Core mould material(casting)

[BrianGar]

Hey guys, quick one regarding material suitable for use as a core moulding agent to be used in a sand casting process.

Basically Im making a pretty basic but irregular core.
My thoughts were to make the core pattern from wood or hard foam(easily worked)and then pour plaster of paris around that in a two pour setup using a release agent on the the first pour half.Then filling the remainder.
Mould will then be able to be split and pattern removed.
On re-assembly the mould halves will then be able to be packed with casting sand, perhaps oil or c02 cure type, in order to produce core.
Is plaster an ok material to use?
I have done a lot of reading and it looks like most big places make the core moulds from either cast alloy or machined from billet, which to me is a costly and long process for just a one off casting.
But at the same time it would be nice to be able to reproduce a core if i wanted to.
I have a casting book on order aimed at one-off casts to get one or two more answers to questions i have but it wont be here for a few days and ide just like to have something to think about.
I have a lot of reading done besides and im pretty happy with everything else, shrinkage, sand choice, binders, dimensional change, etc but alot of whats out there is aimed at high volume, hence the alloy core moulds.

Thanks in advance, Brian.
ps, the part in question in intend making is a cylinder head.

 

[arunmrao]

You can try a broken graphite electrode ,cheap and easily machinable. else,polystyrene material for core.


If you think education is expensive, try Ignorance.
- Andy McIntyre


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[BrianGar]

Andy, thanks for the prompt reply, i was just thinking when you mentioned polystyrene, i wonder if a 2 pack pourable resin could be used to make the core moulds?
Just to make things clear, when i say core mould, i mean the mould used in which the sand is placed in order to make the sand core, sadly, im not to familiar with casting terms so i do apologise.
Brian.

 

[MikeHalloran]

If you insist on making sand cores, the mold materials you can use for the cores are basically unlimited, including resins, plaster, metal, and concrete.

For a one-off, you might consider machining or otherwise fabricating a complete part pattern from polystyrene foam or CNC proving wax, and doing some variation of lost-foam or investment casting, instead of dealing with the complexities of making sand cores.

Or ask a professional metal founder to propose and bid on a process, assuming you have other things to do than learn foundry skills the hard way.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianGar]

Thanks Mike, mould advice taken on board.
This isent a profit thing, Its just something i want to do, and do it all myself for myself.
Its been clawing away at the back of my mind for a long while, and at last, im going to make it happen, its going to be a fine learning rollercoaster, but im looking forward to it immensely!
Brian.

 

[MikeHalloran]

Ah, you _are_ intending to learn it the hard way.
Could be fun.

I could swear that I had a Boy Scout Merit Badge Handbook that covered foundry science pretty well. I don't think it was just a chapter in 'metalworking', but I could be wrong; it's been fifty years since I've seen it.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianGar]

Ya! the hard way, hmmmm, ive read quite a few books, but many parts at aimed at high volume so, dont really apply, theres 1 book im waiting for on one-off low volume casting so hopefully ill ''know it all'' then!!!!!(i joke)
If a suitable core moulding process or indeed actual core material comes into your minds for the skeleton core needed to create internal waterways im all ears! Keeping cast material shrinkage in mind, as far as i know the core material must be able to contract as surrounding metal cools and also allow the transition of hot gasses, and of course be easily evacuated without the use of expensive machinery/processes once the casting has cooled. Yes, i know, i have much to learn yet.
Brian.

 

[ScottyUK]

A guy called B. Terry Aspin wrote a few articles on foundry work for 'Model Engineer' magazine in the 70's or 80's and at least a couple of very practical books for the backyard foundry around the same time. They were't particularly expensive as I recall. Google will find them quickly enough.


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If we learn from our mistakes I'm getting a great education!

 

[BrianGar]

Thanks a lot Scotty, ill check them out.
Brian.

 

[ajack1]

If you just want a one off polystyrene is ideal as you do not need split lines or draft angles basically you just pack sand around it and pour the polystyrene melts and is filled by whatever you pour in.

Here are a few examples of the type of things that can be done.

 

[Compositepro]

You can find casting videos on You-Tube.

 

[MikeHalloran]

You will also find an incredible trove of printed material at Lindsay Books:

http://www.lindsaybks.com/prod/index.html

Mike Halloran
Pembroke Pines, FL, USA

 

[arunmrao]

Making molds and cores are fine,but get an experienced help when you handle molten metal. It is a different ball game.
Good Luck .


If you think education is expensive, try Ignorance.
- Andy McIntyre


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[BrianGar]

''It is a different ball game'' what comes into your mind most? dangers? molten metal characteristics?

To the above posters, thanks guys, all taken on board.

 

[racookpe1978]

When I teach casting at the local Southern Museum (it has the original 1890's era molds, patterns, and locomotive works from the Glover Machine Shop) here in Kennesaw GA, I start NOT with molten metal but with a rapid-setting two-part pourable epoxy resin from ALUMLITE Corp.

They sell the mold release agent, the liquid silicone mold-making stuff (what replaces "sand" (or other material) in the casting process in the mold for trial and one-of-a kind testing, the resin itself (what replaces the molten metal) and have good instructions on casting.

Alumite is not so good for info about the pattern-making process itself: Look at Astragal Press "The Pattern Maker's Assistant" circa 1889 for extremely detailed info at around 15.00 to 20.00 dollars investment.

Basically, the two-part resin replaces the molten steel - but it is pourable, hardens quickly, is forgiving of mistakes, is not dangerous, mimics the molten metal for trails, shows you where errors in your pattern, vents, split lines, air traps, procedures, methods, cores and patterns etc are before you try the real thing. After the resin hardens, then you can machine it just like steel or aluminum - obviously much easier and faster though - to complete a test installation of whatever you're making.

Couple of reminders: The PATTERN is the key: You need to make every dimension in the pattern accurate exactly accurate - but 1/8 LARGER in all dimensions than the final (pre-machined) product. This allows for metal shrinkage after the solidifies. (The two-0part resin will not shrink as much.)

The PATTERN is the solid outline of the final part - but it can be made of anything convenient and in any way you can invent.

The PATTERN must be built to create an exact indent (#D outline) into the mold material (sand, silicon, dirt, whatever). OK, easy so far.

Now the tricky part: AFTER you make the "3D indent" (recess) into the mold material, then you have to take your pattern OUT of the mold material (without damaging or "breaking" off any corners or edges of the mold "hole" outline!) THEN you have to put the mold BACK TOGETHER in such a way so you can get the liquid molten metal BACK INTO the hollow indent you have created. (All the while venting out the air trapped in the hollow spots AND while allowing the molten metal (or liquid resin) to "overflow" the mold into UP the vent tubes and the downcomers. The DOWNCOMERS create enough additional force from gravity above the mold's outline to force the liquid material into the mold's nooks and crannies, the vents allow air trapped under the (rapidly cooling) liquid to go out and up harmlessly.

The hard part is dividing the pattern and mold into enough pieces at the right places (split lines) so this "little task" can be done at all. Again, a good reason to practice with a flexible (silicon) mold material and wood patterns and rapidly setting resins rather than sand and molten metal.

Cores? A core is a separate part of the molding process: Consider it a "breakable" or throwaway rigid but fragile piece inserted BACK INTO the 3D indent as you put the now-empty mold back together. The "core" reaplces "air" spaces in the pattern so hollows, shafts, inner recesses and such can be made in the final cast product. Example: You want a "hole" through your part for an axle or shaft. But your pattern has to be made solid. So you make a solid pattern, press into into the mold material (sand or resin), then remove the pattern. Now you put the core piece back into the mold, put the upper half of the mold back around the core. Pour in the liquid (two-part resin or metal) and let it harden. Pull off the upper half of the mold, pull out the newly cast part - which will have hardened around the core. Break the fragile core material out of the hardened part and scrape out whatever residue remains on the walls of the part.

Your part is ready for final machining with a hollow interior.

 

[BrianGar]

Excellent, Im forever great full that you took the time to type the above reply.
Brian Garvey.

 

[ajack1]

That is one of the main reasons why they make one offs from polystyrene, everything below the now for the tricky part does not apply, you do not need split lines draft angles etc or to split the sand.