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Author Topic: Moulds for Metal Casting  (Read 4623 times)

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Offline Matt

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Moulds for Metal Casting
« on: August 29, 2011, 16:29:22 PM »
Hi all

Perhaps a question for MikeyJ this one but im making a mould of the open top part of the 4.0 engine and would like a bit made so that it can be welded/brazed into place

how accurate to the moulds have to be? do i need to make it slightly larger due to metal shrinkage?

Shape wise is there any limitations?

Does the mould have to be made out of a certain material? im using fibreglass?

Any idea of cost for something that size?

Best way of fixing said piece to the engine? Glue / Weld / Braze?

Any info greatly appreciated

Matt  ;D


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Offline Cortiworth IV

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Re: Moulds for Metal Casting
« Reply #1 on: August 29, 2011, 17:50:22 PM »
Hi all

Perhaps a question for MikeyJ this one but im making a mould of the open top part of the 4.0 engine and would like a bit made so that it can be welded/brazed into place

how accurate to the moulds have to be? do i need to make it slightly larger due to metal shrinkage?

Shape wise is there any limitations?

Does the mould have to be made out of a certain material? im using fibreglass?

Any idea of cost for something that size?

Best way of fixing said piece to the engine? Glue / Weld / Braze?

Any info greatly appreciated

Matt  ;D

What material do you want to cast? When casting aluminium the sample part need to be about 1.7% bigger than the piece you want to make. The sample is then used to make the cavity in the sand mould where the liquid metal is poured.

The material used to make the sample is not critical. Wood, polyester/fibreglass, automotive polyester body filler, plaster,...you name it.
It need to have a little bit of strength, tho, as you don't want it to crack when the sand is compressed around it.

You have to shape it in a way that allow the sample to be removed from the sand without destroying the imprint. Half the sample part makes an imprint in the lower part of the mould box and the the other half makes an imprint in the upper half. The line separating the upper and lower half is called the parting line. You need positive draft angles on both sides of the parting line in order to let the sample to be removed from the sand without disturbing or destroying the imprint.

You can read more at Wikipedia: http://en.wikipedia.org/wiki/Sand_casting

Cost? If you can make a sample that the foundry can use as it is then my guess is less than £100.

If you want to braze/weld the part in place I would go for cast iron because it has the same expansion rate as the
sorrounding metal. You still have to be very careful when welding/brazing it into place, tho, to avoid cracks.
With aluminium my choice would be to make the cover a millimeter or two smaller than the opening in the block and fill the gap with a
elastic silicone sealer and use screws to hold it in place. Along each cylinder bank the cover can be held in place by the cylinder heads.



Offline Matt

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Re: Moulds for Metal Casting
« Reply #2 on: August 31, 2011, 10:28:47 AM »
Hi

cheers for that mate, i was thining of cast iron as you say it will have the same thermal properties



Matt
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Re: Moulds for Metal Casting
« Reply #3 on: September 06, 2011, 01:29:39 AM »
I was about to answer this but svein beat me to it with an excellent answer. ;)

I might add though that the part will most likely have to have pouring channels, vent channels, cores etc to make it castable. But if you take it to a foundry the guys there will most certainly have an opinion on all that.

During computermodeling of the castingprocess we used 2-3% expansion as a rule of thumb for all materials. It's kind of hard to imagine, but the stuff ACTUALLY shrinks that much from liquid to solid state.

Good luck mate! Even though I think your problem can be solved a whole lot easier.. ;)

Offline Matt

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Re: Moulds for Metal Casting
« Reply #4 on: September 06, 2011, 10:27:42 AM »
Cheers Count

Yea there are probably easier ways of doing it. Tobbe did one with windscreen sealant, but im not sure that would be a good long term solution, as frequent heating and cooling with the engine and the contact with engine oil im assuming would degrade it and then let oil through. The one he did on the 4.1 looked the easiest with just a 'all in one' head gasket, but again im not sure how he sealed it up to stop oil leaking out, or how he got the cylinders to seal up

ive the same feeling with using the block rock (was that Jean-Francois method?)

there is the option of just welding it all up, but if the block warps and needs machining im risking having the pistons smacking the heads, plus the CR will be high as the heads have already been skimmed

ive almost finished the fibreglass mould im doing and hope to have a chunk cast from iron, so should have the same thermal properties as the block, plus if i braze it into place im not using as much heat to melt the brazing wire.

Once fully painted up i hope it will look like a 'standard' 24v Block LOL

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