Toughest non-metal substitute for steel?

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Ambient temperature. One of the uses is for holding an antenna together.

The *weakest* thing I can think of is JB-Weld. I don't know how they say "strong as steel" on the same side of the package it shows "2000PSI". Steel is 30,000PSI *yield*.

The strongest low melting alloy by far would be Zamak 27. Just take a bunch of US pennies, melt, strain out the older copper ones (or sort beforehand,

1983 and newer are zinc) and add 27% aluminum by weight. 30-60ksi (1ksi = 1,000PSI) tensile strength, but almost no elongation (1-3%, although supposedly similar alloys can be hot forged). Melting point is 900°F, so can be done on the stove if you have a substantial enough burner. The aluminum will be a bit more stubborn so you might want to do this on your grill, which I'm sure your landlord has no reservations about.

As for castable plastics or composities, I wouldn't know (I prefer to melt shiny things ;). Anyone?

Tim

-- "California is the breakfast state: fruits, nuts and flakes." Website:

you got to find something that has carbon fibers in it, or that you can add fibers to it if you want to get more strength out of it.

John

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

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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.

Actually I want to make three parts:

1. A mechanical connector to hold antenna elements (alum. tubes).
2. Duplicate a metal lock key, for which blanks are no longer available.
3. Corner joints for an aluminum backpack frame.

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. 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:

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.

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.

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?

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.

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.

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.

Urethane comes in a huge range of formulations and it has some great properties. However, fiber-filled epoxy is stronger than fiber-filled polyurethane.

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.

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

For this I like the idea of fiberglass tape and resin that someone else suggested. That would be easy and plenty strong.

What kind of key is this? Most keys are available if you know where to look. (I am a locksmith)

For this check out the local hardware stores plumbing section. Copper pipe fittings are available in several sizes and if the fit is a little sloppy you could make a shim out of a soda can and epoxy the whole thing together. No need to re-invent the wheel.

Incompatible with tin-cure RTV. Platinum-cure RTV is OK.

is about spincasting, but they have a range of supplies and materials including Zamak and castable urethanes.

Another source of plastic casting materials is

They're primarily into special effects for movies.

More yet at

and Polytek's print catalog is almost a "how to" manual.

"White metal" (which Zamak is) has gotten a bad rep in consumer products because product designers used an absolute minimum of material to keep cost down. It's actually pretty good stuff. See

for some notes on its properties, and favorable comparison to aluminum, magnesium and plastics. A couple of these alloys have higher tensile strength than aluminum and considerably higher yield strength and shear strength.

Scrapyard test for zinc alloy: get some copper sulfate at a garden store and dissolve some in water. A drop of that on Zamak will turn dark, while aluminum won't change color.

You can sand-cast "white metal' and of course you can use the lost-wax process aka investment casting.