Once woven, the mesh goes through a series of processes to stabilise it. Finally, it is inspected by a vision system that identifies any faults and marks the edge. To put the complexity of mesh into perspective, there are 14,161 mesh openings in 1cm2 of 120 mesh, plus 240cm of thread. In 1m2 the figures are 143,976,001 mesh openings and 24,000 metres of thread.

Having established the quality of screen printing mesh, let us examine how best to use it. It has to be stretched over, and adhered to, a frame. The frame must be robust enough to withstand the mesh tension without excessive deformation, plus the stretching must be uniform. Unfortunately, many meshes are not stretched correctly, resulting in production problems. It is not just a matter of achieving a set tension, that tension must be uniform across the screen: ideally better than ± 1N. Also, the threads must be accurately aligned to the frame. Why, as long as the mesh is on the frame, why does alignment matter. Moiré is the answer.

There are three forms of moiré:

• Primary moiré: This form of moiré occurs when two halftones (separations of the same image) at different angles are combined during printing. The respective angles create an undesirable interference pattern
• Secondary moiré: This moiré is seen when halftone angles and/or line counts (rulings) clash with the weave of the screen mesh to produce a patterning effect 

• Tertiary moiré: The final type of moiré occurs when a halftone is printed onto a texture and the texture pattern interferes with the halftone dots, creating unwanted patterning. This is particularly the case when printing textiles. Moiré can also occur when printing solid colours on top of each other when the mesh marking in each ink film can cause patterning

Secondary moiré is the bane of screen printing because mesh has threads at right angles to each other. In four-colour printing (and when printing halftones) it is possible to minimise moiré. To do this the printer must maintain a precise relationship between the lines of dots and mesh. This requires a mathematical calculation which requires the precise mesh angle. If the mesh is at an angle or distorted, moiré will occur at different parts of the image. This can be avoided if the printer knows the mesh threads are parallel to the frame and correctly tensioned. Unfortunately, there are a limited number of sub-contract mesh stretchers who can meet the required standards, plus a small percentage of people who stretch their own mesh.

Of course, for printers fortunate to have a computer-to-screen imaging system, their images should be aligned correctly to the mesh (assuming the mesh is correct.) Align it with a 6in rule, or by eye, and you are bound to introduce misalignment.

Pin registration systems should be used. This is a simple device used throughout the printing industry. The diagram shows how to mount a photopositive on a coated mesh. A punch is required to punch the photopositive, available new and secondhand, as printers move to computerised systems. Always position the image symmetrically in the frame to avoid distorting the image and ending up with varying ink film thickness across the image.

Also, keep away from the frame’s inside edge. A 21 by 27cm image needs >15cm of free area around the image. The greater the free area, the more stable the image. This is definitely the case with high precision industrial printing. Take the example of printing electronic components, such as capacitors, where the free area is greater than the image.

Modern meshes supplied by Sefar, Saati and NBC are superb. The innovation of pre-treating the mesh during production to make it easier to prepare, more receptive to emulsion, easier to de-coat and provided a more consistent ink flow is excellent. I do have reservations about the claim that there is no need to wash mesh before coating. If the mesh was transported, stretched and stored in clean room conditions I would be happy with not washing prior to coating. However this is not the case. I always apply degreaser and wash with clean water before vacuuming with a wet and dry vacuum fitted with a fishtail attachment. Vacuuming is key because it removes dust particles that have lodged in the mesh during stretching and storage.

You can risk not doing this, the choice is yours. I prefer the security of clean mesh to downtime and rejects.

Years ago I said some stencil departments had more bows than Sherwood Forest. At the time I was describing the state of screen frames. The same rule applies. If it is easy to see the bow on the frame it is likely to be too much. The frame is the stencil’s foundation. It has to be strong enough to withstand mesh tension forces without distorting excessively. There will always be some distortion but it must not be sufficient to affect print quality. Twist caused by frame bowing should not exceed 10 per cent of the snap distance. The frame must also be light enough for safe handling. To achieve this it will generally be made of an extruded aluminium alloy profile. Steel frames are still used in some applications but are heavy and can corrode.

Frame profile is important. As a general rule, a sloping frame profile is more resistant to the mesh tension than a rectangle or square.

Things have improved. Printers have recognised the importance of stable frames and now consider them consumable items rather than capital items. My preference is to hand responsibility to a stretched mesh supplier and lease them.

No matter how good the frame, if you drop it on its corner, with a stretched mesh attached, the mesh tension could cause the mesh to split or even spring the frame’s corner welds.

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