Fountain Pen Design

Function, Development, Construction and Fabrication

6.2 Cap Mechanics and Physics

Screw-on Caps

Historically, caps were screwed on. They have a fine thread (typically a so-called “triple-thread” made with three individual threads starting 120° apart) cut inside the cap and a matching screw thread on the section at the end away from the nib or on the barrel. This construction keeps the cap securely in place. When the thread’s location on the section is around the area where the pen is held it can cause considerable discomfort to the persistent, fearless writer, until calluses have grown on their fingers, eventually.  Hence, the famous proverb: “Show me your fingers and I tell you which pen you write with.”

Since the mid-fifties, most fountain pen components are injection moulded, and when the thread is part of the mould no extra manufacturing step is required. A moulded thread has several advantages. The threaded parts on section and cap always match the same way and the threads can have a dead stop; therefore, the orientation of the cap’s clip with regard to the section and barrel always remains the same – if a single-start thread screw is applied.

Photo 1 — Thread between the Grip and the Nib

Furthermore, a dead stop had the advantage of preventing over-tightening and damaging components. Further, the start of the thread can be designed in a way that the two components engage easily. A multi-start thread can close the cap in one turn or less and still provide the same axial holding force.

Some fountain pens have their thread at the front end of the section. In the example, in photo 1, the thread is excessively long and moves the grip area away from the tip of the nib which causes the pen to require more writing pressure for the altering of the writing width.

The proximity of the thread to the sealing shoulder offers the advantage of keeping the dimensions and therefore the tolerances small. Usually, tolerances are compensated by the elastic deformation of the inner cap or a sealing shoulder inside the cap. Small tolerances need less deformation which causes less pumping action. I will refer to this later on.

Photo 2 — Thread behind the Grip

In photo 2 the thread is at the end of a comparatively short grip area where it tends to interfere with the writer’s fingers.

A Kaweco was my first fountain pen and I remember growing callused because we had to write for hours, five to eight, every day. Yes even on weekends because we used to have a lot of homework. We all wrapped a band-aid over the thread, which had to be taken off every time when screwing on the cap. Often the cap stuck because some residue of glue was trapped in the thread.

Photo 3 — Metal Ring

Photo 3 shows a section with a metal ring installed. This design could technically supply the best seal. When dimensioned correctly, the inner cap could hold all the ink the fountain pen would discharge under unfortunate situations, and, as long as the pen was not opened, or if the conscious writer would open it nib-down, spillage of ink could be avoided. But what an unexpected mess when you do open it nib-up! Is this another reason for calling it a fountain pen?

Photo 4 — Metal Band around Cap

Since there are leaking fountain pens, users tried to prevent this by screwing on the cap as tight as possible, which only has an effect when the inner parts of the cap are designed in a particular way. In any case, the excessive force would crack the cap body and accelerate creep and subsequent splitting.

As a precaution, many fountain pen designers reinforced the open end of the cap with a metal band, which consumers raised into a symbol for quality, see photo 4. For some manufacturers, this ring has become a justification for a higher price, and therefore, it is still attached to caps even when unnecessary.


Screw-on caps keep the cap securely in place, and they remain attached to the pen even when an axial pull-off force is applied. For some slip-on caps with a clip, this could turn into a critical situation. When a pen is pulled from a pocket at its cap, which it mostly is, the axial force occurring between the cap and the pen can be larger than the holding capacity of the slip-on clutch. Do I need to say more? Yes.

Slip-on Caps

Maybe, it was the speeding up of time that evoked the slip-on cap. I remember times when the short moment of unscrewing the cap had an almost ritual quality, an action offering a moment of focus and gathering before committing something to paper, for example, a signature on a $10,000 cheque.

In even more olden times, this tradition extended to the moment of collecting one’s thoughts, the premeditation of a whole sentence or a paragraph before committing it to paper. Can we still construct and hold a thought at least for the length of time we need to write it? Would I sit here and write without a computer’s support? My excuse is that I want to hold the composition of the entire chapter or sometimes several in my mind and not be bogged down by details. What if I wouldn’t have the assistance of a computer?

Going even further back into the mists of history, before commencing writing, you had to prepare the ink, select the paper, spread it out on the desk, and secure it.  Not unlike the filling of a pipe with tobacco or rolling a cigarette and lightening either of them with a matchstick.

Photo 5 — Leaf Springs on inner Cap

Initially, the slip-on cap was held by the tightness between the cap and section, providing enough force from the friction between the two conical shapes. Since the force of friction was subjected to wear and the writer’s personal habit, it would reduce after some time and either the cap would fall off or would crack because it was pushed on too hard. I have seen caps with band-aids wrapped around.

In order to increase reliability and reduce wear, all sorts of springs and click mechanisms had been invented. One solution uses leaf springs like in photo 5.


There are examples of screw-on and slip-on caps with no definite stop. The writer receives no tactile feedback from the mechanism to indicate that the seal or the locking mechanism have engaged. As already said, this can lead to the writer applying excessive force when pushing on or overtightening the cap and damage the components, subsequently.

Designers! Don’t make them guess!

When you look at most caps, there is no visual distinction between screw-on or slip-on caps. Innumerable times I have seen pen users (including myself) trying to pull off a screw-on cap or screw off a slip-on cap.  Nothing indicates to the user what type of cap they are faced with which causes considerable confusion or even irritation amongst them.

Whichever type of cap you prefer, I recommend the shape designers to add an obvious visible feature, which tells the user unmistakably which kind of cap they are confronted with.  What this feature should be? I raised the point, you come up with suggestions, and I will gladly display them here.

Unless you are a one-fountain-pen-person and you open your pen habitually, you are spared any puzzlement. But since no fountain pen enthusiast has only one fountain pen, this indicator will spare them the frustration of figuring out how to open any of them. Otherwise, one can never be entirely sure whether turning a tight-fitting screw-on cap and not getting anywhere or believing it is a tight-fitting slip-on cap and crack it through trying to yank off its threaded connection.

In my opinion, fountain pen writers are rather on the patient side, otherwise, they would not write with fountain pens. Still, I believe this aspect of cap design is worth looking at. Hello, you designers out there! What could it be? Don’t ask me, I have done my part.

Opposing Forces

… can cause some dilemma when a fountain pen is kept in a tight-fitting pocket or on a tie in a briefcase and the cap is kept on the pen with a slip-on mechanism.  When taking the pen out of that tight pocket, the pen is pulled by the cap. It has happened that the friction force between pocket and barrel is higher than the force which holds the slip-on cap to the section. Do I need to describe the mess? A screw-on cap would certainly have prevented it.

Why do some Caps go click?

… and why don’t some others?  This question I asked myself in my early days of acquaintance with pens.  After a closer look and wearing my ingeneering hat the answer became clear.  First principle: “Scew-on caps don’t click.”  Second principle: “Only some slip-ons do.”

Let me start by explaining drawing 1, a cross-section through the area of action. “A drawing says more than a thousand words,” is a common phrase amongst ingeneers.

by Amadeus

Drawing 1 — Why Caps Click

Here we go:  Fmove moves the cap towards the right in order to close the cap. The slant pushes the snap mechanism into its recess and the pre-tensioned force Fsnap increases, which, in consequence, increases the friction between the snap and the slant and more Fmove is needed.

Once the cap has travelled for the length of the ramp the travel force Fmove appears to be less even, technically, it remains constant. After moving for the length L2 the snap flicks into the groove.  Thus, suddenly, Fsnap reduces, the friction reduces and therefore, the required force Fmove will be lowered.

Since humans don’t react this fast, the force Fclick makes the end of the cap hit the shoulder on the section (or barrel) when L1 a tiny bit longer than L2.  To make the click louder, you can cause more impact at the shoulder by widening or/and deepening the groove inside the cap so that there is hardly any friction left.

Why would some not click?  Take the shoulder away, or perhaps the geometry is incorrect.  Sometimes, there is a soft stop somewhere inside the cap.  Usually, it is the inner cap.  I talk about this in the chapter about the Inner Cap.

Pumps and Pistons

Consider this: The fountain pen’s grip section being a piston and the cap a cylinder-like shown in figure 1. We all know what happens when the piston moves towards the closed end of the cylinder. The enclosed air will be compressed. The ultimate pressure depends on how much of the trapped air can escape. The fit (gap) between the piston (section) and cylinder (cap), the length of the stroke and the speed at which the piston is inserted into the cylinder permit air to escape.

by Amadeus

Figure 1 — Just remember, the picture describes a pen, a cylindrical product and the gap (g) is a ring.

If the fit is loose and the speed of motion is slow, the air has time to escape through the gap between section and cap; thus, a pressure built-up could almost be avoided. A narrower gap throttles the amount of airflow; the inner pressure increases more and preferably, through slow motion, some time is provided for partial decompression toward atmospheric pressure.

Photo 6 — Lamy Safari (internet)

When section and cap have a different cross-sectional shape like shown in photo 6, air pressure variations can largely even out.  The flat surfaces along the section provide ample space for air pressure built-up to escape.

Torpedo shaped pens where the shape of the cap follows closely the shape of the pen, cause a strong pump effect.

Photo 7 — Pilot M90  (internet)

One of the not so good examples in this regard is shown in photo 7, the Pilot M90. Here the cap has a very long length of engagement and the contour of the cap matches that of the long grip-section, hence, the gap is very narrow. I have to admit this unfortunate lack because otherwise, this fountain pen is one of my favourite shape designs.

Additionally, a high speed of movement limits the backflow because the air cannot escape fast enough and the pressure builds up, even only for a moment which is long enough to cause a problem.

As a good example, try to pump up a bicycle tire when there is a hole in the tube. The tube inflates momentarily. The amount of inflation and the time of deflation depend on the size of the hole and the speed with which you operate the pump.

High speed of pump motion and small hole  high momentary inflation
Low speed and large hole → hardly any inflation

And translated to caps and sections

High speed and narrow gap high momentary pressure built up inside the cap

Low speed and large gap hardly some pressure built up

Now, the same applies in reverse… when you pull the cap off the fountain pen. At the moment no general example springs to mind, so let’s go straight to the fountain pen.

A slow movement of separation and a wide gap  little reduction of pressure inside the cap

Fast movement and a narrow gap larger reduction of pressure, more vacuum inside the cap

Allow me to repeat, this effect is reduced or exacerbated by the length of engagement, the speed of movement and the size of the gap.  The tighter the slip-on locking mechanism, the higher the speed of disengagement.  Screw-on caps disengage much slower than slip-on caps, hence, hardly any pumping effect occurs if any at all.

In summary:

    • Inserting a pen into a cap as well as removing the cap from the pen causes pumping action
    • taking the cap off suction, some vacuum is caused inside the cap
    • putting it on compression of air inside the cap

Pressure Variation

We end up with a pressure variation inside the cap. Why is this important? Perhaps figure 4 would be helpful.  Assuming that the speed of movement with which the cap is pushed on is constant, then the pressure increase shows as a straight line until the peak pressure is reached at the end of the movement when the cap sits in its closed position.

by Amadeus

Figure 4 –— Pressure Variation in Cap

The red line A demonstrates the effect when the cap is pushed on quickly and/or the gap is narrow. The green line B shows what happens at a slower speed and/or a wider gap. After that, the pressure drop back to ambient pressure may follow the shape of some curve depending on the rate of air escape which in turn depends on the individual construction of the fountain pen (more about this later).

For the pulling off action the diagram is identical; only replace the word Pressure with Vacuum.

What does the Feed say about this?

I would like to refer to the article where we talked about The Feed’s Function. One of its functions is to compensate for pressure variations. Here, in our current investigation, are we faced with a momentary pressure variation.

Let’s have a closer look. Pulling off the cap causes a momentary vacuum (against the ambient pressure) around the nib end of the feed and concurrently, a pressure difference builds up between this vacuum and the pressure inside the air pocket in the reservoir.  Since this pressure is higher, an amount of ink is pushed out of the reservoir.

Please note: For the ink to be pushed out, it is irrelevant whether the outer pressure reduction is caused by a climatic change over the entire country or just a small, less than dimple size volume around the opening of the section inside the cap. As long as there is a pressure difference, ink gets pushed out.  The amount of expelled ink depends on the pressure difference and the length of time for which this difference persists.

It would be fair to assume: when the pressure increases, the ink should get pushed back; unfortunately, this is not the case.  When ink is pushed out from the reservoir, a well-designed feed absorbs it in the capillary slits. The only way to empty these slits is through consuming the stored ink through writing or draining it with blotting paper.

When replacing the cap, the pressure inside the cap rises above that in the air pocket inside the reservoir, hence, air is pushed inside the reservoir through the air inlet, bypassing the feed capillaries.  (See: Fountain Pen Feed – Application).

Wait, it gets worse!  So, for a moment the pressure in the air pocket inside the reservoir has been increased.  Once the cap has reached its stop, the momentary pressure increase inside the cap reduces more or less slowly down to atmospheric pressure (through mostly unavoidable leaks). As the increased air pocket pressure inside the reservoir expands while releasing to atmospheric pressure, it pushes out ink.

Summarising: To say it simply, fitting and removing a cap repeatedly will eventually fill up the feed’s slits even when both the cap and feed are well-constructed. It is only a matter of how often it is done, then your pen will drip.  More details on this topic in Section Assembly.

What to do? What to look for?

Inevitably, caps must be put onto a fountain pen, occasionally. You know now, reducing the frequency of capping – uncapping, helps. Sometimes it is better to leave your fountain pen open for a while. Experiment and find out for how long your pen can lay open before the ink may darken or your pen turns reluctant. Furthermore, the speed of movement influences the pressure development. The speed is slower in case of a screw-cap design but faster in pop-ons or click-ons.  Learn to open your snap-on cap with the same hand that holds the pen and you can control the opening speed in particular.  How?

Here is an exercise for you.  Hold the barrel by pushing it with your ring finger and little finger against the ball of your hand.  Hold the cap between thumb and index and maybe the middle finger and move the cap and barrel away from each other.  It works; practice makes perfect.

A small Hole

You may have noticed a small hole in the cylindrical part of the cap, mostly hidden under the clip. It is placed just outside the sealed chamber created by the inner cap.

by Amadeus W.

Figure 5 — Detail around the Air Hole

It has been added to reduce pressure variation during opening and closing by shortening the distance of travel of the effective pump action. It also demonstrates that the ingeneers had been aware of the “side effects” of the pumping action.  Figure 5 shows a breakaway section of the clip, cap body, inner cap and section around the air hole.

In the example I want to demonstrate the usefulness of the air hole:
The area of an air hole of 1mm diameter is about 3.2mm2; to demonstrate the effect of an increase: 1.5mm diameter gives an area of 7mm2 for air to escape.  In comparison:  A gap of 0.1mm between a 10mm section and a 10.2mm cap provides an area of 3.2mm2 for air to escape, and a gap of 0.3mm of a 10.6mm cap gives  9.7mm2.

Table 1 Opening [mm] Area [mm2]
Hole in Cap Diameter = 1 3.2
Hole in Cap Diameter = 1.5 7
Diameter difference
between Cap and Section
0.2  →  Gap = 0.1 3.2
0.6  →  Gap = 0.3 9.7

From this, we can conclude that a diameter difference of 0.6mm is more effective than a hole of 1.5mm.  I investigated seven arbitrary selected fountain pens from reputable manufacturers.  The diameter difference ranged from 0.2mm to 1mm with an average of 0.7mm.  Only one cap had a hole of 1mm but it was close to the open end of the cap, thus would not reduce the pressure buildup much at all.  The cap with the hole also had the closest fit; not a good combination. These findings indicate that the ingeneers don’t consider this aspect of cap construction.


Pointing forward to the next chapter:  The inner cap adds to the pumping action. The impact on the feed is the same as explained above. In the worst case, the consequences can accumulate. The pressure increase happens based on various mechanical criteria. Please visit the article on The Inner Cap.

Above all: Enjoy!


Amadeus W.

10 January 2019

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