Casting aluminium in gypsum (plaster) mold

Hello !

I'm complete newbie at this field.

Imagine a typical heatsink (drafted fins). I make a positive model (ie. from aluminium). I make plaster cast of it. I bake the cast at 100'C and then at 200'C to get out all moisture.
Plaster mold looks good.

How should I pour in the aluminium ? If I make it top-down, then air wont come out from fin bottoms (tried once, complete failure). Should I make air release holes in fin tops and add some bottom plate to the mold, so the aluminium will be cast from the bottom and rises to top of fins ?

I feel very stupid at the moment, please share experience.

There are porous mixes available to assist with venting.

http://www.usg.com/rc/installation-...rocal-gypsum-cements-application-en-IG538.pdf

J.O.

Do not do it. The water retained in the plaster will flash to steam. In investment casting a special plaster is used. Before the metal is poured the plaster mold is held at high temperature long enough to force the water out of the plaster and make a successful casting

never tried to pour something like a heatsink but can't really see what the problem is if you did a slow gradual pour and didn't just dump your whole load on there and expect it to fill the mold. you may still have too much moisture gassing off and blocking the flow. your alloy may be wrong as well. i've only done plaster mold casting with multipart molds and because of all the seams always had good venting.

couple of things you may or may not know already:

plaster should be mixed something like 1:1 with good clean silica sand.

how you bake it depends on the thickness but i always did it at around 450°C for a minimum of 2 hours. yes, plaster should crumble when abused like that, that's what the sand is for.

the mold should be HOT when poured, straight from your bake is ideal.

when pouring something with fine detail or thin walls you need a good alloy designed for casting. scrap like aluminium extrusions aren't going to do the job. 356 is a good common alloy and a good source is auto cylinder heads.

Rollerman said:

Do not do it. The water retained in the plaster will flash to steam. In investment casting a special plaster is used. Before the metal is poured the plaster mold is held at high temperature long enough to force the water out of the plaster and make a successful casting

Click to expand...

when doing lost wax you do need a special "plaster" that will hold up to the high burn out temp. when there is no wax to burn out then regular plaster of paris does just fine as long as the moisture is driven out. and this isn't just some backyard shadetree technique but an accepted industrial process often called "RPM" for "rubber plaster mold" as a rubber mold is often used to cast up the plaster parts that make up the mold used to pour the aluminium.

RPM-Rubber Plaster Mold Casting Process for Aluminum - YouTube

My neigbour casts aluminum parts in plaster every week. He does not mix the plaster with sand but he does bake the moulds in a large oven at about 450 for at least 24 hours some for longer but he allows the mould to cool.

Bottom pour and venting might help solve your problem. Dave pressure casts his parts, which is no solution for you. He uses heavy iron flasks with heavy ribbed iron plates top and bottom all clamped together. A couple of pounds of pressure is all it takes to fill his moulds.

can you recommend a procedure for proper gypsum curing ? ie. x minutes at temp1 and then z minutes at temp2 etc.

Lets assume a gypsum block 300x300x300 mm (about one cubic foot)

Pattnmaker, I saw your post after posting this - is the 450 degrees Celcius or Fahrenheit ? By bottom pour, do you mean that aluminium rises from heatsink bottom and fills rib ends in the end ?

We have pressure casting machinery available, but they can be used only with steel moulds - which is very uneconomical for <100 parts.
Maybe we can use long inlet, so the molten metal column exerces some pressure ?

People who use the word "aluminium" should be taken to a very bad place and kept there for a long time--at least as long as necessary to make the desire to say "aluminium" go away.

Heat sinks are always extruded, not cast.

If you insist on casting them your pattern should not end in a fine structure such as an array of fins. You should have a table on both top and bottom of the pattern and sprues coming out of the table. After the casting is done you cut it in two with a band saw. In other words, each casting is two parts mirror imaged together.

You might look into building a centrifuge and pour into a spinning mold. Its stuff you need is why it was invented.

jscpm said:

People who use the word "aluminium" should be taken to a very bad place and kept there for a long time--at least as long as necessary to make the desire to say "aluminium" go away.

Click to expand...

better be a big place as that's the vast majority of the world. but don't worry, just as soon as we get done shoving the metric system down your throats we'll deal with your bad pronunciation of aluminium. you WILL be assimilated

A lot of heatsinks are cast, want to see some - look under the hood of car, often the control electronics box has fins. Halogen lamp enclosures are another example.
And correct spelling is A-L-U-M-I-N-I-U-M, of course when you're a redneck with mouth full of chewing tobacco, its easier to say aluminum or, better yet with Libyan accent: ALUUUUMINUMM.
And if youre not yet insulted enough - your famous "feet" is'nt even a third of our METRE. And our KILOGRAM beats your pound twofold and still has some overhead ! And your gallon is just a swollen bloated distorted version of out nice tight LITRE.

I must explain that I'm only newbie with casting aluminium into plaster mould. I know quite lot about pressure die casting and machines.

Is there a reason not to use sand?
Tom

I made a few parts from clay master to plaster-supported rubber mold to wax pattern to plaster mold to aluminum casting.

I like the idea of making pattern a double of the finished part ,so that you do not have to vent air from fins individually. Alternatively, you could tilt the patterns in the mold so as to have one corner of all fins high, then tie those corners together with a bar that would have to be cut off the finished casting.

I do not think that pouring metal into an open-top mold with the fins on bottom ad base up would work, because force of flowing metal would break some of the fins in the mold, the base would end up porous and slaggy and not-flat

I would arrange several mold cavities around a central pouring well. Well connects via a gate to a corner of each part base. Top corner of all fins on each part connected to one riser. Top of part at least 2" below top of mold.

You could do all this with cavities formed in plaster by a metal master, if your open-top mold were then inverted on a flat piece of plaster, EXCEPT the risering. An experienced founder, which I ain't, could tell you how to do that. I doubt your molds could be made porous enough to vent without risers. Risers also tell you, while you pour, when all your molds are full.

As for the plaster, I had to burn wax out of my molds as well as moisture, used 900 deg F for several hours. After that, the molds were too fragile to take from the oven in one piece

Eventually settled on about half plaster half sand and 1-2% chopped glass fiber, a type sold wet so it dispersed well in the plaster mix

The plaster set so damn fast I could barely get it mixed. I poured the Al, salvaged from automotive AL sand castings, at about 1400 deg F Melted it in a stainless steel pot made from 8" SS pipe, in a coal-fired furnace with air blown by a bicycle driving an old Buffalo blower. Took 3 hr from cold to bring 100 lb AL to pouring temp. Too bad I did not have digital camera yet when I did it.

DO NOT cast into normal "Plaster of Paris" It will explode in your face -- been there done that with lead.

Normal plaster is calcium sulfate that starts out as hydrated gypsum To make the plaster they heat it to 300F to drive off the water as steam. When you cast the plaster by mixing it with water you are just rehydrating it. The next time it sees 300F the reaction reverses and generates a lot of steam blowing the mold into pieces splashing hot aluminum or in my case lead all over the place.

I believe potters clay in either a solid for slip form could be used once air dried then lightly fired to remove all the moisture -- it is a silicate and a different animal all together.

Craig

If you are just starting out in DIY or small volume casting, there is an enormous amount of info and help here. You will need to sign up but that is straightforward. AlloyAvenue network

investment casting

you should consider investment casting material
Americast Investment, By Ransom & Randolph | Jewelry Making Supplies - Casting - Beading
.
it is designed for casting.
.
i have had a cast iron mold explode when poring iron into it because it was not preheated enough. the sparks was like being in the middle of a fireworks explosion and went at least 10 feet in all directions. lucky i had a face shield, hard hat, welder jacket and apron and heavy work shoes

Madis Reivik said:

Pattnmaker, I saw your post after posting this - is the 450 degrees Celcius or Fahrenheit ? By bottom pour, do you mean that aluminium rises from heatsink bottom and fills rib ends in the end ?

We have pressure casting machinery available, but they can be used only with steel moulds - which is very uneconomical for <100 parts.
Maybe we can use long inlet, so the molten metal column exerces some pressure ?

Click to expand...

450 F . What I mean is the metal fills the mould from the bottom. In sand moulds this is usually done by a runner bar or sprue running down the face of a coreprint or inside a core. It can also be done with a 3 pc mould.

If you already do pressure casting and this is to be a repeat process maybe you could make a blank mould/flask and use the plaster inside that.

I don't need to warn you but I must mention that even a couple of pounds of pressure can spray aluminum across the shop.

ZAGNUT said:

better be a big place as that's the vast majority of the world. but don't worry, just as soon as we get done shoving the metric system down your throats we'll deal with your bad pronunciation of aluminium. you WILL be assimilated

Click to expand...

Tell you what: when you convert the Brits and Canucks to Metric, get back with us.

I subscribe to several British model engineering magazines, and it cracks me up that half the blueprints are in Imperial dimensions, and the other half have hole offsets of 12.7 mm

Madis Reivik said:

A lot of heatsinks are cast, want to see some - look under the hood of car, often the control electronics box has fins. Halogen lamp enclosures are another example.
And correct spelling is A-L-U-M-I-N-I-U-M, of course when you're a redneck with mouth full of chewing tobacco, its easier to say aluminum or, better yet with Libyan accent: ALUUUUMINUMM.
And if youre not yet insulted enough - your famous "feet" is'nt even a third of our METRE. And our KILOGRAM beats your pound twofold and still has some overhead ! And your gallon is just a swollen bloated distorted version of out nice tight LITRE.

I must explain that I'm only newbie with casting aluminium into plaster mould. I know quite lot about pressure die casting and machines.

Click to expand...

You said a "typical" heat sink, which I imagined to be a relatively small, electronics-type heat sink. They are "typically" extruded, not cast.

Production automotive aluminum castings are die cast using high-pressure equipment, a totally different beast than plaster casting.

When plaster casting metals the plaster has to be reinforced with silica and kiln dryed.

There are many tricks to casting that take a long time to learn and master. For example, one trick for creating fine structures that end in a curved surface is you make the mold open at the top and then build a secondary "roof" that sits a few hundreths over the upper surface of the mold. The excess metal rises into the plenum below the roof and is later removed as flash.

Dealing with molten metal is not an area in which you want to "wing it". If you are not familar with the processes get comprehensive guide books and study them closely before attempting a project. Ideally, you want to get an experienced instructor as well.

There are many, many mold materials that are mixed with water and cast like plaster of Paris that are formulated for the temperatures of molten metals. The one I have used with good success is cristobalite

Cristobalite - Wikipedia, the free encyclopedia


It and others are readily available from refractory suppliers and often from hobby craft stores. Why reinvent a process that has been refined for thousands of years?

Bill