IMAGES - THE JOURNAL FOR TEXTILE SCREENPRINTING ,EMBROIDERY, PROMOTIONAL CLOTHING AND GARMENT DECORATION

Author: Will Pearson
Date Published: 2010-03-01

Process Control: Mesh Tension

Will Pearson of Phantom Screenprint, continues his series of articles on process control in screen printing by outlining the reasons why every printshop should be using high mesh tension…

Previously, I outlined the most important areas of the print process that we need to take control of in order to achieve our four values, which are:

1) Improving print quality
2) Improving processing speed
3) Achieving consistency and repeatability
4) Reducing Waste, Cost and labour

This month, I'm going to look at mesh tension and, more specifically, the advantages of using high mesh tension. This is a large topic and one that is often misunderstood. But before I get started, I want to dispel a few myths and establish a few truisms…

Ultimately, your aim is to control the variables in the print process to the point where the day-to-day running of your business is routine and predictable. This will enable you to cost accurately and remain profitable, and also means that the people who work within your system have their roles and procedures clearly defined. That makes for a productive and more autonomous workforce. Your staff need to know what their job is – that is, what's right or wrong today will still be right or wrong tomorrow – and be able to go about their day's work successfully without constantly seeking approval from management.

The first thing I want to establish is that this goal, and achieving the four key values, will be a long term project for many shops, and should be approached incrementally. Whilst bringing any one of the variables under control is a good thing, it's important to bear in mind that the variables are all inextricably linked. So you can't expect miracles until you have taken control over all of them and they are working together. Some of these controls can be achieved with nothing other than a bit of labour, or by introducing a bit of paperwork or implementing a checklist. Others will require investment of hard cash. Controlling mesh tension is one such variable.

Often, I hear people say, "It's not worth the outlay," or, "The customer can't tell the difference," or, "We've been printing just fine for 20 years without worrying about any of that". The point is this: you have to regard implementing process controls as an investment, and as such you can expect a return on that investment, in terms of reduced labour, reduced waste and reduced cost of production. Some customers will be able to tell the difference, many will not, but here's the big secret: you're not doing this solely for the customer's benefit! An improvement in print quality is a happy by-product of what you're really doing, which is tightening your production so that it becomes leaner, works smarter and operates more profitably. The customer gets a better product and more consistency as well, so it's win-win. Of course, you could choose to screen print for the rest of your life without ever paying attention to your process, and that is what you will do: screen print for the rest of your life. My aim is to work smart and retire early!

The importance of high mesh tension

So let's get started. Why is high mesh tension so important? Briefly, if you have high mesh tension, (35 Newton-metres or above), ink transfer starts to work in a very dynamic way: at high tension, the stencil becomes more dimensionally stable, which means greater registration accuracy on multicolour jobs. The stencil is also influenced less by the force exerted on it by the squeegee stroke.
Conversely, in a low mesh tension scenario, registration is hard to control because of the 'squeegee drag' on the mesh in the direction of the stroke, which tends to elongate the image. If each screen is at a different tension, then the rate of elongation will vary. The longer the image, the more apparent this problem becomes. Many shops combat this 'problem' at the separation stage by adding traps. This is not only time consuming, but can also cause other problems on press, such as blurring, ink pick-up, and an inability to print wet-on-wet.

With high mesh tension, the flood bar starts to 'inject' ink into the stencil openings, rather than merely scraping the ink across the top in an uneven layer. This means the squeegee can print with much less force; its role now is mainly to shear off the ink layer, which the flood bar has just injected into the stencil. Conversely, with low mesh tension, the squeegee has to perform the flood bar's role of introducing ink into the stencil, and then it has to drive that ink into the garment. This requires excessive pressure, which means the ink is pushed down into the shirt, leaving fibres to stick up through the ink film and resulting in a print with a rough hand, thick ink deposit and reduced opacity. This is why people resort to double- stroking every colour, and using lower mesh counts than necessary, in an effort to achieve adequate ink transfer in an inefficient (that is, low mesh tension) system.

The benefits of high mesh tension

With high mesh tension, you can expect to print through finer mesh, as your ink-transfer efficiency has become so much higher. This means less ink usage (my printshop realised an approximate 40 percent ink saving when we moved on to high mesh tension, so we're talking significant figures, here) and a better quality print, with dramatically fewer issues when printing multicolour, wet-on-wet or fine detail, 'held out' detail and halftones.

There's pre and post-press benefits too: a tight screen coats more evenly and with greater ease. When reclaiming, it's easier to remove ink and emulsion and stains, as the 'knuckles' of the mesh, which is where particles of ink and emulsion like to collect, are held open. Before proceeding further, it's worth taking the time to summarise the benefits of high mesh tension, as discussed:

1) improved registration, meaning faster set ups and less time spent on
separations, as artwork can now be printed butt-register.
2) Improved ink transfer, meaning a better quality of ink deposit.
3) Lower squeegee pressure, meaning less wear on your squeegee, your
machine/arms and a more opaque print.
4) higher mesh counts, meaning ink savings and fewer problems with
multicolour work, as well as improved 'hand'.
5) Easier and faster coating and reclaiming.

Switching to high mesh tension printing

Firstly, you're going to need some retensionable frames, and be warned, these are costly. There are two main types available: roller frames and ratchet type frames.
The roller type frames retension the mesh by rolling the mesh around the frame's four bars, which can then be bolted tight when the correct tension has been achieved.

Step 1: Mesh is fixed onto the rollers using plastic bars, which sit in a slot along each roller's length.

Step 2: Corners are 'softened' using a piece of stiff plastic.

Step 3: Screen is placed print side down and raised at the corners using squeegee blade off-cuts. The screen is tensioned by hand using a wrench

The ratchet type frames have a bar attached to the mesh, with two grooves on each side of the frame for the bar to sit in. Tensioning is achieved using a jig that pulls the bar into the groove. The frame can be stretched to higher tensions by moving the bar to the second groove, and/ or folding the mesh over the bar before placing it in the groove. Both systems work well. Personally, I prefer the roller frames as they are more 'fine tuneable' and they can be used with standard, off-the-roll mesh, whereas the ratchet frames have their own mesh panels with the bars attached.

Step 3a: Alternatively, the mesh may be stretched using an automatic stretching table.

Step 4: The roller bolts are locked up using a torque wrench.

You are also going to need a high quality tension meter. You should buy this from new, and look after it well. It is a delicate instrument and will not give accurate readings if it is dropped, left standing on its base and so on. Buying one second-hand is like buying a crash helmet second hand: any damage isn't immediately obvious. You'll need to buy some new mesh, either mesh panels or off-the-roll mesh. The roller frames require a good quality torque wrench and a large wrench for turning the rollers. Eventually, you'll want to buy a pneumatic, automatic stretching table too. These are expensive, but much quicker and easier than stretching by hand. The ratchet frames need a jig for stretching. This will be available from the manufacturer. The roller frames will also require a roll of locking strip, which is a thin plastic strip that holds the mesh in the roller by slotting into a channel.

Step 5: The excess mesh is trimmed away.

Mesh protectors are wrapped around the rollers. After five or six retensionings, the mesh will become 'work hardened' and stabilise at high tension.

So, why do so many people continue to use stretch and glue screens?
Using retensionable screens and high mesh tension is a discipline everyone in your shop has to buy into. There's also a bit of a learning curve involved in using them properly. Some people make the investment and then fail to retension the screens; or use them with a non-calibrated press; or just find them too much hassle.

This underlines the point that any move to high performance screen printing requires a business-wide shift in attitude. A good analogy is to start thinking of your process as an F1 car, and your staff as the pit crew. The car will give unbelievable performance, but everything has to be maintained and kept 'just so' in order to realise this performance. The pit crew will get the car winning races, but only if they are highly trained and understand the importance of working to a single goal, as part of a team. I was in one shop where I saw evidence of a press operator who would deliberately damage roller frames and rip mesh, simply because he didn't like using them!

It important to stress that high mesh tension needs to be maintained – at Phantom, we retension all of our screens once a week. Because of this, it is sensible to try to keep your library of screens to a minimum. A good way to work out how many you need is to work out the average amount of screens you process per day, and double it: that's how many frames you should keep. We average 12 screens a day, and we have 26 roller frames. That's all. On a Friday afternoon, all 26 frames are taken out of production and brought back up to tension, along with remaking any screens that have split during the week. Incidentally, this is another big advantage with retensionables – you can restretch them in your own shop, you don't have to send them away. Its much cheaper and you don't end up in the situation where you're using a screen which is way past its sell by date, simply because it still has mesh in it and you're trying to avoid costly send-aways.

Choosing the right mesh tension

It is possible to for mesh tension to be too high. At excessive tensions the screens become fragile and the mesh is stretched to the point where it becomes damaged. The tension within the screen becomes uneven, which can cause problems with halftone printing.
So how much is enough? This will depend on a lot of factors, but we have found that we get our best results at 40 Newtons, particularly with white underbases. We can get 62, 77 and 90 tpc mesh up to 40 Newtons comfortably. The higher the mesh count, the lower its maximum mesh tension will be before failure – I don't know anyone who gets 120 tpc much above 26 Newtons. Each manufacturer's mesh has its own characteristics and some fail at lower maximum tensions than others. You can find the recommended maximum tension charts from the different mesh manufacturers on the internet.

As I said at the beginning of this article, mesh tension is a big subject, and one that I can't hope to cover in its entirety here. My intention with these articles is to introduce the key areas for control and to draw attention to how control can help us with regard to achieving the four key values.
There is a great deal of further research you can do, and the internet is a great place to start. As with any new system, you will want to make sure you've researched it as thoroughly as you possibly can before taking the plunge. My job, here, is simply to convince you to make that start and, ultimately, to make the switch to printing using high mesh tension.

It's interesting to me that the UK as a whole has not embraced retensionable frames in the same way that shops in the States have. High mesh tension printers are few and far between here, whereas in the States it's not unusual to see retensionable screens in the smallest manual, one-man shops.
One thing is for certain, once you've embraced retensionables, and learned to use them to their maximum advantage, there's no turning back! We don't have a single stretch and glue T-shirt screen in our shop these days, and I can't imagine going back to them. Yes, it's possible to produce great work with stretch and glues, but it's much more time consuming, more wasteful and requires a lot more skill to produce consistency. Retensionables allow you the opportunity to fine-tune your process to the point where you can produce consistency and quality quickly, without having to rely on one or two extremely skilled operators to do it. If used correctly and in conjunction with other systems and standardisations in your shop, you can quickly train people who are new to screen printing to produce work to a recognised house standard.

From a print quality point of view, you could test the difference between retensionables and stretch and glues for yourself by taking a set of separations and producing one print using the stretch-and-glues, and another using high tension retensionable screens. I know which one I would want to buy if I was the customer!

Will Pearson has been screen printing since working part time at school. He spent his first five years of employment working his way round various T-shirt and graphic screen printing shops, gaining experience, before starting Phantom Screenprint in 1995. The business quickly became recognised as one of the leading screen printers for independent music merchandise.
Since then, Will has studied the screen process and commercial print management in great depth and currently provides a technical consultation service for other screen printers. He is currently writing a book entitled, Managing for Profit in Screen Process.
Tel: 01209 211673 Email: phantomscreen@btconnect.com

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Will Pearson has been screen printing since working part time at school. He spent his first five years of employment working his way round various T-shirt and graphic screen printing shops, gaining experience, before starting Phantom Screenprint in 1995. The business quickly became recognised as one of the leading screen printers for independent music merchandise. Since then, Will has studied the screen process and commercial print management in great depth and currently provides a technical consultation service for other screen printers.
He is currently writing a book entitled, Managing for Profit in Screen Process.
You can email Will at phantomscreen@btconnect.com.

Most recent articles by Will Pearson:
	Process Control 6: Profit, Pricing (and preparing for a dow
In the sixth of a series of articles on process control in screen printing, Will Pearson, of Phantom Screenprint, suggests ways to organise your business to run profitably when margins become tighter, and discusses how to cost your service effectively…
	Process Control 5: Looking after your most valuable asset
In his latest article on process control in screen printing, Will Pearson, of Phantom Screenprint, focuses on achieving smooth workflow in your printshop, and how to get the best out of your staff…
	Process Control 4: the squeegee and floodbar
In the fourth of his series of articles on process control in screen printing, Will Pearson, of Phantom Screenprint, looks at the squeegee, floodbar and accurate diagnostics…

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Will Pearson	Process Control 3 - The Stencil
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