Fountain Pen Design

Function, Development, Construction and Fabrication

57-2 Caps – Mechanics and Physics


Screw-on Caps

Historically caps were screwed on. They have a fine thread cut inside the cap and a matching screw thread on the section at the end away from the nib or on the barrel. The cap is kept in place, securely. 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 on whose fingers calluses will grow, eventually.  Hence the famous proverb: ‘Show me your fingers and I tell you what 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; therefore, the orientation of the clip in regards to the barrel is always the same.

Another advantage of moulded threads is that they can have a dead stop, which prevents 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.

Photo 1

Some fountain pens have their thread at the front end of the section. On the example, in Photo 1, the thread is excessively long and moves the grip area away from the tip of the nib.

The advantage of this arrangement where the thread is close to the sealing shoulder is that it keeps the dimension and therefore, the tolerances small. The tolerances are usually compensated by the elastic deformation of the inner cap or a sealing shoulder inside the cap. Less deformation means less pumping action. I will refer to this later on.

Photo 2

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. 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

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 certain situations, and, as long as the pen was not opened, the pen would not leak. But what an unexpected mess when you do open it.

Photo 4

Since there are leaking fountain pens, users tried to prevent this by screwing on the cap as tight as possible. The excessive force would crack the cap body or/and the 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 turned it into a symbol for quality, see Photo 4. For some this ring has become a justification for a higher price, and therefore, is still attached to some caps even when it is unnecessary.


Screw-on caps keep the cap securely in place, which can be critical for some slip-on caps with a clip. When a pen is pulled from a pocket at its cap, the axial force occurring between the cap and the pen can be larger than the holding capacity of the clutch. Do I need to say more? Yes.

Slip-on Caps

Maybe, it was the speeding up of time that inspired 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, tradition extended this moment of collecting one’s thoughts. Then it included the preparation of ink, the selection of paper and the spreading of paper on the desk and securing it. Not unlike the filling of a pipe with tobacco or rolling a cigarette.

Photo 5

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

In order to increase the reliability, 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. This can lead to the writer applying excessive force when pushing on or overtightening the cap and damage the components, subsequently.

Don’t make them guess!

When you look at any cap, 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 amongst them.

Whichever type of cap you prefer, I recommend the 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.

This will spare the writer the frustration of figuring out how to open a fountain pen unless you are a one-fountain-pen-person. No fountain pen enthusiast is, therefore, one can never be entirely sure whether one is turning a tight-fitting screw-on cap and not getting anywhere or believing it is a tight-fitting slip-on cap and through trying to yank off a threaded connection and possibly crack the cap.

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.

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.  Firstly, scew-on caps don’t click.  Only some slip-ons do.  Let me start with Drawing 1.  “It 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 into its recess and 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 L2 the snap pushes into the groove.  Thus, suddenly, Fsnap reduces, the friction reduces and therefore, the force Fmove will be lower.

Since humans don’t react this fast, the surplus force Fclick makes the end of the cap hit the shoulder on the section (or barrel) when L1 a 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.  Generally, there is somewhere inside the cap a soft stop.  Usually, the inner cap.  I talk about this in the Inner Cap chapter.


Consider this: The fountain pen’s grip section being a piston and the cap a cylinder like 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 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 and how much of the trapped air can escape.

by Amadeus

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

If the fit is loose, air has time to escape through the gap between section and cap; thus, almost avoids a pressure built-up. A narrower gap throttles the amount of air flow; the inner pressure increases more and preferably some time is provided for the decompression to atmospheric pressure.

Photo 6 — Lamy Safari

When section and cap have a different cross-sectional shape it permits air pressure variations to even out.  This aspect of the Lamy Safari is very good. See Photo 6. The flat surfaces 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

One of the not so good examples in this regard is shown in Photo 7. The cap Pilot M90 is very long, therefore, the length of engagement is very long and since the outer contour of the cap matches that of the long section, the gap is very narrow. I have to admit this, unfortunately, because otherwise, it is one of my favourite designs.

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

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 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 any 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.

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 and the speed of movement.

Screw-on caps disengage slower than slip-on caps.  The tighter the slip-on locking mechanism, the higher the speed of disengagement.

In summary:

Inserting a pen into a cap as well as removing the cap from pen causes pumping action

taking the cap off =► suction

putting it on = ►compression

Pressure Variation

by Amadeus

Figure 4 –— Pressure Variation in Cap

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 we end up with a straight line until the peak pressure is reached at the end of the movement when the cap sits in its closed position.

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 lower speed and/or a wider gap. After that, the pressure drops may follow the shape of curve or rate depending on the amount of air can escape which depends on the individual construction of the fountain pen (more about this later).

For the pulling off action the diagram is almost 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 Function of the Feed. One of its functions is to compensate for the pressure variations. Here we are faced with a momentary pressure variation.

Let’s have a closer look. A momentary vacuum is caused around the nib end of the feed/section (the outer pressure) when pulling off the cap.  This causes a pressure difference between this vacuum and the pressure inside the air pocket in the reservoir.  Since it is higher, ink is pushed out of the reservoir. It is irrelevant whether the outer pressure reduction/vacuum is caused by an atmospheric climatic change over your country or just a small, less than dimple size volume around the opening of the section.

The amount of expelled ink depends on the pressure difference and the length of time for which this difference persists.

It is 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 stores it in the capillary slits. The only way to empty these slits is through consuming the ink through writing or draining it with blotting paper.

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

It gets worse!  So, for a moment the pressure in the air pocket inside the reservoir increases.  Once the momentary pressure increase inside the cap reduces more or less slowly to atmospheric pressure (through mostly unavoidable leaks), the increased air pocket pressure pushes out ink, again. To say it simply, fitting and removing a cap which is not well designed will slowly fill up the feed and your pen will drip.

And worse? Can it?  Yes!  As the volume of the air pocket increases the amount of ink pushed out per cap opening or closing will also increase.  More details on this topic in the article Section Assembly.

What to do? What to look for?

Caps must be put onto a fountain pen, occasionally. You know now, reducing the frequency of capping – uncapping, helps. The speed of movement influences the pressure development. The speed is generally slower in case of a screw-cap design but faster in a pop-on or click-on designs.  Learn to open your snap-on cap with the same hand that holds the pen.  How?

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


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. It also demonstrates that the designers 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, 1.5mm diameter gives 7mm2. The gap of 0.1mm between a 10mm section and a 10.2mm cap provide 3.2mm2 for air to escape, and a gap of 0.3mm of 10.6mm cap gives  9.7mm2.


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 different mechanical criteria. Please visit the article on The Inner Cap.

Above all: Enjoy!

Amadeus W.

10 January 2019

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