Sculpture Mold Help

James Bray

I'm currently working on a character sculpt with plans of casting in resin. My experience in silicone molds/resin casting is intermediate, but I'm somewhat concerned about casting this piece.
Potential problem areas I'm seeing are:
1. Hollow interior, solid base
2. Hollow eyes/mouth
3. Tree limbs at the bottom


I was hoping someone more knowledgeable could provide some tips for getting the best silicone mold/resin cast from this sculpt.
Any help is greatly appreciated

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Chris Ellerby

That does look like a somewhat difficult shape to mold due to all the undercuts.

Do you have photos from other angles?   (looks like there may be a few undercuts not visible in this photo.

One option would be to cut off the limbs and mold them individually.  If you leave a small socket where each limb was, and then add a plug on the individual limb molds that matches those sockets you should be able to plug the cast limbs right into the cast base, and then do some seam cleanup to make them appear as one piece.

Another option would be to do a silicon glove mold like this: 
https://www.youtube.com/watch?v=FJ5q0sZgcx8

And here is the video for a resin mother mold for the silicone glove mold:
https://www.youtube.com/watch?v=Ut0fuAR_4QE

Glove molds work great for parts with a lot of overhangs/undercuts, but there are still limits to how complex the piece can be, as you eventually need to get the casting out of the mold intact, without damaging the mold.   And if the piece is too complex the outer jacket (mother mold) would need to be more than just 2 pieces.

This also depends a little on what material you want to cast the final piece in.

/Chris

Chris Ellerby

One more thing to add, that is a super cute character.  Nice work!

/Chris

James Bray

Thanks for the feedback, Chris! 
I updated the post to include additional angles of the sculpt. 

Haven't tried a Glove Mold before, but I can see the benefits from that video. The piece that still worries me, is the hollow interior (basically a cylinder) and slightly concave base (under the limbs). How that works in the mold is melting my brain. Would a Two Part mold have any benefits to this?

Casting the limbs separately does sound like a good option. Keeping the "bark" detail and hiding the seam will be a challenge. 

Up until now I've been using EasyFlo - 60. If you have any other suggestions though, I'm open to alternatives.

Chris Ellerby

Ok, now that I've seen the other angles I understand your concern!  The hollow nature of the top cavity really does complicate things.

Visualizing how to break a sculpt down for molding is a difficult process, and this shape certainly has its challenges.   It's a bit hard to visualize and identify all the possible pain points without seeing the sculpt in person.

One option would be to further break down the sculpture, doing something like slicing it in half from top to bottom, and then making several multi-piece molds.  That would mean more seaming and cleanup/finishing work, but would mean easier demolding and less stress on the silicone.

I think a glove mold might still work, but your mother mold would need to be a few pieces.  One of those pieces would need to be a core that slides down into the large top cavity to help things hold their shape.

You would also want to drill some bleeder holes at any high points (like in the roots) to allow your casting material to displace any air, preventing air pockets from forming.  Those bleeders can be sealed up with some clay during casting after you let them bleed out a bit.  They are also a good indicator for how full your mold is.

It looks like you might be able to have a large hole at the bottom (about twice as large as the pipe you have now) that could allow for the glove mold to turn inside out to release the cast piece.  I'm a little worried as the root portions of the silicone mold would require a lot of stretch to demold.  So you are fighting a battle between the silicone being so thick that it does not want to stretch very far, or so thin that it is prone to tear.

EasyFlow 60 should work just fine for creating your mother mold.  It has a Shore durometer rating of D65, where the example video above uses 1512 which has a Shore rating of D71 so it is a little softer.

/Chris

James Bray

I'm hearing similar advice from other resources, Chris. The multi glove mold may be the way to go.

Just a few questions:
Do you have any examples of a sculpt that uses a core in the mold? 
Would the large hole at the bottom be used to pour in the final casting material?
Would it make the glove mold process any easier, if the limbs were cast separately or is that necessary if it's casted in multi pieces?

I appreciated all your feedback. This has been a big help

Chris Ellerby

The plug would basically just be a small section of the mother mold that fills that top inner cavity so the silicone will properly hold its shape.  It would be its own piece so it can slide down into the cavity and lock into place with the other mother mold pieces.  You would also want to try and add keys so the silicone glove mold could lock into the mother mold, otherwise it could collapse a bit.

Technically it's not a core, as a core is inside the mold (bad choice of words on my part!), but just a plug shaped portion of the mother mold that would slide into the open cavity to help the silicone keep its shape.

The hole at the bottom (under the roots) would be your best bet for pouring the casting material, but it may need to be built up a bit so the area you are pouring into is higher up than the roots, or they will not fill.

Removing the roots would greatly simplify the form, meaning demolding would be easier, and your molds would last for many more castings as they would undergo less stress during the demolding process.   The only down side is it means more molds have to be made, and you'll have some assembly/cleanup on the finished pieces.  (which is pretty common)  In your case that might mean re-carving a bit of bark texture into any seams you fill.

That said, it might be possible to do it all as one glove, but It's really hard for me to wrap my head around it without spending time with the actual sculpt.  As you are new to molding, I would be careful trying an overly complex glove mold, as it could lock up on you if you are not super careful.  I've had it happen, always due to some stupid oversight I made, and it's never a good feeling!

Here is a rough image that shows how the silicone (blue green) glove mold and rigid mother mold (red) might fit around the top portion of the character.  The cylindrical sections of both components would be hollow.   This is just a cutaway for the top portion of the mold, so it does not address how you might handle the roots.

It's important to keep in mind that all your mold components will need to be able to slide off a rigid casting, otherwise the mold could lock up once filled.



/Chris

James Bray

Thanks a lot for spending the time to create the mockup graphic!

Would the plug just be a single piece or would that need to be split in 2?

I may consider the option of casting the limbs separately and maybe adding something to the design that would better hide any  seams that are created.

I'd be interested in hearing your experience with a mold locking up. I have an idea of what may happen, but an example would be great.

Chris Ellerby

The plug could be one piece, but that depends on how you construct it and the rest of the mother mold.  In this example it includes a bit of a lip so it seats on top of the sculpt, but it could also be just the inner cup shape.   How it's constructed would be based on how the mother mold sections for the rest of the sculpt are laid out.

I don't have any photos of locked molds, but I've had my share.  Couple times I forgot to add pry-points to stone molds, so the mold was not technically locked but well sealed and took a bit of a fight to open.

For me a locked mold usually happens when I'm in a rush and don't properly visualize how the demolding process will go once a rigid casting is inside.   I'm now very meticulous with my mold making and take my time to plan out pry points, keys, watching for potential locks with undercuts, etc.

With a glove mold the potential for locking is primarily determined by the rigid mother mold.  (you can always cut the silicone to demold)   As you design and lay up the mother mold just pay close attention to how it will need to slide off.

If the lock up is bad enough, and you have a thin fiberglass/resin/poly mother mold, you can take something like a dremel with a cutting disc and carefully chop up the mother mold into smaller pieces to unlock it.  Then when it's time to run another part you'll have to modify the pieces so they can go back together cleanly.

If it's a stone mold (which is the main type I've locked up in the past) you might just be out of luck if you can't open it without cracking or destroying the casting.  That's one reason I like newer composite molds, but they are much more expensive.

/Chris  

James Bray

There's still a good weeks worth of sculpting left to do, but I may update this post once I begin planning the mold more intently.

If you respond great, if not that's cool too. You've been a big help at this stage and I'll likely use most if not all the advice.

Thanks!