How to manage tension in your shop

What’s ironic about the ‘screen printing’ industry is that so many shops completely ignore the screen itself. It’s funny because it isn’t like this is some sort of deep, dark, industry secret. Yet, I’ll bet at least half the people reading this article (or who need to read this article) either don’t own a tension meter or never use it. Some time ago I visited a shop to help them out with some process and workflow efficiency points. When I walked through the front door, the production manager was screaming at the artist. You could cut the drama with a knife. “Your separations suck! Nothing ever lines up! Do you even know what you are doing?” But I’d seen this movie before. “Hold on”, I said. “It isn’t the seps. It’s your screens. Do you guys have a tension meter?”

They did. It was buried underneath a pile of junk in the back corner on a shelf. The clamshell case was coated with dust and T-shirt lint. Probably like yours. We dug it out, and I calibrated the meter using the thick chunk of glass that comes with the set. The production manager brought the screens over. All were static, aluminium 23” x 31” frames. The table was cleared off and the tension for each of the four screens was measured.

The best tension of any screen in the group averaged out to about 16N/cm2. The lower left corner of the underbase screen was under 11N/cm2. This explained why that job wouldn’t register. It’s like trying to build a house on top of quicksand. There isn’t enough foundational support to do the job right. So, being the curious guy I am, I asked the production manager why they didn’t measure the screen tension. His response: “We don’t have time to measure.” But they did have time to spend over ten minutes per screen per day trying to get jobs to register. Not to mention the previously discussed finger-wagging drama.

Look, I know this is a fairly boring subject. People in this industry normally want to talk about presses or equipment. Those are sexy. Screen tension? It uses numbers. Maths. It’s a step that’s easily forgotten. But what if I told you that screen tension was a secret way that your shop could make more money? Would the enticement of a truckload of cash make it more interesting?

Let’s take that apart for a minute and I’ll explain what I mean. Your screen mesh is comprised of threads that run perpendicular to one another. One set is the warp, and that runs vertically. The other is the weft, and that runs horizontally. 

A calibrated tension meter is the device used to measure tension. This is a precise instrument and care should be taken to ensure the proper storage and use. The meter measures how much the screen fabric gives way when a downward force is applied to it. The downward force when measuring is gravity, and it uses the weight of the meter to calculate that movement. For an analogy, think about how much a trampoline might give way if you are standing in the middle. For our screens, that deflection is expressed in newtons per square centimetre. Yes, newtons as in Isaac Newton with this whole gravity and apple falling from a tree bit. If you are using static frames, 25N/cm2 would be an acceptable tension. For retensionable frames, 35N/cm2 is a good target.

To measure tension, first lay the screen on a flat table with the mesh side up. Measure the warp (vertical direction) and then the weft (horizontal direction) in the middle of the screen. Like a tennis racket, the middle of the screen is the sweet spot. It’s not hard to do and only takes a few moments. Then, measure each of the four corners, about a palm’s width away. More expensive tension meters can measure tension bidirectionally– ie they measure the warp and the weft directions simultaneously. Less expensive tension meters only measure tension one way, so you have to rotate the tension meter each time. You want to measure the centre and the corners, and ideally, they will all be about the same in tension. However, it is possible to have good tension with most of the screen but have one area fail due to any number of reasons.

Your shop should have a floor limit on what is acceptable for tension. The question that I’m asking right now is, “What is your minimum screen tension for your shop?” Do you even know? My recommendation for shops is that any multicolour job should have tensions for all screens on the print run that are over 20N/cm2. And, to ensure easier registration, all of the screens should be within 2N/cm2 of each other. For any screen that has tension under 20N/cm2, they are relegated to only one-colour jobs. I’ve seen shops set their minimum for one-colour screen tension at 16N/ cm2, 17N/cm2 or 18N/cm2. It’s up to you… but this needs to be established. Personally, I’m a fan of 18N/cm2.

Remember, this is the low end. The higher your tension, the better results you are going to get when printing. Let’s examine that next. Have you ever been at a trade show and marvelled at all of those amazing prints on display? The print hand is incredibly soft, the colours vibrant, perfect and tight registration. Those prints are made with all different ink sets, presses, people and shop environments. What could be the one factor that links them together for print quality?

Those shops take incredible care to ensure that they are using the best-made screens for those jobs. This is where tension starts to make you money. Tight screens give you crisp, sharp edges. Prints are smooth, not rough. The ink sits on top of the shirt fabric, not driven through it like a nail, which influences opacity. Tension matters. If you were to peek underneath the press when you are printing the shirt you could see that when your squeegee travels across the mesh during the print stroke, the screen mesh actually touches the fabric of the shirt for an instant.

High-tension When the mesh tension is high, the mesh will snap back up instantly, leaving the ink with a smooth finish on the substrate.

Low-tension With low screen tension, the mesh touches the surface of the shirt longer. It doesn’t snap back fast enough. Due to physics, what’s left behind is a rough, bumpy print. When printing over an underbase, the top colour could print irregularly and leave some small, unprinted dots.

Pick-up For multicolour jobs, because the low-tension mesh has more contact with the wet ink of previously printed colours, it tends to pick up the ink on the back of the screens. This causes opacity issues with printed colours, and sometimes blurry looking edges where the colours might touch in the image.

Poor ink opacity Low-tension can also require you to double stroke the print. Worse, many printers then feel obliged to add more squeegee pressure. I’ve been to shops where every screen hit is a double stroke. Printers commonly blame the ink first, then the press. That’s why for those printers it takes twice as long to print, and they use more ink as well. It is incredibly common for print shops to blame everything but their poor screen room craftsmanship. Here is something to consider. If ink or a particular press works for other people, but not you, maybe the problem isn’t with that product. Are you really using it correctly?

Surfing the mesh wave Another common problem is a combination of low-tension screens and too much squeegee pressure. This creates a ‘wave’ that travels in front of the squeegee during the print pass. This typically causes the ink to smear as it doesn’t drop down perfectly onto the shirt, but prints a little ahead of the squeegee. At the edge of the image, this causes the ink to smear instead of having a crisp edge. Not only that, when the ink isn’t completely shearing through the mesh and onto the garment, it can build up in your screen. This can prevent details from printing, or produce a blurry print.

Here’s a concept that I want you to consider. When your screens are perfect, everything is easier on the print floor. You don’t have to double stroke on an automatic press to get opacity. Screen registration is a breeze. That saves time, and time, as we all know, is money. Plus, on the money side, every time you double stroke your ink you are using twice as much ink to get the same result. Let’s say that your opportunity cost for your press is £300 per hour. For some shops that’s too low but, for this example, it is a good conservative number. £300 per hour is equal to £5 per minute. As everyone probably knows, I like screen set-up time to be under five minutes per screen.

But let’s say we’re dealing with that example shop mentioned before. They took around ten minutes per screen for their set up because nothing was lining up. After adjusting their workflow to take screen tension into consideration, they got their set-up times down to about six minutes per screen. That’s a four-minute per screen saving. Like a lot of shops, they average about three screens per job and print about ten jobs per day. So that can be expressed as: 3 (screens per job) x 10 (jobs per day) x 4 (minutes per screen savings) x £5 (opportunity cost per minute) = £600 per day savings. Per press. Every day. So, tell me again how you don’t have time to measure your tension?

This isn’t anything new, is it? Hopefully not, but there are plenty of people posting online in groups or forums that don’t comprehend the importance of screen tension. So, as people love checklists I’m going to offer one.

■ Regularly use a tension meter. If you don’t have one, buy one. Learn how to use it.

■ Design, as part of your screen room workflow, when the tension should be measured for your screens. Who is doing it? When are they doing it? How are you recording it on the screen?

■ Set a minimum shop tension level. This is the lowest level screen tension that you will use in your shop. Explain to your staff what this means and why you have it.

■ Train your staff. Screen room crew certainly, but your print and art staff as well. Illustrate what success looks like.

■ Reach out to your suppliers for help. You can get better screens. Training is available too. Don’t be in the dark.

■ Don’t make excuses. “It’s how we’ve always done it” is not a valid reason for not having best industry standards in your shop. 

As my friend Alan Howe of SAATI is always fond of saying, “Screens are equipment”. Treat them as such.