[Biggermens]

Hello Everyone

I decided to try lost foam casting but it did not work for me I am looking to see if someone can help me out.

This is what I did
I made my pattern out of pink foam insulations, like in the pictures
I put the pattern in a bucket of fine Sand ( play sand from home depot)
I melt my aluminum and I put a tea spoon of table salt for flux
Then I pour the aluminum on the foam in the sand at about 750oC – 800oC
After cooling I pull out the aluminum and it did not melt all of the foam

What did I do wrong ????

Thanks in advance
Biggs

I put pictuers up so you can see what hapen

[davidmb]

Hi, this looks like a 'cold shut', I have no experiance with lost foam, most of what I do is based on traditional silicate bonded molds, it looks like either:

1) Too much gas is being liberated by the foam in the fins, stopping them from filling, liberated gas may be preventing the alluminium from being able to burn out the foam.

2) There is not enough heat capacity in the alluminium that has started to fill the fins to complete the burn out.

Solutions, maybe.

a) Make the main web heavier, this will feed heat into the fin area helping the burnout, machine off the unwanted thickness afterwards.

b) Precoat the foam pattern with a silica based dip, similar to lost wax casting and burn out the pattern in the furnace before pouring, setup in soft sand and pour.

[nuplowboy]

Welcome to lost foam! 'Tis a fickle mistress, that's for sure. But, so is really any type of metalcasting.

Can you provide some more info, like how the part was oriented in the sand? Lost foam tends to like different orientations and gating than "normal" sand casting would.

davidmb is right about it looking as though the gas in the thin fins blocked the entry of the aluminum. A few things that can help would be the orientation of the pattern in the sand, hotter aluminum, or some venting to allow the gas to escape another way.

[Biggermens]

Thanks for your help David

I think i will try to make them a little thicker and pour my melt hotter maybe at about 1100oC to 1200oc

What do you think of that ?

Do you have a idea how else i can vent the gas out ? i thot all of that vented into the sand


Thanks
Biggs

[Biggermens]

Hello Guys

This is how i placed my foam.
How would you vent that better ???

Thanks nuplowboy
Thanks David

Biggs

[davidmb]

Hotter temperatures will mean more absorbed gas in the ali, if you do go to higher temps ( at higher temps the foam will generate gas much quicker ), also don't let the melt soak at this temp. How fluid is the alluminium when you pour it, maybe a higher, thicker riser will provide enough heat capacity to prevent the metal in the fins solidifying too soon. ( feeding heat into the rest of the metal holding back the start of solidification )

How dense is the foam that you are using, I know my local foundry suppliers stock polystyrene balls to include in made up sand molds, but they look to be very light.

[Biggermens]

Hello

The foam i have is very dense and hard, it is the pink stuff used to insulate basment foundations ect..

This stuff
Owens Corning FOAMULAR® C-200 Extruded Polystyrene Rigid Insulation

When i pour my aluminium is nice and liquiddy

Hotter temperatures will mean more absorbed gas in the ali,
I dont know what ali meens i try to look it up on google all i get is movie stuff hehe

Thanks agine guys
Biggs

[Dennis Rech]

You need to increase the head pressure on the mold inlet. You can do this by placing a tin can with both ends removed over the inlet. You can let this partially stick above the sand.
This will more than double the PSI of the aluminum charge and it will also increase the thermal mass feeding the mold. Right now, you do not have enough pressure forcing aluminum down the mold nor enough hot aluminum to fully melt all the foam.
Another 6 inches of riser will make a big difference.

[Biggermens]

Hi Denis

What do you think of a 12" x 2" tail pipe ???

[paprjam]

By Ali he just meant your Aluminum will get gas bubbles that may show up in the casting.
You could coat the part with drywall mudd and let it dry, then pour acetone (fingernail polish remover will work) into the mold and swish it around and pour it out. Do this a couple times and then let dry totally. Wait for a couple days, I wouldn't want to have it blow back from the acetone. Bury it in the sand again and pour.
Also, the white beaded foam does burn out better than the pink or blue, but it's also much harder to get a good looking pattern because it so crumbly.
Good Luck and be careful.

[Dennis Rech]

Muffler tube will work fine.
Keep in mind that you will now have a two inch diameter steel encased sprue that you will have to cut off. Also, if you get too much head pressure, there is the possibility of a sand blowout.
Those long skinny fins are going to be a tough pour. The sand will be sucking the heat away pretty quickly.
Best of luck,
Dennis

[wjfiles]

In your early post you say that you used play sand.
Sand casting needs a sand mixture, a fine clay mixture and damp.
The correct consistency I squeeze a ball in my hand and then break it in half. If it breaks in two lumps it's fine .
Also in your drawing ,I would add another sprue as a breather from the top of your casting. Don't pore your metal too slow.You must also tamp the sand around the pattern.
WJF

[davidmb]

Hi WJF,

What you are talking about is green sand casting, the method being discussed is lost foam casting, a sacrificial foam pattern including a foam riser & sprue are placed in a loose sand ( straight unbonded silica sand ) and the metal poured straight onto the foam sprue, remarkably the metal burns out the foam, taking its place without the sand shifting, sounds far fetched but is finding some use commercially, not good for high tolerance parts, but is very very cheap, some patterns are made in an injection mold, the injected pattern is usually expanded polystyrene. Fumes are a real issue when pouring as most plastics break down at high temperatures into very unpleasant and harmful by-products. Never tried it myself, but I pour most of my molds inside the garage.

[Dennis Rech]

I found that using swimming pool filter sand gives a nicer finish.
It consists of a variety of sand grits, from 60 to 200, so it packs more uniformly around the foam pattern than standard 100 grit silica play sand.
The local high school shop class brushes melted paraffin wax on the foam patterns to smooth the texture and it does seem to work very well.
I just bought a sheet of the new purple foam from Home Depot last night. It has a very fine grain structure. Have no ideal if it will machine or even work for load foam casting.
Will report the results.
Dennis

[wjfiles]

Yes I was talking about lost foam casting . I use white styrene foam and often coat it with a ceramic paste to retain fine detail. Sprue sizes and venting principles are more or less the same for either method of casting. I am speaking from practical experience as I use both methods and also lost wax casting.
wjf

[wjfiles]

Hi Davidmb,
I started serving my time in the paternshop just over the hill from you at Leyland Motors foundry at Farrington in 1948.
WJF

[davidmb]

Hi WJF,

Nice to be in contact with someone who has really worked in the industry, you are a real rarity these days. I got into casting about 10 years ago, could not find any small shops that would do one offs so I have had to learn what I could, I was fortunate in finding a retired pattern maker who taught me a lot. I switched from petrobond to silicate binder quite quickly mainly because I soon realised I am not skilled enough to make up molds in the old way yet, maybe a couple more years. I live near Keighley west yorkshire, and at one time it was wall to wall with foundries, now there is just one left and a big boiler makers, most of the rest of the town provided ancillary services or used the output from the foundries.

I heard recently that 'Dean Smith & Grace' a one time big lathe maker based in Keighley is making a come back, they hung one by making large precision bearings for the Japanese, Tiawaneese and Chineese markets.

[Biggermens]

Hello Guys

Agine i just wanted to thanks everyone for your help it is very apreciated.

This was my game plan
like in the picther 12"x2" tail pipe and pour at around 1000oC

I was looking at other sites and one sugested to put a vent on the foam patern ?? What do you guys think ??

Thanks very much
Biggs

[davidmb]

Hi Biggs,

Not sure about pouring the alluminium all the way down that pipe, don't forget for good castings you do not want turbulent flow in the metal as the mold fills, especially with alluminium, also the steel pipe is not porous so will not be able to release the gas produced by the foam that is at the bottom of the foam, could be highly dangerous, ever seen an alluminium fountain? Because the foam you are using is quite dense there is probably a lot of material to ablate/ vapourise and this will produce a lot of gas down there in the mold, when you poured the original, did you get a lot of gas released, ie: did the alluminium appear to boil in the sprue area or was it calm? Also what did the top of the sprue look like once the metal had set, if it was convex like the head of a cabbage then you have a big issue with too much gas, it should ideally look flat with a concave dimple in the middle.

Also the vent on your diagram is not going to act as a vent untill the foam has burnt out of it, which is going to be when you have filled the mold, so may not be of much use, with foam casting I guess that the gas has to pass through the sand. It may be that your desired form is not suited to this method of casting, you could use your foam model with a foamed plaster mold, basically mix up some plaster of paris with a small dash of washing liquid, mix with a high speed rotary mixer until it foams and then place your foam model into it, fins down, let the paster set and dry for several days and then burn out the foam with a torch, you then have the equivalent of a single use ingot mold, for the plaster try a dental suppliers for dental investment plaster, it is used to cast gold and has a better temp resistance than plaster of paris.

Sorry for such a long reply.

[Biggermens]

When it harden to a nice concave dimple in the middle

I have a few game plans for the weekend if the snow could stop comming hehe

Thanks everyone for all of your help
Biggs