Author: platypuspens

Early fountain pens (say, in the late 1800’s) often suffered badly from intermittent flow of ink onto the page, and terrible blotting. It was frequently necessary  to shake the pen to get ink to flow down to the nib, but that, of course, risked an unwanted ink splat. 

The problem was that the feeds, insofar as they existed, did not allow for smooth ink-air exchange. As explained in the previous post in this series, air has to enter the ink reservoir of a fountain pen to replace the volume of ink that has been written onto the paper. If not enough air enters then the ink flow dries up, and if the air can enter too easily then ink will flow out in drops. It is a key job of the feed to regulate the air-ink exchange.

Possibly the first feed design that successfully regulated the air-ink exchange was patented by Lewis Edson Waterman in 1884.

That patent describes a feed with a channel cut into the top along its full length with fine sub-channels at either side of the main channel.

The patent explains that ink will flow through the finer channels, assisted by capillary action, and when a slight vacuum occurs in the ink chamber an air bubble can enter via the nib breather hole and travel up into the ink reservoir through the main channel. The passage upwards of air would not prevent simultaneous downwards flow of ink because of the natural preference of air for the large main channel and the preference of ink for the thinner sub-channels. In other words, the Waterman feed allowed for a regulated exchange of ink and air and allowed what the patent describes as “a certain and uniform flow of ink to the pen [i.e. nib].” (At that time the part of a fountain pen that we call a ‘nib’ was described as a ‘pen’.

There have been many, many subsequent patents concerning fountain pen components, including feeds, but that 1884 patent of L.E. Waterman was germinal. Most of the vintage ebonite feeds in my collection have capillary channels inscribed in their upper face that resemble very closely what Waterman described, and ebonite feeds are still made like that [link to FPR].

Despite the fact that we call the hole in a nib at the start of the slit a “breather hole”, very few modern pens breathe through that hole! Instead, they have separate air channel that is directly open to the air, usually on the bottom of the feed opposite the nib, and such feeds do not use the 1884 Waterman arrangement to allow contrary air and ink flow. I imagine that there has been many patents relating to the placement and design of an air channel in the feed separate from the ink channel(s), but have not tried to find them.

Even though modern ebonite feeds can have pretty much all of the design features of modern injection-moulded plastic feeds, those features have to be relatively coarse because ebonite is brittle and has to be machined rather than moulded. For comparison, the following photos show details of the plastic feed from a Schmidt FH452 unit (#6-sized). Notice how narrow the ink channel in the top surface is (about the thickness of two sheets of Rhodia paper). A narrower ink channel will have better capillarity than the wider channels of an ebonite feed, perhaps to compensate for the lower intrinsic hydrophilicity of the plastic compared to ebonite. That capillary channel is also very deep along most of its length (about 2mm), presumably to allow more ink to flow through it as compensation for its narrowness. Feed design is like airplane design in that it consists mostly of balancing compromises!

The biggest compromise in many modern plastic feeds is surely the fact that it has to be designed to work with standard international cartridges and converters. That means that the feed must have very narrow spike that fits inside the nipple that goes into the narrow neck of the cartridge. Ink and air have to go through that nipple in opposite directions. The usual solution to that problem resembles the original Waterman patent of 1884, but it seems likely to me that the small diameter of the system at that point acts as a bottleneck to the smooth flow of ink.

If there is such a bottleneck, how can it be that the ink flows without obvious interruptions from the nibs of Platypus pens that use standard Schmidt nib units and cartridge converters? There is a row of fine fins on the plastic feed inside the housing that provides the answer. The gaps between those fins fill up with ink and act as a local reservoir to support ink flow onto the page even when ink flow along the feed spike is impeded by air. In effect those fins act like the ink collector that makes the original Parker 51 so reliable. If you want to see for yourself how much ink can collect inside the feed, simply remove the cartridge or converter from a filled fountain pen and write with it. You might be surprised by how long you will be able to write before it finally dries up.

What’s next?

What’s next in this series is, of course, an explanation of how and why I made my own feeds for the Platypus Model 20 fountain pen!

We all know what a fountain pen is. It’s a tube filled with ink and a nib that delivers that ink onto paper. Simple.

Hang on, can it really be that simple? No. For proper function of a fountain pen there has to be a close balance between ink flow and a flow of air in the opposite direction. We can start to explore the how and why of that by considering the question of why ink does not flow out of a pen until the writing starts.

Before getting into the nitty-gritty of fountain pen magic, let’s consider a simpler system: a ballpoint. Yep, a standard ballpoint pen like the Bic Cristals that my wife prefers over my pens!

Why does ink not flow out? Part 1: ballpoint pens

Ballpoints pens have a ball in the end that transfers ink from the reservoir to the page by rolling. The ink reservoir in a Bic Cristal is a simple tube open to the air at the top, so why does the ink not flow out of the pen by gravity and, similarly, why does the ink not flow out of the back of that tube when the pen is held tip up? Some ballpoints have pressurised ink reservoirs (e.g. the Fisher Space Pen) so how do they not leak everywhere?

 You probably already know the answers. What stops the ink from flowing out of a ballpoint pen is just the viscosity of that ink. Ballpoint ink is thick and sticky oil-based stuff and so it does not flow readily though the tight space between the ball and its socket. In fact, it is so thick and sticky that it almost doesn’t flow at all and that’s why it doesn’t leak out the back of a Bic Cristal ink tube unless you leave it in a very hot car. (I cannot resist putting in a link to the “pitch drop experiment”) The viscosity of the ink allows the pen to hold ink in all orientations, and its stickiness allows the ball to deposit ink onto the rolling ball and then the page.

[Rollerball and gell pens are closely related to ballpoint pens, but they often (always?) use water-based inks that are less viscous then the oil-based inks of ballpoints. I am not going to discuss them beyond saying that if you are intent on using water-based ink then why not use a fountain pen?]

Why does ink not flow out? Part 2: fountain pens

Fountain pen inks are almost entirely made of water and are neither sticky nor viscous and so the ink regulating task of a fountain pen is entirely different to what happens with a ballpoint pen. Fountain pen ink would certainly run out the back of the unsealed ink reservoir of a ballpoint pen. So how does a fountain pen stop its runny ink from running out all over the place?

It is often said that capillary action and surface tension prevent ink from leaking from a fountain pen. Without explaining either of those (related) things, I will say that they are not the whole story. A simple experiment will prove that. Put a full ink cartridge into a fountain pen and hold the pen nib downwards. Now melt a hole in the back of the cartridge with a hot pin. Make sure that you do that while wearing white clothes because that will help you see what comes next: the ink will now dripping freely from the pen! Perhaps you don’t really need to do the experiment as it is safer to leave it as a thought-experiment (a gedankenexperiment for Pelikan, Lamy, and Kaweko enthusiasts!). And, of course, you can just take my word for what would happen.

When the back of the ink cartridge has a hole air can easily enter (and leave, if it has a mind to) and so the air pressure at the top of the ink in the cartridge will immediately equalise with the ambient air pressure. As the ink flows out of the pen onto your white trousers more air will enter the cartridge to replace the volume of the ink that flow out. If you were now to put your finger onto the end of the cartridge to block the hole the outflow of ink would slow and then stop as a slight negative pressure develops at the top of the cartridge to counterbalance the weight of the ink column.

You can see the pen maker Ian Schon demonstrate a real version of that experiment using a plastic pipette in place of a pen here. If you watch closely you might notice that there is a little ink left inside the tip of the pipette at the end. That little bit of ink is left behind because of the surface tension of the ink and the capillary action of the thin tube at the tip of the pipette. Doing the experiment with a fountain pen will equivalently result in a little ink being retained in the feed and nib and that is totally because of surface tension and capillarity!

How does ink come out when you write?

So, when a fountain pen is full and held nib-downwards the ink is held back by both capillary action and a pressure gradient between the inside of the cartridge and the air outside. When you move a fountain pen nib across paper the ink in the nib tip is left behind on the paper largely because of capillary action. You can think of it as the ink being dragged out of the nib by the paper and you will not be far from the truth. As that ink is dragged out of the pen a slight negative pressure will develop inside the pen (unless you have put a hole in the cartridge!). If nothing else happens, pretty soon that negative pressure will prevent ink from flowing out onto the paper: the nib will “dry up” and the fun stops. Of course that doesn’t regularly happen with a good fountain pen like those made by Platypus Pens. Instead, once the negative pressure reaches a sufficient level a bubble of air will enter the pen by way of a channel in the feed.

Too much air access into the reservoir will lead to ink leaking, and too little will lead to drying out of the nib. The outflow of ink and inflow of air have to be well balanced for reliable writing performance of a fountain pen, and regulation of that is the most important function of the feed. And that is the topic of the next blog post.

The insides of a Platypus Pens Model 20 fountain pen

The Model 20 has more parts than any other Platypus pen, and those parts are made of a range of different types of plastic selected to optimise the function and appearance of the pen. There are so many parts that I decided to make a set of custom trays to held the parts and allow me to easily confirm that I have all of the parts that I need to assemble the pen.

Part 1 is what I call the ‘driver’. It has a crown-like ring of teeth that engage with the top of the bladder, which has indents for each tooth. It allows for the bladder to be twisted and compressed while still letting you unscrew the pen body while it is full of ink. That lets you most reliably see the ink level and thereby let me have a minimal ink window which, in turn, maximised the amount of lead ballast that I could fit into the grip.

Part 2 is the ‘plunger rod’ and it has two distinct threaded portions. The bit nearest the driver has a four-start coarse thread that guides the twisting and compression of the bladder. The upper portion has a two-start left-handed thread that is what the blind cap runs along as it is wound out and in. Part 2 is made of a plastic type that is fairly hard and wear-resistant, PCTG (not PETG). The plunger rod is reinforced with a stainless steel machine screw that runs down its length to a nut captive in the driver.

The plunger rod installation is designed to allow easy replacement. However, even though it did wear noticeable during the 100,000 cycles of testing (see here), but even so it was still fully functional. I now doubt that it will even need replacement.

Part 3 is the inside of the blind cap and it has three threads. The inner thread marries with the two-start thread on the plunger rod and it is retained by a washer under the head of the stainless steel machine screw in the plunger rod.

The second thread of the blind cap inner is a short tapered two-start thread with the same pitch as the plunger rod. That thread engages with minimal turn into a female thread at the end of the body liner. That locks the blind cap in place and prevents my friend, Peter, from breaking another  pen by twisting the blind cap in the wrong direction! Part 3 is made of plain PLA, which is harder than most other 3D printable plastics (but sometimes brittle) and so it is wear-resistant when paired up with the PCTG of the plunger rod. (I use melted beeswax as a lubricant there and on the cap threads because it stays where it is put and wont give you greasy fingers.)

Part 4 is the decorative finial that screws into the third thread of the blind cap inner (that I didn’t explain). That thread is left-handed and fine pitched so that it can hold firmly and it can be removed to give access to the bolt head in case that the drive system needs to be disassembled. I’m not sure that it will ever need to be disassembled, but maybe the plunger rod will need replacement after 20 years of use!

Part 5 is the blind cap outside. It is purely decorative and has no threads.

Part 6 is the ink window and ink reservoir. It’s made of either PETG or CPE. The CPE is a modified PETG that is supposed to have improved layer adhesion and be less brittle. I’m not sure that I can tell the difference between the two types of plastic and so I will probably use up both rolls. The ink window and reservoir are printed in a manner that makes them almost fully waterproof, something that is not always easy with FDM 3D printing. Almost watertight: not exactly leaky, but I found that without further treatment ink can creep into various tiny channels and voids in the walls and then effectively stain the parts. To prevent that I seal the inside of the parts with low viscosity epoxy that runs into the channels and voids to prevent the ink from doing so.

There are two threads in part 6: the male threads to engage the body; and a female tapered thread to engage the white tube that holds the nib and feed. The body thread is pretty ordinary as it is nearly just a 10mm metric thread. The female thread is tapered to maximise strength of the junction between  the nib tube and ink window. (Yes, I broke a few while placing the lead ballast and so I beefed the joint up even though it is well-secured by epoxy and the grip when the pen is finished.)

The nib tube, part 7, is made of ABS plastic so that it can safely resist the heat while I heat-set the nib and feed to each other. The ABS is also useful in being well-behaved while I ream out the tube for a close fit with the feed. The fat region at the bottom of the nib tube in the photo is a temporary extension to help align the reamer.

Part 8 is the cap topper and it has a fine right hand thread to engage with the top of the cap inner while I put the clip in place. The joint is glued with epoxy, so don’t try unscrewing it!

Part 9 is the cap liner. It has the capping thread, triple-start, a waisted portion that engages with the end of the grip to give a reliable seal, and a thread to take the cap topper. The bottom half of the cap inner is made of a relatively soft PLA, Polymaker PolyTerra, and that gives a good feel for the cap threads and helps seal with the grip. The top is made of a stronger plastic (same manufacturer, so the same grey colour) so that there is no problem holding the tension of the clip in the long term.

Part 10 is the outside of the cap. No threads or tricks.

Part 11 is the body liner. It has a female thread to marry up with the ink window thread, and a special four-start female thread through which the fat portion of the plunger rod runs. Part 4 is made of plain PLA for its hardness and excellent wear characteristics.

Part 12 is the body outsides. It has the male triple start thread for the cap.

Part 13 is the grip which has a subtle triple start thread that acts to make the grip ‘grippier’.

Part 14 is the bladder which has a pentagonal star-like cross section and is made out of the very resilient 3D printing filament called TPE. The bladder gets glued to the nipple at the top of the ink reservoir with polyurethane glue (think of the original ‘Gorilla Glue’). The bladder can be removed for replacement if that ever proves necessary but its perfect survival of the 100,000 cycles of stress testing suggests that it is likely to last a lifetime.

That’s it for this post, one photo, one link, and lots of words. It will not do well with Google, so I am glad that you found it!

Seams are one of the shortcomings of many 3D-printed objects—at least for objects printed using the FDM printers like those I use to make the Platypus pens. I have previously written about how I can often avoid them by printing pen parts that consist of a single helical extrusion, a process that is often called “vase mode” printing. Those parts look great but they are not very strong by themselves, and so my pen parts are made in multiple layers glued together with marine epoxy. That makes them ‘plenty strong’, but having to effectively laminate all of the parts is inconvenient and can sometimes limit the designs that I can work with. A better way would be better! At least sometimes…

Enter the scarf seam (not a neck-warmer)

I first devised a way to minimise the visibility of seams more than a year ago and have been using it in a limited way with good results. I called the results a ‘scarf seam’ because of its resemblance to a woodworking method for joining timber end to end, the scarf joint (or sometimes ‘scarph’ joint). Scarf joints can be a simple matched pair of tapers or can have complications that help to hold them together without glue, or while the glue sets.

The picture shows one of many scarf joints used in the reconstruction of a historic sailing ship by the Sampson Boat Co. The construction of the Tally Ho is nearly complete and a long series of YouTube videos document the process. I enjoyed watching them very much and look forward to watching it sail!

The scarf seam that I devised for hiding seams in 3D printed parts is an adaptation of the tapering layers that I use to start and end helical vase-mode extrusions. I realised that if I followed the upward taper with a downward taper then I would have the 3D printing equivalent of a scarf joint. I tried it out and found that it worked remarkably well.

If you have a recently made Platypus Model 10 pen then you will have scarf seams on the grip below the first bulge. You may not be able to see it as, after all, it is designed to be hard to see! There are also scarf seams on parts of the section liners of both models 1 and 10, but they are even less visible as they are glued inside the grip.

How it works

The magic of the scarf seam in the extrusion of the outermost perimeter is that it allows the printer nozzle to be moved away without any need to change the extrusion rate or the height of the nozzle. That results in almost no blob or gap at the seam. Internal perimeters and fill are printed before the nozzle moves back to the outer perimeter still without any vertical movement, and the next scarf seam begins. In that way the seams start and end with zero height and zero extrusion (or very close to it) and, ideally, it is all but invisible.

So you want to print a scarf seam…

It is worth noting that I was able to make scarf seams only because of the unusual way that I generate the g-code that drives my 3D printers. The custom programs that I write to generate the g-code algorithmically give me full control. Most people who print things with their 3D printers work differently.

Conventionally g-code is generated by first modelling a 3D shape using a CAD program or similar. That model does not have any seams or layers. That 3D model is then loaded into a program called a ‘slicer’ that divides the shape into 2D patterns that we call slices or layers, and then makes the g-code that tells the printer how to reproduce the slices. Seams are inevitable because each layer has a start and an end, and the nozzle has to be raised between one layer and the next (unless, of course, the slicer is set to ‘vase mode’). If your slicer program cannot make a scarf seam then you cannot get a scarf seam when you print your model. Until very recently no slicer was able to make scarf seams.

Enter open-source software

Last November I decided to share scarf seams with other 3D printing enthusiasts, with the intention that someone (not me!) would be able to incorporate them into a standard free slicer software package. I posted demonstrations and notes on it on Reddit and a couple of forums dealing with 3D printing and slicer software. Eventually the idea was taken up by a developer of the slicer OrcaSlicer and it is now part of the latest beta release of that software! 

You can read about the scarf joint seam in the OrcaSlicer release notes here and see the first scarf seam YouTube video, by fellow Australian Michael Laws (not me, Michael Lew), by clicking the thumbnail image below.

I am grateful to the person known as @Noisyfox doing so much work to go from the basic idea to working slicer code.

As I mentioned in the previous post , the bladder is both the core and the Achilles heel of the early twist-fill fountain pens, at least from the perspective of today. The rubber tube or sack was not sufficiently robust to withstand the repeated stresses of being twisted and untwisted. The problem was compounded by the fact that the rubber would become stiff and embrittled over time.

Twist-fillers were not the only fountain pens that suffered from the failure of ageing rubber. Replacement of the rubber bladders, tubes, or diaphragms is an everyday occurrence for collectors of vintage fountain pens. Sometimes the replacement is easy (as I found with the bladders of old Conway Stewart pens), but sometimes it is very difficult (as with the Vacumatic Parker 51 that I broke…).

The Platypus Model 20 bladder

The bladder of the Platypus Model 20 will quite likely never need to be replaced. The material is very robust and strong.

The picture here shows a Platypus Model 20 bladder stretched by a dumbbell weighing nearly 10Kg. The bladder was stretched to nearly four times its normal length, and even though it did not return all the way to its initial length after the weight was removed it continued to spring back after being twisted.

The bladder shown in the video was actuated with a stepper motor (driven by a 3D printer control board) repeatedly for more than a day. After 100,000 actuations the bladder was still intact, still moved water in and out when twisted, and was totally undamaged by the experience.

It seems likely that the bladder would withstand far more than 100,000 actuations, but that is far more than any bladder would see in even a lifetime of continuous use.

Polyurethane (TPU)

The Platypus Model 20 bladder is made of polyurethane, a type of plastic that is often called TPU in the context of 3D-printing. Polyurethane is remarkably robust, chemically stable, and resistant to many solvents. Polyurethane’s flexibility and stability make it remarkably useful. It is widely used in many different roles, from stretch fabrics (Spandex, Lycra) to components of car suspension and skateboard wheels.

Polyurethane comes in many variants and different degrees of hardness. The TPU filament used for printing the Platypus Model 20 bladders is softer than many TPU filaments, with a technical Shore hardness rating of 82A, as compared to the most common 95A.  The particular filament used is Filaflex 82A, made by Recreus in Spain.

In contrast to rubber, polyurethane does not suffer from embrittlement much at all. It has even been reported that polyurethane becomes less brittle over time [1], albeit at elevated temperatures. It has also been reported that the presence of water hinders embrittlement from developing with ageing [2], a finding that might be relevant to polyurethane ink bladders.

I do have some limited long-term experience of 3D-printed polyurethane. I still have the tree frog that is the first ‘flexible’ print that I ever made. You can see in the photo that it is not a great print (I like to think that I could do much better now), but it sat on a windowsill for about 7 years and has proved itself both stable and an excellent dust collector.

Polyurethane is not perfect

Not every property of polyurethane is perfect for its role as the bladder for the Platypus Model 20 fountain pen. Unfortunately polyurethane is slightly permeable to water. Not disastrously so, but even a little can lead to drying out of ink if it is left in the pen for long enough. The next blog post in this series will detail my studies of water loss from various bladder constructions and the ways that I have made the problem minor enough to be unproblematical for normal fountain pen use.

References

1. Abbas Tcharkhtchi, Sedigheh Farzaneh, Sofiane Abdallah-Elhirtsi, B Esmaeillou, F Nony, et al.. Thermal Aging Effect on Mechanical Properties of Polyurethane. International Journal of Polymer Analysis and Characterization, 2014, 19 (7), pp.571-584. ff10.1080/1023666X.2014.932644ff. ffhal-01191410f
2. Possart, W., Zimmer, B. Water in polyurethane networks: physical and chemical ageing effects and mechanical parameters. Continuum Mech. Thermodyn. (2022). https://doi.org/10.1007/s00161-022-01082-y

Platypus pens is now offering its pens constructed from a PLA filament that contains 50% terracotta (by weight). The pens look like they are made from terracotta, and feel like it too.

Terracotta infused PLA filament makes a truly lovely pen, but it also offers a few disadvantages for the user and for the penmaker.

For the user

The surface of pens made from the terracotta filament is slightly rough and that means that ink cannot be simply wiped away with a tissue. A wet cloth is usually sufficient to clean the pen after filling, and an occasional purposeful cleaning may be necessary. What’s more, the rough surface is relatively prone to marking when scraped against hard edges, and when scraped against soft surfaces it is slightly abrasive. Cap posting is not recommended for terracotta pens.

Terracotta pens are likely to develop a patina after long use, something that some people might not like, but others will certainly enjoy.

For the designer

Terracotta filled PLA has a matt finish with no sheen or shine. Just like a terracotta pot. The patterns originally developed for Platypus Pens exploit the shininess of some PLA filaments to exhibit sparkles and play with light reflection. They do not work so well with terracotta. Therefore a new pattern has been designed, cleverly called ‘Pattern 4’, a pattern that uses shadows instead of reflections. I call the pattern ‘dimples’ for reasons that may already be apparent. (Dimples works pretty well with the shiny filaments as well and so it is being offered for all pens.)

For the maker

The terracotta filament comes out of the extruder differently from other PLA filaments. It seems more fluid than normal PLA at the optimal printing temperatures and it’s heavy and so it sags during printing when given half a chance. What’s more, it doesn’t weld as well to the non-terracotta filaments used for the contrast bands and so each of the bicoloured components has a weak point. That is not at all a problem for the completed pen because the weak joint is reinforced in the epoxy-glued sandwich of inner and outer parts, but I suffer a much higher failure rate among the components made from terracotta. The component rejection rate is probably about double that of components made of normal PLA.

Why use terracotta at all?

So if terracotta has all of those disadvantages, why bother making (or buying) a terracotta pen? Simple: the terracotta pens feel GREAT!

The micro texture and slightly porous nature of the terracotta feels distinctly different from the  ordinary PLA Platypus pens of otherwise identical design—different in a way that many people find to be better. It’s hard to describe the feel, so perhaps I can just say that it is less ‘plasticy’. I’ve never held a Visconti Homo Sapiens pen, but I suspect that the feel of their lava powder-infused resin might be similar to the terracotta powder infused PLA.

Should you choose a terracotta pen? Not if you would find the extra care needed to keep it free of ink stains, or if you do not like a pen to shows signs of its long use. However, if you want a pen that feels wonderful in the hand and that is unlike any other that you will see then, yes, give it a go.

A pre-review on YouTube

Mick L (Michael Lampard) has just released a YouTube video where he talks about his terracotta Platypus pen (prototype).