Lost foam casting
Other pages deal with my home-made furnace and assembling the foam pattern for a handwheel. This page shows the exciting part - the casting process
The tools for handling the molten metal are shown here. From left to right are the pouring ring, the drossing spoon, the crucible pliers and the stirring rod. The pouring ring was bent up from 6 mm hot rolled steel rod. It should loosely fit around the crucible but when the handles are squeezed together then it should securely grip it. The drossing spoon is an old dessert spoon with an length of steel strip bolted on to extend it. It is used to remove the dross from the surface of the molten metal before pouring.
The crucible pliers are an old pair of pliers with the handles extended by brazing on some suitable sized tube. They are used to lift the crucible from the furnace. The stirring rod is just an old long handle screw driver. It is used to stir the melt and to stir in flux.
The crucible pliers are an old pair of pliers with the handles extended by brazing on some suitable sized tube. They are used to lift the crucible from the furnace. The stirring rod is just an old long handle screw driver. It is used to stir the melt and to stir in flux.
This shows the foam pattern prepared and ready for casting.
The foam pattern has been buried in very dry, sifted builders' sand, with the polystyrene foam sprues projecting above the surface. The sand has been shaken down by tapping the sides of the container and bumping it on the floor. A tin can, open at both ends, has been pushed a third of its length into the sand, enclosing the sprues. The can will act as a reservoir to contain the molten alumiinium as it burns its way down to fill the pattern of the handwheel. To the right is the pouring ring which is resting on two pieces of insulating blocks ready to receive the crucible. The baking tray is used to mould any surplus melt into cakes, ready for next time.
After 20 minutes or so of heat the lid has been removed from the furnace. The crucible - a cheap stainless steel container - is glowing a dull red. Before the crucible is removed from the furnace the drossing spoon is used to to scrape the dross from the top of the molten metal. The dross is dumped onto the metal tray below the furnace. Sometimes, if the melt is very dirty I will add a flux at this stage which helps tp collect the dross together. I use low sodium salt (LoSalt) from the supermarket as a flux.
After skimming off the dross the crucible has been lifted out of the furnace with the crucible pliers and placed in the pouring ring.
The pour - note the essential welders gauntlets and leather apron, and I am also sporting stout boots and a full face visor. The pour should be slow and continuous. Any interruption to the flow of aluminium into the mould may result in the pattern collapsing. A head of molten aluminium is contained by the can, so it will quickly burn down the sprues and fill the pattern proper. Good ventilation is essential, in case the weather turned bad, this casting was made in the garage with the door open, but out in the open air is preferable. Note the flames as the polystyrene burns away.
After 10 minutes the casting can be pulled out of the sand and dunked in water to cool it off. The tin can has been removed from header.
The sprues have been removed using a hacksaw.
This shows the boss side of the casting after machining. It was then reversed in the chuck and gripped by the centre boss for machining the the other face and edge.
The finished hand wheel after machining.
I use scrap aluminium for all my castings. This comes from a variety of sources including things such as scrap alumium tubing from garden furniture, old carpet edging strips, old aluminium guttering, old scrapped tools, scrapped lawn mower engines etc. The list is limitless. I always melt these down and use flux as mentioned earlier to clean the metal. I then cast this into a tart baking tin to make small ingots. I keep a stock of these always available. At the end of a casting session I always melt down a crucible of scrap material and cast these small ingots in order to replenish my stock.
I have also made castings using zinc diecasting alloys. Scrap zinc diecasings are not so readily available as scrap aluminium. These zinc alloys have a lower melting point than aluminium but are almost as strong as mild steel. An old tin can can be used as a crucible for zinc alloys because of the lower temperatures involved. If these alloys become too hot in the furnace they can fume badly ( zinc is volatile and burns readily to zinc oxide fume). Inhalation of zinc fume should be avoided.
I have also made castings using zinc diecasting alloys. Scrap zinc diecasings are not so readily available as scrap aluminium. These zinc alloys have a lower melting point than aluminium but are almost as strong as mild steel. An old tin can can be used as a crucible for zinc alloys because of the lower temperatures involved. If these alloys become too hot in the furnace they can fume badly ( zinc is volatile and burns readily to zinc oxide fume). Inhalation of zinc fume should be avoided.