Tubaing wrote:Will the plastic dent or crack?
Sure, if you hit it hard enough. But PETG (Polyethylene Terephthalate copolyester modified with glycol) is very strong--at least as good as polycarbonate. At 75 pounds/cubic foot, it's less than 1/6 the density of brass. The yield strength is about a third of annealed brass, which means you can make it five times as thick as brass and be stronger and still lighter. The yield strength is nearly as high as the ultimate tensile strength, meaning that when made thick enough to be very strong, it will probably not dent. A big enough hit to dent it would just as likely crack it, though it has good elongation so it will not shatter. Brass, on the other hand, has an ultimate strength anywhere from 50% to 300% higher than it's yield strength, depending on how it's worked.
The big difference is that it is not nearly as stiff as brass. The modulus of elasticity for brass is about 100 times more than for PETG. That means that if you took a bar of brass and a bar of PETG the same size, and loaded them the same way, the PETG would deflect (change shape as a result of the load) 100 times more. Some of this difference is compensated by the PETG being much thicker. If this tuba plays reasonably, then it seems to me that it utterly dispels the myth that one brass alloy can create different acoustical effects compared to another. If red brass were different than yellow brass, for example, this instrument, being thousands of times more different in terms of inherent material stiffness, shouldn't even play like a tuba. Since it, uh, clearly
does play like a tuba, the notion that subtle differences in material properties are important seems to me highly unlikely.
I would expect it to resist damage by about the same margin as a Kelly mouthpiece resists damage compared to a brass mouthpiece.
PETG looks like interesting stuff. It can be sterilized and it's approved for food processing applications. It's a thermoplastic and can be easily extruded, molded from flat sheets (probably how Brian shaped it), or blow molded. Scratches can be melted out of it with a heat gun. It can be heat welded, solvent welded, glued, and mechanically fastened (using the correct screws). It's easy to machine and much more machinable than polycarbonate. The temperature resistance is reasonable for plastic, but it will melt at around 175 degrees, so don't boil it. Static dissipation is good for plastic. It can be painted, but I would not expect paint to adhere really well, so my temptation would be to create a haze surface on the inside using a mild abrasive and then paint it with an epoxy paint. Then, the plastic itself will provide the gloss on the outside and the color will shine through it. You may be able to get it with a color molded into it, but I suspect it will always be transparent.
Chemical resistivity is even pretty good. Keep toluene and acetone away from it, and don't clean it with ammonia glass cleaner, but beyond that nothing to worry about.
I would not use it for valve branches, however. The thermal expansion of plastics is just too much to work well for tuning slides.
I'm no real expert on the variety of plastics, but of those I've looked at, this one seems as good a choice as any and better than most.
Rick "wondering how Brian joined the seams" Denney
