Moulding covers a huge range of technology.  Moulding (foundry sands) and patternmaking (manufacture of master moulds for foundry sand castings) were common trades in industry, now dying out.  I suspect that all of you will frequently need to design moulds and most of you will have to make moulds yourselves.

Most moulds are used for castables. a very high percentage of refractory castable moulds used in production are very poorly designed.

Materials

Moulds can be made of metal, wood, plastic, Perspex, expanded polystyrene, rubber, silicone, polyurethane, cardboard cores, and anything with a convenient shape.  You can’t release a rigid casting from a rigid mould.  Moulds must therefore have a certain amount of flexibility.

METAL

Mild steel is one of the commonest mould materials, mainly because the number of people able to fabricate with steel is very large.  Steel moulds are normally greased because they rust. Therefore, they get really filthy, and yield ugly castings and are unpleasant to work with.

WOOD

Wooden moulds are the quickest and easiest to make. The life span is short because they get wet. They distort easily. Normally they are held together with screws but for mass production toggle clamps are more practical.  Wood expands when it gets wet.  Wood used to make holes is a disaster.  It expands and gets stuck and may even break casting.  Wood impregnated with oil lasts a bit longer.  Clear tape covering the wood is a quick solution for easy release and nice finish.

PERSPEX

Perspex is often used where a perfect surface finish is required.  You can see any defects through the mould.  If you get a very thin layer of detergent on your perspex mould, you can actually watch bubbles being pushed away into the mix.  Oil and grease are pneumophilic and hold onto bubbles.

MELAMINE (Formica)

Sheets of Formica are ideal to produce perfect surface finishes. They need little or no lubrication and can be peeled off the casting so there is no stress caused to the casting.   A melamine board laminated onto a board like a chipboard countertop is not very useful because it's not flexible.

PLASTIC

PVC pipes are very useful for casting refractory pipes.  Usually, the diameter needs to be modified.  To increase the diameter of the I D former, cut one side of the pipe in a straight line down the length, then cut a fillet to fit in the gap and push it open to increase the diameter.  To reduce the diameter of the O D former, slit it down one side, remove a strip and close the gap with jubilee clamps.  Winding a sheet of thin plastic around the inside or outside can also be used to adjust the diameters.

CARDBOARD CORES

Cardboard cores are sometimes used to make holes in castings.  They are normally left in the casting to burn out during preheat.

EXPANDED POLYSTYRENE (EPS)

Expanded polystyrene moulds are rapidly gaining popularity as the number of CNC hot wire cutters grows.  They are nearly all disposable one life moulds.  They are cheap, accurate, versatile, and can be very complex.  If you want to make a large number of intricate castings in a hurry, polystyrene is the answer.  The downside is that removing the polystyrene from the casting is a laborious and messy job.  You can fire the polystyrene away but do this at night when nobody is in the building. Styrene monomer burns your eyes.  EPS allows bubbles to be absorbed, but the casting sticks to it very tenaciously.  If you coat the moulding surface with PVA release agent, the mould is easily released.  In complex castings, it is not possible to remove the EPS.  You can remove it by dissolving it is various solvents.  Xylene works the best.

"RUBBER" 

For refractoramics, use Multimould, silicone or polyurethane. These all come in different hardness’s and you choose the right hardness to suit the product being made. Soft rubber allows complex shapes to be stripped easily.  Hard rubber allows for greater dimensional accuracy.

FIBREGLASS

Fibreglass moulds are sometimes used varying from very stiff and rigid to reasonably flexible.

PLASTER OF PARIS

Plaster moulds are used for slip casting of ceramics.  Never try to make a Plaster of Paris mould for a rigid casting.  You cannot strip a rigid casting out of a rigid mould.

ALLOWING FOR SHRINKAGE 

Most castables have a slight shrinkage on setting.  This means that they grip onto internal mould parts and often break as they try to shrink but can't.  To solve this problem use expanded polystyrene, foam or SFK to allow for slight shrinkage.

Techniques

Crucibles

Long pipes

Pressed bricks