Michael's flutes are attractive, durable, and sound great, but our discussions started me wondering about other polymers that could be used for flutes and other woodwinds. Cronnolly's work convinced me that polymers have many advantages over wood, and I thought it would be interesting to try to identify the ideal polymer for this application.
My first thought was to determine what particular polymers are typically used in commercially produced polymer clarinets oboes, etc.. But upon making some inquiries, I was encouraged to not start with what is being done, but rather start with a clean slate. (These instruments are typically injection molded, while Cronnolly's are machined).
Coincidentally, I happen to know a number of polymers experts, all of whom were fascinated by the challenge of identifying suitable materials for this application. Their interest and assistance stimulated me to record their thoughts in this article in the hope that others might find the information useful
The first question posed by my expert polymer friends was: what characteristics do you need in the polymer? To answer this, we need to examine why anyone would want a polymer flute to begin with, and why they might not want such a flute. We need to examine this question from the perspective of both the flute player and the flute maker.
Polymer flutes also have the potential for being less expensive than wood flutes. Coupled with their durability, this can make polymer a good choice for beginners, young people, and campers.
From the perspective of the flute maker, polymer can be much easier and more predictable to work with, thus reducing scrap and frustration. The low cost and high strength of certain polymers can stimulate more design experimentation on the part of the builder. This is a big plus for novice flute makers like myself. Polymers can also be just plain easier to obtain than certain exotic woods (polymer doesn't grow on trees, you know).
Aesthetics is really the bottom line on this whole polymer flute business. Many players (and maker) still view polymers with disdain; if the polymer doesn't produce an instrument that looks and feels like a wood flute, then nothing else matters to these folks. Now with polymers, a tremendous variety of aesthetic possibilities exist. (How about a transparent fluorescent pink flute? It can be done!) But the majority of customers have set expectations as to what a flute should look and feel like.
Regarding the player's view of the acoustic qualities of polymer instruments, this is either really an important reason for not going polymer, or not an issue at all. Many experiments have indicated that since the body of a flute does not resonate like, say, a violin, the particular material that a flute is made of has very little effect on acoustic qualities. Nevertheless, people usually say that metal flutes, for example, sound "bright" and wooden flutes, in contrast, sound "dark" (or whatever it is that people say, I'm new to all this).
Personally, I think that both views are absolutely correct. Wooden flutes sound different than metal ones not so much because of the acoustic properties of the materials, but because, for example, you can't make wooden flutes with the same thin walls found in metal flutes. This has a major effect on dimensions such as finger hole volume, which does affect sound.
Differences in manufacturing processes and flute geometry and dimensions dictated by the material used are major drivers in acoustical differences between metal and wood flutes. The same reasoning holds true for polymer flutes. So, if you build a polymer flute like you do a wooden flute, then the acoustic properties will be similar.
Why would flute makers stay away from polymers? Probably the main reason is the fear that they couldn't sell the darn things! There are also the issues of adapting machining processes and tools, and altering flute dimensions to accommodate the change in material. Cronnolly has found that in order to produce consistent sounding flutes, he does need to adjust dimensions when going from wood to polymer, or when using different polymers.
Our strategy here is to create a short list of potential materials by examining the "killer" requirements, then examine some basic properties of those materials to narrow the list down further, and then obtain test samples to experiment on. These are the requirements I have used to narrow down the search:
Cost: I'd like the polymer to be less costly than the woods commonly used for this application. This is not an absolute requirement, since even if the polymer was slightly more expensive than wood, Cronnolly's experience indicates the regularity and predictability of polymer translates to less scrap in the form of cracks or flaws which show up after machining begins. This means less total material and labor cost, allowing a higher raw material cost. However, a lower cost for polymers would encourage design experimentation, since the cost of a design error is less. As a novice, this is important to me. Eisenbrand Exotic Hardwoods sells 1-3/8" x 9" squares of African blackwood for $9 per piece (1996 price), which appears to be a pretty good deal for the small-time flute maker. Using this as a benchmark, I set $1 per inch as a maximum cost of the polymer.
Availability: There are dozens of families of polymers with tens of thousands of different formulations, but the only ones of interest to me are those readily available in black rod stock of between about 1" and 1-1/2" diameter. To make up my initial list of materials, I obtained catalogs from several polymer suppliers and determined what they had available.
Moisture absorption: Some polymers (notably the nylons) tend to absorb moisture under humid conditions. This causes swelling, which can cause dimensional stability problems, as well as making disassembly of the flute quite difficult. There are plenty of types of polymers that have low moisture absorption, so we'll stick with these.
Acoustic qualities: Based on my preceding comments, I did NOT consider acoustic properties as a selection criteria.
Aesthetics: Many polymers have fairly well-defined aesthetic characteristics which can be used to identify likely candidates for musical instruments. But the best way to assess this characteristic is to get a sample, machine it, and experiment with finishing to see what kind of look and feel you get. So, aside from requiring the material to be black, no aesthetic requirements were used for this first-cut selection.
For this analysis, I've just dealt with families of polymers, not specific "grades", as they're called. For each family, there may be several manufacturers and hundreds of specific grades, but they'll all have similar basic properties. You should be aware, however, that different grades within a family could produce very different results when you actually start machining flutes!
Each description starts with the scientific name of the polymer and its official abbreviation, where applicable. Some pros and cons of the material are given, followed by some typical applications. The purpose of these definitions is to begin to give you a feel for what each polymer is like.
Acetal: Good fatigue and creep resistant, high tensile strength with rigidity and toughness, good dimensional stability, low coefficient of friction, retains properties at high temperatures. Subject to sunlight degradation, difficult to bond. Applications: gears, bearings, pulleys, pens, plumbing valves, faucets, springs, steering columns, shower heads, video cassette components.
Acrylic: Rigid, with good impact strength, excellent dimensional stability, resistant to marring, excellent optical clarity, excellent weatherability and resistance to sunlight. Poor solvent resistance, subject to stress cracking. Applications: Glazing, head lights, tail lights, name plates, Plexiglas, aircraft canopies and windows.
Acrylonitrile Butadiene Styrene - ABS: Good impact properties, excellent strength, stiffness, and toughness, light weight, good chemical resistance. Attacked by sunlight and solvents, prone to stress whitening during flexing, low continuous service temperature. Applications: Telephones, appliances, business machine housings, safety helmets, pipe fittings, luggage shells, tote trays, boat hulls, pipes.
Polycarbonate - PC: High impact strength, toughness, good optical clarity. Subject to stress cracking, attacked by many chemicals. Applications: storage module housings, appliance and power tool housings, fan and air conditioner grills, instrument panels, mugs, pitchers, milk bottles, tail lights, head lights, helmets.
Polyethylene Terephthalate - PET: Good dimensional stability, wear resistant, good chemical resistance. Relatively low strength. Applications: Motor and wire insulation, containers and beverage bottles.
Polyphenylene Oxide - PPO: Low water absorption, outstanding dimensional stability, good mechanical and thermal properties, resistant to acids, bases, and detergents. Attacked by certain organic chemicals. Applications: Housings, structural parts, tools.
Polyvinyl Chloride - PVC: Excellent chemical and water resistance, good strength, good dimensional stability and resistance to weathering, comparatively low cost. Attacked by certain solvents, poor heat resistance, relatively high density. Applications: Pipe, extruded wire covering, toys, bottles, film and fabric coatings.
In the next installment of this article, we will look at these
materials
in more detail, and narrow down the list to a few materials for
prototyping.