Casting
Sand Casting is a process that has been around for over 5000 years. The process itself is fairly simple. A pattern is pressed into a sand mould to leave an impression. The pattern is then removed and the impression left behind is filled with a molten metal.
Casting
Patterns are generally made from wood however they can be made from metal, plastic or any number of other materials.
When designing a pattern a number of things must be taken into consideration.
Pattern Design
1. All patterns must have a Taper (or Draft) this allows the pattern to be withdrawn cleanly from the mould. This should be between 2˚ & 5˚
2-5
Broken Edges
Square Pattern
Tapered Pattern
Pattern
Sharp Edges
Pattern Design
2. Sharp internal corners should be avoided as these will focus any stresses acting on the casting and can cause it to fracture
Use a wax fillet on internal corners For a stronger castingIf you leave any sharp internal edges This can weaken the final design
SharpCorners
WeakSpot
3. Patterns should be finished with paint or varnish and sanded with fine glass paper. This will help avoid the pattern absorbing moisture form the sand and distorting.
The varnish or paint will give the pattern a much finer finish and consequently the final casting will have a superior finish
Pattern Design
Pattern Design
4. Avoid changes in section because this may cause differential cooling (one part of the casting cooling much faster than the rest) this can lead to internal stresses in the casting. Also try to avoid patterns that are extremely thin as the molten metal can often cool before it has flowed into all areas of the mould.
Pattern CastingCooling has occurred before the mould cavity has filled
Pattern Design
5. Finally remember to make the pattern slightly larger than the required casting. This will allow for any machining and for the fact that as the molten metal cools it will contract.
Aluminium is a lightweight, Non ferrous metal often used for casting . It is the most suitable metal for casting in the school workshop due to its low melting point (around 660˚C) Because the metal cools as it flows through the moulding box it is advisable to heat the metal to 760˚C before pouring.
Casting Metals
Aluminium IngotsAluminium Lamp Aluminium Saucepan
Brass (an alloy of Copper & Zinc) is often used for casting. This yellow, non ferrous metal has many uses, mainly due to its resistance to corrosion. Brass has a higher melting point (905˚C) than aluminium and thus is generally not used as a casting material in schools.
Brass Ingots Brass Ornaments Brass Bell
Casting Metals
Cast Iron is a hard, ferrous metal often used by industry for casting.
Cast Iron melts at about 2270˚C and consequently is not used in the school workshop.
Cast Iron Gate Cast Iron Vice Cast Iron Fire place surround
Casting Metals
Casting Equipment
Riddle Cope
DragBellows
PartingPowder
Degassing Tablets Sprue Pins Rammer
Vent Spike
StrikingOff Bar
GateCutter
RappingIron
DrawPin
The above casting equipment will be used
Casting Equipment
Leather Apron
Full Face Mask
Leather Gauntlets
The above Safety Equipment will be required
Casting
Place the Drag face down on a board and sit the pattern in the centre
DragPattern
Casting
Lightly dust the pattern and board with some parting powder (chalk dust)
Parting Powder
Casting
Carefully riddle some facing sand over the pattern until it is completely covered to a depth of 20mm
Casting
Use your fingers to lightly pack the sand around the pattern.
Casting
Fill the rest of the moulding box and use a rammer to compact the sand.
Rammer
Casting
Use a strike off bar to remove any surplus sand.
Strike Off Bar
Casting
Carefully turn the drag over and place the cope on top.
Cope
Drag
Casting
Again lightly dust with parting powder.
Parting Powder
Casting
Place the Sprue Pins in the box.
Sprue Pins
Casting
Riddle more facing sand onto the underside of the pattern (about 20mm)Then fill the rest of the moulding box.
Casting
Again use a Rammer to pack the sand. Take care to pack around the Sprue Pins.
Casting
Fill the Cope to the top and use the Strike off bar to level the top
Casting
Carefully form a pouring cup around one of the Sprue pins
Pouring Cup
Casting
Gently remove the Sprue pins
Sprue Pins
Use a finger to round over any sharp corners
Casting
Carefully separate the two halves of the moulding box
Pouring Cup
Casting
Carefully cut the in and out gates
In GateIn Gate
Out Gate
Casting
Screw the Draw Pin into the centre of the pattern and gently tap in all directions with a Rapping Iron
Draw PinDraw Pin
Rapping Iron
Casting
Carefully remove the pattern
Casting
Use your finger to round over any sharp edges near the two gates then use the Bellows to blow away any grains of sand that have fallen into the mould
Casting
Carefully put the two halves of the moulding box back together
Casting
Use the Vent Rod to make some holes part way through the sand on the top of the moulding box. This will help any excess gas escape
Vent Rod
Casting
Carefully lift the completed moulding box and sit it in front of the crucible furnace
Casting
Heat the aluminium until it reaches the required temperature (750˚C)This can be checked with a Pyrometer
Casting
Add a Degassing Tablet, this will help remove any excess gas from the aluminium and drive any impurities to the surface
Casting
Carefully skim off the impurities on the surface of the aluminium
Casting
Carefully pour the molten aluminium down the runner stopping when the mould is full (when the molten metal rises up the riser)
Casting
Allow to cool (this will take a number of hours)
Casting
When the casting has cooled open the moulding box
Casting
Carefully scrape the burnt sand into the bin
Casting
Remove the casting (trying not to mix the burnt sand with the rest)
Casting
Use a Hacksaw to cut off the runner & riser
Casting
Finally file off any excess with a Dreadnaught File
Dreadnaught File
The End