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 All patterns must have a Taper (or Draft) this allows the pattern to be withdrawn cleanly from the mould.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