Self-fill pens work by the creation of a difference in air pressure, but there are a variety of mechanisms that can be employed to utilise this effect.
Sac pens
Perhaps the simplest method is to use a flexible sac that can be squeezed to eject air, and then released to create a low pressure volume into which the ink flows. A variety of methods have been employed to crush and release the sac and they may be put into two categories, the ones that use a longitudinal bar to crush the sac laterally, and those that use other means. Levers, buttons and press bars are all mechanisms that use lateral pressure on the sac; they have been used on reservoir pens over a long period (Conklin crescent filler (1897) and Parker 51 aerometric (1954). Some sac squeezing mechanisms use longitudinal compression by means of a button (Swan Visofil V); others use air pressure (Sheaffer TD, Chilton).
Rigid reservoir pens
In the 1930’s many refinements were added to pen design, the chief of these being ink visibility. This quality can be achieved with sac pens (eg Sheaffer transparent sections, Parker 51 transparent ink sacs and Visofil V transparent buttons), but it is better with a transparent barrel, a thing that was made possible with the advent of advanced plastics. Ink capacity is potentially much greater than for a sac pen. The mechanism for filling the barrel may be by piston/plunger or by diaphragm.
‘Diaphragm filler’ is a synonym for ‘bulb filler’, where a length of rubber sac is fitted to the end of the barrel and squeezed to eject air that is subsequently replaced by ink as the bulb returns to its normal configuration. Pens will not take up much ink unless a means is provided to operate the bulb through several cycles without ejecting it again between cycles; the solution is to fit a breather tube connected to the ink channel so that air can escape from the end of the pen as the bulb is compressed. The pen will then fill to the end of the breather tube.
The Parker Vacumatic is a bulb filler; in this example the bulb is turned inside out and compressed endwise. The Blackbird Topfiller and the Visofil VT are variations on the same theme. The Waterman Ink-Vu is another variation, but it uses a lever to squeeze the side of the bulb. This class of pen is very sensitive to air pressure and temperature variations, and they can easily empty their contents into the owner’s pocket during an aeroplane trip!
Piston fillers are upfillers (syringe) or downfillers (vacfillers). The advantage of piston fillers is that there are no flexible components, so pens are less likely to leak due to air pressure variations; however, they are sensitive to temperature variations unless they are doubly insulated. Vacfillers rely on the creation of a vacuum in the barrel end as the piston is pushed down, followed by the sudden release of the vacuum as the piston seal enters a groove near the barrel mouth. The idea dates from a 1905 UK patent. Vacfillers had a remarkable run of success; lasting with their main protagonists (Onoto, Sheaffer and Eversharp) until around 1948. The main disadvantage was reliability in filling, which is critically dependent upon high quality seals. The barrel seal must hold a vacuum, and the piston seal must do the same on the down stroke and be flexible enough to allow fluid to pass it on the up stroke.
The syringe technique is perhaps the obvious way to fill a pen, but unless the barrel is very long, the problem remains of what to do with the withdrawn piston rod. One of the best syringe fillers was introduced as the Ford Patent Pen in 1934. The Ford uses an ingenious sliding seal that allows the plunger to be returned without emptying the pen. Typically, three operations of the plunger are required for complete filling of the reservoir, the ink being able to bypass the seal at the end of each stroke. It is probably the only self-filling system where it is possible to completely fill a full-length reservoir. A screw operated telescopic syringe fill system, where the piston is a two part assembly that retracts within itself, (eg Nozac and Mont Blanc 149) is now the method of choice in many modern pens. These pens typically lack the double insulation and full barrel ink capacity of the Ford.
The capillary filling system (Parker 61) was a brave attempt to produce a leak proof pen. It uses a roll of thin surface-profiled plastic sheeting contained within a rigid open-ended tube, the end being sealed in use by a valve in the pen barrel. By placing the open end into the ink, the pen fills by capillary action into the spaces between the layers of sheeting. The method has very good double insulated properties, but it suffers from low ink capacity and the lack of a method to flush and clean the reservoir.
