What Material is Best for Flex Nibs?
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The performance of flex nibs depends on two factors: (a) a properly shaped nib from a 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
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 good flex nib we need a material with:
Additional criteria that apply to all nibs (flex or not) are:
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 opening of the tines at a force which 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 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 thousands of an inch and often close to the tail it is as thin as 5 thousands 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 is even better than gold (about the same modulus and high strength/fatigue). The difficulty in processing and the high capital cost of tools makes 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 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:
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 contain. The other elements in the alloy (e.g., silver, copper etc.) can affect the properties and may result in large variation of properties. We say that in general 14K is better that 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. TECHNICAL DETAILS
*The wide range of properties indicates variation in composition and processing. Remember we want
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. Antonios Zavaliangos is Associate Professor in Materials Science and Engineering at Drexel University, Philadelphia, PA 19010 An early version of this article appeared on Pentrace and Zoss in October of 2003. |
