The Screen Print Column On press cleaning is easy?
Cleaning on press is probably seen as one of the easiest cleaning processes in screen printing. After all, you just take a rag/tissue soaked in solvent and re-open the mesh ready for printing. William Shorter of SPT Sales + Marketing takes us through this process and explains the technical problems that can occur.
W
hen it comes to screen cleaning respectively mesh opening on the press during long print runs or after an interruption in the printing process, the operators often look for the cheapest press wash.
They want a general-purpose press wash that has the quickest evaporation rate and cuts through the dried inks quickly. Why do they want this? Time lost on press can affect the profits: the quicker they can get back to printing, the more money can be made. Is this false economy though? Cheap fast-evaporating multipurpose press wash solvents, like commonly used cellulose thinners, generally have a low flashpoint, making them highly inflammable compounds. They contain a wild mix of ketones, esters, hydrocarbons etc. and very often also parts of regenerated solvents with water content and acidic pH-value. You can see details regarding these dangerous characteristics and hazard classifications in the relevant Safety Data Sheet (SDS) in section 2 or 3. The apparent advantage to the printer, however, regardless of hazards in use, is the fact that such solvents break down the dried-in inks quickly and open up the mesh.
Failure to comply Unfortunately, many screen printers still today do not know that the use of cleaning solvents with a flashpoint below 40°C is not permitted in the EU according to the EN 1010 Standard ʻSafety of machinery - Safety requirements for the design and construction of printing and paper converting machinesʼ. Failure to comply with this directive might result in loss of insurance coverage in case of a fire in the factory – besides of the vocational health risk for the operator. A consequence of this directive is that inks, thinners and cleaners with flash point below 40°C must only be used in special explosion protected working areas. Such explosion preventions and protective measures on screen printing presses are not required when screen printing inks with a flash point of 40°C to 55°C are used, although there is always a necessity to provide adequate air ventilation in the working area.
Complications from on press screen cleaning with fast evaporating solvents typically do not show up until later on in the process and most commonly at the reclamation stages. It is here that the headaches begin and the extra costs that will affect your profits are often over looked.
Fast evaporating and aggressive press wash, which possess quick mesh-opening properties, will react with stencil materials, especially if they are under-exposed. This chemical reaction causes a hardening (acetate fusing) of the emulsion respectively capillary film and influences significantly the decoating process. Indicator for this is a whitening on the squeegee side of the stencil. Your screen printing emulsion that was originally easy to reclaim with your stencil remover, has become very difficult to remove unless you increase the concentration of your stencil remover or use lots of elbow grease to break down the stencil with your brush or scrub pad. This means, you are using more screen cleaning chemicals and the process of decoating of the screens is taking longer.
This fusing as described above additionally results in a layer of the emulsion being left on the mesh and the only way to
| 88 | October 2018 Now is the time to change
remove this is to use Haze Removers. Continued use of high caustic and fast acting haze removers can damage that all important mesh which governs your ink deposit, increasing the likelihood of you experiencing a split screen during printing.
Further symptoms A further symptom caused by the use of fast evaporating press washes can be seen in heavy ink staining : the solvent is evaporating very fast, the binders from the ink are being removed and absorbed into the rags, while the pigments and the dyes of the inks are left behind and bond to the mesh. Slower evaporating solvents reduce the likelihood of this occurring and you end up using less rags, less solvents and add approximately 30 seconds more to your cleaning time. Your decoating is easier, using less decoating chemicals and finally you reduce the need for highly caustic haze removers. Take a look at your press wash that you are using today and ask yourself, if your current press wash solvent is free of any chlorinated solvents or toxic organic substances that are classified in the legislation for hazardous substances and list of maximum workplace concentrations as damaging to embryos or genetic substance. In respect of the Occupational Exposure Limits (OEL) for airborne workplace chemicals and the safely undercut of upper limits for hazardous substances, the use of cyclohexanone as well as large amounts of aromatic hydrocarbons, like solvent naphtha in the cleaner should be avoided. Ask yourself, if youʼve added costs into the whole process further down the line and thus having created a false economy in screen printing?
If all the above is true for you, then now is the time to change and have a bigger impact on your profits moving forward than purchasing the cheapest press wash in the market place.
www.printwearandpromotion.co.uk
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