A wet epoxy/microsphere mix can be used as a casting compound if you observe a few rules.
First, cured micro is not weak but it doesn't have HUGE strength either. Treat it as a fairing compound, not a structural material. Make a few samples and break them to get a feel for toughness.
Microspheres do not make epoxy thixotropic, so a wet micro mix will cheerfully flow into every nook of a mold. It will also run right out of even the smallest hole in a mold. Don't leave any gaps.
Always use the slowest hardener available, or an epoxy with an overall slow cure time. You should also avoid thick mold sections. The reason for both is fundamental; you must avoid exotherm. Heat is a side effect of epoxy cure. More heat makes epoxy cure faster, and faster cure makes even more heat. It is easy to generate a runaway reaction if the heat cannot escape the curing mix. Microspheres are hollow glass bubbles, a terrific insulator. Heat cannot escape a micro mix quickly enough to avoid exotherm if the section is thick or the epoxy cure time is too short. Serious warning: a bad case of exotherm can burn you, make a lot of smoke in the shop, damage adjacent surfaces, and even set the mix on fire.
Ok, here's an example. Everybody likes speed mods; this a faired pitot tube. The mold to establish basic shape was simple, a square of aluminum sheet with some slick tape applied for mold release, then folded, then fastened shut. it was then spread and the tube was inserted and positioned for a nice symmetrical airfoil shape: Tape up the bottom, pour in the micro:
After cure it is easy to sand to any desired shape:
The slide-down base fairing is a 4-ply fabric layup over a foam form. That's some wet micro painted on the surface just after the layup had jelled, an easy way to save steps and time....you're gonna fill the surface at some point anyway:
Here's the finished assembly on the airplane. Has to add at least 0.25 knots right? Doesn't matter I guess....it keeps me out of the bars.
First, cured micro is not weak but it doesn't have HUGE strength either. Treat it as a fairing compound, not a structural material. Make a few samples and break them to get a feel for toughness.
Microspheres do not make epoxy thixotropic, so a wet micro mix will cheerfully flow into every nook of a mold. It will also run right out of even the smallest hole in a mold. Don't leave any gaps.
Always use the slowest hardener available, or an epoxy with an overall slow cure time. You should also avoid thick mold sections. The reason for both is fundamental; you must avoid exotherm. Heat is a side effect of epoxy cure. More heat makes epoxy cure faster, and faster cure makes even more heat. It is easy to generate a runaway reaction if the heat cannot escape the curing mix. Microspheres are hollow glass bubbles, a terrific insulator. Heat cannot escape a micro mix quickly enough to avoid exotherm if the section is thick or the epoxy cure time is too short. Serious warning: a bad case of exotherm can burn you, make a lot of smoke in the shop, damage adjacent surfaces, and even set the mix on fire.
Ok, here's an example. Everybody likes speed mods; this a faired pitot tube. The mold to establish basic shape was simple, a square of aluminum sheet with some slick tape applied for mold release, then folded, then fastened shut. it was then spread and the tube was inserted and positioned for a nice symmetrical airfoil shape: Tape up the bottom, pour in the micro:
After cure it is easy to sand to any desired shape:
The slide-down base fairing is a 4-ply fabric layup over a foam form. That's some wet micro painted on the surface just after the layup had jelled, an easy way to save steps and time....you're gonna fill the surface at some point anyway:
Here's the finished assembly on the airplane. Has to add at least 0.25 knots right? Doesn't matter I guess....it keeps me out of the bars.
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