Fountain Pen Design

Function, Development, Construction and Fabrication

57-3 Caps – The Inner Cap

Since the inner cap is a critical part of the cap, I dedicate a special article to it. Most modern fountain pens have an inner cap inside the cap body.  Its main function is to provide a sealed off chamber around the nib to prevent the ink from drying out and to hold any ink that may be spilt for various reasons.

In some pens with slide-on caps, the inner cap holds the clutch springs and it may incorporate some sort of snap-on clip feature.

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 1 — Inner Cap – TWSBI

Photo 2 — Inner Cap – Lamy

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

Overcoming the locking force leads to friction and 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 the clutch force and the seal to reduce and to fail, eventually.  Fatigue can lead to cracking of the elastic inner cap.


Recognizing this, some well-designed caps/inner caps (nothing to do with their price) would provide separate components or features for clutch and seal. The Parker in Photo 3 complies with this design principle.

The Trouble with Inner Caps

Photo 4

Often and unfortunately, there are plenty of fountain pens, the elasticity of the inner cap is used as a seal and as the clutch for locking the cap. Often the seal and locking function is demanded from one tiny, elastic, circular lip at the end 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.

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

Photo 5

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

Photo 6

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

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 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 between the inner cap and the section is closed.  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.

Start of Pump Action

Drawing 1 shows the section just touching the seal of the inner cap. To engage the lock, some force F1 needs to be applied. It will increase to F3 eventually, so the separating force generated by the inner cap seal can be overcome.

The volume V1 inside the inner cap is trapped and at its largest and L1 is the longest.

by Amadeus W.

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.  This starts the deformation of the seal and shortens the length of the inner cap.  Thus, the volume of the inner cap is compressed to V2, which results in an increase of pressure. This pressure increase is transferred into the air pocket inside the reservoir via the feed.

The dimension L1 reduces to L2.

by Amadeus W.

Priesing 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 sucks ink from the reservoir.  A well-designed 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.

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. After that, whether through a small leak, a purposeful ventilation passage or the permeability of the inner cap material, the inner pressure returns to atmospheric pressure, and an amount of ink is again pushed/ 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. The opening happens in a pop; the opening speed is quite high. 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. Unfortunately, some inner caps have a sliding seal, which means that at some stage the seal is almost completely closed while the pen is still moving laterally when there is hardly an opportunity for the air to escape. The effect of the pumping action caused by such an inner cap is not as severe but still considerable.

My suggestion?

Photo 7

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

Photo 8

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!)

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  —  A good Example

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

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.
  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, 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.

Above all: Enjoy!


Amadeus W.

10 January 2019

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