Under the Choko Tree By Nevin Sweeney

Metal Casting in the Backyard – Part 5: Coremaking

By Paul

Coremaking is a sort of “reverse mouldmaking”; instead of filling a cavity in a sand mould with metal, a cavity in a metal (or wooden) mould is filled with sand.

A core can be defined as a body of sand placed in the mould to form any openings of holes in the interior of a casting which cannot be formed directly with the pattern. So if you want to make a casting with interior cavities of holes, then by definition you will have to use one or more cores.

Greensand is still sometimes used for simple cores. Increasingly in industry, CO2 sand is used both for cores and in “flaskless” (i.e. without a moulding box) casting. In this process sand is prepared to moulding consistency by mixing with sodium silicate, otherwise known as waterglass. The cores are then moulded and carbon dioxide gas is forced through them under pressure – they harden almost immediately.

For the chemistry buffs, Na2SiO3 + CO2 →Na2CO3 + SiO2

Carbon dioxide converts sodium silicate to sodium carbonate and silica. The carbon dioxide process is certainly not outside the bounds of possibility for the serious backyard operator.

However, coremaking using oilsand (see the section on oilsand in part 4) is by far the most convenient method for the small scale foundryperson.

Cores can be produced by extrusion, where sand is forced through a narrow opening and comes out as a “sausage” which is cut into appropriate lengths; or by moulding in some type of corebox. The latter method is well suited to small scale work.

The above figure shows a simple wooden corebox designed in this case for a cylindrical core. Note the holes and pegs for accurate registration. When the core is being made, the two halves would be held together with a clamp. Similar types of coreboxes can be used for most varieties of core the small operator is likely to encounter.

Another “box” for cylindrical cores is a plastic tube lit down its length and held together with a couple of screw driven hose clips (“jubilee” clips) while the core is rammed. A “box” for rectangular cores is just as simple to make, need only two “L” shaped sections of wood or metal, clamped together when the core is being rammed. The surface of the workbench acts as the bottom of the “box” and the top is simply trowelled smooth.

Cores with a single flat side can be made in an open type of box, with the core pattern provided by the internal contours of the box. It is important to allow for “draught”, i.e. a slight outward slope from the bottom to the top of the box, so the core slides smoothly out after ramming.

Cores are rammed up in much the same way as moulds, although of course you won’t need a large rammer. A length of ¼” dowel, or ½”for larger cores, should be adequate.

Cores should be well vented before removal from the corebox, as they are otherwise likely to be dislodged or damaged by the gases generated during casting. A single vent is sufficient for most cores, but large cores may need two or three. Venting is accomplished in the same way as for moulds, with the aid of a pointed wire.

Odd shaped cores present a problem here, mainly because a straight piece of wire won’t go around corners. The usual trick is to coat a piece of string with wax and place it in the middle of the core (following the contours) as you are ramming up. Use enough string to leave one end hanging from the end of the core. When the core is baked (see part 4 for instructions on baking oilsand) the string can then be removed, leaving a thin hollow in the core.

Because cores are fragile and have to withstand considerable pressure during casting it is also advisable to strengthen them with reinforcing wire. Even paper clips bent to fit can be used. Again, with the intricate cores the wire should follow the contours of the core and is incorporated into the core during ramming.

When cores are used in a mould, “core prints” are generally provided on the pattern. These are projecting pieces of wood on the pattern which leave a corresponding hollow in the mould for the core to fit in. Obviously the print must match the core (ever tried to put a square peg in a round hole?”

Where it is not possible to hold a core securely in place by means of core prints, “chaplets” are used. Nails make ideal chaplets – copper ones for bronze or brass, steel ones for iron. Never use rusty nails. Since the rust reacts with the molten metal to form carbon monoxide gas, which is likely to dislodge the core and wreck the casting. The chaplets should be inserted from the inside of the mould before fitting the core.

Sometimes an intricate core is best made in separate pieces, which are then bonded together with dextrin gum. Even a simple core is often easier to make in two halves.

In the illustration above, a hollow core has been used. The hollow acts as a large “vent”, and should be connected to the cavity which the metal will fill. In the above example fine vents will allow the gas out but will not allow the metal into the interior of the core.

Cores are best given a “wash” of plumbagoe or refractory material (zirconia etc.) mixed with metho before use, to ensure a smooth surface. Make sure it is fully dry before casting.

I haven’t said much about the design of cores or coreboxes for specific purpose here, as this series is concentrating on general moulding methods rather than the making of particular castings. In the next article – patternmaking – I will be following the same general approach.

 

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