"Bruce W.1" wrote in message
...
Ed Huntress wrote:
"Bruce W.1" wrote in message
news
I need to make some small parts. Ideally they would be made of steel.
And I'd really like to setup a home melting furnace, but my landlord
would not share my enthusiasm.
Low temperature metals are a possibility, like pewter, whitemetal, or
solder. However these aren't very strong.
I can do lost wax or RTV mold making. What's the toughest non-metal
material that I could pour into it?
The toughest thing I can think of is JB-Weld glue. It's just a little
too thick and doesn't flow very well. Epoxy is pretty weak. Then
there's urethane plastic...
Can anyone recommend a tough material for pouring into a gravity mold?
Thanks for your help.
First off, "tough" and "strong" are not the same thing. Among plastics,
Nylon is tough. Epoxy is strong.
JB Weld is a filled epoxy, and not a particularly strong one. Epoxy
filled
with milled carbon fibers will be at least three times as strong, and
many
times as tough. Epoxy filled with milled glass fibers also will be much
stronger and tougher than JB Weld.
However, epoxy filled with milled fibers does not pour well. When it's
molded, it's injected under pressure. There are very short carbon fibers
and
glass fibers that will pour, and their strength potentially could be
almost
as great. But, when you get enough of the fiber into the mix to make a
really strong composite, it's too thick to pour. You're limited to a
lower
percentage of fibers than the ideal, and considerably less strength.
Your application, holding antenna parts together, sounds like it needs
strength rather than toughness. You can get 5,000 psi or so with filled
epoxy. Zinc casting alloys can be six or eight times higher than that.
The
Zamak 27 mentioned by Tim has 55,000 psi yield and 61,000 psi ultimate
tensile strength, but it creeps over time, and you have to get it up
over
800 deg. F or so to melt it.
Solder may be 5,000 psi or so, en-masse. I don't know about the low-temp
alloys, which are bismuth-based. Look up Cerro Metal Products Co. They
probably have specs for Cerrobend and so on, online.
All of these materials can be molded in plaster, and all but the Zamak
can
be molded in silicone; even the Zamak is good for a few shots in a
high-temperature, high-rigidity industrial molding silicone compound.
They
even use it for a few shots of molded aluminum. Watch out molding
polyurethane in silicone. If my memory isn't failing me, they're
incompatible.
But are you sure you have to mold these parts? Is it something you're
doing
in volume? If not, wrapping fiberglass tape, soaked in laminating epoxy
(WEST System is one good brand) will be much stronger than any plastic
part
you can mold, of any plastic you can buy. Pound-for-pound, it will be
stronger than most metals. It will just have to be thicker for a given
strength.
--
Ed Huntress
================================================== ==
Not sure how you define tough and strong. In all cases I would prefer
the material to bend when it fails, rather than snap and break.
Materials that do that usually are not strong, in comparison to strong
materials of the same type. Toughness has two common meanings but in the
materials business, technically, it generally refers to the material's
ability to withstand an impact. What you're talking about is ductility, or
elongation.
It may be helpful to keep these things in mind as you experiment.
Actually I want to make three parts:
1. A mechanical connector to hold antenna elements (alum. tubes).
It depends on the configuration. Are you securing crossed elements, as with
a multi-element beam? Or are you clamping tubular pieces that slip together?
2. Duplicate a metal lock key, for which blanks are no longer available.
That's going to be very tough to do with plastics, although a
high-fiber-density carbon-fiber fabric layup in epoxy may stand a chance.
Unidirectional carbon-fiber roving is stronger in one direction, but it
wouldn't stand the lateral loading on the projections between the notches.
In general, plastics and composites don't have the wear resistance to take a
lot of use as a regular key. And casting Zamak or other capable metal is
going to require precision casting, like jewelers do with their little
spin-casters.
3. Corner joints for an aluminum backpack frame.
Some of those use high-strength plastics. Composites would be good
candidates. Keep in mind that glass is nearly as strong as carbon fiber.
Carbon's big advantage is in stiffness rather that strength.
None of these parts will be made in volume.
I will definitely research Zamak 27.
RTV rubber molds sound easy, but not good for hot metal of course.
The silicone material I was talking about is an industrial material
formulated for making metal molds. It's used extensively in spin-casting
zinc. I've watched a special version of it take seven shots of diecasting
aluminum, but that's working at the edge of performance.
The RTV silicone that comes in caulking tubes at your hardware store is
nowhere near as tough, nor as temperature-tolerant, as the materials I'm
talking about. But it is used some for casting thermoplastics, such as
polyester and epoxy. Again, I don't think it's used with polyurethane.
I've
used fiberglass cloth and Bondo when doing auto body repair. And I'm
reading up on urethane resins. This sounds like a fun kit:
http://www.ares-server.com/Ares/Ares...oduct&ID=82698
Urethane comes in a huge range of formulations and it has some great
properties. However, fiber-filled epoxy is stronger than fiber-filled
polyurethane.
I'm forever making stuff. A fabrication method I've used a lot in the
past is soldering together printed circuit board. Now I'd really like a
way to mold and cast high strength parts, at least as high strength as I
can get them without working with high temperature metals.
If you cast zinc, do it outdoors. Zinc-fume fever is very unpleasant,
although not permanent.
Of course, we don't have to tell you melting about lead or cadmium-bearing
alloys. g
Don't overlook Plaster of Paris as a mold material for thermosets, including
composites and filled plastics. It's dirt cheap, it holds detail and
dimensions well, and it's widely used for this purpose. It's just
complicated to use if you have undercuts. If you do, silicone rubber is far
better, if you can use it, although silicone for moldmaking costs at least
50 times as much. There are special plaster formulations for casting metal,
and there are many "gypsum cements" that are much stronger than plain ol'
Plaster of Paris.
Excuse me if I'm telling you something you already know, but you're going to
be confined to thermosetting plastics for this kind of casting. They happen
to be among the highest-performance plastics in general, but they're also
the most expensive. Besides epoxy and polyurethane, there is vinylester,
polyester, and two or three other ones that are less common. Polyester is
the cheapest and it's strong enough for many applications. That's the resin
in Bondo, although Bondo is an intentionally weak formulation, to make
sanding easier.
Although there are very tough and bendable industrial epoxies, they are
nothing like the stuff you'll find in a hardware store. Polyurethanes can be
brittle but most formulations sold to the public are tougher and more
bendable than epoxies.
There's a lot of room for experimenting in what you're planning to do.
There's also a lot of information about casting thermoset plastics and
making composite moldings on the web. Have fun. I have dabbled in it myself,
and it can be very enjoyable.
--
Ed Huntress