News & Events
Solving problems in the real world
Solving problems in the real world
In this third extract from the ‘How to be a great Screen printer’ e-book produced by MacDermid Autotype, Professor Steven Abbott takes a look at a selection of typical screen faults that can occur during screen printing.
Over the years we’ve built up a large number of case studies of problems found in the real world. Here we share some of our experience of the things that have caused screen printers real problems, and the solutions we have found.
These faults are listed in no particular order and we’re sure you may just recognise some of them!
Dirt and contamination from a number of sources can mess up your print in many ways. Dirt on the mesh during coating can cause streaks. Dirt on the film positive or on the stencil during exposure can cause pinholes. Dirt during printing gives pinholes and streaks. It is not surprising then that the highest quality industrial screen printers do their work in clean rooms. It’s amazing how much easier life is when you don’t have to fight with dirt. If you don’t have the luxury of a clean room then here are ten tips we’ve found make a real difference.
- Install a simple filter in the water supply used for your stencils and meshes – and remember to change it regularly. It costs pennies yet can save large amounts of money in avoidable rework.
- Clean all surfaces and floors, with a damp cloth or a vacuum cleaner that is fitted with a HEPA filtered exhaust.
- Keep doors and windows closed whilst ensuring correct local exhaust ventilation (LEV) around printing machines and cleaning equipment. If adding an additional enclosure, take care that air movement into the area is from a clean source. For instance, shutting the door will be a waste of time if the air is then going to be pulled from a dirty air space above ceiling tiles.
- People are the biggest source of dust, so ensure that you minimise the number of people moving around in the area. Use tack mats to minimise dust and dirt entering the screen printing area.
- Use lint free wipes for cleaning to prevent creating more dust.
- Avoid fibrous packaging. Sheets should be removed from the transit box prior to use. Card and paper should be kept away from the printing area if possible.
- Raise the humidity (this reduces static). But remember if you dampen down the floors, take care to avoid slip hazards.
- Wear proper clean room overalls.
- Filter air conditioning. If you turn off air conditioning on the days when it is not necessary, make sure that the alternative source of air is not dust laden.
- 10. Use an ionising air-gun to clean your mesh. An ordinary air-gun can create static (the movement of air alone will create static that can be measured) and attract dirt back onto the mesh. The ionising air-gun gives a really clean result.
Static causes dust to be attracted to surfaces and is produced in three ways:
The simple act of removing a sheet of film from a stack, or passing a film through a rubber nip-roller produces static by separation and friction. Static is hard to eliminate once it has been produced, so the best thing to do is to reduce the chances of creating it in the first place.
An environment with a relative humidity greater than 50% always helps. Reduce the amount of film handling and the number of rubber nip-rollers to a minimum. Then make sure you have antistatic devices at critical positions. An ionising air-gun is useful for spot work (especially for the final cleaning of a screen) and an ionising air-system installed on a press keeps this critical area static free. Although “Christmas tinsel” does a reasonable job of removing static, it doesn’t look too professional and easily breaks into small lengths. Modern antistatic bungee cords are a better alternative.
The wrong mesh
It is surprising that some printers still choose the wrong mesh for high quality screen printing. The most common error is to use white mesh, then complain about the lack of resolution/screen life. Next is the inappropriate choice of mesh type, count or diameter. If the print contains a lot of fine detail, then the rule-of-thumb is ’2.5x the thread diameter is the minimum size of a printable line’ this will get you fairly close to the right answer. Refer to the mesh manufacturers’ data for thread diameter information.
If critical sheet-to-sheet registration is the main requirement, then stainless steel or specialised liquid-crystal polymer meshes are your most likely choices, as polyester simply does not have sufficient long-term stability.
For demanding fine line industrial printing, the best choice is the finest stainless steel mesh you can handle.
The other aspect of mesh choice is more subtle. For example you must also take into consideration mesh moiré if you are printing halftone images. We also have a section on moiré in the ‘How to be a great Screen printer’ e-book that can help you find the right mesh-count for your 4-colour settings, visit macdermid.com/autotype.
Image too close to the frame
If you have a few mm snap-off (off-contact distance), the pressure from the squeegee needed to force the mesh into contact with the substrate might be low and acceptable in the middle of the mesh, but will be higher as the squeegee gets closer to the edge of the frame, see figure 1. This comes from simple geometry. It’s therefore important to make sure your image area is not too close to the frame, both in the print and crosswise directions.
So what does happen if you get too close to the frame? Firstly, you get large image distortion. Secondly, you are forced to use a higher squeegee pressure which reduces the screen life, damages the squeegee, causes judder and gives extra dot gain in graphics printing or positive sawtoothing with technical Applications.
There is also evidence that the squeegee gets distorted near the edge and does not perform well at scraping off the excess ink, thereby giving a higher ink deposit in these areas. Of course, as you reduce snap-off distance the problems become less, so you can go to a larger image size for the same frame size, see figure 2.
We all know that failure to properly clean and reclaim a mesh can give you a ghost image in the print. However, under microscopic examination of the mesh you can barely see this ghost image, so how can it have an effect on the print? The answer is, that the knuckles of the mesh are where most of the ghosts hide out. But why is this important? Because the amount of ink held in the mesh, and the amount of ink remaining in the mesh when it is deposited on the substrate depends strongly on the knuckles. A small amount of ghost, hiding in the corner of a knuckle is enough to change the printed ink volume. There is a whole section on mesh marking in the e-book that explains this further. The simple remedy is to use a haze remover to remove all the ghost images.
Manual or Automatic coating?Excellent results can be achieved with both manual and automatic coating, however, automatic coating machines typically allow a greater degree of consistency than manual coating. Coating machines are of particular benefit when processing very large screens, or where the screens are all the same size.
Coating trough selection. Selecting and then maintaining the coating trough is critically important for achieving a quality result. Troughs are available with either a sharp or round edge profile depending on what deposit is required. A sharp edge trough will deposit significantly less emulsion per coat than a round edge trough. The coating edge of the trough must be inspected regularly for any nicks or profile irregularities and replaced if damaged.
Coating techniques.Coating pressure, angle, speed and amount of emulsion in the coating trough will all affect the amount of emulsion that is deposited with each pass.
Below is an overview of each technique:
Pressure. A consistent pressure is actually more important than absolute pressure. Use sufficient pressure to ensure that the trough edge is in perfect contact with the mesh and that this contact is maintained throughout the stroke. The pressure may have to be increased slightly when using a high viscosity emulsion, to prevent it ‘pooling’ in the centre of the screen. A mesh deflection of 1 to 3mm at the trough edge is fairly typical depending on emulsion viscosity.
Angle. Typically an angle of 15 to 30° from the horizontal is used. Once again maintaining a consistent angle is the most important factor. Many troughs have the end caps pre-cut to the appropriate angle.
Speed. Always try to coat as slowly as possible as this will help minimise the amount of air trapped in the mesh openings. The coarser the mesh and the thicker the emulsion, the slower you need to coat. However, if you manually coat slowly, be careful not to introduce judder lines across the mesh. This picture shows air trapped during coating. As a rough guide, speeds can range from 80 to 300cm/min depending on the viscosity of the emulsion.
Did you know that quite small variations in the depth of the emulsion in the coating trough can have a significant impact on the coating thickness? Where coating thickness is critical, e.g. 4-colour halftone printing, ensure that the troughs are always filled to the same depth for each screen in the set.
Always wipe the coating edge clean immediately after coating.
Coating Regime. The number of coats you apply and the application technique used are the best way to control the stencil Rz and EOM. For most screen printing applications the objective is to produce a screen that has a thin, flat stencil surface on the print side, with just a thin, fully cured layer of emulsion on the squeegee side. Stencil build comes primarily from applying multiple wet coats from the squeegee side, whilst stencil smoothness comes from applying thin, face-coats of emulsion to the print side.
The typical coating regime for manual coating is to apply 1 or 2 base coats of emulsion to the print side of the screen which ‘fills’ the mesh and then immediately apply 1 or 2 coats to the squeegee side. This pushes the wet emulsion back through the mesh to the print side, leaving only a thin, sealing layer on the squeegee side. With coating machines you have the capability to coat both sides with each pass, this is a very effective method, however, ensure that the final pass for the base coat is always from the squeegee side.
There are obviously many variations to this theme depending on frame size, mesh count, image requirements etc. but the principle requirement is to start on the print side and finish on the squeegee side.
Face-coats. All emulsions lose volume during drying as the water evaporates and consequently they shrink back around the mesh fibres to leave an uneven surface. Face-coating is used to fill in these hollows in the uneven emulsion surface on the print side, to create a flat stencil surface (low Rz) which minimises sawtoothing during printing. Face-coating can only be applied to screens that have already been fully dried. Typically 2 face-coats are applied and then the screen is dried again. If an even lower Rz is required, then additional face-coats can be applied. Some coating machines are fitted with IR driers which facilitate intermediate drying and allow face-coats to be applied as part of a continuous production process.
For high specification applications, a sharp edged trough can be used in conjunction with a round edge trough. Use the round edge trough for the base layer and the sharp edge trough for the wet-on-dry face-coats. This reduces the surface roughness (Rz) without significantly increasing the stencil profile (EOM).
Drying of Stencils
Did you know that drying the emulsion after coating is just as important as exposure when it comes to screen life? A poorly dried screen will be easily damaged during printing as its solvent and mechanical resistance has been dramatically reduced.
The theory of drying says that ‘air flow is usually more important than mere temperature’. Just think of the difference of drying your hair with a cool hair dryer (plenty of air flow but little heat) and with an electric heater (plenty of heat but little air flow). The hair dryer wins every time. A hot box stuffed with wet screens is a sure fire way to get breakdown on the press as the warm moist air simply won’t dry the screens enough. And remember that overheating the stencil will cause it to fuse and give poor reclaim.
A quick tip to help with a poor screen drying setup is to install a de-humidifier in the system. You can then turn that ‘hot box’ into an efficient and effective drier and you will be amazed at just how much water even a small de-humidifier can pull out of the air.
We hope that you enjoyed this selection of some of the screen printing faults featured in the ‘How to be a great Screen printer’ e-book and maybe it has sparked a couple of ideas on how to resolve some of your own problems.
If you want more helpful advice from the technical team at MacDermid Autotype the ‘How to be a great Screen printer’ e-book is downloadable from macdermid.com/autotype.
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