Steven Abott- Director Research & Technical Autotype International Ltd.,UK Steven Abott recieved his Oxford PhD in Chemistry for work carried out at Havard. After a Post-Doctoral position in Strasburg he went to work at ICI on new product developments in electronics, imaging & data storage business. As research & technical director of autotype he has been responsible for ensuring a constant stream of new product, & also for providing the Science behind the coating and printing techniques used. A lot of this science has come from a fruitful partnership with professor Phil Gaskels team at University of Leeds where Steven is visiting Professor. He is a frequent speaker at International confrences where he is passionet About the need to apply good science to coating & printing

In the previous article we discussed how moiré effects are created. We now move on to consider what causes moiré to occur in screen printing and provide some proven recipes for success. These solutions have been thoroughly cross-checked against production moiréd prints and the conclusions tested using a sophisticated computer model - the Moiré Mesh Calculator*(MMC) - which examines moiré at a fundamental level. It should be noted that these solutions assume that you are not printing onto a regularly corrugated surface and that there is no moiré in the film positives. Mesh Moire From the previous article we saw that in screen printing the regular pattern of the screen mesh can clash with the regular pattern of the print image to form moiré patterns. Typically, a coarse mesh with an image at high lpi can give particularly strong moiré. To compensate, it is a natural step to change to a finer mesh; yet, paradoxically, this can cause still more moiré patterns to form. The problem with moiré effects is that they can come and go and can occur in particular tints, as waves, and in only one or across several colours. Moiré effects can be predicted using complex mathematical calculations or softwaresuch as The Mesh Moir Calculator (MMC). These show that at a reasonably high ratio of mesh count to halftone ruling (e.g. a 150 tpcm mesh with a 39 lpcm (380tpi:100lpi) halftone, giving a ratio of 3.8:1) you are likely to find that at three screen angles there is no visible moiré, but that at the fourth angle problems will occur. Unfortunately in the UK we mix both imperial and metric measurements for halftone screen rulings (lines per inch LPI) and mesh count (threads per cm tpcm) which can complicate the calculations. Therefore in the interests of clarity all measurements will be given in threads per inch. These problems will occur irregularly; for example, in some instances a small change in the mesh (tension or angle) will have little impact, while on other occasions small changes can produce major wave patterns. In addition, calculations show that certain dot shapes and sizes will make patterns more of less visible. Mesh moiré can easily be identified as it appears when a single colour is printed onto a flat substrate and will generally fade in and out at different densities of tint. If a thin, low Rz (flat) stencil is used then the moiré effect can be significantly reduced; thinning the ink can reduce it still further. However, if a thick or high Rz stencil is being used then there is little that can be done to change the effect.

CURES Firstly, it is essential to ensure that the moiré is not in the film positives for the colour in question. The ideal cure for mesh moiré is to use the MMC to find a mesh/lpi combination that eliminates any effects. For example, a 63 lpi print with a 355 mesh results removes moiré for the standard 7.5, 22.5, 37.5 and 67.5 set. If just one colour is suffering from a moiré effect, then a good solution is to keep the original mesh count for the other three colours and find a nearby mesh count for which the MMC predicts no moiré. For example, using a 63lpi print, this time with a 305 mesh, ensures that no mesh moiré is seen except for the 37.5 set. If this set only is changed to a 355 mesh then no moiré will be present.

Workable solutions range from printing the least noticeable or weakest colours (usually yellow) at the angles that cause mesh moiré, to using the smallest possible thread diameter to reduce the visibility of the moiré pattern. Additionally, if calculations show that there is strong moiré with a set of film positives at 0, 15, 30 and 60 then change
them to 7.5, 22.5, 37.5 and 67.5 or vice versa. However, beware of the simple moiré that can occur between the 0 colour and the mesh. If the mesh moiré is seen as lines rather than dots, make the printed dot-shape rounder. This will make the lines weaker and should result in the dots being less visible. The other possibility is that the squeegee is amplifying the problem in the direction of the squeegee stroke, so reducing
pressure and/or stencil Rz will reduce the moiré. Other solutions include using new specialised films for example Capillex CP, which give extremely low emulsion over mesh (EOM) and a low controlled Rz. That should immediately give you a major improvement.
Additionally, try using a thinner ink if a low Rz stencil is used; if you are using a high Rz
stencil, then try to reduce the Rz without going too thick with the stencil, then use a thinner ink. Alternatively, try and work the design and colour saturation so that the presence of the other three colours will mask the pattern in the fourth colour.

If none of the above solutions can be implemented and if an unwanted wave pattern is present, then stretching the screen better will help; it wont eliminate the moiré effect but it will reduce it. A common myth is that if only the perfect mesh existed then moiré would not be a problem. Although it is true that imperfections in the mesh may make some moirés patterns more visible it is also true that even if the mesh was perfect the moiré would still exist.

15 MOIRE
It is an unfortunate fact that in any screen setone of the colours must be at 15° to the others. This clash of dots is guaranteed to give moiré and is characterised by a pattern that is at an angle of 7.5 (i.e. halfway) with a repeat every 3.8 dots on a 4-colour print. This can be seen as a pattern of dots or lines, with the intensity of the moiré effect varying across the print; this can be seen clearly through a loupe or microscope. Note that if you do not have a four colour print then the phenomenon cannot be 15° moiré (although if you are printing a duotone make sure that the separations are at 30).

Glossary Below is a brief explanation of the terms used in this article . EOM: Emulsion over mesh or stencil Profile . The extra thickness provided by the stencile material,i.e. the thickness of the screen with the stencil thickness of the screen without stencil. Rz: A measurement of the roughness of stencil. Dot Shape:The printer can choose the shape of the halftone dots. They can be circular, elleptical,rhomboid etc. Loupe & Microscope: Most printer have a hand-held magnifier, commonly called a loupe. For greater magnification, a microscope is used.

Cures The ideal cure is to replace the 15° component with a different frequency. This is extremely risky unless either carried out by an expert or a modern RIP that will replace it with a stochastic equivalent. There are also a number of other solutions: reduce the visibility by switching the 15° component to a less visible colour or reduce the visibility by printing the 15° component last. Its possible that it will find a more random surface because of the variety of dots underneath and the moiré wont be so intense. As with mesh moiré, if 15° moiré is seen as lines rather that dots, make the printed dotshape rounder to make the lines appear weaker. Also, consider reducing squeegee pressure and/or use a low EOM, low controlled Rz film such as Capillex CP. Additionally if you have a low Rz stencil, consider using a thinner ink; if you have a high Rz stencil, try to reduce the Rz without going too thick with the stencil, then use a thinner ink.

Recipe for success
Products such as the Moiré Mesh Calculator will enable the right screen sets to be identified, while the use of a good stencil system with low EOM and Rz will also be extremely beneficial. Furthermore, much lower squeegee pressures, lower viscosity inks and lower snap-off will mean that the mesh is treated much more gently. This will enable finer mesh diameters to be specified that, in turn, will reduce the intensity of any mesh moiré to provide a larger, more dynamic print range.
It is rather surprising that the virtuous cycle exists; but that’s what the maths say and that’s what we’ve found in real life. This is the real recipe for success.

* The Moiré Mesh Calculator is part of the Autotype DSP (Digital Screen Printer) suite of programs. For a FREE CD containing the DSP software, contact Autotype at screen@autotype.com

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