polystyrene cylinder head

[S8ight]

picked some of my engine bits up today. was chatting while i was there and got told to go and have a look at the polystyrene heads  dont know if its the same as normal Vaux heads, they where from 2.2, but theyre obviously pressing heads from ally beads instead of normal casting. the heads looked like they where made of polystyrene then painted silver

[Simon H]

The heads and block from the 2.2 engine look like that because they have a polystyrene form used when they are cast and it leaves the surface looking that way.

[DickieB]

I'd do anything to lose weight. Where can I get one?

[S8ight]

you sure about that ? it does look like its made from pressed beads

[JonnyBoy]

it does look like its made from pressed beads

investment casting gives fantastic detail, so if the invested part was made from pressed PS like simon says (good game) then the cast part would look exactly the same but in metal.

I just had an intricate wax part investment cast, and i can testify to the exact replica nature of the resulting ally part!

[Chris Elworthy]

The process that you have seen is called "Lost Foam".

The process involves constructing an exact replica of the form to be cast in dense polystyrene moulds, this includes runners, risers and vents, machining stock and cast in pick-ups. Fragile areas can be supported with thin pillars called Chaplits which look like a drawing pin with a head at both ends, these last just long enough before melting into the parent metal.

The foam structure is coated with a few layers of ceramic slip, which cures before it is placed in its cope. The ceramic layer prevents the sand burning too much and is sufficient just enough to support the sand surface.

The structure, in its cope is then surrounded by sand, this is filled under vibration which makes it very dense, but there is no bonding agent, it remains fluid, hence the need for the ceramic coating.

When poured in, the molten metal instantly melts the polystyrene (to about 1% of its original volume) leaving a metal form exactly matching that of the polystyrene. A port is then opened at the base of the cope and the sand is poured out, the Ceramic layer is usually removed by vibration.

The pattern that you saw is a positive copy of the polystyrene, the negative being in the Ceramic coating.

I am fairly certain that Corsa 12V heads around 98 and 99 were made in this way amongst others.