Complex casting

[dmac]

Watching with interest

 

[kvom]

Novel approach. The cardboard part in itself is pretty cool.

 

[Jasonb]

1mm was added to the valve cover surface and 1mm (2 sections of card) was added to the head surface. shrinkage is added by layers of U-POL high build primer number 5.

matt+

Missed that bit about waxbut you need to think again about shrinkage, your primer may be OK on the thickness of the parts but it won't account for overall shrinkage, you need to make evey thing approx 2% larger as the casting will shrink in all directions.

J

 

[Nickademusss]

This is an interesting way to make castings!
THe other posts about metal shrinkage are right, my own experiments in casting soft metals like brass and aluminum, show that the metal pulls away from the mold as it cooled, not much bt some, did you make your positive a little smaller so that you final cast will have enough material for machining?

You should also look into the 3D printer, looks like great fun to play with !

 

[matt-jaysey]

Yes I have made allowances for shrinkage, the casting is 30mm wide, 2% of this is 0.6mm the filler used to even out the texture with the addition of the high build primer adds about 0.8mm each side of the pattern. The model itself was scaled up universally by 102%. This addition also makes allowances for the shrinkage of the wax when it itself cools. The machinable surfaces have 1.5mm excess so they can be planed. Ive done some test runs before doing this version and they range between +0.2mm to -0.1mm out of spec.

matt+

 

[John S]

Matt,
Colour me impressed, a very novel and unique way to obtain various shapes that may not be related to casting.
From halfway down the first page I was hooked.

John S.

 

[Billzilla]

Hey Matt, if you're interested I could make most of those parts with my 3D printer.
I've read that the ABS plastic is uses just burns away without any residue when used in sand casting, so it might be a good thing for you. It'll print objects up to about 130 mm cubed.

And my usual car is a 2.5 litre WRX.


Edit - If you want to see what the printer can do, please send me a copy of something, say a conrod file in .STL format, and I'll print it for you as a sample.

 

[Anzaniste]

Im then going to submerge the wax in green sand and turn the sand box upside down and melt the wax away in the oven. Once this is done i'll have a void in the shape of the head. Then in goes the aluminium and bam! i'll have a perfect copy of the head in aluminium.

Here's where I think you will have a problem.

In the days when my company used to make investment castings we used to coat the wax pattern with a ceramic slurry which was then allowed to dry. The dried ceramic covered wax pattern was then baked which had the effect of making the ceramic shell hard and also completely removing all traces of wax by vapourising. The thin delicate shell was then supported in packed sand for the metal to be poured.

With your more direct approach I fear some of the melted wax will soak into the green sand waiting to vaporise violently when the molten metal is poured. I remember as youngster the narrow escape I had when experimenting with casting lead in damp sand moulds.

It is some years since I was involved so I referred to wikipedia to refresh my memory with reference to terminology. Look up both investment and lost wax casting for a much mor detailed explanation than mine.

Your idea seems to me a super novel way of producing the wax pattern that I shall follow with interest.

 

[JorgensenSteam]

I am not sure your process for making the cores will work.

I am no casting expert, just a beginner, but it seems like submerging wax in green sand and then heating it will just cause the wax to soak into the green sand and make a big mess.

Many (including myself) use a fine sand mixed with sodium silicate core binder, which can then be solidified with CO2. If you use the correct amount of binder, and a fine sand, you can produce complex cores with very little trouble.

 

[matt-jaysey]

Thanks for the comments

Its a very good point, but thats what im doing, giving it a go and see what works. Im not sure that the wax will vapourize voilently?! Look at when lost foam is used, this remains in the mould and foam is a polymer. Most of the wax will be melted from the sand then the metal poured. Because of the density of the alumiunium in comparison to the wax the wax should defuse further through the sand and not into the mould, but we'll just have to waiting and see. Im probably going to have to create a custom sand blend for this kind of casting method.

matt+

 

[Tin Falcon]

Another non expert but we do have a couple guys here very experienced in lost wax /invesment casting.
in the reading I have done investment casting is suited to complex shapes. and a smooth finish.
so that would likely be the way to go.
tin

 

[90LX_Notch]

It's been awhile so some of this is fuzzy; as I remember it:

The foundry that I worked in used autoclaves with high pressure steam to melt the wax out. The ceramic shell molds were then fired in kilns to set the ceramic. Plaster molds were baked for days. Both types of molds were preheated before the pours to ensure that any moisture was removed from the molds. Water/moisture is the real "bomb". It flashes to steam and causes violent results (Ceramic and plaster don't make good pressure vessals). Residual wax caused more of a bad casting (voids/inclusions).

-Bob

 

[matt-jaysey]

Like i said all I can do is give it a go. Obviously being very careful. I have tried this in a simpler fashion before and the casting wasn't very good but that was probably down to the speed and lack of prep. But anyways im gonna have a go and report back with what happens.

One thing i was thinking of would be to 'spray' the wax with somekind of barrier agent before putting it into the sand. They do this with investment casting, i was thinking something like charcoal or something, not too sure yet. At present the best thing to do is to cast first and work out what needs to be changed in what way. Thanks for the comments though, they are really appreciated.

matt+

 

[JorgensenSteam]

I think heating green sand will cause it to loose its shape.
It is difficult enough to keep green sand in the proper mold shape without heating it.

Pouring molten metal onto a combustible material such as wax is a sure way to disaster, no doubt about that. (Get lots of life insurance if you decide to use that method.)

Don't forget that the sand has to vent also, and so if you seal the mold with melted wax, then no venting will occur, and you will either blow out the mold, or have tons of holes.

I have seen many play around with custom green sand and various baked core mixes, but often people come around to petrobond and sodium silicate core binder because the petrobond makes for a very fine finish, and does not have most of the gassing problems since it contains no moisture.

The sodium silicate makes a quick and easy core in minutes with no baking, and if used with a fine sand such as 400 grit, it makes a very good finished surface.

Experimentation is good, just don't take chances when pouring molten metal.
It does not like the slightest bit of moisture, or it blows up in your face, so be prepared for that event (full face shield and leather coverings on everything, including the tops of your shoes and your neck).

 

[matt-jaysey]

After talking with some of you im kinda going off the idea. Can i ask one thing though. I wet my green sand before to make it bondable again when used several times, surely this is adding moisture to the mold when pouring?

matt+

 

[matt-jaysey]

Just been doing some more research, in loss wax casting, the wax replica in covered in a silica slurry then dipped in sand, this is repeated until a inch thick coating covers the wax. The wax is then heated and it melts away. The silica shell is then cooked for a couple of hours and sometimes water tested to reveal cracks. It is then dried out and molten metal is poured in. The silica shell doesn't vent any gas out apart from afew vent holes and the spru. There's no mention of retained wax, pressure explosions or anything. The silica shell is the only thing stopping the molten metal escaping, but no venting through the actual material. Green sand vents slightly, so this surely wouldn't hinder the casting. The point about heating, i fully understand what you mean but when a cast has been poured, the sand seems to hold pretty firm until you give it a good tap, but thats the same before a regular pour. I not saying any of you are wrong and your words of caution are for my own benefit. Because of this im going to try a very small piece first. Im going to set up a 2 inch deep crucible and rig up and rod operated pouring craddle, just incase it goes pop. I away pour with rigger boots on full apron and face shield, and 2 pair of welding gloves.

If it goes pop 4 feet away is too close but better than over the top of the damn thing! if the sand doesn't hold then its back to the drawing board or get some silica and start dipping. One other thing. The wax im using melts at around 75 degrees centigrade, which possible might not disrupt the integrity of the sand but i break one apart before pouring my first cast just to have a look. I'll post picture of how this works out. Thank you all for your input.

matt+

 

[John S]

Matt.
Please keep us informed, I for one are very interested in that i can see a commercial application for this process on parts that cost 5K upwards by conventional means

 

[RonGinger]

Just been doing some more research, in loss wax casting, the wax replica in covered in a silica slurry then dipped in sand, this is repeated until a inch thick coating covers the wax. The wax is then heated and it melts away. The silica shell is then cooked for a couple of hours and sometimes water tested to reveal cracks. It is then dried out and molten metal is poured in.

I believe the shell is embedded into a box of sand to support the shell.

I have done several small investment casting, and have had many made in a commercial foundry. These use the old jewelers process of investing the wax in a plaster like product. The investment cures in an hour or so then goes into an oven where the temperature is raised slowly. After an hour or so, with the temp at about 200 dF the wax melts out. I stand my molds upside down to just let the wax run out into a pan. Then they are placed upright and the temp raised to abut 1200 dF over 5-6 hours. At that point the metal is poured in. You do need to keep a vacuum under the investment to help get the gasses out and the metal in.

This process will result in extremely fine detail look as your wife's jewelery collection to see some of the detail possible.

 

[90LX_Notch]

I never suggested pre heating sand molds; that's a whole different animal. The foundry I worked in did lost wax in two different "flavors". One was the ceramic type and the other was plaster of paris with chopped fiber glass. We also did RPM. Rubber Plaster Molding where a silicone rubber pattern is coated in corn oil and placed in a flask. Plaster is then poured around it and once it is sets up the pattern is removed. The corn oil acts as the release agent. These were for centrifugal supercharger impellers.

The ceramic molds never had residual wax because the ceramic and sand molds are fired in a kiln. Any wax is totally burned off. The Plaster Molds rarely had wax residue. Even then it's trace amounts. As stated above, the wax was melted out in an autoclave. Nothing ever exploded from wax residue.

Water is what you have to worry about. Once it turns to steam the volume increases by some crazy number. Hence the danger of explosion.

It's been a long time since I worked there; so I might not have it all correct. It was a neat place to work but the health hazards were frightening. The sand is Free Silica Sand. Very Bad stuff before it's fired; causes lung cancer. Chopped fiber glass floated through the air etc.

 

[JorgensenSteam]

Here is a good site for some good casting information, since there are a number of methods covered.

I got into casting recently, and am still early in the learning curve, but as I understand it, the water-based green sand does have water mixed in with it, but it is not a large amount of water, and your venting is much more critical if you use water-based sand.

Pouring molten metal into ingot molds can be dangerous if you don't pre-dry the mold. An ingot mold may look dry, but unless you put it into the flue gases and get it hot, it is not dry, and it may eject the metal into your face. Likewise with any other metal, crucibles, etc. It is not dry until you put it either into the oven or in the flue stream for a while. Crucibles absorb moisture, but you can't see that.

My experience with petrobond (oil-based green sand mix) is that you can get a very fine finish with it, and little if any gassing problems. You do have to mull 30 weight non-detergent oil into it every so often, as it will dry out even though it is oil-based.

I looked at many different methods before choosing petrobond (green sand) casting.

The whole lost-wax process is too complex for my home shop.
You have to cast the wax, dip the wax into ceramic slurry and then sand multiple times, have a bake-out oven that operates at 1200 degrees to get the wax out, and then you have to break off all the solidified sand/slurry after you are done, and you can't reuse it I don't think.
Your propane usage would probably double with the bake-out.

I really don't have time to spend a day making and baking the molds for the lost wax process. With green sand, I can start the furnace and put in some AL, then while the metal is melting, ram up a mold in 15 minutes, make a core in 15 minutes, be pouring in 45 minutes, and be cooling and trimming the part to size within an hour. Can't do that with any other method that I am aware of.

Accuracy with petrobond and fine sand cores is very good (if you are careful in your setup), and too good to justify lost wax or other methods (for me anyway). Petrobond can be reused repeatedly.

Here is a link with many casting methods examined.
http://www.sfsa.org/tutorials/index.html

Here is a US supplier for casting supplies.
http://budgetcastingsupply.com/

I don't use flux with AL, and you don't need to use flux or degasser if you heat quickly, don't stir, and pour at the right temperature (not too cold).

Pat J