white metal casting probs

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Spinal - 13 Sep 2005 21:17 GMT

A while ago, i started making some parts for my models i done, and
started making models of them and casting them in white metal. When i
first started i used the metal that came with the handcasting starter
kit i bought. It was OK for some things but as it cools with a matt
finish it wasnt as good for other things (such as the lightbars, which
get painted metalic blue, so a shiny silver base gives much better
results) so i bought some some different metal which was KA Low Melt
Alloy, which cools with a nice shiny surface - perfect just what i was
after, apart from one promlem i just can get any castings to come out
good.

Ever lightbar ive tried to cast, comes out with dimples in it. At first
i thought i migt be the mold so i tried the older (matt finish wm) in
the mold and it gives a perfect smooth casting, new metal dosent.

I check the info for the metal and it says porosity problems indicate
the metal has been over heated - which is the problem i am having but
cant under stand because as soon and the metal has melted in the ladel i
pour it into the mold.

Ive tried load of times, and at first thought i could be air getting
trapped in the middle of the mold/lightbar so made small channel for the
air to escape into - still didnt work, tried moving the model side to
side to help the metal flow still didnt work.

This metal should have better flow properties better for finder/smaller
detals yet it seems to be worse and musch more of a pain than the
cheaper metal.

Anyone got any suggestings as what could be causing the problem
-------------
Diecast Fire & Police Service Models
Collection and Custom Conversions
www.spinal.34sp.com

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mark johnston - 14 Sep 2005 02:31 GMT

My experience with casting is with mainly with superalloys (turbine blades).
I'm not sure if this applies to the alloys you are using, but one problem
that we can have is shrinkage as the alloy solidifies. One design practice
is to consider how the part cools off or "freezes". You like to have a
reservoir of molten metal to supply the part as solidifies and shrinks,
filling in the voids. If for example, the ends solidify first and leave a
pocket of molten metal trapped in the middle, as it cools off it will shrink
and pull away from the mold surface.

I'm not sure if this is the problem you are having, but I would suggest that
you look at your mold design. You may need to add some gates or risers to
feed the metal to the mold differently to prevent sections from freezing
off.

Not a definitive answer, but something to try.

Good luck,

Mark

Signature

>A while ago, i started making some parts for my models i done, and started
>making models of them and casting them in white metal. When i first
[quoted text clipped - 29 lines]
> Collection and Custom Conversions
> www.spinal.34sp.com

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Les Pickstock - 14 Sep 2005 02:40 GMT

>>A while ago, i started making some parts for my models i done, and started
>>making models of them and casting them in white metal. When i first
[quoted text clipped - 25 lines]
>>
>> Anyone got any suggestings as what could be causing the problem

Have you tried pre warming the moulds I have had much better results from
hot moulds. If you start casting in a cold mould and consign the first few
casting stright back into the pot the mould soon warms up. You might also
try hollowing out the masters, so instead of a moulding a big block you cast
a shell. The thinner section may help the casting to cool evenly. HTH

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rwsmithjr@rcn.com - 14 Sep 2005 03:43 GMT

> A while ago, i started making some parts for my models i done, and
> started making models of them and casting them in white metal. When i
[quoted text clipped - 5 lines]
> Alloy, which cools with a nice shiny surface - perfect just what i was
> after, apart from one promlem i just can get any castings to come out good.

So polish the one.

> Ever lightbar ive tried to cast, comes out with dimples in it. At first
> i thought i migt be the mold so i tried the older (matt finish wm) in
> the mold and it gives a perfect smooth casting, new metal dosent.

Probably from shrinkage while cooling.

> I check the info for the metal and it says porosity problems indicate
> the metal has been over heated - which is the problem i am having but
> cant under stand because as soon and the metal has melted in the ladel i
> pour it into the mold.

That doesn't mean you're not using too high a heat.

> Ive tried load of times, and at first thought i could be air getting
> trapped in the middle of the mold/lightbar so made small channel for the
> air to escape into - still didnt work, tried moving the model side to
> side to help the metal flow still didnt work.

Shrinkage for sure.

> This metal should have better flow properties better for finder/smaller
> detals yet it seems to be worse and musch more of a pain than the
> cheaper metal.

Try R8 alloy.

> Anyone got any suggestings as what could be causing the problem

Other than custom cutting an aluminum mold with a large reservoir for
molten metal and preheating the mold....nope.

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Rob de Bie - 15 Sep 2005 16:06 GMT

>Anyone got any suggestings as what could be causing the problem

As it happens I just subscribed to the Yahoo 'Casting' group. It has 3000
subscribers, and covers white metal too. Perhaps worth a try?

Rob de Bie

My models: www.xs4all.nl/~robdebie/models.htm
Me 163B site: www.xs4all.nl/~robdebie/me163.htm
AQM-34 site: www.xs4all.nl/~robdebie/aqm34.htm

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Model Forum / General / Models / September 2005

white metal casting probs