Fountain Pen Design

Function, Development, Construction and Fabrication

7.3 Self-filling pens

By the time Waterman’s patent was published, there were already other patents and descriptions of what were to be known as “self-filling” pens: pens that could fill themselves. Although there are many different mechanisms that have been developed over the years that followed, they can be divided into two main groups:

  • Those with a flexible ink bladder (or “ink sac(k)”)
  • Those with a rigid reservoir
  • It can easily be argued that there is a third category that is a hybrid of these two; combining both a rigid and a flexible portion in the same reservoir.

Flexible Bladder

An eyedropper is based on a simple principle: you collapse a rubber bulb by squeezing it. This expels a portion the air from within it. When you release the pressure, the rubber moves back to its original shape, creating a vacuum inside it. If the glass tube connected to the open end of the bulb is immersed in a liquid, the liquid will be pulled up into the tube due to the lower pressure inside the tube and bulb than outside, similar to that which was shown in the illustration of the bird bath in Figure 1 on this page.

A latex ink sac on a section assembly

In an eyedropper, the reservoir is the glass tube, and the bulb is merely there to create the vacuum. If you replace the glass tube with the section assembly of a fountain pen, and make the the bulb much longer, you have the basis for a fountain pen with a self-filling system based on a flexible ink sac. The primary difference is that the bulb is both the device” that creates the vacuum to pull in the ink and the reservoir itself. An example of this can be seen in the figure above which shows a latex ink sac on the section assembly from a Penol Ambassador SeniorSack-Filler” made in Denmark in 1945. Typically, the latex is affixed to the section using shellac, and covered in chalk powder which acts both as a dehumidifier and a lubricant.

The darker barrel is celluloid discolouration caused by decades of off-gassing from the latex ink sac

Initially, ink sacs were make of latex rubber. Although this is an excellent material for the purpose initially, it does suffer from issues with longevity, both with hardening and with off-gassing, possibly resulting in the discolouration of surrounding components in the pen, an example of which can be seen in the figure above. (The original colour was the bright green still seen near the cap bands.)

Consequently, although latex is still used in ink sacs, particularly for the restoration of older pens, other materials such as silicone and PVC have been introduced over the years to avoid these problems.

Although the core component of a filling system with a flexible bladder is the sac itself, the remaining discussion is about the surrounding mechanism that is used to squeeze it when filling.

The first step in using a flexible sac to pull in ink is to collapse it, and this can be done either pneumatically (using compressed air) or mechanically (by squeezing it with an object).

Pneumatic systems

When a flexible ink sac with an outlet (through the feed) is placed inside an air-tight container (the barrel of the pen) and the pressure of the air inside the barrel is increased, the latex bladder collapses, pushing air out through the section and feed. The only question is how to apply the increase in air pressure inside the pen barrel. There are three common methods for doing this:

1. Blow Filler

Blow filler

A blow filler pen is possibly the simplest self-filling mechanism. The barrel and section form an air-tight seal, and a small hole is drilled in the end of the barrel. In order to fill the pen, the user:

  • submerges the feed into the ink
  • blows into the hole, which increases the air pressure inside the barrel, squeezing the ink sac, and thereby evacuating the air inside it through the section and feed.
  • When the seal around the hole is broken, the excess air inside the barrel escapes, the ink sac returns to its original size, and ink is pulled into the sac.

This filling method was patented by Seth Crocker in 1901 with an associated improvement in 1904 (US Patent 678,547 and US Patent 767,208).

Although a blow filler is a very simple mechanism, it is certainly far from elegant. Imagine yourself, taking minutes in the board meeting, and your pen runs out of ink. So, you uncap your ink bottle and your face turns red while you blow into your pen, whilst sticking it into the bottle, like sticking a drinking straw into a soda. To solve this problem, Seth Crocker came to the rescue again, patenting a squeezable rubber bulb that could be placed over the hole on a blow filler (US Patent 1,181,574).

2. Pneumatic Piston

Pneumatic piston

Crocker’s idea of using an external method of applying air pressure (instead of the mouth) was incorporated into the pen itself with the introduction of a method of increasing a pressure inside the barrel by changing its volume. In this method, patented by Julius Abegg in 1915 (US Patent 1,134,936), the ink sac is housed by two tubes: an inner tube, and the outer barrel of the pen. These two are able to slide telescopically relative to each other, and one of them is sealed at the end of the pen, except for a small hole.

In order to fill the pen, the user:

  • extends the inner (or outer) sleeve,
  • places a thumb on the hole to close it, and
  • pushes the sleeve back down, which increases the air pressure inside the barrel, squeezing the ink sac, and thereby evacuating the air inside it through the section and feed.
  • The section and feed are submerged into the ink,
  • the finger or thumb is released, allowing excess air inside the barrel to escape, so the ink sac returns to its original size, and ink is pulled into the sac.

Note that, In Figure \ref{thumb_pneumatic} I have drawn the mechanism with a capped inner sleeve extending outwards, however, it could just as easily have been drawn with a capped outer barrel extending over an interior sleeve instead.

3. Pressure Touchdown

In 1949, Sheaffer introduced the new Touchdown model of pens, which is closely related to the pneumatic piston design. The principal difference is that, instead of using a finger to seal and open the hole in the movable piston, this was done automatically by a clever placement of the hole in the side of the sleeve. When the tube is collapsed, the hole is opened and the pen fills with ink.\footnote{US Patents 2,610,612 and 2,769,427}

For more detailed information and details on pneumatic self-filing fountain pens, a good place to start is Richard Binder’s article on the topic.

Mechanical systems

Of course, instead of using air pressure, the ink sac can be compressed by simply squeezing it mechanically. Many different methods of doing this have been invented, most of them purely for reasons we would today call a “USP” (Unique Selling Proposition), or even, hyperbolically, “a Technological Innovation”: “Our product is better than the competitor’s because of this feature that is different.”

1. Pressure Bar

Pressure bar filler

Possibly the simplest (mechanical) mechanism for squeezing the ink sac consists of a pressure bar that is approximately the same length as the sac. Pressing on the bar squeezes the ink sac against the inner wall of the pen barrel, pushing the air out and creating the vacuum that will pull ink in when the pressure is released.

In its simplest incarnation, the bar is pressed directly using a finger through a rather large hole in the side of the pen barrel, as described in a patent for Waterman from 1905 (US Patent 799,897). Other versions with a smaller hole or a slit in the barrel require the use of a matchstick or coin to press the bar. In yet another earlier version patented by Roy Conklin 1901 (US Patent 695,258), an extension is connected to the pressure bar, protruding from the side of the pen barrel, as shown in Figure \ref{pressure_bar}.

In order to avoid the extension being accidentally pressed and squirting ink on your documents, this design includes a rotatable collar that slides through the hole in the extension, locking the entire mechanism. A second advantage of this protrusion was that it stops the pen from rolling off the desktop, and was therefore dubbed by Mark Twain to be a “profanity-saver”.

A number of companies offer a pressure bar variant of the removable converter, which can be used in place of a pre-filled cartridge.

2. Lever

Lever filler

Although Sheaffer is widely credited for inventing the lever filler, the idea was first patented by John Barnes in 1903 (US Patent 738,876), 5 years before Sheaffer (US Patent 896,861).

This mechanism consists of two main moving parts: a lever pivots at or just inside the barrel, and is pulled away from the pen. This causes the inner portion of the lever to push against a metal pressure bar that rests on the side of the ink sack, thus squeezing it along its length against the inner wall of the barrel. The lever usually attaches to the pressure bar using two small extensions that slide along tracks in the sides of the bar. Often, the pressure bar is attached to the bottom of the barrel to prevent it from moving backwards and forwards on the ink sac, as can be seen in the figure below.

Figure 1 from Barnes’s 1903 patent for a lever filler pen. Note the small arm connecting the pressure bar to the end of the barrel to prevent it from sliding along the ink sac.

3. Button

Button filler

Like the lever filler, the button filler uses a pressure bar that squeezes the ink sac between the inner wall of the pen barrel along its length. However, instead of being pushed from the side of the pen, it is attached to a length of spring steel that extends from the section to a button at the end of the barrel. When the button is pushed, the spring bends inwards, pushing the pressure bar against the ink sac. When the button is released, the spring straightens, and the ink sac returns to its original shape, pulling ink in as it does.

The history of the button filling mechanism is similar to many other inventions. Although advertised (and repeated) as being originally developed by William E. Moore for Parker in 1920 (US Patent 1,346,045), it was actually patented 15 years earlier by John T. Davison (US Patent 787,152).

Figure 1 from Davison’s 1905 patent for a button filler pen.

Rigid Ink Reservoir

Pens based on a flexible ink bladder rely on the restorative powers of the sac material to provide the vacuum needed to pull the ink into the reservoir. However, in the case of pens with a rigid reservoir, the vacuum must be created using a piston instead.

1. Syringe piston

Figures showing the top-feed variant from Prince’s 1855 patent for a piston filler pen.

The earliest known patent for a piston-filled pen was published by Newell Prince in 1855 (US Patent 12,301). This mechanism worked exactly like a medical syringe. A rod, extending out the back of the barrel, connects to a plug that seals against the inner barrel. The rod is pushed into the barrel of the pen, the nib and section are dipped in ink, and the rod is pulled back out, sucking ink into the pen. It may be interesting to note that Prince described two versions of the pen, one with a feed below the nib, and the other with the feed above, as can be seen above.

2. Screw piston

Piston filler

In 1905, the simple push-pull mechanism was upgraded to use a threaded rod that, when turned, extended and retracted the plug in the reservoir. This idea was first patented by James Morris (US Patent 834,373), and variations on this design can easily be said to be the most popular filling system in use, since it also forms the underlying mechanism of most removable piston converters.

There are different variations on this type of filling system, but all have the same basic underlying logic. In all versions, the cork or rubber plug must be prevented from rotating with the threaded rod. This is typically done using one of three methods:

  • friction between the plug and the inner wall of the barrel
  • a non-circular guide that prevents the nut from rotating with the threaded rod
  • a protrusion that runs along a slot in a tube surrounding the mechanism.

One particularly clever implementation of the screw mechanism for piston fillers uses a design with a threaded rod inside a threaded rod. This is often found on Pelikan and Montblanc piston fillers, an older example of which is shown in the figure below. Notice that this mechanism is prevented from turning by the protrusion guided along the slot in the outer tubing. This arrangement ensures a maximal extension of the piston, providing a large filling capacity.

Montblanc telescopic piston showing the figures from the US patent and a production example of it.

3. Vacuum Filler

Vacuum filler

Almost all piston-based filling systems work on the same principle as a syringe: pulling the piston upwards creates a vacuum in front of it, thus pulling ink into the reservoir. There is one design, the vacuum filler, that works in the opposite direction. In this design, the piston plug is a cupped rubber washer that flexes asymmetrically. It moves inside a tubular reservoir that has two different diameters: a long, narrower one at the back of the barrel (further from the nib) and a wider, short section near the section assembly. When the piston is pulled back, the washer flexes downwards, and thus lets air escape from the barrel around it, as shown on the left side of Figure \ref{vacuum_filler}. When the piston is pushed downwards, the washer flexes to a larger diameter, sealing against the reservoir walls, and thus creating a vacuum behind it. When it reaches the larger diameter portion of the reservoir, the vacuum that has been created pulls ink around the washer and upwards into the barrel of the pen.

This action of filling a vacuum filler should be done only once, since, if the plunger is pulled upwards too quickly when the reservoir contains ink, the liquid cannot escape around the washer fast enough, and thus will likely be pushed out through the top seal washer (shown in green in Figure \ref{vacuum_filler}), potentially damaging it, besides making a mess…

Flexible / Rigid Hybrids

1. Bulb Filler

Bulb filler

A bulb-filler pen works on almost exactly the same principle as an eyedropper: it has a rigid reservoir for the ink (typically the barrel of the pen) and a small rubber bulb at the top. When the bulb is squeezed, it expels the air inside the reservoir, and when it’s released, the vacuum pulls ink upwards into it. The only problem with this basic design is that, if this has been done once, then the next time the bulb is squeezed, the ink inside the barrel is expelled, and you don’t get more ink into the reservoir on subsequent actions.

To avoid this problem, a bulb filler pen includes a “breather tube” that extends up into the reservoir. When the bulb is compressed, the air is through the tube and out of the section assembly. Then, when the bulb is released, the ink that is pulled into the reservoir enters via the tube. This means that, on the second squeeze, the air at the top of the reservoir is pushed down through the breather tube, thus preserving the ink inside. It should be noted, however, that this also pushes out the ink remaining in the tube itself, so it’s necessary that the volume of air being moved is greater than the total volume of the breather tube. This also means that the reservoir can be filled to a maximum defined by the length of the breather tube.

2. Button

Figure from H. Taylor’s 1905 patent for the bulb filler.

Huston Taylor’s 1905 patent (US Patent 802,668) for a bulb filler design described the “breather tube” improvement on the earlier 1903 patent by Perks and Thacker (US Patent 723,726). Along with the squeezable bulb, it also included a second method of filling the pen by the use of a small piston that could be repeatedly pumped to pull ink into the reservoir. This is shown in the figure above.

Vacumatic filler

This mechanism of using a small piston to create the vacuum necessary to fill the pen can be implemented using a flexible diaphragm instead of a rigid plug to seal the back of the reservoir. This is the core design of the Parker Vacumatic filler (US Patent 1,904,358), which uses a folding rubber sac that unfolds when the button is pressed, extending into the reservoir and reducing its volume to create the necessary vacuum when it’s released. While the Parker Vacumatic mechanism relies on a spring to push the button back upwards, a variation on this design called the “stretch vacumatic” (patented by Johannes Iverson in Denmark for Miller (US Patent 2,143,601)) relies on the stretched rubber diaphragm to provide the restoring force on the button.


There have been many other self-filling mechanisms introduced over the years, however, they will not be described in detail in The Richard’s Pens Guide to Fountain Pens – Volume 3: Filling Systems is highly recommended reading for anyone wishing to dive deeply into the varied history of the topic.

Capillary reservoir on a Parker 61

The one system that stands out, and is separate from the three categories mentioned above was introduced on the Parker 61 model in 1956. This system pulls the ink into the reservoir using capillary action instead of a vacuum. The reservoir consists of a tube containing a plastic sheet that was punched with holes and rolled up around a central feed that delivers the ink to the nib. The tube is made of Teflon so as to repel the ink on the outside of the tube (to avoid stained fingers after lifting it out of the ink bottle). The rather large holes at the bottom of the tube through which the ink enters are sealed by a spring-loaded cap inside the barrel when the pen is assembled.

For more information about the history of the development of this filling system, the article on Richard Binder’s website is a good place to start.