Mould size

[FoundrySE]

Im working on a mould (will propbably be cast like split mould but cut ompletely aprt into a 2-part mould) for some 28mm legs.

How much mould meterial do you recommend between the edge of the widest component to the mould box wall?

I don't have the means to post a picture of the parts nor do I have the skills to draw something on my tablet but I'll try to illustrate with text...

|LEGO| space_a | object | space_a |LEGO|
|LEGO| space_c | space_c | space_c |LEGO|
| BOTTOM_OF_BOX | BOTTOM_OF_BOX |

object(multiple objects on sprue) varies from 8mm to 22mm
space_a is space from objet to box wall
space_c is space from sprue to bottom of mold.

Or should I try to remake the mould into a proper splitmould?
If so, do i pur into the "waist" and down into the legs or reverse?

[mangozac]

Split moulds work great for things like legs, but only if you're using vacuum in your casting procedure, otherwise you'll have big air pocket issues. People generally sprue legs so that the feet are closest the sprue.

[space_c] doesn't need to be much at all, and for a split mould could be as thin is a couple of millimetres.

[space_a] you want to be wide enough to help with mould alignment on a split mould. At least 5mm, but 10+ is better. It's kind of proportional to the volume of the parts being cast.

For a 2-part mould both [space_a] and [space_c] need to be wide enough to have some locating lugs - generally at least 10mm.

[FoundrySE]

I intended to cure under pressure but a two part mould for legs should rather be cured under vacuum?

[FoundrySE]

What are the recommendations for sprues with arms, torsos, heads?
Important for me with the arms/torso casts is that I get good holes for magnets

[mangozac]

The problem is that you're going to have trouble getting the resin down into the legs. Pressure will crush small bubbles but can't remove large air cavities. You might have some success squeezing the mould a bit as you pour the resin in, but the downside to that is that it can cause mould slip...

[FoundrySE]

My sprue is currently modelled from Paulson's guide.

With the legs oriented upside down (feets toward the vent)

[mangozac]

Ahh ok. If you have the separate entry and vent channels you should be fine

[FoundrySE]

Sweet! so then just pressure is acceptable?

Now I JUST
need finish the work on the masters,
And prepare the other sprues,
And get some new RTV,
And get some new resin.

And find a way to pay for it all...

[mangozac]

Well I mean vacuum and pressure is ideal, but sprued like you've shown you should be able to get acceptable results using just pressure

[FoundrySE]

I am aiming for a vacuum chamber but it is not likely to be ready until this fall

[FoundrySE]

Vacuum ventuuri pump sourced today...

[mangozac]

You're going to use a venturi device, huh? This will be interesting - I don't know of anybody else currently using one. Do you have a compressor that puts out a significant flow rate?

[FoundrySE]

We'll se if it's beefy enough, I'll figure something out otherwise, My brother has a bigger one that's too big for his needs. It used to run a pneumatic robot for sharpening saw chains for large scale harvesting machines. It used to run for like 6 hours continously every day...

[mangozac]