off-axis undercuts

[madmagician]

I posted this on a different forum that I know some of you frequent, but this looks like a better place to talk it over.

First off I want to be perfectly clear. This is NOT a post about recasting. It is a question of technique.

I have been bitten by the 3D bug and although most of my initial attempts have been relative failures, I have a few that have come back from Shapeways recently that I am really happy with. (I will share them when I am more confident in my design skills.)

So this has led me to look back upon some of my prior experiences casting custom bitz in a 2-part Silicone/Resin method. Long story short I pulled out some of my Forgeworld pieces to get a better vibe on mold lines and cut depth, etc. One of the first pieces I looked at was my precious Lucius Reaver. (cold dead hands, boyz...) I was immediately appalled by what I saw. the biggest no-no in casting, a off-axis undercut! (GASP!)



So......the question is how the heck do they even manage this? I thought this was no-no number 1 in casting? Is this a technique that can be mastered? If so, it will open up a new dimension (cough) to my 3D work.

mm

[mangozac]

Hi Madmagician, welcome to RA!

There's several attributes of that cast that could confuse beginners - which area exactly is your problem? The good news is that none of it is what I would deem non-trivial

[madmagician]

So I ran it through MS paint to highlight a couple of things

[url]
https://app.box.com/s/1sirz6m856dzspi1z87fy253dd9mpbsl[/url]

As best I can tell, the green line is the parting line (assuming a 2-piece mold) (I know from lurking that FW tends to use uses split 1 part molds). This line is technically not fully within axis but I have successfully cast items that had 2-part molds that were not flat (within a few mm of the axis). The main part that is confusing me is the cable bundles (red circles). Also are the triangular supports actually supposed to be filled or are they intended to be struts?

Thanks

MM

[mangozac]

OK well to start with casting with silicone moulds allows for massively more elaborate parting lines than hard moulds. The green line is indeed the parting line and is really very simple - it could quite easily be far more "off-axis" than that! I have split moulds where the split is cut in almost a C-shape.

The cables are something that I would personally probably avoid doing, but again they're not a big deal. See the one towards the top of the picture has a lot of flash on the inside? That shows you where there's a cut in the silicone so that it will release. Remember, if you use a silicone around ShoreA 22 hardness it is quite flexible.

Hope that helps!

[Fleafa]

They have just made two small cuts along the cables, perpendicular to the main split. I think those triangular bits are intended as detail, not support material.

[madmagician]

Thanks for the tips!

So, basically, in a 1-part split you would make some crazy careful cuts while removing the master.

[mangozac]

Yep! If it helps, you might want to try out a clear silicone to begin with, so that you will be able to see where you're cutting. I've never bothered though and don't have any issues cutting properly with opaque silicone.