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, dry!! 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).]
Great read. sounds like quite a project. I have to ask though Alan, can you explain 'tin-pest'?
I built a Christmas display for a building owner a few years a ago and it required guy wires to hold a frame to a rooftop. I used #14 STR stripped the ends, wrapped them around an eyelet, fluxed and soldered. I used regular plumbing solder, not sure of the %'s though. Now I'm wondering if this was a bad idea because Christmas season is well below 55 F.
#71123 - 10/23/0606:18 PMRe: Casting metals in your back yard.
jps; 'Tin-pest' is where the pure tin metal literally crumbles to powder. At or below 55F, the metal can very slowly change from a bright metal phase to a dull gray one, and it is reputed that once started, it rots into the rest of the piece like a canker. Remelting removes the problem, for instance if it occurs in ingots- not much help in an art piece! Very small additions [ say 1 or 2%] of other metals like lead, silver or antimony prevent the 'disease', so lead-bearing solders are not at risk. The 0.5% copper in my alloy may be enough- I don't know- but 'Castin' is definitely not affected. Any lead bearing tin solder is a good choice for outside work; it goes grey with age but will last for centuries.
Joe; Yes,this is a hood ornament, it just says 'RENAULT' in a version of the Company 'diamond' logo. The originals were clear plastic, probably perspex; [these tractors were made in the fifties]. They painted part of the inside surface with a block color, and part of it was 'chromed', so that it looked like a metal badge with enamelled relief. They all fell off as the puny 1/16" diameter cast-plastic rivets snapped! The block colors, BTW, yellow/blue/magenta/green denoted the engines fitted, choice of 4 basic models from 16 thru 35hp. And yes, now that I will have an RTV mould, I can try a casting in clear resin to better mimic the originals.
Wood work but can't!
#71125 - 10/24/0607:19 AMRe: Casting metals in your back yard.
I second the emotion for lindsay books, have quite a library of their titles. Get Castain's two casting books and the reprinted US Navy foundry manual.
I have a small foundry setup, can melt bronze and brass and have made belt buckles, 3" long anvil paperweights and brass bobeches for a chandelier I built and other similar sized items. Great fun, you could cast the hotter metals easily now that you understand the complexities of mould and pattern making. I started out with pewter, then moved on to brass & bronxe. My source of metal is scrap valves and other plumbing stuff and I usually throw in a handful of copper wire to deepen the color. My next project is to whittle some patterns for some double and triple barrell lugs because the local supply houses only carry aluminum units. I still have little experience in making cores and cored patterns and the lug project is a good place to start. I have a friend who recently cast a bronze 11 foot diameter sphere, fabricated of 24 x 30 inch plates that were welded together. As patterns he used rubber stamp technology to produce a prayer for peace in about 22 languages. The whole unit is assembled as a fountain, the sphere will turn by hand. It is a pretty awesome sight to behold, took tons of silicon bronze and a small crew of smoke crazed metal heads.
[This message has been edited by Almost Fried (edited 10-24-2006).]
#71127 - 11/10/0610:35 AMRe: Casting metals in your back yard.
6. Problem: Air-bubbles in RTV moulds. The uncured RTV had such a high viscosity, it needed a 29”mercury vacuum to de-gas it. My hand-pump only made 7”, so the fine detail [‘RÉGIE NATIONALE’] is a separate centrifuged graven plaque, making a 2-part badge assembly. By careful mold design, mixing and pouring, bubbles can be minimised or kept off important surfaces. Spincasting mould pattern laid up ready for pouring the RTV. Engraved master in brass. Note ‘dummy’ casting opposite for balance and the ‘dirt pads’ -this is an old Alfred Herbert trick- dross and air are driven through the impression into a dead-end for a better casting.
7. Spin-casting mold. I hand-cut the runners with a sharp blade, it’s easier than trying to mould them.
8. Spincasting die, assembled and ready for mounting on the machine. I dusted molds with talc; fine graphite powder is supposed to be superior. I raided the bathroom. The discs are ‘Duralumin’, 4% copper aluminum-alloy, as strong as steel.
9. Home made spincasting machine. 40 yr. old 230v 1Ø 1480rpm 4-pole motor, set vertical in oak frame. Die fixes with a set-screw to the motor shaft. Note: motor is open-framed, hence the aluminum slot cover. Cooling is not a problem; the motor only runs short bursts and has virtually no load. Gives about 18 psi hydraulic pressure on metal at a 1” radius, equal to about a 4 ft head of metal.
10. Machine fitted with ‘coffee can’ guard and running at speed. Concentricity is vital, so accurate RTV mold and die parts were turned on the lathe. Guard needed to be 1” higher than the header: Molten metal can eject up as well as out!
[This message has been edited by pauluk (edited 11-10-2006).]
Wood work but can't!
#71129 - 11/10/0610:39 AMRe: Casting metals in your back yard.
11. Pouring alloy, main casting. 99.9% tin, 0.5% copper at 511°F. TheRTV,[ ‘Rhodia’ 3255, there are other brands ], has a limit of 572°F. -Hence the need for accurate temperature control to conserve mold life.
12. Infra red thermometer measures melt temperature.
13. Problem: IR thermometer cannot ‘see’ liquid metal due to it’s low emissivity. My solution was to float some chunks of graphite on the melt and measure their temperature.
14. Die showing hollow sprue. Anything over 32 grams [1.13 oz] was ejected! -Hence need for higher guard! In future I need to make rotating headers with a smaller hole at top and with a larger recess under, to allow more metal to be poured and retained as the sprue.
15. Die opened showing castings in impressions. Zero flash, no leaks and pads filled.
16. Plaque / balancer / pads / runners assembly, on hollow sprue. ‘Regie Nationale’. Reproduction is astounding- it cast my thumbprint left in the release agent!
[This message has been edited by pauluk (edited 11-10-2006).]
Wood work but can't!
#71130 - 11/10/0610:44 AMRe: Casting metals in your back yard.