Everything starts with the pattern. The essence of sand casting is to take a pattern, pack sand around it, then remove the pattern from the sand and pour molten metal into the resultant hole. After cooling the sand is knocked away to leave the required metal shape.

Sand castings cannot be made without patterns. The patterns for sand casting are positive. That is the shape of the pattern is the shape of the casting. Patterns are normally made by professional pattern makers. We do not have a pattern maker on the foundry staff but work closely with a number of firms asking each to make the patterns best suited to their specialisation's.

However White Eagle prides itself on being able to work from any pattern, not just professionally made ones. We frequently use amended or repaired original castings. Click here to go to notes designed to help the non professional pattern makers who wish to make their own.

Once the pattern has been received it will be placed on a board and surrounded by a steel frame (box). The box will be filled with sand and packed hard.

The box will then be turned upside down and another box located by pins placed on top of it. This box will be packed with sand.

The plane between the pattern boxes is the parting line. It is very difficult to remove all traces of the parting line from the final casting.

The boxes are parted and the pattern removed and a link with the outside cut. This will be used to pour the metal in later. The boxes are put back together giving a mould.

Metal is then poured into the mould through the metal entrance.

We use a variety of different sands with different grain size, make up, bonding methods and costs. The best type is chosen to fit the circumstances of each pattern. The most commonly used sand is Mansfield sand. This occurs naturally and was first discovered by the Romans. It is silica sand with 12-14% clay mixed in. It is bonded by water and can be reused by the replacement of the water lost as steam when the molten metal is poured into the mould. This is called greensand.

The aluminium or bronze alloys are melted in gas fired furnaces and the molten metal carried round to the mould by hand. Eight different aluminium alloys are regularly poured. Click here to see their names. On the yellow metal or copper based alloy side a number of gunmetals, leaded gunmetals, phosphor bronzes and aluminium bronzes and one high tensile brass (HTB1) are regularly poured. Click here to go to a list of their names and chemical composition. In addition to these, arrangements can be made to buy and cast any British or foreign standard alloy or any privately specified alloy.

Click here to download a PDF file of Aluminium and Bronze alloys regularly cast. To view this document you will need to have the latest version of Adobe Acrobat.

Once the metal in the mould has frozen the sand is knocked away and the casting goes to be fettled. The runners (channels for metal to enter the mould) and risers (extra shapes added to the mould to feed the molten metal as it contracts on cooling) are cut off, sorted by alloy and recycled. The fettling removes any unwanted metal from the casting, in particular flash from around the parting line.

After fettling the casting goes for finishing. Here it may be left with its as cast finish or may be shot blast or given a polished finish. The shot blasting gives the casting an even finish (light grey when done with steel balls or powdery white when done with aluminium grits). This gives a good key for subsequent painting and identifies any air bubbles just below the surface skin. The polishing ranges from just making the casting very smooth to touch, through various brushed and matte finishes to a full mirror finish.

Crucial points to remember with pattern making are:
What you see is what you get
Subject to shrinkage, covered in point 2, and a slight loss of definition due to the sand grains not touching every part of the pattern. Therefore the detail on what is to be used as a pattern should be as clear as possible. Any machining or thickening required must be added to the pattern.
Shrinkage

The casting is always smaller than the pattern. The traditional amount is 1/75th or 1.3% although this varies with the pattern shape. The reason for this is the difference in metal size between room temperature and melting point. This shrinkage cannot be satisfactorily compensated for by over-rapping the pattern.

One area where consideration of shrinkage is particularly important is in casting covers from original castings. Here I find it useful to imagine all the holes drifting out from the centre to visualise whether or not they will still be surrounded by sufficient metal.
Draw

The pattern must be able to be drawn out of the mould from both the top and the bottom without disturbing the sand. If the sand is disturbed the cast shape will not be what it was intended.

A part that does not draw is called an undercut. It must be removed by a core.
Slick Surface
The pattern must come away from the sand without any adhering to it. If it does this will result in a casting with a rough finish. Patterns are normally sealed with a cellulose based paint.
Wear and Tear
The pattern must be able to withstand sand being packed hard around them, once for each casting required. For example Plaster of Paris patterns will normally only be for 2 or 3 castings.
Cores

Cores are used to avoid undercuts and to hollow castings. They are made by packing sand into a core box (negative shape) and then inserting this block of sand into the casting mould. Where there is sand there will be no metal.

The most important things to bare in mind over cores are:

1. Sand ingress: the box must be left open so that the entire box can be packed with sand.

2. Solid location: the core must be able to be placed securely in the correct location in the casting mould. This is done by adding core prints to the pattern and matching additions to the core box.

Aluminium Alloys
LM4, LM5, LM6, LM10, LM18, LM25, LM31, L99.

Bronze Alloys
LB2, PB1, PB2, G1, LG2, LG4, AB1, AB2, AB3, HTB1, HCC, CMA1.

If you have a problem where making a pattern I would be happy to advise you. Please call me, Andrew Sharp on: 01273 832062

 

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