The performance of flex nibs depends on two factors: (a) a properly shaped nib from good material to withstand flexing and (b) a proper feeding system not only to supply adequate ink but also to follow the rapid changes of flow requirements flex nibs. The amount of flexing (the opening of the tines) depends on the geometry and the material of the nib. The focus of the discussion here is going to be the nib material.
The discussion on best materials for flex nibs is often clouded by a number of misconceptions or unclear use of terms. For example, the stiffness of a nib is confused with its strength. For a flex nib, we want
- Low stiffness, so that a small force can produce large reversible deflections of the tines.
- High strength, so that after large deflections the tines return to their original shape (i.e. do not deform permanently.
The stiffness or the strength of a nib can be adjusted by changing its geometry, e.g., by changing the thickness of the nib. For a fixed geometry, however, the performance of a nib depends on the material. For a good flex nib we need a material with:
- Low elastic modulus, to get low stiffness which allows for large tin opening at low force
- High yield strength and fracture strength: which allows for large openings of the tines without permanent deformation or cracking at the tip of the breather hole.
- High fatigue resistance: to avoid opening of cracks at the breather hole due to repeated flexing of the pen.
Additional criteria that apply to all nibs (flex or not) are:
- Weldability of tip alloy (this essentially excludes plastics, composites and aluminum)
- Corrosion resistance to inks (this excludes a number of otherwise good materials)
- Ease of manufacturing.
There are steel alloys with excellent strength and fatigue performance but the modulus of steel is 2-3 times that of gold alloys (~200GPa versus 60-100GPa). Therefore any advantage offered by steel due to high strength/fatigue performance is lost due to the high modulus (stiffness) of steel. The strength and fatigue performance of some gold alloys is quite remarkable. The low stiffness of gold is its biggest advantage. In simple words, if you had two nibs of identical dimensions, the gold one would give you the opening of the tines at a force that is half or a third of the force needed to flex the steel nib to the same tine opening. As a result, the stresses that may cause fatigue will also be 2-3 lower in the gold than in the steel nib.
It is possible to compensate for the high modulus of steel by decreasing the thickness of the nib/tines (or other geometric characteristics such as the length of the tines, the curvature of the nib, the width of the shoulders, etc). A thinner steel nib can match the opening of a thicker gold nib. Steel nibs with some flex exist (e.g., 9128, 9048 Esterbrooks). The thickness of nibs, however, is ~25 thousand of an inch, and often close to the tail it is as thin as 5 thousand of an inch. Getting such thickness in high-performance steel is much more difficult than in gold. Nib punching from a metal sheet will cause high wear on the tools. The problem is similar with titanium nibs – in fact, the properties of titanium are even better than gold (about the same modulus and high strength/fatigue). The difficulty in processing and the high capital cost of tools make the processing of steel and titanium nibs unfavorable given the small production sizes.
The advantage of gold is even stronger if you consider the corrosion resistance which excludes some other interesting materials such as memory alloys. Stainless is more sensitive than gold to acids and titanium is slightly worse than gold to bases and acids. I would rank the material selection criteria for flex nibs in terms of importance (high first) in the following way:
- Weldability of tip alloy
- Corrosion resistance to inks
- Low Modulus
- Ease of manufacturing
- Fatigue resistance
Therefore gold is better than steel for flex nibs because of the low modulus (stiffness), reasonable strength/fatigue, excellent corrosion resistance, and good formability.
There are two other facts that also lead to confusion in the discussions on the best material for flex nibs:
1. A single material can have a range of properties depending on processing (rolling + heat treatment). In simple words, we can change the properties of the metal by rolling the sheet before stamping the nibs or by heating the nibs to a high temperature than induces changes in the internal structure of the alloy.
2. Generic materials designations are not enough to specify the material. For example, when we say 14K this includes a very large range of materials. The karat designation only specifies the gold contained. The other elements in the alloy (e.g., silver, copper, etc.) can affect the properties and may result in a large variety of properties.
We say that in general 14K is better than 18K for flex nibs because we can make 14K gold alloys that have lower elastic modulus and higher strength than the 18K alloys. This is shown in the table below that compares some of the common nib materials.
|MODULUS (GPa)||STRENGTH (MPa)||FATIGUE LIMIT (MPa)||CORROSION TOWATER/ACIDS/BASES||FORMABILITY|
|Gold 14K||80-90||200-500*||150-450*||Very good/Very Good/VeryGood||Good|
|Gold 18K||90-100||150-400*||120-350*||Very good/VeryGood/Very Good||Good|
|Stainless 302SS||200||750-900||440-750*||Very good/Good/Very Good||Difficult|
*The wide range of properties indicates variation in composition and processing.
Remember we want
- low elastic modulus
- high strength/fatigue limit
- good corrosion
- good formability
It is interesting to note in the table above that it is possible to get a 14K alloy which is totally inappropriate for flex nibs if its properties (strength and fatigue resistance) correspond to the low end of the range.
There is a lot of room to optimize the composition and the processing of gold alloys for flex but the cost of R&D with gold and the small market size for flex nibs are not favorable for such a pursue. I hope to get back to you with a detailed report on the geometry of flex nibs.