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.
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.
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
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.
between the pattern boxes is the parting line. It is very difficult
to remove all traces of the parting line from the final casting.
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
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.
to download a PDF file of Aluminium and Bronze alloys regularly
cast. To view this document you will need to have the latest version
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.
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.
points to remember with pattern making are:
you see is what you get
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.
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
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.
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
part that does not draw is called an undercut. It must be
removed by a core.
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.
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.
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.
most important things to bare in mind over cores are:
ingress: the box must be left open so that the entire box
can be packed with sand.
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.
LM5, LM6, LM10, LM18, LM25, LM31, L99.
PB1, PB2, G1, LG2, LG4, AB1, AB2, AB3, HTB1, HCC, CMA1.
you have a problem where making a pattern I would be happy to
advise you. Please call me, Andrew Sharp on: 01273 832062
| About | Applications
| The Process | Contact
to the top