Re: Expandable spray foam....lost foam technique

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Posted by bruce paul fink on November 15, 1999 at 20:59:09:

In Reply to: Expandable spray foam....lost foam technique posted by Rob Frink on November 15, 1999 at 19:13:53:

The expandable foam I assume you refer to is a urethane / in the can / normally sold for insulation / foam???

If so, it does burn out but not nearly as well as the styrene or polystyrene or styrofoam types and can tend to leave an ash.

There are alternatives and ways to maybe accomplish this though.

If the amount used is minor in relation to the cavity and mass of the metal to be poured in it could work... it's just hard to judge from here.

If the amount of foam needed to be used pushes the limits try to combine it with the other foams and use them in thin blocks or spots.

Say the wall thickness needs to be 1/4 inch but the masses are low in volume, cut a strip of maybe 1" sticks and then slice off 1/4 inch thick pads of styrofoam into pegs to hold the core in place. May even use a touch of the other foam to 'glue' it or use a thin pin of aluminum welding rod to hold it or... use a thinner pin of stainless steel welding rod to hold it?

An aluminum rod pin will melt but hopefullly after the metal has chilled (risky but can work well) while a stainless rod pin will not melt and can continue to hold the core position... and also leave a nearly imperceptible stainless DOT where the pin was set.

Since you are pouring aluminum as opposed to faster chill metals like bronzes etc. it can do a better job of melting out the 'foam spacers'... but do remember that the aluminum if heated too high will also hold it's liquid state much longer and then the core could float or shift or sink (depending on it's shape and weight in relation to the molten metal).

A MELT CHART FOR ALUMINUM ALLOYS LOOKS ENTIRELY DIFFERENT THAN MOST OTHER METALS This can work for you if you make use of it, or be just a confusing quality if unawares.

BTU HEAT absorption and TIME are consistent but as the aluminum soaks up the heat it seems to stay more nearly hard and then get to be like a softer putty.

TIME PASSES,

BTU's still are absorbed but the metal remains putty like for a longer time.

Minutes pass but all of a sudden checking shows it's liquid.

TIME FOR CAUTION

Do NOT overheat now as the melting curve has

just taken a squared jump up the scale of viscosity. It's time to POUR.

And as the metal sits in the mold it again cools for a longer time than seems normal before it suddenly starts to get putty like and then harden more quickly.

It cools to a harder stage again in a reversed and squared chart line. Had you given it another 10 to 15 minutes of soaking up the heat (like one would do for bronze or most other metals) it would have only further extended the period of time it would sit in the mold as a liquid.

SO here's the rub.

If you heat aluminum like bronze... once it is liquid DON'T give it another 5 or 10 or 15 minutes to soak up the BTU's as it not only does NOT want it or need it but will set in the mold TOO long before solidifying. This gives it too much time to do the things metals do when liquid and are able to molecularly mess around, separate into individual weight layers of metals, open up to later variations of uncontrolled crystal forming and in some cases seek other combinational alloy groupings... and if

there is any H2O or wax residue even way far into the investment body it can do funny things to the surface.

LIKE Create soaking gases and surface wise make one feel real grumpy later when you open the mold.

*** Do note that sand based molds that can pass gases through their bodies,

and plaster investment or ceramic shell type molds that resist gas transferance make a big difference here.

And refrain from getting grumpy if there is still a problem...bpfink


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