Cast Aluminum Handle?

ddavelarsen

ddavelarsen

Joined
Dec 7, 2000
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Cripes, I don't know why I accept these "challenges." :rolleyes: Just can't say "No!" So here I am with a commission to replicate a Gerber Mark II with a few tweaks. One non-tweak however is the cast aluminum handle.

Anyone know how I might approach this? I haven't a clue, honestly. For reminders, the Mark II handle had a rough finish that was, I think, steel particles blown onto the handle and afixed by some sort of technological process that might as well be magic from where I'm sitting. I don't have any ideas how to replicate that except by casting the handle of the knife I have as an example and hoping the mold takes a good enough impression.

All that aside though, I still don't really know how to go about casting the thing in the first place. What should I use for a mold? How do I keep the halves of the mold separate when I make it? And on and on.

This is not a project I have to finish immediately so I have some time to experiment to get it right. I'd really appreciate any and all ideas, links, reprimands for taking on such a project, etc.

Thanks and wish me luck,
 
I've used plaster to cast silver before, and it melts at a higher temp. than aluminum. One thing that helps is to let the plaster harden, then before you cast, cook the mold close to the melting temp. to drive the moisture out and make sure that there isn't a lot of distortion from the liquid aluminum.

To make your mold, you could start with a wooden box that's open on two opposite sides with inside dimensions that match the final size of the mold. I think you should probably end up with at least an inch of plaster all the way around the part. You'll also need a slot about halfway up one side to stick the blade of the Gerber through.

Place the box on a flat surface with one of the open sides down and tape it down around the edges with duct tape. Get some children's modeling clay (I'm serious :D. this is the way the patternmaker where I work does small molds) and fill the box a little less than halfway with it. Then the fun starts.

You need to get the original knife in there (you might put it in before starting with the clay) and embed it half in the clay. Pay attention to the shape of the handle; you don't want any undercuts. Ideally, you'll shore the clay up around the handle so that it's on the same plane as the tang.

There are a bunch of mold parting compounds out there, which you could try, but most of them are just aerosols of dissolved wax. If you can come up with a clever way to apply a thin coat of wax to the knife, clay, and exposed interior of the box then do that and you can avoid using some fancy (expensive) chemical. Thick oil might work, too, but you really want to be able to get most of that combustible stuff out of the plaster before pouring metal into it.

Okay, now you mix some plaster and fill the box up the rest of the way with it. Let it harden over night. That's the first half of your mold.

Next day, pull the box, plaster, clay, and all up off the flat thing you had it on before. Turn it over and remove the clay as completely as you can. Hit everything on this side with your parting compound, then mix some more plaster and fill this side. Then more waiting until it hardens.

After all that, take the box apart and you have your two mold parts! You can sculpt a sprue with a funneled top in the side, then wire them together and go to town.

When I did this before, it was with lost wax. After casting, we just chucked the hot mold in a bucket of water where it disintegrated, leaving the casting.

More later, including pics if you need them, when I get home from work.

-Allin
 
would spray PAM work for the release agent? Spray PAM is to keep things from sticking to cooking pots and pans.
 
I used to work in aluminium foundry (the father of a friend of mine owns it). Unique pieces are always cast in sand.I've seen two types of snad molds used:

1: silica (quartz) sand glued together by some ungodly oozing smelly goo that looked like honey and dried up when exposted to CO2 (and a variation on this with heat to stifften the "glue")

2: that same silica sand but without any glue, it's just thumped together as castle in the sand (you know, the stuff kids make on beaches).

IMHO the latter would be more useful for you because silica sand is easy to come by and all you need to hold it together is some water (it must be just slightly moist, not damp). If youwant the casting to be useful "out of the box" you'll also need a split mold and 2 frames to hold the sand together in the process. It's hard to explain via internet, it would be far easier to simply show you or just do it :D

Now that i think about it ... way better option would be a solid cast aluminium block (which can be purchased at any color metal store), machined to the desired final shape. This way you wouldn't have to worry about pores in material, composition of material, heat and cooling of the casting, making the mold (die of the handle), the frames, etc.
 
How big a piece do you need cast? I have plenty of aluminum here, and can sand cast anything you wish up to about 1'x6"x3". if you have a handle or something the correct shape I can cast it for you.

Sand casting will take a bit of fireclay to mix up with that silica sand as a binder. it is not hard, but if the sand is too wet you can have an explosive result. Nothing like having 1500 degree sand and aluminum coming at you like a swarm of pissed off bees!

Send me an e and I can step you through the process, or we can talk about getting the work done.

Doc
 
I second that Doc - that, or a block of material exploding (aluminium shrapnels and chunk flying everywhere) if not dried completely before it is melted, these things can really ruin one's day. A tin droplet of water can amount to enough steam to blast a couple of kilos (or pounds) worth of material.
 
The only thing I cannot do with the setup I have is harden it. There is a heat treater here who does mold work for a couple of the plastics companies here, but should be unnecessary for the purpose you have for the material. I generally tend to leave the casting in the sand for a couple of hours so they get sufficiently hard for most uses.

I can get it t-6'ed if you like here, but doubt it will be necessary...

Doc
 
Holy Caramba! Man you guys never cease to amaze me. There's more knowledge about arcane stuff floating around here than anywhere else on earth, I swear. :D You've given me a lot to think about so I'm gonna do that while I check the rest of my mail, then I'll be back...

And Thanks!
 
Good links, Howie, thanks! (I just had to find out what the heck Cuttlebone casting was... Not applicable but cool anyhow, and I'm gonna have to try that some time.)

Because I want to get as much detail of the rough finish of the original handle as I can, I'm going to try the plaster method first. But that very detail may prove disasterous to the mold... Just have to try it and see.

I don't know how much detail sand would get, can you give me an idea Doc? I tried that about thirty-something years ago in high school shop making a name plate, and it truly sucked. Had nothing to do with the operator either. Same doofus tried to teach me how to weld, and you can guess where that got us. But I was a screwdriver forgin fool! My mom used those screwdrivers for the rest of her life, and I'm not kidding. :)

Thanks for the help and ideas gents, and feel free to keep em coming. In return I'll relate the least embarrassing of my results.
 
I have an old Gerber skinning knife which had the aluminum handle but I took that off and replaced it with wood !! It was a non skid surface but I think it was some type of epoxy with perhaps sand in it .An old pistol shooters trick for smooth grips was to varnish the grips and while the varnish was still wet sprinkle with fine sand that does work.
 
You can mold the knife and handle without ruining the temper by first making the mold. Then make a wax form of the handle and using that to make a second mold from plaster. Put a small drain in the bottom pf the second mold.

Heat it until the wax is all gone. Put your blade in and pour the aluminum in around the tang. IT helps fill out the mold to heat it (A lot)

Chip the plaster off after it has cooled. It will come right off because the wax acts as a release agent.

Use Brownells Baking lacquer on the molded handle.
 
peter nap said:
You can mold the knife and handle without ruining the temper by first making the mold. Then make a wax form of the handle and using that to make a second mold from plaster. Put a small drain in the bottom pf the second mold.

Heat it until the wax is all gone. Put your blade in and pour the aluminum in around the tang. IT helps fill out the mold to heat it (A lot)

Chip the plaster off after it has cooled. It will come right off because the wax acts as a release agent.

Use Brownells Baking lacquer on the molded handle.
Click to expand...

yes look up lost wax molding
and use investment plaster for your detail..

ck this place out too Dave...
http://www.artmetal.com/w-agora/index.php?bn=techtalk_casting

for supplys you can use
http://www.budgetcastingsupply.com/
http://www.riogrande.com/
 
I agree with faramir and DOC, think sand casting is better, that's how I learned it High School (yes my High School had its own foundry amongst other things).

You could also use a variation of the lost wax casting method. Work wax onto the face of the knife into every crevice you want to mold. Gentily cut the wax into 2 pieces and separate from the handle; you now have a left and right sided negative impression. Coat them with cooking spray and stick them back together. Pour another melted wax, which melts at a lower temperature than the first, into the joined oiled impressions, and remove that when it cools, you now have a solid positive that resembles the original handle. Use that positive to create a 2-piece plaster mold leaving 2 holes to the top -1 a vent the other pour hole. Once the plaster has dried completely, place that plaster mold (wax included) in a kiln and burn out the wax, now you have a reusable plaster mold. Pour in the aluminum, let it cool, separate and clean up the final piece.

Note:
1. Aluimium shrinks a bit, so you'll have to make the mold a bit bigger than the final piece. You' might want to get a "shrink rule" and find out how much it shrinks - I have forgotten. It looks lile a regular scale but has been enlarged proportionally to compensate for the shrinkage.
2. You should always place a vent hole somewhere in the mold parallel to the pour hole so air can escape thru that hole while the aluminum goes in the other or you could easily trap bubbles - think of the sprue-tree the parts come on from building plastic model kits.
3. Be extremely careful pouring the molten aluminum, it WILL burn through shoes or work boots very easily, pours and splashes like water, and sticks like honey.
 
I've never used plaster (even when casting unique pieces they were simply too big for plaster to be practical on industrtial scale) but the sand with tht goo of a glue that hardens when exposed to CO2 was pretty close to plaster. This being said, small details (like letters) wouldn't show up (or at least not nearly as well as they do when steel die is used). The regular sand (clay + silica) will leave tiny grainy texture and the "glued" sand would too. I doubt plaster would leave *exact* impression of your handle either, if you want that you need a steel die made and possibly (low) pressure casting machine (or whatever these are called in english).

My suggestion: cast in sand and use dremel tool to create whatever finish you want to by hand. Aluminium is really soft (depending on contents, obviously - more magnesium means it's considerably harder and far more brittle, ditto for silicon, plus you'llhave varying degree of copper and iron in it).

Oh and one more thing - if i were you i'd put the mold vertically (so that your handle ends standing up rather than being horizontal) to ensure material pours better into the mold. This way you can locate your venting hole at the front or back of the handle where you can sand it off without ruining the finish of the sides of the handle (if it coems out the way you want it). Since it's a small piece you're dealing with you shouldn't have any problems with temperature (with big pieces precaution must be taken to pour material in fast enough and then seal the holes with some burning powdery stuff which keeps material hot longer so it doesn't "freeze" up, before running to every last part of the mold). The temperature of your material should be ~685 degrees C (1265 deg F) or just slightly less, if it's too cold you'll end up with pores forming (this is usually not desireable but then again if you're trying to end up with odd "finish" pores might be just the thing).
 
I dont know if this will be of any help but I used to Galvanise bridge wire and there are temperatures that some metals prefer to be at so that they stick. IIRC Zinc was 545C or thereabouts.

I don't know if Alu will be the same. As for the mold, rather than PAM I would suggest 'Copper Slip' (UK term) its an Ultra high temp grease used in heavy plant (or at least we used it on CAT stuff) its also known as Anti-Seize. Just found it , it goes to 2000 F http://www.saftlok.com/safteze/antiseiz/copper.htm

The place where I live has loads of casting foundries, using both Alu and Iron based stuff. Everyone here seems to use sand.

Hope this helps some

Oh, another thing if you want to keep the blade cold when pouring so as not to alter the temper. You could use the sleeve methof that plumbers use to freeze water in a pipe without shutting and draining/bleeding the system. I think its just a some gas released onto it to freeze it. It may work with the blade, or work enough to keep the important bits unchanged.
 
This is great! Thanks for all the info and help. I think I'll investigate sand casting too, and see where I might find the sand etc. From all the links you folks have left I should be able to get it done. And just think - I'll have added another process to use in future projects. Benefits of learning just never end do they?

I really appreciate the help and admire all the knowledge you folks are carrying around. It never ceases to blow me away how competent knife makers are at virtually everything. One of these days I'll get caught up! :D
 
Do you have an idea of how much detail you want in the handle? depending on the size sand grain used will determine the "out of mold" detail. The more course the sand the more course the casting will be. I have been experimenting with different grain sizes, and haev found that I can make sand powder if necessary with a homemade mortar and pestel (4" pipe welded to a plate for a bowl, and a 3/4" pipe with a pipe cap on it). I do not waste money on fireclay or refractory either since walmart brand dirt cheap kitty litter is the same thing. I pound it out the same way.

I can give you some hints as to how to get the finish you want with lost wax as well, adn on the cheap. Takes a bit of elbow grease, but not very difficult. I use a two part rtv silicone to make a mold for the wax, and then make my wax model from that. relatively simple to do, and the results can be made to be as you want them depending on what you mix with the plaster.

Can you post pics of what you have in mind?

Doc
 
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