Offshoot of Mark [e57]'s 'welding augers' topic.
I’m trying to cast a bonnet badge in a low melt point tin/copper alloy for my old tractor. The originals, for the Renault’s from ‘56 thru ‘61, were made of crummy Perspex, so being dross to start with, they mostly snapped off, got mangled into the turnips and can’t be got for love nor money! Beats me why manufacturers ruin good products with crappy finishing touches just to save a few centimes.
I was prompted to try making a ‘pastiche’ metal badge [ it’s about 8” wide and 3” high] after a visit with my grandchildren to a Bell Foundry not far from here, where the medieval technology still reigns. When I tell you that the [working] overhead gantry was made entirely of oak, you’ll get the picture! Bell bronze is about 80% tin; 20% copper, and melts at around 1700°F. They were actually quite cagey at the foundry about the exact details, [ in case I was a spy from Bow or Ohio?! ]. They were using a mix of techniques, casting into plaster/clay/goat droppings [!] molds, with the lost wax process for lettering and décor appliqué. 1700°F is a bit hot for me as yet, so I plumped for a low melting point lead-free solder, [ 99.5% tin, 0.5% copper, MP 441°F and close to the eutectic ], as it’s a stock UK plumbers’ item. I’d prefer American Iron & Metal’s [AIM] ‘Castin ®’ solder alloy, quatenary eutectic [ 96.2% tin 2.5% silver, 0.8% copper and 0.5% antimony ] - its MP is lower at 422°F, plus, it’s immune to ‘tin-pest’, a form of rot which can attack pure tin items from below about 55°F. But I can’t find any! I can get Sn/Cu badges plated in chrome, or even gold for her 50th jubilee perhaps.
[ Lead is bad news for plating baths, BTW]
I started out intending to make the badge using the lost-wax process, where a wax pattern is buried in a ceramic jacket, the wax subsequently melted out to leave a mold cavity. I duly built a badge model in lead sheet, epoxy putty etc., using pics from the web to get the size & proportions right; and my existing tractor hood for the contours. From this I made a split mold from clear silicone sealant, glass-fibre etc., in order to cast the wax masters. I'm using pure beeswax BTW. As this part of the project neared completion, I discovered high temperature RTV silicone elastomer! Hurrah! I could now build a proper gravity die-casting mold, go into mass production and lose all that long-winded medieval goats’ droppings goo investment fandango used by the Bell founders. The special RTV [ Rhodia 3255 in France ] is good for 570°F in short bursts. I’m currently at the RTV mold stage- ready to cast the silicone rubber die halves, after taking a delivery of a couple of pounds of RTV from Marseilles. I’m using a wax master here, as I found you can carve and engrave the pattern easily to improve detail and get good draft angles. I’m waiting on two items in the post from England- an electronic scale for accurate catalyst dosing, and a laser infrared thermometer for accurate casting temperature - overheated molten metal shortens rubber mold life dramatically. I’ll post more detail when I have some metal badges to show, but basically I’m going all out now for a full ‘vacuum-cast’ piece!
As to general casting -
Aluminum cast alloys [ easy to get hold of scrap castings, but avoid sheet materials, spec. not suitable ] melt around 1200°F. Sand casting is well within the amateurs’ scope with a little gas furnace and if you can find a friendly foundry, the proper sand is cheap for making up the cope and drag. You need patterns of course. One proviso - molten aluminum readily dissolves iron so you need a proper ceramic/plumbago crucible, but you can transfer crucible to mould box with an iron ladle. Items can be lost-wax cast in 'plaster of paris' but it must be bone, ie in an oven,
BTW: There is plenty of detail on the net on ‘how to’ 'DON'TS!' and ‘risk avoidance’. Play safe!
Can’t be easily plated, but can be permanently dyed, anodised or acid-etched.
Zinc base alloys, like Mazak, melt around 693°F, and exist a-plenty in old scrapped auto parts etc. This is quite a strong material, and it is very fluid when molten so it will reproduce fine details from the pattern. Plating by pro’s only. Best painted or left as cast. Sacrificial boat parts a possibility?
Lead is toxic, weak & it creeps; apart from fishing weights, best avoided IMHO. MP 621°F
Copper, MP 1980°F. It is castable, but absorbs furnace gasses and distorts, but it can be cold forged and filed to final form. You will need a bit more ooomph than a gas furnace- probably charcoal and a blower, and casting has to be poured out of the crucible. Plateable, but why bother?
Brasses vary enormously in alloying elements, usually copper-zinc, but many commercial items contain lead to ease machining, so be aware. The MP is about 1650°F.
Iron ladles slowly dissolve [check] as per aluminum, you need a ceramic melt crucible.
As a final note, Google “David Reid melting metals in a domestic microwave oven” and make your wife’s day!!!!
Hey, don’t shoot me, I’m only the piano player!
[This message has been edited by Alan Belson (edited 10-23-2006).]