At the beginning of a structured ingeneering approach to any product stands the design brief. It is a list of function criteria, which the product has to fulfil, ideally, with quantified data attached to each function. If you want to read more about this important process of initiation, Design Innovation is the place.
The Design Brief for a Nib
- the transport of ink,
- the control of the amount of ink
- including the start and stop of the flow
- the variation of the line width between 0.3 and 0.6 mm
The brief continues with telling the designer how these functions are to be instigated or controlled by the user. For example:
- The vehicle is to be steered by turning a wheel inside the cabin.
- There must be proportionality between the angular rotation and the derivation from projected line of movement.
- The user must receive a sensory feedback from the amount of friction force between the wheels of the vehicle and the road surface.
Aren’t we glad, we don’t have to design a car?
Returning to the fountain pen. The functions in the list are to be performed with two actions:
- placing the tip of the nib on paper
- the variation of the width of the slit through applying pressure to the tip
Imagine, you have never seen a fountain pen or a nib in particular. Try to imagine what a device could look like, which is able to satisfy this design brief. No, it is not done with a microprocessor, not even with a miniature tap and a micrometre screw.
Why does the ink flow start when placing the tip of the nib on paper?
The capillary forces around the nib and the slit are so high that they can hold the weight of the hydrostatic column from the ink level in the reservoir to the tip of the nib. Thus, under normal conditions, no ink comes off slit of the nib, unless, it meets paper.
The fibres of the paper have not only much narrower capillaries and therefore higher capillary forces…. more attraction, the fibres also have a type of surface characteristic with a very small contact angle, which causes a higher hygroscopic force than the the nib material.
As a refresher, we know that blotting paper can suck up ink, photo 1.
Photo 2 shows a magnified piece of paper. The fibres’ suction due to capillary action as well as their hydrophilic characteristics is much higher than that of the nib.
Something written with ink on paper, photo 3.
(Those three photos I found on the Internet)
How does the slit widen at the bead with an increase of writing pressure?
Have you ever asked yourself this question? Or have we become so accustomed to this characteristic that we don’t question it any longer? It is not as obvious as it appears, initially.
Have a look at drawing 1.
There is a flat strip of material (we won’t need to waste gold for this experiment) firmly held along the X – Y axis. A force is applied upwards, and the strip bends upwards, as expected.
In drawing 2, the strip has a slit, like in a nib. As long as the two upward forces are equal, the slit stays closed.
In drawing 3, you see the same arrangement as above; however, the strip is slightly bent along the same line of the slit. When you apply a force like in the previous experiment, the loose flaps move apart the more you push upwards, or the arrangement onto a surface (like in the situation of a nib), the slit widens => dimension W.
And in drawing 4, I have attempted to show the above drawing as it would look like when the flaps are shaped like a nib.
Most nib designs are not flat; Parker had one on a slender, elegant pen, which had a very pointy and rigid nib and an untouched iridium bead, I assume, they sacrificed writing quality for appearance. Hard to write with but it was excellent for drawing and sketching.
Keeping the force/pressure at the front edge constant, the experiment demonstrates, the range of slit widening depends on several parameters:
- the angle of the longitudinal bent
- the length of the slit
- the modulus of elasticity of the material
- the pre-set force during the setting process
You can see, all dimensions are all interdependent.
I am aware that I have simplified the mechanics. A nib is not a flat sheet but curved. This makes it stiffer, and the movement becomes rather complicated. Instead of using more words, I would like to invite you to undertake another experiment, yourself.
Then cut it out with a slit and the centre hole. (3) If the hole is too complicated, you don’t need it to understand the mechanics. It does not affect the result much.
When you take your mammoth paper nib, and you place it onto the paper with no pressure nothing much happens (4).
When you apply some pressure, the tip will separate (5). You will observe how increasing the pressure widens the slit at the tip and the deformation it causes on the sides of the nib. As long as you don’t overdo it (with the real metal nib), it will always return back to its original shape.
If you watch closely, you see that the slit does not widen in a straight line but rather a curve. The reason for that is, the material of our paper nib is of the same thickness all along. In the chapter Nib Manufacture, I show, that the material of a real nib is thicker closer to the tip.
In reality, the amount of deflecting a nib is only a few tenths of a millimetre. In materials physics, this is called a deformation within the range of elasticity. After deflection, the nib returns to its original shape. More about this in the chapter Material Technology.
In the experiment, I exaggerated the pressure so you can see the contour the leaves of the nib take on. In the real world, a deflection to such proportions would bend the nib beyond its elastic range. One would end up with a permanent deflection, the nib would be rendered useless.
Here is an example of what not to do with a nib, especially not when it is a gold nib, as it shows through the embossed number 585. A gold nib would plastically deform. If you tried this with a steel nib, you would end up with a hole in your paper.
This expression means, you can pull the fountain pen across the paper without applying any pressure to the pen and it leaves a line. This is achieved by bending the nib at the tip so that the slit looks like a turned over V, diagram 1.
This process is performed during the setting of the nib, which I describe later in the manufacturing process in more detail.
The sketches are overemphasised to make the point I want to make more clearly.
Due to this geometry (turned over V), there is more amount of ink available at the tip, enough to saturate the paper enough to leave a trace without applying pressure, diagram 2. It follows that with less pressure a desired line width can be achieved, writing thus, is less tiring and providing more comfort to the writer.
The larger amount of ink being available at the tip prolongs the time before the ink dries out.
Once the pre-set pressure is overstepped the slit opens, therefore able of delivering more ink to the tip. Adding a comment: The width of the line is more influenced by the hygroscopy of the paper if the feed can supply the ink volume.
Diagram 3 shows the disadvantage if a nib is not set this way. If the pen is at rest with the slit being parallel, the drying out occurs from both sides.
When pressure is applied to such nib the slit widens on the top side more than on the side contacting the paper; in some designs, it actually closes, diagram 4.
This is disadvantageous because with more pressure more ink needed and not less.
More about the Slit
When you look at the slit in our paper roll nib, you will notice that the slit does not widen linear, in a V shape but rather curved. Very obvious it is in this photo 4, which I have shown already above.
This is significant, because the capillary force, which pulls the ink towards the tip needs the slit to narrow towards the tip. In the old ingeneering days, we had special drawing nibs, where the width of the slit was adjustable with a knurled nut.
If the nib was not clean or the gap too wide this would happen: Then ink would not progress to the tip, photo 5.
In the situation of fountain pens, we are lucky. As shown in sketch 1, you see the starting situation:
The slit is filled with ink, and the narrowing increases the capillary force towards the tip, and the ink is pulled forward.
The ink is absorbed by the paper. The compression of the paper fibres by the tip increases the capillary action and spreads the ink out, just a bit beyond the width of the tip.
Sketch 2 shows what happens when the slit widens. Technically speaking the ink should retract like in photo 5. However, the capillary attraction of paper is generally much higher than that of any metal (nib material), therefore, the ink does not retract, up to a certain point.
The ink contact can be broken, and the ink flow interrupted, if the nib is tilted or when the nib is widened extensively, like in photo 4. But such distortion would also damage the nib.
The next chapter will be on Stresses and Strains