This article was first published in 2008.
There are two really good reasons to know about
The first is that knowledge of metal casting will
allow you to better understand how items are produced, and consequently what the
limitations and benefits of this process are. Simply, you’ll be able to look at
castings with a much more educated eye.
The second reason to know about metal casting is
that you may be able to use castings in your own car modifications. If the item
is to be made in batches as small as even five, castings can have a major
benefit over fabrication (eg by welding) or machining (eg on a
computer-controlled machining centre). Those benefits can be in terms of
appearance, strength and cost.
In this three-part series we’ll be concentrating
on casting in aluminium. Aluminium is easily cast, easily machined and
(depending on the grade) can be heat-treated for additional strength. Aluminium
castings won’t rust, can be powder-coated or painted, and are light.
In this article, Part 1, we’ll look at the
background to casting, and specifically sand casting – the cheapest and easiest
way of producing small runs of a casting. We’ll look at patterns and moulds.
In Part 2 we will cover the step-by-step making of
a sand mould from a pattern, using a fairly complex casting as the example – the
production of an aftermarket intake manifold.
Finally, in Part 3, we’ll watch the pouring of the
intake manifold, from the liquid aluminium entering the mould to the casting
finished and ready to be sent off for machining.
So jump on for the ride. I found the process quite
fascinating, and it was an amazing experience watching the pouring of the inlet
manifold. When one is used to idea that metal is a hard substance that can be
cut, filed or machined, watching it suddenly flow like water takes your breath
Patterns and moulds can become complex very
quickly, so let’s start with the absolute simplest.
The fundamental process followed in sand casting
is the creation of an impression in sand into which the molten metal is poured.
The sand that is used goes hard after the impression has been made.
The sand can
be a naturally occurring sand of an appropriate moisture content that is poured
damp (ie soft), or as is almost always the case these days, it can be a silica
sand containing binders and other chemicals that cause the sand to harden after
it is exposed to air, and to go very hard when subjected to the heat of the
casting process. With the latter type of sand, the binding chemicals are mixed
with the sand as it is dispensed from a special machine (pictured).
To create the correctly-shaped impression in the
sand, a pattern is used. In its simplest form, the pattern looks like the
outside shape of the finished casting. Let’s take a look.
Here is the finished product – a small wheel.
The pattern for the casting is shown here. Note
that even on this simplest of patterns, tapers have to be used so that the
pattern will be able to be removed from the moulded sand without damaging the
shape in the sand. As we will see next week, in more complex castings, the
patterns may have to be made with separate parts so that they can be removed
The pattern is placed in the moulding box (usually
made of wood although steel or aluminium can also be used) and sand is packed
around the pattern. Patterns can be made from plastic, wood or steel. Wood is
most often used.
The pattern is removed from the moulding box,
leaving its impression behind. (You can now see the need for the tapers on the
pattern; a normal taper is 1.5 per cent.) The sand hardens, developing the
strength to withstand the force of molten metal being poured into it.
The molten metal is poured into the mould, the
metal flowing and so taking the shape of the impression in the sand.
The metal cools and hardens and then the sand is
broken away, allowing the casting to be removed. (Sand moulds are used just
once.) In this case, quite a lot of machining will be needed before the casting
looks like the finished product, the wheel.
In the casting process described above, the molten
metal was poured straight into the mould. There was also no ‘top surface’ in the
mould – the metal just assumed its natural level. However, in most sand casting,
the mould completely surrounds the casting. This has two implications:
The mould needs to be made in two halves that locate together
Passages need to be created in the mould for the
metal to be able to flow into the mould
Let’s take a look at another casting, but this
time make the mould in two pieces and add the metal-flowing passages.
Here the pattern has been placed in the wood
Now moulding sand has been packed around the
pattern. (The ‘parting sand’ is a fine layer that later allows the pattern to be
more easily withdrawn and prevents the two halves of the mould sticking together.)
Now we see the first difference in this process –
another moulding box has been placed on top and two tapered pins have been
placed in position. These pins act as ‘mini patterns’, giving shape to the
passages that will be formed through the sand.
Sand has been packed around the pins, and then
they have been withdrawn. On the right is the passage through which molten metal
will be added (this is called a ‘sprue’ or ‘runner’). You can also see that a
funnel shape has been created in the sand, simply to allow the metal to be more
easily poured into the mould. On the left is the passage through which molten
metal will rise as the pour is done. You can think of this as the ‘overflow’,
although it shouldn’t actually have metal flow out of its top. (This passage is
called the ‘riser’.) Some vent holes have also been placed in the sand (in
modern sands the displaced air actually flows through the sand – no added vent
holes are needed.)
If you look closely at the above diagram, you’ll
see that the runner (sprue) and riser don’t actually connect to the moulded
impression left by the pattern. This is remedied with these impressions made in
Here is the finished, assembled mould. The top
part of the mould is called the ‘cope’ and the bottom part, the ‘drag’. The
runner (where the metal will be added), the riser (where metal will rise) and
the connecting passages to the part of the mould where the casting will be
created can all be seen.
This is the same diagram as above but the green
colour has been added to show where the metal ends up. As can be seen, quite a
lot of poured (and then solidified) metal is surplus to the required casting –
after the sand is removed from around the casting, these surplus parts need to
be cut off.
The moulds that we’ve seen formed so far have
resulted in castings that are solid. The small wheel produced in the first
example of casting described above not only needs to have its external diameter
machined in a lathe, but it also needs to have a hole drilled through it for the
But forming the internal opening at the time
the casting is made has two major advantages – it saves in waste material
and, much more importantly, complex shape internal openings can be formed. In
many castings, such as this cylinder head, forming the internal openings by
machining after the casting was complete would be impossible.
The openings and passages within a casting are
also formed by shaped sand; these pieces of the mould are called ‘cores’. When
picturing the required shape of a core, think of it like this: the core is
shaped the same way as the air space inside the casting. Cores are
produced in their own patterns (more correctly called ‘core boxes’) before being
inserted inside the main sand mould. Let’s take a look.
Here is the desired finished product. This is a
semi cutaway view – what is wanted is a casting that looks rather like a cotton
A cylindrical opening through the middle of the
casting is needed, so a core of hard sand is required to form this opening.
This core is created in its own pattern – a core
box. The core box is in two halves (remember, the sand core needs to be able to
be removed undamaged) and sand can be inserted into the core box through one
The wooden pattern for the external shape of the
casting is formed in two halves. One half goes in the top half of the mould (the
cope) and the other half goes in the bottom section of the mould (the drag). The
parting line (ie the separation point of the mould) is halfway up the side of
the ‘cotton reel’ shape.
Here is the full mould. The core piece is coloured
orange and where the metal will flow is green. This drawing summarises all that
we’ve covered – a sand mould made in two halves, a runner, risers, and a core
Motorcycle Cylinder Barrels
Here is an example of a simple casting – a
motorcycle cylinder barrel. Look closely at the pic (click on it to enlarge).
The cylinder barrel has a big opening through the middle of it – so the mould
must have used a core. Furthermore, as with the ‘cotton reel’ casting described
above, it must have been made using a two-piece mould. Look closely and you can
see the parting line of the mould replicated on the casting (arrowed).
Here is one half of the pattern for the cylinder
barrel casting. Firstly note that the barrels are being cast in pairs – left and
right. The runner through which the molten aluminium will flow is shown by the
red arrow. Note how at the base of the runner the passage has been shaped to
flow the molten metal around the two corners. The square objects (that will
become square openings in the mould) indicated by the green arrows are for
ceramic filters through which the molten metal flows. The very large risers
above each of the castings are shown by the blue arrows. Note that in addition
to being ‘overflow’ passages, the risers feed metal to the casting as it cools.
This is important because the casting shrinks as it solidifies. (And this
shrinkage means that all patterns are made oversize.)
But what about the core pieces? As shown here,
this part of the pattern (when it’s left its impression in the sand mould) would
simply make the central part of the casting solid.
But here’s the core box. The core box in this case
is required to form only a cylinder of sand, one that can be located in the
middle of the sand mould to prevent metal filling the middle of the casting.
Therefore, this core box is very simple.
Here are both halves of the pattern, minus the
core box. As can be seen, the runner and risers are incorporated in only one
side of the pattern.
The completed castings ready for machining.
Before finishing, let’s take a look at some more
This is the pattern for a design of motorcycle
trailer hub (four at a time being cast)...
...and this shows the patterns for two
metal-working tools resting in half of the sand mould. Note how these patterns
are made of metal and some added body filler.
Finally, giving us a taste of what we will see in
Part 2 of this series, here are some of the patterns for an aftermarket carby
The upper external surface pattern...
...and the corebox for the runners...
... with removable parts to allow the core to be
What seems a very simple idea - just create an
impression in sand and then pour the metal into it – becomes increasingly
complex when the intricacies of shapes, openings and internal passages are taken
into account. However, once you get a fundamental understanding, the rest, er,
flows from there.
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H, Casting, Kaiser Aluminum, 1965
S, Manufacturing Engineering and Technology, Addison-Wesley Publishing