Fountain Pen Design

Function, Development, Construction and Fabrication

Section Assembly

For a typical fountain pen, the section contains the feed and part of the nib, and connects with the barrel and the ink reservoir on one end, and offers means to hold the cap and nib on the other.  Its ergonomic function is to provide the writer with an area to hold the pen comfortably that reduces fatigue, facilitates control of writing and prevents the ink from creeping on the grip area.  Some do it better than others; one can see it on the inky fingertips of the writer.

Somewhere near the nib end is a smooth area to create a sealed-off compartment in conjunction with an inner part of the cap.  In clip-on caps, the clipping mechanism is often found somewhere along the section, often leaving wear marks and eventually calluses at the writer’s fingers.  Some designs combine the clip-on feature with the seal.  Sure, this moves the clip mechanism away from the grip area, but combining two functions into one feature is considered not good ingeneering practice because it causes often problems.  I will expand on this in the chapter on ‘The Cap’.

For screw-on caps, one finds a thread at one or the other far end.  A few good designs move the thread onto the end where the grip connects with the barrel, well out of the way from the grip area.

The Section Assembly

The section housing, the feed and nib unify as a precision mechanism, relying on each other to perform the essential function of a fountain pen, namely, regulating the flow of ink.  Photo 1 shows an assembly, which I would call optimal.  I found it on the internet, unfortunately without any information on the pen depicted.  I am sure someone out there recognises it. Let me know if you do and I will add this information.

Photo 1 — Section Assembly

You can easily recognise the three components; grip, feed and nib.  You can also see the end of the barrel where it screws onto the grip.  Let me say it again, an optimal design.  Firstly, the means of holding the cap is well away from the grip area.  The start of the thread is protected by a chamfered metal ring preventing damage to the start of the thread, its most vulnerable part.

Further towards the nib, you can see two internal metal rings.  They take up the screw forces between the section and the barrel thus preventing the grip from cracking off or splitting.  The screw forces are caused when a new cartridge is inserted and its seal needs to be broken.

In addition, most plastics creep over time, some fatigue crack when held under constant force.  Any good ingeneer would know this fact and prevent this from happening.  Small metal rings or braces do a reliable job.

Central to those rings you can see the extension of the feed reaching into the tank area.  I don’t think it is a cartridge design, because the axial forces to break the seal of the cartridge would push out the feed and since the section is so well designed such a good ingeneer would enclose this pin with a protective sheath around it to pick up the axial load.

Another design feature I really love is the long overlap between the feed and the nib and large extent by the nib reaches inside the section.  This combines two excellent features.  Firstly, it offers a long intimate connection between feed and nib, which facilitates the crossover of the ink, especially at initiation and later at the restart.

And secondly, this style of connection places little stress on the feed, like so many wrap-around designs do.  It distributes the forces applied to the nib along a larger surface, reduces the momentum and thus preventing the feed from breaking, one of the most common problems.

Figure 1 — Cut through the Section

This design is less prone to manufacturing tolerances and actually compensates for them.  Figure 1 shows a cut through the section, feed and nib.  The dark grey in the centre is the feed; the lighter grey is the section.  Since both come out of injection processes they will fit pretty well.  Often there is a tongue and groove which orients the two components.

The radius of the nib is a bit larger, therefore it creates a line of pressure at the top of the feeder, where the ink has to jump over from the feed’s capillary to nib.  Two lines of pressure occur along the side of the nib, they act like a wedge.  Note: Of course the tolerances are much smaller (a few tenths of a millimetre) than shown in the drawing, which is simplified to demonstrate the principle.

The other very good point is the fact that the nib interacting with the grip and feed, it works like a splint and supports the tongue and groove engagement,

With plastics’ tendency to creeping, the interference fit between feed and section will loosen over time.  The preload supplied by the nib will prevent it from falling out.  What more keeps it on is the deposited colour pigment of the ink, which fills the gaps between the nib and feed acting like glue.

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In short (ha, ha, ha!), this is the best ingeneering and ergonomics design I have seen.  Why don’t they all do it this way?  Good question.  Because they don’t understand the function of a fountain pen.  And often it is cheaper to produce.

The Testing the Section Assembly

Once it all fits together the moment of truth has arrived when the functional testing is possible.  Only in this complete configuration, the function of the feed can be tested.  The outer shape of the section need not be the final but the inner opening is, and it must be early enough in the product development to allow modifications of all parts involved, the section, feed and nib.

Alas, the nib requires the most tooling and there is no half-finished nib.  Only if there is a serious mistake, alterations are performed but since nib production is the most established and reliable, this would hardly happen.  In addition, the tests for the function of the nib do not need a fountain pen.  A bib holder in a dip pen arrangement is sufficient.

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There basically two tests.  The first is the flow of ink with the variable parameters being the speed of writing, the width of the line (writing pressure) and the amount of residual ink in the reservoir.  The requirement is simple, no matter what the variations are, no variation in line width and colour density are accepted.

For this test, the standard circle writing machine (as shown in the article “Simply Inks”) is applied with small adjustments so that it can accept fountain pens.  The line width is measured at certain time intervals with a measuring loupe (shown in the article Flex Nibs My Classification Proposal – chapt. 8 photo 8).

Since the ink content only affects the breathing of the feed at lower levels I started the test at the reservoir being half full.

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During the second test, the ambient air-pressure is cycled from room pressure to a reduced pressure.  I chose the air-pressure in the cabin of an aircraft, which is kept at an equivalent of the outside pressure at 2500m (8000feet) at worst which is 76 kPa or 11 psi. Average sea-level pressure is 100 kPa or 14.5 psi,

Applying the equation of the law for ideal gasses pV = mRT[1] and knowing that mRT is constant then, also pV must be constant.  Therefore, as the pressure drops by a quarter (100 kPa – 76 kPa = 24 kPa), the volume increases by a quarter.  Meaning, the pressure in the cabin drops roughly by a quarter… meaning, all the air inside the air-filled pockets, cavities inside the passengers expands by a quarter.

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I started with an almost full reservoir (to save time) and after every cycle, I reduced the ink in the reservoir by two drops, 0.1 mL.  I used blotting paper and sucked off 0.1 mg of ink.  Through holding the fountain pen upside down I assured that the feed was emptied again.  If it wouldn’t empty it was back to the drawing board.

With each cycle, the air-volume inside the reservoir would increase and therefore more ink would be pushed out and into the feed.  With lowered ink volume the test would become more severe.  The aim was to prevent the pen from dropping any ink.

I calculated the volume of the capillary overflow slits of the feed.  Therefore, when the pen would drop ink I could calculate to what percentage the slits had been filled.  Once the ink volume in the reservoir was less than the ink capture capacity of the feed, the excess air-volume was released through the air vent (see “Application to the Feed”).

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As a variation of the second test one could alter the temperature instead of the ambient pressure to cause the volume change in the air-volume inside the reservoir.  As above, both sides of the equation must stay in proportion, therefore, as T increases V must also.  The temperature in the cargo hold of a Boing 767 is maintained above 7°C.  (Where animals are carried it is heated to above 18°C.)

Zero K (Kelvin) of absolute temperature T is about minus 273°C.  A pleasant 25°C converts into 298 K and a drop to 7°C would reduce it to 280 K a variation of only 6%.  Thus, not to worry.

By the way, depending on its composition, ink freezes somewhere between minus 20 to 50°C.   Since ink is mainly water it has its characteristic, namely: it expands by approximately 9% when freezing.  The ink in a reservoir of 5 mm inner diameter would expand to 5.5 mm.  Depending on the material, it could cause it to crack.

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During this testing happened the final tweaking of the feed.  Good utilisation of the slits was around 80% of their volume and when I achieved that the fountain pen would not drop any ink during the pressure cycles.  It turned out, the shape of the feed was important but more the consistent surface preparation and priming (see “Feeds made of Plastic”).

 

[1] pV = mRT … p = pressure,  V = Volume, m = mass, R = gas constant, T = absolute temperature

Ω

Amadeus W.
Ingeneer

30 December 2018

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