William Shorter, Product Manager at CPS, shares his knowledge with a How to Guide to Haze Removal that covers identification of different stains and ghost images and the best methods for their removal.
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 stainTypically 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.
#ScreenPrinitngTopTip: 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.
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 Sodium Hypochlorite based haze remover. 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 recognisable as a lightly coloured, translucent residue left on the screen where the stencil was. These stains can be removed using a Sodium Hypochlorite based haze remover activated with the recommended Screen Wash or ‘Activator’.
Top 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.
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 using an appropriate and effective Screen Wash solvent, which makes removing them straightforward. Stubborn stains may require a more aggressive high caustic and solvent haze remover.
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 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 either a Sodium Hypochlorite type haze remover with Screen Wash ‘Activator’ or a high caustic solvent haze remover, to eliminate them. Therefore, it is best to store these screens under yellow safelight (http://www.cps.eu/guide-detail/44_how-to-coat-cps-ultra-coat-emulsion--screen-printing-guide).
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 Sodium Hypochlorite type with Screen Wash ‘Activator’ or very strong caustic based haze remover. 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.
Haze removers are powerful chemicals and must be handled carefully. Always read the Safety Data Sheet before use and wear PPE when handling them. It is very important to always rinse off any haze remover with a low pressure spray before using the High Pressure Gun, otherwise you will atomise the chemical and expose the operator to this mist. Did you know, when atomised caustic lands on screens that have already been cleaned they will produce pinholes the next time they are used?
All caustics react with aluminium, so take care when applying the low viscosity haze removers to aluminium frames and do not leave in an aluminium scoop coater overnight.
Summary: It is always best to take a proactive approach to screen cleaning, to ensure that every screen is cleaned effectively before it is reused. Knowing the type of stains and the best way to treat them will help you remove them as quickly and efficiently as possible.
The sight of a ghost image on your screen should not be ignored as this stain can, and probably will, cause you problems when that screen is next used, costing you time and money.
Name: William Shorter
Mobile Number: +44 7831 109181