moisture do not cross-link thoroughly. If the drying area has a relative humidity greater than 50%, residual moisture will be present in your coated screens.
Underexposure Unfortunately, most screen printers don’t understand what complete exposure means. Many believe it is the shortest exposure in which the emulsion doesn’t fall off the screen when developing with a garden-hose sprayer. If this is how you pick your exposure times, you are grossly underexposing the emulsion and contributing to your haze problems.
Use of hot solvents Use of hot solvents causes emulsions and polyester mesh to swell, trapping pigments in the weave of the mesh. Haze removers have difficulty reaching trapped pigments, which reduces their effectiveness.
Delayed ink removal The longer you wait to remove inks from screens after production, the more difficult it becomes to remove the resulting ink stains. This is especially true for solvent-based and many UV inks. Applying a slowly evaporating ink wash to screens right after ink is removed is an effective way to reduce ghost images when immediate reclaiming is not feasible.
Incorrect use of reclaiming chemicals Misuse of screen-reclaiming products contributes to ghosting problems. All screen-cleaning products should be applied to both sides of the screen and brushed into both sides of the screen. Remember to remove these products from both sides of the screen with a high-pressure wash . What some perceive as emulsion-haze problem can actually be a procedural issue—for example, pressure washing from only one side of the screen.
Latent or phantom haze This type of haze is virtually invisible on the screen when viewed head on.
There is no apparent stain, and the screen mesh may appear like new. Only when the screen is viewed at approximately 45° does the latent image become apparent. Latent haze may show up in the most unlikely places, such as embedded in the design of a subsequent job. The printed image shows areas of lighter color anywhere the image overlaps a previous job’s latent haze. If screens are not processed correctly and carefully during each step, a thin film of chemical residue may contribute to latent haze. For example, underexposure and improper developing cause difficulties in reclaiming and leave the stencil susceptible to other chemical reactions with inks and ink removers. These reactions can cause physical changes to the mesh that may contribute to latent haze.
Possible remedies for latent ghost haze include keeping a close control over all screenmaking and cleaning procedures, using mesh-preparation products that rely on surface treatments and wetting agents to balance the surface tension of the mesh, and using a slow-acting, two-part haze remover that contains sodium hypochlorite and can be dried on the screen.