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Improving the screen process
With too many years trouble shooting screen printing behind me (and probably many in front) there are certain recurring critical issues which I face. These being the summer months, whilst you watch the weather you may wish to divert your mind to screen printing improvements by Peter Kiddell
One of the most common questions I’m asked by people keen to start up in screen printing is ‘what machine do I buy’? My response is that it’s the wrong question to ask. The limiting factor in choice of machine can be how the ink is going to be dried or cured. This is particularly the case in industrial applications where temperatures of >120°C for extended periods are sometimes necessary. The temperature triggers a chemical reaction in the printed film, while the time is based on pre-production trials in static ovens.
In non-UV curing printing mediums (inks) there is always some form of solvent used to make the mixture printable. Removing solvents from printed films requires temperature and airflow. Just think washing on a clothesline: it dries in no time on a warm, windy day. With high humidity and little wind, it never dries.
An efficient dryer has accurate temperature control, high velocity controllable air movement and, if using infra red drying, the correct wavelength of infrared energy. It is not just a matter of high air flow because fast moving air isn’t necessarily impinging the printed film’s surface. Think about an insect on a windscreen. The car can travel at 60mph but the insect stays on the windscreen. When air flows over the surface a boundary layer forms close to the surface where there is little air movement.
Dryer designers understand this and design air manifolds and orifices which create airflows that overcome this boundary effect. Incorrect dryer design can actually inhibit drying by forming a skin on the surface of the ink film and blocking the egress of solvents from the wet under layer. UK printers are fortunate to have Nottingham-based Natgraph which understands the science of drying. Its equipment demonstrates the enormous strides made in drying and curing technology in the last ten-years. This also applies to ultra violet curing where both the unit’s effectiveness and efficiency are important. Significant power savings are possible with modern power packs used to drive UV emitters.
Drying technology
The importance of drying technology is increasing, as engineers want to print multiple layers of exotic materials on a range of substrates. Answers to questions like ‘is it dry’ are not sufficient. ‘What is the state of cure’ is the more likely question. Why, because if a film is fully cured it may not be possible for the next layer to form a full adhesive bond. Dryers may have to provide drying and curing from both sides of a substrate. It is possible to have two-side curing, several infrared wavelengths, UV curing and high velocity heated air on the same dryer. Some applications benefit from gas-powered dryers. Then there are chillers to bring the substrate back down to ambient temperature.
Any potential user should carry out drying and curing tests on their printed substrate before producing a dryer specification. Having the wrong dryer can be very costly.
Even in graphics printing using conventional inks (UV or solvent based) the dryer has a tremendous influence on productivity. Maintaining substrate stability is always an issue. Typically, incorrectly set and operated UV emitters produce significant infrared energy which affects substrates. Also, badly focussed lamps mean belt speed has to be slowed, resulting in greater heat problems. A host of issues plague printers with badly maintained UV dryers. The cost of effective maintenance is repaid a hundred fold. There may well be justification for buying a new dryer. If printers run their substrates through Natgraph’s test facilities they would soon see the difference modern technology makes.
A state-of-the-art UV dryer will run cool, cure quickly and lets printers run at maximum speed. Flatbeds up to 1,000 per hour, cylinder presses up to 4,500 per hour. Automatic feeders before the printer and stackers/collection trays after the dryer. The need for auto registration, image recognition and static elimination devices is determined by print speed, which is controlled by how fast the substrate will dry. So, think about the dryer before you think about the printer.
Machine geometry
Having already mentioned the importance of maintaining dryers, screen printing success also relies on correct and consistent printing machine geometry. The screen frame must always be aligned to the printing surface. On a cylinder press this means the stencil must be parallel with the cylinder and its movement tangential to the cylinder. The squeegee also has to be parallel, as does the flood coater. Any miss-alignment means the ink film thickness will vary across the substrate.
Flatbed press geometry is just as important. All too often the screen frame is mounted such that the snap distance varies corner-to-corner, which will compromise the whole printing cycle. Unfortunately, the attitude of many printers is ‘it works so why bother’. This culture is one reason for the decline of screen in the PoS sector. Another problem area is when peel-off is applied. Sometimes it is only applied with one colour, causing registration problems because applying peel-off alters image size. If it is used, it must be applied on every colour. The reason given for using peel-off is it helps lift the stencil off the wet ink. If tension was correct, this mechanical movement would not be necessary.
One of the most abused elements is the flood coater, sometimes known as the flood bar. The device is designed to charge the mesh openings with a controlled amount of ink. It does this before the squeegee deflects the stencil into contact with the substrate and causes the ink to flow through the mesh. Most flood coaters I come across have nicks in the contact edge and are often distorted. The flood coater should have a straight, undamaged edge. To put down a thin ink film it should be a knife-edge. Conversely, a thick ink film requires a rounded edge profile. How sharp the knife-edge and how rounded the profile depends on the film requirement. In simple terms, a four-colour process job would need a knife-edge and a large solid area a rounded profile. When not in use, coaters should be stored with a protective plastic sleeve over the edge.
Just because squeegee material comes rolled in a box, doesn’t mean it should be stored in the box. Either lay it flat or hang it up. If kept rolled, when it is placed in the holder it will try to twist due to elastic memory. Have pity on your squeegee manufacturer’s quality control team. It takes great care to ensure the squeegee edge is perfect and what does the printer do: ram it in a squeegee holder; bite off the ends; and chuck it in an excuse for a squeegee dresser; only to destroy the carefully produced edge. It’s the same as taking a new set of tyres and grinding off the tread. ‘Oh!’ says our hairy friend ‘I ave to do it case the effin things not straight when I puts it in the older!’ Well my effin hairy friend, clean the effin squeegee holder. And then have a shave.
Effin emulsions
For those unaware of the customary discourse heard in screen print shops ‘effin’ refers to flocculation: when pigments separate in a tin of ink and are seen as a lighter colour on the surface before agitation. It has absolutely nothing to do with procreation. Before I leave this sceptred page, a final tip. Make sure your effin emulsions are fully dry before you expose them otherwise you will get pinholes all over a developed stencil.
