5: Casting
Casting is done in a foundry where molten metal is poured into cavities in sand left by a pattern. Foundry work is usually done in a part of the workshop set aside for this purpose.
The metal is melted in a crucible pot which is heated in an insulated crucible furnace. Crucibles are made of plumbago but zinc-based alloys and aluminium alloys can be melted in a cast iron pot (fig. 1).
The patterns are popularly made of soft close-grained wood, a good example being jelutong which is close-grained and easy to work. Wherever possible internal corners are radiused by applying wax or leather fillets or by using plastic wood so that no fragile corners will be left in the sand when the pattern is removed. The pattern is also suitably tapered or drafted. 1/8" per foot is sufficient, but for school work a greater draft is often allowed. This drafting is to facilitate easy removal from the sand. The pattern is finished with glass paper and varnished or painted. Figure 2 shows the pattern for a small vice. Allowance must be made for machining and for the metal shrinking when it cools.
Patterns which do not have one large face, as in figure 2, can be made as split patterns. These are often easier to manage if mounted on a board as in figure 3. If a board is not used, the halves of the pattern can be accurately located by dowel pins as shown in figure 4. When a pattern is made for use by an outside foundry, the various parts must be coloured to indicate to the foundryman where to allow the better metal to be for subsequent machining. These colours are specified in the British Standards No. 467: 1957. Black, for parts to be left as cast; red for core prints; yellow where machining is to be done etc.
FLASKS OR MOULDING BOXES
These are usually made of steel, cast iron or aluminium alloy (fig. 5). The top half is called the cope, the pins of which locate into the sockets of the lower half known as the drag.
SAND
Oilbonded sand such as Petrobond is excellent for beginners because there is no need to worry about the correct moisture content; but it is more expensive than the popular Mansfield, Erith or Belfast sands which are natural sands and are made damp with water.
These natural sands are a mixture of sand grains and clay particles. They are refractory so that the hot metal does not bake or melt them, and they are more or less porous thus allowing the escape of steam and gases. The moisture content must be right. This can be judged by squeezing a handful, which should remain unbroken when the hand is opened and yet break when it is thrown down.
CORES
Cores are used where it is required that parts of the casting should be hollow. Figure 6 shows a hollow casting and figure 7, the pattern. The core prints are for making cavities in the sand in which to place the core. The core is made from sand bonded with oil to give cohesion. The core must be permeable to allow the gases to escape, and strong enough to stand handling, yet sufficiently brittle to crumble when the metal contracts. Cores can be made in a core box (fig. 8) or in metal piping.
For school work the core can be made of beach sand (sharp sand) bonded with linseed oil and baked in an oven until hard.
The linseed oil is mixed, a little at a time, into the sand until a handful squeezed does not exude oil and yet holds together. The core can be baked in an improvised biscuit-tin oven over a soldering stove, or in an old domestic oven, to a temperature of about 200°C. It will first go lighter in colour as the oil dries and then start turning brown. When it is a little darker than when first put in the oven, the baking is finished. Figure 9 shows the core in place in the mould resting in the cavities made by the core prints.
Chaplets
Chaplets are small supports for the core (fig. 10) made of the same metal as the casting. When the metal is poured these fuse into the casting. Although seldom used in school work, they are found necessary in industry where intricate cores are used.
MOULDERS TOOLS
A few of these are shown in figure 11. The large rammer is made of wood and the small one of metal. These are used for ramming the sand all round the pattern. The water brush is soft and comes to a point. It is used for damping the mould where small repairs are made (usually to the edges). The taper trowel is one of the many shaped trowels moulders use to make the sand flat where desired. The spoon tool is often used to make gates. The gimlet is screwed into the pattern and tapped in all directions just above the thread with a metal rod while the handle is held.
This is known as rapping. It loosens the pattern in the sand so that it can be withdrawn easily. Bellows are used for blowing out particles of sand from the mould. The sieve or riddle is used for sifting the facing sand over the pattern. Sprue pins are round tapered pieces of wood used for making . the runners and risers.
THE MOULDING PROCESS
For simplicity let us consider preparing the mould from the pattern for a small vice shown in figure 2 (p. 86).
(a) Choose a flask which allows at least 2" spare round the pattern. Open the flask and turn over the drag on to a flat surface, preferably a flat board (turn-over board) and place the pattern as shown in figure 12. The parting powder, which can be French chalk or a proprietary make, is held in a cotton bag and dusted over the pattern. This prevents the pattern from sticking in the sand.
(b) Riddle sand over the pattern (Petrobond does not need riddling) until it is covered, then add unsifted sand and ram down using the peen end of the rammer. Fill to the top with sand and ram with the butt end of the ram and make level (strickle) by drawing a firm straight edge across the top of the drag.
(c) Turn over the drag and fit the cope. Sprinkle with parting powder. Press the sprue pins into the sand. Their position is determined by experience—this takes into account the flow of the metal and the removal of excess metal afterwards (fettling). Riddle first layer of sand then add more sand, ram and strickle as before.
(d) Make the pouring basin, rap and remove the sprue pins. With a finger smooth off the edges of the holes. Separate the cope from the drag taking care not to jar the cope.
(e) Make the runners from the pattern to the sprue hole as shown. Insert the gimlet and rap in all directions with rod sideways, and remove the pattern carefully. Blow out any loose sand with the bellows and make repairs as required. Remove any sharp edges in the channels by running a finger round.
(f) Take care not to jar the cope when replacing it on the drag. The mould is now ready, but if it is to be left for a time it should be covered (not necessary if Petrobond is used) to prevent loss of moisture.
Odd-side Boxes
If the pattern does not have a flat back an odd-side box has to be made by placing the cope on the turn-over board with its locating pins upwards. Ram the cope with sand and then cut out the sand to take the pattern to half its depth. Then ram the sand round the pattern and strickle. Dust with parting powder and fit the drag, fill with sand, ram and strickle flat. Lift the drag with care and turn over. Knock out the sand in the cope (odd-side). With the pattern in the drag, place the empty cope in position, dust with parting powder, position sprue pins, fill with sand, ram and strickle. Rap and remove spruce pins and continue as for previous mould.
Pouring the Metal
Directly before the metal is poured the dross must be removed with a skimmer (fig. 14). Keep the crucible close to the pouring basin and pour in a continuous unbroken stream, as fast as the mould will take it, until the metal appears at the riser. When the metal has cooled and the casting been removed, it will be as in figure 13. This shows the relative size of the gate. The process of melting the metal prior to pounng must be properly done otherwise faulty castings result.
Melting the Metal
Aluminium alloys and zinc alloys are widely used.
Aluminium alloys melt at about 600°C
Zinc alloys melt at about 390°C
These metals can be melted either in a plumbago crucible or a cast iron pot.
Furnaces for schools usually use gas and air. There are various types available, but whichever kind is used the makers' instructions must be followed regarding the lighting.
Small pieces of metal charged into the crucible will melt more quickly than large pieces. However, once the first charge is molten larger pieces can be added. The metal should never be heated much above its melting point and it should be poured promptly after skimming.
Overheated metal results in the following typical faults: poor quality casting; excessive shrinkage and tearing of the metal.
When melting aluminium, fluxing and degassing are sometimes necessary.
Fluxing
A small amount of powdered flux (proprietary make) is applied to the charge as it becomes pasty. This melts on the surface and protects the metal from the atmosphere.
Degassing
If porous castings are being constantly obtained, degassing should be carried out before pouring. For school work it is convenient to use tablets which are plunged to the bottom of the melt with a domed tool (fig. 15).
