Casting

A specialised part of the manufacturing/engineering world is casting or foundry work as it is properly called. In schools and colleges this usually involves casting molten aluminium. Before any casting can take place a wooden pattern is made precisely. This is called pattern making and in industry this is a very skilful job. Any inaccuracy at this stage will result in the final cast being wrong or even failing. In schools the pattern is usually made from a softwood and its sides are given a draft (an angle) so that it can be removed from the sand easily.

The diagrams to the left shows the pattern on a flat board and a casting box called a ‘drag’ being placed over it.

Special casting sand will soon be packed around the pattern but to ensure to can be removed easily from the sand, parting powder is sprinkled over and around it. (parting powder is similar to talcum powder). It stops the casting sand sticking to the pattern and pulling away with it when the pattern is finally removed from the sand.

The drag is then packed with more casting sand. It is a good idea to sieve all the sand being placed above the pattern and then ram it down firmly using a ramming tool. The tool has two ends, one is cylindrical and is used for general packing down of the sand. The other end is quite pointed and this can be used for packing sand close up to the pattern.

When the drag is packed fully it is levelled off (called ‘strickled off’) using a straight steel bar.

Sprue pins are positioned. One usually on the back of the pattern and the other to the side. These will eventually provide an entrance and exit for the molten aluminium when it is poured into the sand.
The sand is packed/rammed into the cope in the same way as the drag. Parting powder is first applied, followed by facing sand. The sprue pins should be taller than the box and stand out from the sand when it is levelled with a strickling bar.

Small depressions are dug into the sand at the top of the two sprue pins. These are useful when the aluminium is poured. The depressions are called the pouring basin and feeder.

The top box (the cope) is then removed and if all is well the cope with the sand inside should lift off the drag (bottom box) without the sand falling out. A small ‘gate’ is cut below the position of one of the sprue pins. This will help the molten aluminium flow into the cavity left by the mould. Small tools are available or can easily be made to dig a variety of shapes in the casting sand. They are similar to small trowels.

The cope (top casting box) is placed back on top of the drag and the locating pins put in position. Before this is done vents can be created using a thin piece of welding rod, pushing it through the sand . This allows gases to escape once the aluminium is poured.

Appropriate safety must be worn when casting metals. Aprons, gloves and leggings should be leather as this offers the most protection is a spillage of molten aluminium occurs. Normal textile material burns through very quickly and should not be used for the casting process.

In addition, strong, leather shoes should be worn at all times in the workshop as they offer the best protection for feet. In industry shoes with steel toe caps are a basic requirement.

The procedure for preparing aluminium ingots for casting must to be carried out carefully as safety is one of the key concerns, whether in a school workshop or in industry. Outlined below are the stages involved in heating and charging a crucible and then pouring the molten aluminium.

PREHEATING - The crucible is first preheated. This removes any moisture from the furnace and crucible. Usually the gas is turned half on in order to avoid rapid heating. The aluminium ingots to be used are placed on top of the furnace so that they warm up.

CHARGING WITH ALUMINIUM - once the crucible and furnace have been preheated the lid/top is pulled to one side with a steel hook. Aluminium ingots are then placed into the crucible with steel tongs. All steel tools such as tongs are pre-warmed. The gas pressure is turned up to full.

TEMPERATURE CONTROL - Judging the temperature of the molten aluminium is sometimes difficult. Using a pyrometer, the instrument used for testing temperature, allows accurate measurement. The pyrometer is first warmed over the furnace to evaporate any moisture and then it is placed into the molten metal The temperature can be read on the meter, near the handle.

When the temperature reaches 650 degrees it is ready for pouring but first the ‘dross’ (waste that collects on the surface of the aluminium) must be removed with a special tool.

POURING THE ALUMINIUM - The two man ladle is then lifted which raises the crucible from the floor. The teacher must always control the pouring of the molten metal and so holds the two handles. The second person (possibly a pupil) holds the single handle allowing it to revolve when the ladle is turned for pouring by the teacher. A third person stands behind the crucible and uses a steel steady to prevent the crucible from falling out of the ladle.
The aluminium is poured into the runner and when the cavity is full is rises up the riser. The flow of aluminium should be constant, if there is even a short break in pouring the cast aluminium it may cool and the cast may be imperfect.

SAFETY CLOTHING MUST BE WORN !!

A base is prepared. This has two holes that will hold the sprues in place whilst the casting sand is used to fill the box. The sprues will eventually form the holes down which the molten aluminium will be poured and rise during the casting operation.
Small casting boxes are used if small casts are to be manufactured. The top casting box is called the ‘cope’ and the lower box is the ‘drag’.

When the sprues, base and sides are in position, casting sand (normally petrabond) is built up in the casting box (the cope).
A small fine sieve is used to ensure lumps’ of petrabond are broken down and that only fine sand is allowed to build up around the sprues. Hands and fingers can be used to compress the sand.

Sand is then built up around the sprues, above the level of the box. This sand will eventually prevent excess molten aluminium from spilling over the side of the casting box (the cope).

A small trowel or even a spoon is used to ‘dig’ gates around each sprue. The aluminium will be poured into the gate of one of the holes made by the sprues, flowing down into the cavity and rising up the other hole.

The sprues are carefully removed leaving the holes that are now known as the ‘runner’ and ‘riser’.

The molten aluminium will be poured into the ‘runner’ and rise up the ‘riser’.

The COPE - the top casting box is complete.

The sides of the second box are placed on top of a base. Note, the base is plain and does not have holes for sprues, unlike the ‘cope’.

Petrabond casting sand is sieved in the same way as for the ‘cope’. Hands and fingers are used to compress the sand so that it is not loose. If not compressed, the sand will collapse when the box is moved.

The pattern, in this case a letter is pressed into the sand. The pattern must have a draft to each edge. This is a slight incline that allows the pattern to be removed from the sand.

Sometimes it is necessary to pack the drag so that the sand is not highly compressed in the area of the pattern. This allows the pattern to be pressed into the sand more easily.

If necessary, the strickling bar is used to level the surface again. Sometimes pushing the pattern into position disturbs the sand especially around the edges of the pattern. If this happens more fine sand can be placed over the top of the pattern and compressed. The strickling bar can then be used to level the surface for a final time.

The pattern is carefully removed. If the pattern has a pre-drilled hole in its back, a screw can be inserted and the pattern freed by tapping it side to side. The pattern can then be lifted out of the sand. This reveals the cavity into which the molten aluminium will be poured.

The ‘cope’ is then placed on the top of the drag. The cope and drag can be held together by a small weight or by screws. When the aluminium is poured the pressure can lift the cope out of position if the casting boxes are not secured or held together in this way.

Cope and Drag in position ready for the molten aluminium.

The animation below shows pieces of aluminium cut to a size suitable for heating in a ladle. Fire bricks have been arranged around the bowl of the ladle so that the heat from the brazing torch flame is reflected back onto the aluminium. As the temperature rises to 650 degrees centigrade the aluminium begins to become molten and quite quickly forms a liquid.

The molten aluminium is carefully poured into the mould. The casting boxes are set in sand in case any of the molten aluminium runs down the sides. If this happens the molten aluminium will simply stay on the surface of the sand and cool down. The casting boxes should never be placed on a cold surface. If molten metal comes in contact with a cold surface it will ‘splatter’ violently. Anyone close to the area will be in serious danger.

When the aluminium has cooled down it can be removed from the petrabond sand. Great care should be taken because it the aluminium retains heat for sometime.

When completely cool the sprues can be removed with a hacksaw and recycled. The casting can be cleaned up using files, emery cloth and a polishing machine.

The diagram below shows pieces of the pewter ingot cut to a size suitable for heating in a ladle. Fire bricks have been arranged around the bowl of the ladle so that the heat from the brazing torch flame is reflected back onto the ladle. As the temperature rises the pewter begins to melt and quite quickly forms a molten liquid.

The molten pewter is then carefully poured into the mould. The mould is set in sand in case any of the molten pewter runs down the side of the mould. If this happens the molten pewter will simply stay on the surface of the sand and cool down. The mould should never be placed on a cold surface. If molten metal comes in contact with a cold surface it will ‘splatter’ violently. Anyone close to the area will be in serious danger.

Pewter casting is potentially very dangerous because of the high temperatures involved. Safety clothing similar to the type shown below must be worn and it is essential that all possible safety measures/precautions are taken. Pupils / students must be supervised directly during any casting.

Appropriate safety must be worn when casting metals. Aprons, gloves and leggings should be leather as this offers the most protection if a spillage of molten pewter occurs. Normal textile material burns through very quickly and should not be used for the casting process.

In addition, strong, leather shoes should be worn at all times in the workshop as they offer the best protection for feet. In industry shoes with steel toe caps are a basic requirement.

MDF is ideal for use as a mould when casting pewter. The melting point of pewter is low and does not burn or damage the MDF so that it can be reused time and time again.

The sides are generally manufactured from 9mm MDF with the mould being made from 4mm MDF. The shape of the mould can be cut using a coping saw or a fretsaw. The mould is usually quite small so care is needed when cutting and shaping. It can be smoothed with fine glass paper.

2. Draw a simple shape on the MDF include the ‘sprue’. The sprue is the top part of the mould, into which the molten pewter is poured.

3. Cut away the shape using either a fretsaw or a coping saw. Smooth the rough edges with fine glass paper. The mould is now ready.

When the mould is finished it is placed between the two supporting pieces of MDF and secured in a vice. A hot air gun is normally used to strip old paint from wood. However, it is ideal for heating up pewter and usually takes about two to three minutes before it is ready for pouring.

When the pewter is molten it is ready to pour into the mould. Safety equipment should be worn when carrying out this procedure. Leather gloves, a safety visor and leather apron are usually worn.

When pouring is finished a small amount of pewter is normally left above the sprue. This must be removed quickly by pushing a piece of wood along the top. The excess pewter falls to the side and can be recycled later.
The casting is allowed to cool and removed from between the supporting pieces.

If drilling is needed a hand vice is used to hold the casting. This is a safe technique. The hand vice has two jaws that are closed by turning a wing nut. (See below).

The pewter letter is finally cleaned / smoothed by using of emery cloth. A small amount of oil is added and the letter is moved forwards and backwards. Smoothing the surfaces can take a considerable amount of time however, the finish is very good.

If a polished surface is required a polishing machine can be used.