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Substrates are quite simply the surface that we print onto. Achieving perfect results is not as simple as it sounds however, because the range of substrates is constantly increasing.
From a printer’s perspective, generally the substrate is determined by the client. What the printer has to do is decide on the ink and drying/curing system that suits that particular substrate and its working environment. Assuming the substrate ink combination will provide suitable adhesion, the printer still has considerable responsibility in ensuring that the partnership remains satisfactory. Maintaining the condition of the ink and its drying technique is crucial, but so is managing the substrate.
Substrates can be divided into two basic groups: those that absorb moisture and those that don’t. Paper based products will normally be susceptible to moisture absorption, whereas polymers (plastics) and metals are not.
Ditch the dirt
In all cases, keeping the surface clean, free of contaminants and debris is critical to effective ink adhesion. Paper based products will deform if their moisture content changes, unless they have been treated with materials that make it virtually impossible for ink to stick to them. Maintaining temperature and humidity of this material during storage and in the print shop is very important. Using as little heat as possible during the drying and curing process will also keep distortion to a minimum. Plastics are also liable to change in size during drying and this can be an even greater problem when paper and polymer are combined in products like self adhesive vinyl, where the two materials expand and contract at different rates.
Ambient conditions also have a notable effect, but even if conditions are perfect, problems will still arise if the substrate is dirty. Dust on the substrate will cause faults in the print that can be replicated throughout a run and contamination as little as a thumb print can stop the transfer of ink or compromise adhesion.
One common way dust particles are created is guillotining with a knife that has lost its edge. The same applies to any slitting or punching process, so blades must be kept sharp. Perhaps, the greatest hidden danger is to print onto a substrate that has not been allowed to reach the temperature of the print shop. An invisible film of water can form on the surface and act as a barrier to adhesion for the ink.
Static electricity is created in a number of ways: when electrically insulated materials are rubbed together; are heated and cooled quickly; cutting and slitting; being close to an electric field generated by static; or subjected to high levels of ultra violet (UV) energy. This checklist probably ticks all the boxes for screen printers.
The effects on the process can be damaging and include incomplete transfer of ink and feathering on the edge of the printed image as well as attracting dust and debris to the substrate. Static can also cause substrates to stick during their passage through the machine, induce multiple sheets to feed through, or cause substrates to run out of line. Ultimately, static can even discharge through the operator or present an explosion risk due to sparking.
So how do we deal with it? The aim has to be to stop the build up of static or neutralise it when it occurs. Stopping build up is about creating a route to earth for components and materials in which there is likely to be a build up of static electricity. When this is not possible, it is necessary to have an effective means of neutralising static electricity.
One of the experts in this field is Bob Fraser, whose company, Fraser Anti-Static Techniques, produces a range of devices to suit most applications. He stated that screen printing onto plastic substrates has always been affected by static electricity, whether dust attraction, ink misbehaviour or shocks to operators. Digital printing has inherited all of these problems. It works by creating millions of fine ink droplets that can easily be deflected by static charges and because of the cost of inks and substrates, static has an even bigger effect on the profitability.
In screen printing friction is created throughout the process: between the squeegee and mesh, the mesh and the substrate and between sheets of substrate. During drying there is considerable temperature change and/or fast moving dry air. This builds up the amount of static electricity and finally at the exit to the drier there can be a stack of material that is alive with static. By positioning eliminators just before the stack, this massive charge can be safely eliminated.
Various products are available to overcome these static issues, such as ionised airguns for cleaning sheets before printing and static eliminator bars for neutralising static on the machine.
Without these products, static charges can build up to tens of thousands of volts with extremely damaging results, particularly in potentially explosive areas, which are sometimes less than obvious. Clearly high levels of solvent fumes with low flash points are hazardous, but so are dry dusty areas, such as those created when grinding or linishing aluminium screen frames. If the conditions are right, which fortunately is rare, the aluminium dust cloud produced can be ignited by a spark. In an enclosed area, an explosion may result. It is a rare occurrence, but that is a danger in itself, because operators can become complacent and create a hazard.
Environmental issues are a key aspect of managing substrates. It is possible to waste at least 45 per cent of the substrate purchased, with disastrous consequences. Just look how much goes in the skip, calculate how much you paid for it in the first place, add on the overhead and labour plus wasted ink, but by working efficiently, targeting zero defects, right first time and careful inventory control this can be eliminated.