New here, is this reasonable ?

This looks like a nice informative forum!

I have been researching sand casting and watching tons of videos and I'm thinking about doing it. Before I start investing in this, I wanted to get yall's opinion. I do have experience creating silicon molds, though I'm sure that only helps in theory with sand casting.

My original intention to use green sand casting is to help me make molds for plastic injection. I can't machine my own molds, I don't want to pay someone else to machine them, and I can't find anyone that has tried to sand cast aluminum molds for injection molding. However, maybe I don't know how to google that paradox.

I know, I'm heading down a path of making molds of molds to cast molds, to finally cast my part but it makes sense in my head.

My main question is what can I expect in the following scenario, assuming I can eventually get a successful casting.

One of my first tests will be to make an injection mold for a Lego, not the tiny ones, but the medium sized ones that are maybe 2 1/2"L x 1"W x 1"H. I picked a Lego to test with because it is detailed, small, and it needs to keep dimension tolerances enough to work.

1. I'm confident I can make the first 2 halve negative pattern correctly, probably out of plaster of paris.
(at this point I could use the plaster paris mold as injection mold, but I think durability and part release are big concerns.)
2. Then I will need to mold each plaster half in the sand
3. Remove the pattern, and pour aluminum into the sand mold. (This is where I ask for yalls opinion)
4. Repeat these steps for each plaster half.
5. Finally end up with two aluminum halves I can use in plastic injection!

Concerns:
a. thin walls of the Lego
b. shrinkage (could the plastic Lego made with the new mold fit the original Lego considering the shrink. I read it shrinks 1/8" per foot, but Lego's have tight tolerances I assume)
c. surface finish (depends on how well my cast went, but should I expect a smooth surface like the original Lego, seems like wishful thinking)
d. How well does molten aluminum capture detail and sharp angles?

I know I'm looking for a world of hurt doing this, so far I'm just dreaming about it. There has to be a reason I can't find people sand casting to create injection molds, seems like a no-brainer when it comes to cost vs machining.

Let me know if I'm crazy!

Thanks

 

Welcome aboard, you're right, this place is a plethora of knowledge. We don't have anyone here doing injection molding that I know of. As by now, you know their is many methods of casting metal and each has it's advantages and disadvantages.

1. Shrinkage of metal is overcome during designing of molds. This can be worked out.
2. Surface finish will depend on what method of casting you choose. Investment style casting is king here so that means in a flask or ceramic shell and that usually means lost wax. Have you ever seen a plastic injection mold? There is a reason they cost BIG MONEY! They are usually highly polished pieces to aid in the the release. This will require large amounts of hand finishing of the highest level.
3. Thin walls, metal will only CAST SO THIN. But re-reading your post leaves me kinda scratching my head. For injection molding, usually you have two halves that come together and plastic gets squished in. It gets rapidly cooled and popped out. The ones I've seen where not made of cast aluminum. Whoops!
4. Aluminum does very well with detail, but again it comes down to the method of casting. Many of us see finger prints in metal. Good enough? Sharp edges can be more challenging, but there are ways around that too.

I have a friend that has many parts injection molded in china. But he does large volumes of parts and it's worth the money. He pays 10k for the mold and then about 25cents a piece. So the big question is do you need 10 of these parts or 10,000? If you need 10, be ready to give up some of your requirements. If you need 10,000, Send it out to china. Just the way it is.
(and dump the plaster of paris, I would suggest you learn cad and get a 3d printer for your pattern. It will help you alter the dimensions to handle shrinkage)

How are you going to push the plastic?

 

Thanks for the responses!

I should have explained my intentions better. This will be hobby grade and small scale. I've yet to start building anything, it's all in my head. Trying to find reasons to convince myself not to do this before I start something I will feel committed to.

This video will give you a good idea of the size of the injector I plan to build (give or take)
Also the type of mold I aim to make. I don't need ejection pins and all the fancy stuff, and if I need to trim flashing (either aluminum or plastic) that's fine with me. I think I can sum this whole idea up as, proof of concept at this point. If I think it's working well I might try to scale up, but I doubt it. I don't even have good ideas about what to make at this point, just want to have the option!



Jason -
I originally was thinking investment casting the aluminum mold halves, but using plaster of paris and silica sand mix instead of ceramic, again looking small scale to first see if I like this and or its how I imagine.
I agree, I have not seen any made of cast aluminum either, but it seems doable in my head. looking to yalls expertise to confirm that.
I don't plan on cast lots of parts, just things around the house unless I get a great idea that I can sell. I'll cross that road if I ever get there.

Tobho -
The two piece injection mold would be similar to the above video. My pattern would need to be complete with index markers to ensure the mold is put together properly, and I would clamp it (or bolt maybe?) while injecting it.

I'm ok trying lost wax investment casting, in my head the same concerns apply for both methods, but I really don't know as I have never cast anything metal.

Thanks!

 

I think you want to try spin casting for this project. Lego pieces are extremely small and detailed internally and will need a lot of pressure to cast correctly especially in aluminum since it is such a light weight metal. I use spin casting for jewelry projects and casting flowers and plants from the garden into silver using this method.

What you want to do is make a silicone rubber mold of Lego parts, injection mold it with wax using a large steel horse syringe (aka anal lube syringe available on ebay - Jason can help you find the link ) then once you have the wax version of the lego sprue it up and cast it on a tree with several lego copies using the spin caster (centrifugal casting machine). You can probably get several lego parts cast that way at a time per plaster mold and can very likely melt the aluminum with a cheap blow torch mapp gas or oxy/propane setup from home depot.

Total costs would be maybe 50 bucks for silicone rubber, 20 for syringe, 150 for spin caster, 200 for burn out kiln, 40 for plaster investment (silk jewelry investment powder) maybe 20 to 50 for casting wax depending how much you get and maybe 30 to 50 for mapp gas or propane oxy or propane acetylene setup at home depot.

Injection molding directly using liquid metal under pressure into a durable reusable mold is going to cost you a boat load more money since you'd have a specialized way to put the liquid metal under pressure and inject it without freezing up your injector.

Another option is to do all the mold making but use a vacuum setup to draw the metal into the crevices of the plaster mold.

 

as a Master Of sand casting, you are asking more of the medium then it can accomplish.
1) too much detail
2) parts too small
3) dimensional tolerances too tight
4) the requirement for Draft will not get you where you want to go using conventional sand cast.

perhaps some of the high end resin bonded sands can do what you want, but that is big money and a huge learning curve, have your Molds machined .

if you where going to try something like making fishing sinker molds, sand cast will get you there, https://www.google.com/search?q=fis...WIm1kKHcJnC74Q_AUoAnoECA4QBA&biw=1920&bih=937


But to try and create the Mold for this, from sand cast , not happening


V/r HT1

 

I hear y'all with the sand, thanks for correcting my thoughts. I'm thinking about lost wax now.

Dumb question I'm pretty sure I know the answer to...
If I pour melted wax onto cool wax, is there a way to separate the halves? I'm pretty sure it would melt the surfaces together, ruining the first wax half and fusing the two together. Doh!

Some responses sound like there is some confusion. I don't want to cast metal Legos.
I want to cast a metal Lego two piece mold. That way I can use it repeatedly to inject plastic into it. And make plastic Legos from recycled HDPE plastic . Again the Lego mold is a test to see if I'm crazy, I won't be trying to duplicate all the Lego pieces

That being said, I really appreciate the feedback and I have some more thinking to do!

Good tip about the hungry Machinists, that might be an option later.

 

I like the idea of epoxy!

Checked prices and it's $80-$100 a gallon, not bad. Then I stumbled on Bondo... Hmm $25 for a gallon, that's more appealing and for my test I'll buy less, maybe a quart.

I think I'll try a Bondo mold first, if it fails I won't feel bad. I also see people using it as a mold for lead. I'll have to let y'all know.

This topic took a turn from metal, sorry about that. I'll update when I've tried it.

Unless y'all have worked with this and it's hopeless to try

 

Actually, I think I'll try plaster of Paris as the mold first, it's thinner and will probably mold better, and I'll take the idea of putting it in some sort of box like that video.

Who knows...

 

"lost wax...." We don't need no stinking draft!

 

The only thing stopping me from trying lost wax is buying the silicone to make the mold

Because if it works perfectly than I'd want to buy silicone for all the other molds I want to make and before you know it I might as well have bought a CNC machine!!!
Maybe not that bad but you get my point

 

If your plaster of paris skills is up to speed.. You can soak PoP in water and pour wax into it. It will release easily and there you go. Silicone is not really that bad.
Check out brush on rebound 25 or 40 for the firmer stuff. You don't need super thick silicone. Back it up with a mother mold and save some coin.

 

And buy some thixotropic too to thicken the non detail layers of silicone or you'll be painting hundreds of layers of silicone on.

 

Could try to cast a milling machine to mill your molds.
Patrick-C