Two-material meltdowns

I was thinking about ways to design a two-material RepRap (seems like a fashionable subject around here just now ) and it seems to me that even if you were able to extrude two materials, aluminum and HDPE, for example, the wildly different melting points (130* C and 660* C respectively) would lead to problems. When depositing the aluminum its temperature will be WAY higher than the melting point of the HDPE in the layer beneath it. Even if the model was kept frozen (a significant task considering it will be basically touching a very hot extruder) the heat from the extruder's barrel would conduct through the already-laid aluminum, considering how heat-conductive aluminum is, and melt down the HDPE around it just the same, correct?

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-Kyle

i dont think anyone was talking about extruding aluminum directly. i think i made a comment about it in passing... but it was describing a process where you use reprap to print the object in wax, then use that wax object in a process called 'lost wax casting' where you pour molten metal (aluminum) into a box of (fancy) sand that contains the molt. it melts the wax and takes the shape of it.

another way would be to use some sort of ceramic composite material that comes out as a liquid and when it solidifies, its strong and can withstand high temps.

although if we ever do get to the stage where we're extruding aluminum directly, it would probably be a separate step. ie: print plastic parts. then print aluminum parts, then finally assemble them when they're both cooled.

I've successfully extruded common 60/40 Sn/Pb solder onto HDPE. It etches its way into the HDPE quite uniformly.

It's good to remember that plastics have a much higher specific heat than metals, anywhere from 4-8 times depending on the polymer/alloy combination. What that means is that if you extrude metal onto a polymer the polymer tends to soak up a lot of the molten metal's energy and it solidifies almost instantly.

Mind, I'm not making a case that you should be pouring molten aluminum onto HDPE, but there are any number of eutectic metal alloys that you can quite successfully.

Ah, that makes sense. Those are both good solutions, and now that I think about it pretty much everything involving two materials that can be fabricated WITHOUT a RepRap can be made WITH one using one of those techniques.

Anyone else see advertisements for that soldering gun that cools down as quickly as it heats up? I'm guessing it'd be based on a thermocouple, myself. If so, a ring of thermocouples, with the hot "side" facing in, and the cold "side" facing out, could allow you to selectively apply heat while ensuring the ambient temperature isn't raised.

Of course, now I'm talking fusing two dissimilar metals, and applying a complex shape to them.

Dealing with two materials where the higher melt point material MUST be laid second is one of two hurdles I can see for fabbing just about anything (made of common materials.) The other is moving parts.

Of course, a friend of Richard Feynman's discovered how to electroplate plastics back before WWII, and it involved chemistry, not heat.

Better read up on the cold heat, my son used one and said it didn't work very well. I read up on it on the internet and the reviews are scathing. Not easy to use and the "heat" comes from running a current through the material to be soldered.

bartlee45 Wrote:
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> Better read up on the cold heat, my son used one
> and said it didn't work very well. I read up on it
> on the internet and the reviews are scathing. Not
> easy to use and the "heat" comes from running a
> current through the material to be soldered.


Its true that Cold Heat has a bad reputation, and a well deserved one. I got one as a present a few years ago, and found that it has no real benefits over a conventional soldering iron, not to mention that the tip is extremely brittle . Its been sitting in the back of my workshop gathering dust since then. But the concept behind it is actually pretty cool, IMHO. Isn't that how a lot of welders work, heating the wire by running high voltage through it? The idea itself is very nifty, the issue was just in their implementation. I definitely wouldn't discount anything this early in the game.

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-Kyle