Date Published: 2000-09-01
A Guide to Mesh Selection.
The subject of mesh parameters is one that is often overlooked by many printers, particularly novices. I still find customers who are printing everything with a 43 “because that’s what we were told to use when we started”. I still find folks struggling to develop fine detail stencils because they don’t appreciate the difference dyed fabric makes. And I still have blank looks when I ask them if the mesh they have just ordered should be in an S or a T thread.
This is especially sad because this is where good screen printing begins. You can make a mammoth investment in state-of-the-art exposure units, printing equipment and other expensive hardware and still be producing mediocre work if you overlook the basics of mesh selection. Focussing attention on this area can greatly benefit even the most basically-equipped print shop. For those of you who are au fait with all of this, many apologies and just skip to the last paragraph. For everyone else, let’s start at the beginning.
The Basics.
It’s been a long time since silkscreen printing was done with silk. Most of the mesh you will come across is woven from monofilament polyester. Prior to the introduction of polyester, nylon was a common material, but suffered from poor dimensional stability and rapid tension loss. Although still used for specialist industrial applications, it is not really suitable for quality textile work. It is, however, cheaper than polyester. So if someone offers you a great deal on cheap mesh, make sure this is not the reason why. You get what you pay for.
Similarly, multifilament meshes were common at one time, because the threads were not strong enough unless braided together. These were popular for yardage printers at one time, but have largely been superceded.
Nowadays, the first major choice you will make is between standard and Low Elongation (or LE) mesh. Even polyester mesh has a certain innate degree of elasticity. This means that, no matter how tight you stretch it, it will relax back to lower tensions over time. The introduction of LE meshes in the early 80’s brought a new type of thread that had a much lower modulus of elasticity. That means that when you stretch it, it does not lose anything like so much tension afterwards and will stabilize at a much higher level. This offers a much better platform for quality printing.
Of course, to take advantage of the higher tension possibilites of LE mesh, you have to use a frame strong enough to support it. For example, LE mesh even in fine counts such as 120T will support 30 - 40n with a minimal loss thereafter. But a cheap aluminium frame with a thin sidewall is not capable of handling the force this puts on it without the sides bowing inwards. To get the best that LE mesh offers, you should be using a reinforced frame such as a HiTech or Hurtz, or a retensionable model.
For my money, LE mesh is the only way to go these days, and even restocking on stronger frames will be a good long-term investment for any printshop.
I assume you all know that mesh is classified primarily by the mesh count. This is the number of threads per linear centimetre. (But don’t forget that our American cousins still use threads per inch. So where we say 120, they say 305. It’s useful to bear this in mind when reading technical articles in US trade journals!) The lower the number, the coarser the mesh. The coarser it is, the more ink it will let through but the less detail it will support. This is the constant tradeoff to consider when selecting what count to use, especially on dark shirts. Coarser threads will also support higher tension, as they are stronger and can be stretched tighter.
However, within each mesh count, there are also a number of different thread diameters to choose from. These are most commonly referred to as S (or thin) T (or normal) and HD (for thick, or heavy duty). You may also find additional classifications such as M (somewhere between S and T). Anyone who can tell me what all the letters originally stood for wins a prize. I’ve never found out! Just to complicate matters, there is no standard diameter for each of the above classification letters, so, for example, one manufacturer’s T thread could be nearer that of another’s S. So the most accurate data to go on is to ask you fabric supplier for a chart showing the full specification of their range. Comparing them can make quite interesting reading (OK, I admit I don’t get out much).
Why do we need all these variants and what do they mean? Well, the thinner the thread diameter, the greater the size of the holes in between. (See diagram).
This means that, broadly speaking, a 43S will let more ink pass through than a 43T. Yes. The thinner the diameter, the weaker the filament. So a 43S will not support the same tension level as a 43T.
And does this matter? Again we come to another tradeoff - tension v. mesh opening. I hope you all know by now the importance of high tension and that the tighter the screen, the better the opacity and print quality. But does this mean that a 43T at 40n is better than a 43S at 20n?.
Like a deconstructionist philosophy class, the screen room is full of imponderables!
The answer used to be that, as far as plastisol inks with a relatively high viscosity were concerned, a highly tensioned T would often yield better results than the optimally-tensioned S version. The reason for this was that, until fairly recently, the tensile strengths of S threads were substantially lower than the T, and even when stretched to maximum were not capable of anything like the tension levels of thicker diameter cousins. And, as lower tension levels mean greater resistance to ink flow, the potential offered by the bigger openings could be offset by this inertia (ie your opacity would not be so good!).
Happily, emerging new types of mesh can now offer significantly higher strength in the S strands (more of this later), and this ability to offer higher tensions with larger open areas makes them very attractive to t-shirt printing.
The final choice to make in your mesh is white or dyed. Dyed mesh is often refered to as amber (hence suffixes such as 90TA). While the actual colour does vary from manufacturer to manufacturer, and can cover yellow, goldorange and even red, the purpose is the same - to produce higher resolution stencils.
How does this work? Well, when you are exposing a screen, the UV rays from the lamp penetrate through the layer of emulsion and hit the threads of the mesh. If the mesh is white, it will tend to bounce the light back upwards into the emulsion layer - white reflects, remember? This phenomenon is known as light scatter,or undercutting and means that, in effect, you are exposing from behind the positive as well as above it. This is why fine lines exposed on white mesh are sometimes difficult to develop properly - they are getting more exposure than you think. So why not just cut down the exposure time to compensate?
This is a common practice, but will actually risk reducing the strength of the stencil, leading to premature breakdown. Contrary to popular belief, the optimum exposure of emulsion is the point at which all the components cross-link to produce the toughest mechanical and solvent resistance, not necessarily the point at which you can get all the detail out.
Conversely, dyed mesh tends to absorb rather than reflect the UV, so you can give a good exposure time for a tough stencil whilst still holding the finest halftones and lines. (Don’t forget though that a dyed mesh will typically take 30 - 50% longer exposure time than white).
Most of you will associate amber mesh with the finer counts such as 90 and above, and indeed many suppliers don’t seem to stock dyed in the coarser grades. So is there any advantage in using it for counts below a 77?
I believe the answer is an unequivocable ‘yes’. This was brought home to me many years ago when I was involved in flock transfer production. The adhesive was printed using a 24S mesh. With the standard-issue white, even relatively modest detail was a problem. Switching to amber brought transferrable flock lines of one millimetre thick well within our grasp. We had to try several suppliers before we even found one prepared to order a roll in for us, but ever since then I have recommended dyed mesh for everything. Yes, it costs a little bit more and you may have to shop around. Yes, your exposure times will be longer, but your overall print quality will improve effortlessly.
Try it next time you send some screens back for stretching.
Mesh Selection.
Having now established the basic parameters of mesh count, thread diameter and colour, let’s see how these factors affect our choice of which mesh is best for which job and what the arcane information on a mesh specification chart means.(See Diagram 2)
Looking at the artwork, you should be considering such factors as level of detail, opacity required and type of surface being printed on. Let’s take a typical bread-and-butter job such as a heraldic-type school crest in chrome yellow on a navy sweatshirt. You may initially want to reach for something like a 47T to get a good ink deposit down, but find that this does not quite resolve the fine detail properly. This is the tradeoff we mentioned above.
Using a finer mesh count but a in thinner thread diameter can give you the best of both worlds.For example, looking at our chart, we see that a 53S has a mesh opening of 143microns, not a lot different to the 149micron opening on the 47T, but the finer count will support higher resolution.. Selecting it in amber is even better.. the stencil will have improved detail and edge definition, but the open area will still be sufficient to get a good covering deposit down. And as a modern 53SA will easily support 30n, that would allow good tension for ease of printing.
Examining the spec is also very important when deciding the optimum mesh for halftone work. Using the thinnest thread possible will help reduce moire and allow good coverage at the higher mesh counts needed if you are printing very fine separations such as 85 line. Also at this level, to ensure easy passage of process inks, you need to know the average particle diameter of the coarsest ink colour and multiply that by 3 to give you the smallest mesh opening that will print well. So if your ink supplier tells you that their cyan has an average particle size of 10 microns, you’ll be advised to select a mesh opening of at least 30. A glance at our chart will show that while the 140T just about cuts it at 37microns, the 150S is even better at 40. And the 150 will support finer dots than the 140.
Studying the tables also explains such anomalies as why 120 mesh is stronger than 110. Although you might think that the coarser grade would be stronger, this is not the case. Both are woven with a 35 micron diameter thread, but as the 120 has more threads per centimetre, it is actually a tougher fabric and will support higher tensions. What a good read these mesh spec charts are!
The future.
The biggest breakthroughs in mesh since the inception of the LE thread has undoubtedly come from Japan. Here, such companies as NBC have taken away the traditional technological pole position from the Swiss and gone onto second and third generation fabrics.The trend has been for finer and finer thread diameters with increased tensile strength - culminating in the recent launch of the V-Screen range at FESPA this summer, which offers 120 mesh with thread diameters well below 30 microns that can be stretched to levels approaching 60n.
Although this mind-blowing mesh is expensive and aimed squarely at the specialist electronics and industrial sector, the pioneering work has resulted in ever-better “regular” LE meshes. The most important development for the t-shirt printer being that the thinner threads (‘S ‘and’ M ‘ranges - no pun intended:-) will now support much greater tension levels, making them much more durable for both automatic and manual use. As an example, I frequently recommend the use of a 79SA for soft-handled but opaque underbases for large image areas. The fact that you can now easily stretch this type of mesh to 30n makes it an excellent choice for plastisols, which will still shear well in production. This would not the case several years ago, when such counts would struggle to achieve 15n before breaking.
This should give you an inkling that mesh selection can have a profound impact on your print and should not be treated lightly. It should not be random, but can be a predictable science.
If your choice of mesh is largely based on the first screen that comes to hand, or you blithely re-order the same thing from your supplier for no other reason than that’s what you’ve always had, I would urge you to spend the traditionally slower winter months experimenting with new meshes. Try shooting the same test positive on three or four different counts and diameters to see the print difference. It could be the best payback for a small investment in time and money you have had for a long time.
If you would like further information on mesh technology, you can contact Peter at Active Screen Europe Ltd on 01634 - 719400, or email him at peter@cyberscreen.com
