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Thread: hot wire cutting foam

  1. #31
    Registered User Jay Kempf's Avatar
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    Re: hot wire cutting foam

    Quote Originally Posted by DangerZone View Post
    The voltage of 70V is a bit exaggerated for nichrome wire unless your wire is thick and long. The impedance of the wire (inner resistance) varies with the tension of the wire too, thus a variable voltage power source would be a better solution to have clean foam cuts.

    I see people use nichrome wire quite a lot but there is a better wire for foam cutting if smooth surfaces are needed. Nichrome has an inner specific resistance of 1.08 Ohm while Kanthal wire has 1.45 Ohm per mm2 per meter of length. The composition of nichrome wire is nickel and chrome while the Kanthal wire is iron, chrome, aluminum and cobalt. The aluminum part in the Kanthal wire allows to have smoother cuts because molten foam will not stick to the wire surface. This wire is less strong than the nichrome wire but it is intended for heating so has a better resistance to heat. Thus a Kanthal wire if the calculated resistance and voltage are met will almost never break and will cut smoothy. The thinner the wire the more resistance it has, so an optimum is to balance between resistance and voltage to get the right wire temperature. Tensioning the wire (be it nichrome or kanthal) impacts the thin structure of the walls and adds resistance to the wire. Basically, the higher the resistance the electricity 'does not want to go through' so you have to increase the voltage. As you increase the voltage in a high resistance wire the heat goes up. With a good Kanthal wire and a variable voltage power source you could make a foam cutter which could set an ideal temperature for the foam you intend to cut.

    The 0.5mm kanthal wire has an impedance of 6 Ohm while the 0.6mm wire has 0.4 Ohm. So you see the thinner you get the more voltage you will need. When tensioning with a spring the 0.6mm wire extends with the heat and it's resistance increases almost to the one of the 0.5mm wire. So, every spring that is put in a system might influence the heat of the foam cutter wire temperature and has to be taken into consideration.
    I have been using 120V line power and a dimmer switch for a long time with .032 stainless safety wire. Cuts beautifully and you don't have to worry about putting tension on it. I bought a pound spool of the stuff like 20 years ago and still haven't gone through it.
    Jay K.

    VT USA

  2. #32
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    Re: hot wire cutting foam

    Quote Originally Posted by Jay Kempf View Post
    I have been using 120V line power and a dimmer switch for a long time with .032 stainless safety wire. Cuts beautifully and you don't have to worry about putting tension on it. I bought a pound spool of the stuff like 20 years ago and still haven't gone through it.
    As long as it works and you are happy&safe with it, it's good. However, bear in mind that a dimmer switch does not turn the voltage down but just chops the wave leaving the peak voltage at 120V. Touching the wire when grounded could lead to an electric shock which is risky if you have someone assisting you in cutting long foam wing profiles.

  3. #33
    Registered User Pops's Avatar
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    Re: hot wire cutting foam

    Started cutting foam model airplane wings in 1970 when someone gave me a Variac. Make the bow, and use aircraft .032 stainless safety wire. I usually run the Variac around 20-24 volts. Also for small parts, its nice to make a small hot wire table with a rip fence.
    Pops

  4. #34
    Registered User Jay Kempf's Avatar
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    Re: hot wire cutting foam

    Quote Originally Posted by DangerZone View Post
    As long as it works and you are happy&safe with it, it's good. However, bear in mind that a dimmer switch does not turn the voltage down but just chops the wave leaving the peak voltage at 120V. Touching the wire when grounded could lead to an electric shock which is risky if you have someone assisting you in cutting long foam wing profiles.
    To clarify that I have a transformer for output with the dimmer on the input. The kit used to come from Aircraft Spruce for cheap like $30. The output side is safe and I only use it at 25% scale most times for cutting slow and clean.
    Jay K.

    VT USA

  5. #35
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    Re: hot wire cutting foam

    Quote Originally Posted by Pops View Post
    Started cutting foam model airplane wings in 1970 when someone gave me a Variac. Make the bow, and use aircraft .032 stainless safety wire. I usually run the Variac around 20-24 volts. Also for small parts, its nice to make a small hot wire table with a rip fence.
    Such voltage from a variac/transformer is quite safe. For small parts a soldering iron can also be used with a copper wire in it, copper is conducting heat well. For example, to make intake manifolds from carbon fibers the foam pipes can be formed with a copper loop rounded around a pipe of the diameter wanted. After the carbon and epoxy have bonded the foam is flushed out with gasoline.

  6. #36
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    Re: hot wire cutting foam

    Quote Originally Posted by Jay Kempf View Post
    To clarify that I have a transformer for output with the dimmer on the input. The kit used to come from Aircraft Spruce for cheap like $30. The output side is safe and I only use it at 25% scale most times for cutting slow and clean.
    Yeah, that makes more sense. Today there's lot of transformers that can be used from old laptops, battery chargers, most in the range from 12V to 24V. Even the wattage does not have to be high if the proper wire is used and the right temperature achieved.

  7. #37
    Registered User cavelamb's Avatar
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    Re: hot wire cutting foam

    Quote Originally Posted by DangerZone View Post
    Such voltage from a variac/transformer is quite safe. For small parts a soldering iron can also be used with a copper wire in it, copper is conducting heat well. For example, to make intake manifolds from carbon fibers the foam pipes can be formed with a copper loop rounded around a pipe of the diameter wanted. After the carbon and epoxy have bonded the foam is flushed out with gasoline.
    Sounds like fun.
    Until you see how rough the inside surfaces turn out.
    Which would not be a really great feature in an intake manifold!

  8. #38
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    Re: hot wire cutting foam

    Quote Originally Posted by cavelamb View Post
    Sounds like fun.
    Until you see how rough the inside surfaces turn out.
    Which would not be a really great feature in an intake manifold!
    If smoother surfaces would be needed on the inside, another layer could be inserted on the inside with peel ply around and a balloon/condom inserted inside and then blown. The tension of the balloon presses on the peel ply and this one on the fibers, the ends should be blocked so any excess epoxy would go out the upper side and not drip down by gravity. The walls would be as smooth as the usual peel-ply-ed surfaces.

    Now, that is some fun... Don't ask me how I cam to that idea.

  9. #39
    Registered User Pops's Avatar
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    Re: hot wire cutting foam

    Could always wrap the foam with Saran Wrap and lay up over top and then melt the foam out and pull the Saran Wrap out for a smooth surface.
    Pops

  10. #40
    Registered User cavelamb's Avatar
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    Re: hot wire cutting foam

    Quote Originally Posted by Pops View Post
    Could always wrap the foam with Saran Wrap and lay up over top and then melt the foam out and pull the Saran Wrap out for a smooth surface.
    Actually Saran Wrap will react some and you will get a bunch of wrinkles.

    I've never found a workable way to make a smooth inner surface with a foam core.

  11. #41
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    Re: hot wire cutting foam

    Quote Originally Posted by cavelamb View Post
    Actually Saran Wrap will react some and you will get a bunch of wrinkles.

    I've never found a workable way to make a smooth inner surface with a foam core.
    Making Re-Usable Molds wtih CNC Machined Foam and Stretchelon Bagging Film
    Aude somniare

  12. #42
    Registered User Pops's Avatar
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    Re: hot wire cutting foam

    If you cut the Saran wrap in thin strips and wrap the foam core like wraping with tape the only thing that I ever had was the small steps the thickness of the Saran Wrap. If you want it perfect, thats something else, but it will be a lot better than what you get with the rough raw foam for the inside wall of an intake manifold. If the very light steps from the thickness bothers you, just take a small drum sander on a shalf and smooth the inside to suit.
    I just built a fiberglass tube from my airbox to the carb on the engine of my single seat Cub using this method.

    Quote Originally Posted by cavelamb View Post
    Actually Saran Wrap will react some and you will get a bunch of wrinkles.

    I've never found a workable way to make a smooth inner surface with a foam core.
    Pops

  13. #43
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    Re: hot wire cutting foam

    Quote Originally Posted by cavelamb View Post
    Actually Saran Wrap will react some and you will get a bunch of wrinkles.

    I've never found a workable way to make a smooth inner surface with a foam core.
    Not even with peel ply, plastic isolation tape and wax..? What kind of inner surfaces did you have to do?

  14. #44
    Registered User Hugh Lorimer's Avatar
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    Re: hot wire cutting foam

    hot wire cutting foam-001.jpghot wire cutting foam-002.jpghot wire cutting foam-003.jpghot wire cutting foam-004.jpghot wire cutting foam-005.jpgSome photos of my hot wire bow. Wire is nichrome3,collets help prevent sticking on cutting templates. Spring maintains tension
    Hughie.

  15. #45
    Registered User cheapracer's Avatar
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    Re: hot wire cutting foam

    Quote Originally Posted by oriol View Post

    Most Folks seem to do it with a current car battery.
    I have always used a car battery but I always run jumper leads a few feet away from the battery and connect to the end of those leads. Reason being is there is a genuine danger of a battery exploding from sparks at the terminal posts igniting gases from the cap vents. It is rare but does seriously hurt people on occasion.

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