Haze Removal (Ghost Images)
Using dirty, stained screens can lead to low quality prints and expensive stencil breakdown. This How to Guide provides essential advice on how to choose the best haze removal system to remove screen stains and ghost images.
The importance of haze removal:
Most ghost images are made up of microscopic residues on the mesh left over from the previous job. These residues not only prevent the stencil from adhering properly, they can even affect the way in which the ink flows through the mesh to cause a latent ‘ghost’ image in the print. Therefore, it is very important that these residues are removed from the mesh during the cleaning process.
Prevention is better than cure:
Before going into detail about haze removal, it is worth stating that a few simple steps can be taken to reduce, or even eliminate, the formation of screen haze and stains. Exposing screens correctly, cleaning them as soon as possible after printing and using the right Screen Wash, can minimise and even eliminate the need for an additional haze removal process, saving time and money.
Identifying the type of stain
Typically ghost images come from one or more of the following: fused Acetate ‘(locked-in screens’) or Diazo from the stencil, ink that has dried-in, ink staining of the mesh fibres, or even mechanical abrasion of the mesh itself. Knowing the root cause of the stain will then help you choose the ideal haze remover to clean it.
Pro tip: As a simple guide if the stain is the same as the print, then it has come from the ink and if it is a negative of the print, it has come from the stencil.
There are two types of stencil stains:
Diazo stains - Diazo sensitisers are chemical dyes and are therefore, very effective at dyeing Polyester mesh. The yellow/brown stain left by Diazo can easily be removed by using a low caustic haze remover, such as Autokleen Plus. If the Diazo stain is very noticeable, it is usually an indication that the stencil has been under-exposed.
Fused Acetate stains - These are easily recognizable as a lightly colored, translucent residue left on the screen where the stencil was. These stains can be removed using a low caustic haze remover such as Autokleen Plus activated with Autosolve Industrial AF Screen Wash.
Pro tip: It is much easier to remove stencil stains immediately after decoating the stencil whilst the screens are still wet. If they are allowed to dry out fully, then the acetate hardens and becomes more difficult to remove. Use the High Pressure Gun on both sides of the screen for the best effect.
There are five main types of ink stains:
Dried-in ink - If the ink has been allowed to dry in the mesh after printing it is very easy to see. Most inks can be re-dissolved in a powerful cleaning solvent, such as Autosolve, which makes removing them straightforward. Stubborn stains may require the use of a more aggressive solvent and low caustic blend, such as Quick Clean.
Top tip: Applying a ‘stain preventer gel’ to the screen immediately after printing can make cleaning much easier.
Hardened ink - Two-pack catalysed inks are notoriously difficult to remove, as they are formulated to be highly resistant once they have hardened. You will need to use an aggressive very high caustic and solvent haze remover, such as Autohaze, to have any chance of breaking these down. The longer the catalysed inks have been allowed to react the harder they will be to remove.
UV cured inks - Screens that have been used for printing UV curing inks should not be left in a white light area as they will quickly harden and become much more difficult to remove. They will require a high caustic and solvent haze remover, such as Autohaze Extra to remove them. Therefore, it is best to store these screens under yellow safelight (go to macdermid.com/autotype How to Guide – Coating PLUS Emulsions for advice on safelights).
Ink staining of the mesh fibres - Some inks will actually dye polyester thread during printing. If this does happen the stain can only be removed with a very strong caustic haze remover, such as Autohaze. Although this type of stain does not reduce the mesh opening diameter it can cause problems during the exposure of subsequent stencils due to differential UV light absorption.
Mechanical abrasion of the mesh fibres - Although not strictly mesh staining, these ghost images are quite rare and are typically caused by printing very long runs with an abrasive ink. For example, ceramic inks contain a glass frit which will micro-abrade the mesh as it flows through the image. If the is screen is then reused to print a sensitive ink, such as a transparent, then the previous image may appear in the print as a ghost. Haze removers will have no effect on this type of stain and it is best to discard the mesh after printing.
How to remove common ink stains
Most inks can be removed completely without mesh staining if they are cleaned immediately after printing with the right cleaning solvents.
The Alkemirange of screen cleaning solvents are specially formulated to combine cleaning power with low evaporation so they work efficiently and stay longer in the screen.
Use a brush to apply your solvent directly onto the screen. The right brush has soft chemically resistant fibers that help hold the solvent without dripping and will not damage even the finest mesh fibers.
Alkemi screen wash solvents can be rinsed away with water to leave a clean screen ready to be degreased for the next job.
How to use screen cleaning solvents
Brush the solvent thoroughly into both sides of the screen until all the ink is dissolved and then immediately rinse with cold water before using a High Pressure Gun.