The Polarizing Truth: Why Your TWSBI Eco Won’t Break

Thankfully, debates about fountain pens rarely reach chair-throwing intensity. Occasionally, though, a topic comes along that’s tougher to unpack than aesthetic preference or writing experience. Something that gnaws at us, stubbornly refusing to yield a root cause, leaving us to build opinions on mounds of anecdotal evidence…

TWSBI Diamond 580 + EcoThe durability of TWSBI’s acrylic fountain pens is a prime example. Some peoples’ TWSBIs break. Most don’t. Everyone’s use cases and environments are different. The 540 and Mini models seem particularly susceptible, but I’ve seen complaints about cracked 580s and VAC 700s, too. TWSBI have told us about manufacturing improvements, such as relocating injection mold feed points. Still, the parade of broken pens marches on… Pen Addict Podcast host Myke Hurley recently mentioned that his girlfriend’s TWSBI Mini had joined the casualty ranks.

Thankfully, TWSBI offer incredible support. From every account I’ve seen, they’ll happily provide free replacement parts (or, if needed, a replacement pen) to anyone who experiences a break.

Enter the Eco

TWSBI recently released a new pen, the “Eco” – their least-expensive offering yet. Weighing in at US$29, many wondered: Will the Eco’s durability suffer relative to its more-expensive (and already-anecdotally-problematic) cousins? After doing some analysis of my own, I can confidently tell you: No, it won’t.

I’ve been the happy owner of two TWSBI 580 models (the original “Diamond” 580 and a colorful 580USA) and have had no issues with them. Given its price point, I’d meant to pick up an Eco, but quickly filed that purchase away to “when-I-get-around-to-it” status. Then, while reading about polymer stress analysis for work, I happened upon a method to compare the two pens – so an Eco quickly made its way to my Anderson Pens shopping cart, and then to my doorstep.

Crossing the … Polarizers?

"Spoon and Reflection" by Dan Weidbrauk
“Spoon and Reflection” by Dan Weidbrauk (CC-BY)

When I learned about crossed-polarizer stress analysis, my mind immediately time-warped to a demonstration I saw years ago at the Corning Museum of Glass. Explaining the special properties of Pyrex, the presenter showed two sheets of glass, suspended between two enormous sheets of polarite – much like the stuff they use for polarizing camera filters. Between these polarizing sheets, one of the glass panes appeared clear, and the other was filled with beautiful multi-colored swirls. These indicated stress in the material – at the molecular level, explained the presenter – making it more prone to breaking. She then applied some external stress – maybe a localized heat source? I don’t remember. But I do remember the swirled glass shattering dramatically, while the clear, stress-free sheet stayed intact.

Cut back to present-day: Searching the web for ways to visualize stress in plastic, I came across the site of one of my dad’s colleagues at RIT – Professor Andrew Davidhazy. An expert on technical photography, he has a page explaining the technique on display at Corning all those years ago.

Trying it Out

As my new TWSBI Eco traveled from Wisconsin to New York, I took the technique for a test-drive. Instead of sheets of polarite, my crossed-polarizers would be a pair of polarizing camera filters1. I pulled the nib out of my Diamond 580 and lined everything up: it worked beautifully! The stresses in the plastic of the barrel and grip section became immediately apparent:

TWSBI 580 - Polarized Light Stress Analysis
TWSBI 580 – Polarized Light Stress Analysis (Click to Enlarge)

There are a few important take-aways here:

  • The obvious one: This plastic is stressed. That can happen for various reasons, including the rate and distribution of cooling or curing, or the flow of liquid material through the mold. I’m sure the mechanical engineers among us can provide some more insight.
  • Stress isn’t necessarily bad: Pretty much all molded plastics are under some amount of internal, material stress. In spite of this, we know that the vast majority of 580 pens live long and happy lives.
  • In this pen, the stress lines bend to form concentrations: From my [limited, electronics engineer’s] understanding, the convergence of “lines” or “bands” of stress in the material represents a concentration of stress at a particular point, making that point more prone to breakage.
  • There are stress concentrations near the threads: Looking closely, you can see that the material stress seems to change – from linear, along the barrel’s length, to circumferential – near the threads.
  • This pen is disassembled, so none of the stress we’re seeing is due to over-tightening or outside forces.

Given I have zero experience with broken TWSBI 580s, I can’t say whether there’s any causal connection between these stress concentrations and cracks or breaks. But the presence of stress concentrations in areas prone to breakage gives us at least a point of correlation, and warrants further exploration…

The Stress-Free Eco

When I unboxed the TWSBI Eco and – before even inking it up or writing with it! – dropped it onto my polarizers, I expected to see some stresses. Maybe they’d be oriented differently, or have different concentrations. What I saw, though, surprised me:

TWSBI Eco - Polarized Light Stress Analysis
TWSBI Eco – Polarized Light Stress Analysis (Click to Enlarge)

Almost no stress at all. Given the Eco’s smooth, rounded barrel – contrasting the 580’s facets – the plastic is under very little stress. The colorful bands seen in the 580 are caused by more severe filtering of extreme wavelengths of light (the red and violet ends of the visible spectrum) – indicative of higher local stress.

Looking closely, you can still see some stress lines in the Eco’s barrel, particularly where it meets the threads. But, in relative terms, if the 580 is a Wall Street day-trader, the Eco is relaxing by the pool with a margarita.

What to Conclude?

Let’s start off by saying, right off the bat: This is not a scientific analysis! At worst, it’s one more point of anecdotal evidence to add to the pile. With a whopping sample size of one each, we certainly can’t generalize to make conclusions about entire product lines or brands. But we can say, with decent confidence, that the TWSBI Eco will tend to be a more durable pen than the 580.

Polarized Light Stress Analysis - TWSBI Eco / 580 Side-by-side
Polarized Light Stress Analysis – TWSBI Eco / 580 Side-by-side (Click to Enlarge)

In my opinion, this is no reason not to buy a TWSBI 580 or Mini. They remain great pens, and great values. Without total sales numbers from TWSBI, the community really has no idea what percentage of owners experience problems – but we can be confident that those who do will be well supported by TWSBI.

The most important thing to take away from this analysis, I believe, is confidence. The sort of confidence objective analysis2 allows: New fountain pen users considering a TWSBI can choose an Eco with far fewer reservations about cracking or breaking. Enthusiasts and Addicts can recommend it without the caveats they might have previously applied to the TWSBI brand. And, as usually happens with debates in the pen world, we’ll all get back to putting ink to paper soon enough…

To be sure, some Ecos will crack3 – some already have. Just as, I’m sure, some demonstrators from Pilot, Sailor, Monteverde, Lamy, Kaweco or any number of other brands have: in small-enough numbers to avoid garnering a reputation. If material stress concentration is any indicator, the likelihood of this happening to a TWSBI Eco should be comparable.

Want More Data?

Looking at just two pens doesn’t tell us a whole lot; I would certainly be interested in applying this technique to other pens in the TWSBI lineup, other demonstrators (perhaps from brands with less of a reputation for breakage), or to pen barrels or sections that have either cracked, or appear ready to. If TWSBI have replaced a part of your pen, and you still have the broken bits, I’d like to get a look at them through this crossed-polarization setup. Please get in touch using the handle @mtbkrdave via Twitter, Instagram or on the Pen Addict Slack.

If I learn more, I’ll post it here.

  1. For the optics nerds among us: Yes, I realize that these are circular-polarized, and stress-analysis is typically done using linear polarite. In this case, I’m analyzing in strictly relative / comparative terms. 

  2. While this analysis isn’t necessarily scientific, it is objective – far more so than simply saying “some of the TWSBIs I’ve owned have cracked, therefore all of their pens must be prone to it.” 

  3. Heck, by posting this, I’m probably guaranteeing that mine will! 

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  22 comments for “The Polarizing Truth: Why Your TWSBI Eco Won’t Break

  1. September 23, 2015 at 7:35 pm

    Thank you for sharing this! I was avoiding the Eco after hearing about people’s 540s & 580s but I’m going to take a risk with one now based on this. What a cool blog post!

    • Dave
      September 23, 2015 at 7:37 pm

      Thanks – Glad to hear that – I’m looking forward to inking mine up for the first time!

  2. Klundtasaur
    September 23, 2015 at 9:14 pm

    Awesome post! Thanks for the pics and the writeup. Also, just FYI, the FPN has an informal poll up about cracking TWSBI’s at Anecdotally, it seems that the percentage of pens cracking seems to vary by model (TWSBI’s newer models seem to crack less).

  3. September 24, 2015 at 3:44 am

    I remember that explanation in the Corning Museum ( I been there ) and what it means is that your won’t easily break but it will take more scratches. The stressed material make it harder to scratches.

  4. September 24, 2015 at 2:43 pm

    This is a very cool post, Dave.

    • Dave
      September 24, 2015 at 2:50 pm


  5. February 15, 2016 at 1:41 am

    Thanks for the fascinating analysis Dave. One minor question: Are the pens made of acrylic plastic or polycarbonate plastic. Thanks for any clarification you can provide.

    • Dave
      February 17, 2016 at 5:18 pm

      Glad you liked the post Barry! Honestly, I have no idea… I don’t even know the difference between the two (I’m an electronics guy, only dabbling in materials here). I assumed the clear plastic is acrylic because it’s so commonly used in the pen world – but clear polycarbonate is certainly a possibility too.

      • April 27, 2016 at 11:00 am

        according to TWSBI it is polycarbonate which in a sense is “not much different than acrylic” but I would say it is still different than acrylic

      • Douglas
        May 18, 2017 at 8:21 pm

        This is great information and very insightful. TWSBI, indeed, confirmed that they’re using polycarbonate, which is not a cheap plastic at all. TM names for polycarbonate are Lexan and Makrolon whereas Lucite is a TM name for acrylic. They are quite different. Polycarbonate (lexan) is enormously strong but it isn’t really scratch resistant. It’s also clear that Lamy dealt with the surface of the Lamy 2000 (Makrolon) quite differently (by applying a texture) than TWSBI’s preferences. Manufacturers that require ongoing clarity in the polycarbonate typically bake a coating on it which makes the surface much more scratch resistant. That may be the challenge that TWSBI is dealing with because in heating a complex shape so that surface is bonded with the scratch resistant compound it may also introduce undesirable stresses in the material itself. Some of you may be old enough to remember Lexan being used in kitchenware (eg bowls, cups, and plates) in the 60’s & 70’s and then reports that they occasionally explode or shatter. LOL! This is possibly due to similar causes. Polycarbonate has also been successfully used in eyeglass lenses with scratch-resistance being critical as well. Your pics suggest that with the ECO they’ve made a lot of progress.

        Thanks for the article.

  6. Brian
    April 9, 2016 at 5:17 am

    Hello Dave,
    Amazing post! It’s well detailed and explained. I have never used TWSBI fountain pens, since you say that they replace damaged fountain pen parts so is this warranty covered for a lifetime or a limited period like Parker? If I am going to buy a TWSBI in the future, it will be an Economic. Thanks to you.


  7. Piers Harris
    September 21, 2016 at 11:16 am

    Politely. So what. I have over time had 8 Ts. I had issues with 4. Replacing broken pens for those who can be bothered with the hassle isn’t a good business model in my book!

    • December 29, 2016 at 5:04 am

      Yet you’ve bought EIGHT of them.

  8. May 13, 2017 at 4:14 pm

    I’m not an engineer (I studied business in undergrad. and ended up in law school), but I wonder if the “stress” you’re seeing is, in fact, material with a lesser density? That would account for the colors under the polarizers since different densities have different refractive indices and would refract light at different angles. When you line up your polarizers, they block one angle of light but not others, causing some light to appear colored. This could also account for the cracking since a lower density medium wouldn’t expand at the same rate in response to heat, nor would it flex in the same way when put under shear stresses. Perhaps with the Eco TWSBI figured out a way to more consistently cool the plastic?

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