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

Author: Will Pearson
Date Published: 2010-04-01

Process Control 3 - The Stencil

In the third of his series of articles on process control in screen printing, Will Pearson, of Phantom Screenprint, turns his attention to stencil-making…

This month I'm going to look at the stencil, and how taking proper control over this aspect of the screen printing process can help to achieve the four key values that I've discussed previously.

These are:

1) Improved print quality
2) improved processing speed
3) Achieving consistency and repeatability
4) Reducing waste, cost and labour.

In this white halftone print on a black shirt, sharp detail is maintained by making a stencil with the correct EOM ration. This has a dramatic effect on minimising dot gain and loss of detail in both the shadow and highlight areas.

I'm sure you're all familiar with explaining the screen printing process to customers or other people who show an interest in 'how it works'. A typical explanation goes something like this:
"We coat the screen with a light sensitive emulsion and then expose this to our film positive using a UV light source. The stencil is then sprayed with water. The areas that were exposed to UV light go hard and do not dissolve, and the 'image' areas that were masked from the UV light by the film positive remain unexposed and dissolve in water and are washed away. The end result is a photo stencil that we can print ink through."
In essence, this is exactly what happens, and a good enough description for those with a casual interest. However, to achieve a quality stencil requires standardisation and a great deal of control. Before we look at what constitutes good stencil-making practice, let's first examine some of the problems that arise from poor stencil making.
These include:

Loss of detail and sharpness
Images that print heavy and 'gain' as the print run progresses
Pinholing and progressive stencil breakdown during a long print run
Tonal jumps and lack of highlight/shadow detail in halftone images
Uneven colour, especially when using transparent inks
A rough quality to the cured ink layer
Moiré patterns
Poor reclaiming performance

The list above should provide ample incentive to take control over your stencil-making. So, what are the ingredients for good, consistent stencilmaking?

The film positive

A screen print specific RIP coupled with a high quality inkjet printer will produce the elliptical 60/40 dot needed for screen printing stencils.

Up until around eight years ago, it was common for screen printers to have all their film work done out of house, by a bureau or litho printer. The positives supplied would be high quality, silver-based film produced on an imagesetter. The film was very clear in the non-image areas, giving good transmission of light, and very dense black in the image areas, allowing virtually no transmission of light. Line edges and halftones would be extremely sharp and well defined, too.

Now, it's far more common for screen printers to produce their own film using an inkjet printer. There is a compromise in definition, and the film's ability to transmit light, but in general the compromise is worth it in terms of convenience and reduced costs.
When shopping for an inkjet film system, ask prospective suppliers/vendors to send you a set of films produced on their machine for you to look at – anyone that's serious about selling a film system should be more than happy to oblige. Ask for a set of A3 films for a halftone job, run at 85 lpi and at 65 lpi. If, say, it's a 4-colour separation job ask the supplier to produce three of the films one day, then produce the fourth film the next day, after the machine has been turned off overnight. This will give you a strong indication of the machine's ability to maintain accurate register whether the machine is warm or cold, and of the film's stability.
Get sample films from a couple of different systems and, using a 30x magnifier, compare the dots and lines. This may sound over the top, but spending the time at this stage, and risking making yourself unpopular with the salesman, is essential as your ability to make faithful prints can only ever be as good as your film output.

The emulsion

This 4- colour process print shows good colour balance and detail. Fleshtones can be particularly difficult to reproduce accurately. Getting this type of print to work well is only possible if you have a good quality stencil.

Photo emulsion has some very specific tasks to carry out, and the quality of an emulsion can be defined by the following parameters:
The resolution and the definition that the emulsion is able to resolve
Resolution may be defined as how thin a line the emulsion will be able to copy; and definition, as how sharp that line will appear. The emulsion's mesh bridging ability The emulsion's ability to 'bridge' a gap between two strands of mesh, rather than reticulate and follow the matrix of the mesh. An emulsion with good bridging qualities will help to reduce 'saw-tooth' edges in the print, and will be capable of a much higher resolution and definition. Emulsions that have a high solids content will always exhibit better bridging than those with low solids content.
The emulsion's build performance
Again, this is directly linked to the solids content of the emulsion. A high solids emulsion can be defined as one that has at least 40 percent solids content. Emulsion is coated onto the screen as a liquid, and then dried, and so when the screen is coated and the emulsion is still wet, you have a thicker layer of emulsion than when it is dried. The dried coating has had the liquid content evaporated out of it and so the solids content only remains, which will mean a degree of reticulation of the coating. The higher the solids content, the less reticulation occurs. As a general rule of thumb, for T-shirt printing, you'll want a stencil thickness of around 25 percent of the thickness of the mesh it's adhered to – often referred to as Emulsion Over Mesh, (EOM) – on the printing side of the mesh. A high solids emulsion will achieve this value more easily than a low solids emulsion. It really is a false economy to save money on emulsions, and I'll be taking a closer look at why a little later on.
The emulsion's ability to form a flat surface on the print side
This is known as the Rz value, whereby a low Rz value signifies a very flat stencil surface. A low Rz value is less important for T-shirt printing than it is for graphics printing, (on substrates such as vinyl or paper and board), and again, I'll examine why later in this article.
The exposure unit

A single point light source with a good bulb, clean unmarked glass and a strong vacuum is essential for achieving a quality stencil.

For the very best exposures, you'll need a single point light source – that is, the light emitting from a single bulb. In my opinion, multiple tube or bulb arrangements do not produce even exposures, and they make fine halftone work virtually impossible. The bulb must also produce a high degree of actinic light. This is the area of the Ultra Violet spectrum where photosensitive products are most acutely affected. Metal halide is the no-nonsense source for good actinic UV.
The bulb's ability to produce UV in the required range drops off over time, so you need to have a routine for replacement. Once a year is a good starting point. Avoid turning the exposure unit on and off many times during the day. If you know you have exposures later on, leave the unit on; or if it has the option, turn the lamp power to 'low', rather than turning off altogether. This will prolong the bulb's active life. It's important for quality stencil-making that the exposure unit has a light integrator, and measures the exposure time in lumens, or light units, rather than being on a timer. The integrator compensates for bulb strength drop-off, automatically adjusting exposure times as the bulb ages, or if the bulb receives electrical 'spikes' during the exposure. The distance of the bulb from the glass and screen is critical in determining an even exposure across the face of your stencil. If you have a self-contained unit, this distance is already set. If you have a separate lamp/glass frame arrangement, the correct distance from the bulb to the glass can be calculated thus: measure the diagonal distance of the largest stencil you want to make; this will be the correct distance of the glass from the lamp.

If you have a self contained unit and you're now wondering why the distance from bulb to glass seems less than the above calculation, this is because in self-contained units, you have special reflectors that are designed to give a wide, even spread of light despite a shorter glass to bulb distance.

The exposure unit must have a powerful vacuum and rubber blanket, which will hold the glass, film positive and emulsion coating together very tightly. The rubber blanket must be completely clean and free of any lumps or bumps on the inside. It's important that everyone makes sure screen frames are scrupulously clean and free of old ink before making exposures, in order to keep the blanket in top condition. We replace our blanket every two years as a matter of routine – just to be on the safe side. Many an otherwise good halftone print has been ruined by nonexposure of a few dots that were sat next to a lump of old dry ink on the inside of the vacuum blanket!

Pulling it all together

So, how do you pull these three elements together and start making repeatable, high quality stencils, and how will this help you to achieve our four key values?
Well, using a reliable and repeatable system will obviously speed up production. Having to re-shoot a screen because it didn't give the results we were after should be an extreme rarity. Re-shoots are incredibly time consuming, and besides, if you don't know why it didn't work the first time, you're counting on nothing more than luck to ensure that it will work correctly on the re-shoot.

Quality stencils save time pre-press, too. Have you ever seen a piece of artwork that had some very small text or detail 'held out' of a large body of printable area? If you spotted it on the computer file, you might go in and add white strokes or expand it in the hope that it won't fill in on the press, which is time consuming. Or, if you didn't spot it at this stage, you would then have to perform some magic on the press to try to hold that detail out – speeding up the squeegee, endlessly fiddling with squeegee/ flood bar pressure, adapting the ink and so on. Alternatively, you could just start all over again with a new separation, or worse still carry on printing and hope your customer won't notice! All of these scenarios can be avoided by simply exercising control over the stencil making process.

Earlier on, I touched on the subject of Emulsion Over Mesh. If there's one common error in the T-shirt shops I visit, it's that people don't put enough emulsion on their screens. To build a good EOM, you need multiple strokes; these must start on the print side of the screen, and your final strokes must be on the squeegee side. The screen must then be dried print side down, in gentle heat not exceeding 38°C. You should be aiming to coat with your scoop coater using the slowest stroke you can manage whilst maintaining a smooth action. This takes practice! Your scoop coater blade must be perfectly smooth, with no nicks. We polish our scoop coaters periodically using very fine wet and dry paper, used wet. We also make the passes with the wet and dry all in one direction, all the way along the blade, rather than rubbing along the edge in a sanding motion. This means we'll maintain flatness along the length of our coater. Incidentally, we always do this job outside, to save introducing metal dust in our work area. Dust is the enemy of stencil-making and aluminium dust is particularly unwelcome.

How many coats you make to achieve the magic 25 percent EOM will depend on the solids content of your emulsion, and the mesh count you are coating. Once you've found the coating strategies that work best, make a poster with the coating procedure for each mesh count you use in your shop and display it in the screen room. Make sure everyone adheres to it rigidly.

How will you know if you've achieved the right thickness: by using a densitometer, or simply looking at your prints and experimenting. Make a screen that has a band of tone and some progressively small fonts held out of a body of ink and test different coating techniques. Incidentally, keeping your emulsion in a fridge will ensure it doesn't go off in the warmer weather, and will mean that you're always coating with emulsion of the same viscosity. This will make a difference to EOM build-up.
You may have heard of face-coating – that is, coating a screen, drying it, then adding further coats to the print side. There are two reasons to do this: the first is to create a very deep stencil – for high build printing, for example; the second is to make a smoother bottom to your stencil, and lower the Rz value. This is important when printing smooth substrates, such as vinyl or coated papers, as it helps the stencil to 'gasket' the ink it is depositing: it helps reduce gain from ink escaping under the edges of a rough-surfaced stencil. Don't worry about RZ value for T-shirt printing, though: the incredibly rough surface of a T-shirt will negate any attempt to improve gasketing by creating a smoother stencil.

A drying cabinet with an accurate thermostat helps to dry coated screens thoroughly. Screens must be dried level and print side down, to help the emulsion form on the print side of the screen.

The stencil coating must be completely dry before you expose it: don't be tempted to rush and expose a screen too soon after it was coated. You can use a digital hygrometer in your drying cabinet to ensure screens are dry, (only expose screens once the hygrometer is reading less than 25 percent Relative Humidity).

Exposure

On to exposure, then: clean the exposure glass daily, to avoid contaminants and pinholes. These following issues are the most common causes of pinholes:

1) Dust and contaminants on the glass
2) Inadequate degreasing or ghost images on your mesh
3) Underexposure

This last issue (exposure) tends to produce the sort of pinholing that appears progressively during a print run: it's stems from underexposure causing the stencil to break down. T-shirt printers have a tendency towards underexpose, partly because the inks we use are fairly inert and non-aggressive, unlike the solvent inks used in graphic printing. T-shirt printers tend to 'get away' with underexposures more because of this. The other problem with underexposure is that, when the screen is developed, you end up washing away a lot of that precious EOM build-up that you took so much care achieving in the first place.

There really is no other way to ensure correct exposure times than by buying or borrowing an exposure calculator, testing with your different mesh counts, and then displaying a poster with the correct exposure times for each mesh count on it. Autotype do a superb 'one shot' calculator, which is quick and easy to use. ) Once the screen is exposed correctly, you can wash it out. If a high quality emulsion has been used on a well degreased screen, exposed at the correct setting, there's no reason why you can't use a pressure washer on fan setting to develop it. This speeds up the developing process and is more thorough on halftones. Use a wet and dry vacuum cleaner to remove excess moisture and unexposed emulsion debris from the stencil. Now it's time to dry the stencil; again, print side down in gentle heated air.

Correct EOM

In Diagram A, (above left), the stencil is too thin and has reticulated into the mesh openings. This means that ink is blocked by the mesh from 'flowing out' to fill the stencil correctly. Also, there is no edge to the stencil to stop ink leaking out under the edges of the stencil.

Diagram B, (above right), shows a stencil with correct EOM (Emulsion Over Mesh). Note that the ink has room to flow under the mesh and into the entirety of the open areas of stencil.

I'd like to quickly explain why the correct EOM is so desirable before I leave this topic. A stencil that is too thin sits too close to the matrix of the mesh, and it is in this situation that the mesh starts to influence the printed image. Held out details and shadow dots fill in; straight lines print with saw tooth edges; and radii exhibit stair-stepping. Also, the depth of the stencil directly affects the ink deposit. Getting the right EOM will mean that the ink deposit is more uniform and there is less need for multiple squeegee strokes.

Once the correct EOM is established, the mesh count typical for any given job can often be increased, allowing for tighter quality control, reduced ink deposit and so on. For example, if you normally use a 55 tpc mesh for underbases, you may be able to move on to a 62 tpc successfully. The great thing about improving your stencil-making skills is that it doesn't have to happen all at once; you can do it incrementally and test and experiment when you can make time for it.

If you've ever looked at some of the high end prints on the high street and asked yourself "how do they do that?" there's one thing you can be sure of: the shops producing the best quality simulated process halftone jobs, or very fine detail prints, have total understanding and mastery of stencil-making techniques.
I have a friend who is regarded as the best stencil-maker in the world. He doesn't print T-shirts; he prints decals for model aircraft. I have watched him produce stencils that can print legible type at 1 pt, and lines that are 25 microns thick. It's no coincidence that for the first 15 years of his career, he was a stencil-maker. He didn't get involved in any other part of the process; he just made stencils. The point is that quality stencil making is a much bigger and more important subject than a lot of people realise, and it's another major player in the quest for high performance screen printing.

We can all up the bar in our own shops, and I hope this article has given you some useful pointers for doing just that in your own business. In the next instalment, I'll be looking at the squeegee and floodbar, and examining the dynamic of the ink-transfer process itself. See you then!

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.

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