Fountain Pen Design

Function, Development, Construction and Fabrication

6.3 The Inner Cap

Since the inner cap is a critical part of the cap, I dedicate a special chapter to it. Most modern fountain pens have an inner cap inside the cap body.  It forms a sealed-off, sort of air-tight chamber around the nib to prevent the ink from drying out or at least to reduce the speed of drying out.  Since there are always two to Tango, before honing in on the inner cap, let’s turn our attention to the ink, for a while, so we can tick it off.  Admittedly, the inner cap has another purpose: to hold any ink that may be spilt for various reasons.

Drying of Ink

When I was developing the inner cap for my fountain pen I was confronted by prerogatives which had been immovably set in marble by the shape designer who had been supported by the almighty boss. The seal of the inner cap and the lock of the cap had been designed to be the same mechanism. It is the quality of good ingeneers that they would keep functions separate. This problem occurs when shape designers believe they can do “this bit of ingeneering”.

May this be as it is, some things one must accept. When realised how vital a certain degree of airtightness was, how difficult it was to obtain and maintain with this given design, I opened the ink books again.

In order to reduce the susceptibility to drying while the ink was inside and on the pen, I asked myself if there was an opportunity to prolong the ink’s drying time without affecting any of its other characteristics. Initially, this question sounded contradicting because when the ink hits the paper, you want it to dry as quickly as possible.

During speculations about polyethylene glycol, PEG, I learned about organic, large molecule materials that have a strong dipole character just like water which causes them to repel water. They are commonly used at the dry-cleaners such as ethanol, PEG, and glycerol. In that order, they also increase the viscosity (resistance to flow) when added to water but luckily only at higher concentrations.

Amongst other conditions, evaporation depends on the size of a liquid’s molecules. Small ones puff off fast, bigger ones more slowly. Molecules can have opposite electric polarity at either end.  For some reason (unknown to me but certainly others), such dipole molecules don’t like water, and eventually, they all meet at the water surface, sit tightly next to each other and create a molecule thick film, which is VERY thin.

Gradually I added tiny amounts of PEG to the ink and recorded the drying times.  At some point, the drying time increased rapidly, went up with a jump. This must have been the concentration which closed the film. Adding more PEG didn’t increase the drying time any further. Concerning the fountain pen: when using ink with added PEG significantly prolonged the drying of ink at the air-exposed areas at the nib and feed.

And what effect did this have on the drying time on paper?

Sometimes, luck favours the sedulous, “Glück folgt dem Tüchtigen” as we say in Germany. Once the ink hits the paper it disperses quickly along all the fibres in reach, and the surface it covers is about 10 000 times than those air-exposed areas on a fountain pen, as mentioned before.  Therefore, there are hardly any large molecules present which could act as a barrier and the ink (the water) can dry as quickly as before.

Once I had discovered this, experiments lead me to the optimum, minimum percentage of ethylene glycol needed to provide the desired effect: to make a fountain pen less susceptible to drying out.

And mind you, drying of ink not only depends on evaporation but also on the ink absorption of the paper, which is highly influenced by the type and quality of the paper, which is another unfathomable field of ancient alchemy, in which I have not entered, but will gladly leave to someone else to write about. If you are this someone, know of someone or know a link, please tell me, and I will include it here.

Back to Mechanics

In some pens with slide-on caps, the inner cap incorporates a clutch spring, and some other kind of snap-on feature.

Photo 1 — Inner Cap – TWSBI

Where the inner cap touches the grip section or barrel, an elastic, deformable shoulder performs the seal function once the contact pressure is high enough. I marked this contact area with a purple line in photos 1 and 2.


Photo 2 — Inner Cap – Lamy

The compression force for maintaining the seal action is caused by the elastic deformation of the inner cap. Often and unfortunately, this elastic feature is also used as the clutch for locking the cap in its stop position.

Overcoming the locking force leads to friction and consequently to unavoidable wear, either on the inner cap, the section or both.

Photo 3 — Metal Spring – Parker

Through this and the inevitable creep, the engagement force will relax causing a reduction of the clutch force and the seal to fail, eventually.  Some plastic materials used for the inner cap tend to fatigue which leads to its cracking.

Recognising this, some well-designed caps/inner caps (nothing to do with the price of the fountain pen) keep components or features for the clutch and seal separate. The Parker in photo 3 complies with this design principle.

The Trouble with Inner Caps

Photo 4 — Inner Cap – Lamy

Often and unfortunately, there are plenty of fountain pens, where the elasticity of the inner cap is used as a seal as well as the clutch for locking the cap. Often the seal and locking function is demanded from one tiny, elastic, circular lip at the opening of the inner cap. Photo 4 shows such an example where the 12 locking lugs are very tiny.  A vent passage was added to prevent condensation of the trapped air which becomes superfluous once the cap is worn out.

Photo 5

A typical matching fountain pen section is shown in photo 5 (not the same brand pen!)

Photo 6 — Rubberised plastic Deterioration from left to right over a period of 5 years

In addition, the elastic materials used in my days were rubberised plastics, PU or EVA.  They lost their elasticity and gradually, compromising at least the seal’s efficacy. After five years (approximately) these plastics change their integrity, turn brittle and fall to bits or get slimy (PU).  Needless to say, at that stage, the locking mechanism had gone, too.  See photo 6.

The most amazing point is that even though the transient nature of these types of plastics (PU, EVA) is well known in the industry, however, the manufacturers still produce these materials, what’s worse, that product producers still use them, and it is embarrassing that there are ingeneers out there who apply them in their designs. Why was/is such a construction accepted by those who know better? Some ingeneers should give their money back to their schools.

One of the basic rules for good design I have mentioned over and over again: “Don’t combine two functions into one feature of a component!” How hard is that to comply with?  Because it happens so often I will demonstrate this problem explicitly. Perhaps, this will stop designers and ingeneers to choose such solutions.  After reading this, they can’t tell they didn’t know.

The Inner Cap: A Pump

Through repeated closing and opening the cap from the fountain pen, the pump-action will gradually empty the ink reservoir. I talked about this “side effect” in the chapter on Cap Mechanics and Physics.  I will demonstrate here that an inner cap using the seal as a snap-on clutch adds another pumping action, thus exacerbating the problem.

Let’s begin.  For the lock to hold, a locking force must be overcome, which, in a fountain pen starts when the seal of the inner cap and the section touch.  From this moment onward, the air inside the inner cap is trapped and the pump-action commences. I have prepared a series of drawings to demonstrate this behaviour. For clarity purpose, I have exacerbated the proportions.

by Amadeus W.

Drawing 1 — Start of Pump Action

Drawing 1 shows the section just touching the seal of the inner cap. The volume V1 inside the inner cap is trapped and at its largest and L1 is the longest.  To engage the lock, some force F1 needs to be applied. It will increase to F3 eventually (drawing 3), so the separating force generated by the inner cap seal can be overcome.

by Amadeus W.

Drawing 2 — Compression of Inner Cap

The increase of the longitudinal force F2 as in drawing 2, pushes the section against the seal of the inner cap.  The dimension L1 reduces to L2This starts the deformation of the seal and compresses the length of the inner cap.  Thus, the volume of the inner cap reduces to V2, which results in an increase of pressure inside the cap. This pressure increase is transferred into the air pocket inside the reservoir via the feed.

by Amadeus W.

Drawing 3 — Prising the Seal

A speed peak occurs when force F3 has pried the seal apart as shown in drawing 3.  Depending on the actual dimensions of the components involved V3 can be larger or smaller than V2.  In case it is larger, the pressure drop inside the inner cap sucks ink from the reservoir.  A well-constructed feed would absorb this ink in the capillary slits.

In reverse, when V3 is smaller than V2 the pressure increases and is transferred to the air pocket inside the reservoir.

by Amadeus W.

Drawing 4 —  Section and Inner Cap are engaged

Once the locking components have engaged (the seal slips into the groove of the section), F4 is zero. See drawing 4.

In any case, V4 is the smallest, resulting in the highest pressure increase, which is transferred into the reservoir where the pressure is now at its highest. After that, whether through a small leak, a purposeful ventilation passage or the permeability of the inner cap material, the inner pressure slowly returns to atmospheric pressure, and an amount of ink is pushed out of the reservoir and sucked into the capillary slits of the feed, hopefully.

In reverse, when opening a snap-on cap applying the above construction, the locking force elongates the inner cap and reduces the pressure and ink is sucked out of the reservoir. The opening happens with a pop; the opening speed is quite high and more ink is sucked out. Compensating for this shortfall is a big job for the feed.

More on Mechanics

Some cap designs show separate components for sealing and holding the cap in place. Excellent!  Unfortunately, some inner caps have a sliding seal, which means that at some stage the seal is completely closed while the pen and cap are still moving longitudinally towards each other while there is no way for the air to escape. The pumping action caused by such an inner cap/section construction can be very severe depending on the actual dimensions.

My suggestion?

Photo 7 — One good Example

1. A half-turn multi-start thread for holding the cap onto the pen. (Has been done! See photo 7.

2. For the seal: two matching flat surfaces, one being a metal ring and the other a neoprene type ring. (Has been done! Photo 8. I only show the metal ring.  Not the inner cap because I am not sure about its design.)

3. Plenty of space between the section and the cap. (Has been done!)

Photo 8 — Another good Example

These are traditional solutions which are all quite simple and you may (I do) wonder why not all caps designs incorporate these features.

Photo 9 shows one example of cap construction, which shows an innovative solution, and it deserves to be promoted here. It is produced by the Japanese Platinum Pen Company. What more? It’s all done at a reasonable price.

Photo 9  —  Yet another good Example

Why have I not shown a complete photo of this fountain pen? There are other features, which could have been worked out better.

Let’s begin pointing out the details:

  1. There is a reasonable gap between the section and the inner diameter of the cap.
  2. The cap is attached via a multi-threaded screw action.
  3. The seal pressure is caused by a spring and not the deformation of the inner cap, thus, no compression, no increase in pressure.  Only small contact pressure reduces wear and fatigue.
  4. Once the contact pressure between inner cap and section is high enough the inner cap rotates with the section. Then there is no relative movement, thus, highly reduced wear.

Unfortunately, at a closer look, I noticed the interference between the inner cap and the section, which is a disadvantage.  A flat surface contact could have provided enough seal.  I wonder why the designer has chosen to go this way?

Initially, I considered discussing a few not as successful examples; however, I didn’t want to spoil my enthusiasm.

The next chapter is on Fountain Pen Cap — The Clip

Above all: Enjoy!


Amadeus W.

10 January 2019

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